Cover illustration illustration
Acknowledgements
Tub Tubular lar reactor loop loops and dist istillat illation columns are much in i n evide vidence in this view of an ethanol plant at Gran G rangemouth, Scotland. Scotland.
Austin Austin,, D.G. and J effreys ffreys,, G.V. The The manufacture of methy ethyl ethy ethyl ketone tone from from I nstitution titution of Chemical ical 2-butanol. The Ins Engineers in association with Georg George Godwin Godwin L td, td, 1979: figu fi gure 3.1. 3.1. Baclrn Baclrnurst, rst, J .R. and Harke Harker, J .H. Proce Pr ocessplant ss plant fi gure 5.12 design. Heinemann, 1979: figu Photogr Photogra aphs courte courtesy BP Chemicals: cover, contents page, figure f igure 1.2 Coulson Coulson, J .M. and Richa Richardso rdson, J .F. Chemical engineering. gineering. Pergamon Press L td, 1971: figu fi gures res 4.14, 4.15 4.15 Ju-Ch Ju-Chin Chu Chu Vapou Vapour/l r/liquid iquid equilibrium uili brium data. data. V an Nostra Nostrand Reinho Reinhold Co. L td, td, 1950: figu fi gures res 5.2, 5.6 Photogr Photogra aphs courtesy Esso: Esso: figu fi gures 1.1,3.2 Photographs courtesy I mperial Chemical I ndustries pic, Mon M ond d Division Di vision:: figu fi gures 3.8, 4.9,5.11 Photographs courtesy I mperial Chemical I ndustries pic, Pha P harmaceuticals Division: Di vision: figure 4.6 Mannin Manning, g, J . An introduct introduction to chemical Perga amon Pre P ress L td: td: figu fi gure 6.5 industry. Perg Shell I nternational Petroleu Petroleum Compa Company L td. Oil. Shell Education Service, 1981: figure 4.19 Photographs courtesy Shell: ll : figu fi gures res 4.18, 6.3 Photograph courtesy Whessoe Heavy Engineering L td: td: figure figure 3.6. 3.6.
L ongman Group L imited imited L ongm ongman House, Burnt Mill Mi ll,, Harlow, Harlow, Essex CM20 CM 20 2JE, 2JE , England and A ssociated Companies tluoug tluoughout hout the World. First Fi rst publish li shed 1971 Revised editi edition on first fi rst published 1984 © Nuffi eld-Chelsea Curriculum Trus T rust 1971,1984 Desig Design n and art direction ction by Ivan Dodd Dodd Printed in Great Britain Britain by Georg George e Over L imited, L ondon and Rugby ISBN 0 582 3892 38925 9 A ll rights reserved. No part of this this publication may bere be repro produ duced, store stored in a retrieval syst system, or trans transmitted in any form or by any any means - electronic, mechanical, photocopying, or otherwise - withou without the the writte written prior prior permission ission of the Publishe Publi sher.
The The ne new dra drawings are are by Oxfo Oxford rd Illustrators L imited.
,,'? ,'::".
~ -,~
,
The The au author would like to express his sin since cere tha thanks nks to the followin foll owing g for the their help in the preparation of this this material. V era Ryba R ybalka, K are aren George George, R Barnett, N. Colenut Colenutt, t, Dr M.W. M .W. Brimicom Brimicombe, Dr R.J R .J.. Nevill Neville e, and othe other collea colleagues at The The Ced Cedars Upp Upper Sch School, Leig Leigh hton Buzzard Dr A.M . Mearns rns and K .E. Pee Peet (Depa (Department of Chemical Engineering, ering, University of N ewcastle-upon- Tyne Tyne) C.S. Ga G amage and Dr R.T .W. Ha H all (Esso Petroleu Petroleum Compa Company L imited) imited) S. Wrighto Wrighton n (B.P. Education Service) R. Chapman (The I nstitution titution of Chem Chemical ical Engineers) G. V enn (Sharnbr rnbroo ook k Uppe Upper School) ool) Dr B. B . Hitchen (W.R. T uson Colleg College) C.J . J ohns ohnson (Alce (A lcest ste er Grammar School) P.R. L uton (Richm (Ri chmond-up ond-up on-T on-Tha hames College Col lege) B. Robinson Robinson (Queen's Coll C olle ege, Tau T aunton) G. Cooke (The (T he Harve Harvey Grammar School, ool, Folkestone) D.H.Ma D.H.M ansfield (The Harve Harvey Gra G rammar School, Folkestone) Adrian Wistreich (Education Adviser, Esso Petroleu Petroleum Company L imited). imited). Dr T.P. Borro Borrows ws (Chairman of the ASE A SE Safety Committee) reviewed the experim riments and his hi s safety notes have been incorp incorporated in the the text. text.
2
A cknowled cknowledgement is also due due to those those who helped with wi th the original developm velopment of this Study. Study. The text published in 1971 was writte written by: by: Dr R.J . Dalto Dalton, n, G.R. Grace, E.K. Hayt Hayton on, Dr J . Manning, Dr A .M. M earns rns, K .E. Pee Peet, J .G. Raitt, Raitt, Professor J .D. Tho T horn rnton ton, and K . Wats Watson on.
Cover illustration illustration
Acknowledgements
Tub Tubular lar reactor loop loops and dist istillat illation columns are much in i n evide vidence in this view of an ethanol plant at Gran G rangemouth, Scotland. Scotland.
Austin Austin,, D.G. and J effreys ffreys,, G.V. The The manufacture of methy ethyl ethy ethyl ketone tone from from I nstitution titution of Chemical ical 2-butanol. The Ins Engineers in association with Georg George Godwin Godwin L td, td, 1979: figu fi gure 3.1. 3.1. Baclrn Baclrnurst, rst, J .R. and Harke Harker, J .H. Proce Pr ocessplant ss plant fi gure 5.12 design. Heinemann, 1979: figu Photogr Photogra aphs courte courtesy BP Chemicals: cover, contents page, figure f igure 1.2 Coulson Coulson, J .M. and Richa Richardso rdson, J .F. Chemical engineering. gineering. Pergamon Press L td, 1971: figu fi gures res 4.14, 4.15 4.15 Ju-Ch Ju-Chin Chu Chu Vapou Vapour/l r/liquid iquid equilibrium uili brium data. data. V an Nostra Nostrand Reinho Reinhold Co. L td, td, 1950: figu fi gures res 5.2, 5.6 Photogr Photogra aphs courtesy Esso: Esso: figu fi gures 1.1,3.2 Photographs courtesy I mperial Chemical I ndustries pic, Mon M ond d Division Di vision:: figu fi gures 3.8, 4.9,5.11 Photographs courtesy I mperial Chemical I ndustries pic, Pha P harmaceuticals Division: Di vision: figure 4.6 Mannin Manning, g, J . An introduct introduction to chemical Perga amon Pre P ress L td: td: figu fi gure 6.5 industry. Perg Shell I nternational Petroleu Petroleum Compa Company L td. Oil. Shell Education Service, 1981: figure 4.19 Photographs courtesy Shell: ll : figu fi gures res 4.18, 6.3 Photograph courtesy Whessoe Heavy Engineering L td: td: figure figure 3.6. 3.6.
L ongman Group L imited imited L ongm ongman House, Burnt Mill Mi ll,, Harlow, Harlow, Essex CM20 CM 20 2JE, 2JE , England and A ssociated Companies tluoug tluoughout hout the World. First Fi rst publish li shed 1971 Revised editi edition on first fi rst published 1984 © Nuffi eld-Chelsea Curriculum Trus T rust 1971,1984 Desig Design n and art direction ction by Ivan Dodd Dodd Printed in Great Britain Britain by Georg George e Over L imited, L ondon and Rugby ISBN 0 582 3892 38925 9 A ll rights reserved. No part of this this publication may bere be repro produ duced, store stored in a retrieval syst system, or trans transmitted in any form or by any any means - electronic, mechanical, photocopying, or otherwise - withou without the the writte written prior prior permission ission of the Publishe Publi sher.
The The ne new dra drawings are are by Oxfo Oxford rd Illustrators L imited.
,,'? ,'::".
~ -,~
,
The The au author would like to express his sin since cere tha thanks nks to the followin foll owing g for the their help in the preparation of this this material. V era Ryba R ybalka, K are aren George George, R Barnett, N. Colenut Colenutt, t, Dr M.W. M .W. Brimicom Brimicombe, Dr R.J R .J.. Nevill Neville e, and othe other collea colleagues at The The Ced Cedars Upp Upper Sch School, Leig Leigh hton Buzzard Dr A.M . Mearns rns and K .E. Pee Peet (Depa (Department of Chemical Engineering, ering, University of N ewcastle-upon- Tyne Tyne) C.S. Ga G amage and Dr R.T .W. Ha H all (Esso Petroleu Petroleum Compa Company L imited) imited) S. Wrighto Wrighton n (B.P. Education Service) R. Chapman (The I nstitution titution of Chem Chemical ical Engineers) G. V enn (Sharnbr rnbroo ook k Uppe Upper School) ool) Dr B. B . Hitchen (W.R. T uson Colleg College) C.J . J ohns ohnson (Alce (A lcest ste er Grammar School) P.R. L uton (Richm (Ri chmond-up ond-up on-T on-Tha hames College Col lege) B. Robinson Robinson (Queen's Coll C olle ege, Tau T aunton) G. Cooke (The (T he Harve Harvey Grammar School, ool, Folkestone) D.H.Ma D.H.M ansfield (The Harve Harvey Gra G rammar School, Folkestone) Adrian Wistreich (Education Adviser, Esso Petroleu Petroleum Company L imited). imited). Dr T.P. Borro Borrows ws (Chairman of the ASE A SE Safety Committee) reviewed the experim riments and his hi s safety notes have been incorp incorporated in the the text. text.
2
A cknowled cknowledgement is also due due to those those who helped with wi th the original developm velopment of this Study. Study. The text published in 1971 was writte written by: by: Dr R.J . Dalto Dalton, n, G.R. Grace, E.K. Hayt Hayton on, Dr J . Manning, Dr A .M. M earns rns, K .E. Pee Peet, J .G. Raitt, Raitt, Professor J .D. Tho T horn rnton ton, and K . Wats Watson on.
Contents CHAPTE CHAPTER R ONE ONE
CHEMICAL CHEMICAL ENGINEER ENGINEERS S AND THE CHEMICAL CHEMICAL INDUS INDUSTRY page 4
CHAP CHAPTE TER R TWO
FLUID FLUID FLOW FLOW page 8
CHAPT CHAPTER ER THRE THREE E CHAPT CHAPTER ER FOUR FOUR CHAPTER FNE CHAPTE CHAPTER R SIX
UNIT UNIT OPE OPERATIO RATIONS NS page 11 THE THE CHE CHEMICAL MICAL REACT REACTO OR page 17 DISTILLATION
page 27
THE DEVELOP DEVELOPMENT MENT OF A PRO PROCESS CESS page 32 REVIEW SECTION SECTION page 38
A gene general view view of an anhydride plant (BP Chemicals, icals, Hull).
