1.2.1 Fluid mechanics

The branch of engineering that investigates the behavior of fluids is called fluid mechanics. It is a part of a larger branch of engineering called continuum mechanics, which deals with the behavior of fluids as well as stressed solids. A fluid is a substance that does not permanently resist distortion. An attempt to alter the shape of a mass of fluid results in layers of fluids sliding over one another until a new shape is attained. During the change in shape, shear stresses exist depending upon the viscosity of the fluid and the rate of sliding, but when the final shape is achieved all the shear stresses disappear. A fluid in equilibrium is free from shear stresses.

Fluids may be compressible or incompressible. If the density of a fluid changes slightly with the changes in temperature and pressure then such a fluid is called incompressible and if the density changes significantly then such a fluid is said to be compressible. Gases are generally considered to be compressible while liquids are considered incompressible.

The behavior of fluids is very important in chemical engineering. It is a major part of unit operations principle. Understanding of fluids is essential not only for accurately treating problems in the movement of fluids through pipes, compressors, pumps, and all kinds of process equipment but also for the study of heat flow and many separation principles that depend on diffusion and mass transfer. Design and study of measuring devices (such as flow meters, area meters, pressure gauges), transportation equipment (such as compressors and pumps), and mixing and agitation equipment (such as mixers and agitators) are considered in fluid mechanics.

Fluid mechanics can be divided into two branches:

• Fluid statics
• Fluid dynamics

Fluid statics deals with the fluids at rest or in equilibrium state with no shear stress. It is concerned with the properties and behavior of fluids. In the case of liquids this subject is called hydrostatics and in the case of gases it is called pneumatics.

Fluid dynamics, also called fluid flow, deals with the flowing fluids or with fluids when section of the fluid is in motion relative to the other parts. The flow of a fluid is of two types:

• Laminar flow: The flow in which the layers of the fluid are flowing parallel to the axis of the pipe or conduit
• Turbulent flow: The flow in which the layers of the moving fluids are not parallel to the axis of the pipe and the fluid is disturbed from point to point

Chemical engineers are continuously involved with the flow of fluids. In industrial applications, they have to transport fluids from one point to another through pipes or open ducts which require the determination of pressure drops in the system, selection of a proper type of pump or compressor, power required for pumping or compression, and measurement of flow rates. All these aspects are studied in fluid flow. A major portion of fluid flow deals with the transportation, metering, and mixing & agitation of fluids.

1.2.2 Heat transfer

Heat transfer is the branch of engineering that deals with the rates of heat exchange between hot and cold bodies. The driving force for heat transfer is the temperature difference per unit area or temperature gradient. In a majority of chemical processes heat is either given out or absorbed. Most of the times the fluids must be heated or cooled in a variety of equipment such as boilers, heaters, condensers, furnaces, dryers, evaporators, and reactors. In all of these cases the fundamental problem encountered is the transferring of heat at the desired rate. Some times it is necessary to prevent the loss of heat from vessels or pipes.

The control of flow of heat at the desired rate is one of the most important areas of chemical engineering. The principles and laws governing the rates of heat flow are studied under the heading of heat transfer. Even though the transfer of heat is involved in every unit operation, in evaporation, drying, and combustion the primary concern is the transfer of heat rather than the transfer of mass and these operations are governed by the rate of heat transfer. Laws and equations of heat transfer are used for the designing of the equipment required for these processes.

Evaporation is the process used to concentrate a solution consisting of a non-volatile solute and volatile solvent. In a majority of evaporations the solvent is water.

Drying is the removal of relatively small amounts of water or other liquid from the solid material to reduce the content of residual liquid to a low value.

Heat transfer can take place through the following three modes of (heat) transfer:

• Conduction
• Convection
• Radiation

However, most of the processes are a combination of two or more modes of heat transfer.

1.2.3 Mass transfer

Mass transfer is involved with the transfer of a component in a mixture from a region in which it has a high concentration to a region in which its concentration is lower. This process can occur in a gas, liquid, or vapor. It can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion). Like temperature gradient is the driving force for heat transfer, the driving force for mass transfer is the concentration gradient. Many unit operations such as distillation, absorption, extraction, leaching, membrane separation, dehumidification, crystallization, ion exchange, and adsorption are considered as mass transfer operations. Even though transfer of heat is also involved in these operations but the rate of mass transfer governs the rate phenomena in these processes. Unlike purely mechanical separation processes, which utilize density difference and particle size, these methods make use of differences in vapor pressure, solubility, or diffusivity.

1.2.4 Solid particulate operations

Also termed as particle technology, this branch of unit operations deals with solid handling and is mainly concerned with the mixing, size reduction, and mechanical separation of solids. Solids in general are more difficult to handle than fluids. In processing solids appear in a variety of forms such as angular pieces, continuous sheets, finely divided powders. They may be hard and abrasive, tough and rubbery, soft or fragile, dusty, cohesive, free flowing, or sticky. Whatever their form, means must be found to handle these solids.

Mixing of solids resembles to some extent with the mixing of low-viscosity liquids, however, mixing of solids requires much more power. In mixing two or more separate components are intermingled to obtain a uniform product. Some of the mixers and blenders used for liquids are also used for solids. Solid mixers mainly used are kneaders, dispersers, masticators, mixer-extruders, mixing rolls, pug mills, ribbon blenders, screw mixers, tumbling mixers, and impact wheel.

1.5.1 Process selection

The selection of a process is one of the most important and time-consuming activities undertaken by a chemical engineer. One process may be energy efficient than the other but the other may be less polluting or may have its raw materials readily available. Selecting a process out from the available options is no easy job because each process has certain advantages and disadvantages. To select a process many constraints have to be faced such as time, available data, investment, and economics. As all industries are mainly concerned with profits, out of all the constraints economics always remains the chief factor in selecting a process.

Selecting a process to be in batches or to be continuous is another critical task. Early chemical processing was normally done in batches and much continues to be done in that way. Batches can be measured correctly and are much suitable for small-scale production. However, the temperature and pressure control can be troublesome.

Furthermore time and resources lost in attaining the required conditions such as temperature and pressure, limits the use of batch processes. On the other hand, continuous processes require far smaller and less expensive equipment, have much less material in process, have less chance to ruin large quantities, have more uniform operating conditions, and give more uniform products than batch processes.

Continuous processes are very suitable for large-scale productions. However, these require concise control of flow-rates and conditions, which is impossible without high quality instrumentation. The reduction in plant cost per unit of production is often the major force in selecting a process to continuous or in batches.

1.5.2 Operation

Operation of a process plant is another important activity carried out by a chemical engineer. Chemical processing of a raw material into the desired product can only be achieved by operating the chemical plant. The quality and quantity of the product is directly dependent on the efficient operation of a plant. The smooth operation of a plant is a very difficult task and requires close attention of the engineer at all times. Many problems like temperature and pressure control, maintenance, and safety continue to arise during the plant operation. Experience and application of engineering principles is always needed to shoot out these problems. Negligence of a small problem can often lead to bigger, more complex problems and can cause unnecessary halts in production.

In order to be able to handle plant operation smoothly, a chemical engineer should start early to become familiar with the industrial equipment such as pumps, compressors, distillation columns, filter presses, and heat exchangers, etc. Almost every industry wants its engineers to be intimately familiar with every pipe & gauge of that industry. That is why every industry makes its new engineers spend their earlier time in tracing pipelines, an activity known as line tracing. The reason behind this practice is to intimately familiarize the engineers with all the pipelines, gauges, valves, and equipment of that industry so that whenever there is any fault in any section he should be able to identify the location and to work out its solution immediately.

In fact, troubleshooting is the core of plant operation. Successful plant operation of a chemical plant does not only depend upon the original strength of the materials of construction but also upon the affects of corrosion. Constant check-ups and inspection must be maintained to avoid corrosion. Mechanical failures are seldom experienced unless there has been previous corrosion or weakening by chemical attack.

Chemical manufacturing process can be divided into the following steps:

• Raw material
• Physical treatment steps
• Chemical treatment steps
• Recycle
• Product

1.5.3 Instrumentation and control

In commercial scale continuous operations the function of the workers and the supervising chemical engineer is to maintain the plant in proper running conditions. Maintaining required temperature, pressure, flow-rates, and other conditions is a very difficult task. Quality instrumentation is a must for maintaining these conditions. Instruments are the essential tools for modern processing. A chemical engineer must have the proper knowledge of the instruments involved for controlling and measuring process variables. Adequate ability to design control systems for processes and workout problems faced in controlling process operations is also essential.

Batch operation requires few instruments and hence more supervision on the part of the workers and the chemical engineer because the conditions and procedures differ from the start to the finish. Programmed instruments can solve even these problems if the expense can be justified. Instrument costs, once a trivial part of the total plant investment, have risen up to 25% of the total investment.

The use of computers has reduced this cost to some extent. Earlier plants used mechanical control instruments. These were replaced by pneumatic control systems, which were replaced by electronic control systems. Currently, plant-control is being done by DCS (Distributed Control System) using computers. DCS incorporates the use of electronic control devices but it utilizes computers to monitor and control process conditions. Even though many industries continue to use pneumatic and electronic control systems, however, the global trend is towards DCS because of its ability to handle plant operation more smoothly. Instrumentation has been forced into this position of eminence by the increase in continuous processes, increase in labour and supervision costs, the relative unreliability of human actions, and by the availability of many types of instruments at decreasing price and increasing reliability.

Instrument types include indicating instruments, recording instruments, and controlling instruments. Two types of instruments are generally used: analog and digital. Analog instruments such as pressure spring thermometer and Bourdon pressure gauges show results by mechanical movement of some type of device (e.g. spring or Bourdon tube), which is proportional to the quantity being measured. Digital devices generally utilize a transducer, a device that converts the measured signal into some other type of signal usually electronic or pneumatic. These devices also use electronic circuits, which convert the signal to readable numerical figures (digits), which are then displayed and may be recorded.

1.5.4 Chemical process economics

Economics is a vital part of an engineers work. Engineers are distinguished from scientists by their consciousness of costs and profits. Economics plays a vital role in the operation, design, and maintenance of every chemical plant. A good chemical engineer always gives economics top priority in his every effort. Every engineering decision involves cost considerations. Engineers must continue to keep up with the economic changes that may affect their products.

The primary objective of an engineer’s efforts must be to deliver safely the best product or the most efficient services at the lowest cost to the employer and the consumer. Since change is an outstanding characteristic of the chemical procedures, hence potential alteration of a process is of importance not only when the plant is being designed but continuously.

Decisions based on comparative facts must be exerted in most of the important discussions of a chemical engineer. Careful calculations using local parameters generally lead to clear and just decisions. Yield and conversions of the chemical process form the basis for the material and energy balances, which in turn are the foundation for the cost determination. Primary stress must be laid on these balances to keep the plant operation economical and profitable. Economic conditions and limitations are one of the most important factors of any plant design activity. The engineer must consider costs and profits constantly throughout his work. Cost per unit product always turns out to be the key issue for any business enterprise and an engineer should always work to keep it as economical as possible. It’s almost always better to sell many units of product at a low profit per unit than a few units at a high cost. An engineer must take into account the volume for production when determining costs and total profits for various types of designs, keeping in view customer needs and demands.

1.5.5 Marketing

Whenever a new product is under assessment, market evaluation for that product becomes essential. The job of a chemical engineer then leads to the market estimation of that product. The factors generally considered in the market evaluation are the present and future supply and demand, present and future uses, new uses, present buying habits, price range for products and by-products, character, location, and number of possible customers.

The marketing of a product does not only depend upon its advertisement but also on the quality of the product, its physical conditions, and it’s packing. Good firms rarely compromise on quality. Proper instrumentation, uniform plant conditions, good operators, and careful supervision lead to quality production. The physical conditions of the products have a very strong impact on the marketability. The physical conditions involve crystal structure, particle size and shape, colours, and moisture content.

Packaging of the product also plays an important role in the marketing of a product, especially the consumer products. However, packaging is often expensive. The most economical containers are refillable bulk ones such as tanks, tank-ships, and tank-cars. But these cannot be used for the consumer products since the container appearance is very important to the customers. For consumer products quality packing with attractive colours, designs, and materials has to be used.

Price of a product is the real concern for a customer. Prices should be maintained within the affordable range of a large number of people, since bigger markets lead to larger profits. To enhance the marketing of a product, an engineer should listen to the suggestions and the information brought to him by the salesperson, since he is the link between the company and the customer.

1.5.6 Safety

Chemical engineer also has role to perform in plant safety. Nothing is more dangerous to a plant than fire. Precautions to prevent and extinguish fire must be taken. Employees must be protected against toxic chemicals. Safety measures not only keep the employees out of danger but also save money and time by reducing accidents and any unnecessary halts in the production. Even though every human being is bound to err sometime, but at times he gets careless too. Sometimes too much familiarity with chemicals breeds’ carelessness, hence well-run plants have safety devices and continuing programmes for alerting those working with a given process to its hazards. Adequate safety and fire protection measures require expert guidance. There is considerable difference of opinion in rating certain chemicals as hazardous and their degree of toxicity. There are different standards for many toxic and harmful substances, however nowadays the governments decide these standards and are very severe on their implementation.

1.5.7 Construction of a plant

The construction (erection) of a plant is another activity carried out by a chemical engineer. The presence of the chemical engineer is essential during the erection of the plant in order to implement the design standards and interpret technical and design data whenever needed. The chemical engineer should always work closely with the construction team during the final stages of construction and purchasing designs. Construction of the plant may be started before the final design is completed.

During plant erection, the chemical engineer should visit the plant site to assist in the interpretation of plans and learn methods for improving future designs. The engineer should also be available during the initial start-up of the plant and early phases of operation. During the erection of a plant the engineer becomes intimately connected with the plant and hence learns the internal structure of the plant. The chemical engineer becomes involved with the installation of every pipe and gauge of the plant and this helps greatly while running the plant and eliminating problems faced during operation.

1.5.8 Research and development

Adequate and skilled research with patent protection is required for future profits. The chemical process industry has certain salient characteristics such as rapidly changing procedures, new processes, new raw materials and new markets. Research creates or utilizes these changes. Without forward-looking investigation or research a company would lag behind in the competitive progress of its industry. Development is the adaptation of research ideas to the realities of production and industry. The progress of industry opens up new markets for even the most fundamental, established products.

1.5.9 Management

Due to the dramatic rise in productivity and the recent technological advances in the chemical process industries, this sector has become very complex. The complexity of this industry has made it very difficult for business graduates, who do not have any knowledge of chemicals and equipments, to handle it. Now the chemical companies like to have chemical engineers as their mangers.

Management is a very important aspect of plant operation. Handling the personnel, most importantly the workers is one of the most difficult jobs but a chemical engineer is always in contact with his workers and most of the time has to rely on them. Dealing the personnel is often called Human engineering. The job of a chemical engineer is to control and run machines effectively and efficiently, and there is no machine better or more complex than the human being. Controlling this machine is perhaps the most difficult task a man has to perform. But as an engineer is in constant interaction with his workers and personnel so he has to perform it effectively. Hence, a good engineer must be a good manager as well and has to listen to their opinions and understand their attitude. Keeping the personnel in high spirits and motivate them is very important.

1.5.10 Process system engineering

Many engineers are realizing that they can no longer think of a process plant as a collection of individually designed operations and processes. It is becoming increasingly evident that each separate unit of a plant influences all others in subtle ways. It is also true that the plant is a part of an ecological system extending well beyond its boundaries. The general availability of the computers has made it possible to study the dynamic behaviour of plants as well as their static or steady state behaviour. Such intense studies have shown new possibilities for plant operation not previously conceived. The next generation of engineers will be studying, analyzing, and optimizing such interacting and complex systems. This is a major improvement over envisioning design as involving simple, non-interacting, static systems that use only operations and unit processes.

1.5.11 Environment

The role of a chemical engineer in controlling pollution and waste generation can hardly be over emphasized. Chemical engineers are concentrating in the area of environmental engineering to develop new methods and techniques to treat wastes generated by the process industries, minimize waste generation and develop renewable sources of material and energy.

These engineers are working towards developing sustainable and renewable technologies. Their role in the earlier design phases of process industries has now led to new practically fume-less chemical plants.

1.5.12 Design

Design of a chemical process plant is the one activity unique to chemical engineering and is the strongest reason justifying the existence of chemical engineering as a distinct branch of engineering. Design is a creative activity and is perhaps the most satisfying and rewarding activities undertaken by a chemical engineer. It is the synthesis, the putting together of ideas to achieve a desired purpose. It is perhaps the most important task undertaken by a chemical engineer. The design does not exist at the commencement of a project. The designer starts with a specific objective in mind, a need, and by developing and evaluating possible designs, arrives at what he considers best way of achieving that objective.

A principle responsibility of the chemical engineer is the design, construction, and operation of chemical plants. In this modern world age of industrial competition, a successful chemical engineer needs more than a knowledge and understanding of the fundamentals sciences and related engineering subjects such as thermodynamics, reaction kinetics, and computer technology. The engineer must also have the ability to apply this knowledge to practical situations for the purpose of accomplishing something that will be beneficial to society. However, in making these applications, the chemical engineer must recognize the economic implications, which are involved and proceed accordingly.

Plant design includes all engineering aspects involved in the development of either a new, modified, or expanded industrial plant. In this development the chemical engineer makes economic evaluations of new processes, designs individual pieces of equipment for the proposed new venture, or develops a plant layout for coordination of the overall operation. Because of these design duties, the chemical engineer is many times referred to as design engineer. On the other hand, a chemical engineer specializing in the economic aspects of the design is often referred to as cost engineer. Chemical engineering design of new chemical plants and the expansion or revision of the existing ones require the use of engineering principles and theories combined with a practical realization of the limits imposed by individual conditions.

1.6.1 The atom

Biology, medicine, metallurgy, and power generation have all been transformed by the capability to split the atom and isolate isotopes. Chemical engineers played a significant role in achieving both of these results. Early on chemical facilities were used in warfare, which ultimately resulted in the production of the atomic bomb. Today, these technologies have found uses in more peaceful applications. Medical doctors now use isotopes to monitor bodily functions; quickly identifying clogged arteries and veins. Similarly, biologists gain invaluable insight into the basic mechanisms of life and archaeologists can accurately date their historical findings.

1.6.2 The plastic age

The start of 19^th Century witnessed tremendous achievements in polymer chemistry. However, it required the vision of chemical engineers during the 20^th century to make bulk produced polymers a viable economic reality. When a plastic called Bakelite was introduced in 1908 it heralded the dawn of the “Plastic Age” and quickly found uses in electric insulation, plugs & sockets, clock bases, iron cooking handles and fashionable jewelry. Now, plastic has become so ubiquitous that we hardly notice it exists. Yet, nearly all aspects of modern life are positively and deeply impacted by plastic.

1.6.3 The human reactor

Chemical engineers have been engaged in detailed study of complex chemical processes by breaking them up into smaller called-“unit operations.” Such operations might comprise of heat exchangers, filters, chemical reactors and the like. Subsequently, this concept has also been applied to the human body. The implications of such analysis have aided to improve clinical care, suggest improvements in diagnostic and therapeutic devices and led to mechanical wonders such as artificial organs. Medical doctors and chemical engineers continue to work in tandem to help us live longer fuller lives.

1.6.4 Wonder drugs for the masses

Chemical engineers have been adept to take small quantities of antibiotics developed by distinguished researchers such as Sir Arthur Fleming (who discovered penicillin in 1929) and increase their yields several thousand times through mutation and special brewing techniques. Today’s low price, high volume, drugs owe their existence to the work of chemical engineers. This ability to bring once scarce materials to all members of society through industrial creativity is a defining characteristic of chemical engineering.

1.6.5 Synthetic fibers

Right from blankets and clothes to beds and pillows, synthetic fibers keep us warm, cozy and provide a good night’s rest. Synthetic fibers also help reduce the strain on natural sources of cotton and wool, and can be tailored to specific applications.

1.6.6 Liquefied air

When ambient air is cooled to very low temperatures (about 320 deg F below zero) it condenses into a liquid. Chemical engineers are then capable to separate out the different components of air. The purified nitrogen can be used to recover petroleum, freeze food, produce semiconductors, or prevent unwanted reactions while oxygen is used to make steel, smelt copper, weld metals together and support the lives of patients in hospitals.

1.6.7 The environment

Chemical engineers furnish economical solutions to clean up yesterday’s waste and prevent tomorrow’s pollution. Catalytic converters, reformulated gasoline and smoke stack scrubbers all help keep the world clean. Additionally, chemical engineers help reduce the strain on natural materials through synthetic replacements, more efficient processing and new recycling technologies.

1.6.8 Food

Plants require large quantities of nitrogen, potassium and phosphorus to grow in abundance. Chemical fertilizers can help provide these nutrients to crops, which in turn provide us with a bountiful and balanced diet. Fertilizers are especially important in certain regions of our earth where food can sometimes be scarce. Advances in biotechnology also offer the potential to further increase worldwide food production. Finally, chemical engineers are at the forefront of food processing where they help create better tasting and most nutritious foods.

1.6.9 Petrochemicals

Chemical engineers have assisted to develop processes like catalytic cracking to break down the complex organic molecules found in crude oil into much simpler components. These building blocks are then separated and recombined to form many useful products including: gasoline, lubricating oils, plastics, synthetic rubber and synthetic fibers. Petroleum processing is therefore recognized as an enabling technology, without which, much of modern life would cease to function.

1.6.10 Synthetic rubber

Chemical engineers play a prominent role in developing today’s synthetic rubber industry. During World War II, synthetic rubber capacity suddenly became of great importance. This was because modern society runs on rubber. Tires, gaskets, hoses, and conveyor belts (not to mention running shoes) are all made of rubber. Whether you drive, bike, roller-blade, or run; odds are all are running on rubber.

The meaning of a chemical equation

Chemical equations give information in three major areas:

• They tell us what substances are reacting (those being used up) and what substances are products (those being made)
• The coefficients of a balanced equation tell us in what ratio the substances react or are produced
• The relative amounts of all substances involved in the reaction

Sample problem

As a sample exercise, consider the equation given below. To determine whether this reaction is balanced you must first determine how many atoms of each type are on the reactant side (left-hand side) of the equation and how many atoms of each type are on the product side (right-hand side).

In this example, you have two nitrogen atoms and two hydrogen atoms on the reactant side but only one nitrogen atom and three hydrogen atoms on the product side. For balancing the equation, we are not concerned what molecules these atoms are in, just the number of atoms of each type.

To balance this reaction, it is best to choose one kind of atom to balance initially. Let’s choose nitrogen in this case. To obtain the same number of nitrogen atoms on the product side as on the reactant side requires multiplying the number of product NH₃ molecules by two to give:

As you can see above, once we know what the molecules are (N₂, H₂, and NH₃ in this case) we cannot change them (only how many of them there are). The nitrogen atoms are now balanced, but there are six atoms of hydrogen on the product side and only two of them on the reactant side. The next step requires multiplying the number of hydrogen molecules by three to give:

As a final step, make sure to go back and check whether you indeed have the same number of each type of atom on the reactant side as on the product side. In this example, we have two nitrogen atoms and six hydrogen atoms on both sides of the equation. We now have a balanced chemical equation for this reaction

Avogadro’s number

6.022 x 10²³ is so important that it has a name. It is called Avogadro’s number and has the symbol N. It is so named in honor of Amedeo Avogadro, an Italian chemist, who, in 1811, made a critical contribution (recognized only in 1860 after his death) which helped greatly with the measurement of atomic weights.

Solution

The ratio from the problem will have N₂ and NH₃ in it.

How is it possible to know which number goes on top or bottom in the ratios? Answer: it does not matter, except that you observe the next point all the time.

When making the two ratios, be 100% certain that numbers are in the same relative positions. For example, if the value associated with NH₃ is in the numerator, then MAKE SURE it is in both numerators.

Use the coefficients of the two substances to make the ratio from the equation.

Why isn’t H₂ involved in the problem? Answer: The word “sufficient” removes it from consideration.

Let’s use this ratio to set up the proportion: NH₃/ N₂
That means the ratio from the equation is: 2 / 1
The ratio from the data in the problem will be: x / 2
The proportion (setting the two ratios equal) is: x / 2 = 2 / 1
Solving by cross-multiplying gives x = 4.00 mol of NH₃ produced.

Remarks

• Double check the equation. Lots of students go right ahead and solve using the unbalanced equation supplied in the problem (or test question for that matter).
• DON’T use the same molar mass in steps two and four.
• Don’t multiply the molar mass of a substance by the coefficient in the problem BEFORE using it in one of the steps above. For example, if the formula says 2 H₂O, DON’T use 36.0 g/mol, use 18.0 g/mol.
• Don’t round off until the very last answer.

A graphical representation is given below.

Molarity

The molarity of a solution is calculated by taking the moles of solute and dividing by the liters of solution.

Dilution: Definition and Calculations

To dilute a solution means to add more solvent without the addition of more solute. Of course, the resulting solution is thoroughly mixed so as to ensure that all parts of the solution are identical. The fact that the solvent amount stays constant allows us to develop calculation techniques.

