Cover illustrationTubular reactor loops and distillation columnsare much in evidence in this view of anethanol plant at Grangemouth, Scotland.

,,'?,'::".

~ ,-,~

2

AcknowledgementsAustin, D.G. and Jeffreys, G.V. Themanufacture of methyl ethyl ketone from2-butanol. The Institution of ChemicalEngineers in association with George GodwinLtd, 1979: figure 3.1.Baclrnurst, J .R. and Harker, J .H. Process plantdesign. Heinemann, 1979: figure 5.12Photographs courtesy BP Chemicals: cover,contents page, figure 1.2Coulson, J.M. and Richardson, J.F. Chemicalengineering. Pergamon Press Ltd, 1971:figures 4.14, 4.15Ju-Chin Chu Vapour/liquid equilibrium data.Van Nostrand Reinhold Co. Ltd, 1950:figures 5.2, 5.6Photographs courtesy Esso: figures 1.1,3.2Photographs courtesy Imperial ChemicalIndustries pic, Mond Division: figures 3.8,4.9,5.11Photographs courtesy Imperial ChemicalIndustries pic, Pharmaceuticals Division:figure 4.6Manning, J. An introduction to chemicalindustry. Pergamon Press Ltd: figure 6.5Shell International Petroleum Company Ltd.Oil. Shell Education Service, 1981:figure 4.19Photographs courtesy Shell: figures 4.18, 6.3Photograph courtesy Whessoe HeavyEngineering Ltd: figure 3.6.

The new drawings are by Oxford IllustratorsLimited.

The author would like to express his sincerethanks to the following for their help in thepreparation of this material.Vera Rybalka, Karen George, R Barnett,N. Colenutt, Dr M.W. Brimicombe,Dr R.J. Neville, and other colleagues atThe Cedars Upper School, LeightonBuzzardDr A.M. Mearns and K.E. Peet (Departmentof Chemical Engineering, University ofNewcastle-upon- Tyne)C.S. Gamage and Dr R.T.W. Hall (EssoPetroleum Company Limited)S. Wrighton (B.P. Education Service)R. Chapman (The Institution of ChemicalEngineers)G. Venn (Sharnbrook Upper School)Dr B. Hitchen (W.R. Tuson College)C.J. Johnson (Alcester Grammar School)P.R. Luton (Richmond-up on-Thames College)B. Robinson (Queen's College, Taunton)G. Cooke (The Harvey Grammar School,Folkestone)D.H.Mansfield (The Harvey Grammar School,Folkestone)Adrian Wistreich (Education Adviser, EssoPetroleum Company Limited).

Dr T.P. Borrows (Chairman of the ASESafety Committee) reviewed the experimentsand his safety notes have been incorporatedin the text.

Longman Group LimitedLongman House, Burnt Mill, Harlow,Essex CM20 2JE, Englandand Associated Companiestluoughout the World.

First published 1971Revised edition first published 1984© Nuffield-Chelsea Curriculum Trust1971,1984

Design and art direction by Ivan Dodd

Printed in Great Britain by George OverLimited, London and Rugby

ISBN 0 582 38925 9

All rights reserved. No part of thispublication may be reproduced, storedin a retrieval system, or transmitted in anyform or by any means - electronic,mechanical, photocopying, or otherwise- without the written prior permission ofthe Publisher.

Acknowledgement is also due to those whohelped with the original development of thisStudy. The text published in 1971 waswritten by: Dr R.J. Dalton, G.R. Grace,E.K. Hayton, Dr J. Manning, Dr A.M. Mearns,K.E. Peet, J .G. Raitt, Professor J.D. Thornton,and K. Watson.

Contents

CHAPTER ONE CHEMICAL ENGINEERS AND THE CHEMICAL INDUSTRY page 4

CHAPTER TWO FLUID FLOW page 8

CHAPTER THREE UNIT OPERATIONS page 11

CHAPTER FOUR THE CHEMICAL REACTOR page 17

CHAPTER FNE DISTILLATION page 27

CHAPTER SIX THE DEVELOPMENT OF A PROCESS page 32

REVIEW SECTION page 38

A general view of an anhydride plant (BP Chemicals, Hull).

I~Iilillfllir ~lflll[II~1N12969 3

CHEMICALENGINEERING

CHAPTER ONE1.1WHAT IS CHEMICAL ENGINEERING?In our modern industrial society there is an enormousdemand for substances which do not occur naturally buthave to be made from raw materials found in the earth, sea,and air. Such substances include petrol, paint, plastics, fert-ilizers, steel, glass, paper, cement, and pharmaceuticals, andare produced by a wide variety of different manufacturingprocesses. The technology underlying such processes isknown as chemical engineering. It is the applied scienceconcerned with changes in the composition or physical stateof materials in bulk, and is both an academic discipline and avitally important profession.

The chemist demonstrates the feasibility of a chemicalreaction in the laboratory and specifies the conditions underwhich it will take place. The chemical engineer designs andsupervises the construction and operation of the large-scaleplant required to convert a laboratory synthesis into anindustrial process producing hundreds or even thousandsof tonnes of material a year. This calls for a thoroughunderstanding of the chemistry of the process. Other skillsare required too. The chemical engineer must understand thephysics and mathematics underlying the problems of heatand mass flow which arise when large quantities of materialhave to be heated or moved about. He or she must also knowthe properties of the materials used to build the plant itself,such as how they will stand up to high pressures and temp-eratures, and how they will withstand corrosion and wear.

Chemical engineers are employed in a wide variety ofprocess industries from brewing and baking to petrochemicalsand plastics. Within these industries, the type of activity withwhich they are involved may vary considerably.

Some chemical engineers may spend much of theirworking lives in the field of research and development(R & D), either in the large research laboratories operated byindustrial companies or in universities. Their job is toinvestigate and develop new processes and products and totry to modify existing processes to make them more efficient.Others are engaged in plant design and construction, perhapsworking for a contracting firm which specializes in the designand construction of plant for chemical manufacturers. Oncea plant is built and successfully in operation, chemicalengineers are responsible for keeping it running at maximumefficiency and for making arrangements for maintenance andmodifications to be carried out as necessary.

Wherever they work, chemical engineers are usuallymembers of a team. They are often required to co-ordinatethe activities of members of other specialist disciplinesinvolved in the construction and maintenance of chemicalplant. These include chemists, mechanical engineers, civilengineers, control engineers, electrical engineers, and so on.To get the best from such a team requires considerablemanagement skill, and such experience often leads eventuallyto senior positions in industry.

Thus chemical engineering is a career for men andwomen who can accept challenge and responsibility

4

extending far beyond the confines of the laboratory. It isfounded upon a thorough knowledge and understanding of thefundamental sciences of chemistry, physics, and mathematics.

1.2WORKING ON THE LARGE SCALEImagine that you have been asked to prepare a I-gram sampleof sodium hydroxide in the laboratory, starting from otherchemicals of your choice. You might begin by thinking of allthe chemical reactions you have met which produce sodiumhydroxide and choosing the one which seems most conven-ient. Try to list some of the possible reactions, and notethe advantages and disadvantages of each.

A student's plan for a preparation might read as follows.'Sodium hydroxide can be made in the laboratory by pouringsome sodium carbonate solution into a test-tube and addingsome solid calcium hydroxide. The test-tube is shakento mix the reactants and heated over a Bunsen burner.

A precipitate of calcium carbonate is formed which is filteredoff to leave a clear solution of sodium hydroxide. Solidsodium hydroxide may be obtained from this solution bycareful evaporation to dryness.'

This sounds quite reasonable for a I-gram laboratorypreparation, but the World demand for sodium hydroxide isabout 30 million tonnes per year.

Now imagine that you are a chemical engineer and havebeen asked to report on a possible process to produce just asmall proportion of this total World demand, say 10000tonnes per year . You will note that 10 000 tonnes is 1010 gramor if you like, ten thousand million times more than yourI-gram laboratory sample.

In order to appreciate the extra problems which this enor-mous scale of operations presents to a chemical engineer, letus break down the simple laboratory preparation into stages.

StorageIn the laboratory report, it is simply assumed that thesodium carbonate solution and calcium hydroxide are firstcollected from their storage places, usually bottles on shelves.Such details cannot be left unmentioned when hundreds oftonnes of materials are needed every day.

It is important to make planned decisions about theamounts of the raw materials to be stored. The followingfactors must be considered.The cost of the storage tanks, especially if the materials arecorrosive, highly flammable, or toxic.The value of the land required for storage.The value of the materials stored and the working capital tiedup with them.The cost to the company should stocks run out and produc-tion be brought to a halt. (If this occurs, materials may haveto be bought expensively from a competitor in order tohonour marketing contracts.)

Figure 1.1 The Esso refinery and chemical manufacturing complex atFawley, Southampton. (Aerial view from the east.)

Transport of materialsThe next stage in the laboratory preparation is to carry thesodium carbonate solution across the laboratory and poursome of it into a test-tube. Energy must be supplied to dothis, and on an industrial scale this may well involve usingan electrically driven pump to move the liquid from thestorage tank to the reactor vessel through a series of pipes.

In an industrial plant, materials have to be movedbetween different stages: reactor vessels, distillation columns,and so on. This is particularly important in continuousprocesses where there must be a steady flow of raw materialsinto the plant and finished prc<lucts out of the plant. Thiscontinual flow of materials is called the process stream.Because fluids (of which gases are a special case) are easier tometer and to control than solids, most continuous processesinvolve material in the liquid or gaseous state. A detailedknowledge of fluid flow is therefore essential in designingan industrial plant, and this important topic is considered inChapter 2.

MixingIn the laboratory we often mix reactants together by shakingthem up in a test-tube, another process which requires thesupply of energy. It is clearly not practical to have largereaction vessels bouncing up and down in a chemical plant.

The energy required for mixing must be supplied in otherways.

Reactants may be mechanically stirred using large rotaryagitators driven by electric motors, rather like scaled-uplaboratory stirrers. Another common method of mixingmakes use of a phenomenon of fluid flow - turbulence -which is particularly relevant to continuous processes. Theconditions under which turbulence occurs are discussed inChapter 2. This is important because in a large-scale processthe rate of mixing may control the rate of reaction to agreater extent than changes in concentration.

Transfer of heatIn our laboratory process, heat energy is supplied in a veryinefficient manner by holding the test-tube containing thereactants in a Bunsen flame. An industrial process mayinvolve thousands of times as much heat energy as largeamounts of materials are heated up or cooled down duringthe various stages of manufacture. Heat energy is expensiveand so energy transfer must be carried out as efficiently aspossible.

One of the first stages in designing a chemical piant is todraw up an energy balance showing clearly how much energymust be given to and taken from each process stage. This,together with an estimate of the materials involved (the massbalance), allows the first rough calculations of the cost ofrunning the plant to be made.

5

The ways in which heat may be supplied to the processinclude: the combustion of a fuel, the use of an electriccurrent, superheated steam, and energy from a chemicalreaction.

Heat may be transferred to the process stream by directtransfer. This happens when a fuel is burned in direct contactwith the material to be heated, as in a blast furnace orcement kiln.

Alternatively, indirect transfer may occur when .there is aphysical barrier between the source of heat and the materialto be heated, for example in steam boilers and heat exchangers.Such indirect transfer is normally more significant,.and heatexchangers are considered further in Chapter 3.

SeparationGoing back to our laboratory preparation: after heating, theprecipitate of calcium carbonate is filtered off using a filterpaper and funnel, leaving a solution of sodium hydroxide.On the industrial scale, this corresponds to the end of thesynthesis stage of the process and the beginning of theseparation stage.

Filtration appears to be a fairly simple operation, but tryto list the reasons why the normal laboratory method isgenerally unsuitable for use in a large-scale continuous pro-cess. Sketch a design for a suitable piece of equipment whichovercomes the shortcomings of simple laboratory apparatus.

Most separation techniques used in the laboratory, suchas centrifuging, distillation, and solvent extraction, have theirindustrial equivalents. In addition, chemical engineers havedeveloped other separation techniques which are not suitablefor laboratory use; some of these are discussed in Chapter 3.

However, all separation techniques, whether on a labora-tory or industrial scale, have one thing in common: they relyfor their success on a difference in properties between thematerials to be separated.

Waste productsIn the laboratory, most unwanted materials can be simplythrown away in the waste-bin or down the sink withoutfurther consideration.

In industry, as much use as possible is made of all theproducts of the process, and one of the functions of theresearch department is to find uses for waste products(perhaps by recycling valuable materials). This is partly foreconomic reasons, but there are also practical and environ-mental problems involved in disposing of very large quantitiesof materials such as slag from furnaces, cooling water, orobnoxious gases and smoke. In general, the closer one gets toremoving all the impurities, the more difficult it becomes andtherefore more expensive.

Process controlIn a laboratory we normally rely on our senses to tell uswhen things are not going according to plan, so that we canquickly decide what adjustments are necessary and carrythem out. For instance, our eyes enable us to see when thecontents of a test-tube are being overheated and our brainresponds by instructing our hand to move the test-tube outof the flame.

With a large-scale continuous process it is neither practicalnor desirable to use this kind of manual control, and one ofthe characteristics of a modern chemical plant is a sophisticat-ed automatic control system. Sensors are used around the

6

Figure 1.2 The control room of an ethanoic acid plant.

plant to measure variables such as temperature, pressure, andflow rate. This information is transmitted to an automaticcontroller (often an on-line computer) which is programmedto adjust control valves in order to maintain the desired operat-ing conditions and to ensure a consistent quality of product.

For maximum efficiency, automatic process control hasbecome increasingly important. A major design considerationon any plant is how and where to incorporate instrumentssuch as flow meters, thermometers, pressure gauges, andautomatic analysers into the plant equipment.

SummaryWe have seen that the same chemical and physical principlesunderlie both large-scale and small-scale operations but that,in industry, the use of large quantities of materials may intro-duce factors which are unimportant or even non-existent ona laboratory scale. Amongst the problems which the chemicalengineer must consider are:methods of storagemethods of transferring materialsvessel design and materials of constructionmethod of operation (batch or continuous)methods of heating and coolingoptimum economic conditions (not necessarily the same asoptimum chemical conditions)toxicity and fire hazardsinstrumentation and controlwaste product disposal.

1.3THE CHEMICAL INDUSTRYThe chemical industry is essentially concerned with theefficient conversion of raw materials found in the earth, sea,and air into new substances of greater value to mankind.Such operations are often carried out on a vast scale whichcan be difficult to comprehend fully; for instance, theannual World production of nitrogenous fertilizers is inexcess of 50 million tonnes. The continuing availability ofsuch materials at reasonable cost is essential for the survivalof our society, few aspects of which are untouched by theproducts of the chemical industry.

Q1To gain an understanding of the structure ofthe modern chemical industry, it is useful toexamine a few everyday objects and to attemptto trace the substances from which they aremanufactured back to the original naturally-occurring raw materials. Consider a varietyof products such as a glass beaker, a detergent,a polythene bag, a record, a synthetic fabric,a saucepan, a fertilizer, and an aspirin tablet.

For a selection of such objects, try to identifyall the raw materials and chemical processesinvolved in their manufacture, consultingtextbooks and other source materials asnecessary. Vou may find it convenient towrite each manufacturing route as aflowscheme, but try not to be satisfied with asuperficial resPonse. For instance, whenexaminillg a nylon fibre it is tempting to say

'nylon comes from petroleum'. However, it isapparent that one of the elements in nylon(nitrogen) is not present in petroleum;therefore some other raw material must alsobe involved. As an example, a suitableflowsheet for nylon production is shown infigure 1.3. The manufacture of nylon is acomplex process and most of your flow-schemes will be much simpler than this.

nylon

Figure 1.3 Flowscheme for the manufacture of nylon 66.

ammonia

cyclohexane

benzene

hydrogenwater

air

petroleum

When you have completed the above exercise for a variety ofsubstances and discussed the results, you should appreciatethe following points.a The basic raw materials of the chemical industry arepetroleum, coal, air, water, vegetable materials, and mineralssuch as metal ores, salt, limestone, and gypsum.b Many products are made from these raw materials viaimportant intermediate chemicals such as ethene, ammonia,chlorine, sulphuric acid, sodium hydroxide, and sodiumcarbonate. These materials, although rarely seen by theaverage citizen, are made in very large quantities and areknown as heavy chemicals.c Most heavy chemicals are used to make many differentproducts. Each manufacturing process forms part of a com-plex network of processes which are inter-related chemicallyand economically, connecting raw materials to final products.