I~ I il i l fl l ir
~ lf l l [I I~ 1
N12969
3
C H E M IC A L E N G IN E E R IN G
C HAP TER ONE 1.1 1.1 W H A T IS C H E MI MIC A L E N G IN IN E E R IN IN G ? In our modern industria triall socie ociety ty there is an enorm ormous demand for substance nces which which do not occu occurr natura turall lly y but have to be ma made from raw raw materials found in the the earth, rth, sea, and air. air. Such Such substa bstance nces include include petrol, trol, paint, plast plastics, ics, fertfertilizers, il izers, steel, glass, glass, pape paper, cem cement, and pharm pharmace aceuticals, uticals, and are produce uced by a wide variety variety of different manufactu facturing ring proc proce esses. The techno chnolog logy y underlying such proc proce esses is know known n as chemical engineering. ri ng. It is the applied science conc conce erned with cha changes in the the com composition position or phy physica sical sta state of materials rials in bulk, and is both both an aca academic discipline discipline and a vitally vitall y im importa portant nt profess profession. ion. The The chemist ist demonstrates the fea feasibilit ibility y of a che chemica ical rea reaction in the the labora laboratory tory and specifie cifi es the cond conditions itions unde under which which it will wil l take place place. The chemical engine nginee er designs signs and and supe supervises the the cons constru truct ction ion and ope operation ration of the la l arge-scale plan lant required ired to con convert a la l abora oratory tory synthesis into into an indust industrial rial proce process produ producing cing hund hundre reds ds or even thou thousa sands nds of ton tonnes of material rial a yea year. This calls for a th thorou orough understa rstanding of the the che chemistry istry of the the proc proce ess. Other skil skills ls are required required too. too. The chemical enginee ngineer must ust unde understand the phys physics ics and mathe thematics unde underlying rlyi ng the the proble problem ms of heat and mass flow fl ow which which arise when large large qua quantiti ntitie es of material have to be heated or move moved d about bout.. He or she must ust also know the the prope properties of the materials use used to build the the plant plant itself, lf , such such as how the they will wil l sta stand up to high pressure ssures and temperatu rature res, s, and how the they will wil l withst withsta and corros corrosion ion and wear. Chemical ical engine gineers are employe ployed in a wide wide variety riety of proces process industries industries from brewing and baking to petrochemicals and plast plastics. ics. Within the these ind i ndus ustrie tries, s, the the type type of activity activity with which they are involved nvolved may may vary vary conside considerab rably. Som Some che chemical ical engineers may spend much uch of the their workin working g lives lives in the field field of research rch and developm lopment (R & D), either in the the large large rese research labor labora atorie tories s ope operate rated by ind industrial rial companies ies or in universitie rsities. The Their job is to investiga stigate te and develop new proce processe sses and produ product cts s and to try to modif odify y existing proce processe sses to make the them more efficie effi cient nt.. Others are engaged in plant plant design and and construct construction, ion, perhaps workin working g for a cont contra ract cting ing firm which speciali cialize zes in the the design sign and const construc ruction tion of plant plant for chemical manufac nufactu ture rers. rs. Once Once a plan lant is built and succe ccessfully in operation tion, chemical engine nginee ers are resp respon onsible sible for kee keeping it running at maximum ximum effici ff icie ency and for making arrangements for maintenance and modif odificat ications ions to be carried out out as necessa ssary. Whereve rever the they work, work, che chemical ical engineers are usually ll y members of a tea team. They They are often often requ required ired to co-ordina co-ordinate the activitie ctivities of members of oth other specialist disciplin iscipline es involved in the the cons constru truction ction and ma maintenance nce of che chemical plant plant.. These These include include che chemists, ists, mecha chanical engine nginee ers, civil enginee ngineers, control control enginee gineers, electrical electrical enginee gineers, and so on. on. To get the best fro from such a team requires conside iderable management skill, skil l, and such such experien experience often often leads leads eventually to senior senior position positions s in industry. stry. Thu Thus chemica ical engine ineering ring is a career for for men and wom women who can acce ccept challenge and responsibility
4
exte xtending ding far far beyond yond the the con confines fines of the labor labora atory. tory. I t is founded upon upon a thoroug thorough knowledg knowledge e and unde understanding nding of the fundamental sciences of chemistry, physi physics, and and mat mathe hem mati atics.
1.2 W O R K IN IN G O N T H E L A R G E S C A L E I magine tha that you have bee been asked to prepare a I -gram sample of sodiu sodium m hyd hydroxide roxide in the the labora oratory, tory, sta starting rting from othe other che chemicals icals of your your choice choice. Y ou might might beg begin by thinkin thinking g of all the the che chemical rea reactions you have met which which produ produce sodium sodium hydro hydroxide xide and choo choosing sing the the one one which which seems most most conv conve enien ient. Try to list some of the possible reaction ctions s, and note ote the advantages and disadvantages of each. A stud student's nt's plan plan for a preparation ration might rea read asfoll as follow ows. s. 'Sodiu 'Sodium m hydr hydrox oxide ide can be made in the la l abora borato tory ry by pou pouring som some sodium sodium carbona rbonate solution solution into a test-tub st-tube e and adding dding some solid calcium lcium hydro ydroxid xide e. The test-tu t-tube is shaken to mix the reactants and heate ated over a Bunsen Bunsen burner. burner.
A pre precipita cipitate te of calcium calcium carbon rbonate is formed which which is filte fil tered red off to lea leave a clea clear solutio olution n of sodium hydro ydroxid xide e. Solid Solid sodiu sodium m hyd hydroxide roxide may be obt obta ained from this solution solution by careful careful evapo vaporat ration ion to drynes dryness.' This This sounds quite ite reasonable for for a I-gra I-gram lab laboratory preparation ration,, but but the the World demand for sodium sodium hydro hydroxide xide is about bout 30 milli il lion on tonn tonne es per year. Now imagine tha that you are a che chemical engine nginee er and have been aske sked to repo report rt on a possible proc proce ess to produce just just a small prop propor ortion tion of this this tota total World demand, say 10000 tonn tonnes per year . Y ou will note note tha that 10 000 tonnes is 1010 gram or i f you you like, like, ten thousand million ill ion times more ore than you your I-gram labora laboratory tory sample. I n orde order to appre appreciate ciate the the extra proble problem ms which which this this enornormous ous sca scale of ope operation rations s presents nts to a che chemical engine nginee er, let us break down down the the simple simple labora laboratory tory preparation ration into sta stages.
Storage In the lab laboratory tory report, ort, it is simp imply assumed that the sodiu sodium m carbon rbonate solut solution ion and calcium hydr hydrox oxide ide are first f irst collec collecte ted d from their storag storage places, usua usually ll y bottles bottles on she shelves. Such Such details il s cannot be left left unm unmentione ntioned whe when hundre ndreds ds of tonne tonnes of materi teria als are needed every very day. It is imp importa ortant to make plan lanned decision cisions s about the amount ounts s of the the raw raw materials rials to be store stored. d. The follow foll owing ing factors factors must ust be conside considered red. The The cost of the storage tanks, especially ially if the materials are corros corrosive, ive, highly highly fla fl ammable, ble, or toxic. The The value lue of the lan land required for for storage. The The value lue of the ma materials stored and the workin rking g capita ital tied ied up with wi th them. The The cost to the company should stocks run run out and prod roduction be bro broug ught to a halt. (If this this occu occurs rs,, materials rials may have to be bought expe xpensively from from a com competitor titor in ord order to honour marketing contracts.)
Figure 1.1 The Esso refinery and chemical manufacturing complex at Fawley, Southampton. (Aerial view from the east.)
Transport of materials The next stage in the laboratory preparation is to carry the sodium carbonate solution across the laboratory and pour some of it into a test-tube. Energy must be supplied to do this, and on an industrial scale this may well involve using an electrically driven pump to move the liquid from the storage tank to the reactor vessel through a series of pipes. In an industrial plant, materials have to be moved between different stages: reactor vessels, distillation columns, and so on. This is particularly important in continuous processes where there must be a steady flow of raw materials into the plant and finished prc
Mixing In the laboratory we often mix reactants together by shaking them up in a test-tube, another process which requires the supply of energy. It is clearly not practical to have large reaction vessels bouncing up and down in a chemical plant.
The energy required for mixing must be supplied in other ways. Reactants may be mechanically stirred using large rotary agitators driven by electric motors, rather like scaled-up laboratory stirrers. Another common method of mixing makes use of a phenomenon of fluid flow - turbulence which is particularly relevant to continuous processes. The conditions under which turbulence occurs are discussed in Chapter 2. This is important because in a large-scale process the rate of mixing may control the rate of reaction to a greater extent than changes in concentration.
Transfer of heat In our laboratory process, heat energy is supplied in a very inefficient manner by holding the test-tube containing the reactants in a Bunsen flame. An industrial process may involve thousands of times as much heat energy as large amounts of materials are heated up or cooled down during the various stages of manufacture. Heat energy is expensive and so energy transfer must be carried out as efficiently as possible. One of the first stages in designing a chemical piant is to draw up an energy balance showing clearly how much energy must be given to and taken from each process stage. This, together with an estimate of the materials involved (the mass balance), allows the first rough calculations of the cost of running the plant to bemade.
5
The ways in which heat may be supplied to the process include: the combustion of a fuel, the use of an electric current, superheated steam, and energy from a chemical reaction. Heat may be transferred to the process stream by direct transfer. This happens when a fuel is burned in direct contact with the material to be heated, as in a blast furnace or cement kiln. Alternatively, indirect transfer may occur when .there is a physical barrier between the source of heat and the material to beheated, for example in steam boilers and heat exchangers. Such indirect transfer is normally more significant,.and heat exchangers are considered further in Chapter 3.
Separation Going back to our laboratory preparation: after heating, the precipitate of calcium carbonate is filtered off using a filter paper and funnel, leaving a solution of sodium hydroxide. On the industrial scale, this corresponds to the end of the synthesis stage of the process and the beginning of the separation stage. Filtration appears to be a fairly simple operation, but try to list the reasons why the normal laboratory method is generally unsuitable for use in a large-scale continuous process. Sketch a design for a suitable piece of equipment which overcomes the shortcomings of simple laboratory apparatus. Most separation techniques used in the laboratory, such as centrifuging, distillation, and solvent extraction, have their industrial equivalents. In addition, chemical engineers have developed other separation techniques which are not suitable for laboratory use; some of these are discussed in Chapter 3. However, all separation techniques, whether on alaboratory or industrial scale, have one thing in common: they rely for their success on a difference in properties between the materials to be separated.
Figure 1.2 Thecontrol room of an ethanoic acid plant.
plant to measure variables such as temperature, pressure, and flow rate. This information is transmitted to an automatic controller (often an on-line computer) which is programmed to adjust control valvesin order to maintain the desired operating conditions and to ensure a consistent quality of product. For maximum efficiency, automatic process control has become increasingly important. A major design consideration on any plant is how and where to incorporate instruments such as flow meters, thermometers, pressure gauges, and automatic analysers into the plant equipment.
In the laboratory, most unwanted materials can be simply thrown away in the waste-bin or down the sink without further consideration. In industry, as much use as possible is made of all the products of the process, and one of the functions of the research department is to find uses for waste products (perhaps by recycling valuable materials). This is partly for economic reasons, but there are also practical and environmental problems involved in disposing of very large quantities of materials such as slag from furnaces, cooling water, or obnoxious gases and smoke. In general, the closer one gets to removing all the impurities, the more diffi cult it becomes and therefore more expensive.
Summary We have seen that the same chemical and physical principles underlie both large-scale and small-scale operations but that, in industry, the use of large quantities of materials may introduce factors which are unimportant or even non-existent on a laboratory scale. A mongst the problems which the chemical engineer must consider are: methods of storage methods of transferring materials vessel design and materials of construction method of operation (batch or continuous) methods of heating and cooling optimum economic conditions (not necessarily the same as optimum chemical conditions) toxicity and fire hazards instrumentation and control waste product disposal.