Moles before dilution = moles after dilution

From the definition of molarity, the moles of solute equals the molarity times the volume.

Hence it is now possible to substitute MV (molarity times volume) into the above equation, like this:

M₁V₁ = M₂V₂

In the following sections we will review the other measurements that a chemical engineer uses in a process plant.

Base units or units

These units indicate the dimensions of mass, length, temperature, time, electrical current and light intensity.

Multiple units

They are defined as multiples or fractions of base units such as minutes, hours and milliseconds, all of which are defined in terms of the base unit of a second. Multiple units are defined for convenience rather than necessity.

Derived units

They are obtained in one of two ways:

• By multiplying and dividing base or multiple units (cm², ft/min, kg m/s², etc). Derived units of this type are commonly referred to as compound units
• As defined equivalents of compound units (e.g. 1 erg = 1g cm/s²)

During the 1960’s, an international conference proposed a system of metric units that is widely accepted in the scientific and engineering field. It is known as the “Systeme Internationale d’Unites,” or the SI system for short. Unit prefixes are used in the SI system to indicate powers of ten. The CGS (Centimeter, Gram, Second) system is almost identical to the SI system; the main difference being that grams (g) and centimeters (cm) are used instead of kilograms and meters as the base units of mass and length. The base units of the American engineering system are the foot (ft) for length, the pound mass (lb_m) for mass and the second (s) for time.

F = ma / g_c

The weight of an object is the force exerted on the object by the gravitational attraction of the earth. Consider an object of mass m, is subjected to a gravitational force W (W is by definition the weight of the object). If this object were falling freely its acceleration would be g. The weight, mass and free fall acceleration of the object are related by the following equation:

W = mg / gc

The value of g at sea level and 45⁰ latitude and corresponding value of g/g_c are given below in each system of units:

g = 9.8066 m/s² => g/g_c = 9.8066 N/kg

g = 980.66 cm/s² => g/g_c = 980.66 dyne/g

g = 32.174 ft / s² => g/g_c = 1 lbf / lb_m

Mass and volumetric flow rate

Continuous processes involve the movement of material from various process units. The rate, at which material is transported through a process line, is the flow rate of that material.

The flow rate of a process stream may be expressed as a mass flow rate (mass/time) or as a volumetric flow rate (volume/time). Consider a fluid (gas or liquid) which flows in a cylindrical pipe as shown in Figure 2.5, where the shaded area represents a section perpendicular to the direction of flow.

If the mass flow rate of the fluid is m (kg/sec), then every second m kilograms of the fluid pass through the cross section. If the volumetric flow rate of the fluid at the given cross-section is V (m³/sec), then every second V cubic meters of the fluid pass through the cross section. However, the mass m and the volume V of a fluid (in this case, the fluid that passes through the cross section each second) are not independent quantities but are related through the fluid density ρ:

ρ = m / V

Fluid pressure and hydrostatic head

A pressure is the ratio of a force to the area on which it acts. Accordingly, pressure units are force units divided by area units (e.g. N/m², dynes/cm²….). The SI pressure unit (N/m²) is called a Pascal (Pa).

Consider a fluid (gas or liquid) contained in a closed vessel or flowing through a pipe and suppose that a hole of area A is made in the wall of the containing vessel, as in Figure 2.6. The fluid pressure may be defined as the ratio (F/A), where F is the minimum force that would have to be exerted on a plug in the hole to keep the fluid from emerging.

Consider a vertical column of fluid is h meters high and has a uniform cross sectional area A(m²) Fig 3.3. Further, assume that the fluid has a density of ρ(kg / m³) and that a pressure p_o (N/m² ) is exerted on the upper surface of the column. The pressure P of the fluid at the base of the column is called the hydrostatic pressure of the fluid (F/A). F thus equals the force on the top surface plus the weight of the fluid in the column.

P = P₀ + ρ(g/g_c) h

An iron bar rusts

The iron reacts with oxygen in the air to make rust.

Methane burns

Methane combines with oxygen in the air to make carbon dioxide and water vapor.

As a general rule, biochemical process makes poor examples of basic chemical reactions because the actual reaction is carried on within living things and under enzyme control.

Advantages

A continuous process operates at a steady state in which all the variables go to stable value. How fast this steady state is reached depends on the residence time distribution behaviour of the process and this varies from a single mean residence time for a process with ideal plug flow and multiple mean residence times for a fully back mixed reactor like for instance a CSTR. After the steady state is reached, the heat production becomes constant making accurate temperature control much better.

Because of the time independent conversion-space relationship it is possible to fine-tune the exact location and flow rates of additional feeds. In a traditional semi-batch process these additions are generally made on a time based profile because there are not sufficient robust on-line sensors to allow feedback control. For the continuous process, the slower off-line measurements can be used to adjust for instance temperature or feed flow rates. This way, the product properties can be kept constant. This contrasts sharply with a semi-batch process where minor disturbances in the early stages of the reaction (caused by for instance poor temperature control) result in a different conversion-time history and in different product properties because the additions are made at the “wrong” time.

Disadvantages

Emulsion polymerisation can be very sensitive to residence time distribution, especially if particle nucleation is involved. It has been shown that a reactor system with too much back mixing (the ultimate case being a single CSTR) can result in non-steady behaviour. In that case the particle number and the conversion can start to oscillate resulting in a variation in product properties. Even if these oscillations do not occur, the resulting number of particles (and because of this also the volumetric production rate) will be considerably lower.

Another negative effect of residence time distribution is that if a grade change is required also some “twilight” product will be produced which has some intermediate properties and, depending on what change is imposed, has to be considered as waste or low quality product. This limits the flexibility of reactor systems with considerable back-mixing to applications where large amounts of a single grade are required and where the different grades differ only slightly. In all other cases, only a continuous reactor system with near plug flow behaviour will be suitable.

Palladium

• Good Selectivity
• Suitable Micro porous Structure
• High Strength
• Long Life (over Two Years)

Platinum

Platinum is used as a catalytic agent in processing of nitric acid, fertilizers, synthetic fibers, and a variety of other materials. In catalytic processes, the catalyst material is not consumed and can be recycled for future use. This makes chemical demand for platinum quite volatile. Platinum is essential in many of these processes and there are few satisfactory substitutes.

Rhodium

Rhodium catalyst gives extremely high activity in hydrogenation of an aromatic compound. It hydrogenates many compounds at room temperature and atmospheric pressure. Normally, a palladium metal catalyst is used for hydrogenation of olefins, but rhodium catalyst gives even higher activity than palladium metal catalyst in this reaction.

Iridium

The iridium catalyst proved to be more stable under a wide range of conditions, and more soluble so that it has no tendency to precipitate out of solution. This means that the catalyst can be continuously recycled within the plant. New catalyst does not need to be added. As well as this iridium is considerably cheaper.

The rarest of the PGMs, iridium is second only to osmium as the densest element and is the most corrosive resistant known. It is white with a yellowish hue.

Although brittle, it is extremely hard (over 4 times that of platinum itself) and with its high melting point, temperature stability and corrosion resistance is used in high-temperature equipment such as the crucibles used to grow crystals for laser technology.

Its biological compatibility is what we owe most to iridium as this enables it to be used in a range of medical and surgical applications. Iridium can be found in health technology combating cancer, Parkinson’s disease, heart conditions and even deafness and blindness.

Its durability prolongs the life of electronic components and products, which exploit iridium’s conductivity and stability.

A shiny, oxidation-resistant metal, iridium also adds to the brilliance and durability of jewels. It also has industrial applications such as the production of chlorine and caustic soda.

Osmium

Osmium is the densest substance known and the hardest of all Platinum Group Metals (PGMs.). It is 10 times harder than platinum itself.

It is these extraordinary qualities that see osmium used in a range of applications in which frictional wear must be avoided, including fountain pen nibs, styluses, and instrument pivots. Especially when alloyed to other PGMs.

Its conductivity means it can be used as a more effective and durable alternative to gold as plating in electronic products.

Like the other PGMs it is an extremely efficient oxidation catalyst and contributes to the environment through use in fuel cells. This quality is also uniquely applied in forensic science for staining fingerprints and DNA (as osmium tetroxide).

Ruthenium

Ruthenium’s catalytic qualities make it a key element in catalysts for fuel cells. Due to its hardness and corrosion resistance, ruthenium is used to coat electrodes in the chloralkali process, which produces chlorine and caustic soda for a wide range of industrial and domestic applications.

In the future, the use of ruthenium in alloys for aircraft turbine blades will help reduce the CO₂ impact of air travel on the environment. If current prototypes are successful, their high melting points and high temperature stability will allow for higher temperatures and, therefore, a more efficient burning of aircraft fuel.

Beverages

Beer and lager are alcoholic drinks made in the following way:

• A source of sugar (barley) is added to water and allowed to grow (barley is a plant!). Enzymes in the mixture start to make sugars
• The mixture is then flavoured with hops
• After a bit of a technical process, this mixture has yeast (a living fungus and there are lots of them) added to it
• The yeast ‘eats’ the sugar and gives out alcohol as a waste product

Baking

Bread is a food made in the following way:

Flour, water, salt, yeast and other ingredients (to give flavour) are mixed together to give ‘dough’. The dough is left to ‘rise’. Both of the reactions described above start to happen. The carbon dioxide gas bubbles get trapped in the dough and it starts to get a bit bigger (a bit like a balloon). Alcohol is also made in the dough. Then the dough is ‘kneaded’ to get rid of any big gaps. It may be allowed to rise again, followed by kneading again. The dough is shaped and ‘baked’ at over 100°C. During the baking, the yeast is killed (remember, it starts as a living fungus), the alcohol evaporates away (no getting drunk on bread then!) and sugars remain, giving extra flavour to the bread once it has cooled. Fermentation takes place from when the flour and yeast meet, right up to the temperature at which the yeast dies (about 46°C).

When milk ‘goes off’, this means certain bacteria from around the milk start to multiply, using the milk as a food. The waste materials given out by the bacteria are often nasty and, what’s more, if we drink the milk, the bacteria may multiply in us! Even some harmless bacteria will ‘sour’ milk.

Yoghurt

Milk contains sugar. The bacterium Lactobacillus bulgaricus is added to the milk and its enzymes ferment the sugar (at about 43°C for 4 to 5 hours) into lactic acid. It is this acid the gives yoghurt its pleasant acidic taste. It is also this acid, which stops harmful bacteria from multiplying. The yoghurt is now more like a watery paste.

Fruit and their juices can be added to give a range of flavoured yoghurts.

Most yoghurt has been pasteurized (heated to the kill the bacteria), but this is not done to ‘live’ yoghurts, which still have the bacteria doing their job.

Another microbe called Streptococcus thermophilus is also present in live yoghurt giving it its creamy flavour. Some yeast may also be present.

Cheese

Traditionally, an enzyme called rennin was taken form the stomach juices of calves. When this enzyme is added to milk, it starts to ‘clot’ (form a paste). If the watery part of the mixture (the ‘whey’) is removed, the part, which is left behind, is the ‘curd’, the raw material of cheese. Salt is then added.

Some specially designed Lactobacillus bacteria are also used instead to make the curd.

There are many cheeses. The curd can be allowed to age slightly to give cream cheeses or it can be squeezed and left much longer to slightly decompose giving Cheddar and Cheshire cheeses. If the curd is left long enough, decomposition goes so far that the cheese starts to turn back into a liquid.

Fractional Conversion

It is the fraction of a reactant that has undergone a chemical change at a particular stage of the reaction process.

Yield

The yield of a product is the ratio of the quantity of the product actually obtained to its maximum obtainable quantity.

Selectivity

Total or integral selectivity, is the ratio between the amount of reactant used up in a desired reaction and the total amount of the same reactant used up during the overall reaction.

Throughput and Production Rates

Throughput is the quantity of the product obtained per unit time. The maximum throughput of a chemical plant is normally referred to as its design capacity.

Production rate is defined as the throughput per unit of some quantity characterizing the standard geometry of the equipment, such as its volume, cross-sectional area, etc.

4.2.1 Ideal gas law

The ideal gas law is based on the kinetic theory of gases by assuming that gas molecules have a negligible volume, exert no forces on one another and collide elastically with the walls of their container.

PV= nRT

P = absolute pressure of a gas
V = volume or volumetric flow rate of the gas
n = number of moles or molar flow rate of the gas
R = the gas constant, whose value depends on the units of P, V, n, and T
T = absolute temperature of the gas
The equation may also be written as PV=nRT
Where = (V/n) is the molar volume of the gas.

A gas, whose P-V-T behavior is well represented by the above equation, is said to behave as an ideal gas or perfect gas.

4.2.2 Standard conditions

To perform P-V-T calculations utilizing the ideal gas law requires values of R with different units and it can be made easy by the following method:

PV = nRT

and for a set of arbitrarily chosen reference conditions

P_sV_s = n_sRT_s

and then to divide the first equation by the second:

PV / P_sV_s = nT / n_sT_s

4.2.3 Ideal gas mixtures

Suppose n_A moles of substance A, n_B moles of B, n_c moles of C, etc are contained in a volume V at a temperature T and total pressure P. The partial pressure p_A and partial volume v_A of A in the mixture are defined as follows:

p_A = The pressure that would be exerted by n_A moles of A alone in the same total Volume V at the same temperature T.

v_A = The volume that would be occupied by n_A moles of A alone at the total pressure P and temperature T of the mixture.

Consider each of the individual mixture components and that the mixture as a whole behaves in an ideal manner (this is the definition of an ideal gas mixture). If there are n moles of all species in the volume V at a pressure P and temperature T, then

PV = nRT

In addition, from the definition of partial pressure,

p_AV = n_ART

Dividing the second equation by the first yields

4.3.1 Types of liquid-column manometers

Open manometer

In the “open” type, air exerts pressure on the liquid in one arm of the U-tube. The difference in liquid level in the two arms is a measure of the gas pressure relative to air pressure.

Mercury manometer

The mercury manometer employs two mercury reservoirs. The mercury in one reservoir is displaced by gas pressure changes resulting from water-level fluctuations over the orifice. This displacement activates a motor that moves the other reservoir to balance the pressure change. This movement is converted to a shaft rotation for recording.

Well type manometer

Well manometers are direct reading device used for process monitoring, general-purpose production testing or laboratory measurement. These instruments may also be used for tank level, flow measurement and leak detection. Well manometers are constructed of aluminum channel, stainless steel end blocks and stainless steel manometer well. The glass tubing is a yoke packed with viton gaskets at each end block and is supported at spaced intervals to prevent distortion. In most cases, the uncertainty of a manometer reading is + 1/2 of the smallest scale graduation. This is due to the human eye’s ability to interpolate between graduations

Inclined type manometer

Inclined manometers offer greater readability by stretching a vertical differential along an inclined indicating column, giving more graduations per unit of vertical height. This effectively increases the instrument’s sensitivity and accuracy. Scales are typically graduated to the hundredth of an inch.

4.3.3 Limitations of manometer

• While it can be adapted to measure very small pressure differences, it cannot be used conveniently for large pressure differences – although it is possible to connect a number of manometers in series and to use mercury as the manometric fluid to improve the range
• Some liquids are unsuitable for use because they do not form well-defined menisci. Surface tension can also cause errors due to capillary rise; this can be avoided if the diameters of the tubes are sufficiently large – preferably not less than 15 mm diameter
• A major disadvantage of the manometer is its slow response, which makes it unsuitable for measuring fluctuating pressures
• It is essential that the pipes connecting the manometer to the pipe or vessel containing the liquid under pressure should be filled with this liquid and there should be no air bubbles in the liquid

4.4.1 Types of mechanical pressure gauges

Bourdon and Diaphragm Gauges that show both pressure and vacuum indications on the same dial are called compound gauges.

Bourdon-tube gauge

Bourdon tubes are circular-shaped tubes with an oval cross-sectional appearance. The pressure of the media acts on the inside of this tube, which results in the oval cross section becoming almost round. Because of the curvature of the tube ring, the bourdon tube bends when tension occurs. The end of the tube, which is free, moves, thus being a measurement for the pressure. A pointer indicates this movement. Bourdon tubes bent at an angle of approx. 250° are used for measuring pressures up to approximately 60 bar.

For higher pressure, Bourdon tubes are utilized which has a number of superimposed coils of the same diameter (i.e. helical coils) or helical-shaped coils (i.e. helical springs) at one level. Bourdon tubes can only be protected against overload to a limited extent. For specific measuring operations, the pressure gauge can be provided with a chemical seal as a separation or protection system.

The pressure ranges are between 0…0.6 and 0…4000 bar with a reading accuracy (or accuracy class) from 0.1 to 4.0%.

Diaphragm gauge

Diaphragm elements are circular-shaped, convoluted membranes. They are either clamped around the rim between two flanges or welded and exposed t to the pressure of the media acting on one side. The deflection caused in this way is used as a measurement for the pressure and is indicated by a pointer. When compared with bourdon tubes, these diaphragm elements have a relatively high activating force and because of the circular clamping of the element, they are insensitive to vibration. The diaphragm element can be subjected to higher overload through load take-up points (by bringing the diaphragm element against the upper flange).

The measuring instrument can also be protected against extremely corrosive media by coating with special material or covering with foil. Pressure ranges are between 0…16 mbar and 0…40 bar in accuracy class 0.6 to 2.5.

4.4.2 Pressure gauges with capsule element

The capsule element comprises of two circular-shaped, convoluted membranes sealed tight around their circumference. The pressure acts on the inside of this capsule. A pointer indicates the stroke movement as a measurement of pressure. Pressure gauges with capsule elements are more suitable for gaseous media and relatively low pressures. Overload protection is possible within certain limits. The activating force is increased if a number of capsule elements are connected mechanically in series (a so-called capsule element “package”). Pressure ranges are between 0…2.5 mbar and 0…0.6 bar in the accuracy class 0.1 to 2.5.

Absolute pressure gauges

These instruments are employed where pressures are to be measured independently of the natural fluctuations in atmospheric pressure. As a rule, all the known types of element and measuring principles can be applied.

The pressure of the media to be measured is compared against a reference pressure, which at the same time is absolute zero. For this purpose, an absolute vacuum is given as reference pressure in a reference chamber on the side of the measuring element not subject to pressure. Sealing off the appropriate measuring chamber or surrounding case accomplishes this function. Measuring element movement transmission and pressure indication follow in the same way as with the already described overpressure gauges.

Differential pressure gauges

The difference between two pressures is evaluated directly and shown on the differential pressure gauge. Two sealed medium chambers are separated by the measuring element or measurement elements, respectively. If both operating pressures are the same, the measuring element cannot make any movement and no pressure will be indicated. A differential pressure reading is only given when one of the pressures is either higher or lower. Low differential pressures can be measured directly in the case of high static pressures.

Very high overload capability is reached with diaphragm elements. The permissible static pressure and the overload capability on the + und – side must be observed. Transmission of the measuring element movement and pressure indication is the same as with the already described overpressure gauges in the majority of cases. Pressure ranges are between 0…16 mbar and 0…25 bar in the accuracy class 0.6 to 2.5.

The typical areas of application are:

• Filter technology (monitoring filter pollution)
• Filling level measurement (in enclosed containers)
• Flow measurement (pressure drop)

4.5.1 Variable head meters

Head meters are the most common types of meter utilized to measure fluid flow rates. They measure fluid flow indirectly by creating and measuring a differential pressure by means of an obstruction to the fluid flow. Using well-established conversion coefficients, which depend on the type of head meter used and diameter of the pipe, a measurement of the differential pressure may be translated into a volume rate.

Head meters are simple, reliable, and offer more flexibility than other flow measurement methods. The head-type flow meter usually consists of two components:

• Primary device
  The primary devices are placed in the pipe to restrict the flow and develop a differential pressure. The primary device can be selected for compatibility with the specific fluid or application
• Secondary devices
  The secondary device measures the differential pressure and provides readout or signal for transmission to a control-measuring device is not required in the field. The secondary device can be selected for the type or readout of signal transmission desired

Orifice meter

The orifice meter consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. There are three recognized methods of placing the taps. And the coefficient of the meter will depend upon the position of taps.

The principle of the orifice meter is identical with that of the venturi meter. The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by a manometer. Bernoulli’s equation provides a basis for correlating the increase in velocity head with the decrease in pressure head.

A practical advantage of this device is that cost does not increase significantly with pipe size.

Flow nozzle

Flow nozzles may be assumed a variation on the venturi tube. The nozzle opening is an elliptical restriction in the flow but with no outlet area for pressure recovery. Pressure taps are located approximately 1/2 pipe diameter downstream and 1 pipe diameter upstream.

The flow nozzle is a high velocity flow meter used where turbulence is quite high (Reynolds numbers above 50,000) such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls between that of the venturi tube and the orifice plate (30 to 95 percent).

Pitot tube

The pitot tube is a device to measure the local velocity along a streamline. The pitot tube has two tubes: one is static tube (b), and another is impact tube (a). The opening of the impact tube is perpendicular to the flow direction. The opening of the static tube is parallel to the direction of flow. The two legs are connected to the legs of a manometer or equivalent device for measuring small pressure differences. The static tube measures the static pressure, since there is no velocity component perpendicular to its opening. The impact tube measures both the static pressure and impact pressure (due to kinetic energy). In terms of heads, the impact tube measures the static pressure head plus the velocity head.

The pitot tube measures the velocity of only a filament of liquid, and hence it can be used for exploring the velocity distribution across the pipe cross-section. If, however, it is desired to measure the total flow of fluid through the pipe, the velocity must be measured at various distances from the walls and the results integrated. The total flow rate can be calculated from a single reading only of the velocity distribution across the cross-section is already known.

4.5.2 Area meters

These meters consist of devices in which the pressure drop is nearly constant. The area through which the fluid flows varies with the flow rate. The area is related through proper calibration to the flow rate.

Rotameters

Rotameters (also known as variable-area flowmeters) are typically made from a tapered glass tube that is positioned vertically in the fluid flow. A float that is the same size as the base of the glass tube rides upward in relation to the amount of flow. Because the tube is larger in diameter at the top of the glass than at the bottom, the float resides at the point where the differential pressure between the upper and lower surfaces balance the weigh if the float. In most rotameter applications, the flow rate is read directly from a scale inscribed on the glass; in certain cases, an automatic sensing device is used to sense the level of the float and transmit a flow signal. These “transmitting rotameters” are often made from stainless steel or other materials for various fluid applications and higher pressures.

It comprises of a gradually tapered glass tube mounted vertically in a frame with the large end up. The fluid flows upward through the tapered tube and freely suspends a float. The float is the indicating element. The entire fluid stream must flow through the annular space between the float and the tube wall. The tube is marked in divisions and the reading of the meter is obtained from the scale reading of the float. A calibration curve must be available to convert the observed scale reading to flow rate. Rotameters can be used for either liquid or gas flow measurements.

Rotameters may range in size from 1/4 inch to greater then 6 inches. They measure a wider band of flow (10 to 1) than an orifice plate with an accuracy of ±2 percent, and a maximum operating pressure of 300 psig when constructed of glass. Rotameters are commonly used for purge flows and levels.

Target meters

In a target meter a sharp-edged disk is fixed at right angles to the direction of flow and the drag force exerted by the fluid is measured. The flow rate is proportional to the square root of this force and to the fluid density. Target meters are rugged and inexpensive and can be used with a variety of fluids, even viscous liquids and slurries. The bar mechanism, however, tends to clog if the solids content of the slurry is high.

Target flowmeters may be used where rough accuracy is required or where the fluid is extremely dirty. A disk or body is immersed into the fluid stream perpendicular to the flow. The differential pressure forces acting on the target are sensed by a strain either gage or force balance method. The magnitude of the strain gage signal or energy required to maintain balance is proportional to the fluid flow. Target flow meters may be used in applications where the flowing fluid has sufficient momentum to cause the required pressure differential.

The target flowmeter is located where turbulence, pulsation, or vibrations are minimized. If mass flow rate outputs are required, then the target flowmeter requires other readings to infer mass flow. Manual or computer calculations incorporating physical process measurements such as absolute pressure, differential pressure, temperature and viscosity readings must be applied to the output signal to obtain the actual flow rate. These meters typically have a turn down ratio of 10:1.

Vortex-shedding meters

As the fluid flows over a bluff body, vortices are alternately formed downstream on either side of the bluff body. The frequency of the vortices is proportional to the fluid velocity.

Various sensing methods can be applied to measure the frequency of the vortices. If mass flow rate outputs are required, then the vortex shedder flowmeter requires other readings to infer mass flow. Manual or computer calculations incorporating physical process measurements such as absolute pressure, differential pressure, temperature and viscosity readings must be applied to the output signal to obtain the actual flow rate.

In a vortex-shedding meter, the “target” is a bluff body, often trapezoidal in cross section. The body is designed to create, when flow is turbulent, a “vortex street” in its wake. Sensors close to the bluff body measure the pressure fluctuations and hence the frequency of the vortex shedding from which the volumetric flow rates may be inferred.

These meters are suitable for many types of fluids, including high-temperature gas and steam. The minimum Reynolds number required for a linear response is fairly high, so the flow rate of high viscous liquids cannot be measured by this type of instrument.