Having examined the manufacturing routes for some indiv-idual products and after doing a little research on the otheruses of the heavy chemicals mentioned above, you shouldattempt to produce a combined flowscheme connectingthe principal raw materials, heavy chemicals, and final pro-ducts which form the basis of the modem chemical industry.

When complete, your flowscheme should clearly illustratethe dependence of the industry on a relatively small numberof raw materials and the enormous diversity of productswhich can be manufactured from them.

It is important to realize that the route by which aparticular chemical is manufactured may change over theyears for a variety of reasons such as availability of rawmaterials, new technology, energy costs, the changingdemand for by-products, and so on.

For instance, considerable quantities of sodiumhydroxide were once manufactured using the 'lime-soda'process which involved reacting sodium carbonate withcalcium hydroxide in a manner similar to that described inthe laboratory process earlier in this chapt~r. However, thegrowth in demand for chlorine manufactured by theelectrolysis of brine produced abundant supplies of cheapsodium hydroxide as a by-product. A consequence of this

was that by 1970 virtually all thelime-sodaplants throughoutthe World had been closed down.

Significant changes in the structure of the chemicalindustry are likely to occur as raw materials such as petroleumand certain minerals become scarcer. Considerable research isbeing directed into ways of producing many organicchemicals from coal or plant material (biomass) opening upexciting possibilities for the chemical engineers of the future.

Because of the inter-relationships that exist, any attemptto divide the chemical industry into sections is necessarilyarbitrary. However; some of the traditional 'divisions' of theindustry together with their principal products are listed infigure 1.4.

Figure 1.4 Traditional divisions in the chemical industry.

HEAVY INORGANIC CHEMICALSChlorine, sulphuric acid, sodium hydroxide, sodium carbonate,ammonia, nitric acid.

HEAVY ORGANIC CHEMICALSEthene, ethanol, ethanoic acid, ethan-1,2-diol, propene,propanone,benzene, phenol.

INDUSTRIAL POLYMERSPoly (ethene), poly(chloroethene), poly(propene), poly(phenylethene),thermosetting plastics, synthetic rubbers.

AGRICULTURAL CHEMICALSFertilizers, pesticides, herbicides.

PHARMACEUTICALSAnalgesics, antibiotics, antiseptics, anaesthetics.

EXPLOSIVESNitroglycerine, trinitrotoluene(TNTl, trinitrophenol (picric acid),

BUILDING PRODUCTSCement, plaster, bricks, and blocks.

PETROLEUMPetrol, diesel, fuel oil, lubricating oil, bitumen, LPG.

SYNTHETIC FIBRESNylon, polyester, acrylic, acetate.

DETERGENTSSoapy and soapless detergents.

7

CHEMICALENGINEERING

CHAPTER TWO

where

Figure 2.1 Flow at low and high Reynolds Numbers.

uniform (average)velocity across most

of the section

u = mean fluid velocityp = fluid densityd = diameter of pipeIJ. = fluid viscosity

Re = updIJ.

distribution in figure 2.1b shows a fairly uniform fluidvelocity across most of the pipe diameter but, no matter howvigorous the turbulence, there is always a narrow butimportant boundary layer adjacent to the wall where viscousforces and the rigid wall successfully preserve streamline flow.

Turbulence only occurs as a result of viscosity. Whetherflow is streamline or turbulent largely depends on the ratioof inertial to viscous forces. This important ratio is given bythe Reynolds Number.

For flow in a pipe the Reynolds Number (Re) is given by:

Since the Reynolds Number is a ratio of forces, it isdimensionless. (Clearly, u, p, d and IJ. must be expressed inconsistent units.) In general, when the value of the ReynoldsNumber exceeds a certain critical value, the nature of thefluid flow in the pipe will change from streamline toturbulent.

Other things being equal, it requires less power to pumpa fluid in streamline than in turbulent flow. This is becausemuch energy is wasted creating turbulent eddies. However,chemical engineers generally prefer turbulent flow for thefollowing reasons.

streamline flow in a boundary layer whereviscous-forces and the pipe wall preventeddy formation

b Turbulent flow at high Reynolds Number

a Streamline flow at low Reynolds Number

{2.1HOW FLUIDS FLOW THROUGH PIPESFluids are generally moved about in pipes, and a chemicalreactor may be nothing more than a pipe or an enlargedsection of one. A heat exchanger in which fluids are heatedand cooled is usually a complicated system of pipes.

The force of gravity may be enough to cause a liquid toflow from one vessel to another, but generally it is necessaryto supply energy by means of a pump at some stage. Thechemical engineer must be able to calculate the resistanceto flow through a pipe in order to specify the size and typeof pump required and the power of the motor needed todrive it.

The total resistance is made up of two parts: the resis-tance arising from viscous friction within the pipe, and theheight through which the fluid is to be lifted. Further resis-tance occurs, of course, if the fluid is to be pumped into avessel at a higher pressure. The viscosity factor is frequentlythe more important. Viscosity results from the intermolecularforces which exist within the liquid (see Topic 10). Problemsoften arise with very viscous liquids such as heavy oils.Pumping may also be difficult when a fluid has to flowthrough obstructions in the pipe such as a bed of catalystpellets or a heat exchanger made of small bore pipes withmany bends.

The frictional forces arising from the viscosity of aliquid can act in two different ways.

When the flow velocity is small and the fluid is viscousand nowhere very far from the pipe walls, the fluid flows asif it were in layers sliding over one another. The fluid velocityis at. a maximum in the centre of the pipe and decreases in afairly uniform manner to zero in contact with the pipe walls.This is known as streamline (or laminar) flow because allelements of fluid move in orderly lines along the pipe, with avelocity distribution as shown in figure 2.1a.

If, however, the flow velocity is high, the fluid viscositylow, and the stabilizing pipe walls are far distant, then smalldisturbances upset the streamline path of the fluid, adjacentelements interfere with one another, and swirling eddiesdevelop. This is known as turbulent flow. The velocity

This chapter is concerned with the theory which explainssome aspects of the behaviour of fluids: how they flowthrough pipes, around particles, and through loosely packedsolids. This theory can be used in the design of equipmentfor moving fluids about, measuring flow rates, separatingproducts as in filtering, mixing immiscible fluids, and so on.

Physicists refer to this field of study as fluid dynamics;civil and marine engineers who are concerned with movementof or through water refer to it as hydraulics; and aeronauticalengineers interested in the same sort of problems in air talkabout aerodynamics. The chemical engineer has borrowedfrom all these sources, but there are many problems whichare peculiar to chemical processes. These include novelfeatures such as the flow of reacting liquids, the physicalproperties of which are changing as the reaction proceeds.

8

a It is often necessary to mix fluids in a pipe, and this isdone effectively by turbulent eddies. Mixing in streamlineflow is largely a result of diffusion which is a slow process.The effect of turbulent flow along a few metres of pipe isequivalent to shaking reagents together in a laboratorytest-tube.b It is much easier to heat or cool fluids flowing throughpipes when the flow is turbulent. This is because, in streamlineflow, heat can only pass into the fluid by conduction whichis slow through most fluids. In turbulent flow, bulkmovement occurs, so that hot fluid at the walls is movedbodily into the main stream and mixed. This forced convec-tion is very effective in transferring heat throughout thefluid.c A length of pipework is normally designed to carry a

specified mass flow rate of fluid (m), say 2000 kg hr-1 • Thismass flow rate is related to the mean fluid velocity (u) andpipe diameter (d). Thus the expression for the ReynoldsNumber may be rewritten:

_ 4mRe---rrd/1

This expression shows that for a fixed mass flow rate, alow ReynJlds Number (and hence streamline flow) calls fora large pipe diameter. However, the cost of increasing thepipe diameter quickly outstrips the cost of pumping, andeven if the other advantages of turbulent flow are notimportant, the minimum overall cost is often obtained usingfairly small diameter pipes with turbulent flow.

concentrated potassiummanganate(vlI) solution

Figure 2.2

to sink

screw clipto controlflow rate

glass pipe

observe here

Q1How does your result compare with othergroups in the class? Does pipe diameteraffect the flow pattern for a given flow rate?

Q2Is there a critical Reynolds Number at whichflow suddenly changes from streamline toturbulent?

4m1Td/1

Re =

dyenozzle

turbulent. What difficulties do you encounter?Measure the maximum flow rate for

streamline flow using a measuring cylinderand stopwatch.

d Use this mass flow rate (mlkg S-'), theinternal diameter of the tube (dim), andthe viscosity of the water OJ/kg m-1 S-1)to calculate the Reynolds Number at whichthe flow pattern begins to change fromstreamline to turbulent in your experiment.

Experiment 2.1Investigation of flow patternsIn this experiment, water is discharged froma cons tan t head device through a circular glasspipe of known diameter. The water flow rateis varied and the flow pattern within the glasspipe is observed by injecting a fine tracer streamof coloured liquid into the water. In thismanner, the relationship between pipe diameter,flow rate and flow pattern may be investigated.

Procedurea Assemble the apparatus as shown in figure2.2. Check that the dye nozzle is correctlyaligned with the end of the glass pipe. Eachgroup in the class may use a pipe of differentdiameter. The bottom of the air inlet tubemust be at least 5 cm above this pipe to give areasonable head of pressure.b Open the screw clip slightly to give a smallflow rate of water through the glass pipe.Once a stream of bubbles begins to emergefrom the bottom of the air inlet tube, theeffective head of pressure of water within theapparatus will remain constant until thewater level falls to this point.c Slightly open the tap on the funnelcontaining the dye (potassium manganate(vn)solution) so that a fine stream of colouredliquid is injected into the water as it enters theglass pipe. Observe the flow pattern within thepipe as you gradually increase the water flowrate. Attempt to identify the flow rate at whichthe flow pattern changes from streamline to

2.2HOW FLUID FLOW IS MEASUREDIt is necessary to measure flow rates in order to monitor theoperation of a chemical plant. Generally the overall process iscontrolled by adjusting flow rates using automatic controlvalves. For example, the temperature in a reactor may belowered by increasing the flow of cooling water.

Many flow-measuring instruments used in large-scalecontinuous processes depend upon the application ofBernoulli's Law. This is a special case of the Law of Conser-vation of Energy and states that, if friction losses are ignored,the energy per unit volume is constant along any streamlinein a liquid. Hencl'l for the flow of fluids along a horizontalpipe:

Gain in kinetic energy per unit volume= Loss in potential energy per unit volume

If there is a constriction in the pipe, the fluid flows fasterand its kinetic energy increases. The loss in potential energyis shown by a related drop in pressure. If the fluid slowsdown, the pressure is observed to increase.

A venturi flow meter is shown in figure 2.3. This consistsof a smooth contraction within the pipe, followed by asmooth expansion to the original diameter. The reduced

manometer

flow •--~--------------Figure 2.3 Venturi flow meter.

9

float stop

11

float

graduated glasscone--.--,

)--,1---IIJmanometer

section or 'throat' causes an increase in fluid velocity with acorresponding decrease in pressure, in accordance withBernoulli's principle. This pressure difference betweenthe throat and the upstream pipe gives a measure of the fluidvelocity and hence flow rate.

A cruder variation of this type of instrument is theorifice meter, which consists simply of a plate containing amachined hole (or orifice) placed across the pipe (figure 2.4).The pressure drop across the orifice is also a measure of theflow rate within the pipe.

11Figure 2.4 Orifice flow meter. Figure 2.6 Variable area meter.

However, whereas the overall pressure loss across aventuri meter may also be as low as 1 %, a typical orificeplate might give a 5 to 6 % pressure loss as a result of themuch larger frictional losses due to turbulence. Despite thisdisadvantage, orifice meters are frequently used because theyare easier and cheaper to install and require less space. Forinstance, they may be incorporated in flanged pipe joints.

If flow in an open channel is to be measured, Bernoulli'sprinciple may again be used by making the liquid flow overa weir, as in figure 2.5. The height of liquid standing over theweir is a measure of the pressure drop and hence the liquidflow rate.

Figure 2.5 .A weir as a meter in an open channel.

Finally, a very convenient meter that is commonly usedon a chemical plant is the variable area meter or rotameter.This consists of a vertical transparent tube, the diameterof which increases slightly with height. A bullet-shaped bobis contained in the tube, and upward-flowing fluid lifts thisuntil the annular gap is wide enough for the pressure drop to

just support the bob. The tube is graduated and so theinstrument can be calibrated. The bob can be made of anysuitable material, so that the meter can be used with corro-sive fluids. It is suitable for gases or liquids and can be usedfor small flow rates. (See figure 2.6.) )

This is by no means a complete list of all the methods ofmeasuring flow, and many ingenious devices, such as ultra-sonic and electromagnetic flow meters, are used for specialpurposes.

2.3OTHER APPLICATIONS OF FLUID FLOWAn understanding of the behaviour of fluids in motion isalso important to the chemical engineer when consideringthe flow of fluids around particles, particularly in a 'packedbed' consisting of many solid particles touching each other.Flow through packed beds is very common in the chemicalindustry. Most catalytic reactors involve a bed of catalystpellets or powder. Gases are dried by blowing them throughbeds of silica gel or activated alumina. Filtration ofteninvolves the flow of liquid through a bed of loosely packedsolid particles.

Fluid which finds its way through the interstices of apacked bed of particles is rather like fluid flowing through anarrow tortuous pipe with rough walls and, as we mightexpect, the laws of fluid flow are similar. Thus, flow througha pipe and flow around a particle are at the two ends of acontinuous 'spectrum' of conditions, and are sufficientlyrelated for much of the mathematics to be common to both.

10

CHEMICALENGINEERING

CHAPTER THREE3.1THE MANUFACTURE OF BUTANONEIn this chapter we shall begin by examining a chemicalprocess for the manufacture of butanone and see how such aprocess may be conveniently broken down into a series ofsteps or unit operations.

Butanone (commonly known as methyl ethyl ketone orM.E.K.) is an important industrial solvent with an annualworldwide production figure in the region of 100 000tonnes. The process described involves the catalytic dehydro-genation of butan-2-o1 for which the overall equation is:

Vapour phase------, CH 3CH 2CO CH3 + H2ZnO or brass

catalyst

A simplified flow diagram for a typical butanone plant isshown in figure 3.1.

Every .chemical process can be broken down into a seriesof unit 0rerations carried out on the process stream.Hundreds of operations may be involved in a complexprocess such as nylon manufacture and each one requires thedesign, construction, and maintenance of a separate item ofequipment. Chemical engineering places great emphasis onthe study of these unit operations, because the same theoryis applicable to a particular operation (e.g. distillation)whether on a butanone plant or a hydrogen cyanide plant.Thus the concept of unit operations provides a frameworkfor the study of the technology of chemical processes whichspreads across the boundaries of different manufacturingindustries.

Unit operations may be classified into three main groups:transport of materialsheat transferseparation.

Figure 3.1 The manufacture of butanone from butan-2-ol.

coolingwater

-hydrogen

vaporizer

pre-heater

pump

butan-2-01storage

butan-2-01

butanonesteam

condensatereceiver

distillationcolumn

recycled butan-2-01

FeedThe butan-2-01 is pumped at a carefully controlled flow rate fromstorage tan ks.