Process control
1.3
In a laboratory we normally rely on our senses to tell us when things are not going according to plan, so that we can quickly decide what adjustments are necessary and carry them out. For instance, our eyes enable us to see when the contents of a test-tube are being overheated and our brain responds by instructing our hand to move the test-tube out of the flame. With alarge-scale continuous process it is neither practical nor desirable to use this kind of manual control, and one of the characteristics of a modern chemical plant is asophisticated automatic control system. Sensors are used around the
THE C HE M IC AL IN DU S TR Y The chemical industry is essentially concerned with the efficient 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 which can be difficult to comprehend fully; for instance, the annual World production of nitrogenous fertilizers is in excess of 50 million tonnes. The continuing availability of such materials at reasonable cost is essential for the survival of our society, few aspects of which are untouched by the products of the chemical industry.
Waste products
6
Q1
To gain an understanding of the structure of the modern chemical industry, it is useful to examine a few everyday objects and to attempt to trace the substances from which they are manufactured back to the original naturallyoccurring raw materials. Consider a variety of products such asa glass beaker, a detergent, apolythene bag, arecord, a synthetic fabric, a saucepan, a fertilizer, and an aspirin tablet. petroleum
For a selection of such objects, try to identify all the raw materials and chemical processes involved in their manufacture, consulting textbooks and other source materials as necessary. Vou may find it convenient to write each manufacturing route as a flowscheme, but try not to be satisfied with a superficial resPonse. For instance, when examinillg a nylon fibre it is tempting to say
'nylon comes from petroleum'. However, it is apparent that one of the elements in nylon (nitrogen) is not present in petroleum; therefore some other raw material must also be involved. As an example, a suitable flowsheet for nylon production is shown in figure 1.3. The manufacture of nylon is a complex process and most of your flowschemes will be much simpler than this.
benzene cyclohexane
water
hydrogen
nylon ammonia
air
Figure 1.3 Flowscheme for the manufacture of nylon 66.
When you have completed the above exercise for a variety of substances and discussed the results, you should appreciate the following points. The basic raw materials of the chemical industry are a petroleum, coal, air, water, vegetable materials, and minerals such as metal ores, salt, limestone, and gypsum. b Many products are made from these raw materials via important intermediate chemicals such as ethene, ammonia, chlorine, sulphuric acid, sodium hydroxide, and sodium carbonate. These materials, although rarely seen by the average citizen, are made in very large quantities and are known as heavy chemicals. c Most heavy chemicals are used to make many different products. Each manufacturing process forms part of a complex network of processes which are inter-related chemically and economically, connecting raw materials to final products. Having examined the manufacturing routes for some individual products and after doing a little research on the other uses of the heavy chemicals mentioned above, you should attempt to produce a combined flowscheme connecting the principal raw materials, heavy chemicals, and final products which form the basis of the modem chemical industry. When complete, your flowscheme should clearly illustrate the dependence of the industry on a relatively small number of raw materials and the enormous diversity of products which can be manufactured from them. It is important to realize that the route by which a particular chemical is manufactured may change over the years for a variety of reasons such as availability of raw materials, new technology, energy costs, the changing demand for by-products, and so on. For instance, considerable quantities of sodium hydroxide were once manufactured using the 'lime-soda' process which involved reacting sodium carbonate with calcium hydroxide in a manner similar to that described in the laboratory process earlier in this chapt~r. However, the growth in demand for chlorine manufactured by the electrolysis of brine produced abundant supplies of cheap sodium hydroxide as a by-product. A consequence of this
was that by 1970 virtually all thelime-sodaplants throughout the World had been closed down. Significant changes in the structure of the chemical industry are likely to occur asraw materials such as petroleum and certain minerals become scarcer. Considerable research is being directed into ways of producing many organic chemicals from coal or plant material (biomass) opening up exciting possibilities for the chemical engineers of the future. Because of the inter-relationships that exist, any attempt to divide the chemical industry into sections is necessarily arbitrary. However; some of the traditional 'divisions' of the industry together with their principal products are listed in figure 1.4. Figure 1.4 Traditional divisions in the chemical industry. HEAVY
INORGANIC
CHEMICALS
Chlorine, sulphuric acid, sodium hydroxide, ammonia, nitric acid.
sodium carbonate,
HEAVY ORGANIC CHEMICALS Ethene, ethanol, ethanoic acid, ethan-1,2-diol, benzene, phenol. INDUSTRIAL POLYMERS Poly (ethene), poly(chloroethene), thermosetting plastics, synthetic
propene,propanone,
poly(propene), rubbers.
poly(phenylethene),
AGRICULTURAL CHEMICALS Fertili zers, pesticides, herbicides. PHARMACEUTICALS Analgesics, antibiotics,
antiseptics,
anaesthetics.
EXPLOSIVES Nitroglycerine,
trinitrotoluene(TNTl,
trinitrophenol
(picric acid),
BUILDING PRODUCTS Cement, plaster, bricks, and blocks. PETROLEUM Petrol, diesel, fuel oil, lubricating
oil, bitumen,
LPG.
SYNTHETIC FIBRES Nylon, polyester, acrylic, acetate. DETERGENTS Soapy and soapless detergents.
7
C H E M IC A L E N G IN E E R IN G
C HAP TE R TW O This chapter is concerned with the theory which explains some aspects of the behaviour of fluids: how they flow through pipes, around particles, and through loosely packed solids. This theory can be used in the design of equipment for moving fluids about, measuring flow rates, separating products as in filtering, mixing immiscible fluids, and soon. Physicists refer to this field of study as fluid dynamics; civil and marine engineers who are concerned with movement of or through water refer to it as hydraulics; and aeronautical engineers interested in the same sort of problems in air talk about aerodynamics. The chemical engineer has borrowed from all these sources, but there are many problems which are peculiar to chemical processes. These include novel features such as the flow of reacting liquids, the physical properties of which are changing as the reaction proceeds.
2.1 H O W F L U ID S F L O W TH R O UG H P I P E S Fluids are generally moved about in pipes, and a chemical reactor may be nothing more than a pipe or an enlarged section of one. A heat exchanger in which fluids are heated and cooled is usually a complicated system of pipes. The force of gravity may be enough to cause a liquid to flow from one vessel to another, but generally it is necessary to supply energy by means of a pump at some stage. The chemical engineer must be able to calculate the resistance to flow through a pipe in order to specify the size and type of pump required and the power of the motor needed to drive it. The total resistance is made up of two parts: the resistance arising from viscous friction within the pipe, and the height through which the fluid is to be lifted. Further resistance occurs, of course, if the fluid is to be pumped into a vessel at a higher pressure. The viscosity factor is frequently the more important. Viscosity results from the intermolecular forces which exist within the liquid (see Topic 10). Problems often arise with very viscous liquids such as heavy oils. Pumping may also be difficult when a fluid has to flow through obstructions in the pipe such as a bed of catalyst pellets or a heat exchanger made of small bore pipes with many bends. The frictional forces arising from the viscosity of a liquid can act in two different ways. When the flow velocity is small and the fluid is viscous and nowhere very far from the pipe walls, the fluid flows as if it were in layers sliding over one another. The fluid velocity is at. a maximum in the centre of the pipe and decreases in a fairly uniform manner to zero in contact with the pipe walls. This is known as streamline (or laminar) flow because all elements of fluid move in orderly lines along the pipe, with a velocity distribution as shown in figure 2.1a. If, however, the flow velocity is high, the fluid viscosity low, and the stabilizing pipe walls are far distant, then small disturbances upset the streamline path of the fluid, adjacent elements interfere with one another, and swirling eddies develop. This is known as turbulent flow. The velocity
8
a Streamline flow at low Reynolds Number
{ streamline flow in a boundary layer where uniform (average) viscous-forces and the pipe wall prevent velocity across most eddy formation of the section b Turbulent flow at high Reynolds Number
Figure 2.1 Flow at low and high Reynolds Numbers.
distribution in figure 2.1b shows a fairly uniform fluid velocity across most of the pipe diameter but, no matter how vigorous the turbulence, there is always a narrow but important boundary layer adjacent to the wall where viscous forces and the rigid wall successfully preserve streamline flow. Turbulence only occurs as a result of viscosity. Whether flow is streamline or turbulent largely depends on the ratio of inertial to viscous forces. This i mportant ratio is given by the Reynolds Number. For flow inapipe the Reynolds Number (Re) isgiven by: Re = upd IJ . where
u = mean fluid velocity p = fluid density
d = diameter of pipe I J . = fluid viscosity Since the Reynolds Number is a ratio of forces, it is dimensionless. (Clearly, u, p, d and I J . must be expressed in consistent units.) In general, when the value of the Reynolds Number exceeds a certain critical value, the nature of the fluid flow in the pipe will change from streamline to turbulent. Other things being equal, it requires less power to pump a fluid in streamline than in turbulent flow. This is because much energy is wasted creating turbulent eddies. However, chemical engineers generally prefer turbulent flow for the following reasons.
It is often necessary to mix fluids in a pipe, and this is done effectively by turbulent eddies. Mixing in streamline flow is largely a result of diffusion which is a slow process. The effect of turbulent flow along a few metres of pipe is equivalent to shaking reagents together in a laboratory test-tube. It is much easier to heat or cool fluids flowing through b pipes when the flow is turbulent. This is because, in streamline flow, heat can only pass into the fluid by conduction which is slow through most fluids. In turbulent flow, bulk movement occurs, so that hot fluid at the walls is moved bodily into the main stream and mixed. This forced convection is very effective in transferring heat throughout the fluid. c A length of pipework is normally designed to carry a a
Experiment 2.1 Investigation of flow patterns
specified mass flow rate of fluid (m), say 2000 kg hr-1 • This mass flow rate is related to the mean fluid velocity (u) and pipe diameter (d). Thus the expression for the Reynolds Number may be rewritten:
_
4m
R e rrd/1
This expression shows that for a fixed mass flow rate, a low ReynJlds Number (and hence streamline flow) calls for a large pipe diameter. However, the cost of increasing the pipe diameter quickly outstrips the cost of pumping, and even if the other advantages of turbulent flow are not important, the minimum overall cost is often obtained using fairly small diameter pipes with turbulent flow.
concentrated potassium manganate(vlI) solution
In this experiment, water is discharged from a constant head device through a circular glass pipe of known diameter. The water flow rate is varied and the flow pattern within the glass pipe is observed by injecting afine tracer stream of coloured liquid into the water. In this manner, the relationship between pipe diameter, flow rate and flow pattern may beinvestigated. Procedure a Assemble the apparatus as shown in figure
2.2. Check that the dye nozzle is correctly aligned with the end of the glasspipe. Each group in the class may use a pipe of different diameter. The bottom of the air i nlet tube must beat l east 5 cmabove this pipe to givea reasonable head of pressure. b Open the screw clip slightly to give a small flow rate of water through the glasspipe. Once a stream of bubbles begins to emerge from the bottom of the air inlet tube, the effective head of pressure of water within the apparatus will remain constant until the water level falls to this point. c Slightly open the tap on the funnel containing the dye (potassium manganate(vn) solution) so that a fine stream of coloured liquid is injected into the water as it enters the glasspipe. Observe the flow pattern within the pipe asyou gradually increase the water flow rate. Attempt to identify the flow rate at which the flow pattern changes from streamline to
observe here
screw clip to control flow rate
dy e nozzle glass pipe to sink Figure 2.2
turbulent. What difficulties do you encounter? Measure the maximum flow rate for streamline flow using a measuring cylinder and stopwatch. d Usethis mass flow rate (mlkg S-'), the internal diameter of the tube (dim), and the viscosity of the water OJ/kg m-1 S-1)
to calculate the Reynolds Number at which the flow pattern begins to change from streamline to turbulent in your experiment. Re
=
How does your result compare with other groups i n the class? Does pipe diameter affect the flow pattern for a given flow rate? Q2
Is there a critical Reynolds Number at which flow suddenly changes from streamline to turbulent?