Turbine meters

Turbine flowmeters consist of inlet flow conditioners, rotor, rotor supports, rotor bearings, housing, and signal pick-off coil. A turbine rotor has multiple blades, and the velocity of rotation sensed by the pick-off coil is proportional to flow.

Turbine flowmeters are sensitive to density and viscosity fluctuations. If mass flow rate outputs are required, then the turbine flowmeter requires other readings to infer mass flow. Manual or computer calculations incorporating physical process measurements such as absolute pressure, differential pressure, temperature and viscosity readings must be applied to the output signal to obtain the actual flow rate. Clean fluids are required to prevent contamination of the bearings unless sealed bearings are used. Turbine meters typically have a turn down ratio of 10:1, but with special care, it is possible to achieve 20:1. Other rotational meters are the propeller, paddle wheel, impeller, rotor and rotating cup flowmeters. They are exceptionally accurate when used under proper conditions but tend to be fragile and their maintenance costs may be high.

4.5.3 Positive displacement meters

Positive Displacement (PD) flow meters measure the volumetric flow rate of a liquid or gas by separating the flow stream into known volumes and counting them over time. Vanes, gears, pistons, or diaphragms are used to separate the fluid. PD flow meters provide good to excellent accuracy and are one of only a few technologies that can be used to measure viscous liquids. Positive displacement flow meters may incorporate oval gears, helical gear, pistons, lobed impeller, sliding vanes, or nutating disks. This type of flow meter entraps a known quantity of fluid per pulse and by totaling up the pulses over time the fluid flow rate is known. If mass flow rate outputs are required, then the positive displacement flow meter requires other readings to infer mass flow. Manual or computer calculations incorporating physical process measurements such as absolute pressure, differential pressure, temperature and viscosity readings must be applied to the output signal to obtain the actual flow rate.

They are very accurate and suitable for clean gases and liquids, even viscous ones. They perform well under high viscosities and can handle dirty liquids or slurries. They are relatively expensive and may be costly to operate.

Reciprocating piston meters

Reciprocating piston meters are of the single and multiple-piston types. The specific choice depends on the range of flow rates required in the particular application. Piston meters can be used to handle a wide variety of liquids. Liquid never encounters with gears or other parts that might clog or corrode.

Oval-gear meters

Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.

Nutating-disk meters

Nutating-disk meters have a moveable disk mounted on a concentric sphere located in a spherical side-walled chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.

A pin extending perpendicularly from the disk is connected to a mechanical counter that monitors the disk’s rocking motions. Each cycle is proportional to a specific quantity of flow. As is true with all positive-displacement meters, viscosity variations below a given threshold will affect measuring accuracies. Many sizes and capacities are available. The units can be made from a wide selection of construction materials.

The most common type of displacement flow meter is the nutating disk, or wobble plate meter.

This type of flow meter is normally used for water service, such as raw water supply and evaporator feed. The movable element is a circular disk, which is attached to a central ball. A shaft is fastened to the ball and held in an inclined position by a cam or roller. The disk is mounted in a chamber, which has spherical sidewalls and conical top and bottom surfaces. The fluid enters an opening in the spherical wall on one side of the partition and leaves through the other side. As the fluid flows through the chamber, the disk wobbles, or executes a nutating motion. Since the volume of fluid required making the disc complete one revolution is known, the total flow through a nutating disc can be calculated by multiplying the number of disc rotations by the known volume of fluid.

To measure this flow, the motion of the shaft generates a cone with the point, or apex, down. The top of the shaft operates a revolution counter, through a crank and set of gears, which is calibrated to indicate total system flow. A variety of accessories, such as automatic count resetting devices, can be added to the fundamental mechanism, which performs functions in addition to measuring the flow.

4.5.4 Magnetic meters

Magnetic flow meters follow Faraday’s law of Electromagnetic induction. In a magnetic flowmeter, an electromotive force (flux) is generated perpendicular to the conductive fluid as it passes through a magnetic field in a nonmagnetic conduit. An electromagnet is excited by a pulsed DC either current or sinusoidal AC current. Through ion exchange an electro-motive-force (emf) is produced across an electrode pair, thereby providing the magnetic flowmeter with an emf output signal proportional to the fluid velocity. Magnetic flowmeters are used in conductive liquid applications exclusively. Normally, when installed they provide an unobstructed flow. Typically the minimum conductivity of the fluid is in the order of 0.1 microsiemns/cm so magnetic flowmeters won’t work for most gases and petroleum products. These meters typically have a turn down ratio of 10:1.

Electromagnetic meters

Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive. The flow tube mounts directly in the pipe. Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable

Electromagnetic meters are used widely in urban and wastewater systems and in industrial applications where a high degree of accuracy is required. They could be used in similar configurations to ultrasonic meters.

Electromagnetic flowmeters have major advantages:

• High degree of accuracy (<0.5%±) and consistent over full flow range when calibrated correctly
• No obstructions to flow
• Robust with only minimal routine maintenance
• They can measure difficult and corrosive liquids and slurries
• They can measure forward as well as reverse flow with equal accuracy

Disadvantages:

• Relatively high cost
• Power supply required
• Electronic components liable to lightning damage
• Repairs require skilled technician and specialized equipment

These meters are non-intrusive. They create no pressure drop in the fluid. The rate of flow is measured from outside the tube. Commercial magnetic meters can measure the velocity of almost all liquids except hydrocarbons, whose electrical conductivity is too small.

4.5.5 Ultrasonic meters

The ultrasonic meter can measure water, wastewater, hydrocarbon liquids, organic or inorganic chemicals, milk, beer, lube oils and the list goes on. The basic requirement is that the fluid is ultrasonically conductive and has a reasonably well formed flow. Clamp-on ultrasonic flowmeters measure flow through the pipe without any contact with the process media, ensuring that corrosion and other effects from the fluid will not affect the workings of the sensors or electronics.

Doppler and transit-time flowmeters are two types of ultrasonic flow meters that have been extensively used in liquid applications. Both transit time and Doppler ultrasonic flowmeters may use clamp-on sensors with their associated assemblies and detect flow rate from the outside of the pipe without stopping the process or cutting through the pipe.

The ultrasonic transducers can be mounted in one of two modes. The upstream and downstream ultrasonic transducers can be installed on opposite sides of the pipe (diagonal mode) or on the same side (reflect mode)

The electronics unit will measure internally, the time it takes signals to transmit from one transducer to another. At zero flow, we see no difference in time, but when flow is introduced, time for the transmission of signal from the downstream transducer to the upstream transducer will take longer than the upstream to downstream. Hence, we will see a time differential, which has a relationship with the velocity of the fluid being measured. Knowing the internal diameter of the pipe, we can now calculate a volumetric flow for the liquid.

Obviously there are factors such as solid and liquid refractive angles, error transmissions and poor sonic conductivity at times, but the inbuilt software will normally deal with these by telling us how we should space the transducers and by rejecting erroneous data it receives.

It is important when installing an ultrasonic transit time flow meter to select a location where we would find the most fully formed flow profile, this means that we should avoid bends and try to install our meters on straight runs of pipe. A rule of thumb in the industry is to give at least 10 diameter lengths upstream and 5 lengths downstream. If we are measuring liquids such as water with known properties and sonic velocities, we can further check our measurements by several diagnostic methods. Using the keypad we can see what the actual sonic velocity of water is being measured on our installation. This is one of the simplest and quickest methods.

4.5.6 Coriolis meters

Coriolis meters measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.

Coriolis meters are also available in various designs. A popular unit consists of a U-shaped flow tube enclosed in a sensor housing connected to an electronics unit. The sensing unit can be installed directly into any process. The electronics unit can be located up to 500 feet from the sensor.

Inside the sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a magnetic device located at the bend of the tube. The vibration is similar to that of a tuning fork, covering less than 0.1 in. and completing a full cycle about 80 times/sec. As the liquid flows through the tube, it is forced to take on the vertical movement of the tube. When the tube is moving upward during half of its cycle, the liquid flowing into the meter resist being forced up by pushing down on the tube.

Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. The amount of twist is directly proportional to the mass flow rate of the liquid flowing through the tube. Magnetic sensors located on each side of the flow tube measure the tube velocities, which change as the tube twists. The sensors feed this information to the electronics unit, where it is processed and converted to a voltage proportional to mass flow rate. The meter has a wide range of applications from adhesives and coatings to liquid nitrogen.

4.5.7 Calorimetric meter

Calorimetric or energy balance thermal mass flowmeters require one heating element and two temperature sensors. Although many design variations exist, they all have a similar operating method. Typically, the heater is attached to the middle of a flow tube with a constant heat input. Two matched RTDs or thermocouples are attached equidistant upstream and downstream of the heater.

The temperature differential at flowing conditions is sensed, producing an output signal. Because both temperature sensors see the same temperature and pressure effects, the design is inherently unaffected by density changes and the result will be a true mass flow output. Limitations of this flowmeter design would commonly be a maximum flow rate of 200 liters per minute, non-industrial packaging, and a tendency to clog in dirty fluids. These meters typically have a turn down ratio of 10:1.

Constant power thermal mass flow meters

Constant power thermal mass flowmeters are thermal (heat loss) mass flowmeters and require three active elements. A constant current heating element is coupled to an RTD. This heated RTD acts a heat loss flow sensor while a second RTD operates as an environmental temperature sensor. When the fluid is at rest the heat loss is at a minimum. Heat loss increases with increasing fluid velocity. In this method of operation the mass of the sensor must change, it’s temperature, making it slow to respond to fluid velocity changes. In addition, this method of operation has a limited useful temperature range due to the constant current applied.

The dynamic temperature range may be widened by applying more power (current) to the heater, but this can result in excessive heat applied to the heater when the fluid is at rest. The effects of variations in density are virtually eliminated by molecular heat transfer and sensor temperature corrections. These meters typically have a turn down ratio of 100:1.

Constant temperature thermal mass flow meters

Constant temperature thermal mass flowmeters are thermal mass flowmeters (heat loss). There are three basic operating methods, which are commonly used to excite the sensor. Constant temperature thermal mass flowmeters require two active sensors (typically platinum RTDs) that are operated in a balanced state. One acts as a temperature sensor reference; the other is the active heated sensor. Heat loss produced by the flowing fluid tends to unbalance the heated flow sensor and it is forced back into balance by the electronics. With this method of operating the constant temperature sensor, only the skin temperature is affected by the fluid flow heat loss. This allows the sensor core temperature to be maintained and produces a very fast response to fluid velocity and temperature changes. Additionally, because the power is applied as needed, the system has a wide operating range of flow and temperature.

The heated sensor maintains an index of overheat above the environmental temperature sensed by the unheated element. The effects of variations in density are virtually eliminated by molecular heat transfer and sensor temperature corrections. These meters typically have a turn down ratio of 1000:1 when properly sized.

4.5.8 Open channel meters

The common method of measuring flow through an open channel is to measure the height or HEAD of the liquid as it passes over an obstruction (a flume or weir) in the channel. Using ultrasonic level technology, open channel flow meters include a non-contacting sensor mounted above the flume or weir. By measuring the transit time or time of flight from transmission of an ultrasonic pulse to receipt of an echo, the water level or “Head” is accurately measured.

Flumes and weirs are specially designed channel shapes that characterize the flow of water. The choice of flume or weir type depends on the application: flow rate, channel shape and solids content of the water.

The traditional method to measure flow in man made channels is to introduce a restriction into the section. The liquid flowing in that channel must rise as the flowing volume is constant upstream and downstream of that section. By measuring that rise, the flow rate can be deduced. Such devises are known as flumes and weirs.

Weirs

Weirs operate on the principle that an obstruction in a channel will cause water to back up, creating a high level (head) behind the barrier. The head is a function of flow velocity, and, therefore, the flow rate through the device. Weirs consist of vertical plates with sharp crests. The top of the plate can be straight or notched. Weirs are classified in accordance with the shape of the notch. The basic types are V-notch, rectangular, and trapezoidal

A notch is an opening in the side of a measuring tank or reservoir extending above the free surface. A weir is a notch on a large scale, used, for example, to measure the flow of a river, and may be sharp edged or has a substantial breadth in the direction of flow.

Flumes

Flumes are generally used when head loss must be kept to a minimum, or if the flowing liquid contains large amounts of suspended solids. Flumes are to open channels what venturi tubes are to closed pipes. Popular flumes are the Parshall and Palmer-Bowlus designs.

The Parshall flume consists of a converging upstream section, a throat, and a diverging downstream section. Flume walls are vertical and the floor of the throat is inclined downward. Head loss through Parshall flumes is lower than for other types of open-channel flow measuring devices. High flows velocities help make the flume self-cleaning. Flow can be measured accurately under a wide range of conditions.

Palmer-Bowlus flumes have a trapezoidal throat of uniform cross section and a length about equal to the diameter of the pipe in which it is installed. It is comparable to a Parshall flume in accuracy and in ability to pass debris without cleaning. A principal advantage is the comparative ease with which it can be installed in existing circular conduits, because a rectangular approach section is not required.

Rectangular and trapezoidal flumes function by having a constriction at the throat and/or a raised invert (bottom) at the throat. Either feature can cause critical flow at the throat in a properly operating flume. These flumes are simpler to construct, can be more easily fit into an existing channel, and can trap less sediment than a Parshall flume. However, the methodology relating discharge to measured head is more complex.

U-flumes, similar to Palmer-Bowles flumes but with a semi-circular throat, are ideal for use in culverts or pipes. Critical flow is achieved by narrowing the throat or by raising the bottom of the flume at the throat. Analysis of U flumes is similar to that of the trapezoidal flume.

Discharge through weirs and flumes is a function of level, so level measurement techniques must be used with the equipment to determine flow rates. Staff gages and float-operated units are the simplest devices used for this purpose. Various electronic sensing, totalizing, and recording systems are also available.

4.5.9 Insertion meters

A few insertion meters measure the average flow velocity but the majority measures the local velocity at one point only.

4.6.1 Flow control valves

Flow control valves are used to regulate the flow of fluids in fluid-power systems. Control of flow in fluid-power systems is important because the rate of movement of fluid-powered machines depends on the rate of flow of the pressurized fluid. These valves may be operated manually, hydraulically, electrically, or pneumatically.

Gate valves

Gate valves are used when a straight-line flow of fluid and minimum flow restriction are needed. Gate valves are so-named because the part that either stops or allows flow through the valve acts somewhat like a gate. The gate is usually wedge-shaped. When the valve is wide open, the gate is fully drawn up into the valve bonnet. This leaves an opening for flow through the valve the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve.

Gate valves are not suitable for throttling purposes. The control of flow is difficult because of the valve’s design, and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.

Gate valves are classified as either rising-stem or non rising-stem valves. The stem is threaded into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will usually have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate. Valves with rising stems are used when it is important to know by immediate inspection whether the valve is open or closed and when the threads (stem and gate) exposed to the fluid could become damaged by fluid contaminants. In this valve, the stem rises out of the valve when the valve is opened.

Globe valves

Globe valves are probably the most common valves in existence. The inlet and outlet openings for globe valves are arranged in a way to satisfy the flow requirements. Figure 4.34 shows straight-, angle-, and cross-flow valves.

The moving parts of a globe valve consist of the disk, the valve stem, and the hand wheel. The stem connects the hand wheel and the disk. It is threaded and fits into the threads in the valve bonnet. The part of the globe valve that controls flow is the disk, which is attached to the valve stem. (Disks are available in various designs.) The valve is closed by turning the valve stem in until the disk is seated into the valve seat. This prevents fluid from flowing through the valve (Figure 4.35 view A).

The edge of the disk and the seat are very accurately machined so that they form a tight seal when the valve is closed. When the valve is open (Figure 4.35 view B), the fluid flows through the space between the edge of the disk and the seat. Since the fluid flows equally on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the seat. The valve is commonly used as a fully open or fully closed valve, but it may be used as a throttle valve. However, since the seating surface is a relatively large area, it is not suitable as a throttle valve, where fine adjustments are required in controlling the rate of flow.

The globe valve should never be jammed in the open position. After a valve is fully opened, the hand wheel should be turned toward the closed position approximately one-half turn. Unless this is done, the valve is likely to seize in the open position, making it difficult, if not impossible, to close the valve. Many valves are damaged in this manner. Another reason for not leaving globe valves in the fully open position is that it is sometimes difficult to determine if the valve is open or closed. If the valve is jammed in the open position, the stem may be damaged or broken by someone who thinks the valve is closed, and attempts to open it.

It is important that globe valves be installed with the pressure against the face of the disk to keep the system pressure away from the stem packing when the valve is shut.

Needle valves

Needle valves are similar in design and operation to the globe valve. Instead of a disk, a needle valve has a long tapered point at the end of the valve stem. A cross-sectional view of a needle valve is illustrated in figure 4.36. The long taper of the valve element permits a much smaller seating surface area than that of the globe valve; therefore, the needle valve is more suitable as a throttle valve. Needle valves are used to control flow into delicate gauges, which might be damaged by sudden surges of fluid under pressure. Needle valves are also used to control the end of a work cycle, where it is desirable for motion to be brought slowly to a halt, and at other points where precise adjustments of flow are necessary and where a small rate of flow is desired.

Hydraulic and pneumatic globe valves

The valve consists of a valve body and a stem cartridge assembly. The stem cartridge assembly includes the bonnet, gland nut, packing, packing retainer, handle, stem, and seat. On small valves (1/8 and 1/4 inch) the stem is made in one piece, but on larger sizes it is made of a stem, guide, and stem retainer. The valve disk is made of nylon and is swaged into the stem, either for 1/8- and 1/4-inch valves, or the guide, for larger valves. The bonnet screws into the valve body with left-hand threads and is sealed by an O-ring (including a back-up ring).

The valve is available with either a rising stem or a non-rising stem. The rising stem valve uses the same port body design, as does the non-rising stem valve. The stem is threaded into the gland nut and screws outward as the valve is opened. This valve does not incorporate provisions for tightening the stem packing nor replacing the packing while the valve is in service; therefore, complete valve disassembly is required for maintenance. Figure 4.37 illustrates a rising stem hydraulic and pneumatic globe valve.

4.6.2 Pressure control valves

The safe and efficient operation of fluid power systems, system components, and related equipment requires a means of controlling pressure. There are many types of automatic pressure control valves. Some of them merely provide an escape for pressure that exceeds a set pressure; some only reduce the pressure to a lower pressure system or subsystem; and some keep the pressure in a system within a required range.

Relief valves

Some fluid power systems, even when operating normally, may temporarily develop excessive pressure; for example, when an unusually strong work resistance is encountered. Relief valves are used to control this excess pressure. Relief valves are automatic valves used on system lines and equipment to prevent over-pressurization.

Most relief valves simply lift (open) at a preset pressure and reset (shut) when the pressure drops slightly below the lifting pressure. They do not maintain flow or pressure at a given amount, but prevent pressure from rising above a specific level when the system is temporarily overloaded.

Main system relief valves are generally installed between the pump or pressure source and the first system isolation valve. The valve must be large enough to allow the full output of the hydraulic pump to be delivered back to the reservoir. In a pneumatic system, the relief valve controls excess pressure by discharging the excess gas to the atmosphere.

Smaller relief valves, similar in design and operation to the main system relief valve, are used in isolated parts of the system where a check valve or directional control valve prevents pressure from being relieved through the main system relief valve and where pressures must be relieved at a set point lower than that provided by the main system relief.

Figure 4.38 shows a typical relief valve. System pressure simply acts under the valve disk at the inlet to the valve. When the system pressure exceeds the force exerted by the valve spring, the valve disk lifts off its seat, allowing some of the system fluid to escape through the valve outlet until the system pressure is reduced to just below the relief set point of the valve.

All relief valves have an adjustment for increasing or decreasing the set relief pressure. Some relief valves are equipped with an adjusting screw for this purpose. This adjusting screw is usually covered with a cap, which must be removed before an adjustment can be made. Some type of locking device, such as a lock nut, is usually provided to prevent the adjustment from changing through vibration. Other types of relief valves are equipped with a hand wheel for adjusting the valve. Either the adjusting screw or the hand wheel is turned clockwise to increase the pressure at which the valve will open. In addition, most relief valves are also provided with an operating lever or some type of device to allow manual cycling or gagging the valve open for certain tasks

In some hydraulic systems, there is a pressure in the return line. This backpressure is caused by restrictions in the return line and will vary in relation to the amount of fluid flowing in the return line. This pressure creates a force on the back of the valve element and will increase the force necessary to open the valve and relieve system pressure.

4.6.3 Directional control valves

Directional control valves are designed to direct the flow of fluid, at a desired time, to the point in a fluid power system where it will do work. The driving of a ram back and forth in its cylinder is an example of a directional control valve application. Various other terms are used to identify directional control valves, such as selector valve, transfer valve, and control valve. They are ideal for machine tools, production and material handling equipment, marine auxiliary power controls, off-highway and heavy construction equipment, oilfield, and farm equipment.

Directional control valves for hydraulic and pneumatic applications are similar in design and application, with one major difference: the return port of a hydraulic valve is ported through the return line to the reservoir, while the return port of a pneumatic valve is exhausted to the atmosphere. There are a number of valve porting options available, depending upon the needs of a given application.

However, they vary considerably in physical characteristics and operation. The valves may be a:

• Poppet type, in which a piston or ball moves on and off a seat
• Rotary-spool type, in which a spool rotates about its axis
• Sliding-spool type, in which a spool slides axially in a bore

Directional-control valves may also be classified according to the method used to actuate the valve element:

• Hydraulically operated
• Manually (lever or plunger action) operated
• Mechanically (cam or trip action) operated
• Electrically (solenoid action) operated

Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position. For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths.

Poppet valve

It consists primarily of a movable poppet that closes against a valve seat. Pressure from the inlet tends to hold the valve tightly closed. A slight force applied to the poppet stem opens the poppet. The action is similar to the valves of an automobile engine. The poppet stem usually has an O-ring seal to prevent leakage. In some valves, the poppets are held in the seated position by springs. The number of poppets in a valve depends on the purpose of the valve.

Spool valve

The valve element slides back and forth to block and uncover ports in the housing. Sometimes called a piston type, the sliding-spool valve has a piston of which the inner areas are equal. Pressure from the inlet ports acts equally on both inner piston areas regardless of the position of the spool. Sealing is done by a machine fit between the spool and valve body and sleeve.

Check valves

Check valves are used to “check” (prevent) flow in one direction while allowing flow in the opposite direction. Check valves are available in a variety of different configurations. They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve. The valve element may be a sleeve, cone, ball, poppet, piston, spool, or disc. Force of the moving fluid opens a check valve; backflow, a spring, or gravity closes the valve.

4.8.1 Working mechanism of a centrifugal pump

A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head.

4.8.2 Components of centrifugal pumps

A centrifugal pump has two main components:

• A rotating component comprised of an impeller and a shaft
• A stationary component comprised of a casing, casing cover, and bearings.

4.8.3 Generation of centrifugal force

The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string. Figure 4.42 depicts a side cross-section of a centrifugal pump indicating the movement of the liquid.

4.8.4 Conversion of kinetic energy to pressure energy

The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.

Pump curves relate flow rate and pressure (head) developed by the pump at different impeller sizes and rotational speeds. The centrifugal pump operation should conform to the pump curves supplied by the manufacturer. In order to read and understand the pump curves, it is very important to develop a clear understanding of the terms used in the curves.

4.8.5 Cavitation

In designing any installation in which a centrifugal pump is used, careful attention must be paid to check the minimum pressure, which will arise at any point. If this pressure is less than the vapor pressure at the pumping temperature, vaporization will occur and the pump may not be capable of developing the required suction head. Moreover, if the liquid contains gases, these may come out of solution giving rise to packets of gas. This phenomenon is known as cavitation and may result in mechanical damage to the pump as the bubbles collapse. The onset of cavitation is accompanied by a marked increase in noise and vibration as the bubbles collapse, and a loss of head.

4.9.1 Rotary pumps

All rotary pumps have rotating parts, which trap the fluid at the inlet (suction) port and force it through the discharge port into the system. Gears, screws, lobes, and vanes are commonly used to move the fluid. Rotary pumps are positive displacement of the fixed displacement type. Rotary pumps are designed with very small clearances between rotating parts and stationary parts to minimize slippage from the discharge side back to the suction side. They are designed to operate at relatively moderate speeds. Operating at high speeds causes erosion and excessive wear, which results in, increased clearances.

There are numerous types of rotary pumps and various methods of classification. They may be classified by the shaft position—either vertically or horizontally mounted; the type of drive—electric motor, gasoline engine, and so forth; their manufacturer’s name; or their service application. However, classification of rotary pumps is generally made according to the type of rotating element. A few of the most common types of rotary pumps are discussed in the following paragraphs.

Gear pumps

Gear pumps are classified as either external or internal gear pumps. In external gear, pumps the teeth of both gears project outward from their centers. External pumps may use spur gears, herringbone gears, or helical gears to move the fluid. In an internal gear pump, the teeth of one gear project outward, but the teeth of the other gear project inward toward the center of the pump. Internal gear pumps may be either centered or off-centered.