CoolThe hot gases leaving the reactor are cooled in the heat exchangerused to vaporize the butan-2-01 feed.

Pre-heatThe reaction is to take place in the vapour phase. The butan-2-01 isfirst heated to its boiling point of 100°C using superheated steam.

CondenseOn further cooling"in a water-cooled condenser, most of the butanoneand unreacted butan-2-01 condense to liquids. The other reactionproduct, hydrogen, remains as a gas.

VaporizeThe hot gases leaving the reactor are used to vaporize the boilingbutan-2-01 before it enters the reactor. The vaporizer is designed toachieve the desired reactor inlet temperature.

CompressThe hot vapour is compressed to force it through the plant atoptimum pressure.

SeparateA condensate receiver is used to separate the liquid and gas in theprocess stream. However, some of the butanone and unreactedbutan-2-01 remain in the vapour phase and are carried off with thehydrogen gas. (Possible methods for their recovery are discussed laterin the chapter.)

ReactButan-2-01 vapour is passed over a zinc oxide, or brass, catalyst bedat 400 to 500°C and undergoes dehydrogenation to butanone with atypical yield of about 90 %.

DistilThe butanone product is separated from unreacted butan-2-01 bydistillation. Heat energy must be supplied to boil the mixture of thetwo liquids, and the butanone vapour emerging from the top of thedistillation column must be cooled and condensed. The butanoneproduct is run to storage tanks and any recovered butan-2-01 isrecycled.

11

Figure 3.2 The butanone plant at Fawley, Southampton.The lefthand tower is the main butan-2-o1 purification tower, withthe butanone product tower on the right.

3.2TRANSPORT OF MATERIALSThis group of operations is concerned with the bulkmovement of materials between and through the differentitems of chemical plant. Materials in fluid form are generallymuch easier to handle, but chemical engineers often have todeal with sticky, powdery, or lumpy solids, and highlycorrosive or flammable gases.

SolidsSolids may vary in many ways: in particle size and range,density, moisture content, free-flowing tendency, and so on.They are commonly moved by conveyor belt, although it issometimes difficult to achieve the accurate control of flowrate necessary for continuous processes by this method.

An alternative way of transporting solids for shortdistances is by a screw feeder. These are frequently used tofeed polymer granules to moulding machines or coal tofurnaces, and can give very accurate control of flow rates.(To see a screw feeder in operation you should examine adomestic kitchen mincer.)

Another useful way of moving solids is in suspension ina fluid. For example, coal and china clay may be transportedin pipes over considerable distances as fme particles suspendedin a fast-moving stream of water, an operation called hydrau-lic conveying. Transporting granular or powdered solids in afast-moving air-stream is called pneumatic conveying, and haslong been used for loading and unloading grain ships. It isnow extensively used in the chemical industry, for examplein transferring catalyst particles between reactor and regener-ator in fluidized catalytic cracking units (see figure 4.19).

12

FluidsGenerally speaking, materials in liquid form are easiest totransport and store, although those which are corrosivetoxic, or flammable require special precautions. '

Liquids are normally moved to and from chemical plantsin large tanks mounted on lorries, railway wagons, or ships. Ifvery large quantities of material are involved, an overlandpipeline may be constructed. These are expensive to installbut relatively. cheap to run. They are particularly favoured bythe petroleum industry to transport crude petroleum fromoilfield to refinery or to convey products to distributiondepots.

Within a chemical plant, considerable movement ofliquid takes place from one vessel to another, and throughheat exchangers, reactors, filters, and pipes. Where possible,use is made of gravity, but often energy must be supplied andsuitable pumps are required.

Two basic types of liquid pumps are in common use:centrifugal pumps and positive displacement pumps.

.. !airair Signal (?';\)I t0J controller

transmitter !air to valvecentrifugal air-operatedpump valve

As liquidis pumped through the orifice plate

there is a pressure drop. The transmittersends a signal (air) to the controller wh ichcompares the signal with the set point and

alters the control valve until the correctpressure drop (and hence flow) is obtained.

Figure 3.3 Automatic flow control using a centrifugal pump andcontrol valve.

3.3HEAT TRANSFERMany chemical reactions and separation operations rely fortheir success on the accurate control of temperature. Heat-ing and cooling the process stream at various stages is veryimportant. On a chemical plant this is brought about by usingheat exchangers to transfer heat energy from one fluid toanother.

One of the simplest heat exchangers is the Liebig water-cooled condenser, commonly used in laboratories. Thisconsists of a 'tube' through which the fluid to be cooled ispassed, surrounded by a 'shell' through which the coolingfluid, usually water, flows.

This type of 'shell and tube' heat exchanger finds muchuse in industry, though in a considerably modified form. Inorder to appreciate the design of industrial heat exchangers,we shall first develop the theory of heat transfer for a simplelaboratory device.

The theory of heat transferConsider a stream of hot liquid which is to be cooled in asimple shell and tube heat exchanger using cold water. Thereare two basic methods of carrying out this operation.

a Parallel-current flow where both the hot liquid and thecooling water flow through the exchanger in the samedirection.b Counter-current flow where the hot liquid and coolingwater pass through the exchanger in opposite directions.

Figure 3.4 shows these two modes of operation, togetherwith typical temperature profiles for the fluids within theexchanger. Study these diagrams and try to suggest thepossible advantages and disadvantages of each method.

Mm = (T1 - t2) - (12 - t 1)In [(11 - t2) / (T2 - td]

Q = the duty of the exchanger: the amount of heat tobe transferred (in kJ hr-1)

U = the heat transfer coefficient; a measure of theefficiency of the process (in kJ hr-1 m-2 K-1)

A = area of surface across which heat transfer takesplace (in m2)

M = temperature difference (in K)

where

Q = UAM

In practice, counter-current flow is generally preferredand the following theory applies to this method. The basicequation which describes the performance of a heat exchangeris:

Sometimes M can be taken simply as the temperaturedifference between the two fluids. However, the temperatureof both fluids usually varies throughout the exchangeras shown in figure 3.4. Under these circumstances the mostuseful value for /:;t is known as the log. mean temperaturedifference, Mm, which is calculated from the inlet and outlettemperatures as follows:

distanceb Counter-current flow

distancea Parallel current flow

Figure 3.4 Temperature variation in a heat exchanger. T1, T2, t1, and t2 are as shown in figure 3.4b.

Q = UA/:;tm

A = rrdav X I

The performance of a heat exchanger isdescribed by the equation:

t coolingI water out

•

The area of the heat transfer surface(A/m') is calculated from the tube length(Ifm) and average diameter (dav/m).

/:;tm, the log mean temperature differenceacross the heat exchanger, is calculated fromtl't"TI andT2•

••

t2

T,

-hot liquid out

cooling twater In I

Figure 3.5

the mass flow rate of hot liquid and coolingwater through the heat exchanger. Afterdismantling the apparatus, measure the lengthand average diameter of the 'tube' acrosswhich heat transfer takes place.

Treatment of resultsThe 'duty' of the heat exchanger (Q) is theamount of heat being transferred per hour(in kJ hr-I). This is calculated from theresults for the hot liquid as follows:

Heat transfer per hour (Q)/kJ hr-I= mass flow per hour/kg hr-IX specific heat capacity of liquid/kJ kg-II("IX temperature change (TI - T, )/K

Experiment 3.3Investigating heat transfer in a laboratoryLiebig condenserIn this experiment you will use a laboratorywater-cooled Liebig condenser to reduce thetemperature of a stream of hot liquid. The'duty' of the heat exchanger will be determinedfrom the inlet and outlet temperatures andthe mass flow rate of the liquid stream. Thismay be used to estimate the heat transfercoefficient across the heat exchanger surfaceunder the conditions of the experiment.

Procedurea Fit a laboratory condenser with thermo-meters at each inlet and outlet so that thetemperature of both the hot liquid stream andthe cooling water may be measured beforeand after passing through the apparatus. Thisis readily achieved by fitting plastic T-piecesinto the rubber tubing, as shown in figure 3.5.Take care to avoid leaks at joints.

b The hot liquid which is to be cooledshould be passed through the central 'tube' ofthe heat exchanger, and cooling water passedthrough the outer 'shell' in a counter-currentdirection. The flow rates of both liquids maybe controlled by means of screw clips attachedto the outlet hoses. Ideally, both liquidsshould be supplied from constant head tanksso that their flow rates remain steadythroughout the experiment.

c Adjust the flow rates of both liquidstreams to give a temperature drop of at least5 °C for the hot liquid. When conditions aresteady, record the hot liquid inlet temperatureTI and outlet temperature T,; also thecooling water inlet temperature tI and theoutlet temperature t,.Use an appropriatemeasuring cylinder and stopclock to measure

13

QlUse the values for Q.A, and 6tm obtainedfrom the experimental results to calculate thevalue of U, the heat transfer coefficient, foryour apparatus under the conditions of theexperiment.

How does your answer compare with thevalues obtained by other groups?

Can you explain any differences?

Q2What factors affect the heat transfercoefficient across the tube?

Q3What modifications to the design of yourheat exchanger would incre~se its potentialduty?

Industrial heat exchangersThe duty of a heat exchanger depends upon:a the heat transfer coefficientb the surface area across which heat is transferredc the temperature difference.In the design of heat exchangers, chemical engineers attemptto achieve optimum values for each of these variables.

a Heat transfer coefficient (U)High operating values for the heat transfer coefficient areobtained by the following.i Ensuring that the fluid flow is turbulent. This keeps to aminimum the thickness of the film at both surfaces of thetube. Here the fluid is in streamline flow (or even stationary)and heat transfer can be by conduction only (see figure 3.7).

fluid near wall in streamline flow- heat transfer bV convection on IV

fluid in a transition region- heat transfer bV

and convection

wallFigure 3. 7 Heat transfer between fluids in turbulent flow.

ii Constructing the tubes of a material with a high thermalconductivity. (Look up the values for aluminium, copper,steel, and glass in your Book of data.)

14

Figure 3.6 Shell and tube heat exchanger under construction,showing the arrangement of tube bundles and internal baffles.

III Keeping the walls of the tubes clean and free fromcoatings of 'scale' or other solids. Where fouling of this typeis likely, the exchanger must be designed for ease of cleaningand maintenance.

b Surface area (A)A large surface area of tube is desirable, and to achieve thismany small tubes are used rather than a single large one.However, the larger the number of tubes the greater thecapital cost of the exchanger and the pumping costs tooperate it. In practice, an optimum value is specified to giveminimum overall costs. Some tubes have special 'fins' attachedto increase the effective surface area for heat transfer.

c Temperature difference (/':,t)The higher the temperature difference between the twofluids, the greater the heat transfer. However, the value ofthis variable is often dictated by the heating or cooling agentavailable.

River water at 5 to 15°C is often used as a cooling agent,with a maximum discharge temperature of 50 °c or less. Insome locations, suitable cooling water is not available and air-cooled heat exchangers must be used.

The most common heating agent on chemical plants ishigh pressure steam at about 150°C, often produced at acentral location on site.

A typical industrial shell and tube exchanger is shown infigure 3.8. It consists of dozens or even hundreds of small-bore tubes (the tube bundle) through which the processstream passes. A second fluid, perhaps cold water for coolingor steam for heating, passes over the outside of the tubeswithin the shell. Its path is directed backwards and forwardsover the tubes by means of baffles.

solid

~

circular valve: remains stationaryas segmented outer drum rotates

screen oscillates to ensure movement of solids

Separation operations may be divided into two maincategories: mechanicalseparation and mass transfer operations .

Mechanical separationMechanical separation operations depend on differences inbulk properties, such as density or particle size, to bring .about the separation of different components of a mixture.Typical examples include the following.a Screening or sieving is based on size differences betweenthe components of a solid-solid mixture, and is the simplestoperation. (See figure 3.10.)

Figure 3.10 Size separation of solids by screening.

-

b The continuous vacuum filter is the industrial equivalentof a laboratory suction ftlter. In figure 3.11 the mixture ofliquid and solid ('slurry') is fed into a trough. A large hollowdrum is suspended in the trough as shown. The outside of thedrum is perforated metal or woven wire string, and is coveredwith a ftlter cloth, on top of which are closely spaced strings.The pressure inside the drum is reduced by suction so thatthe liquid ('mother liquor') is sucked inwards and the solidforms a cake on the outside. When the ftlter is running, thedrum rotates and a 'cake' of solid is formed. As this comesout of the slurry it is washed, both washings and motherliquor being sucked inside the drum and run off.

The ftlter cake has to be removed before the next cycle,and in figure 3.11 this is shown being done by leading thestrings around an external roller so that the cake falls offinto a container. This diagram also shows how suction isapplied selectively to only two-thirds of the circumference ofthe drum by means of a special valve.

Figure 3.11 Filtration usipg a continuous vacuum filter.

coolingwater

feed

gas in

bottomsproduct

productcooler

water out

condenser

feed preheaterliquid

,r--,. ,...,.j/?tS" j/",.., ••

Ii \\ II I \\ n I , \t:I \ I I j/ I \" I j/ j/ I \ I13 II I II HI u];I ,WI ,VI

'-.A

distillationcolumn

gas out water in

t t

Figure 3.8 Shell and tube heat exchanger.

reboiler

steam

Figure 3.9 Distillation column with ancillary heat exchangeequipment.

Heat exchangers perform a wide variety of functions indifferent situations and are often given names to indicate thisfunction. Thus, pre-heater, reboiler, condenser, cooler,vaporizer, and economizer are all names for heat exchangersused for different applications. Figure 3.9 shows the heatexchangers commonly used in association with a distillationcolumn. cooling water

Although usually studied in school physics rather thanchemistry courses, heat transfer is a very important aspectof industrial chemistry. The chemical and economic viabilityof a process may well depend upon the efficient use andrecovery of heat energy.

3.4SEPARATIONSeparation of the products of a chemical reaction does notusually present too much difficulty on a laboratory scale un-less a very high degree of purity is required. Thus solids canbe separated from liquids by ftltering or centrifuging. A singlecomponent may be isolated from liquid mixtures by distilla-tion or solvent extraction. All such techniques take advantageof differences in properties of the substances to be separated.

The chemical engineer uses these same principles todesign equipment which can perform the task on a largescale, frequently on a continuous basis, at rates of hundredsof tonnes of product per day. Much of the equipment seenon a typical chemical plant may well be concerned with suchseparation operations.

condensate

15

organicsolvent

aqueoussolution

.t organic layer

Figure 3.14 Mixer/settler unit for solvent extraction.

Mass transfer coefficients may be derived to describe theefficiency of all mass transfer operations. Just as heat transferequipment is designed to give an optimum value for the heattransfer coefficient (U) so chemical engineers must designmass transfer equipment to obtain maximum values for thema$S transfer coefficient (K).

frequently used to recover the absorbed gas from a solvent.Figure 3.13 shows an absorber/stripper system which mightbe used to bring about complete separation of gases X and Y.

The gas mixture and solvent are passed through theabsorber in opposite directions (counter-current flow) tomaintain the maximum 'driving force' for mass transferbetween phases. A large surface area of contact betweenphases is achieved by using trays or packings similar to thoseused in distillation columns. (See Chapter 5.)

In the butanone plant discussed at the beginning of thechapter, butaI10ne and butan-2-o1 vapours niay be recoveredfrom the hydrogen gas stream by scrubbing with water.However, butan-2-01 cannot be separated from water bystripping as both liquids have the same boiling point (100°C).Solvent extraction must be used instead.

b Solvent extraction is used for liquid-liquid separation anddepends on the partition effect of a solute between twoimmiscible liquids.

For instance, if a mixture of butan-2-01 and water isagitated with a suitable solvent such as 1,1 ,2-trichloroethane,most of the butan-2-01 but virtually none of the water willenter the trichloroethane layer. To increase mass transfer,and hence approach equilibrium conditions more rapidly, theinterfacial area between the two phases is made as large aspossible by mechanical agitation.