4m
1Td/1
2.2 H OW F L U I D F L OW I S M E A SU R ED It is necessary to measure flow rates in order to monitor the operation of a chemical plant. Generally the overall process is controlled by adjusting flow rates using automatic control valves. For example, the temperature in a reactor may be lowered by increasing the flow of cooling water. Many flow-measuring instruments used in large-scale continuous processes depend upon the application of Bernoulli's Law. This is a special case of the Law of Conservation of Energy and states that, if friction losses are ignored, the energy per unit volume is constant along any streamline in a li quid. Hencl'l for the flow of fluids along a horizontal pipe: Gain in kinetic energy per unit volume = Loss in potential energy per unit volume
Q1
If there is a constriction in the pipe, the fluid flows faster and its kinetic energy increases. The loss in potential energy is shown by a related drop in pressure. If the fluid slows down, the pressure is observed to increase. A venturi flow meter is shown in figure 2.3. This consists of a smooth contraction within the pipe, followed by a smooth expansion to the original diameter. The reduced
manometer
flow
--~-------------•
Figure 2.3 Venturi flow meter.
9
section or 'throat' causes an increase in fluid velocity with a corresponding decrease in pressure, in accordance with Bernoulli's principle. This pressure difference between the throat and the upstream pipe givesa measure of the fluid velocity and hence flow rate. A cruder variation of this type of instrument is the orifice meter, which consists simply of a plate containing a machined hole (or orifice) placed across the pipe (figure 2.4). The pressure drop across the orifice is also a measure of the flow rate within the pipe.
11 float stop
float
--.
graduated glasscone
,
--,1 )- - - I I J
manometer
11 Figure 2.4 Orifice flow meter.
However, whereas the overall pressure loss across a venturi meter may also be as low as 1 %, a typical orifice plate might give a 5 to 6 % pressure loss as a result of the much larger frictional losses due to turbulence. Despite this disadvantage, orifice meters are frequently used because they are easier and cheaper to install and require less space. For instance, they may be incorporated in flanged pipe joints. If flow in an open channel is to be measured, Bernoulli's principle may again be used by making the liquid flow over a weir, as in figure 2.5. The height of liquid standing over the weir is a measure of the pressure drop and hence the liquid flow rate.
Figure 2.5 .A weir as a meter in an open channel.
Finally, a very convenient meter that is commonly used on a chemical plant is the variable area meter or rotameter. This consists of a vertical transparent tube, the diameter of which increases slightly with height. A bullet-shaped bob is contained in the tube, and upward-flowing fluid lifts this until the annular gap is wide enough for the pressure drop to
10
Figure 2.6 Variable area meter.
just support the bob. The tube is graduated and so the instrument can be calibrated. The bob can be made of any suitable material, so that the meter can be used with corrosive fluids. It is suitable for gases or liquids and can be used for small flow rates. (See figure 2.6.) ) This is by no means a complete list of all the methods of measuring flow, and many ingenious devices, such as ultrasonic and electromagnetic flow meters, are used for special purposes.
2.3 O T HE R A P P L IC A T IO N S
O F F L U ID F L O W
An understanding of the behaviour of fluids in motion is also important to the chemical engineer when considering the flow of fluids around particles, particularly in a 'packed bed' consisting of many solid particles touching each other. Flow through packed beds is very common in the chemical industry. Most catalytic reactors involve a bed of catalyst pellets or powder. Gases are dried by blowing them through beds of silica gel or activated alumina. Filtration often involves the flow of liquid through a bed of loosely packed solid particles. Fluid which finds its way through the interstices of a packed bed of particles is rather like fluid flowing through a narrow tortuous pipe with rough walls and, as we might expect, the laws of fluid flow are similar. Thus, flow through a pipe and flow around a particle are at the two ends of a continuous 'spectrum' of conditions, and are sufficiently related for much of the mathematics to be common to both.
C H E M IC A L E N G IN E E R IN G
C H A P T E R T H R E E 3.1 TH E MA NU F AC TU R E OF BU TA NO NE In this chapter we shall begin by examining a chemical process for the manufacture of butanone and see how such a process may be conveniently broken down into a series of steps or unit operations. Butanone (commonly known as methyl ethyl ketone or M.E.K .) is an important industrial solvent with an annual worldwide production figure in the region of 100 000 tonnes. The process described involves the catalytic dehydrogenation of butan-2-o1 for which the overall equation is: Vapour phase ZnO or brass catalyst
------,
CH 3CH 2CO CH3 + H 2
A simplified flow diagram for a typical butanone plant is shown in figure 3.1.
Every .chemical process can be broken down into a series of unit 0rerations carried out on the process stream. Hundreds of operations may be involved in a complex process such as nylon manufacture and each one requires the design, construction, and maintenance of a separate item of equipment. Chemical engineering places great emphasis on the study of these unit operations, because the same theory is 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 framework for the study of the technology of chemical processes which spreads across the boundaries of different manufacturing industries. Unit operations may beclassified into three main groups: transport of materials heat transfer separation.
Figure 3.1 The manufacture of butanone from butan-2-ol. cooling water
butan-2-01
butan-2-01 storage
pre-heater pump
hydrogen
vaporizer
butanone steam condensate receiver distillation column recycled butan-2-01 Feed The butan-2-01 is pumped at a carefully storage tan ks.
controlled
flow rate from
Pre-heat The reaction is to take place in the vapour phase. The butan-2-01 is first heated to i ts boiling point of 100°C using superheated steam. Vaporize The hot gases leaving the reactor are used to vaporize the boiling butan-2-01 before it enters the reactor. The vaporizer is designed to achieve the desired reactor inlet temperature. Compress The hot vapour is compressed optimum pressure.
to force it through
the plant at
React Butan-2-01 vapour is passed over a zinc oxide, or brass, catalyst bed at 400 to 500°C and undergoes dehydrogenation to butanone with a typical yield of about 90 %.
Cool The hot gases leaving the reactor are cooled in the heat exchanger used to vaporize the butan-2-01 feed. Condense On further cooling"in a water-cooled condenser, most of the butanone and unreacted butan-2-01 condense to liquids. The other reaction product, hydrogen, remains as a gas. Separate A condensate receiver is used to separate the liquid and gas in the process stream. However, some of the butanone and unreacted butan-2-01 remain in the vapour phase and are carried off with the hydrogen gas. (Possible methods for their recovery are discussed later in the chapter.) Distil The butanone product is separated from unreacted butan-2-01 by distillation. Heat energy must be supplied to boil the mixture of the two liquids, and the butanone vapour emerging from the top of the distill ation column must be cooled and condensed. The butanone product is run to storage tanks and any recovered butan-2-01 is recycled.
11
Fluids Generally speaking, materials in liquid form are easiest to transport and store, although those which are corrosive toxic, or flammable require special precautions. ' Liquids are normally moved to and from chemical plants in large tanks mounted on lorries, railway wagons, or ships. If very large quantities of material are involved, an overland pipeline may be constructed. These are expensive to install but relatively. cheap to run. They are particularly favoured by the petroleum industry to transport crude petroleum from oilfield to refinery or to convey products to distribution depots. Within a chemical plant, considerable movement of liquid takes place from one vessel to another, and through heat exchangers, reactors, filters, and pipes. Where possible, use is made of gravity, but often energy must be supplied and suitable pumps are required. Two basic types of liquid pumps are in common use: centrifugal pumps and positive displacement pumps.
.. I
air Signal transmitter centrifugal pump
!air
(?';\)
t0J
controller
!air to valve air-operated valve
Figure 3.2 The butanone plant at Fawley, Southampton. The lefthand tower is the main butan-2-o1 purification tower, with the butanone product tower on the right.
As liquid is pumped through the orifice plate there is a pressure drop. The transmitter sends a signal (air) to the controller which compares the signal with the set point and alters the control valve until the correct pressure drop (and hence flow) is obtained.
3.2 TR AN S P O R T O F MAT E R IALS This group of operations is concerned with the bulk movement of materials between and through the different items of chemical plant. Materials in fluid form are generally much easier to handle, but chemical engineers often have to deal with sticky, powdery, or lumpy solids, and highly corrosive or flammable gases.
Solids Solids 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 is sometimes difficult to achieve the accurate control of flow rate necessary for continuous processes by this method. An alternative way of transporting solids for short distances is by a screw feeder. These are frequently used to feed polymer granules to moulding machines or coal to furnaces, and can give very accurate control of flow rates. (To see a screw feeder in operation you should examine a domestic kitchen mincer.) Another useful way of moving solids is in suspension in a fluid. For example, coal and china clay may be transported in pipes over considerable distances asfmeparticles suspended in a fast-moving stream of water, an operation called hydraulic conveying. Transporting granular or powdered solids in a fast-moving air-stream is called pneumatic conveying, and has long been used for loading and unloading grain ships. It is now extensively used in the chemical industry, for example in transferring catalyst particles between reactor and regenerator in fluidized catalytic cracking units (see figure 4.19).
12
Figure 3.3 Automatic flow control using a centrifugal pump and
control valve.
3.3 HEAT TRANSFER Many chemical reactions and separation operations rely for their success on the accurate control of temperature. Heating and cooling the process stream at various stages is very important. On a chemical plant this is brought about by using heat exchangers to transfer heat energy from one fluid to another. One of the simplest heat exchangers is the Liebig watercooled condenser, commonly used in laboratories. This consists of a 'tube' through which the fluid to be cooled is passed, surrounded by a 'shell' through which the cooling fluid, usually water, flows. This type of 'shell and tube' heat exchanger finds much use in industry, though in a considerably modified form. In order to appreciate the design of industrial heat exchangers, we shall first develop the theory of heat transfer for a simple laboratory device.
The theory of heat transfer Consider a stream of hot liquid which is to be cooled in a simple shell and tube heat exchanger using cold water. There are two basic methods of carrying out this operation.
a Parallel-current flow where both the hot liquid and the cooling water flow through the exchanger in the same direction. b Counter-current flow where the hot liquid and cooling water pass through the exchanger in opposite directions. Figure 3.4 shows these two modes of operation, together with typical temperature profiles for the fluids within the exchanger. Study these diagrams and try to suggest the possible advantages and disadvantages of each method.
In practice, counter-current flow is generally preferred and the following theory applies to this method. The basic equation which describes the performance of aheat exchanger is:
Q = UAM where
Q = the duty of the exchanger: the amount of heat to be transferred (in kJ hr-1) a measure of the U = the heat transfer coefficient; 1 efficiency of the process (in kJ hr- m-2 K- 1) A = area of surface across which heat transfer takes place (in m2) M = temperature difference (in K) Sometimes M can be taken simply as the temperature difference between the two fluids. However, the temperature of both fluids usually varies throughout the exchanger as shown in figure 3.4. Under these circumstances the most useful value for /:;t is known as the log. mean temperature difference, Mm, which is calculated from the inlet and outlet temperatures as follows:
M
distance
distance
a Parallel current flow
b Counter-current flow
m
Figure 3.4 Temperature variation in a heat exchanger.