External-gear pumps are used for flow rates up to about 400 m³/hr working against pressures as high as 170 atm. The volumetric efficiency of gear pumps is in the order of 96 percent at pressures of about 40 atm but decreases as the pressure rises.

In the internal-gear pump a spur gear, or pinion, meshes with a ring gear with internal teeth. Both gears are inside the casing. The ring gear is coaxial with the inside of the casing, but the pinion, which is externally driven, is mounted eccentrically with respect to the center of the casing. A stationary metal crescent fills the space between the two gears. Liquid is carried from inlet to discharge by both gears, in the spaces between the gear teeth and the crescent.

Lobe pumps

In principle the lobe pump is similar to the external gear pump; liquid flows into the region created as the counter-rotating lobes unmesh. Displacement volumes are formed between the surfaces of each lobe and the casing, and the liquid is displaced by meshing of the lobes. Relatively large displacement volumes enable large solids (nonabrasive) to be handled. They also tend to keep liquid velocities and shear low, making the pump type suitable for high viscosity, shear-sensitive liquids.

The choice of two or three lobe rotors depends upon solids size, liquid viscosity, and tolerance of flow pulsation. Two lobes handle larger solids and high viscosity but pulsates more. Larger lobe pumps cost 4-5 times a centrifugal pump of equal flow and head.

Screw pumps

A most important class of pump for dealing with highly viscous material is the screw pump. Designs employing one, two and three screws are in use.

Multiple screw pumps operate as follows:

• The displacement volume is opened at the suction as the counter-rotating screws unmesh
• Subsequent meshing of the screws produces a displacement volume bounded by the thread roots, the thread flanks, and the pump casing
• Continued rotation of the screws translate the displacement volume to the pump discharge
• At the pump discharge the volume is displaced by the meshing of the screw ends

In single screw pumps, the fluid is sheared in the channel between the screw and the wall. Flow is produced because of viscous forces. Pressures achieved with low viscosity materials are negligible.

4.9.2 Reciprocating pumps

Reciprocating pumps operate by displacing a fixed volume through the reciprocating motion of either a piston, a diaphragm, or a bellows. In the simplest example, a piston is drawn back in a closed chamber, creating a vacuum which draws in a fixed volume of fluid. The piston then moves forward and expels the fluid. In this way, by either controlling the stroke length of the piston or the piston stroking speed, accurate flow control can be achieved. The reciprocating motion can be supplied by a motor driven eccentric or a liner magnetic drive (solenoid).

Piston pumps typically require seals or close clearances around the piston to operate accurately. This introduces the problems of seal and piston wear, contamination of the pumped fluid by wear particles, and limitations on the material selections for optimum chemical resistance.

The simplest form of the piston pump is the syringe pump, which is designed to accurately meter up to the volume of one full stroke of the syringe. By accurately stepping the piston on a syringe pump, very accurate flow rates in microliters can be obtained. The major disadvantage of this type of pump is that once the syringe is empty, the refill period allows no flow from the pump. Thus, a syringe pump is not suitable in continuous metering applications.

Diaphragm metering pumps eliminate some of the disadvantages of piston style pumps by replacing the piston with a flexible diaphragm. Because the diaphragm is sealed by clamping around the edge, the pump uses no dynamic seals, which can wear, eliminating leakage or contamination of the pumped fluid.

They may be single-cylinder or multi-cylinder design and of the following three types:

• Piston pumps
• Plunger pumps
• Diaphragm pumps

Diaphragm pump

The diaphragm pump has been developed for handling corrosive liquids and those containing suspensions of abrasive solids. It is in two sections separated by a diaphragm of rubber, leather, or plastics material. In one section, a piston or plunger operates in a cylinder in which a non-corrosive fluid is displaced. The movement of the fluid is transmitted by means of flexible diaphragm to the liquid to be pumped. The only moving parts of the pump that are in contact with the liquid are the valves, and these can be specially designed to handle the material. In some cases, the movement of the diaphragm is produced by direct mechanical action, or the diaphragm may be air actuated.

4.10.1 Static or inline mixers

Static mixers consist of a series of mixing elements arranged axially in a pipe or duct and are widely used in the process industry for a large variety of mixing applications. The energy for mixing is derived from the kinetic energy of the fluid stream. Hence, there is an increase in the pressure drop relative to that of empty pipes.

There are several advantages to static mixers:

• No moving parts
• Simple construction
• Lower capital and operating costs
• Lower energy requirements
• Narrow residence time distributions

The main disadvantages of static mixers are increased pressure drop and fouling problems. Though there are numerous static mixer designs, only two of the more common designs are shown below.

Helical mixers

Helical mixers are comprised of a series of mixing elements with the leading edge of one element being perpendicular to the trailing edge of the previous. Each mixing element is a metal or polymeric ribbon with a 180 degree helical twist that measures approximately one and a half pipe diameters in length. These mixers can be used for laminar, transitional, and turbulent flow applications and are suitable for most blending and dispersion processes involving liquids and gases. The mixing elements can be permanently mounted inside a tube or removable to allow for frequent cleaning and inspection.

High Efficiency Vortex (HEV) mixers

High Efficiency Vortex (HEV) mixers consist of a series of baffles or tabs inclined relative to one another and at an angle relative to the pipe axis. The mixer elements are rotated by 90 degrees and arranged successively in the pipe. This design can be used in all turbulent-flow mixing applications regardless of line shape or size, and has pressure losses 75% less than conventional static mixers. Mixing is achieved by controlled vortex structures generated by the baffle geometry that requires a mixer length less than 2 pipe diameters. Typical applications include low-viscosity liquid-liquid blending processes, as well as gas-gas mixing.

4.10.2 Dynamic mixers

A dynamic mixer consists of an impeller, mixer shaft, support structure, gear or bearing housing, and a motor. Though side-entry and bottom-entry mixers are occasionally used, most mixers are top-mounted (shown to the left) with the motor and support housing located above the fluid level. The motor power required for proper mixing can range from less than 1 hp for small vessels containing low viscosity fluids to more than 5000 hp when mixing viscous fluids in large vessels. The extent to which fluid mixing occurs is controlled by the design, size, location, and speed of rotation of the impeller. Some of the more common metal fabricated impeller designs are shown below, but other highly efficient designs that incorporate advanced composites are also widely used in industry.

Marine and pitched blade impellers

The marine impeller (left) is the classical design used for axial mixing. This design, often pitched for downward pumping action, provides a uniform discharge and is most often used with low viscosity fluids (less than 1,000 centipoises). In most cases, vessel baffling is required for the optimal performance of this impeller design. A similar design is the pitched blade impeller (right). This design produces axial flow and is well suited for applications requiring high speeds to disperse liquid/solid mixtures in non-baffled vessels.

Curved and straight blade impeller

The curved blade or backswept turbine (left) is used with highly viscous mixtures where power consumption is a concern or with liquid/brittle solid mixtures. The straight blade turbine (right) is designed for gas/liquid applications requiring high shear at high speeds. Frequently, these two designs produce radial fluid flow and require vessel baffling to achieve optimum performance.

Helical impellers

Helical impellars are used in applications involving highly viscous fluids, such as polymer melts and carmalized sugars. These designs incorporate outer (left and right) helical bands with minimal vessel wall clearance to achieve axial flow at low agitator speeds. In order to further enhance the axial flow patterns of non-Newtonian fluids, an additional inner helical flight with opposite handedness is attached to the impeller shaft (left). The inner flight produces downward pumping action, while the outer flight pumps in the upward direction.

Anchor impeller

The anchor impeller design (left) is best suited for mixing of high viscosity fluids. The design produces radial flow at low speeds. These types of impellars often incorporate wipers that remove material from the vessel walls during agitation, which enhances heat transfer.

Classification of heat exchangers

The following types of heat exchangers are presently used in process industries:

• Double pipe heat exchangers
• Shell and tube heat exchangers
• Fixed tube sheet
• Outside packed floating head
• Internal floating head
• U-Tube type
• Re-boilers

Special types of exchangers

• Pipe coils
• Spiral heat exchanger
• Plate type exchanger
• Finned tube exchanger
• Graphite heat exchanger
• Air-cooled exchanger

Applications

For heating or cooling chemical or hydrocarbon fluids, condensing air or gases.

Construction

Removable bundle, pull-through bolted internal floating head cover.

Advantages

• Allows for differential thermal expansion between the shell and tubes
• Bundle can be removed from shell for cleaning or repairing without removing the floating head cover
• Provides multi-tube pass arrangements
• Provides large bundle entrance area
• Excellent for handling flammable and/or toxic fluids

Limitations

• For a given set of conditions, it is the most costly of all the basic types of heat exchanger designs
• Less surface per given shell and tube size than C500

Applications

For heating or cooling chemical or hydrocarbon fluids, condensing air or gases.

Construction

Removable bundle, pull-through bolted internal floating head cover.

Advantages

• Allows for differential thermal expansion between the shell and tubes.
• Bundle can be removed from shell for cleaning or repairing without removing the floating head cover
• Provides multi-tube pass arrangements
• Provides large bundle entrance area
• Excellent for handling flammable and/or toxic fluids

Limitations

• For a given set of conditions, it is the most costly of all the basic types of heat exchanger designs
• Less surface per given shell and tube size than C500

Applications

For tube side circulation of corrosive liquids, high-fouling fluids, or gases and vapors.

Advantages

• Floating end allows for differential thermal expansion between the shell and tubes
• Shell side can be steam or mechanically cleaned
• Bundle can be easily repaired or replaced
• Less costly than full internal floating head-type construction
• Maximum surface per given shell and tube size for removable bundle designs

Limitations

• Shell side fluids limited to non-volatile and/or non-toxic fluids, i.e., lube oils, hydraulic oils
• Tube side arrangements limited to one or two pass
• Tubes expand as a group, not individually (as in U-tube unit); therefore, sudden shocking should be avoided
• Packing limits design pressure and temperature

Thermosyphon re-boilers

Thermosyphon re-boilers are generally heat exchangers used to provide vapor boil-up to a distillation column. They can be provided in either a vertical position with the boiling fluid in the tubes or in the horizontal position with the boiling fluid in the shell side. In normal operation, between 10% and 33% of the liquid entering the unit is boiled.

Kettle re-boilers

Kettle re-boilers are used in services where 20% to 100% boil-up is required. The enlarged outer shell provides a separation between the boiling fluid and the exit nozzle reducing the amount of liquid entrainment in the exiting vapor stream. In cases where the amount of liquid entrainment must be minimal, a demister pad is provided inside the shell.

Advantages

• Optimal Use of Pressure Drop
• Differing Flow Rates in Each Channel
• High Heat Transfer Coefficients
• Minimum Fouling & Self Cleaning Effect
• Compactness of a Spiral
• Utilize the Smallest of Temperature Differences

Applications

• Chemical Industry
• Municipal/Sewage Treatment Plants
• Sterilization Plants
• Alcohol Plants
• Textile Mills
• Cellulose Industry
• Aluminum Plants
• Sugar
• Food Industry

Evaporator designs

Types of single effect evaporators:

• Batch Pan
• Rising film
• Falling film
• Plate evaporators
• Scraped surface

5.4.1 Batch pan

These evaporators are the simplest and oldest. They consist of spherical shaped, steam jacketed vessels. The heat transfer per unit volume is small requiring long residence times. The heating is due only to natural convection; therefore, the heat transfer characteristics are poor. Batch plants are of historical significance; modern evaporation plants are far-removed from this basic idea. The vapors are a tremendous source of low-pressure steam and must be reused.

They are currently used in making jams and jellies but mostly outdated by more efficient means of evaporation. A batch pan evaporator is one of the oldest methods of concentrating.

5.4.2 Rising film

They consist of a heat exchanger isolated from the vapor separator. The heat exchanger, or calandria, consists of 10 to 15 meter long tubes in a tube chest, which is heated with steam. The liquid rises by percolation from the vapors formed near the bottom of the heating tubes. The thin liquid film moves rapidly upwards. The product may be recycled if necessary to arrive at the desired final concentration. This development of this type of modern evaporator has given way to the falling film evaporator.

Dating back to the early 1900s, this equipment uses a vertical tube with steam condensing on its outside surface. Liquid on the inside of the tube is brought to a boil, with the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapors are formed resulting in a higher central core velocity those forces the remaining liquid to the tube wall.

5.4.3 Falling film

These types of evaporators are the most widely used in the food industry. They are similar in components to the rising film type except that the thin liquid film moves downward under gravity in the tubes. A uniform film distribution at the feed inlet is much more difficult to obtain. This is the reason why this development came slowly and it is only within the last decade that falling film has superceded all other designs. Specially designed nozzles or spray distributors at the feed inlet permit it to handle more viscous products. The residence time is 20-30 sec. as opposed to 3-4 min. in the rising film type. The vapor separator is at the bottom which decreases the product hold-up during shut down. The tubes are 8-12 meters long and 30-50 mm in diameter.

5.4.4 Falling film tubular

Similar to the rising film unit, this unit has several advantages. First, because the vapor is working in the same direction as gravity, these units are more efficient. Second, to establish a well-developed film, the rising film unit requires a driving film force, typically a temperature difference of at least 25 degrees across the heating surface whereas the falling film evaporator is not limited by this permitting many more multiple effect stages of evaporation. Therefore, with this technology, it is feasible to have as many as ten effects in a process.

5.4.5 Forced circulation

The forced circulation evaporator was developed for processing liquors, which are susceptible to scaling or crystallizing. Liquid is circulated at a high rate through the heat exchanger, boiling being prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor.

The main applications are in the concentration of inversely soluble materials, crystallizing duties and in the concentration of thermally degradable materials, which result in the deposition of solids.

5.4.6 Plate equivalents of tubular evaporators

Plate type evaporators were initially developed by APV in 1957 to provide an alternative to tubular systems to meet the growing challenges of the process industries. The plate evaporator offers full accessibility to the heat transfer surface. It also provides flexible capacity merely by adding more plate units, shorter product residence time resulting in a quality concentrate, a more compact design with low headroom requirements and low installation cost.

5.4.7 Scrapped surface thin film evaporators

They are designed for evaporation of highly viscous and sticky products, which cannot be otherwise evaporated. This type of evaporators have been specially designed to provide high degree of agitation, effecting heat transfer as well as scrapping the walls of the evaporator to prevent deposition and subsequent charring of the product.

In circumstances in which a scale is formed the tubes of the calandria can be scraped to remove the scale. Such evaporators are called Scrapped surface tube evaporators. Inside the tubes are rotating devices, which scrape off any scale formed with spring-loaded blades. These scraping devices can be used in long tube or short tube evaporators, however, particularly in long tube evaporators there can be problems of poor liquid distribution. The scale that is removed must be separated from the product later in the process. Other types of evaporator are often more efficient for use with liquids that form scale.

5.4.8 Vertical agitated thin film evaporators (ATFE)

They contain a rotor designed to produce and agitate a thin film between the rotor and the heated wall of the evaporator. The agitation of the film on the heated surface promotes heat transfer and maintains precipitated or crystallized solids in a manageable suspension without fouling the heat transfer surface. This capability makes ATFEs particularly suited for volume reduction of radioactive wastes that contain suspended solids. The tests were conducted with surrogates that contained no radionuclides. The results of the tests indicated that a variety of products could be produced with the ATFE. It was possible to vary the consistency from a highly concentrated liquid to a completely dry powder. Volume reductions ranged from ~20 to 68% and decontamination factors in the range of 10,000 to 100,000 were achieved.

5.4.9 Multiple effect evaporators

Two or more evaporator units can be run in sequence to produce a multiple effect evaporators. Each effect would consist a heat transfer surface, a vapor separator, as well as a vacuum source and a condenser. The vapors from the preceding effect are used as the heat source in the next effect.

There are two advantages to multiple effect evaporators:

• Economy – they evaporate more water per kg steam by re-using vapors as heat sources in subsequent effects
• Improve heat transfer – due to the viscous effects of the products as they become more concentrated

Each effect operates at a lower pressure and temperature than the effect preceding it so as to maintain a temperature difference and continue the evaporation procedure. The vapors are removed from the preceding effect at the boiling temperature of the product at that effect so that no temperature difference would exist if the vacuum were not increased. The operating costs of evaporation are relative to the number of effects and the temperature at which they operate. The boiling milk creates vapors, which can be recompressed for high steam economy. This can be done by adding energy to the vapor in the form of a steam jet, thermo compression or by a mechanical compressor, mechanical vapor recompression.

5.4.10 Thermo compression (TC) evaporator

Involves the use of a steam-jet booster to recompress part of the exit vapors from the first effect. Through recompression, the pressure and temperature of the vapors are increased. As the vapors exit from the first effect, they are mixed with very high-pressure steam. The steam entering the first effect calandria is at slightly less pressure than the supply steam. There are usually more vapors from the first effect than the second effect can use. Usually only the first effect is coupled with multiple effect evaporators.

To reduce energy consumption, water vapor from an evaporator is entrained and compressed with high-pressure steam in a thermo compressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapor. A thermo compressor is similar to a steam-jet air ejector used to maintain vacuum in an evaporator.

Only a portion of the vapor from an evaporator can be compressed in a thermo compressor with the remainder condensed in the next-effect heat exchanger or a condenser. A thermo compressor is normally used on a single-effect evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate Delta-T’s in the heat exchanger to minimize the compression ratio.

5.4.11 Mechanical vapor recompression (MVR)

Whereas only part of the vapor is recompressed using TC, all the vapor is recompressed in an MVR evaporator. Radial compressors or simple fans using electrical energy mechanically compress vapors.

There are several variations; in single effect, all the vapors are recompressed therefore no condensing water is needed; in multiple effect, can have MVR on first effect, followed by two or more traditional effects; or can recompress vapors from all effects

Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapor from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.

A simplified flow sheet of a single-effect recompression evaporator illustrates why mechanical recompression is energy efficient

Mechanical recompression is not limited to single-effect evaporation. It is sometimes economical to compress vapor from the last effect of a double- or triple-effect evaporator so that the vapor can be condensed in the first-effect heat exchanger.

Bubble cap trays

A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapor. Vapor rises through the chimney and is directed downward by the cap, discharging through slots in the cap, and finally bubbling through the liquid on the tray.

The conventional bubble cap tray is well proven in applications having:

• Very large loading ranges
• Very low liquid loadings
• Very low gas loadings
• Constant liquid holdup
• Low leakage rates

Valve trays

In valve trays, perforations are covered by lift able caps. Vapor flows lift the caps, thus self-creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.

Due to the wide operating range they can handle, valve trays are now the most commonly used type of mass transfer trays. The valve tray differentiates itself from other trays by its ability to handle high capacities at excellent mass transfer efficiencies over a wide operating range. Depending on the application, a specific type of valve is selected.

Sieve trays

Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters.

The sieve trays are very cost-competitive mass transfer trays. The operating range of these sieve-perforated trays is less than that of valve trays.

Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.

Chimney trays

A purpose designed Chimney Tray must be installed to function as a collector device either for feeding to a liquid distributor (particularly flashing feeds) or for a liquid draws off. A leaky chimney tray will result in liquid by-passing the distributor causing maldistribution in the packed bed. To avoid this, all chimney trays, which are used to collect liquid to feed a liquid distributor, should have all joints seal welded after installation.

When installed below the bottom packed bed, Chimney Trays perform a useful function as a vapor distributor. This is especially important where high vapor rates are encountered due to the low-pressure drop across most packed beds.

Random packings

Random packings are an inexpensive packing alternative to increase a tower’s capacity and efficiency

Metal packings

Metal packings are lighter and resist breakage better than ceramic packings, making metal the choice for deep beds. Metal also lends itself to packing geometries that yield higher efficiencies than ceramic or plastic packing shapes. Compared to most standard plastic packings, metal packings withstand higher temperatures and provide better wetting ability.

High-performance random tower packing

• Provides greater capacity and efficiency than fractionation trays and other random packings
• Exhibits roughly 40% lower pressure drop than equal-sized pall rings
• Minimizes liquid residence time through low liquid holdup
• This type of packing is ideal in a wide range of mass transfer services. It is used extensively in distillation towers from deep vacuum towers where its low pressure drop is crucial, to high-pressure towers where its capacity easily surpasses that of trays

High-strength pall ring & raschig rings

• Physical strength without thickness and weight
• Economically priced
• Our high-strength pall ring combines flared ends with a latch that holds the ring seams together for uniform closure and added strength for use in beds up to 50 feet (15m). The high-strength pall ring is currently available in a 2-in. (50mm) size in a broad range of metals

Support plates

They provide structural support for packing. A well-designed packing support plate provides structural support for the tower packing. Weight loads associated with installation, liquid holdup during operation and the potential for scaling or buildup of solids enter into the design considerations. The support plate must minimize resistance to the flows of rising vapor and down coming liquid. Additionally the support plate must be easy to install.

Liquid distributors

Proper liquid distribution represents one of the most important aspects of packed tower design. For critical applications in large towers, we recommend the trough distributor.

Liquid re-distributors

Optimum tower performance depends upon uniform liquid distribution though multiple packed beds. Re-distributors accomplish this task by accumulating the liquid leaving an upper bed and distributing it in a uniform patter over a lower bed.

Collecting trays are required if more than one liquid is used in a multiple bed packed tower. The collecting tray collects and removes liquid from the tower while permitting vapor to move upward though the successive packed bed.

Collecting trays

Optimum tower performance depends upon uniform liquid distribution though multiple packed beds. Re-distributors accomplish this task by accumulating the liquid leaving an upper bed and distributing it in a uniform patter over a lower bed.

Collecting trays are required if more than one liquid is used in a multiple bed packed tower. The collecting tray collects and removes liquid from the tower while permitting vapor to move upward though the successive packed bed.

Bed limiters

Bed limiters hold packings in their proper location. The gas velocity and the packing material determine whether a given tower requires a bed limiter. High gas velocities can lift the packing, dislocating it into the liquid distributor or beyond. This adversely impacts the performance of the liquid distributor and the efficiency of the tower.

Mist eliminators

Carry over of liquid particulate matter by gas or vapor is generally termed as ‘Entrainment’, which is commonly encountered in Gas-Liquid separations

Mist eliminators are a highly efficient, low cost means of removing entrained liquid droplets from gas and vapor streams.

Examples and applications

• Petroleum Processing – Flash Distillation is used to separate petroleum in a process called ‘cracking’. The products produced include: gasoline, kerosene, lubricating oils, asphalt, etc
• Alcohol Production – Flash Distillation is used to separate the ethanol (vapor phase) from the water (liquid phase) in order to form a more concentrated product
• Desalination of Water – Flash Distillation is used to purify salt water by boiling off pure water, leaving salt behind in the remaining water
• Perfume Spray – Flash Distillation is used to extract the essential oils of flowers and cedar to make perfume

Multi-stage flash distillation

Multi-Stage Flash (MSF) is the most commonly used process for seawater desalination. A MSF facility is typically located so that it uses steam from a nearby electricity generation facility. Seawater is heated in a ‘brine heater’ and proceeds to another receptacle, called a stage, where it immediately boils (flash) due in part to the ambient pressure. The steam yielded is the condensed on heat exchanger tubes that in turn heat up the incoming water, thereby decreasing the amount of thermal energy needed to heat the Feed water.

It is an advantage to perform flash-distillation in several steps. This is called multi-stage flash evaporation or MSF for short. This is where the preheated liquid passes through a series of stages or chambers with each successive stage at a lower vapor pressure so some of the liquid will flash at each stage.

Flash distillation begins when the salt water enters a bundle of tubes which are located in the vapor space of the preheat chamber. The water then flows into a heater consisting of a bundle of tubes, which are heated externally by steam. Here, the water is heated to 100 degrees C, but it does not boil because the pressure is above 1 atm. The hot seawater then enters a flash chamber, which is kept under reduced pressure. The vapors, which are flashed off, are then condensed on the tubes carrying the incoming flow of cold seawater. The distillate and the remaining salt water are then restored to atmospheric pressure by pumps. Condensing the water by heat exchange with the incoming flow in one of the economical advantages of this process.

The multiple stage evaporators is able to produce more distillate per unit of heating steam because the flashing occurs in more than one stage since the flashed vapors are used to heat the incoming water.

Mechanism

Water moves across the Osmotic Distillation (OD) membrane by evaporating, diffusing through the pores, and condensing on the other side of the pores. The liquid is prohibited from entering the pores by membrane design. The membrane is designed such that the liquids cannot exceed the capillary forces required to enter the pores. Design factors include surface tension, contact angle, capillary pressure, and pore radius. The heat of vaporization is supplied by conduction or convection from the upstream liquid through the membrane. The temperature gradient across the membrane is typically less than 2 ⁰C making the process nearly isothermal.

Figure below shows a typical setup used in osmotic distillation.

Osmotic distillation provides a means of purifying heat sensitive substances including pharmaceuticals and biological products.