Because they are immiscible and have different densities,the water and trichloroethane separate into two layers whenagitation ceases. Most of the butan-2-01 is now in thetrichloroethane layer from which it may be separated bydistillation. You may have carried out this kind of separationoperation in the laboratory using a tap funnel. (See Topic 9,Experiment 9.4.)

Solvent extraction may be carried out on a continuousbasis using mixer-settler units. These consist of two tanks, ofwhich one is agitated to bring the two liquid phases intocontact and the other is calm to allow them to settle out(figure 3.14). The solvent phase which now contains thedissolved solute is called the extract and the residual phasefrom which solute has been removed is called the raffinate.

gasY

gasX

steam

-circumferential collecting boxes

solvent + X

scrubber

Figure 3.13 Gas separation by selective absorption.

mixed gasx·+ y

feed

Figure 3.12 Solids separation in a centrifuge.

a Gasabsorption is used to separate a mixture of gases usinga selective solvent in an absorption tower or 'scrubber'. Forinstance, a mixture of two gases X and Y might be separatedin this way, by using a packed tower to bring the gas mixtureinto contact with a solvent in which gas X is soluble but gasVis not.

Gas absorption (or 'scrubbing') is characterized by masstransfer in one direction only - from the gas to the liquidphase. The reverse process, where mass transfer occurs fromthe liquid to the gas phase, is called 'stripping', and is

Mass transfer operationsMass transfer operations are characterized by the movementof one substance through another on a molecular scale. Suchseparation techniques are based on the principle thatsubstances tend to distribute themselves in different concen-trations in different phases. Thus distillation takes advantageof the difference in composition usually found between aliquid mixture and the vapour with which it is in equilibrium.Distillation is the most important of the mass transferseparation operations, and will be investigated in some detailin Chapter 5.

Other important mass transfer operations include thefollowing examples.

pusher blade

product wash motherliquor liquor

c The centrifuge is used for solid-liquid separation, on thesame principle as a laboratory centrifuge. Figure 3.12 showsa continuous centrifuge. As it rotates, the solid collects onthe lining of the cylindrical basket and the mother liquor andwashing water pass through the perforated basket intocollecting boxes. A reciprocating pusher blade graduallymoves the solid layer through the washing zone and out t~ adischarge point as shown.

p~rfor~ted ~etal ts~etl

feed (crvstals in mother liquor)--washliquor

16

CHEMICALENGINEERING

CHAPTER FOUR

Figure 4.3 Continuous flow tubular reactor.

-.-..products out

stirrer

•products out

\~b~;l~il'

Figure 4.2 Continuous stirred tank reactor.

reactants in••

c The continuous tubular reactorIn this type of reactor, the reactants are fed continuouslyinto one end of a tubular vessel and products flow out at theother end (figure 4.3). This is a steady rate operation. Withconstant flow rates, the conditions at any particular pointremain constant with time. At a distance x downstream from

the inlet, reactants have spent a time.£ in contact, where v isvthe flow velocity through the reactor. Thus changes in timein a batch reactor become identical with changes in position(x) in a tubular reactor. The significant characteristic oftubular reactors is that no attempt is made to mix togethermaterials which are at different stages of reaction. Theoverall length of the reactor is determined by the contacttime needed to achieve the desired concentration of product.

rA = k[A]

It is possible to derive the integrated form of the rate law asshown in Appendix 1 to Topic 14.

4.2DESIGN EQUATIONSFor each type of chemical reactor it is possible to derive ageneral design equation. This relates the residence time (t)which the chemicals must spend in the reactor, to the requiredchange in the concentration of reactants and the rate con-stant for the reaction.

In a batch reactor, the percentage conversion of reactantsto products in time t may be calculated simply from therate expression.

For a first order reaction of the typeA -+ productsthe rate of reaction, rA, is the rate of change of concen-tration of A.

stirrer

4.1TYPES OF REACTORThe function of the reactor is to produce a certain productfrom given reactants at the required rate. There are threemain types of chemical rellctor commonly used to achievethese objectives. These are:a the batch reactorb the continuous stirred tank reactorc the continuous tubular reactor.

reactant

Most chemical processes may be divided into two main stages:the synthesis stage in which the required product is formedfrom reactant materials;the separation stage in which the required product isseparated from the rest of the reaction mixture.

The synthesis stage is carried out in a vessel called thereactor. On a chemical plant this may often appear small andunimpressive compared with some of the other items ofequipment present. However, the performance of the reactorinfluences the design and operation of almost every otherpart of the plant. Thus the reactor lies at the heart of anychemical process. Its design must be undertaken early inthe development stage and will often dictate the capital costand economic viability of the overall plant.

Figure 4.1 Batch reactor.

a The batch reactorIn this type of reactor, all of the reactants are placedtogether in a vessel, and the mixture is stirred and heated asappropriate until the reaction is sufficiently complete. (Seefigure 4.1.)

In a batch reactor the rate of reaction falls as thereactants are used up. At any particular instant all thematerial present has reached the same stage of reaction.

b The continuous stirred tank reactor (CSTR)An alternative to batch operation is to feed reactantscontinuously into the reactor at one point and withdrawproducts at an equal flow rate elsewhere. Thus the chemicalsreact as they flow through the system.

A typical continuous stirred tank reactor is shown infigure 4.2. Reactants flow continuously into a vigorouslystirred vessel and products are withdrawn at the same rate sothat a steady state is maintained. The main characteristic'ofthis type of reactor is that the contents are thoroughly mixedto give a uniform composition throughout. Thus thecomposition of the outlet stream will be the same as that inthe bulk: of the vessel.

17

t =-!. In [A]ok [A]

[A] = concentration of reactants at time tk = rate constant for reaction.

where: This expression may be used as the design equation for a firstorder batch reactor.

[A]o = initial concentration of reactants

QlUse the design equation for a batch reactor tocalculate the time taken to achieve 50 %,

90 %, and 100 % conversion of A if thereaction is first order and the rate constant (k)is 0.04 min-I. (Note that, in solving such

problems, absolute values of concentrationare not required, only the ratio of [A] 0 to[A].)

This is the general design equation for continuous stirredtank reactors.

If the reaction is first order:

The designequation becomes:

rA = k[A]

Dividingby ut

[AJ. = [AJ + (A x : )

+ (rA x Vt)= [A]ut

[A]o - [A]T =u

V

[AJout

Thus:

If the concentration of A entering the reactor is [AJo,then the number of moles of A entering the reactor in timetis [A]out.

Similarly, if the concentration of A leavingthe reactor is[A] , then the number of moles of A leaving the reactor intime tis [A]ut.

For a continuous process, the reactants arebeingcontinuouslyadded and the reaction mixture is being continuouslyremoved. Thus flow data must be incorporated into thedesign equation to allow for the effect which flow has uponconcentration. Consider the reaction: A -+ products. SupposeA is being fed into a continuous stirred tank reactor in whichperfect mixing is taking place.

Let the volume of the reactor be V dm3 , and the volumeflow rate through the reactor be u dm3 min-I. Then themean residence time (T) of material in the reactor is givenby:

T = VU

The number of moles reacting in time t = rA x Vt V-=u

[A]o - [A]T = k[A]

Applying a massbalance over the reactor for component A:

number of molesentering

number ofmoles leaving

+ number of moleswhich have reacted

Thus if [A]o, [A] , and k are known, the required flow ratethrough a reactor of volume V may be calculated.

Thus the volume of thiosulphate solutionrequired to discharge the blue colour is ameasure of the number of moles of hydrogenperoxide which have reacted.

HP2 + 21- + 2W ->-12 + 2H20 [4.1](bluewithstarch)

Experiment 4.2aUsing a batch reactor to obtain kineticdata for a reactionIn acid solution hydrogen peroxide willoxidize iodide ions to produce iodine.

The iodine produced gives an intenseblue colour if a little starch is present. Inthis experiment, the reaction will be carriedout in a simple batch reactor. The rate ofchange of concentration of hydrogen peroxidewill be followed by progressively titratingthe iodine produced with sodimn thiosulphatesolution.

Part 2 Batch determination of reactionkineticsa Put 500 em' of 0.02M potassium iodidesolution in a large beaker. This vessel is toserve as a batch reactor, and must be stirredconstantly during the experiment. Add10 em' of 5M sulphuric acid and 10 cms of1 % starch solution.

b Fill a burette with 0.2M sodiumthiosulphate solution and arrange this overthe batch reactor.

c Using a measuring cylinder, add 50 em' of'1 volume' hydrogen peroxide solution andsimultaneously start a stop clock. A bluecolour should appear in the stirred reactionmixture as iodine is produced.

d Immediately add 1.0 em' of thiosulphatesolution to the contents of the reactor. Thisshould cause the blue colour to disappearuntil sufficient iodine has been produced bythe peroxide/iodide reaction to reactcompletely with this thiosulphate solution.

concentration in mol dm-' of your 'I volume'hydrogen peroxide solution.

b Using a measuring cylinder, add 50 em' of'1volume' hydrogen peroxide solution.

c Warm the reaction mixture to about 50 DCand allow it to stand for at least 30 minutesto ensure that the reaction is complete. (Part 2of the experiment should be attempted duringthis time. Alternatively, add 5 drops of 3 %ammonium molybdate solution whichcatalyses the oxidation of iodide by peroxide,so that there is no need to wait for 30minutes.)

d Titrate the liberated iodine with 0.2Msodium thiosulphate solution, adding a fewdrops of starch solution to enhance the colourof the iodine as you approach the end-point.

e Record the volume of thiosulphate solutionused. Let this be a cms • This is a measure ofthe number of moles of hydrogen peroxideinitially present in 50 em' of '1 volume'solution. Use this to calculate the

Part 1 Standardization of hydrogen peroxidesolutiona Add about 4 g of solid potassium iodide toabout 25 em' of l.OM sulphuric acid in aconical flask and dilute to 100 ems withwater.

[4.2]2S20;- ....•21- + S40~-(colourless)

12 +(blue withstarch)

18

Plot a graph of t (y axis) against In _a - (x axis).a-x

e Note the time when the blue colourreappears, and add a further 1.0 cm3 ofthiosulphate solution to the reaction mixture.

f Repeat until a total of 12.0 cm3 ofthiosulphate solution has been added, notingthe total time from the start of the experimentas the blue colour reappears after each1.0 cm3 addition of thiosulphate. Recordyour results carefully.

Volume ofthiosulphateaddedx cm3

Figure 4.4

Timet/min

aa-x

a-xa

a-xIn a

a-x

Treatment of resultsIf the reaction is first order with respect tohydrogen peroxide, then:

The design equation for the batch reactor is:

a is the volume of thiosulphate solutiQ,nequivalent to the initial number of moles ofH202•

x is the volume of thiosulphate solutionadded at time t.

Thus the design equation becomes:

Q2Confirm that the reaction is first order withrespect to hydrogen peroxide.

Q3Calculate the rate constant (k) for thereaction under these conditions from thegradient of the graph.

1 In[H202]ok [H202]

[H2 °2] 0 is the initial hydrogen peroxideconcentration, and[H202] is the hydrogen peroxideconcentration at time t.

Expressing hydrogen peroxide concentrationin terms of the volume of thiosulphatesolution used:

=!-In ak a-x

If the reaction is first order, a graph of

t against In _a_ should be a straight line witha-x

d" Igra lent -k

Draw up a table of results as shown infigure 4.4 above.

Q4Why can the effect of iodide concentrationon reaction rate be ignored in this experiment?Look carefully at Equations [4.1] and [4.2].

Q5Using the design equation, calculate the timetaken for 10, 20, and 30 % conversion of theinitial hydrogen peroxide in your batchreactor.

Figure 4.5

Vk[A][A]o - [A]

1.0 X 0.03 X 0.00600.0015

then [A] = 0.0060 mol dm-3

• '. if [A]o = 0.0075 mol dm-3

k = 0.03 min-1

Target conversion = 20 %

Rearranging the design equation gives:

u = total volume flow rate in dm3 min-1

u

[A]O - [A]k[A]

O.02M acidifiedpotassium iodidesolution+ starch

own solution using the results of Experiment4.2a.

V--

d Decide upon the degree of conversion ofperoxide for which you will design (between10 % and 30 %). Each group in the classshould aim for a different target conversion.

e Use these conditions in the design equationfor a continuous stirred tank reactor tocalculate the flow rate of reagents required.

u

Example:V = 1.0dm3

Experiment 4.2bThe continuous-flow stirred tank reactorIn this experiment you will design acontinuous stirred tank reactor (CSTR) toproduce a certain percentage conversion ofreactants to products. You will then constructthe reactor to your own specifications andcompare its operating performance with yourdesign calculations. The reagents will be'I volume' hydrogen peroxide solution andacidified potassium iodide solution (0.02M) asused in the batch reactor experiment.

a Design a reactor vessel of capacity between0.5 and 1 dm3 which will enable reactants tobe added continuously and products to bewithdrawn at the same flow rate. It must bepossible to agitate the contents of the reactormechanically so that they are thoroughlymixed at all times. (Check the design withyour teacher before construction.)

b Determine the working capacity of yourreactor by filling it with water and switchingon the stirrer. Water will overflow until asteady state is reached. Switch off andmeasure the volume of water left in the vessel.This is the working volume of the reactor(V dm3).

c For the sake of comparison, aim to work atthe same initial concentrations as in the batchreactor experiment, so the inlet streamshould be 'I volume' ('" 0.083M) hydrogenperoxide mixed with 0.02M acidifiedpotassium iodide solution in a volume ratio of1: 1O.Allowing for dilution, this would makethe initial hydrogen peroxide concentration

0.083 x--lI = 0.0075 mol dm-3

Note. As the concentration of hydrogenperoxide solution may change significantlyduring storage, you should standardize your

19

= 0.12 dIn3 min-I

So the required total flow rate is 120 cm3

min-I.

To give peroxide/iodide flow rates in the ratioof 1:10, the peroxide flow rate should be

120 x ..!- = 11 cm3 min-I11

And the iodide flow rate should be

120 x 10 = 109 cm3 min-I11

f Put about 9 dm3 of 0.02M acidifiedpotassium iodide solution (containing 10 cm3

of 1 % starch solution) into a constant headreservoir. Position the reservoir above thereactor vessel and adjust the flow rate to thedesired value using a measuring cylinder andstopclock. (109 ± S cm3 min-I in the aboveexample.)

g Set up asimilar reservoir containing about2 dIn3 of '1 volume' hydrogen peroxidesolution and adjust the flow rate to thecalculated value. (11 ± 1 cm3 min-I in theabove example.)

h Allow the reactor vessel to fill up andreach equilibrium. This will take approximatelyfour times the mean residence time (7) afterthe reactor is full. Since

7 =~u

the residence time in the above example is

1000 = 8.3 minutes.120

Thus at least 30 minutes should be allowed ifpossible.

i While the system is coming to equilibrium,drops of saturated sodium thiosulphatesolution should be added to remove the bluecolour of the iodine each time it appears. Thiswill ensure that the iodide concentration inthe reaction mixture remains constant.

i Once the reactor has reached a steady state,then O.IM sodium thiosulphate from aconstant head device should be carefully runinto the reaction mixture at such a rate thatthe colour of the reactor contents appears to'hover' between blue and colourless. It maytake a few minutes to determine thisequilibrium flow rate. Measure the rate of flowof thiosulphate solution required using ameasuring cylinder and stopwatch.

Treatment of resultsWhen the reaction mixture 'hovers' betweenblue and colourless:

rate of production of iodine from hydrogenperoxide= rate of removal of iodine by thiosulphate

Assuming that the reaction between iodineand thiosulphate is instantaneous, use yourresults to calculate the percentage conversionin the reactor.