Ex periment 3.3 Investigating heat transfer in a laboratory L iebig condenser In this experiment you will use a laboratory water-cooled Liebig condenser to reduce the temperature of a stream of hot liquid. The 'duty' of the heat exchanger will be determined from the inlet and outlet temperatures and the mass flow rate of the liquid stream. This may be used to estimate the heat transfer coefficient across the heat exchanger surface under the conditions of the experiment.
Procedure a Fit a laboratory condenser with thermometers at each inlet and outlet so that the temperature of both the hot liquid stream and the cooli ng water may be measured before and after passing through the apparatus. This is readily achieved by fitting plastic T-pieces into the rubber tubing, as shown in figure 3.5. Take care to avoid leaks at joints.
T 1, T 2,
= (T 1
-
t2) -
In [(11 - t2)
(12 - t1) / (T 2 - td]
t1,and t2 are as shown in figure 3.4b.
T,
t cooling
t2
I water out
-
••
•
hot liquid out
cooling water In
t
I
b The hot liquid which is to be cooled
should be passed through the central 'tube' of the heat exchanger, and cooling water passed through the outer 'shell' in a counter-current direction. The flow rates of both liquids may be controlled by means of screw cli ps attached to the outlet hoses. Ideally, both liquids should be supplied from constant head tanks so that their flow rates remain steady throughout the experiment.
c Adjust the flow rates of both liquid streams to give a temperature drop of at least 5 °C for the hot liquid. When conditions are steady, record the hot li quid i nlet temperature T I and outlet temperature T,; also the cooling water inlet temperature tI and the outlet temperature Use an appropriate measuring cylinder and stopclock to measure
t,.
Figure 3.5 the mass flow rate of hot liquid and cooling water through the heat exchanger. After dismantling the apparatus, measure the length and average diameter of the 'tube' across which heat transfer takes place.
The performance of a heat exchanger is described by the equation:
Treatment of results The 'duty' of the heat exchanger (Q) is the amount of heat being transferred per hour (in kJ hr-I ). This is calculated from the results for the hot l iquid as follows:
(A/m')
Heat transfer per hour (Q)/kJ hr-I = mass flow per hour/kg hr-I X specific heat capacity of liquid/kJ kg-II("I X temperature change (T T, )/K
/: ; tm, the log mean temperature difference across the heat exchanger, is calculated from andT2• tl't"TI
I
Q = UA/:;tm
The area of the heat transfer surface is calculated from the tube length (Ifm) and average diameter (dav/m). A = rrdav
X
I
-
13
Ql Usethe values for Q. A, and 6tm obtained from the experimental results to calculate the value of U, the heat transfer coeffi cient, for your apparatus under the conditions of the experiment.
How does your answer compare with the values obtained by other groups? Can you explain any differences? Q2
What factors affect the heat transfer coefficient across the tube?
Figure 3.6 Shell and tube heat exchanger under construction,
Industrial heat exchangers The duty of a heat exchanger depends upon: the heat transfer coefficient a the surface area across which heat is transferred b c the temperature difference. In the design of heat exchangers, chemical engineers attempt to achieve optimum values for each of these variables.
a Heat transfer coefficient (U) High operating values for the heat transfer coefficient are obtained by the following. i Ensuring that the fluid flow is turbulent. This keeps to a minimum the thickness of the film at both surfaces of the tube. 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 onI V fluid i n a transition region - heat transfer bV and convection
wall Figure 3. 7 Heat transfer between fluids in turbulent flow.
ii Constructing the tubes of amaterial with a high thermal conductivity. (Look up the values for aluminium, copper, steel, and glassin your Book of data.)
14
Q3
What modifications to the design of your heat exchanger would incre~se its potential duty?
showing the arrangement of tube bundles and internal baffles.
Keeping the walls of the tubes clean and free from coatings of 'scale' or other solids. Where fouling of this type is likely, the exchanger must be designed for ease of cleaning and maintenance.
III
b Surface area (A) A large surface area of tube is desirable, and to achieve this many small tubes are used rather than a single large one. However, the larger the number of tubes the greater the capital cost of the exchanger and the pumping costs to operate it. In practice, an optimum value is specified to give minimum overall costs. Some tubes have special 'fins' attached to increase the effective surface area for heat transfer. c Temperature difference (/':,t) The higher the temperature difference between the two fluids, the greater the heat transfer. However, the value of this variable is often dictated by the heating or cooling agent available. River water at 5 to 15°C is often used as a cooling agent, with a maximum discharge temperature of 50 °c or less. In some locations, suitable cooling water is not available and aircooled heat exchangers must be used. The most common heating agent on chemical plants is high pressure steam at about 150°C, often produced at a central location on site. A typical industrial shell and tube exchanger is shown in figure 3.8. It consists of dozens or even hundreds of smallbore tubes (the tube bundle) through which the process stream passes. A second fluid, perhaps cold water for cooling or steam for heating, passes over the outside of the tubes within the shell. Its path is directed backwards and forwards over the tubes by means of baffles.
gas out
water in
t
gas in
t
Separation operations may be divided into two main categories: mechanicalseparation and mass transfer operations .
M echanical separation ,r--,.
,...,.
j/?tS"
Ii
\ \
t:I
\ II
1 3
];I
II
j/I
II I II ,W I
Mechanical separation operations depend on differences in bulk 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 between the components of a solid-solid mixture, and is the simplest operation. (See figure 3.10.)
j/",.., • •
nI,\
I \ \ \"
I
j/ j/ I \ I
H I u ,V I '-.A
Figure 3.8 Shell and tube heat exchanger.
water out
Heat exchangers perform a wide variety of functions in different situations and are often given names to indicate this function. Thus, pre-heater, reboiler, condenser, cooler, vaporizer, and economizer are all names for heat exchangers used for different applications. Figure 3.9 shows the heat exchangers commonly used in association with a distillation cooling water column.
-
condenser
distillation column
product cooler
cooling water
feed
steam
reboiler
liquid condensate
screen oscillates to ensure movement of solids
feed preheater bottoms product
Figure 3.9 Distillation column with ancillary heat exchange equipment.
Although usually studied in school physics rather than chemistry courses, heat transfer is a very important aspect of industrial chemistry. The chemical and economic viability of a process may well depend upon the efficient use and recovery of heat energy.
3.4 SEPARATION Separation of the products of a chemical reaction does not usually present too much difficulty on a laboratory scale unless a very high degree of purity is required. Thus solids can be separated from liquids by ftltering or centrifuging. A single component may be isolated from liquid mixtures by distillation or solvent extraction. All such techniques take advantage of differences in properties of the substances to be separated. The chemical engineer uses these same principles to design equipment which can perform the task on a large scale, frequently on a continuous basis, at rates of hundreds of tonnes of product per day. Much of the equipment seen on a typical chemical plant may well be concerned with such separation operations.
Figure 3.10 Size separation of solids by screening.
b The continuous vacuum filter is the industrial equivalent of a laboratory suction ftlter. In figure 3.11 the mixture of liquid and solid ('slurry') is fed into a trough. A large hollow drum is suspended in the trough as shown. The outside of the drum is perforated metal or woven wire string, and is covered with a ftlter cloth, on top of which are closely spaced strings. The pressure inside the drum is reduced by suction so that the liquid ('mother liquor') is sucked inwards and the solid forms a cake on the outside. When the ftlter is running, the drum rotates and a 'cake' of solid is formed. As this comes out of the slurry it is washed, both washings and mother liquor 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 the strings around an external roller so that the cake falls off into a container. This diagram also shows how suction is applied selectively to only two-thirds of the circumference of the drum by means of a special valve. circular valve: remains stationary as segmented outer drum rotates
solid
~ Figure 3.11 Filtration usipg a continuous vacuum fil ter.
15
c The centrifuge is used for solid-liquid separation, on the same principle as a laboratory centrifuge. Figure 3.12 shows a continuous centrifuge. As it rotates, the solid collects on the lining of the cylindrical basket and the mother liquor and washing water pass through the perforated basket into collecting boxes. A reciprocating pusher blade gradually moves the solid layer through the washing zone and out t~a discharge point as shown. p~rfor~ted ~etal ts~etl
-
pusher blade
-
feed (crvstals in mother liquor)
b Solvent extraction is used for liquid-liquid separation and
wash liquor
circumferential collecting boxes product
wash liquor
frequently used to recover the absorbed gas from a solvent. Figure 3.13 shows an absorber/stripper system which might be used to bring about complete separation of gases X and Y . The gas mixture and solvent are passed through the absorber in opposite directions (counter-current flow) to maintain the maximum 'driving force' for mass transfer between phases. A large surface area of contact between phases is achieved by using trays or packings similar to those used in distillation columns. (See Chapter 5.) In the butanone plant discussed at the beginning of the chapter, butaI10ne and butan-2-o1 vapours niay be recovered from the hydrogen gas stream by scrubbing with water. However, butan-2-01 cannot be separated from water by stripping as both liquids have the same boiling point (100°C). Solvent extraction must be used instead.
mother liquor
Figure 3.12 Solids separation in a centrifuge.
M asstransfer operations Mass transfer operations are characterized by the movement of one substance through another on a molecular scale. Such separation techniques are based on the principle that substances tend to distribute themselves in different concentrations in different phases. Thus distillation takes advantage of the difference in composition usually found between a liquid mixture and the vapour with which it is in equilibrium. Distillation is the most important of the mass transfer separation operations, and will be investigated in some detail in Chapter 5. Other important mass transfer operations include the following examples.
a Gasabsorption is used to separate amixture of gases using a selective solvent in an absorption tower or 'scrubber'. For instance, a mixture of two gases X and Y might be separated in this way, by using a packed tower to bring the gas mixture into contact with a solvent in which gas X is soluble but gas Vis not. Gas absorption (or 'scrubbing') is characterized by mass transfer in one direction only - from the gas to the liquid phase. The reverse process, where mass transfer occurs from the liquid to the gas phase, is called 'stripping', and is
depends on the partition effect of a solute between two immiscible liquids. For instance, if a mixture of butan-2-01 and water is agitated with a suitable solvent such as 1,1,2-trichloroethane, most of the butan-2-01 but virtually none of the water will enter the trichloroethane layer. To increase mass transfer, and hence approach equilibrium conditions more rapidly, the interfacial area between the two phases is made as large as possible by mechanical agitation. Because they are immiscible and have different densities, the water and trichloroethane separate into two layers when agitation ceases. Most of the butan-2-01 is now in the trichloroethane layer from which it may be separated by distillation. Y ou may have carried out this kind of separation operation in the laboratory using a tap funnel. (See Topic 9, Experiment 9.4.) Solvent extraction may be carried out on a continuous basis using mixer-settler units. These consist of two tanks, of which one is agitated to bring the two liquid phases into contact and the other is calm to allow them to settle out (figure 3.14). The solvent phase which now contains the dissolved solute is called the extract and the residual phase from which solute has been removed is called the raffinate. aqueous solution
organic solvent
gasY gasX
scrubber
.t organic layer Figure 3.14 Mi xer/settler unit for solvent extraction. feed
steam
mixed gas
x·+y solvent + X
Figure 3.13 Gas separation by selective absorption.
16
Mass transfer coefficients may be derived to describe the efficiency of all mass transfer operations. Just as heat transfer equipment is designed to give an optimum value for the heat transfer coefficient (U) so chemical engineers must design mass transfer equipment to obtain maximum values for the ma$Stransfer coefficient (K).