Forced draft cooling towers

The forced draft tower, shown in figure 6.38 has the fan, basin, and piping located within the tower structure. In this model, the fan is located at the base. There are no louvred exterior walls. Instead, the structural steel or wood framing is covered with paneling made of aluminum, galvanized steel, or asbestos cement boards

During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. Water entrained in the air is removed by the drift eliminators located at the top of the tower. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems

Induced draft cooling towers

The induced draft tower illustrated in figure 6.39 has one or more fans, located at the top of the tower, that draw air upwards against the downward flow of water passing around the wooden decking or packing. Since the airflow is counter to the water flow, the coolest water at the bottom is in contact with the driest air while the warmest water at the top is in contact with the moist air, resulting in increased heat transfer efficiency.

The fans at the top discharge the hot, moisture laden air upward and away from the air entering at the bottom of the tower, thus preventing any recirculation of warm air. Warm water from the building enters the distribution system located just under the drift eliminators. The fans and their drive are mounted on the top deck.

A schematic drawing of another type of induced draft tower, called the crossflow is shown in figure 6.40. Crossflow towers provide horizontal airflow as the water falls through the packing. Single and double airflow designs are constructed to suit the job location and operating conditions.

The fans, located at the top, draw air through cells or packing that are connected to a suction chamber partitioned midway beneath each fan. The water falls from the distribution system in a cascade of small drops over the packing and across the horizontal flow of air. The total travel path of the air is longer and there is less resistance to air flow than in the counterflow design.

A newer type of induced draft cooling tower design is illustrated in Fig.6.40. The tower consists of a venturi-shaped chamber, a spray manifold, and a sump. Neither fill nor fan are required in this tower.

The water to be cooled is injected at the narrow end of the venturi by spray nozzles, inducing a large airflow into the tower, which mixes intimately with the fine water spray. Heat transfer by evaporation of a small part of the water takes place while the remaining water drops in temperature. The cooled water falls into the sump and from there flows to the suction of the cooling water circulating pump. The air containing the water vapor leaves the tower via the eliminators and is discharged upward through a cowl.

The advantages of this tower are its quietness of operation due to the absence of any moving parts and their associated noise and vibration problems, the elimination of the need for electrical connections, starters, etc., the elimination of fill, and the reduced maintenance requirements. A cross-sectional view of this tower is shown in figure 6.42.

Rotating horizontal bed

In this device, dry, granular desiccant is held in a flat, segmented rotary bed that rotates continuously between the process and reactivation airstreams. As the bed rotates through the process air, the desiccant adsorbs moisture. Then the bed rotates into the reactivation airstream, which heats the desiccant, raising its vapor pressure and releasing the moisture to the air.

The process and reactivation air heats and cools the desiccant to drive the adsorption-desorption cycle. The moisture is removed through a process of continuous physical adsorption on a continuous basis (both, counter flow and parallel flow options are available).

The adsorption of moisture and reactivation of desiccant take place continuously and simultaneously without any cross mixing of the process and reactivation air streams.

To increase capacity, the manufacturer can either increase the diameter of the rotating bed to hold more desiccant, or increase the number of beds stacked on top of one another. Both options are not practical if very large volumes of air need to be dehumidified. If the desiccant is evenly loaded through the trays, the rotating horizontal bed provides a constant outlet moisture level, and a high airflow capacity can be achieved in less floor space than with dual-tower unit. The rotating horizontal bed design offers a low first cost. The design is simple, compact and easy to produce as well as install and maintain.

Modular Vertical Bed (MVB)

The Modular Vertical Bed (MVB) design is a ‘fairly new’ but ‘proven’ concept with the combined better features of packed tower and rotating horizontal bed designs in an arrangement that is well suited to atmospheric pressure dehumidification applications, and yet can achieve very low dew points. The single or double tower is replaced by a circular carrousel with eight or more vertical beds (towers) that rotate, by means of a drive system, between the process and reactivation air streams.

This design can achieve low dew points because leakage between process and reactivation air circuits is almost negligible. Also because the beds are separate and sealed from one another, the pressure difference between process and reactivation is not so critical; so airstreams can be arranged in the more efficient counter-flow pattern for better heat and mass transfer. Like the rotating bed, the ratcheting, semi-continuous reactivation of the desiccant provides a relatively constant outlet air moisture condition on the process side, reducing the ‘saw tooth’ effect that can occur in packed tower units.

The ‘MVB’ design allows for low replacement cost of desiccants as well as large savings in energy and performance improvements at low dew points, especially if the equipment incorporates a heat pipe heat exchanger in the regeneration air circuit.

Open system

Featuring atmospheric air in a once-through system where water is to be removed. Normally a push-pull system is used to balance the pressure to be slightly negative in the free board of the fluid bed. Depending upon the product and available heat source, direct or indirect heating may be applied. The exhaust air is cleaned by e.g. bag filter, cyclone with or without wet scrubber.

In cases where products pose a dust explosion risk, open cycle systems feature pressure shock resistant components or alternatively semi-closed cycle, self-inertizing layouts can be considered.

Closed cycle system

This features drying in an inert gas atmosphere (usually nitrogen) recycling within the system. It must be used for drying feedstocks containing organic volatiles or where the product must not contact oxygen during drying. Closed cycle systems are gastight, and addition of inert gas is controlled by monitoring the oxygen content of the drying gas and the system pressure, which is kept positive. The evaporated volatiles are recovered in a condenser.

Batch fluid bed systems

Batch fluid bed processing allows several process steps (mixing, agglomerating, drying and cooling) to be carried out in a single unit. The batch process assures uniformity of all products within a batch and allows every unit of final product to be traced to a given batch run.

Direct rotary cascade dryers

The most common type of rotary dryer, direct rotary cascade dryers have internal lifters or flights to elevate the feed and drop it in a curtain from the top to the bottom, cascading along the length of the dryer hence the name rotary cascade dryers. These flights need to be carefully designed to prevent cross-sectional asymmetry of the curtain. The flights are arranged in repeating patterns, and the dryer should have several rows of distinctively designed flights that are indexed and offset to form numerous simultaneous curtains along the drum length.

Direct rotary cascade dryers have internal lifters or flights to elevate the feed and drop it in a curtain from the top to the bottom, cascading along the length of the dryer. The carrier stream (hot gas) may be co- or countercurrent with the primary flow being through the ‘bed’ or curtain, and, in this instance, multiple curtains in the longitudinal direction. As you can imagine, the formation of each curtain is intermittent. Therefore the design should allow for successive curtains to be formed in advance, promoting a continuous exposure of the feed to the carrier. Secondary crossflow occurs on the surface of the bed material on the bottom of the drum.

Rotary steam tube dryers

Rotary steam tube dryers operate in a similar fashion to conventional rotary cascade dryers with the exception that the heat transfer is indirect (principally conductive) with the material cascading through a rotating nest of tubes that are internally heated by steam or other thermal transfer fluid. The lifters, if used, are on the peripheral circumference of the drum. Otherwise, the tubes actually act as the lifters or conveying medium that elevate the feed to the top of the drum and release it to contact and tumble through the tube bundle directly. Many have spirals installed to assist in moving the materials forward. Only evolved vapors are exhausted from the drum, requiring a lower volume of air for the process.

Rotary louver dryers

This type of rotary dryer has the feed materials supported and moving over a set of louvers mounted to an external rotating drum. The hot gas is introduced into a tapered bustle below the louver ring. The air passes through the louvers and the product (through the bed) before being exhausted from the dryer in a co-current or countercurrent flow. The drum rotation causes the material to roll and mix, providing intimate contact with the drying gas. There is a certain amount of fluidization that occurs in a rotary louver dryer, leading to this technology being thought of as a combined fluid-bed rotary dryer. The technology provides a very gentle method of handling the material and is especially well suited for fragile and crystalline materials.

Rotary tube furnace dryers

An indirect dryer that allows a high degree of temperature control, a Rotary Tube Furnace (RTF) dryer consists of a muffle furnace with a steel drum passing through it. Tumbling or rolling flights rather than the lifting flights such as those in the cascade rotary dryer are fitted to the inside of the drum. In operation, the particles are exposed to the drum surface, which is heated from the outside by a suitable heat source such as gas burners or electric elements. The internal flights tumble and mix the product, constantly exposing new surfaces to the heated drum surface. In addition, there is a high degree of conduction between the product particles to enhance operation efficiency.

In all rotary dryers the speed of advancement of the material and hence its retention time in the dryer is determined by the rotational speed of the drum as well as its angle of inclination. By varying these parameters, the residence time can be controlled accurately. The amount of material in the drum at any one time the drum fill is relatively low as a percentage of the cross-sectional area or total volume of the drum. They are typically of the order of 8 to 15 percent of the total volume of the drum.

Rotary dryers are continuous processing machines that can effectively process feeds that are classified as powders, granules, nonfriable agglomerates and large solid particles. Some — the direct cascade and tube furnace, for example — are able to cope with wide variations in the feed such as particle size and moisture. Depending on the configuration of the particular dryer, the feed and carrier inlets, discharges (or both) need to be well sealed to prevent the introduction of cold air into the system or the expulsion of hot, dust-laden air to the atmosphere. They operate at varying feed rates from several pounds to hundreds of tons per hour.

Fluid bed types

Fluid bed processors may be configured for either continuous or batch operation.

Transport of the solids through the fluid bed may be achieved either by the fluidization alone or a combination of fluidization and vibration.

The flow of gas relative to the solids is characterized either as cross flow in a single tier fluid bed or as cross/counter-current in a multi-tier fluid bed.

There are two types of basic fluid bed designs according to the solids flow pattern in the dryer.

Continuous back mix flow

The continuous back-mix flow design for feeds that require a degree of drying before fluidization is established.

These are applied for feeds that are non-fluidizable in their original state, but become fluidizable after a short time in the dryer, e.g. after removal of surface volatiles from the particles.

The condition of the fluidizing material is kept well below this fluidization point. Proper fluidization is obtained by distributing the feed over the bed surface and designing the fluid bed to allow total solids mixing (back-mix flow) within its confines. The product temperature and moisture are uniform throughout the fluidized layer. Heating surfaces immersed in the fluidized layer improve the thermal efficiency and perfomance of this system. Back-mix fluid beds of both rectangular and circular designs are available

Plug flow design

The plug flow design for feeds that are directly fluidizable on entering the fluid bed.

These are applied for feeds that are directly fluidizable. Plug flow of solids is obtained by designing the fluid bed with baffles to limit solids mixing in the horizontal direction. Thereby the residence time distribution of the solids becomes narrow. Plug flow fluid beds of either rectangular or circular designs are especially used for removal of bound volatiles or for heating and cooling. The volatile content and temperature vary uniformly as solids pass through the bed, and the plug flow enables the solids to come close to equilibrium with the incoming gas.

Plug flow may be achieved in different ways depending on the shape and size of the bed.

• In rectangular beds, baffles are often arranged to create an alternating flow of solids from side to side
• In circular beds, baffles are spiral
• In relatively small circular beds with high powder layers, baffles are radial

Vibrating fluid beds

This design, is basically of the plug flow type. It is especially applied for drying and cooling products that fluidize poorly due to a broad particle size distribution, highly irregular particle shape, or require relatively low fluidization velocities to prevent attrition. It operates with a shallow powder layer of less than 200 mm. This gives a much lower product residence time per unit bed area than non-vibrating beds which can have powder layers up to 1500 mm.

These fluidizers incorporate pressure shock resistance and sanitary features if clean operation is required.

Contact fluidizers

This is a rectangular fluid bed dryer incorporating back-mix and plug flow sections. A rotary distributor disperses the wet feed evenly over the back-mix section equipped with contact heating surfaces immersed in the fluidized layer.

The heating surfaces provide a significant portion of the required energy, and therefore, it is possible to reduce both the temperature and the flow of gas through the system. This is particularly important for heat sensitive products.

Subsequent plug flow sections are used for postdrying and cooling, if required.

Advantages of the Contact Fluidizer – compared to fluid beds without heating surfaces, two-stage flash/fluid bed dryers, or rotary dryers – include its compact design, high thermal efficiency, and low gas throughput.

Multi-tier fluid beds

These fluid beds consist of two or more stacked fluid beds. The upper tier (back-mix or plug flow) is for predrying and the lower tier (plug flow) for postdrying. The drying gas travels counter-current to the solids. The gas leaving the lower tier contains sensible heat, which is transferred to the upper tier. Furthermore, each fluid bed may be provided with immersed heating surfaces. These designs result in a low gas throughput and high thermal efficiency which are of great importance in closed cycle drying systems.

Conventional two-stage dryer

This is a spray dryer layout featuring either a rotary atomizer or spray nozzle atomizer. Powder discharged from the drying chamber can be further dried and cooled in a vibrating fluid bed. This two-stage drying concept achieves better overall heat economy and is suitable for many food and dairy products. When non-agglomerated powders of non-fat products are dried, a pneumatic transport system can replace the fluid bed.

Compact dryer

This is a space saving spray dryer with integrated fluid bed. Atomization is created by either a rotary atomizer or spray nozzle atomizer. The location of the fluid bed within the drying chamber permits drying at lower temperature levels, resulting in higher thermal efficiencies and cooler conditions for handling powders. The plant can be equipped with pneumatic transport system for many powders or with an external vibrating fluid bed for agglomerated powders.

Tall form dryer

A tower spray dryer with a top mounted nozzle assembly featuring a fines return capability. The atomized droplets dry while gently falling down the tower. Further drying and cooling are carried out in a vibrating fluid bed located under the tower. The tall form dryer is suited for both non-fat and fat-containing products, producing non-agglomerated and agglomerated free-flowing powders.

Multi-stage dryer

This is a spray dryer with an integrated fluid bed. The spray is created by a spray nozzle atomizer. Operational flexibility enables production of a wide range of physical properties. The process produces non-dusty, free flowing agglomerated powders with high flavor retention. It operates with low outlet temperatures, achieving high thermal efficiency. Dry ingredients for additional flavor or nutrient fortification can be added within the system to further promote capabilities and improve formulation efficiencies. This design concept is successful for drying high fats, hygroscopic, and sticky products that are difficult to handle in more conventional designs.

Features of inductive heating

Thin film equipment with an inductive heating system is designed without any jacket in the evaporation zone. Inductive heating is generated in the inner shell by wrap-around copper coils. Inductive heating permits temperatures of up to 500 °C for any type of thin film equipment, either of cylindrical or conical design of the evaporation zone.

Thin film technology is commonly the last stage of a thermal separation process requiring the highest temperature. The possibility of avoiding installing a special hot oil system for this last stage can often prove an economically interesting alternative for the following reasons:

• High operational safety (no rotating parts, e. g. pumps, no oil leakages)
• Easy erection (control cabinet only)
• Smaller space requirements (no hot-oil unit, no piping)
• Reduced inspection and testing by minimising the number of pressure retaining parts
• High specific heat transfer rates
• Short heat up time
• High overall thermal efficiency due to reduced radiation from a hot oil unit and associated pipe work
• Fine temperature control
• Safety requirements for hot oil are not required

7.1.1 Nozzles

A device, in which kinetic energy and internal energy of a fluid are interchanged as a result of changing cross-sectional area available for flow, is termed a nozzle. It increases the velocity of the fluid at the expense of pressure. Nozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft, and even garden hoses

A convergent nozzle is one in which with an incompressible fluid the nozzle area continues to decrease and velocity increases as the pressure decreases. On the other hand, for compressible fluids, the area first decreases and then increases so that the nozzle has a converging section followed by a divergent section. Such a nozzle is called a convergent divergent nozzle.

A converging duct is a nozzle for subsonic flow (M < 1).

A diverging duct is a nozzle for a supersonic flow (M > 1).

7.1.2 Diffusers

The diffuser is the gradually expanding passage following the test section in which the flow speed decreases and the pressure rises. The recovery of pressure from kinetic energy reduces the power needed to drive the tunnel: in the case of open-circuit tunnels the diffuser also reduces drafts in the laboratory. The pressure rise is less than that given by Bernoulli’s equation, because of losses due to skin friction and resulting growth of boundary-layer displacement thickness.

The cross-sectional area of a diffuser increases in the flow direction for subsonic flows and decreases for supersonic flows.

A converging duct is a nozzle for a supersonic flow (M > 1).

A diverging duct is a nozzle for a subsonic flow (M < 1).

Supersonic diffuser

Supersonic tunnels, in which a diverging diffuser after the test section would produce a further increase in Mach number, are equipped with a second throat at the end of the test section figure 7.5. The first throat is the one upstream of the test section through which the flow accelerates through the speed of sound. In the converging section leading to the second throat the flow is decelerated to slightly above sonic speed (obeying the one-dimensional inviscid compressible flow equations to a first approximation); in the diverging section downstream of the throat the Mach number rises again, until a shock wave or waves produce a reduction to subsonic speed. It may be shown that a shock wave in the converging portion of the second throat would be unstable, and in practice the second-throat Mach number is chosen large enough for the breakdown shock system to be located well downstream of the throat, to ensure stability under all operating conditions.

When the tunnel is started up, the second throat must be rather larger than the first throat in order for the latter to choke first, so that supersonic flow can be established in the test section. The inviscid-flow equations are not accurate during startup because a strong shock wave passes down the test section, reducing the speed below that of sound and usually causing massive boundary-layer separation. Therefore the second throat has to be significantly larger than the first, and to achieve the best possible supersonic diffusion in tunnels for high supersonic speeds the second throat is closed in after starting (reducing the local Mach number), by adjusting the shape of the walls. This is cost-effective only in large tunnels or if drive power is restricted.

Subsonic diffuser

The usual design rule for subsonic diffusers is that the total included angle of a portion of a circular cone with the same length and area ratio as the diffuser should not exceed 5 deg. This is well below the angle for maximum pressure recovery, which is nearer 10 deg., but at angles of more than about 5 deg. the boundary layer is close enough to separation for the flow to be unsteady. The 5 deg. rule will fail if the test section is unusually long, so that the boundary-layer thickness at entry to the diffuser is unusually large. (We expect the behavior of boundary layers in adverse pressure gradient to depend on some dimensionless parameter such as (δ/U_CL)dU_CL/dx, which must be small enough to avoid separation. Here δ and U_CL are strictly the local boundary layer thickness and centerline velocity – related by Bernoulli’s equation to the pressure gradient, which is what really matters – but the evolution of δ is determined by its value at the beginning of the diffuser.)

7.1.3 Throttling valve

The critical flow nozzle allows the steam to reach sonic velocity at the throat of the nozzle at a very low upstream pressure. Due to the laws of physics the sonic velocity at the throat cannot be exceeded. As the upstream pressure increases the volumetric flow rate does not change, but the density of the steam increases with increasing pressure and thus the mass flow rate is proportional to the upstream pressure. A conical diffuser reduces the sonic velocity at the throat back down to the flow velocity in the pipe while recovering up to 90% of the upstream pressure.

Because of the pressure loss across the nozzle this type of meter can only be used in applications where steam is injected into a lower pressure area. Typical applications include humidification, surging tanks, atmospheric blanchers, steam eductors, steam injection into process ovens, and any other application where steam is exhausted through a manifold or nozzles to a lower pressure.

7.2.1 Types of compressors

7.2.2 Compressing devices

The different types of compressing devices are:

Centrifugal fans

A centrifugal fan has an impeller with a number of blades around the periphery, and the impeller rotates in a scroll or volute shaped casing, and it is this casing, which identifies the centrifugal fan. As the impeller rotates, air is thrown from the blade tips centrifugally into the volute shaped casing (snail shell) and out through the discharge opening, and at the same time more air is drawn into the ‘eye’ of the impeller through a central inlet opening in the side of the casing, thus creating a continuous flow of air through the fan impeller and casing.

The volute shape of the casing helps to transform some of the velocity pressure of the air leaving the impeller into useful static pressure to overcome resistance to airflow in the ducting system to which the fan is connected. In normal ventilation work, a centrifugal fan would be used for static pressures (system resistances) up to about 750 Pa(=N/m²). A point to note is that the air flow through a centrifugal fan cannot be reversed.

The advantages are:

• Provides pulsation-free air 24 hours a day
• 100% continuous duty
• Quiet operation
• Energy efficient at full load
• Extended service intervals
• Reliable long life
• Improved air quality

Centrifugal compressors or turbo-compressors

Centrifugal compressor process simulation can be configured to compress a variety of process gases by varying the molecular weight. The default configuration is for dry air.

Air passes through a suction valve before entering the suction drum. If the process gas demand is less than the minimum recommended compressor flow (surge point), then the makeup gas will mix with the kickback flow. The gas then passes through a cooler, where the temperature is lowered to prevent excessive compressor discharge temperatures. Any condensate present in the gas will be knocked out in the suction drum before the gas enters the compressor.

The compressed gas is then drawn off from the discharge side of the compressor by users. In the event of a decrease in process gas demand by the users, a minimum compressor flow line (kickback or spillback) is provided to allow the recycling of gas to prevent compressor surging. A vent/flare line is also provided to prevent an over-pressuring of the system.

Axial compressors

The axial compressor is one of the main components of modern gas turbines used as drive for generators in the energy supply and as jet engines in aviation, respectively. For reasons of a reduction of weight and costs a reduction of the stage number by an increase of the stage pressure ratio is desired along with constantly high stage efficiency. This tendency increases the danger of flow instabilities, which means a considerable risk for the axial compressor and which may lead to aggravating damages.

An axial flow compressor generally contains several ‘stages’. Each stage consists of a rotor, which is mounted on a central shaft, and rotates at high speed, together with a stator, which is fixed to the casing. Both rotor and stator have many aerodynamically profiled blades designed to generate a pressure difference across the stage. The attainable pressure rise per stage is limited by the aerodynamic efficiency of the blade and many stages are required to build up a useful pressure difference.

The axial-flow compressor is not a positive-displacement device and will not operate for all flow conditions. At high loading and low flow rates it will surge as the flow pattern through the blading breaks down.

Advantages of axial compressors:

• High overall π
• High η
• Small front area (High mass flux)

Reciprocating compressors

A reciprocating compressor is used to boost the pressure of the flow media for use as a seal gas by the flow compressor. This unit can be used to pressurize air for use in the loop. (Air is sometimes used for testing that does not involve in-line inspection tools.) Another reciprocating unit is used for pressurizing fuel gas from its supply pressure of under 50 psi to the pressure required by the flow compressor, nominally 170 psi. Each compressor system is self-contained with its own coolers, scrubbers, and filters.

The process gas to be compressed enters the suction of the first stage of the reciprocating compressor through a suction pressure control valve. The make-up process gas is mixed with the kickback flow before passing through the inlet cooler.

The process gas leaving the first stage of the compressor passes through an inter cooler before being compressed by the second stage of the compressor.

The compressed process gas is then drawn off by users from the discharge of the second stage. Excess gas may either by sent back to the suction of the first stage through the discharge pressure control valve, or may be vented.

Rotary compressors

The compression chamber is a toroidal channel in which an impeller rotates. The gas trapped between the vanes is centrifugally forced to the periphery of the chamber and swirls around the core before it is caught again by the next vane on the wheel, repeating the process all around the channel, and transforming the kinetic energy into pressure. A stripper separates the inlet from the outlet ports and helps in guiding the gas flow from suction to discharge. The clearance between the impeller and the stripper is very tight to limit the gas slippage.

The location, orientation, area of inlet/outlet ports, the geometry of the vanes (bending radius, height, penetration, angle), the shape and area of the chamber, and the shape of the channel impart different compressor characteristics (flow rate, pressure, efficiency), offering multiple solutions for dealing with varied process conditions.

The benefits of rotary compressors over reciprocating compressors are:

• Continuous and smooth operations
• Higher efficiency as a result of less power consumption
• Fewer moving parts leading to greater reliability
• Lighter weight and more compact size
• Quieter operation

Jet compressors

Jet compressors utilize the energy of steam, gas, or air, under pressure, to circulate steam, boost low-pressure steam, and mix gases in desired proportion. In all of the processes where jet compressors are applied, they not only perform the primary function of compressing and mixing gases, but also take the place of a reducing valve and salvage much of the energy lost in the reduction of operating medium pressure.

Basically, jet compressors consist of an expanding nozzle, diffuser, and body. They are also equipped with manually or automatically controlled adjusting spindles for regulating the volume of flow through the nozzle. Manually controlled units are recommended for use when pressures do not vary. Automatically controlled units are recommended when capacities or discharge pressures vary. Units can be made of corrosion-resistant materials.

In operation, jet compressors utilize a jet of high-pressure gas as an operating force to entrain a low-pressure gas, mix the two, and bring the pressure to an intermediate point. Gases can be steam, air, propane, or others. When both fluids are steam, the unit is known generally as a Thermo compressor. These Thermo compressors are used as energy savers, recovering low-pressure ‘waste’ steam by combining it with higher-pressure steam to raise the discharge pressure so the mixture can be re-used in a process.

7.4.1 Heat engines

Heat engines convert heat energy into mechanical energy. Examples include steam engines, steam and gas turbines, spark-ignition and diesel engines, and the ‘external combustion’ engine or Stirling engine. Such engines can provide motive power for transportation, to operate machinery, or to produce electricity.

All heat engines operate in a cycle of repeated sequences of heating (or compressing) and pressurizing the working fluid, the performance of mechanical work, and rejecting unused or waste heat to a ‘sink.’ At the beginning of each cycle, energy is added to the fluid forcing it to expand under high pressure so that the fluid ‘performs’ mechanical work. The thermal energy contained in the pressurized fluid is converted to kinetic energy. The fluid then looses pressure, and after unused energy (in the form of heat) is rejected, it must then be reheated or recompressed to restore it to high pressure.