The method is as follows:i Calculate the number of moles ofthiosulphafe added per minute.ii Hence calculate the number of moles ofiodine being produced per minute, usingEquation [4.2].iii Hence calculate the number of molesof hydrogen peroxide reacting per minute inyour reactor, volume V dIn3, using Equation[4.1].iv Calculate the rate of reaction in molesdm-3 min-I.v Using your value fOIthe rate constant andthe rate expression for the reaction

calculate the concentration of hydrogenperoxide in the reaction mixture and hencealso in the exit stream.

Q6Compare the actual percentage conversionwith your design conversion. Try to accountfor any discrepancies which exist.

Q7Consider the likely effect of the followingchanges of conditions on the percentageconversion within the reactor:a increased reactant concentration in feedb increased total flow rate through reactorc increased reactor volumed increased temperature.

The average residence time 7 is

For a first order reaction in a batch reactor the designequation is

Then

= 13.3 minutes

= 2..- In [Alok [A]

t

v = 22u 1.65

[A]o = 1 mol dm-3

k = 0.122 min-1

t = 13.3 minutes (same time interval as CSTR)

If:

v = 7 = [A]o - [A]u k[A]

4.3BATCH OR CONTINUOUS OPERATION?In the design of any chemical reactor, two factors - thekinetics of the reaction and the required output of product -are normally fIXed from the outset. Using all the availableinformation, the chemical engineer must make decisionsconcerning the type of reactor to be used, its physicaldimensions, and the optimum conditions under which it is tooperate.

The design equations developed earlier in this sectionenable comparisons to be made between theoretical yields ofproduct from a continuous stirred tank reactor and a batchreactor during the same time interval.

For a fust order reaction in a continuous stirred tankreactor, the design equation is

... [A]

If:Volume (V) = 22 m3

Flow rate (u) = 1.65 m3 min-IRate constant (k) = 0.122 min-I[A] 0 = 1 mol dm-3

In 1[A]

= 0.122 x 13.3

= 0.20 mol dm-3

Then using the design equation[A] = 0.38 mol dm-I

This represents a 62 % yield of products.

Since [A]o was 1 mol dm-3, this represents an 80 % yield ofproducts.

The batch reactor gives a larger percentage conversionthan the continuous stirred tank reactor, using the same sizevessel over the same period of time. Normally, a manufac-

20

Figure 4.6 Batch reactors for the production of pharmaceuticals.They produce a range of different products including an

anti-convulsant, a cardio-vascular drug, and a veterinary wormmedicine.

Q8Calculate the time required for the batchreactor to achieve 62 % conversion in theexample on the previous page.

Q9Why does the reaction proceed more rapidlyin a batch reactor than in a continuousstirred tank reactor?

Figure 4. 7 Comparative performances of batch and continuousprocesses.

turing process has a target yield of product which the batchreactor will reach with a shorter residence time than thecontinuous reactor.

The major disadvantage of batch reactors is that manyancillary operations are necessary both before and after thereaction tak~s place. The reactor vessel must be filled withmeasured quantities of reactants, the batch must be tested toensure that it has reached the desired percentage conversion,and the vessel must then be emptied completely. The timespent on these operations is called 'shut-down' time. Themanufacture of a large quantity of product requires verymany batches, and it is the overall time of the cycle of alloperations which must be considered when comparing batchand continuous processes. (See figure 4.7.) For most proces-ses the shut-down time would be so large that a greaterthroughput can be obtained from a continuous reactor.

The decision whether to operate on a batch or continu-ous basis is also influenced by factors such as the following.

Batchprocess

Continuousprocess

charging ofreactorreactiontimedischargingreactor

I- -Imean residence time to achieve the same% conversion as in the batch process

Manpower The manpower required to operate a process isrelated to the number of times an operating condition has tobe changed. Which type of processing requires the greaternumber of men to operate it?

Automation This relies on instruments, and instrumentsrequire conditions which are as steady as possible. Whichtype of process is more easily automated?

Degree of control Control over a process, whether manual orby instruments, is the result of a series of adjustments. Theeffect of an adjustment is noted and subsequently a fineradjustment is made. The longer the time available understeady conditions, the more refined the adjustment. Whichtype of process allows the greater control?

Cost of plant In a continuous process, conditions at anypoint in the system are constant and the equipment is 'tailor-made' for those conditions. In batch processing, multi-purpose units are frequently used which are the large-scaleequivalent of laboratory apparatus and are obtainable 'offthe shelf' from chemical plant manufacturers. Which type ofprocess is likely to have the higher capital costs?

Generally speaking, batch operation is used for processeswhich produce relatively small quantities of material such asin the pharmaceutical, fine chemicals, or dyestuffs industry.A well-equipped batch reactor (or autoclave) allows greatflexibility of operation, as it may be used to produce adifferent product each day. Batch reactors are also frequentlyused for polymerization and fermentation processes wherethe shut-down time allows thorough cleaning of the reactionvessels to avoid build-up of unwanted by-products or harmfulbacteria. However, for most other lar~e-scale processes con-tinuous operation is generally favoured.

21

4.4CONTINUOUS REACTOR DESIGN

Experiment 4.4The continuous-flow tubular reactorThe two main types of reactor in which achemical reaction may be carried out on acontinuous basis are the stirred tank reactorand the tubular reactor.

In this experiment you will operate atubular reactor and compare its performancewith that of the tank reactor studied inExperiment 4.2b.

a Construct a tubular reactor using atransparent glass or rigid plastic tube 3 or 4 cmin diameter and 1.5 m long. The tube shouldbe clamped at a slight incline and be fittedwith an 'inlet manifold' at the lower end toenable peroxide, iodine, and thiosulphatesolutions to be introduced at controlledflow rates. The upper end of the tube shouldbe fitted with an exit pipe discharging into asink or bucket. (See figure 4.8.)

To make the flow pattern along thereactor tube more turbulent it should befitted with a series of 'baffles' at 2 or 3 cmintervals along its length. These are readilymade from thin discs of plastic, perforatedwith a few holes and threaded onto a glass rod.

b Three constant reservoirs should be filledwith the same solutions as in Experiment 4.2b.

Reservoir A O.02M acidified potassium iodidesolution and starch.

Reservoir B '1 volume' hydrogen peroxidesolution.

Reservoir C O.lM sodium thiosulphatesolution.

c Set the flow rates of the three solutions sothat they are the same as in the previousexperiment (4.2b) when the reaction mixturewas 'hovering' between blue and colourless.Then introduce these solutions into thetubular reactor via the inlet manifold.

d While the reactor is filling and reaching asteady state, calculate the total flow ratethrough the reactor. If possible check this atthe exit pipe. Use this flow rate, the diameterof the reactor tube, and the results of thebatch reactor (Experiment 4.2a) to predictthe position in the tube where the reactionmixture should first turn blue.

e When the system has reached equilibrium,measure the actual distance along your

reactor tube at which the blue colour appears.How does this compare with your predictedresult?

QIOHow does the volume of the tubular reactorcompare with the volume of the stirred tankreactor used to bring about the samepercentage conversion in Experiment 4.2b?

QllWhat explanation can you offer for anydifference in volume required?

Q12How would you expect the position of thecolour change (and hence the volume ofreactor required) to be affected by:a increased total flow rate of reactantsb increased concentration of hydrogenperoxidec increased temperature?

Ql3What are the possible advantages anddisadvantages of the tubular reactor comparedwith the stirred tank reactor?

O.02Macidifiedpotassium iodidesolution

~

O.lMsodiumthiosulphatesolution

~

'1 volume'hydrogen perox idesolution

Figure 4.8 Apparatus for tubular reactorexperiment.

baffles

'Inlet manifold' Tubular reactor(at slight incllneto remove air)

to Isink +

The two principal types of continuous reactor, stirred tankand tubular, have rather different performance characteristicswhich determine their suitability for use in particularchemical processes. Whendesigninga continuous reactor, thechemical engineer must consider factors such as reactorvolume, selectivity of product, temperature control, optimumphysical conditions, and the use of catalysts.

Reactor volumeFor a given production target, the size of reactor requiredwill depend upon the rate at which the reaction occurs. Sincereaction rate isnormally dependent on reactant concentration,the volume of a tubular reactor required to.bring about acertain percentage conversion is significantly different fromthat of a stirred tank reactor.

22

In an 'ideal' tubular reactor, all elements of the reactionmixture are assumed to take the same time to pass along thereactor tube (figure 4.10). This situation is known as 'plugflow', and no 'back-mixing' occurs between materials atdifferent stages of reaction. The chemicals react as theyproceed along the reactor tube, and thus the reactantconcentration falls steadily from its initial value [A]0 at theinlet to its final value [A] at the exit. Consequently thedesign equation for a tubular reactor is similar to that for abatch reactor.

If V is the reactor volume and u is the flow rate throughthe reactor, then the residence time t is givenby

t = Vu

Figure 4.9 A large-scale continuous tubular reactor.

For a first order reaction it has been shown that:

t = ~ In [A]ok [A]

Thus the design equation for a frrst order tubular reactormay be written:

V = t = ~ In [A]ou k [A]

Q14In a previous example, the volume of stirredtank reactor needed to give 62 % conversionwas 22m3•

Use the above expression to calculate thevolume of tubular reactor required to achievethe same percentage conversion.

(Flow rate u = 1.65 m3 min-I, rate constantk = 0.122 min-I.)

For a given flow rate and percentage conversion, a tubularreactor has a smaller volume than the equivalent stirred tankreactor. This may have a significant bearing on the capitalcost of the reaction vessel.

Reactions take place more slowly in stirred tank reactorsbecause the reactant concentration is at the low exit value

Inlet

reactantconcentration[AJo

1ft Ii ~ Outlet

reactantconcentration[A]

throughout the residence time (figure 4.11).A partial solution to this problem is to use stirred tank

reactors in series. The outlet stream from one tank becomesthe inlet stream for the next (figure 4.12 on the next page).The reactant concentration falls step-wise from tank to tank(figure 4.13 on the next page). Thus the average reaction rateis higher and the total reactor volume required is lower thanif a single tank had been used. In the extreme case, a tubularreactor may be regarded as equivalent to an infinite numberof stirred tank reactors in series.

Residence time

c:0

";::;c:

[A] 0 ~0";::; c:

~ '"uc:c: 0'" uu •..c:0 c:u ~•.. uc: co~ '"u a:co [AJ'"a:

Distance along reactor x

[AJ 0

[A]

Figure 4.10 Plug flow in an 'ideal' tubular reactor. Figure 4.11

23

Reactor selectivityIt is not uncommon for by-products to be formed in areaction mixture due to the occurrence of undesired chemicalreactions. In these circumstances, the reactor design mayconsiderably influence the nature of the products formedand hence the type of separation equipment required to dealwith them.

Such unwanted products may arise in two ways.

First stage

pump products out

Second stage' Reactions in series (consecutive reactions)Consider the reaction scheme

Figure 4.12 Two-stage continuous stirred tank reactor.

Here the reactant A produces the desired product B, but thismay itself undergo further reaction to form the undesiredproduct C. In order to suppress the conversion of B to C, theconcentration of B must be kept as low as possible within thereaction mixture. Thus where B is the desired product, atubular reactor will give the best performance, whereas astirred tank reactor will tend to favour the formation of C.

.2 [AJ 0•..~cQ)ocoo•..c~o'"&: [A]

Areactant

Bdesiredproduct

C

undesiredproduct

Residence time

Figure 4.13 Concentration changes in tubular reactor and two-stagecontinuous stirred tank reactor.

Q15Benzene can enter into substitution reactionswith chlorine as follows:

C6HsCImonochlorobenzene

C,H4CI2

dichlorobenzene

Which type of reactor would you specify tofavour the formation of:a monochlorobenzeneb dichlorobenzene?

If the reaction A -----+ B is first order then:

If the reaction A -----+ D is second order then:

rate of formation D = k2 [A] 2

Temperature controlMost chemical reactions involve a significant energy change,either exothermic or endothermic, which will tend to alterthe temperature of the reaction mixture as reaction proceeds.If no attempt is made to compensate for this by heating orcooling the reaction mixture then the reactor is said to beoperating adiabatically. This may be used to advantage formoderately exothermic reactions, where the increase intemperature will maintain the reaction rate as the reactantconcentration falls.

However, with highly exothermic reactions, a significantrise in temperature will occur unless heat is removed fromthe mixture during reaction. For many chemical systems, therate of reaction doubles for every 10°C rise in temperatureand this can quickly lead to a ;runaway' situation withdisastrous consequences.

With most chemical reactions an optimum temperaturerange needs to be maintained and the reactor design mustincorporate provision for heat transfer. In the extreme casewhere the temperature of the reaction mixture is heldconstant throughout, the reactor is said to be operatingisothermally.

Thus the choice of reactor type depends upon thekinetics of the two competing reactions. A tubular reactorwill favour the higher order reaction and a CSTR will favourthe lower order reaction, assuming the rate constants aresimilar in each case.

=rate of formation of B

rate of formation of D

Hence

rate of formation of B = k1 [A]

To favour the production of B, the concentration ofreactant A must be kept as low as possible, a situation bestachieved in a stirred tank reactor. However, if D is thedesired product, the concentration of A should be kept at amaximum and a tubular reactor will give the best performance.

Reactions in parallel (competing reactions)Consider the situation where a reactant A may form twopossible products Band D.

24

-productsout

t t fuel burners

Figure 4.15 Heat transfer in tubular reactors.a single tube with heating or cooling jacketb multi-tube reactor; tubes in parallel give low tube velocity forreactantsc pipe furnace; tubes usually in series; uses include 'stearn cracking'of hydrocarbons.

-inheatingor cooling

___ out agent

a reactants ain t

-h ~ ;~--reactants products- in I out

heatingheating

_ or cooling b ior cooling

,agentagent

l productsout -

b reactantsproducts

inout

iheating

_out or cooling

heatingc flue gases agent

or coolingto stack

agent t t_in

-reactants convection

~ productsin section

out

c radiantsection

products Iout +

pump

Figure 4.14 Heat transfer in stirred tank reactors.a jacketed; b internal coils; c external heat exchanger.

Accurate temperature control is readily obtainable in astirred tank reactor, where the contents are thoroughly mixedand uniform throughout. However, deviations from 'plugflow' in a tubular reactor can lead to the formation of 'hotspots' in the reaction mixture, where the temperature andconsequently the rate of reaction cannot be accuratelypredicted.

Operating conditionsMany chemical reactions are reversible, and at first sight itmight appear that conditions within a reactor should alwaysbe designed to favour a high equilibrium yield of the desiredproduct. In practice, the situation is often more complexthan this. Consider the Haber process for the manufacture ofammonia:

Nz (g) + 3Hz(g) "'" 2NH3(g) ~H~8 = -92.1 kJ morl

The equilibrium data shown in figure 4.16 suggests thatthe best percentage yield of ammonia will be obtained byoperating at a low temperature and high pressure. However,at low temperatures the rate of reaction is far too slow (achemical factor) and the operation of high pressure plant isvery expensive (an economic factor). This problem isresolved in most ammonia manufacturing plants by using acompromise temperature of about 450°C, a pressure ofabout 250 atmospheres, and a catalyst to speed up the rateof reaction. The reaction mixture is not allowed to reachequilibrium but is removed from the reactor at 12 to 15 %conversion. Ammonia is separated by liquefaction, andunreacted nitrogen and hydrogen recycled. The recyclingof unconverted reactants in this manner is common practicein the chemical industry .

The reactor conditions and percentage conversion perpass are designed to give the lowest production costs takinginto account the kinetic, thermodynamic, and economic datafor the system. Problems such as this are readily investigatedusing mathematical models on a computer.