C H E M IC A L E N G IN E E R I N G
C H A P T E R F O U R Most chemical processes may be divided into two main stages: the synthesis stage in which the required product is formed from reactant materials; the separation stage in which the required product is separated from the rest of the reaction mixture. The synthesis stage is carried out in a vessel called the reactor. On a chemical plant this may often appear small and unimpressive compared with some of the other items of equipment present. However, the performance of the reactor influences the design and operation of almost every other part of the plant. Thus the reactor lies at the heart of any chemical process. Its design must be undertaken early in the development stage and will often dictate the capital cost and economic viability of the overall plant.
4.1 TYPES OF REACTOR The function of the reactor is to produce a certain product from given reactants at the required rate. There are three main types of chemical rellctor commonly used to achieve these objectives. These are: a the batch reactor b the continuous stirred tank reactor c the continuous tubular reactor.
a
Thebatchreactor
In this type of reactor, all of the reactants are placed together in a vessel, and the mixture is stirred and heated as appropriate until the reaction is sufficiently complete. (See figure 4.1.) In a batch reactor the rate of reaction falls as the reactants are used up. At any particular instant all the material present has reached the same stage of reaction. reactant
~il'
•
products out Figure 4.2 Continuous stirred tank reactor.
c
Thecontinuous tubular reactor
In this type of reactor, the reactants are fed continuously into one end of a tubular vessel and products flow out at the other end (figure 4.3). This i s a steady rate operation. With constant flow rates, the conditions at any particular point remain constant with time. At a distance x downstream from the inlet, reactants have spent a time.£ in contact, where v is v the flow velocity through the reactor. Thus changes in time in a batch reactor become identical with changes in position (x) in a tubular reactor. The significant characteristic of tubular reactors is that no attempt is made to mix together materials which are at different stages of reaction. The overall length of the reactor is determined by the contact time needed to achieve the desired concentration of product. reactants in ••
- .- ..
products out
Figure 4.3 Continuous flow tubular reactor.
4.2 stirrer
Figure 4.1 Batch reactor.
b
\~b~;l
stirrer
Thecontinuous stirredtank reactor (CSTR)
An alternative to batch operation is to feed reactants continuously into the reactor at one point and withdraw products at an equal flow rate elsewhere. Thus the chemicals react as they flow through the system. A typical continuous stirred tank reactor is shown in figure 4.2. Reactants flow continuously into a vigorously stirred vessel and products are withdrawn at the same rate so that a steady state is maintained. The main characteristic'of this type of reactor is that the contents are thoroughly mixed to give a uniform composition throughout. Thus the composition of the outlet stream will be the same as that in the bulk: of the vessel.
DESIGN EQUATIONS For each type of chemical reactor it is possible to derive a general design equation. This relates the residence time (t) which the chemicals must spend in the reactor, to the required change in the concentration of reactants and the rate constant for the reaction. In a batch reactor, the percentage conversion of reactants to products in time t may be calculated simply from the rate expression. F or a first order reaction of the type A -+ products the rate of reaction, r A, is the rate of change of concentration of A. rA
=k[A]
It is possible to derive the integrated form of the rate law as shown in Appendix 1 to Topic 14.
17
t
=-!.
In [A]o
k
[A]
= concentration of reactants at time t = rate constant for reaction.
[A] k
where: [A]o = initial concentration of reactants Ql Usethe design equation for a batch reactor to calculate the time taken to achieve 50 %,
This expressionmay be used as the designequation for afirst order batch reactor.
For acontinuous process,thereactantsarebeingcontinuously added and the reaction mixture is being continuously removed. Thus flow data must be incorporated into the designequation to allow for the effect which flowhas upon concentration. Considerthereaction: A -+products. Suppose A is beingfedinto a continuous stirred tank reactor in which perfect mixingistaking place. Let thevolumeof the reactor be V dm3, andthe volume flow rate through the reactor be u dm3 min-I. Then the mean residencetime (T ) of material in the reactor is givenby:
Thus: [AJ out
If the concentration of A entering the reactor is [A J o, then the number of moles of A entering the reactor in time tis [A]out. Similarly, if the concentration of A leavingthereactor is [A] , then the number of moles of A leavingthe reactor in time tis [A]ut.
Thenumber of molesreactingin time t =r A x Vt Applyinga massbalanceover the reactor for component A: number of moles entering
+ number of moles
number of moles leaving
Dividingby ut [AJ. = [AJ + V T
=
(A
x : )
[A]o - [A]
In acid solution hydrogen peroxide will oxidize iodide ions to produce iodine. + 21- + 2W ->-12 + 2H20 (blue with starch)
[4.1]
rA
=
k[A]
Thedesignequation becomes: V
[A]o - [A] k[A] =
=
u
T
Thus if [A]o, [A] , and k are known, the required flowrate through areactor of volume V maybecalculated.
Part 1Standardization of hydrogen peroxide solution a Add about 4 g of solid potassium iodide to about 25 em' of l.OMsulphuric acid in a conical flask and dilute to 100 ems with water. b Using a measuring cylinder, add 50 em' of
'1volume' hydrogen peroxide solution.
The iodine produced gives an intense blue colour if a little starch is present. In this experiment, the reaction will be carried out in a simple batch reactor. The rate of change of concentration of hydrogen peroxide will be followed by progressively titrating the iodine produced with sodimn thiosulphate solution. + 2S20;- ....•21- + S40~12 (blue with (colourless) starch)
This is the general design equation for continuous stirred tank reactors. If the reaction is first order:
which have reacted
Ex periment 4.2a U sing a batch reactor to obtain kinetic data for a reaction
[4.2]
Thus the volume of thiosulphate solution required to discharge the blue colour is a measure of the number of moles of hydrogen peroxide which havereacted.
18
+ (rA x Vt)
= V U
HP2
[A]ut
=
u T
problems, absolute values of concentration are not required, only the ratio of [A] 0 to [A].)
90 %, and 100 % conversion of A if the reaction is first order and the rate constant (k) is 0.04 min-I. (Note that, in solving such
c Warm the reaction mixture to about 50 DC and allow it to stand for at least 30 minutes to ensure that the reaction is complete. (Part 2 of the experiment should be attempted during this time. Alternatively, add 5 drops of 3 % ammonium molybdate solution which catalyses the oxidation of iodide by peroxide, so that there i s no need to wait for 30 minutes.) d Titrate the liberated iodine with 0.2M
sodium thiosulphate solution, adding a few drops of starch solution to enhance the colour of the iodine asyou approach the end-point.
e Record the volume of thiosulphate solution used. Let this bea cms •This is a measure of the number of moles of hydrogen peroxide initially present in 50 em' of '1 volume' solution. Use this to calculate the
concentration in mol dm-' of your 'I volume' hydrogen peroxide solution.
Part 2 Batch determination of reaction kinetics a Put 500 em' of 0.02M potassium iodide solution in a large beaker. This vessel is to serveasa batch reactor, and must be stirred constantly during the experiment. Add 10 em' of 5M sulphuric acid and 10 cms of 1 % starch solution. b Fill a burette with 0.2M sodium
thiosulphate solution and arrange this over the batch reactor.
c Usinga measuring cylinder, add 50 em' of '1 volume' hydrogen peroxide solution and simultaneously start a stopclock. A blue colour should appear in the stirred reaction mixture asiodine is produced. d Immediately add 1.0 em' of thiosulphate
solution to the contents of the reactor. This should cause the blue colour to disappear until sufficient iodine has been produced by the peroxide/iodide reaction to react completely with this thiosulphate solution.
e Note the time when the blue colour reappears, and add afurther 1.0 cm3 of thiosulphate solution to the reaction mixture.
f Repeat until a total of 12.0 cm of 3
thiosulphate solution has been added, noting the total time from the start of the experiment as the blue colour reappears after each 1.0 cm3 addition of thiosulphate. Record your results carefully.
Treatment of results If the reaction is first order with respect to hydrogen peroxide, then:
Volume of thiosulphate added x cm3
Time t/min
a a- x
a a- x
a is the volume of thiosulphate solutiQ,n equivalent to the initial number of moles of H202•
added at time t. Thus the design equation becomes:
Expressing hydrogen peroxide concentration in terms of the volume of thiosulphate solution used:
E xperiment 4.2b T he contin uou s-fl ow
(x axis).
Q2
Confirm that the reaction is first order with respect to hydrogen peroxide. Q3
Calculate the rate constant (k) for the reaction under these conditions from the gradient of the graph.
t against In _a_ should bea straight line with a- x
Q5
[H202]
°
Plot agraph of t (yaxis) against In _aa- x
If the reaction is first order, agraph of
a-x
k
[H2 2] 0 is the initial hydrogen peroxide concentration, and [H202] is the hydrogen peroxide concentration at time t .
a a- x
Q4 Why can the effect of iodide concentration on reaction rate beignored in this experiment? Look carefully at Equations [4.1] and [4.2].
= ! -In a
1 In[H202]o
k
In
Figure 4.4
x is the volume of thiosulphate solution
The design equation for the batch reactor is:
a- x
" I gradlent k
Draw up a table of results as shown in figure 4.4 above.
Using the design equation, calculate the time taken for 10, 20, and 30 % conversion of the initial hydrogen peroxide in your batch reactor.
stir r ed tank r eactor
In this experiment you will design a continuous stirred tank reactor (CSTR) to produce a certain percentage conversion of reactants to products. You wil l then construct the reactor to your own specifications and compare its operating performance with your design calculations. The reagents will be 'I volume' hydrogen peroxide solution and acidifi ed potassium iodide solution (0.02M) as used in the batch reactor experiment.
O.02M acidified potassium iodide solution +starch
a Design areactor vessel of capacity between 0.5 and 1 dm3 which will enable reactants to be added continuously and products to be withdrawn at the same flow rate. It must be possible to agitate the contents of the reactor mechanically so that they are thoroughly mixed at all times. (Check the design with your teacher before construction.) b Determine the working capacity of your reactor by filling it with water and switching on the stirrer. Water will overflow until a steady state i sreached. Switch off and measure 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 at the same initial concentrations as in the batch reactor experiment, so the inlet stream should be 'I volume' ('" 0.083M) hydrogen peroxide mixed with 0.02M acidified potassium iodide solution in a volume ratio of 1: 1O.Allowing for dilution, this would make the initial hydrogen peroxide concentration 0.083
x--lI
Figure 4.5 own solution using the results of Experiment 4.2a. d Decide upon the degree of conversion of
peroxide for which you will design (between 10 % and 30 %). Each group in the class should aim for a different target conversion.
e Use these conditions in the design equation for a continuous stirred tank reactor to calculate the flow rate of reagents required.