Heat engines cannot convert all the input energy to useful mechanical energy in the same cycle; some amount, in the form of heat, is always not available for the immediate performance of mechanical work. The fraction of thermal energy that is converted to net mechanical work is called the thermal efficiency of the heat engine. The maximum possible efficiency of a heat engine is that of a hypothetical (ideal) cycle, called the Carnot Cycle. Practical heat engines operate on less efficient cycles (such as the Rankine, Brayton, or Stirling) but in general, the highest thermal efficiency is achieved when the input temperature is as high as possible and the sink temperature is as low as possible.

The ‘waste’ or rejected heat (to the ‘sink) can be used for other purposes, including pressurizing a different working fluid, which operates a different heat-engine (vapor turbine) cycle, or simply for heating. Renewable sources of heat or fuels, such as solar or geothermal energy and biomass (as well as fossil fuels) can power heat engines. The following is a brief description of four types of heat engines, the Rankine, Stirling, Brayton, and the newly developed, and highly efficient, Kalina, that can be used or are being investigated for converting renewable sources of energy to useful energy.

7.4.2 Internal combustion engines

An internal combustion engine burns a mixture of fuel and air. Typical IC engines are classified as Spark and Compression ignition engines.

The simplest model for IC engines is the air-standard model, which assumes that:

• The system is closed
• Air is the working fluid and is modeled as an ideal gas throughout the cycle
• Compression and expansion processes are isentropic
• A reversible heat transfer process characterizes the combustion of fuel and air
• Heat rejection takes place reversibly and at constant volume

The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.

Spark ignition engine (Otto cycle)

The most common type is a four-stroke engine. A piston slides in and out of a cylinder. Two or more valves allow the fuel and the air to enter the cylinder and the gases that form when the fuel and air burn to leave the cylinder. As the piston slides back and forth inside the cylinder, the volume that the gases can occupy changes drastically.

The process of converting heat into work begins when the piston is pulled out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve. This motion is called the intake stroke or induction stroke. Next, the fuel and the air mixture are squeezed together by pushing the piston into the cylinder. This is called the compression stroke. At the end of the compression stroke, with the fuel and the air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. The hot burning fuel has an enormous pressure and it pushes the piston out of the cylinder. This power stroke is what provides power to the engine and the attached machinery. Finally, the burned gas is squeezed out of the cylinder through another valve in the exhaust stroke. These four strokes repeat over and over again. Most internal combustion engines have at least four cylinders and pistons. There is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes. The maximum efficiency of such an engine is e_max = (Tignition – T_air)/T_ignition where T_ignition is the temperature of the fuel-air mixture after ignition. To maximize the fuel efficiency, you have to create the hottest possible fuel air mixture after ignition. The highest efficiency that has been achieved is approximately 50% of e_max.

Compression ignition engines (Diesel cycle)

In the Diesel, the fuel is not mixed with the air entering the cylinder during the intake stroke. Air alone is compressed during the compression stroke. The Diesel fuel oil is injected or sprayed into the cylinder at the end of the compression stroke. The Diesel Cycle differs from the Otto Cycle only in the modeling of the combustion process. In a Diesel Cycle, it is assumed to occur as a reversible constant pressure heat addition process, while in an Otto Cycle, the volume is assumed constant.

The four steps of the air-standard Diesel Cycle are outlined below:

• (1-2) Isentropic Compression (Compression Stroke)
• (2-3) Reversible, constant pressure heat addition (Ignition)
• (3-4) Isentropic expansion to initial volume (Power Stroke)
• (4-1) Reversible constant-volume heat rejection (Exhaust)

In diesel engines, compression ratios are as high as (22.5 to 1) and provide pressures of (500psi) at the end of the compression stroke. Through the compression process, the air can be heated up (1000 degrees F). This temperature is high enough to spontaneously ignite the fuel as it is injected into the cylinder. The high pressure of the explosion forces the piston down as in the gasoline engine.

7.4.3 Steam turbine

The steam turbine obtains its motive power from the change of momentum of a jet of steam flowing over a curved vane. The steam jet, in moving over the curved surface of the blade, exerts a pressure on the blade owing to its centrifugal force. This centrifugal pressure is exerted normal to the blade surface and acts along the whole length of the blade; the result of this centrifugal pressure plus the effect of change of velocity is the motive force on the blade. This motive power enables the turbine to drive the electric generator.

A steam turbine consists of a pair of blade rings consisting of a fixed ring of blades and moving ring. Both the moving and fixed blades are designed so that the jet shall not strike the blade but will merely glide over it in parallel direction. The fixed blades are fixed to the turbine casing and face the moving blades in opposite direction. The object of fixed blade is to receive steam jet discharging from the moving blade ring and to divert it to the next ring of moving blade by changing its direction. This diversion may continue over several rings of moving and fixed blades until the whole of K.E. of the steam is expanded.

Afterburners

In addition to the basic components of a gas turbine engine, one other process is occasionally employed to increase the thrust of a given engine. Afterburning (or reheat) is a method of augmenting the basic thrust of an engine to improve the aircraft takeoff, climb and (for military aircraft) combat performance.

Afterburning consists of the introduction and burning of raw fuel between the engine turbine and the jet pipe propelling nozzle, utilizing the unburned oxygen in the exhaust gas to support combustion. The resultant increase in the temperature of the exhaust gas increases the velocity of the jet leaving the propelling nozzle and therefore increases the engine thrust. This increased thrust could be obtained by the use of a larger engine, but this would increase the weight, frontal area and overall fuel consumption. Afterburning provides the best method of thrust augmentation for short periods.

Afterburners are very inefficient as they require a disproportionate increase in fuel consumption for the extra thrust they produce. Afterburning is used in cases where fuel efficiency is not critical, such as when aircraft take off from short runways, and in combat, where a rapid increase in speed may occasionally be required.

7.4.4 Jet engines

The main function of any aero plane propulsion system is to provide a force to overcome the aircraft drag, this force is called thrust. Both propeller driven aircraft and jet engines derive their thrust from accelerating a stream of air – the main difference between the two is the amount of air accelerated. A propeller accelerates a large volume of air by a small amount, whereas a jet engine accelerates a small volume of air by a large amount. This can be understood by Newton’s 2nd law of motion which is summarized by the equation F=ma (force = mass x acceleration). Basically the force or thrust (F) is created by accelerating the mass of air (m) by the acceleration (a).

Given that thrust is proportional to airflow rate and that engines must be designed to give large thrust per unit engine size, it follows that the jet engine designer will generally attempt to maximize the airflow per unit size of the engine. This means maximizing the speed at which the air can enter the engine, and the fraction of the inlet area that can be devoted to airflow. Gas turbine engines are generally far superior to piston engines in these respects; therefore piston-type jet engines have not been developed.

Air passing through the engine has to be accelerated; this means that the velocity or kinetic energy of the air must be increased. First, the pressure energy is raised, followed by the addition of heat energy, before final conversion back to kinetic energy in the form of a high velocity jet.

The basic mechanical arrangement of a gas turbine is relatively simple. It consists of only four parts:

• The compressor, which is used to increase the pressure (and temperature) of the inlet air
• One or a number of combustion chambers in which fuel is injected into the high-pressure air as a fine spray, and burned, thereby heating the air. The pressure remains (nearly) constant during combustion, but as the temperature rises, each kilogram of hot air needs to occupy a larger volume than it did when cold and therefore expands through the turbine
• The turbine, which converts some of this temperature rise to rotational energy. This energy is used to drive the compressor
• The exhaust nozzle which accelerates the air using the remainder of the energy added in the combustor, producing a high velocity jet exhaust

7.4.5 The compressor

In the gas turbine engine, compression of the air is effected by one of two basic types of compressor, one giving centrifugal flow and the other axial flow. Both types are driven by the engine turbine and are usually coupled direct to the turbine shaft.

The centrifugal flow compressor employs an impeller to accelerate the air and a diffuser to produce the required pressure rise. Flow exit’s a centrifugal compressor radially (at 90° to the flight direction) and it must therefore be redirected back towards the combustion chamber, resulting in a drop in efficiency. The axial flow compressor employs alternate rows of rotating (rotor) blades, to accelerate the air, and stationary (stator) vanes, to diffuse the air, until the required pressure rise is obtained.

The pressure rise that may be obtained in a single stage of an axial compressor is far less than the pressure rise achievable in a single centrifugal stage. This means that for the same pressure rise, an axial compressor needs many stages, but a centrifugal compressor may need only one or two

An engine design using a centrifugal compressor will generally have a larger frontal area than one using an axial compressor. This is partly a consequence of the design of a centrifugal impeller, and partly a result of the need for the diffuser to redirect the flow back towards the combustion chamber. As the axial compressor needs more stages than a centrifugal compressor for the equivalent pressure rise, an engine designed with an axial compressor will be longer and thinner than one designed using a centrifugal compressor. This, plus the ability to increase the overall pressure ratio in an axial compressor by the addition of extra stages, has led to the use of axial compressors in most engine designs, however, the centrifugal compressor is still favored for smaller engines where it’s simplicity, ruggedness and ease of manufacture outweigh any other disadvantages.

Multiple chambers

This type of combustion chamber is used on centrifugal compressor engines and the earlier types of axial flow compressor engines. It is a direct development of the early type of Whittle engine combustion chamber. Chambers are disposed radially around the engine and compressor delivery air is directed by ducts into the individual chambers. Each chamber has an inner flame tube around which there is an air casing. The separate flame tubes are all interconnected. This allows each tube to operate at the same pressure and also allows combustion to propagate around the flame tubes during engine starting.

Can-annular chamber

This type of combustion chamber bridges the evolutionary gap between multiple and annular types. A number of flame tubes are fitted inside a common air casing. The airflow is similar to that already described. This arrangement combines the ease of overhaul and testing of the multiple systems with the compactness of the annular system.

Annular chamber

This type of combustion chamber consists of a single flame tube, completely annular in form, which is contained in an inner and outer casing. The main advantage of the annular combustion chamber is that for the same power output, the length of the chamber is only 75 per cent of that of a can-annular system of the same diameter, resulting in a considerable saving in weight and cost. Another advantage is the elimination of combustion propagation problems from chamber to chamber.

7.4.6 Rocket engines

One of the most amazing efforts man has ever undertaken is the exploration of space. A big part of the amazement is the complexity. Space exploration is complicated because there are so many problems to solve and obstacles to overcome.

The gray areas to be confronted are:

• The vacuum of space
• Heat management problems
• The difficulty of re-entry
• Orbital mechanics
• Micrometeorites and space debris
• Cosmic and solar radiation
• The logistics of having restroom facilities in a weightless environment

But the major problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in.

The basics

Rocket engines are fundamentally different. Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle that ‘to every action there is an equal and opposite reaction.’ A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.

This concept of ‘throwing mass and benefiting from the reaction’ can be hard to grasp at first, because that does not seem to be what is happening. Rocket engines seem to be about flames and noise and pressure, not ‘throwing things.’

A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass. If you burn a pound of rocket fuel, a pound of exhaust comes out the nozzle in the form of a high-temperature, high-velocity gas. The form changes, but the mass does not. The burning process accelerates the mass.

Solid-fuel rockets

Solid-fuel rocket engines were the first engines created by man. They were invented hundreds of years ago in China and have been used widely since then. The idea behind a simple solid-fuel rocket is straightforward… Here’s a typical cross section:

On the left is the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When the fuel is lighted, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine or in a tiny bottle rocket the burn might last a second or less. In a Space Shuttle SRB containing over a million pounds of fuel, the burn lasts about two minutes.

The propellant mixture in each SRB motor consists of an ammonium per chlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star-shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.

The idea is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust. As the fuel burns the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.

An ‘11-point star-shaped perforation’ might look like this:

Solid-fuel rocket engines have three important advantages:

• Simplicity
• Low cost
• Safety

They also have two disadvantages:

• Thrust cannot be controlled
• Once ignited, the engine cannot be stopped or restarted

The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system.

Liquid-propellant rockets

In most liquid-propellant rocket engines, a fuel and an oxidizer (for example, gasoline and liquid oxygen) are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then they leave the engine. The following highly simplified diagram shows you the basic components.

This diagram does not show the actual complexities of a typical engine (see some of the links at the bottom of the page for good images and descriptions of real engines). For example, it is normal for either the fuel of the oxidizer to be a cold liquefied gas like liquid hydrogen or liquid oxygen. One of the big problems in a liquid propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them. The pumps have to generate extremely high pressures in order to overcome the pressure that the burning fuel creates in the combustion chamber.

The main engines in the Space Shuttle actually use two pumping stages and burn fuel to drive the second stage pumps. All kinds of fuel combinations get used in liquid propellant rocket engines. For example:

• Liquid hydrogen and liquid oxygen – used in the Space Shuttle main engines
• Gasoline and liquid oxygen – used in Goddard’s early rockets
• Kerosene and liquid oxygen – used on the first stage of the large Saturn V boosters in the Apollo program
• Alcohol and liquid oxygen – used in the German V2 rockets
• Nitrogen tetroxide/monomethyl hydrazine – used in the Cassini engines

7.5.1 Plate heat exchangers in industrial refrigeration systems

The plate heat exchanger concept, with flow-trough channels formed by corrugated plates and the heat transfer taking through the thin plates, is an extremely efficient heat exchange technique.

The turbulent flow, coupled with fouling factors and high heat transfer coefficient, means that it is possible to operate with a small temperature difference in evaporation and chilled water temperatures. This in turn provides a good operational economy with high C.O.P. values.

It also means that a plate heat exchanger becomes much more compact than a shell & tube heat exchanger for the same duty. The practical advantages are:

• Lower weight
• Smaller space requirements
• Lower refrigerant filling

The most common plate heat exchanger in industrial refrigeration is the Semi Welded Plate Heat Exchanger (SWPHE), which alternates welded channels and traditional gasketed channels.

The refrigerant flows in welded channels and the only gaskets in contact with the refrigerant are two circular porthole gaskets between the welded plate pairs. These gaskets are made from highly resistant materials, attached for easy replacement by a glue free construction.

The secondary medium flows in channels sealed by traditional elastomer gaskets.

The SWPHE is very flexible and variable and can be arranged in Twin or two-in one design, e.g. Desuperheater/Condenser, Oilcooler/Condenser. These features give the possibility to manufacture two duties in one frame at a lower cost, smaller volume and shorter footprint.

The plate heat exchanger concept allows the SWPHE to be opened and reclosed several times.

The SWPHE is not sensitive to temperature shocks and due to the turbulent flow freezing risks are small, but the flexible design will accommodate expansion and no damage will be caused should freezing occur.

The Semi Welded Plate Heat Exchangers are used as evaporators and condensers for refrigeration systems in a whole series of applications, e.g.:

• Dairy, brewery and wine yard production
• Marine
• Fishing vessels and fish processing
• Slaughter houses
• Chemicals and pharmaceutical industries
• Ice manufacturing, ice-skating rinks
• Food retail outlet

When the gasketed side is food approved it could be used in direct cooling of food liquids, e.g. NH₃/beer, juice or water.

Other application like Heat Pumps, Organic Rankine Cycles and Absorption Systems could also request SWPHE for different duties.

7.5.2 Cryogenics

Cryogenic Separation is a distillation process that occurs at temperatures close to -170 degrees Celsius. At this temperature, air starts to liquefy.

Before separation can occur, there are specific operation conditions that must be achieved. Distillation requires two phases, gas and liquid. Air must be very cold for this to happen. For this instance, at one atmosphere, nitrogen is a liquid at -196 degrees Celsius. A pressure 8-10 times atmospheric pressure is required for this process. These conditions are achieved via compression and heat exchange; cold air exiting the column is used to cool air entering it. Nitrogen is more volatile than oxygen and comes off as the distillate product.

Air separation plants

A cryogenic air separation plant is expensive and large; the distillation column is several stories high and must be well insulated. Consequently, it only becomes economically feasible to separate air this way when a large amount is needed. Cryogenic separation is also capable of producing much purer nitrogen than either of the other two processes because the number of trays in the distillation column can be increased

Process description

The exhaust air containing pollutants is preheated up to the ignition temperature of the catalyst. This is done by waste heat utilization from the clean gas. In stationary operation power requirement is in most cases limited to the exhaust air blower. The process data are determined by the reactor type and the catalyst applied.

Typical examples of exhaust air streams that can be treated by a thermal or catalytic process are:

• Residual gases from pharmaceutical plants
• Off-gases from tank aeration
• Process gases with odour load
• Exhaust-air with solvent load

8.2.1 General design procedure

It is difficult to suggest a standard design procedure. Such a procedure, however, would involve the following steps:

• Specifying the problem
• Analyzing the probable solution
• Applying chemical process principles and theories of mechanics satisfying the conditions of the problem
• Selecting materials and stresses to suit processing conditions
• Evaluating and optimizing the design
• Preparing the drawings and specifications

Each step is to be checked both in terms of mathematical calculations and engineering feasibility. It is necessary to ascertain whether the results are consistent with experience and feasibility. It may take several iterations before the satisfactory solution is obtained.

8.3.1 Equipment classification

The classification of chemical equipments is normally based on the specific type of unit operation. Equipments may also be defined to emphasize certain common features, which require similar design procedures. These result in the following classification:

8.5.1 Choice of material

Choice of the material cannot be made merely by choosing a suitable material having the requisite mechanical behavior and anticorrosive properties, but must be based on a sound economic analysis of competing materials.

8.6.1 Uniform attack

Uniform attack is a form of electrochemical corrosion that occurs with equal intensity of the entire surface of the metal. Iron rusts when exposed to air and water, and silver tarnishes due to exposure to air. Potentially very risky, this type of corrosion is very easy to predict and is usually associated with ‘common sense’ when making material decisions.

8.6.2 Galvanic corrosion

Galvanic corrosion is a little more difficult to keep track of in the industrial world. Galvanic corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte. The more reactive metal will experience severe corrosion while the more noble metal will be quite well protected. Perhaps the most infamous examples of this type of corrosion are combinations such as steel and brass or copper and steel. Typically the steel will corrode the area near the brass or copper, even in a water environment and especially in a seawater environment. Probably the most common way of avoiding galvanic corrosion is to electrically attach a third, anodic metal to the other two. This is referred to as cathodic protection.

8.6.3 Crevice corrosion

Another form of electrochemical corrosion is crevice corrosion. Crevice corrosion is a consequence of concentration differences of ions or dissolved gases in an electrolytic solution. A solution became trapped between a pipe and the flange on the left. The stagnant liquid in the crevice eventually had a lowered dissolved oxygen concentration and crevice corrosion took over and destroyed the flange. In the absence of oxygen, the metal and/or it’s passive layer begin to oxidize. To prevent crevice corrosion, it is recommended to use welds rather than rivets or bolted joints whenever possible. Also consider nonabsorbing gaskets. Remove accumulated deposits frequently and design containment vessels to avoid stagnant areas as much as possible.

8.6.4 Pitting

Pitting, just as it sounds, is used to describe the formation of small pits on the surface of a metal or alloy. Pitting is suspected to occur in much the same way crevice corrosion does, but on a flat surface. A small imperfection in the metal is thought to begin the process, and then a ‘snowball’ effect takes place. Pitting can go on undetected for extended periods of time, until a failure occurs. A textbook example of pitting would be to subject stainless steel to a chloride-containing stream such as seawater. Pitting would overrun the stainless steel in a matter of weeks due to its very poor resistance to chlorides, which are notorious for their ability to initiate pitting corrosion. Alloy blends with more than 2% Molybdenum show better resistance to pitting attack. Titanium is usually the material of choice if chlorides are the main corrosion concern. (Pd stabilized forms of Ti are also used for more extreme cases).

8.6.5 Intergranular corrosion

Occurring along grain boundaries for some alloys, intergranular corrosion can be a real danger in the right environment. On the left, a piece of stainless steel (especially susceptible to intergranular corrosion) has seen severe corrosion just an inch from a weld. The heating of some materials causes chromium carbide to form from the chromium and the carbon in the metals. This leaves a chromium deficient boundary just shy of the where the metal was heated for welding. To avoid this problem, the material can be subjected to high temperatures to redissolve the chromium carbide particles. Low carbon materials can also be used to minimize the formation of chromium carbide. Finally, the material can be alloyed with another material such as Titanium, which forms carbides more readily so that the chromium remains in place.

8.6.6 Selective leaching

When one element or constituent of a metal is selectively corroded out of a material it is referred to as selective leaching. The most common example is the dezincification of brass. On the right, nickel has been corroded out of a copper-nickel alloy exposed to stagnant seawater. After leaching has occurred, the mechanical properties of the metal are obviously impaired and some metal will begin to crack.

8.6.7 Erosion corrosion

Erosion-corrosion arises from a combination of chemical attack and the physical abrasion as a consequence of the fluid motion. Virtually all alloy or metals are susceptible to some type of erosion-corrosion as this type of corrosion is very dependent on the fluid. Materials that rely on a passive layer are especially sensitive to erosion-corrosion. Once the passive layer has been removed, the bare metal surface is exposed to the corrosive material. If the passive layer cannot be regenerated quickly enough, significant damage can be seen. Fluids that contain suspended solids are often times responsible for erosion-corrosion. The best way to limit erosion-corrosion is to design systems that will maintain a low fluid velocity and to minimize sudden line size changes and elbows. The photo above shows erosion-corrosion of a copper-nickel tube in a seawater surface. An imperfection on the tube surface probably causes an eddy current which provided a perfect location for erosion-corrosion.

8.6.8 Stress corrosion

Stress corrosion can result from the combination of an applied tensile stress and a corrosive environment. In fact, some materials only become susceptible to corrosion in a given environment once a tensile stress is applied. Once the stress cracks begin, they easily propagate throughout the material, which in turn allows additional corrosion and cracking to take place. Tensile stress is usually the result of expansions and contractions that are caused by violent temperature changes or thermal cycles. The best defense against stress corrosion is to limit the magnitude and/or frequency of the tensile stress.