25

Pressure/atmospheres

-heavygasoil

-gases andgasoline

spentcatalyst1 375°C

flue gas

Figure 4.19 Catalytic cracking: the catalyst powder passes to thereactor, in the centre, where the cracking process takes place. Thecracked vapours then pass to a fractionating column, on the right. Theused catalyst returns to the regenerator, at left, where it is cleaned forre-use.

-freshfeed

Figure 4.18 Overall view of the catalytic cracking unit at Cura~aooil refinery.

Such fluidized systems overcome many of the problemsof mass transfer and temperature control which may beassociated with fixed bed reactors. However, they areexpensive to construct and require careful control tomaintain a uniform fluidized bed of active catalyst.

productsout

Figure 4.17 Fixed bed catalytic reactor.

'filted bed' of catalyst

Homogeneous and heterogeneous reactorsMost of the discussion so far has assumed that the chemicalsystem within the reactor is homogeneous, which means thatall the substances involved are present· in the same phase.However, many reaction systems are heterogeneous, withmaterials in two or more phasestaking part. Thisisparticularlysignificant for gas or liquid phase reactions which take placeat the surface of a solidcatalyst. If so, the reactor performancemay well be determined, not by reaction kinetics, but by therates of mass transfer of reactants to the catalyst surface andproducts away from it.

Heterogeneous reactions involving solid catalysts aregenerally carried out in tubular reactors packed with catalystpellets through which the reactants must pass (figure 4.17).This is known as a 'fixed bed' system and is favoured becauseof its simplicityand the flexibility of its operating conditions.

Figure 4.16 Equilibrium data for the synthesis of ammonia.

One heterogeneous system which poses particularproblems is the catalytic cracking of hydrocarbons frompetroleum. This may be carried out by passing hydrocarbonvapours over a silica-alumina catalyst at about 500oe.However, during the cracking process the surface of thecatalyst becomes fouled with deposits of carbon whichreduces its activity and hence reactor performance. Thisproblem has been solved by using 'fluidized bed' reactors inwhich the catalyst is suspended as small granules in thestream of hydrocarbon vapour (figure 4.19). In this fluidizedstate the catalyst may be regenerated on a continuous basisby passing it through a vessel in which the carbon depositsare 'burned off' using air at about 600 °e. The hot, cleancatalyst is then recycled back to the reactor.

reactantsin

l!! 100 100°C...•~ 200°C..

)(

'E 90E.:2 300°C~ 80':;C'

70III

.:••'c 600 400°CEE•• 50~III'0

40~500°C

30

20

10

26

CHEMICALENGINEERING

CHAPTER FIVE

Figure 5.1 Temperature/composition diagram for a mixture of liquidsAandB.

Thus a liquid of composition Xl boils at a temperatureT1 to give a vapour of composition Xz. Notice that thevapour produced is richer in the more volatile componentthan the original liquid mixture. Such temperature/compo-sition diagrams can be obtained by measuring the boilingpoint of various liquid mixtures and analysing the compo-sition of the vapour produced in each case.

Distillation is without doubt the most important of all theseparation techniques used in the chemical industry. It is amass transfer operation which has a firm quantitative basisand can be controlled to a very high degree. In this chaptersome of the important factors in distillation are investigatedin a semi-quantitative manner to determine the conditionsfor efficient separation.

5.1L1QUID/VAPOUR EQUILIBRIUMMost mixtures of liquids can be separated by distillation.This is possible if the liquid mixture and the vapour withwhich it is in equilibrium at its boiling point have differentcompositions. For an ideal mixture the difference incomposition between liquid and vapour may be predictedusing Raoult's Law. (See Topic 10.)

At a fixed pressure, the boiling point of a liquid mixturedepends on its composition. The liquid line shown in figure5.1 relates boiling point and composition for a mixture oftwo liquids, A and B. The composition of the vapour whichexists in equilibrium with each liquid mixture may also beshown on the same diagram and gives rise to a correspondingvapour line.

boiling pointpure B

~::J•..~'"a.E'"I-

o 20

100 80

40

60

IIIIIIIX2

60 80

40 20

boiling pointpure A

100 Mole % A

o Mole % B

Experiment 5.1To determine the temperaturecomposition diagram for the system:ethanoic acid/waterIn this experiment you are to determine theboiling point and the composition of thevapour produced for various mixtures ofethanoic acid and water. The vapourcomposition may be accurately determinedby condensing a sample and titrating the acidcontent with sodium hydroxide solution usingphenolphthalein indicator.

Caution. Ethanoic (acetic) acid is corrosiveand its vapour is unpleasant. It should onlybe used in a fume cupboard or a well-ventilated laboratory, and contact with theskin or eyes should be very carefully avoided.

Before the experiment, the thermometersto be used should be standardized. This isreadily achieved by suspending them in a largebeaker of distilled water which is heated untilit boils steadily. Any thermometer whichdoes not indicate 100 °C should be marked

with the appropriate correction factor.Equilibrium data for some ethanoic

acid/water mixtures is shown in figure 5.2.Aim to complete this table by experiment,each group in the class dealing with adifferent mixture.

a Assemble the apparatus as shown in figure5.3. To avoid errors due to superheating ofthe liquid, a short length of thin capillarytubing should be sealed at one end in aBunsen flame. Attach this sealed tube to the

Mole % ethanoicacid in liquid100.090.080.070.060.050.040.030.020.010.00.0

Mole % ethanoicacid in vapour100083.369.857.547.0

0.0

Temperature (boilingpoint);oC118.1113.8110.1107.5105.8

100.0

Figure 5.3 Apparatus to deterrnineliquid/vapour composition curve.

thermometer- 5 to 105 °C x 0.1 °C

Figure 5.2 Liquid/vapour equilibrium data for mixtures of ethanoicacid and water at atmospheric pressure.

anti-bumpinggranules

- - ----,---ceramicI centred

heat gauze

coolingwater

~

graduatedcollectingvessel

27

Mole % ethanoic acid

moles ethanoic acid X 100=-------------(moles ethanoic acid + moles water)

Share your class results to complete figure5.2 and plot a temperature/compositiondiagram for mixtures of ethanoic acid andwater showing both liquid and vapour lines.Your axes should be as shown in figure 5.4,with 100 % ethanoic acid on the lefthand side.

d Use the results to calculate the percentagecomposition of your distillate sample asfollows.

Calculate the number of moles and hencethe mass of ethanoic acid in your 1.00 gsample. Subtract to determine the mass andhence the number of moles of water presentin the sample. Calculate the mole percentageof ethanoic acid present using the expression:

thelmometer, open end downwards, using arubber or plastic ring as shown. Thethermometer bulb should be positioned inthe liquid, not the vapour. (Why?).

b Put about 50 cm3 of the mixture in theflask with some fresh porous pot or anti-bumping granules. Ensure that the still-headand upper portion of the flask are well laggedto prevent fractionation within the apparatus.Bring the mixture to the boil, heating stronglyat first but gently as the boiling point isapproached. As the liquid heats up thereshould be a slow escape of bubbles from theopen end of the capillary tube.

When the boiling point is reached, a rapidstream of bubbles will begin to emerge fromthe tube. Remove the source of heat andwatch the stream of bubbles carefully. Recordthe thermometer reading as the last bubbleemerges just before the liquid slicks back upthe capillary tube. This is the boiling point ofthe liquid mixture.

c Reboil the mixture steadily and collect thefirst 1.5 cm3 of distillate in a suitable clean,dry receiver. The composition of this distillatemay now be analysed by titration. Using a drydropping pipette carefully transfer exactly1.00 g of your distillate sample into a dryconical flask placed on a top-pan balance(accurate to 0.01 g). Add 20 cm3 of distilledwater and 3 drops of phenolphthaleinindicator to the contents of the flask, thenaccurately titrate with 0.50M sodiumhydroxide solution from a burette. Theend-point is when the first permanent pinkcolour appears in the solution.

uo-..,:;•..

III

tCoE.,I-

a100

Figure 5.4

50

50

100 Mole % water

a Mole %ethanoic acid

What information can temperature/composition diagrams give?Use your graph to answer the followingquestions.

Q1At what temperature does a mixturecontaining 50 mole % ethanoic acid boil?

Q2What is the composition of the vapourobtained when this mixture boils?

Q3What is the highest quality distillate obtainablefrom a single simple distillation of a mixturecontaining 50 mole % ethanoic acid?

Q4If this first distillate were placed in a seconddistillation apparatus and heated until itdistilled, what would be the composition ofthe second distillate?

Q5If this second distillate were placed in a flaskand distilled, what would be the new distillatecomposition?

Q6How many such simple distillations would berequired to give a mixture containing 90 mole% water (i.e. only 10 mole % ethanoic acid)?

Q7How many such distillations would be requiredto produce pure water as the distillate?

flowing down it means that a fractionating column givesthe same overall effect as a number of successive simpledistillations.

Bubble caps are expensive on a laboratory scale, and thesame effect may be obtained using a column packed withglass beads or rings. This provides a large area of wet surfacefor vapour and liquid to approach equilibrium at all heightsin the column.

Figure 5.5 A plate and bubble cap column.

1

vapour

vapourliquid

liquid5.2FRACTIONATING COLUMNSWe have seen that simple distillation produces a distillatewhich is richer in the more volatile component than theoriginal mixture. Successive simple distillations bring aboutfurther separation but such a procedure would be veryinefficient to operate. A fractionating column is a devicewhich accomplishes in one operation the equivalent of manysuccessive simple distillations.

The easiest to understand is probably a bubble-capcolumn as used in some large-scale distillations (see figure5.5). Vapour boiled off from the liquid in the kettle (theindustrial equivalent of a flask) passes up the column andcondenses on the first plate. This condensate is of the samecomposition as the rising vapour and thus one distillationstage has been completed.

The bubble caps force following vapour to bubblethrough the condensed liquid on the plate and the heat fromthis vapour causes the liquid to boil. This gives off a newvapour of a composition even richer in the more volatilecomponent which condenses on the second plate, completinga second distillation stage. In this simple treatment eachdistillation stage or 'step' on the temperature/compositiondiagram corresponds to one theoretical plate in a fractionatingcolumn.

In practice no plate performs as efficiently as this. Eachplate receives liquid from the plate above by means of anoverflow weir, and thus the composition of liquid on a plateis not identical to the vapour rising from the plate below.Nevertheless, the interchange of components (mass transfer)between the vapour rising up the column and the liquid

28

the column. This is known as reflux. The ratio of reflux flowrate to distillate flow rate is known as the reflux ratio.

In fractional distillations which you may have conductedpreviously, almost all of the vapour which reaches the top ofthe column has passed into a side-arm condenser and beenremoved from the system. However, it is normal industrialpractice to return some of the condensed vapour back down

Reflux ratio = reflux rate

distillate rate

Figure 5.8 Fractional distillation column.

watercondenser

the thermometer you have previouslystandardized at the top of the column. In theflask put a mixture of 1 mole of ethanol and1 mole of propanone with a few pieces offresh porous pot to prevent superheating.Heat the water bath strongly at first, butmore gently when it reaches 60 to 70°C.When the bath reaches approximately 80 °c,distillation will begin. Observe the counter-flow in the column with vapour going up and

Volume ofdistil/ate/cm3

Vapourcomposition/mole per centpropanone

liquid running down. Good contact betweenthese two flows is an essential feature offractionation.

b After observing the column in action,insulate it using cut lengths of domestic pipeinsulation. Extend the insulation to the refluxhead, but leave a 'window' to allow the refluxdrip rate to be counted. Once insulation iscomplete, allow conditions to become steadyunder total reflux (do not remove anydistillate). The boil-up rate should be briskbut not so fast that 'flooding' occurs in thecolumn. This happens when the condensate isprevented from flowing down the column byexcessive vapour flow up the column. Only asmall flame is needed under the water bath.When steady conditions have been establishedfor a few minutes, record the kettle andvapour temperature at total reflux.

c Now set the reflux ratio to your desiredvalue by opening the still-head tap slightlyand counting the number of drops of distillateand the number of drops of reflux during a30-second interval. If necessary, adjust the tapand recount the drop rates until you are closeto your target ratio (this should not takelonger than 5 minutes).

Collect the distillate in a graduatedcontainer, recording the kettle temperatureand the vapour temperature in the still-headwhen 5,10,15,20, and 25 cm3 of distillatehave been collected. Check the reflux ratioperiodically. If it has changed, it may bebecause the kettle is no longer boiling asvigorously as before. Before shutting down,turn the reflux head tap off to give totalreflux again. Observe the effect on the vapourtemperature.

d Compile your results as in figure 5.9.Use the liquid line on your temperaturecomposition diagram to determine thecomposition of liquid in the kettle from itsboiling temperature. Similarly, use the vapourline to determine the composition of thevapour which is condensed to producedistillate at the top of the column.

Construct a graph showing composition(0 to 100 % propanone) on the vertical axisand volume of distillate (0 to 25 cm 3) on thehorizontal axis. Use your results to plot a lineshowing the variation of distillate compositionwith volume of distillate collected. On thesame graph plot the results obtained by othergroups using different column conditions.

Use the results obtained to answer thefollowing questions.

VapourtemperaturerC

graduatedcollecting

vessel

still-head tap(distillate

rate)

Kettlecomposition/mole per centpropanone

KettletemperaturerC

fractionatingcolumnpacked withglass beads

windowto observedrop rate(reflux rate)

Refluxratio

Reflux Column Packingratio height type

Group A 1:1 25 cm Glass beadsGroup B 5:1 25 cm Glass beadsGroup C 1:1 IOcm Glass beadsGroup D 1:1 25cm Glass rods

Mole % Mole % Temperaturepropanone propanone /oCin liquid in vapour

0.0 0.0 78.310.0 26.2 73.020.0 41.7 69.030.0 52.4 65.9

40.0 60.5 63.650.0 67.4 61.860.0 73.9 60.470.0 80.2 59.180.0 86.5 58.090.0 92.9 57.0

100.0 100.0 56.1

Figure 5.6 Liquid/vapour equilibrium datafor mixtures of propanone and ethanol atstandard atmospheric pressure.

Experiment 5.2To investigate the factors whichinfluence the effectiveness of a fractionaldistillation columnIn this experiment you will investigate theeffect of reflux ratio, column height, andcolumn packing on the performance of afractionating column used for batchdistillation.

It is possible to use the ethanoic acid/water system studied in the previousexperiment, but more interesting results areobtained using the system: ethanol/propanone.Equilibrium data for this system is given infigure 5.6. Use this data to plot thetemperature/composition diagram, drawingboth liquid and vapour lines as accurately asyou can, with a flexible curve if available.

Other variations of these factors may bestudied if there are more than four groups inthe class.

Use your graph to determine the numberof theoretical plates (i.e. distillation stages)necessary to obtain a distillate containing95 mole % propanone starting from a 50 mole% mixture. Carefully draw the appropriatesteps on your graph, using a sharp pencil andruler. You will use this diagram later tointerpret your experimental results.

Each group in the class should study adifferent set of conditions. (See figure 5.7.)

Caution. Mixtures of ethanol and propanoneare highly flammable and care must be takento ensure novapours escape into the laboratory.

Figure 5. 7

a Assemble the apparatus as shown in figure5.8, but without the insulation at first. Use

Figure 5.9

29

Q8What effect does column length have on thequality of distillate obtainable from a givenkettle liquid composition?

Q9What effect does the surface area of thecolumn packing have on the quality of thedistillate obtainable?

Q10What effect does reflux ratio have on thequality of the distillate obtainable?

QllWhat conditions would you choose to achievethe most efficient separation of a mixture?

Q12How does reflux ratio affect the rateof distillation? Assuming all experimentswere carried out at approximately the sameboil-up rate, you may compare the timeeach group took to collect 25 cm 3 distillate.

Q13In chemical manufacture, a certain quantityof material of a given target quality mustbe produced in a fixed time. What differenceswould there be between distillation columnsoperating at high and low reflux ratios?