-[A ]O V
=
0.0075 mol dm-3
Note. As the concentration of hydrogen peroxide solution may change significantly during storage, you should standardize your
u
- [A ]
k[A]
Example: V = 1.0dm3
k
= 0.03 min-1
Target conversion = 20 % •'. if [A]o = 0.0075 mol dm-3 then [A] = 0.0060 mol dm-3 u
= total volume flow rate in dm3 min-1
Rearranging the design equation gives:
u
Vk[ A] [A]o - [A]
1.0 X 0.03 X 0.0060 0.0015
19
= 0.12 dIn3 min-I
The method is as follows:
the residence time in the above example is
i Calculate the number of moles of
So the required total flow rate is 120 cm3 min-I.
thiosulphafe added per minute. ii Hence calculate the number of moles of iodine being produced per minute, using Equation [4.2]. iii Hence calculate the number of moles of hydrogen peroxide reacting per minute in your reactor, volume V dIn3, using Equation [4.1]. iv Calculate the rate of reaction in moles dm-3 min-I. v Using your value fOIthe rate constant and the rate expression for the reaction
1000 = 8.3 minutes. 120
To giveperoxide/iodide flow rates in the ratio Thus at least 30 minutes should be allowed if of 1:10, the peroxide flow rate should be possible. 120 x
..!- = 11
11 cm3 min-I
And the iodide flow rate should be 120 x 10 = 109 cm3 min-I 11
f Put about
9 dm3
of 0.02M acidifi ed potassium iodide solution (containing 10 cm3 of 1 % starch solution) into a constant head reservoir. Position the reservoir above the reactor vessel and adjust the flow rate to the desired value using ameasuring cylinder and stopclock. (109 ± S cm3 min-I in the above example.)
i While the system iscoming to equilibrium,
drops of saturated sodium thiosulphate solution should be added to remove the blue colour of the iodine each time it appears. This will ensure that the iodide concentration in the reaction mixture remains constant.
i Once the reactor has reached a steady state, then O.IM sodium thiosulphate from a constant head device should be carefully run into the reaction mixture at such a rate that the colour of the reactor contents appears to 'hover' between blue and colourless. It may take a few minutes to determine this equilibrium flow rate. Measure the rate of flow of thiosulphate solution required using a measuring cylinder and stopwatch.
calculate the concentration of hydrogen peroxide in the reaction mixture and hence also in the exit stream. Q6
Compare the actual percentage conversion with your design conversion. Try to account for any discrepancies which exist.
g Set up asimilar reservoir containing about
2 dIn3 of '1 volume' hydrogen peroxide solution and adjust the flow rate to the calculated value. (11 ± 1 cm3 min-I in the above example.)
h Al low the reactor vessel to fill up and reach equilibrium. This will take approximately four times the mean residence time (7) after the reactor is full. Since 7=~
u
Treatment of results When the reaction mixture 'hovers' between blue and colourless:
Assuming that the reaction between iodine and thiosulphate is instantaneous, use your results to calculate the percentage conversion in the reactor.
The average residence time
B A T C H O R C O NT IN U O U S O P E R A T IO N ? In the design of any chemical reactor, two factors - the kinetics of the reaction and the required output of product are normally fIXed from the outset. Using all the available information, the chemical engineer must make decisions concerning the type of reactor to be used, its physical dimensions, and the optimum conditions under which it is to operate. The design equations developed earlier in this section enable comparisons to be made between theoretical yields of product from a continuous stirred tank reactor and a batch reactor during the same time interval. For a fust order reaction in a continuous stirred tank reactor, the design equation is
v u
= 7 = [A]o - [A]
k[A]
Volume (V) = 22 m3 = 1.65 m3 min-I Flow rate (u) Rate constant (k) = 0.122 min-I = 1mol dm-3 [A] 0
Then using the design equation [A] =0.38 mol dm-I This represents a 62 % yield of products.
20
Consider the likely effect of the following changes of conditions on the percentage conversion within the reactor: a increased reactant concentration in feed b increased total flow rate through reactor c increased reactor volume d increased temperature.
rate of production of iodine from hydrogen peroxide = rate of removal of iodine by thiosulphate
4.3
If:
Q7
v
=
u
7 is
22 1.65
= 13.3 minutes For a first order reaction in a batch reactor the design equation is
t
=
[A]o k
=
2..-
In [Alo [A] k
If: 1 mol dm-3 = 0.122 min-1 = 13.3 minutes (same time interval as CSTR)
t
Then
In
1
= 0.122 x 13.3
[A] ... [A]
= 0.20 mol dm-3
Since [A]o was 1 mol dm-3, this represents an 80 % yield of products. The batch reactor gives a larger percentage conversion than the continuous stirred tank reactor, using the same size vessel over the same period of time. Normally, a manufac-
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 aveterinary worm medicine.
Q8
Q9
Calculate the time required for the batch reactor to achieve 62 % conversion in the example on the previous page.
Why does the reaction proceed more rapidly in a batch reactor than in a continuous stirred tank reactor?
turing process has a target yield of product which the batch reactor will reach with a shorter residence time than the continuous reactor. The major disadvantage of batch reactors is that many ancillary operations are necessary both before and after the reaction tak~s place. The reactor vessel must be filled with measured quantities of reactants, the batch must be tested to ensure that it has reached the desired percentage conversion, and the vessel must then be emptied completely. The time spent on these operations is called 'shut-down' time. The manufacture of a large quantity of product requires very many batches, and it is the overall time of the cycle of all operations which must be considered when comparing batch and continuous processes. (See figure 4.7.) For most processes the shut-down time would be so large that a greater throughput can be obtained from a continuous reactor. The decision whether to operate on a batch or continuous basis is also influenced by factors such as the following.
Batch process
charging of reactor reaction time discharging reactor
Continuous process
I-
-I
mean residence time to achieve the same % conversion as in the batch process
Figure 4. 7 Comparative performances of batch and continuous processes.
Manpower The manpower required to operate a process is related to the number of times an operating condition has to be changed. Which type of processing requires the greater number of men to operate it? This relies on instruments, and instruments Automation require conditions which are as steady as possible. Which type of process is more easily automated?
Degree of control Control over a process, whether manual or by instruments, is the result of a series of adjustments. The effect of an adjustment is noted and subsequently a finer adjustment is made. The longer the time available under steady conditions, the more refined the adjustment. Which type of process allows the greater control? Cost of plant In a continuous process, conditions at any point in the system are constant and the equipment is 'tailormade' for those conditions. In batch processing, multipurpose units are frequently used which are the large-scale equivalent of laboratory apparatus and are obtainable 'off the shelf' from chemical plant manufacturers. Which type of process is likely to have the higher capital costs? Generally speaking, batch operation is used for processes which produce relatively small quantities of material such as in the pharmaceutical, fine chemicals, or dyestuffs industry. A well-equipped batch reactor (or autoclave) allows great flexibility of operation, as it may be used to produce a different product each day. Batch reactors are also frequently used for polymerization and fermentation processes where the shut-down time allows thorough cleaning of the reaction vessels to avoid build-up of unwanted by-products or harmful bacteria. However, for most other lar~e-scale processes continuous operation is generally favoured.
21
4.4 CONTINUOUS REACTOR DESIGN Experiment 4.4 The continuous-flow
b Three constant reservoirs should be filled
with the same solutions as in Experiment 4.2b.
tubular reactor
The two main types of reactor in which a chemical reaction may be carried out on a continuous basis are the stirred tank reactor and the tubular reactor. In this experiment you will operate a tubular reactor and compare its performance with that of the tank reactor studied in Experiment 4.2b.
Reservoir A O.02M acidified potassium iodide solution and starch. Reservoir B '1 volume' hydrogen peroxide solution. Reservoir C O.lM sodium thiosulphate solution.
a Construct a tubular reactor using a transparent glassor rigid plastic tube 3or 4 cm in diameter and 1.5 m long. The tube should be clamped at a slight incline and be fitted with an 'inlet manifold' at the lower end to enable peroxide, iodine, and thiosulphate solutions to beintroduced at controlled flow rates. The upper end of the tube should be fitted with an exit pipe discharging i nto a sink or bucket. (See figure 4.8.) To make the flow pattern along the reactor tube more turbulent it should be fitted with a series of 'baffles' at 2 or 3 cm intervals along its length. These are readily made from thin discs of plastic, perforated with afew holes and threaded onto aglassrod. O.02M
O.lM
acidified potassium iodide solution
sodium thiosulphate solution
~
c Set the flow rates of the three solutions so that they are the same as in the previous experiment (4.2b) when the reaction mixture was 'hovering' between blue and colourless. Then introduce these solutions into the tubular reactor via the inlet manifold. d While the reactor is filling and reaching a
steady state, calculate the total flow rate through the reactor. If possible check this at the exit pipe. Use this flow rate, the diameter of the reactor tube, and the results of the batch reactor (Experiment 4.2a) to predict the position in the tube where the reaction mixture 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 predicted result? QIO
How does the volume of the tubular reactor compare with the volume of the stirred tank reactor used to bring about the same percentage conversion in Experiment 4.2b?
Ql l What explanation can you offer for any difference in volume required?
Q12 How would you expect the position of the colour change (and hence the volume of reactor required) to beaffected by: a increased total flow rate of reactants b increased concentration of hydrogen peroxide c increased temperature?
Ql 3 What are the possible advantages and disadvantages of the tubular reactor compared with the stirred tank reactor?
Figure 4.8 Apparatus for tubular reactor experiment.
'1 volume' hydrogen perox ide solution
~
baffles
'Inlet manifold'
Tubular reactor (at slight incll ne to remove air)
to
sink
The two principal types of continuous reactor, stirred tank and tubular, haverather different performancecharacteristics which determine their suitability for use in particular chemical processes. Whendesigningacontinuous reactor, the chemical engineer must consider factors such as reactor volume, selectivityof product, temperaturecontrol, optimum physical conditions, andthe useof catalysts.
Reactor volume For a given production target, the size of reactor required will dependupon the rate at whichthe reaction occurs. Since reaction rateisnormally dependentonreactant concentration, the volume of a tubular reactor required to.bring about a certain percentage conversion is significantly different from that of astirred tank reactor. 22
I +
In an 'ideal' tubular reactor, all elementsof the reaction mixture are assumed to take the sametime to passalongthe reactor tube (figure 4.10). This situation is known as 'plug flow', and no 'back-mixing' occurs between materials at different stages of reaction. The chemicals react as they proceed along the reactor tube, and thus the reactant concentration falls steadily from its initial value [A]0 at the inlet to its final value [A ] at the exit. Consequently the designequation for a tubular reactor is similarto that for a batch reactor. If V is the reactor volumeandu is the flowratethrough the reactor, then the residencetime tisgivenby
t = V u
Figure 4.9 A large-scale continuous tubular reactor.
For a first order reaction it has been shown that: t=~
k
Thus the design equation for a frrst order tubular reactor may be written:
In [A]o [A]
V = t = ~ In [A]o k [A] u
Q14
In a previous example, the volume of stirred tank reactor needed to give 62 % conversion was 22m3•
Use the above expression to calculate the volume of tubular reactor required to achieve the same percentage conversion.
For a given fl ow rate and percentage conversion, a tubular reactor has a smaller volume than the equivalent stirred tank reactor. This may have a significant bearing on the capital cost of the reaction vessel. Reactions take place more slowly in stirred tank reactors because the reactant concentration is at the low exit value Inlet reactant concentration
1ft
Ii
[A J o
~
Outlet reactant concentration [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 becomes the 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 rate is higher and the total reactor volume required is lower than if a single tank had been used. In the extreme case, a tubular reactor may be regarded as equivalent to an infinite number of stirred tank reactors in series. c:
0
";::; c:
0
'c:"
~ c:
u
0 u
•.c:.
'u " c:
0 u
~ u
•.c:.
co
~ u
'" a:
[AJ
~ c:
0 [A] 0
";::;
co
(Flow rate u = 1.65 m3 min-I , rate constant k = 0.122 min-I.)
'" a:
[A]
[A J
Distance along reactor x
Figure 4.10 Plug flow in an 'ideal' tubular reactor.
Residence time
Figure 4.11
23
R eactor selectivi ty It is not uncommon for by-products to be formed in a reaction mixture due to the occurrence of undesired chemical reactions. In these circumstances, the reactor design may considerably influence the nature of the products formed and hence the type of separation equipment required to deal with them. Such unwanted products may arise in two ways. pump
products out
Reactions in series (consecutive reactions) Consider the reaction scheme
Second stage'
First stage
Figure 4.12 Two-stage continuous stirred tank reactor.