8.8.1 Compressors and vacuum pumps

• Fans are used to raise the pressure about 3% (12 in. water). Blowers raise to less than 40 psig, and compressors to higher pressures, although blower range commonly is included in the compressor range
• Vacuum pumps: reciprocating piston type decrease the pressure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary down to 0.0001 Torr; steam jet ejectors, one stage down to 100 Torr, three stages down to 1 Torr, five stages down to 0.05 Torr
• A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr
• Efficiencies of reciprocating compressors: 65% at compression ratio of 1.5, 75% at 2.0. And 80-85% at 3-6
• Efficiencies of large centrifugal compressors, 6000-100,0000 ACFM at suction, are 76-78%
• Rotary compressors have efficiencies of 70%, except liquid liner type which have 50%

8.8.2 Conveyors for particulate solids

• Screw conveyors are suited to transport of even sticky and abrasive solids up inclines of 20 or so. They are limited to distances of 150 ft or so because of shaft torque strength. A 12 in. dia conveyor can handle 100-3000 cu, ft/hr, at speeds ranging from 40 to 60 rpm
• Belt conveyors are for high capacity and long distances a mile or more, but only several hundred feet in a plant), up inclines of 30 maximum. A 24-inch wide belt can carry 300 cu. ft/hr at a speed of 100 ft/min, but speeds up to 600 ft/min are suited to some materials. Power consumption is relatively low
• Bucket elevators are suited to vertical transport of sticky and abrasive materials. With buckets 20 × 20 inches, capacity can reach 100 cu. ft/hr at a speed of 100 ft/min, but speeds to 300 ft/min are used
• Drag-type conveyors (Redler) are suited to short distances in any direction and are completely enclosed. Units range in size from 3 in, square to 19 in square and may travel from 30ft/min (fly ash to 250ft min (grains). Power requirements are high
• Pneumatic conveyors are for high capacity, short distance (400ft) transport simultaneously from several sources to several destinations. Either vacuum or low pressure (6–12 psig) is employed with a range of air velocities from 35 to 120 ft/sec depending on the material and pressure, air requirements from 1 to 7 cu. ft/cu. ft of solid transferred

8.8.3 Cooling towers

• Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature
• In commercial units 90% of saturation of the air is feasible
• Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures:
• T (F) 5 15 25
• Relative volume 2.4 1.0 0.55
• Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 2 inches of water
• Water circulation rate is 1–4 gpm/sqft and air rates are 1300–1800 lb/(hr)(sq. ft or 300–400 ft/min)
• Chimney-assisted natural draft towers are of hyperboloidal shapes because they have greater strength for a given thickness; a tower 250 ft high has concrete walls 5–6in. thick. The enlarged cross section at the top aids in dispersion of exit humid air into the atmosphere
• Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 2 deg F of the wet bulb
• Evaporation losses are 1% of the circulation for every 10 deg F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3%. Blow down of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup

8.8.4 Crystallization from solution

• Complete recovery of dissolved solids is obtainable by evaporation but only to the eutectic composition by chilling. Recovery by melt crystallization is also limited by the eutectic composition
• Growth rates and ultimate sizes of crystals are controlled by limiting the extent of super saturation at any time
• In crystallization by chilling, the temperature of the solution is kept at the most at 1–2 deg F below the saturation temperature at the prevailing concentration
• Rate of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/hr. The growth rates are approximately the same in all directions
• Growth rates are influenced greatly by the presence of impurities and of certain specific additives that vary from case to case

8.8.5 Disintegration

• Percentages of material greater than 50% of the maximum size are about 50% from rolls, 15% from tumbling mills, and 5% from closed circuit ball mills
• Close circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to design of air classifiers. Closed circuit is most common with ball and roller mills
• Jaw crushers take lumps of several feet in diameter down to 4 in stroke rates of 100–300/ min. The average feed is subjected to 9–10 strokes before small enough to escape. Gyratory crushers are suited to shabby feeds and make a more rounded product
• Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps 14in. dia. Smooth rolls affect reduction ratios up to about 4. Speeds are 50–900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon of material passing through the rolls
• Road mills are capable of taking feed as large as 50 mm and reducing it to 300 meshes, but normally the product ranges is 8-65 mesh. Rods are 25-150 mm dia. Ratio of rod length to mill diameter is about 1.5. About 45% of the mill rods occupy volume. Rotation is at 50-65% of critical
• Ball mills are better suited than rod mills to fine grinding. The charge is of equal weights of 1.5, 2 and 3in ball for the finest grinding. Volume occupied by the balls is 50% of the mill volume. Rotation speed is 70-80% of critical. Ball mills have a length to diameter ratio in the range 1–1.5. Tube mills have a ratio of 4–5 and are capable of very fine grinding. Pebble mills have ceramic grind elements, used when contamination with metal is to be avoided
• Roller mills comply with cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 meshes are made

8.8.6 Distillation and gas absorption

• Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization, or others
• Tower operating pressure is determined most often by the temperature of the available condensing medium, 100-120 F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 366 F
• Sequencing of columns for separating multicomponent mixtures: (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last; (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products; (c) When the adjacent ordered components in the feed vary widely in relative volatility; (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed
• Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio
• The economically optimum number of trays is near twice the minimum value
• A safety factor of 10% of the number of trays calculated by the best means is advisable

8.8.7 Drivers and power recovery equipment

• Efficiency is greater for larger machines. Motors are 85–95%; steam turbines are 42–78%; gas engines and turbines are 28–38%
• For under 100 HP, electric motors are used almost exclusively; they are made for up to 20,000 HP
• Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited, for example, for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, e.g. weatherproof
• Steam turbines are competitive above 100 HP. They are speed controllable. Frequently they are employed as spares in case of power failure
• Combustion engines and turbines are restricted to mobile and remote locations
• Gas expanders for power recovery may be justified at capacities of several hundred HP; otherwise any required pressure reduction in process is effected with throttling valves

8.8.8 Drying of solids

• Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even days in tunnel shelf or belt dryers
• Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying times in the range of 10–200 min
• Rotary cylindrical dryers operate with superficial air velocities of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 50–90 min. Holdup of solid is 7–8%; an 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–20 deg C above the solid; in parallel flow, the temperature of the exit solid is 100 deg C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25
• Drum dryers for pastes and slurries operate with contact times of 3–12 sec, produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m²hr. Diameters are 1.5–5.0 ft; the rotation rate is 2–10 rpm. The greatest evaporative capacity is to the order of 3000 lb/hr in commercial units
• Pneumatic conveying dryers normally take particles 1–3 mm dia but up to 10mm when the moisture is mostly on the surface. Air velocities are 10–30 m/sec. Single pass residence times are 0.5-3.0 sec but with normal recycling the average residence time is brought up to 60 sec. Units in use range from 0.2 m dia by 1 m high to 0.3 m dia by 38 m long. Air requirement is several SCFM/lb of dry product/hr
• Fluidized bed dryers work best on particles of a few tenths of an mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1–2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2–3 hr
• Spray dryer: Surface moisture is removed in about 5sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012–0.15in. and operate at pressures of 300–4000 psi.
  Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250–600 ft/sec. With nozzles, the ratio is 0.5–1.0. For the final design, the experts say pilot tests in a unit of 2m dia should be made

8.8.9 Evaporators

• Long tube vertical evaporators with either natural or force circulation are most popular. Tubes are 19–63 mm dia and 12–30 ft long
• In forced circulation, linear velocities in the tubes are 15–20 ft/sec
• Elevation of boiling point by dissolved solids results in differences of 3–10 deg F between solution and saturated vapor
• When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4–6
• When the boiling point rise is small, minimum cost is obtained with 8–10 effects in series
• In backward feed the more concentrated solution is heated with the highest temperature steam so that heating surface is lessened, but the solution must be pumped between stages
• The steam economy of an N-stage battery is approximately 0.8 N lb evaporation/lb of outside steam
• Inter stage steam pressures can be boosted with steam jet compressors of 20–30% efficiency or with mechanical compressors of 70–75% efficiency

8.8.10 Filtration

• Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid 0.1–10.0 cm/scc; medium, 0.1–10.0 cm/min; slow, 0.l–10.0 cm/hr
• Continuous filtration should not be attempted if 1/8 inch cake thickness cannot be formed in less than 5 min
• Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges
• Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges
• Slow filtering slurries are handled in pressure filters or sediment centrifuges
• Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters
• Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters; when cake washing is critical; when cake drying may be a problem; or when pre-coating may be needed

8.8.11 Heat exchangers

• Take true countercurrent flow in a shell-and-tube exchanger as a basis
• Standard tubes are 3/4 in. OD, an inch triangular spacing, 16 ft long; a shell 1 ft dia accommodates 100 sq. ft; 2ft dia, 400 sq. ft, 3ft dia, 1100sqft
• Tube side is for corrosive, folding, scaling, and high pressure fluids
• Shell side is for viscous and condensing fluids
• Pressure drops are 1.5 psi for boiling and 3–9 psi for other services
• Minimum temperature approach is 20 deg F with normal coolants. 10 deg F or less with refrigerants
• Water inlet temperature is 90 deg F, maximum outlet 120 F
• Double-pipe exchanger is competitive at duties requiring 100–200 sq. ft
• Compact (plate and fin) exchangers have 350sqft/cuft about 4 times the heat transfer per cuf. t of shell and tube
• Plate and frame exchangers are suited to high sanitation service, and are 25–50% cheaper in stainless construction than shell-and-tube units

8.8.12 Mixing and agitation

• Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 0.1-0.2ft/sec, and intense agitation at 0.7-1,0ft/sec
• Intensities of agitation with impellers in baffled tanks are measured by power input, HP/1000 gal and impeller tip speed

• Proportions of a stirred tank relative to the diameter D: liquid level=D; turbine impeller diameter=D/3; impeller level above bottom=D/3; impeller blade width=D/15; four vertical baffles with width=E/10
• Propellers are made a maximum of 18in., turbine impellers to 9ft. Gas bubbles sparged at the bottom of the vessel will result in mind agitation at a superficial gas velocity of 1ft/min, severe agitation at 4ft/min
• Suspension of solids with a settling velocity of 0.03 t/sec is accomplished with either turbine or propeller impellers, but when the settling velocity is above 0.15ft/sec intense agitation with a propeller is needed
• In-line blenders are adequate when a second or two contact time is sufficient, with power inputs of 0.1-0.2 2 HP/gal

Measurement

This is essentially an estimate or appraisal of the process being controlled by the system. In this example, this is achieved by the right hand of the operator.

Comparison

This is an examination of the likeness of the measured values and the desired values and is carried out in the brain of the operator.

Computation

This is a calculated judgment that indicates how much the measured value and the desired values differ and what action and how much should be taken. In this example, the operator will calculate the difference between the desired temperature and the actual one.

Accordingly the direction and amount of the adjustment of the valve are worked out and the order for this adjustment is sent to the left hand from the brain of the operator.

If the outlet water temperature is lower, then the brain of the operator will tell the left hand to open the steam valve wider. If there is any disturbance, or variation of flow rate in water to the shower inlet, some adjustment must be made to keep the outlet water temperature at a desired value.

Correction

This is ultimately the materialization of the order for the adjustment. The left hand of the operator takes the necessary actions following the order from brain.

Therefore, for a control system to operate satisfactorily, it must have the abilities of measurement, comparison, computation and correction.

Of course, the manual operation has obvious disadvantages e.g. the accuracy and the continuous involvement of operators. Although accuracy of the measurement could be improved by using an indicator, automatic control must be used to replace the operator. In industry, it is automatic control that is widely used.

Positioning of elements

One of the problems with designing control systems is that, as in any design problem, we are faced with alternatives. We have an alternative here in the positioning of the elements.

The measurement element should either be placed upstream of the valve as shown in the figure 9.3 or be placed downstream.

Consideration of the properties of flow meters and valves suggests that the measurement element be placed upstream. If the valve were upstream of the flow meter then there are a number of ways in which it might affect the flow meter calibration.

Control algorithms

In the simple illustrative example of the water heater the rule for making the adjustment was:

If the temperature is too high then turn the heater off.

If it is too low then turn the heater on.

This is an example of an on-off control algorithm. The heater is either on (full) or off (completely).

If the flow is too high then shut the valve off.

If it is too low then open it.

Clearly, this is unlikely to serve, as rather than maintaining a specified flow the conditions will switch between zero and some maximum value. To achieve a specified steady flow we require something like:

If the flow is too high then shut the valve some more.

If it is too low then open it more.

This is a proportional control algorithm; the larger the error in the measured quantity, the larger will be the adjustment. This arrangement should result in the system settling at or near the required flow.

In practice, on-off control is seldom used. Most adjustment elements are valves, or occasionally other mechanical elements. These do not take kindly to being regularly or rapidly swung across their full range of adjustment; they very quickly wear out or break down.

In most cases, therefore, proportional control or some variant is used. More detailed investigation of control algorithms requires quantitative information about the process. This aspect will be dealt with in a later section.

Level control systems

Here we will consider simple feedback control of the level in a tank. This being the case it is necessary to measure the level directly and adjust the flow into or out of the tank to keep it constant.

Alternative control systems

Figure 9.4 shows the two alternative control systems available for feedback control of the level. Both are equally valid and the decision as to which to use is based on

What is upstream or downstream of the tank?

Which streams are already being controlled?

The relative sizes of the flow rates, if there are several input or output flows.

As can be seen the control system consists of:

• A Level Transducer denoted by LT in the diagram
• A Control Valve
• A Level Controller denoted by LC: The level can be regulated by altering the flow via the adjustment of the valve position
• Control Algorithms: It is possible to control the level in a tank using
  • On/Off control
  • Proportional control
  • Extensions to proportional control

The theory behind the algorithms will be found in a later section. There is also a level control experiment based on an actual experiment carried out by the undergraduates in the laboratory. Note that a link to this can be found in the Case Study Section

Pressure control systems

In gas or vapour systems we regulate inventory as pressure. A typical system is shown in figure 9.5. Both the inlet and outlet are gas or vapour. Therefore if the control valve is shut then the pressure in the tank will rise and vice versa.

In principle we might, like the level control system, have the valve either upstream or downstream of the tank. In practice in gas systems it is more likely to be downstream for the following reason.

Raising the pressure of a gas requires energy, and normally some mechanical device, such as a compressor, imparts this energy. Both the compressor itself, and the energy to drive it, is expensive. To minimise the first cost we try to minimise the number of compressors in a process. Where possible we would use only one, locate it at the front of the process, and perform any subsequent manipulations to obtain the required pressure by downstream valves.

The energy used in compression is expensive, and throttling through a control valve throws this energy away. Therefore in processes where compressor costs are very significant we may sometimes avoid such valves and manipulate the compressor speed in order to maintain the system at the required pressure. This control system is shown in the figure 9.6

When we have vapour we usually also have liquids. Regulating pressure in two-phase systems can be somewhat different. This is dealt with later.

Diagrams of these alternative schemes can be found below.

Simple composition control problem

To illustrate composition control considers the simplest process in which composition can be changed, namely blending. Here two streams of different compositions are mixed together e.g. a concentrate and a diluent as shown in the figure 9.8.

Either of the above schemes could be used although the first is preferred. The reasons are discussed in a later section. It is worth mentioning that the composition of a stream is rarely measured directly.

Typical composition analyzers include:

• Gas chromatographs
• Spectroscopic analysers

Features of this type of hardware, which make them ineffective for control purposes, are:

• Large time delay in their response
• Low operational reliability
• Relatively high cost

Thus an alternative method has to be sought to control the composition. This could be via the:

• Temperature of the mixture
• Pressure of the mixture

Example 1 – Reactor temperature control

In this example the aim is to keep T2 at its set point. The primary control loop detects and eliminates changes in T1, the temperature of the reactants. The secondary control loop detects changes in the temperature of the cooling water. Hence it can adjust the flow accordingly before the effects are detected by the primary control loop. If there was no second controller the effect of the cooling water would take a long time to materalise and hence eliminated

Example 2 – Distillation bottoms temperature control

In this example the primary loop detects changes in the temperature brought about by changes in composition, pressure, etc. The secondary loop detects changes in the steam flow rate and hence eliminates anticipated effects on the temperature.

Example 3 – Heat exchanger temperature control

This is similar to example 2. The aim is to keep T2 constant. Again the secondary loop is used to compensate for flow rate changes.

Example 1 – Control of pressure in a reactor

The aim of this loop is to control the pressure in the reactor. It may be possible to operate this system with only one of the valves but the second valve is added to provide additional safety and operational optimality.

In this case the action of the two valves should be coordinated. Thus for example if the operating pressure is between 0.5 and 1.5 bar then the control algorithm could be

• If the pressure is below 0.5 bars then valve 1 is completely open and 2 is completely closed
• If the pressure is between 0.5 and 1 bar then valve 1 is completely open while 2 is opened continuously as the pressure rises. Note that both these actions lead to a reduction in pressure
• If there is a large increase in pressure and it rises to above 1 bar then valve 2 is completely open while 1 is closed continuously
• If the pressure reaches 1.5 bar then valve 1 is shut and 2 is open

A graph of these valve positions with respect to pressure is shown below.

Example 2 – Control of pressure in a steam header

The aim of this control loop is to maintain a constant pressure in the steam header subject to differing demands for steam further downstream. In this case the signal is split and the steam flow from every boiler is manipulated. An alternative manipulated variable could be the steam production rate at each boiler via the firing rate. A similar control scheme to the above could be developed for the pressure control of a common discharge or suction header for N parallel compressors.

Control mode

In the proportional control mode, the final control element is throttled to various positions that are dependent on the process system conditions. For example, a proportional controller provides a linear stepless output that can position a valve at intermediate positions, as well as “full open” or “full shut.” The controller operates within a band that is between the 0% output point and the 100% output point and where the output of the controller is proportional to the input signal.

Proportional band

With proportional control, the final control element has a definite position for each value of the measured variable. In other words, the output has a linear relationship with the input. Proportional band is the change in input required to produce a full range of change in the output due to the proportional control action. Or simply, it is the percent change of the input signal required to change the output signal from 0% to 100%.

The proportional band determines the range of output values from the controller that operate the final control element. The final control element acts on the manipulated variable to determine the value of the controlled variable. The controlled variable is maintained within a specified band of control points around a set point.

Consider the example (Figure 9.27.) of a proportional level control system; the flow of supply water into the tank is controlled to maintain the tank water level within prescribed limits. The demand that disturbances placed on the process system are such that the actual flow rates cannot be predicted. Therefore, the system is designed to control tank level within a narrow band in order to minimize the chance of a large demand disturbance causing overflow or run out. A fulcrum and lever assembly is used as the proportional controller. A float chamber is the level-measuring element, and a 4-in stroke valve is the final control element. The fulcrum point is set such that a level change of 4-in causes a full 4-in stroke of the valve. Therefore, a 100% change in the controller output equals 4-in.

The proportional band is the input band over which the controller provides a proportional output and is defined as follows:

For this example, the fulcrum point is such that a full 4-in change in float height causes a full 4-in stroke of the valve.

Therefore:

The controller has a proportional band of 100%, which means the input must change 100% to cause a 100% change in the output of the controller.

If the fulcrum setting were changed so that a level change of 2 in, or 50% of the input, causes the full 3-in stroke, or 100% of the output, the proportional band would become 50%. The proportional band of a proportional controller is important because it determines the range of outputs for given inputs.

Proportional-derivative

Proportional plus rate describes a control mode in which a derivative section is added to a proportional controller. This derivative section responds to the rate of change of the error signal, not the amplitude; this derivative action responds to the rate of change the instant it starts. This causes the controller output to be initially larger in direct relation with the error signal rate of change. The higher the error signal rate of change, the sooner the final control element is positioned to the desired value. The added derivative action reduces initial overshoot of the measured variable, and therefore aids in stabilizing the process sooner.

This control mode is called proportional plus rate (PD) control because the derivative section responds to the rate of change of the error signal.

Definition of derivative control

A device that produces a derivative signal is called a differentiator. Figure 9.29 shows the input versus output relationship of a differentiator.

The differentiator provides an output that is directly related to the rate of change of the input and a constant that specifies the function of differentiation. The derivative constant is expressed in units of seconds and defines the differential controller output.

The differentiator acts to transform a changing signal to a constant magnitude signal as shown in figure 9.30. As long as the input rate of change is constant, the magnitude of the output is constant. A new input rate of change would give a new output magnitude.

Derivative cannot be used alone as a control mode. This is because a steady-state input produces a zero output in a differentiator. If the differentiator were used as a controller, the input signal it would receive is the error signal. As just described, a steady-state error signal corresponds to any number of necessary output signals for the positioning of the final control element. Therefore, derivative action is combined with proportional action in a manner such that the proportional section output serves as the derivative section input.

Proportional plus rate controllers take advantage of both proportional and rate control modes.

As seen in figure 9.31, proportional action provides an output proportional to the error. If the error is not a step change, but is slowly changing, the proportional action is slow. Rate action, when added, provides quick response to the error.

Reset control (Integral)

Integral control describes a controller in which the output rate of change is dependent on the magnitude of the input. Specifically, a smaller amplitude input causes a slower rate of change of the output. This controller is called an integral controller because it approximates the mathematical function of integration. The integral control method is also known as reset control.

Definition of integral control

A device that performs the mathematical function of integration is called an integrator. The mathematical result of integration is called the integral. The integrator provides a linear output with a rate of change that is directly related to the amplitude of the step change input and a constant that specifies the function of integration.

For the example shown in figure 9.31, the step change has amplitude of 10%, and the constant of the integrator causes the output to change 0.2% per second for each 1 % of the input.

The integrator acts to transform the step change into a gradually changing signal. As you can see, the input amplitude is repeated in the output every 5 seconds. As long as the input remains constant at 10%, the output will continue to ramp up every 5 seconds until the integrator saturates.

With integral control, the final control element’s position changes at a rate determined by the amplitude of the input error signal. Recall that:

Error = Set point – Measured Variable

If a large difference exists between the set point and the measured variable, a large error results. This causes the final control element to change position rapidly. If, however, only a small difference exists, the small error signal causes the final control element to change position slowly.

Figure 9.33, illustrates a process using an integral controller to maintain a constant flow rate. Also included is the equivalent block diagram of the controller.

Initially, the system is set up on an anticipated flow demand of 50 gpm, which corresponds to a control valve opening of 50%. With the set point equal to 50 gpm and the actual flow measured at 50 gpm, a zero error signal is sent to the input of the integral controller. The controller output is initially set for a 50%, or 9 psi, output to position the 6-in control valve to a position of 3 in open. The output rate of change of this integral controller is given by:

Output rate of change = Integral constant × % Error

If the measured variable decreases from its initial value of 50 gpm to a new value of 45 gpm, as seen in figure 9.34, a positive error of 5% is produced and applied to the input of the integral controller. The controller has a constant of 0.1 seconds-’, so the controller output rate of change is 0.5% per second.

The positive 0.5% per second indicates that the controller output increases from its initial point of 50% at 0.5% per second. This causes the control valve to open further at a rate of 0.5% per second, increasing flow.

The controller acts to return the process to the set points. This is accomplished by the repositioning of the control valve. As the controller causes the control valve to reposition, the measured variable moves closer to the set point, and a new error signal is produced. The cycle repeats itself until no error exists.

The integral controller responds to both the amplitude and the time duration of the error signal. Some error signals that are large or exist for a long period of time can cause the final control element to reach its “fully open” or “fully shut” position before the error is reduced to zero. If this occurs, the final control element remains at the extreme position, and the error must be reduced by other means in the actual operation of the process system.

Properties of integral control

The major advantage of integral controllers is that they have the unique ability to return the controlled variable back to the exact set point following a disturbance.

Disadvantages of the integral control mode are that it responds relatively slowly to an error signal and that it can initially allow a large deviation at the instant the error is produced. This can lead to system instability and cyclic operation. For this reason, the integral control mode is not normally used alone, but is combined with another control mode.

Chemistry of fire

There are three ingredients, which must simultaneously exist to cause a fire:

• Spark
• Air
• Fuel

To prevent fire, it is therefore necessary that one or more of the above three ingredients should be so controlled as to be non-existent. In case of a fire, one or the other of the three factors must be removed to extinguish it. The fire pyramid explains the necessary components of a fire.

Some of the possible causes of fires are:

• Smoking and Matches: Smoking is a dangerous practice where flammable or combustible substances are handled. This can be prevented by strictly enforcing “no smoking regulations”
• Open Flame: Use of open flame, gas cutting and welding can cause hazards. Such jobs should only be permitted after making the area safe or with proper hot work permit and adequate precautions
• Friction: Hot bearing, misaligned or broken machine part, jamming of material can create a spark or temperature. Proper maintenance and inspections could be preventing this
• Electrical: These are principal causes of Industrial fires. Electrical equipment should be properly designed and installed in flammable area. Electrical equipment and wiring should be properly maintained to avoid any short-circuiting or spark. Non-Flameproof Electrical equipment should not be brought into flammable area
• Hot Surface: Hot surfaces of Heaters, furnaces, Boilers can be a source of fire in the event of leakage of flammable liquid or gases. It could be prevented by avoiding any leakage from equipment/pipe line at the source of ignition
• Foreign Substances: Presence of foreign substances in process can cause fire
• Spontaneous ignition: Oily waste and rubbish or presence of combustible material in the storage of material can cause spontaneous ignition
• Overheated Material: Abnormal rise in temperature in process or moving machine can cause ignition
• Static Electricity: Static charge can develop during flow of liquids/vapors (hydrocarbon, acids steam etc) or powder/solid substances

Classification of flammable liquids

As per NFPA -USA code the flammable liquids are divided into two classes.

• Class A Liquids, those having flash point below 100 deg F and may be sub divided as follows:
• Class 1-A Liquids, those having flash point below 73 deg F and having a Boiling point below 100 deg F
• Class 1-B Liquids, those having flash point below 73 deg F and having a Boiling point above 100 deg F
• Class 1-C Liquids, those having flash point at or above 73 deg F and below 100 deg F
• Class B Liquids, those having flash point at or above 100 deg F and below 140 deg F

Flash point

The flash point of flammable liquid is the lowest temperature at which it gives off enough vapors to form a flammable mixture with air near the surface of the liquid or within the tank or container

Fire point

The lowest temperature, at which a liquid in an open container will give off enough vapors to continue to burn once ignited, is a fire point. It is generally slightly above the flash point

Explosive limit

The explosive range or limit includes all concentrations of mixtures of flammable vapor or gas in air (usually expressed in % by volume) in which a flash will occur or a flame will travel if the mixture is ignited. The lowest percentage at which this occurs, is the lowest explosive limit and the highest percentage is the upper explosive limit. If such a mixture is confined and ignited, an explosion results.

Auto ignition temperature

The auto ignition temperature of a substances whether solid, liquid or gaseous is a lowest temperature required to initiate or cause self-sustained combustion in the absence of a spark or flame. The temperature varies considerably depending upon the nature, size and shape of igniting surface and other factors

Vapor pressure

Vapor pressure of a liquid is the pressure of vapor at any given temperature at which the vapor and liquid phases of the substances are in equilibrium in a closed container. Vapor pressure varies with temperature and is useful in evaluating the relative tendency of any given liquid to evaporate under any unknown set of conditions

Threshold Limit Values (TLV)

Threshold Limit Values are set by the American Conference of Government Industrial Hygienist (AGIH). According to the AGIH, these values represent conditions under which it is believed that nearly all workers may be repeatedly exposed daily without adverse effect

Interpretation of hazard code symbol

• H – Health
• F – Fire
• R – Reactivity
• 1 – Low
• 2 – Medium
• 3 – High
• 4 – Extreme

For gas and vapors, the TL value is usually expressed in parts per million (ppm), i.e. part of the gas or vapor per million parts of air.

For fumes and mists and for some dust, the TLV is usually given as milligrams per cubic meter.

Minimal Lethal Dose and LD 50 Test

In experimental toxicology, it is common practice to determine the quantity of poison per unit of body weight of an experimental animal, which will have a fatal effect. The amount per unit of body weight, which will cause even one fatality in a group of experimental animals, is known as Minimal Lethal Dose (MLD). A more commonly figure in experimental industrial toxicology is the amount which will kill 1/2 of a group of experimental animals. This is known as LD 50 test representing 50% fatalities.