Q14What is the effect on the composition of thekettle liquid when distillate is removed?

Q15What is the effect on distillate composition ofcontinuously removing distillate from the topof the column?

Q16If a product of fixed quality is required,what changes in conditions, made during thecourse of a batch distillation, would enablea distillate of constant composition to beobtained?

5.3THE CONDITIONS FOR CONTINUOUS DISTillATIONA fractional distillation column designed for continuousoperation is shown diagrammatically in figure 5.10. Themixture to be distilled is fed into the system at a steady rate,and product is continuously removed both at the top andbottom of the column. Heat input is either by pre-heatingthe feed or re-boiling the bottom residue, using a suitable heatexchanger.

The less volatile component is removed as liquid frombelow the bottom plate and the more volatile component isremoved as vapour from the top plate. The boiling liquid on

reflux condenser

vapour from top plate

rectificationsection

each intermediate plate becomes progressively richer in themore volatile component as the column is ascended. Thus,there is a corresponding temperature gradient within thecolumn.

The feed must be introduced into the side of the columnat a height where its composition corresponds to thecomposition of the liquid on the plate. In this way, thesteady state within the column is not disturbed. The part ofthe column above the feed point is called the rectificationsection. In this section the feed is concentrated to the desireddistillate quality. Below the fee.d point is the strippingsection where the more volatile component is progressivelystripped out until an acceptable lower limit of concentrationis obtained. The residue is removed as a liquid. In manyprocesses, the liquid residue is as saleable as the distillate, andsufficient plates are present in the stripping section to bringthe residue liquid up to the customers' product specificationfor purity.

Batch and continuous fractionating columns operate bythe same mechanism but have one major difference. In abatch still, the conditions gradually change with time becausematerial is being removed as distillate from a fixed quantityof liquid being distilled. In a continuous still, the conditionsremain steady because a steady feed of uniform compositioncompensates for the removal of distillate and residue.

feed••

strippingsection

steamheilting coil residue

5.4COLUMN EFFICIENCYThe internal structure of a fractionating column must bedesigned to bring ascending vapour and descending liquidinto intimate contact, so that mass transfer of the compon-ents may readily occur. Both packed columns and platecolumns are used in industry .

Packed columns consist of a hollow shell filled with alarge number of specially shaped rings made from ceramic,glass, metal, or plastic. Some common examples are shown infigure 5.12 on the next page.

Plate columns contain trays on which liquid rests andthrough which ascending vapour rises. Traditionally, thesehave been constructed with bubble caps, but the high cost of

Figure 5.10 Arrangement for continuous fractional distillation.

Q17The relative efficiency of different packingsmay be compared using a term known as the'height equivalent to a theoretical plate'(HETP.)

30

HETP = = Total height of packingNumber of theoretical plates

Use this expression to calculate theHETP values for the packings used in your

experiment. The number of theoretical platesshould be determined from the steps on yourtemperature/composition diagram, using theinitial vapour composition obtained at totalreflux.

valve tray

bubble-captray

sieve tray

vapouroverflowweir

'downcomer'pipe

The capital cost is high for very low reflux ratios, since alarge number of plates would be required. The capital cost isalso high for very high reflux ratios, since a large plate areawould be required to produce the desired flow rate of product.

The running costs increase with increasing reflux ratio,since a higher proportion of liquid is flowing back down thecolumn and the distillation is slower. At total reflux therunning costs are infinitely large, since no product at all isobtained. This is shown in figure 5.14.

The sum of these two curves gives the total cost, and thevalue of the reflux ratio which corresponds to the minimumtotal cost is the optimum reflux ratio.

Figure 5.13 Three types of tray commonly used in distillationcolumns. All the trays in one column are normally of the same type.

) )) ) )

Pall ringsLessing ring

Figure 5.11 Industrial distillation columns.

Raschig ring )

Figure 5.14 The cost of operating a distillation column.

5.6MUlTI·COMPONENT DISTillATIONThe discussion so far has concentrated on the use ofdistillation to separate binary mixtures, i.e. those containingtwo components only. Distillation may also be used toseparate mixtures containing several components intodifferent boiling ranges, although the theory of multi·component distillation is much more complex.

The primary distillation of crude oil is an importantexample. The oil is pre-heated in a furnace, then fed contin-uously into a bubble-cap fractionating column. The liquidcontents of each plate represent a boiling range of the mixture,and liquid is removed continuously from several of the platesin the column. Each/raction obtained in this way has its owncharacteristics and will be further processed and sold aspetrol, paraffin (kerosine), fuel oil, and so on.

Figure 5.12 Some common types of tower packing.

these has resulted in their replacement, for many applications,by modem devices such as valve trays and sieve trays (figure5.13). In their simplest form, sieve trays consist of steelplates drilled with holes. The liquid on each plate is preventedfrom flowing down through these holes only by the upwardflow of vapour from below. Consequently, precise control ofconditions within the column is essential. With valve traysthese holes are closed off if the vapour flow rate falls.

Plate columns have the main advantages that they cancope with a wide range of conditions (including liquids whichfoam), are readily cleaned, and enable side-streams to beremoved at intermediate points in the column if desired.Packed columns are generally cheaper to construct for smalldiameters (less than 1 m) and have superior corrosionresistance since inert ceramic packing materials may be used.

The main disadvantages of packed columns are thatliquid tends to flow down the walls of the tower instead ofthrough the packing, and for large columns the sheer weightof packing may impose severe structural loads.

5.5ECONOMICS AND OPTIMIZATIONThe cost of operating a distillation column can be brokendown into two principal parts: the capital cost of buildingthe column, and the cost of running it once it has started tofunction. These depend upon the number of plates, and uponthe reflux ratio.

•..:sCJ

total cost

capital cost

Reflux ratio

31

CHEMICALENGINEERING

CHAPTER SIX

sales development

project committee

Thus, while a small item of equipment may operate in asatisfactory manner, the effect of. doubling its size may bedisastrous unless the consequences of scaling up are antici-pated and compensated for. How would you solve theproblem in the above example?

If a process still appears viable after laboratory tests, thenext step is generally the construction and running of a small·scale pilot plant. Proceeding to the pilot plant stage involvesa considerable increase in expenditure on the project. Thisimportant decision is often taken by a project committeeformed from representatives of all the company departmentsconcerned. The pilot plant is operated on a continuous basis

II

I---2 metres -----lvolume 8 m3

surface area 24 m2

r1 metre,

volume 1 m3

surface area 6 m2

Figure 6.2 The effects of scaling up.

The preliminary investigation involves an examination ofthe scientific and economic soundness of each of the possibleroutes by which the product can be manufactured. Therequirements of each process, such as raw materials, equip-ment, heat, and electricity are listed together with the pre-dicted yields of products and by-products. This informationis used to prepare a mass balance for the material flow and anenergy balance for the energy flow within the proposed plant.Thus an estimate of the capital and running costs of a processemerges; this usually enables the best production route to beselected.

If the preliminary investigation establishes the existenceof a viable production route, laboratory tests are carried outin order to gather as much information as possible about thechemistry of the process. Kinetic and equilibrium data for thereactions will be established, possible by-products will beidentified, and the effects of scaling up will be investigated.

The consequences of scaling up are of critical importancein process development. Consider the simple case or"a cubicstirred tank reactor 1 m wide in which an exothermic reactionis carried out (figure 6.2). The volume of the reactor contentsis 1 m3 and the surface area of the reactor through whichheat may be lost is 6 m2 • Now suppose a similar reactor 2 mwide is to be constructed. The volume of the reactor contentswill now be 2 x 2 x 2 = 8 m3 , with a surface area of 24 m2

•

Scale-up has increased the volume of the reaction mixturewhich is producing heat by a factor of eight. However, thesurface area of the reactor through which this heat canescape has only increased by a factor of four. Clearly, thescaled-up reactor will operate at a higher temperature and,since this will increase the rate of reaction and hence the rate ofheat evolution, a temperature 'runaway' becomes a realpossibility .

engineeringand construction

full-scale Jproduction

~X~

project committee project committee

j cost estilate from Ifirst flowsheet

trials of pilot plant pilot plantlproduct I I

I . f IL --.costestlmate rom .•.. _-.J

second flowsheet

Idirectors directors directors directors

I

-------idea X -----

6.1EXAMINING A NEW PROJECTAll new industrial processes begin with an 'idea' which mayinvolve an entirely new product or a better way of making anexisting product. Many ideas originate from pure researchcarried out in the laboratories of chemical companies, univer-sities, and government establishments. Others stem fromexperience gained during industrial production and marketing,which can lead to an awareness of the need for a new ormodified product.

Once a production ide~ has been established, it is usuallycarefully examined in three stages:a preliminary investigationb laboratory testsc pilot plant trials.

market research preliminary laboratory

L __ .. rough costing from massi:Jstilgations

and energy balances

laboratoryscale tests

technical services(helping customers to use the product tobest advantage)

Figure 6.1 The development of a process.

How does a large, complex, expensive industrial plant comeinto being? In this section we shall consider how a companydevelops a new idea, from preliminary work in the laboratory,to a pilot plant producing small amounts of product, andfinally to the design and construction of a full-scale plant.

32

Figure 6.3 A pilot plant.

and usually gives much valuable information on the effects ofscale-up, engineering design of equipment, materials of cons-truction, and corrosion problems. The small amounts ofmaterial produced may be used by the sales department for apreliminary market evaluation of the product.

The information gathered during' pilot plant operationenables a pictorial flowsheet to be drawn up for the full-scaleplant. A typical example is shown in figure 6.5. This showsthe major items of the plant and gives the principal technical

information from which detailed engineering drawings areprepared and capital costs are estimated.

Considerable thought must be given to the materials ofconstruction, for excessive corrosion not only causes theplant to wear out rapidly but may contaminate or discolourthe product. The uses and limitations of some common fab-rication materials are given in figure 6.4 below.

The pictorial flowsheet also enables an estimate to bemade of the day-to-day running costs of the plant, includingconsumption of steam, electricity, and water, maintenancecosts, cost of labour, and so on. The sum of the capital andrunning costs gives the amount of money needed to make theproduct.

A predicted value of sales is made by the sales depart-ment and this, minus the cost of making and marketing theproduct, represents the potential profit. If the project provesto be a borderline case, the flowsheet can be used to identifythe main items of cost and further work can be concentratedon trying to reduce these.

Ultimately, the decision whether or not to build theplant is made by the company directors who sift the evidencefor and against a particular project. They obtain this evidencefirstly from the various departments within the company,secondly from their inside information concerning thecompany's financial position, and thirdly from theirknowledge of industry in general, developments in Worldtrade, and political and economic trends at home.

6.2BUILDING A PLANTOnce the decision to go ahead has been made, the buildingprogramme is planned in great detail so that items of equip-ment are available when required and costly delays areavoided. It is usual to prepare a schedule for each section ofthe plant, showing the times of ordering, the expected deliverydate, and the dates when erection will be started and com-pleted. This enables the whole of the work to be coordinated.

MaterialMild steel

Stainless steel

Aluminium

Copper

Glass

Synthetic polymers

Advantagescheap;good mechanical strength;easily fabricated;resistant to most organic liquids and dilute alkalis;widely used for general purpose construction

good corrosion resistance under oxidizing conditions;withstands nitric and organic acids

significantly lighter than steel;easily fabricated;good corrosion resistance to organic acids, nitricacid, and nitrates

easily fabricated;resistant to acids and alkalis

relatively cheap;transparent;resists corrosion by almost all chemicals includingbromine;may be used to line vessels and pipes

very light;often inexpensive and easily fabricatedgood resistance to corrosion by inorganic chemicals;may be used to line steel vessels

Disadvantagesreadily corroded by dilute acids and moisture inatmosphere

several times more expensive than mild steel;corroded by acids under reducing conditions, andby solutions containing chloride ions

relatively expensive;corroded by alkalis and halogen acids

very expensive;corroded by ammonia and amines;may discolour certain products

susceptible to mechanical and thermal shock;corroded by hydrofluoric acid

may soften and melt at moderate temperatures;often corroded by organic solvents

Figure 6.4 Some common materials of construction.

33

Raw materials

liquid ammonia

Item no.

i.e. 40 000 T /year avg

Description

11 200 ton nes/year

power

96 % plant efficiency

34 tonnes/day

24 hours/day

330 days/year

rate or capacity

of HNO, as 60 %

& 130 max

Output running

no. off

at 120 T Iday avg

cooling water

pressure

temperature

material ofconstruction

steam

dimensions

analysis

drawn by: A. Smithdate: 1.4.84

capacity2 X 15 tonnesHNO; as60%

27 28 29 30

Feed water Feed water Condensate Acid(to absorber) injection tank tankscooler pump

1W1S 2

12 m2 1.6 m high 7.5 m dia.su rface 1.7 m dia. X9m

mild steel cast iron mild steel 18/8/titaniumstainless steel

336000 1Y, to 4 m' 3 m' 25 tonneskJ/hr /hr max. 60 % acid

80-30°C 80°C 80 °c 25°CC.W. 26°C

150 m head 1 atm 1 atm

condensate condensate 60 % HNO,

4kW

15m3/hr

~---!: 0

CD I ;;--~

air

+ + +,Item no. 1 2 3

Ammonia Ammonia Air filtertrip valve pressure

controller

1 1 1

280 m2

mild steel mild steel cloth bagsm.s. case

2000 2000 16000m3/hr m3/hr m3/hr

atmos atmos atmos

1 atm 1 atm 1 atm

NH3 gas NH, gas air

- - -

- - -

- - -

solenoid to maintainoperated constant

NH3 gaspressure

PICTORIAL FLOWSHEET

Figure 6.5 Part of a pictorial fJowsheet for design data.

Progress is constantly monitored, and any delays minimizedby the rescheduling of other operations. The work can thenproceed as smoothly as possible: first the site work anddrainage, then the foundations, then the steelwork, buildings,vessels, machines, and pipes, and finally the motors andinstruments. A few months' delay in production may costhundreds of thousands of pounds, for while a plant is underconstruction interest charges are being paid on the capitalinvestment without any return being made.

Once construction is complete, the plant is ready for'start-up'. Whenever possible, items of equipment are firstrun under minimum load conditions: the pumps and tanksare tested with water and the blowers and fans are testedwith air. This is to ensure that the plant is correctly assembledand will run as intended when the chemical materials, whichmay be poisonous or corrosive, are introduced.

Serious problems during start-up are not uncommon.There are few operations which are not affected in one way

or another by scaling up, and it may take weeks or evenmonths to commission a complex new plant.

Even when the plant is running continuously it will bekept under constant review, both to improve the efficiencyof the process and to make any product modification necess-ary to suit customer requirements.

Meanwhile, research continues to develop new productsand processes in order to ensure the future survival of thecompany.

6.3MASS BALANCESThe preparation of mass and energy balances is an essentialfeature of almost all chemical engineering design where flowsof material are involved.

Mass balances are based on the principle (sometimescalled the Law of Conservation of Matter) that during physicaland chemical changes matter is neither created nor destroyed.

34

Ethanol Water Stream Ethanol column Ethanol Water Stream/kg h(l /kg hr-1 totals /kg hr-1 /kg h(l totals

Stream 240 5 45I

50 .75 125 Stream 1 I JlI Stream 3

10 70 80,

50 75 125 Component totals/kg hr-1 50 75 125

We can say that during a given time interval:

Figure 6.6 Mass balance of distillation column seRarating ethanoland water.

Consider a system which is exchanging matter with itssurroundings:

-+ (CH3COhO [6.2]ethanoicanhydride

CH3C02Hethanoicacid

+

Propanone at atmospheric pressure is vaporized and fedinto a tubular reactor which is heated to between 650 and800°C; thermal cracking occurs as shown in equation [6.1] .