Furnace defined

Furnace may be defined as an enclosed space in which heat is produced from the chemical oxidation of a fuel. The geometry of the furnace also depends on the type of burners and services, which are utilized within a given furnace.

Furnaces look very dangerous when they are in operation and appear to be harmless when not in operation. But as a matter of fact, a furnace is most hazardous when it is not in operation and attempts are made to light it. Safe heater operation is completely dependent upon the controlled release of fuel into a confined space, furnishing a strong source of ignition and maintaining the fuel air ratio within explosive limits.

Furnace lighting and shutdown

The following should be done before lighting any type of furnace burner:

• Look into the firebox to be sure that there are no flammable materials such as wood, paper or rags. Be very sure there is no accumulated oil in the firebox. Remember that oil or gas can seep into the firebox through the floor of a furnace setting on the ground
• Brick up access openings if required and close all observation doors and access openings. Header boxes and plates should be closed tightly. Air should only enter the furnace through burner registers
• Re-check fuel system valves for proper setting. All defective valves in the fuel gas and fuel-oil lines should have been repaired and tested during the shutdown
• Be sure that fuel valves and cocks at the burners are tightly closed
• Adjust the draft gauges to proper zero setting and open them to the furnace. Be certain that all air and fuel-gas ducts are free from oil and water
• Be sure that all burner air registers and the stack damper is wide open. Furnaces designed to operate with forced or induced draft fans, should never be lighted off or operated under natural draft condition
• Start the fans that provide air to the firebox as soon as possible after the firebox is closed up. Operate the fans at the specified rate for the specified time to obtain a thorough purge of the firebox
• Remove the blind from the furnace fuel-gas line
• Check the fuel gas system. Drain any liquid accumulation accordingly to safe procedure. Do not leave the drain open
• Purge of furnace
  On natural draft furnaces, use a steam purge to heat the air in the firebox and produce a draft as follows:
  • If the furnace has combination burners, shove the oil gun forward into firing position and open the steam valve wide. Don’t open the oil valve
  • If the furnace only has gas burners, use steam lances inserted through burner openings
  • Steam long enough according to instructions to obtain an adequate draft and free the firebox of possible flammable mixtures
• When you are ready to light the first burner
  • Ensure that the number of people in the vicinity of the furnace is at an absolute minimum
  • Adjust both the air flow through the burner to be lighted and the total air flow in accordance with unit operating instructions, so that a stable flame can be obtained
• Ensure that the fuel oil pump is running and circulating oil at the required temperature and pressure if fuel oil is to be used

Lighting gas burners

The following rules are to be followed for lighting gas burners:

• Prepare the furnace as outlined in the above section
• Shut off purge steam. Be sure the firebox is warm enough to create a draft before you shut off steam
• Open the main block valve in the gas line. This will put full line pressure against the control valve blocks and bypass, if any
• Shut off primary air requesters on premix gas burners to prevent flash back when lighting
• Standing to one side, insert the torch through the opening of the burner to be lighted and open the burner gas valve slowly. If the burner does not light in the short period of time specified, shut off the burner gas valve and remove the torch. The firebox must be re-purged before attempting the next light-off
• When the fuel ignites, open the primary air requesters until the yellow flame turns blue. Don’t extinguish the flame by opening the requesters too much
• Never light one burner from another

Lighting fuel oil burners

• Adjust atomizing steam/air to the burner to the minimum requirement
• Ensure that pilot burner is on
• Gradually open the fuel oil valve so that fuel immediately catches fire. Wait for a few seconds and finally adjust the atomizing steam/air and fuel oil to a suitable stable flame
• If the burner does not light immediately and oil is sprayed from the top of the burner, shut off the oil and purge with the steam for a few minutes
• Check for an adequate fuel oil temperature, stable flame of hand torch, dryness of atomizing steam and presence of water in the fuel oil etc
• Relight the burner as set out above
• Ensure a staggered pattern of burners to maintain equal heat distribution inside the furnace

Switching fuels

The steps in switching from oil to gas on combination burners are as follows:

• Increase gas to ensure gas burner will stay lighted
• Close the oil block valve on the header
• Open the steam bypass and flush the oil out of burner to prevent coking and plugging. Flush the oil cock, moving the handle back and forth
• Shut off steam entirely, unless a little is required to prevent freezing during winter or to keep the tip cool
• Adjust the gas to meet heat requirements

Normal shutdown

• Maintain good heat distribution with staggered pattern of burners while individual burners are shut down to reduce heat rate
• Close the oil burner valves and gradually reduce the atomizing steam and close the valve. Ensure that pilot flame is not extinguished and that there are no flashbacks in premix burners
• When the main fuel valve for gas burner seems closed, it is necessary to shut some burners off entirely to continue adequate fuel pressure on the rest of the burner. This prevents all burners from extinguishing as well as flash-backs in premix burners
• Close the fuel gas to pilots
• Ensure that all the valves in fuel lines are closed properly
• Keep air flow on for some time

Normal operation

When the furnace is in service a regular check on the following is necessary

• Pressure of fuel gas/fuel oil/steam
• Draft of the furnace
• Burner and flame condition/impingement on the tubes
• Refraction lining of the furnace
• Tube condition inside the furnace, hot spots and carbon deposition
• Oil/Gas/Steam leaks around the furnace

Hot spots and soot formation

Hot spots on the tubes inside the furnace are observed due to improper flow of the fluid within the tube caused by to some obstructions like catalyst crumbling, coke deposition etc and consequently has an effect on heat transfer. If hot spots are noticed and corrective actions are not taken in time, tubes are over-heated and due to thermal stresses, it leads to failures and ruptures. Whenever hot spots are detected the flames within the vicinity of the tubes should be controlled and skin temperature of the tube should be maintained low during soot blower operations. Draft is to be adjusted and flames stability should be checked. Any rise in temperature of tubes should be watched and controlled.

Some points to remember

• Use a firing shield when looking through furnace peepholes
• Light each burner the way you lit the first one. Never try to light one burner from another. Light all pilots before lighting any main burner
• If one burner goes out while the others remain lit, shut off fuel to that burner for five minutes and then relight with a torch
• Prepare a check-list for startup or shut down of furnaces so that nothing is forgotten

Causes of explosion of boiler firebox

• Insufficient purge of firebox and fuel passage causes the boiler start up explosion. Conditions promoting this cause are:
  • Leaking fuel valves to burners and pilots
  • Ignition system failure with repeated attempts to relight pilots and burners
  • Adequate manual purge procedures not followed
• Fuel/air ratio not properly balanced accounted during the start-up. Conditions promoting this cause are:
  • Improper air flow when lighting first burner resulting in accumulation of fuel rich atmosphere followed by increased air flow for lighting other burners
  • Interruption of fuel or air supply resulting in concentration of explosive mixture
  • Flame failure without automatic shut down of one or more burners in the presence of other burners operating normally
  • Unknown fuel leakage into firebox through valves of unlit burners or pilots
  • Sudden shut down of one or more boilers with other boilers trying to pick up the load without proper combustion control limits
  • Partial loss of draft without a corresponding reduction of fuel gas flow
• Failure to follow light off procedures. Conditions promoting this cause are:
  • Lack of communication between operations.
  • Lack of complete knowledge of start up procedure
  • Short cut of the procedure
  • Lack of sufficient training
• Component failure of combustion and safety control systems. Conditions promoting this cause are:
  • Draft system malfunction
  • Instrument air system malfunction
  • Main fuel gas control valve failure
  • Leaky block valves in fuel gas line
  • Liquids in fuel gas line
• On-line explosions during normal operation are operator errors. Conditions promoting this cause are:
  • Lack of thorough and frequent training
  • Lack of proper instrumentation that will give the operator the information he needs to make correct decisions
• Improper draft caused by equipment failure. Conditions promoting this cause are:
  • Steam and hot water backing into the drum through these connections can cause injury and death
  • Ensure that the number of people in the vicinity of the steam generator at start up is at an absolute minimum

Good safety practices

• Safety – It is everybody’s business. Safety is your responsibility. Get into the habit of thinking of Safety as you do of your technical duties
• Don’t Let Familiarity Breed Contempt
• Experience shows that most of the accidents or near accidents are the direct result of operations/experiments which have been carried out many times before
• Check all new operations carefully
• Consult the literature and superiors and obtain all possible information on new operations that you plan
• Familiarize yourself with Safety and Fire Equipment
• Practice ‘Good Housekeeping’
• Obey the rules outlined
• ‘Don’t Be Afraid To Ask If You Don’t Know’

General safety precautions

• All the tests and experiments must be carried out by following the safety methods as directed. Performing any test or experiment outside the scope of approved laboratory facilities and procedures or without permission of Supervisor is forbidden
• Before leaving work, ensure all appliances and equipment not required to be used, are properly turned off, disconnected or otherwise rendered safe; particularly electrical/gas equipment
• Never distract the attention of another employee unnecessarily. Employee should give their fullest attention to their job at all times to prevent accident and mistakes
• Do not allow gas to escape from gas burners. It can cause an explosion hazard. When gas burner is to be lighted, open gas tap and apply lighter spark simultaneously
• Be sure that the chemical reaction will not develop pressure or create any hazard to person, appliances and in the work area
• Bottles should be carried in the carriers
• Tests requiring the use of an open flame, shall be restricted and only allowed in designated location
• Never pick up dry ice with bare hands. Use a spatula
• Use hand gloves, apron and eye protection while handling acids and corrosive chemicals to avoid injuries
• When drawing samples of toxic and corrosive products use proper protective equipment
• While drawing samples from operating units, inform the Shift Engineer or Operator in charge of the unit before drawing the sample
• All containers of chemicals and samples should be labelled clearly and correctly. All unlabeled materials should be discarded under the direction of the supervisor
• Liquefied hydrocarbon sample must not be stored in glassware
• Draw samples slowly and cautiously, particularly the samples with high temperature and pressure
• When taking samples, never stand on the side of the opening where escaping vapors or liquid will flow towards you. Keep sampling valves closed at all the times, except when actually sampling
• Cork borer shall be kept sharp and lubricated with glycerol-alcohol mixture when used. Always bore through from both sides to meet in the center and rotate the stopper occasionally during the boring to aid in the palm while boring
• Before inserting glass tube into stopper hole, moisten the hole and the tube with water or glycerine. Hold the stopper firmly with the thumb and forefinger and gradually push it by rotating the tube
• Air pressurizing system in the laboratory should be kept operative to maintain positive air pressure within. This will prevent entry of flammable gases from outside
• All laboratory personnel must be familiar with location and the use of fire extinguishers, safety showers, eye baths, fire blankets, gas masks and electrical switches. Fire/safety equipment will not be blocked by placement of materials
• When using compressed air for cleaning/drying/glass ware, crack opens the valve. Excess pressure can cause hazard

Hygiene

The following rules should be adhered to with regard to personal and general hygiene

• Food must not be stored in laboratory fridges or iceboxes and must not be left on laboratory benches. Prior to taking meals hands must be washed thoroughly with soap and water
• It is advisable to protect hands with barrier cream before handling solvents
• Hands should be washed promptly after handling chemicals
• Laboratory glassware must not be used for handling chemicals
• Do not use compressed air to clean clothing or the skin
• When new or unfamiliar chemicals are encountered make every effort to determine their hazards before handling them. In case it cannot be determined, treat them as toxic or otherwise hazardous materials

Eye protection

All laboratory personnel will be provided with safety goggles (spectacle type):

• Working with any glass apparatus maintained under positive pressure or negative pressure such as vacuum distillation etc
• Using acid bath melting point apparatus which may break and splatter hot acid into the employees eyes
• Opening bottles of any acids, alkalies or other corrosive chemicals. Pressure is frequently built-up, especially during warm weather
• Opening glass stoppers of containers containing corrosive material may cling to the glass stoppers and splatter upward on opening, creating hazard to the openers eyes
• While diluting concentrated acids, especially sulfuric, the heat generated often results in splattering. This hazard can be mainly reduced by adding the acid to water, stirring slowly
• Working with perchloric acid or its salt, it should be remembered that certain commercial drying agents are salts of perchloric acid. Perchloric acid or perchlorates will explode on contact with organic matter
• Working with strong alkalies and acids (or solutions)
• Exposed to danger of chemicals splashing
• Chipping dry ice etc

Use of hood

Hoods should be used for any operation, which may give off flammable vapours, poisonous or obnoxious odors:

• The efficiency of its exhaust systems should be checked periodically to ensure that adequate airflow is maintained. (Do not allow materials like paper bits being sucked into duck. It may cause choking)
• Over a period of time ‘peroxides’ can form in the hoods and ducts. To avoid peroxide hazards in hoods and ducts, they should be washed periodically in water (a record of such cleaning should be maintained)

Procedure for taking samples from plant

• Only authorized persons of plant should be allowed to take sample from the plants
• The person collecting the sample should be fully aware of the hazard of the plant and also of the properties of sample chemicals to be collected
• Whenever new persons are allowed to take samples, they should be properly instructed by the process staff
• When sampling hot or corrosive liquids, the operator must wear safety equipment like face shield or goggles, hand gloves, aprons etc
• Great care must be taken to ensure that undue strains are not put on the sample point, valves and their connection. Any damage to the sample point can do as much harm as the bursting of a pipe line, resulting in immediate plant stoppage or plant having to be stopped for repairs
• The damage to the sample line can also cause serious injuries to the operating personnel due to sudden splashing of chemicals
• There can also be a serious fire hazard in cases of damage to the sampling line of Flammable liquids or gases
• Sampling valve should be opened slowly and in case nothing is coming out, then close the valve immediately. Try to remove the chocking slowly by putting rod or wire inside the sampling point, keeping the valve close. Do not try to remove the chocking by keeping valve in full open condition
• At the time of removing chocking, try to stand away from the point and at the opposite direction of air. Wear complete protective equipment for such jobs
• If sampling points become completely obstructed then the sample collector from the other department should not try to attempt to clear it but should bring the defect to the notice of the Shift Engineer concerned
• Before samples are actually taken, it is necessary to purge the sampling toxic or harmful gases in a confined area, collect a volume of gas sufficient to purge the line in a sample bottle or rubber bag and release this outside the building. Sampling line must not be purged into confined space
• When collecting samples of highly toxic or poisonous chemicals, make positive use of gas mask or oxy breathing apparatus
• Sampling containers should be placed a few inches below the sample cock so that if any liquid flows out under pressure, it will not splash out of the container
• When collecting samples in glass bottles full line pressure must not be put into the bottle. When taking such samples, a Tee (T connection) with rubber should be used, keeping one branch of the Tee wide open the other being connected to the bottle. The open end of the rubber tube should be pinched down slowly until the sample bottle is filled at the desired rate
• Sample or containers of different chemicals should be kept separately and properly labeled. The sample bottles of one chemical should not be used for taking samples of other chemical without thoroughly cleaning and washing the bottle

Process data sheets

They define the operating and design parameters for an equipment or a rotating machinery envisaged for a plant or unit operation. They form the basic document to consolidate on the basic engineering package for any project.

Instrument data sheets

These formats explain the instruments involved in a project and the control philosophy adopted for plant operation and safety.

Mechanical data sheets

It explains the mechanical aspects for all the stationary and moving machinery including heavy equipments like cranes, trucks etc.

Electrical data sheets

All the motors, switches, transformers and other circuits are indicated in these formats.

Civil data sheets

Plant layouts designs, foundations, structural details etc are mentioned in these formats.

Bulk storage hoppers

Vibratory bulk storage hoppers are used to load parts into a bowl feeder, giving large storage capacities and providing several hours running time. The hoppers can also be used to trickle-feed awkward components (i.e. components that easily tangle) into bowl feeders, therefore increasing feed rates and efficiency from the feeder. These units are best applied when the bowl feeder is situated at comparatively low level.

Elevating hopper loaders

Elevating hopper loaders are used to load heavy (or where appropriate light) parts to bowl feeders that because of design layout parameters is situated at a high level. The storage bin is floor mounted and therefore at a low convenient level for refilling. The storage bin can be either static or driven by vibration or a conveyor belt and these options are selected according to the components handling characteristics.

High-speed impact mills

These include hammer mills, pin mills, counter-rotating pin mills, cage mills, turbo mills, and universal mills. A high-speed impact mill (figure 12.17) reduces non-friable and friable materials such as wood waste, sheet pulp, plastics, coal, chemicals, limestone, and fertilizer to medium-fine and fine (10- to 200-mesh) pieces. The material, which is to be reduced, enters the mill’s housing, which is impacted by a rotating assembly of hammers, pins, or cages. As it rotates, the assembly throws the material centrifugally outward where the hammers, pins, or cages grind it against a perforated screen for further size reduction. The final product’s size is controlled by the assembly’s rotating speed and the perforated screen at the discharge port. A high-speed impact mill is available in several sizes ranging from small laboratory equipment up to large production machines.

Hammer and cage impact mills

Use fixed or swinging hardened steel hammers, chain or a cage for coarse crushing to fine milling. Hammer crushers and cage mills are available in vertical and horizontal rotor configurations with one or many rows of hammers.

Agitators

Agitators are used for mixing a product inside a vessel.

Static or motionless mixers

They are fins, obstructions, or channels mounted in pipes, designed to promote mixing as fluid flows through the mixer. Most static mixers first divide the flow, then rotate, channel or divert it, before recombining the flow. Some static mixers create additional turbulence to enhance mixing

Extruder feed mixers

They have an integral extruder screw to mix and then extrude its contents.

Turbine mixers

They include a wide range of general purpose mixing equipment, operating at reduced speeds provided by an enclosed gear drive with one or more multi-bladed impellers mounted on an overhung shaft. These mixers may be used on open tanks, when supported by a beam structure, or in closed tanks with a variety of seal and support arrangements. Because of the general-purpose capabilities of turbine mixers they may be used on almost any shape tank, of any size, with other drives or impellers

Magnetic drive mixers

They are ideally suited for Continuous Stirred Reactors (CSTR) and batch reactors where mixing and agitation must be contamination-free and leakage cannot be tolerated. The magnetic drive eliminates seals, and the problems associated with rotating seals, such as leakage, contamination, and constant maintenance.

Impeller/propeller mixers

They pump out in a radial direction generating a re-circulating mixing pattern above and below the disc. This high shear, high power design is stable under varying liquid depths and is an excellent choice as a rapid mixer in shallow basins, solids suspensions in shallow or varying water depths, and is often used as the lower impeller in a multiple impeller design.

Kneaders

They provide a kneading motion to mix the contents of the mixer.

Screw mixers

They use a rotating screw that progresses around the periphery of a conical hopper. The screw lifts solids from the bottom of the hopper to the top, where the mixture flows by gravity back into the screw. Mixing occurs around the open screw, where the solids are transported by the screw exit at various levels and are replaced by other solids at that level. The screw’s shearing action also intimately mixes the various components. Gross mixing action also occurs within the mixture by the velocity profile created in the conical hopper as it feeds the screw. This gross mixing action is most effective when the solids move along the conical hopper walls.

Planetary mixers

They have two mixing blades that rotate around individual shafts and the two blades further rotate around a center axis. The net effect is intermixing, stirring, and shear.

Turbo mixers

They have a circular trough with a housing in the center around which revolves a spider or a series of legs with plow shares or mold boards on each leg. This type of mixer is also known as a plow mixer.

Sieves

Sieves, primarily for coarse through fine grades of material use screens with a specified mesh size to separate the particles, and vibration or air fluidization is applied to the sieves to maintain particle flow through the screens. The toll processor can stack the sieves to classify a range of particle sizes greater than 100 microns.

Air classifier

An air classification machine uses air velocity to separate materials based on particle weight and size. It classifies particles ranging from 1 to 100 microns. Many air classifiers use a vane wheel to control the particle size distribution.

Air classifiers, cones or cyclones employ the spiral airflow action or acceleration within a chamber to separate or classify solid particles. Powders suspended in air or gas enters the cyclone and the heavier particles spiral out and down where they are collected. The air and finer particles flow up to the top where they may be passed to another cyclone with finer classification capability. A cyclone is essentially a settling chamber where the effects of gravity (acceleration) have been replaced with centrifugal acceleration. An air-classifying mill reduces friable materials such as polyesters, epoxies, acrylics, and sugar to fine and superfine (1 50- to 400-mesh) pieces. The material to be reduced first enters the mill’s high-speed impact grinding chamber where a fixed-speed, rotating grinding plate with fixed hammers reduces it.

Air moving through the mill then carries the particles to the classifying chamber where the classifier wheel rejects oversized particles and directs them back to the grinding chamber for further size reduction. The material circulates through this closed-loop environment until it’s been reduced to the appropriate particle size. The classifier wheel’s speed and the mill’s airflow rate are adjustable to allow for a wide range of particle sizes. Heated or chilled air can enhance an air-classifying mill’s performance.

Water classifiers

Water classifiers such as elutriators and classifying hydro cyclones use settling or flow in water or a liquid to separate or classify powdered materials based on particle size or shape.

Rake, spiral and bowl classifiers use mechanical action to dewater, de-slime or separate coarse bulk materials from finer materials or liquids. Rake classifiers lift solid-liquid mixtures up onto a plate with a screen or rake.

Spiral classifiers use an Archimedes pump screw to lift solid-liquid mixtures up onto a screen for dewatering. Drag classifiers consist of a chain-link conveyor or endless belt that is dragged through a solid-liquid mixture.

Electrostatic separators

They employ preferential ionization or charging of particles to separate conductors from dielectrics (nonconductors). The charged dielectric particles are attracted to an oppositely charged electrode and collected. The particles may be charged through contact electrification, conductive induction or high tension (ion bombardment).

Concentrating tables or density separators

They screen bulk materials or minerals based on the density (specific gravity), size and shape of the particles. This group includes jigging equipment, hindered-bed settling devices, shaking table, spiral concentrators, concentrating or wet tables, hydraulic concentrating tables, constriction plate separators or specialized settling vessels. Most concentrating or density separation equipment are hydraulic or water-based, although pneumatic or air-based systems are also available.

Magnetic separators

They use powerful magnetic fields to separate iron, steel, ferrosilicon or other ferromagnetic materials from non-magnetic bulk materials. The magnetic field may be generated by permanent magnets or electromagnets.

Trommel

Trommels are large rotary drum shaped with a grate-like surface with large openings. Trommels are used to separate very coarse materials from bulk materials such as coarse plastics from finer aluminum recycled material, coarse inorganic materials from organic wastes or large ore chunks from finer minerals.

Thickeners

Thickeners are used to separate solids from liquids by means of gravity sedimentation. Most thickeners are larger, continuous operation pieces of equipment. They are used for heavy-duty applications such as coal, iron ore taconites, copper pyrite, phosphates and other beneficiation processes. Common thickener types include conventional thickeners, high rate thickeners, lamella thickeners and tray thickeners.

The candle filter

The Candle Filters are, as all pressure filters, operating on a batch cycle and may be seen in process lines handling titanium dioxide, flue gas, brine clarification, red mud, china clay, fine chemicals and many other applications that require efficient low moisture cake filtration or high degree of polishing.

Candle Filters are also used for thickening to produce a concentrated flowable slurry by partial removal of the liquid phase as filtrate. This mode of operation is possible since Candle Filters may operate on very short cycle times taking advantage of the high filtration rates whilst the cakes are still thin.

Manufacturing costs

• Raw material
• Shipping containers
• Operating costs
• Operating labor
• Operating supervision
• Operating supplies
• Utilities
  • Electricity
  • Steam
  • Water
  • Fuel
• Control laboratory
• Miscellaneous
• Overhead costs
  • Employee benefit
  • Medical service
  • Cafeteria
  • Purchasing
  • Shops
  • Property protection
  • General plant supervision
• Depreciation
• Property taxes and insurance

General expenses

• Freight and delivery
• Administrative expense
• Sales expense
• Research expense

Selling price

Market analysis

• Price-volume relationship, present and anticipated
• Application of products
• Competition

Income taxes

It is the revenue to be paid to the government based on the applicable tax structure.

Net and new earnings

Profitability

The profitability of a venture is influenced by the ratios mentioned below.

• Return on investment
• Cost and profit charts

Other methods of economic analysis

• Pay-out time
• Accounting method
• Investor’s method
• Project present worth

The capital and total product costs are estimated from process and mechanical design flow sheets and specifications, once a market and sales analysis has been conducted to establish the annual plant capacity.

The latter is a function of a marketing analysis group, composed of engineers and economists who have developed methods of forecasting production statistics and demands for chemicals under consideration for manufacture. To fully appreciate the problems encountered in marketing and sales forecasting, a study of any process industry must include a discussion of competition and use patterns characteristic of that industry.

Choice between alternative processes

In the case of many major chemical products, there is variety of alternative routes available to a potential producer depending on the cost and availability of raw materials, power etc. Hence, to arrive at an economic process route under the exiting conditions, it is always beneficial to study various manufacturing methods. Once the optimum scheme is finalized, the associated cost factors can be worked out.