Unfortunately, as is often the case, other undesirablereactions also occur such as the two competing reactions:

mass out)

mass in ~-----~)'~

mass in = mass out + accumulation within the system[6.3]

If the system is operating in the 'steady state', then there isno accumulation of material within the system, and thisexpression simplifies it to:

(CH3hCOpropanone

3H2 + CO + 2C [6.4]

mass in per unit time = mass out per unit time

If the operations carried out within the system do notinvolve chemical change but are of a physical nature only,such as distillation or solvent extraction, then a simpfe massbalance may also be drawn up for each component withinthe system. A mass balance for a continuous distillationcolumn separating ethanol and water is shown diagrammati-cally in figure 6.6. Note that:

mass of ethanol in per hour = mass of ethanol out per hour

However, if the materials undergo a chemical changewhile within the system (e.g. in a reactor) then the masses ofthe individual components will change as they pass throughthe system. Thus a component balance will need to take intoaccount the kinetics of the reaction (see page 18). However,there will still be no change in the total mass of the materialsand an overall mass balance may still be applied. (Note thatwhilst the total mass always remains constant, the totalnumber of moles may not. Why?)

To illustrate these ideas, let us consider a mass balancefor a proposed chemical plant designed to produce 2600 kgof ethanoic anhydride per hour. A partially completedflowsheet for such a process is shown in figure 6.7. Thisrepresents an accounting system for all the flows within theplant. In this section all material flows are measured in kgper hour.

The starting materials for this production route arepropanone and ethanoic acid, and the main reactions involvedare:

This gives rise to a variety of substances in the processstream which must be separated from the anhydride product.The gases which leave the reactor are cooled very quickly in aquench unit by mixing them with a mixture of ethailoicanhydride and ethanoic acid (see the flowsheet in figure 6.7).The gaseous mixture then passes to a packed tower in whichthe same mixture of acid and anhydride is used to cool thehot gases further.

The vapour stream then passes to a shell and tubecondenser, where 90 % of the ketene in the reactor effluentreacts with ethanoic acid to form ethanoic anhydride as inequation [6.2] . The remainder of the gas passes to an absorp-tion unit where most of the residual ketene is absorbed in(and reacts with) recycled ethanoic acid. The liquid streamsfrom the condenser and the absorption unit are both mixedin a crude product storage vessel.

The crude product is then fed to a distillation columnwhere essentially pure propanone is recovered as an 'over-heads' product. The 'bottoms' product from the propanonestill then passes to another still where ethanoic acid isrecovered overhead and recycled to the absorber and quenchunit. The ethanoic anhydride product, as 'bottoms' from theanhydride still, then passes to storage through a cooler.

Details of streams 12 and 15 are not yet shown on theflowsheet and must be calculated from mass balances. Thecomposition of stream 12 may be obtained from a massbalance on the propanone still.

mass in per hour = mass out per hour

stream 10 = stream 11 + stream 12 + stream 13

or

(CH3hCOpropanone

thermal)cracking

CH4 +methane

[6.1]stream 12 = stream 10 - stream 11 - stream 13

Since no chemical change occurs, this is true not only for thetotal streams but also for each component present.

35

Thus:

For propanone:stream 12 = 6641 - 6621 - 6

For ethanoic acid:stream 12 = 3786 - 1 - 1149

= 14 kg hr-1

= 2636 kg hr-1

For ethanoic anhydride:stream 12 = 3786 ~ () - 1151 = 2635 kg hr-1

Check total:stream 12 = 14213 - 6622 - 2306 5285 kg hr-1

16

Figure 6. 7 Flowsheet for the manufacture of enthanoic anhydride.

15 onhydride----,1'~I product

14

13

obsorber

5

crude product storage

2

quench unit

propanonefood

1 2 3 4 S 6 7 8Component Propanone Ethanoic Feed to Cracker Total Liquid pro- Feed to Off-gas

feed acid cracker products product duct from absorber frommake-up ex qup.nch condenser absorber

Propanone 2245 8866 6650 6650 5670 980 9

Ethanoic acid 1842 1 1600 32 142 142

Anhydride 1152 3515 71 71

Ketene 1120 1120 112 1

Methane 599 599 600 600

Unsatu rates 176 176 176 176

Carbon monoxide 296 296 296 296

Carbon dioxide 22 22 22 22

Hydrogen 4 4 4 4

Total kg hr-1 2245 1842 8867 8867 11619 9217 2403 1321

9 10 11 12 13 14 15Component Liquor ex Feed to Propanone Anhydride Anhydride Ethanoic Anhydride

absorber propanone recycle still feed recycle acid productstill recycle

Propanone 971 6641 6621 6 14Ethanoic acid 3753 3786 1 1149 2495Anhydride 269 3786 1151 9Ketene

Methane

Unsaturates

Carbon monoxide

Carbon dioxide

Hydrogen

Total kg h(' 4993 14213 6622 " 2306 2518

36

Q1Using the composition for stream 12justderived, calculate the composition of stream15 by means of a mass balance on theanhydride still. State:a whether the plant will achieve itsproduction target of 2600 kg hr-I of ethanoicanhydride

b the percentage purity of the product bymass.

Q2If the product specification demands 99 %pure ethanoic anhydride, then a more efficientanhydride still will be required. Assuming nochange in the number of kg hr-1 of ethanoicanhydride in the product stream, calculate:

a the new composition of stream 15b the composition of stream 14 from thenew still.

Q3By performing a mass balance over theabsorber, calculate the flow rate of stream 16,assuming it to be pure ethanoic acid.

energy input = energy output + energy accumulation

For a given system during a fixed time interval:

As for mass balances, during steady state operation theaccumulation term is zero.

Energy may enter and leave a chemical system in manyforms, such as kinetic, potential, electrical, and heat energy.However, in the simple example which follows, only heatenergy is involved.

Heat content of input cooling water= 8000 x 4.2 x 15= 504 000 kJ hr-1

Heat content of output cooling water= 8000 x 4.2 x 30= 1 008 000 kJ hr-1

By applying a heat balance over the heat exchanger, itis possible to calculate the exit temperature of the ethanoicanhydride. This may be summarized on a diagram such asfigure 6.8.

\For example, if the specific heat capacity of ethanoicanhydride is taken as 2.0 kJ kg-1 K-1, then the heat contentofthe input stream is 2600 X 2.0 x 140 = 728 000 kJ hr-1

•

This ethanoic anhydride is to be cooled using river waterat 15 DC, with a maximum discharge temperature of 30 DC.Suppose the optimum water flow rate through the heatexchanger is 8000 kg hr-1 •

Specific heat capacity of water = 4.2 kJ kfl K-1

energy outputenergy input

6.4ENERGY BALANCESJust as mass balances provide an accounting system for theflow rate of material through a chemical plant, so energybalances enable chemical engineers to predict the energytransfer requirements at each stage. The high cost of energydemands great efficiency in this area, and every effort mustbe made to ensure that surplus energy from one section ofthe plant will be used elsewhere with a minimum of waste.

Example: Heat balance over the product coolerConsider a heat exchanger designed to cool 2600 kg hr-1 ofethanoic anhydride, initially at 140 DC. It is common practiceto calculate the heat content of a process stream relative to adatum temperature at which the materials are said to possesszero heat content. (273 K is often used by chemicalengineers.) The heat content of a stream above this tempera-ture may be calculated using the expression:

Total heat input = total heat output

728 000 + 504 000 = 1 008 000 + heat content ofanhydride exit stream

heat content of anhydride exit stream224 000 kJ hr-1

Let the temperature of the anhydride exit stream be t °cHeat content =mass flow X specific heat capacity X temperature above datum

Figure 6.8 Heat balance over anhydride product cooler •

224 000 = 2600 x 2.0 x t

.'. t 43 DC

.

IN Anhydride product cooler OUT

Material Flow Temperature Heat content Material Flow Temperature Heat content/kg hr-I I"c /kJ hr-I /kg hr-1 /0 C /kJ hr-I

Coolingwater 8000 15 504000 ~

I-Ethanoic

Ethanoic anhydride 2600anhydride 2600 140 728 000 I--

I Coolingwater 8000 30 1 008 000

Input 1 232 000 Total Output 1 232 000

Q4Calculate the ethanoic anhydride exittemperature in the above example if:

a the maximum water exit temperature isonly 25°C (assuming unchanged flow rates)b the cooling water flow rate is 9000 kg hr-'(assuming unchanged water temperatures).

Q5Construct a heat balance table similar tofigure 6.8, using your results from the heatexchanger experiment (4.2).

37

CHEMICALENGINEERING

REVIEW QUESTIONS

At 20°C the value of the rate constant k is 0.11 min-1 •

2CH3COzH(l)ethanoic acid

(CH3C010(l)ethanoic anhydride

If the reaction is carried out with a large excess of water, thekinetics are pseudo first order with respect to ethanoicanhydride.

Q1Ethanoic anhydride reacts with water as follows:

Process controlSpecial safety precautions necessaryDisposal of waste productsGeographical location of plant, with reasonsMarket value of productPrincipal uses of product.

It is unlikely that you will be able to obtain informationabout every one of these aspects of your chosen chemicalprocess. Alternatively, additional information may beavailable which is not included in the list. The importantpoint is that you should take this opportunity to conductyour own investigation into the chemical engineeringprinciples on which a modern manufacturing process is based.

Your teacher should be able to suggest sources ofinformation to you and give some additional guidance on theform your written report is to take.

a If the initial concentration of ethanoic anhydride is1 mol dm-3 , calculate the residence time required to achieve75 % conversion at 20°C in a batch reactor vessel of capacityIOdm3•

b The reaction in part a is to be carried out in a continuousstirred tank reactor, also of 10 dm3 capacity. Use the designequation for a CSTR to calculate the mean residence time(7) required to give 75 % conversion in this type of reactor.c Explain why the reaction proceeds more slowly in theCSTR than in the batch reactor. What are the advantagesof the CSTR which make it a more attractive propositionfor large·scale production?d Calculate the flow rate of reactants through the CSTRin part b.e When estimating the throughput of a batch reactor,allowance must be made for the 'shut·down time' betweenbatches. Calculate the maximum shut-down time whichwould still allow the batch reactor in part a to equal thethroughput of the CSTR in part b.

A CASE STUDYIn this Special Study we have examined some of the problemsencountered when large quantities of materials have to bechemically reacted or physically separated on a continuousbasis, and we have looked at the ways in which chemicalengineers have tackled these problems.

The object of the Case Study is to give you an oppor-tunity to investigate further the applications of these ideas.You are expected to write a report on a chemical manufac-turing process. This report may be based upon a visit to achemical plant in your vicinity or can be prepared solelyfrom the material produced by several of the larger companiesabout the chemical processes which they operate.

Whatever your sources of information, you should try totreat the chemical process which you choose to study as aseries of 'unit operations' , and use the principles developed inthis book to describe the scientific basis underlying each stepin the process.

The style and presentation of your report will obviouslydepend upon the nature of the process which you choose toinvestigate. However, a typical report should contain someconsideration of most of the following points.

GeneralProcess energy requirementsTypes of pumps and other ancillary equipment necessaryInstrumentation and measuring devices used

Synthesis stageChemical changes to be performedKinetic and equilibrium considerationsPossible side reactions and by-productsReactor type and conditions used, with reasonsUse of catalystsReactor design, dimensions, flow rates, special featuresMaterials of construction.

Separation stageSubstances present after reactionTypes of separation operations used, with reasonsSeparation equipment details, including materials of cons·tructionHeat transfer operations during separation.

OverallProcess outline including flowschemeMode of operation: batch or continuousAlternative processes available to manufacture productAdvantages of chosen manufacturing route, includingeconomic considerations.

Raw materialsSources of raw materials and approximate cost per tonneMethod of transport of raw materials to siteStorage capacity for raw materialsSeparation/purification operations before reactionHeat transfer operations before reaction.

38

Q2Ethene may be manufactured by 'the thennal 'cracking' ofethane gas at temperatures in the region of 900 DC.

Using this information, and by considering the characteristicsof batch, CSTR, and tubular reactors, decide which typeof reactor you would recommend for the large-scale manu-facture of ethane·l,2-diol. Explain how you arrive at youranswer.

Q4Temperature/composition data for mixtures of methanol andwater at standard atmospheric pressure is given in the tablebelow.

a Use this data to construct an accurate temperature/composition diagram for mixtures of methanol and water,showing both the liquid and vapour curves.b If a mixture containing 25 mole per cent methanol isheated, what is the composition of the vapour obtained fromthe liquid as it boils?c If this vapour is condensed and redistilled, what is the com-position of the liquid obtained after two simple distillations?d How many simple distillation stages are required toproduce a liquid containing 95 % methanol?e If a packed column of heightl5 cm is required to bringabout the degree of separation in part d, what is the 'heightequivalent to a theoretical plate', (HETP) of the columnpacking?f During a batch distillation, the percentage of methanolin the distillate tends to fall as the distillation proceeds. Whatcauses this effect? What adjustments can be made to maintainthe quality of product during such a batch distillation?

Studies have shown that this reaction is first order withrespect to ethane. Rate = k[C2H6]. The value of the rateconstant k is 770 min-1- at 900 DC.The reaction is carried outin a tubular reactor and, because such cracking is highlyendothermic, a constant temperature of 900 DCis maintainedby heating the outside of the reactor tube strongly(isothermal operation).

Ignoring any changes in volume, use the design equation for acontinuous tubular reactor to calculate:a the residence time required in the reactor for 50 % con·version of ethane to ethene;b the maximum volumetric flow rate of ethane into areactor tube of length 65 m and internal radius 0.1 m, for50 % conversion.

Q3Ethane-l,2-diol (ethylene glycol) is an important industrialchemical used· in the production of polyester fibres and anti·freeze. It is manufactured by reacting the liquid epoxyethane(ethylene oxide) with water.

CH2 -CH2 (1) + H20(1) -+ CH20HCH20H(I)"""0/ ethane-l,2-diol

epoxyethane

However, the ethane-l,2-diol formed may itself react withepoxyethane to produce an undesired by-product commonlyknown as diethylene glycol.

CH20HCH20H (1) + CH2 -CH2 (1)ethane-l,2-diol """0/

epoxyethane

-+ CH20HCH20CH2CH20H(1)diethylene glycol

Both of these reactions may be regarded as first order withrespect to epoxyethane and have similar rate constants at agiven temperature.

Mole per cent methanolin liquid

0.010.020.030.040.050.060.070.080.090.0

100.0

Mole per cent methanolin vapour

0.041.857.966.572.977.982.587.091.595.8

100.0

TemperatureJOC

100.087.781.778.075.373.171.269.367.666.064.5

39

General editor,Revised NuffieldAdvanced ChemistryB.J. Stokes

Editor,Special StudiesJ.A.Hunt

Author of this StudyJohn McLean

ISBN 0 582 38925 9

••••••Longman:

This is the Students' book of CHEMICAL ENGINEERING ,one of the seven SpecialStudies written for the Revised Nuffield Advanced Chemistry course. Each Studyprovides opportunities for students to extend their knowledge in a particular fieldof applied chemistry. Students look at the ways in which chemistry is used inmedicine, industry, and agriculture, and at the economic and soci.al responsibilitiesof the scientist.

Chemical engineering is the applied science associated with changes in thecomposition or physical state of materials in bulk. This Study focuses on theapplication of chemical principles to manufacturing processes, and considers factorsaffecting the design and operation of chemical plant such as mass and energybalances and the effects of scaling up. The work is fully supported by experimentswhich demonstrate some of the processes taking place in a chemical plant. Studentsare expected to make a 'case study' of one particular chemical manufacturingprocess, and the book concludes with review questions.

The other Special Studies are Biochemistry, Chemistry and the environment, Foodscience, Metals as materials, Mineral process chemistry, and Surface chemistry.·