.•.2 .
[A J
0
A
B
C
reactant
desired product
undesired product
~ c Q)
o c
Here the reactant A produces the desired product B, but this may itself undergo further reaction to form the undesired product C. In order to suppress the conversion of B to C, the concentration of B must be kept as low aspossible within the reaction mixture. Thus where B is the desired product, a tubular reactor will give the best performance, whereas a stirred tank reactor will tend to favour the formation of C.
o
•.c.
o
~ o &'" :
[A]
Residence time Figure 4.13 Concentration changes in tubular reactor and two-stage continuous stirred tank reactor.
Q15 Benzene can enter into substitution reactions with chlorine as follows: C6HsCI monochlorobenzene
C,H4CI2 dichlorobenzene
Reactions in parallel (competing reactions) Consider the situation where a reactant A may form two possible products Band D.
Which type of reactor would you specify to favour the formation of: a monochlorobenzene b dichlorobenzene?
Thus the choice of reactor type depends upon the kinetics of the two competing reactions. A tubular reactor will favour the higher order reaction and a CSTR will favour the lower order reaction, assuming the rate constants are similar in each case.
Temperature control
If the reaction A
-----+B
rate of formation of B If the reaction A
-----+D
is first order then:
= k1 [A] is second order then:
rate of formation D =k2 [A]
2
Hence rate of formation of B
=
rate of formation of D To favour the production of B, the concentration of reactant A must be kept as low as possible, a situation best achieved in a stirred tank reactor. However, if D is the desired product, the concentration of A should be kept at a maximum and atubular reactor will givethe best performance.
24
Most chemical reactions involve a significant energy change, either exothermic or endothermic, which will tend to alter the temperature of the reaction mixture as reaction proceeds. If no attempt is made to compensate for this by heating or cooling the reaction mixture then the reactor is said to be operating adiabatically. This may be used to advantage for moderately exothermic reactions, where the increase in temperature will maintain the reaction rate as the reactant concentration falls. However, with highly exothermic reactions, a significant rise in temperature will occur unless heat is removed from the mixture during reaction. For many chemical systems, the rate of reaction doubles for every 10°C rise in temperature and this can quickly lead to a ;runaway' situation with disastrous consequences. With most chemical reactions an optimum temperature range needs to be maintained and the reactor design must incorporate provision for heat transfer. In the extreme case where the temperature of the reaction mixture is held constant throughout, the reactor is said to be operating isothermally.
a
a
reactants in
t
;~ --
h~
-
reactants in
_
l
heating or cooling ,agent
b
I
products out
heating or cooling agent
i
-
products out
products out
reactants in
b
i
_out heating or cooling agent _in
c
flue gases to stack
tt
-
reactants in
~products out
c
heating or cooling agent
convection section
radiant section
-
i n
___
heating or cooling out agent
products out
t t fuel burners
Figure 4.15 Heat transfer in tubular reactors. a single tube with heating or cooling jacket b multi-tube reactor; tubes in parallel givelow tube velocity for
products out
reactants c pipe furnace; tubes usually in series; usesinclude 'stearn cracking' of hydrocarbons.
I
+ 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 a stirred tank reactor, where the contents are thoroughly mixed and uniform throughout. However, deviations from 'plug flow' in a tubular reactor can lead to the formation of 'hot spots' in the reaction mixture, where the temperature and consequently the rate of reaction cannot be accurately predicted.
Operating conditions Many chemical reactions are reversible, and at first sight it might appear that conditions within a reactor should always be designed to favour a high equilibrium yield of the desired product. In practice, the situation is often more complex than this. Consider the Haber process for the manufacture of ammonia: N z (g) + 3Hz(g)
"'" 2NH3(g) ~H~8 = -92.1 kJ morl
The equilibrium data shown in figure 4.16 suggests that the best percentage yield of ammonia will be obtained by operating at a low temperature and high pressure. However, at low temperatures the rate of reaction is far too slow (a chemical factor) and the operation of high pressure plant is very expensive (an economic factor). This problem is resolved in most ammonia manufacturing plants by using a compromise temperature of about 450°C, a pressure of about 250 atmospheres, and a catalyst to speed up the rate of reaction. The reaction mixture is not allowed to reach equilibrium but is removed from the reactor at 12 to 15 % conversion. Ammonia is separated by liquefaction, and unreacted nitrogen and hydrogen recycled. The recycling of unconverted reactants in this manner is common practice in the chemical industry . The reactor conditions and percentage conversion per pass are designed to give the lowest production costs taking into account the kinetic, thermodynamic, and economic data for the system. Problems such as this are readily investigated using mathematical models on a computer. 25
.. l!!...•
100
)(
'E
90
100°C 200°C
~
E
.: 2 ~
300°C
80
':;
C' III
.:
••
'c
0
E E
••
~
70 60
400°C
50
III
'0
~
40 500°C 30 20 10
Pressure/atmospheres Figure 4.16 Equilibrium data for the synthesis of ammonia.
Homogeneous and heterogeneous reactors Most of the discussionso far has assumedthat the chemical systemwithin the reactor ishomogeneous, whichmeansthat all the substances involved are present·in the same phase. However, many reaction systems are heterogeneous, with materials in twoormorephasestakingpart. Thisisparticularly significantfor gasor liquid phase reactions whichtakeplace at the surfaceof asolidcatalyst.If so,thereactorperformance may well be determined, not by reaction kinetics, but by the rates of masstransfer of reactants to thecatalyst surfaceand products away fromit. Heterogeneous reactions involving solid catalysts are generally carried out in tubular reactorspacked with catalyst pellets through which the reactants must pass(figure4.17). This isknown asa 'fixed bed' systemand isfavouredbecause of itssimplicityandtheflexibility of its operatingconditions.
Figure 4.18 Overall view of the catalytic cracking unit at Cura~ao oil refinery. flue gas gases and gasoline
-
products out
reactants in
'filted bed' of catalyst
spent catalyst
fresh feed
1
375°C
-
heavy gasoil
Figure 4.17 Fixed bed catalytic reactor.
One heterogeneous system which poses particular problems is the catalytic cracking of hydrocarbons from petroleum. This may be carried out by passinghydrocarbon o vapours over a silica-alumina catalyst at about 500 e. However, during the cracking process the surface of the catalyst becomes fouled with deposits of carbon which reduces its activity and hence reactor performance. This problem has been solvedby using'fluidized bed' reactors in which the catalyst is suspended as small granules in the stream of hydrocarbon vapour (figure4.19). In this fluidized state the catalyst may be regenerated on a continuous basis by passing it through a vessel in which the carbon deposits are 'burned off' using air at about 600 °e. The hot, clean catalyst is then recycledback to the reactor. 26
Figure 4.19 Catalytic cracking: the catalyst powder passes to the reactor, in the centre, where the cracking process takes place. The cracked vapours then pass to a fractionating column, on the right. The used catalyst returns to the regenerator, at left, where it is cleaned for re-use.
Such fluidized systems overcomemany of theproblems of mass transfer and temperature control which may be associated with fixed bed reactors. However, they are expensive to construct and require careful control to maintain auniformfluidizedbed of activecatalyst.
C H E M IC A L E N G IN E E R IN G
C H A P T E R F IV E Distillation is without doubt the most important of all the separation techniques used in the chemical industry. It is a mass transfer operation which has a fi rm quantitative basis and can be controlled to a very high degree. In this chapter some of the important factors in distillation are investigated in a semi-quantitative manner to determine the conditions for efficient separation.
~ ~ a '" .
•.::J .
I
I I I X2
'"
I-
L1QUID/VAPOUR EQUILIBRIUM Most mixtures of liquids can be separated by distillation. This is possible if the liquid mixture and the vapour with which it is in equilibrium at its boiling point have different compositions. For an ideal mixture the difference in composition between liquid and vapour may be predicted using Raoult's Law. (See Topic 10.) At a fixed pressure, the boiling point of a liquid mixture depends on its composition. The liquid line shown in figure 5.1 relates boiling point and composition for a mixture of two liquids, A and B. The composition of the vapour which exists in equilibrium with each liquid mixture may also be shown on the same diagram and gives rise to a corresponding vapour line.
Ex periment 5.1 To determine the temperature composition diagram for the system: ethanoic acid/water In this experiment you are to determine the boiling point and the composition of the vapour produced for various mixtures of ethanoic acid and water. The vapour composition may beaccurately determined by condensing a sample and titrating the acid content with sodium hydroxide solution using phenolphthalein indicator.
Mole % ethanoic acid in vapour 1000 83.3 69.8 57.5 47.0
0.0
boiling point pure A
I I I
E
5.1
Mole % ethanoic acid in liquid 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
boiling point pure B
o
20
40
60
80
100
Mole % A
100
80
60
40
20
o
Mole % B
Figure 5.1 Temperature/composition AandB.
diagram for a mixture of liquids
Thus a liquid of composition Xl boils at a temperature T 1 to give a vapour of composition X z. Notice that the vapour produced is richer in the more volatile component than the original liquid mixture. Such temperature/composition diagrams can be obtained by measuring the boiling point of various liquid mixtures and analysing the composition of the vapour produced in each case.
Caution. Ethanoic (acetic) acid i s corrosive and its vapour is unpleasant. It should only be used in afume cupboard or a wellventilated laboratory, and contact with the skin or eyes should be very carefully avoided.
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 a different mixture.
Before the experiment, the thermometers to be used should be standardized. This is readily achieved by suspending them in a large beaker of distilled water which is heated until it boils steadily. Any thermometer which does not i ndicate 100 °C should be marked
Temperature (boiling point);oC 118.1 113.8 110.1 107.5 105.8
a Assemble the apparatus as shown in figure 5.3. To avoid errors due to superheating of the liquid, a short length of thin capillary tubing should be sealed at one end in a Bunsen flame. Attach this sealed tube to the Figure 5.3 Apparatus to deterrnine liquid/vapour composition curve.
thermometer - 5 to 105 °C x 0.1 °C
100.0
Figure 5.2 Liquid/vapour equilibrium data for mixtures of ethanoic acid and water at atmospheric pressure.
antibumping granules - - ----,---ceramic
I
heat
cooling water centred gauze
graduated collecting vessel ~
27
thelmometer, open end downwards, using a rubber or plastic ring as shown. The thermometer bulb should be positioned in the liquid, not the vapour. (Why?). b Put about 50 cm3 of the mixture in the flask with some fresh porous pot or antibumping granules. Ensure that the still-head and upper portion of the flask are well lagged to prevent fractionation within the apparatus. Bring the mixture to the boil, heating strongly at first but gently as the boiling point is approached. As the liquid heats up there should be a slow escape of bubbles from the open end of the capill ary tube. When the boiling point is reached, a rapid stream of bubbles will begin to emerge from the tube. Remove the source of heat and watch the stream of bubbles carefully. Record the thermometer reading as the last bubble emerges just before the liquid slicks back up the capillary tube. This is the boiling point of the liquid mixture.
c Reboil the mixture steadily and collect the first 1.5 cm3 of distillate i n a suitable clean, dry receiver. The composition of this distillate may now be analysed by titration. Using a dry dropping pipette carefully transfer exactly 1.00 g of your distillate sample into adry conical flask placed on a top-pan balance (accurate to 0.01 g). Add 20 cm3 of distilled water and 3 drops of phenolphthalein indicator to the contents of the flask, then accurately titrate with 0.50M sodium hydroxide solution from a burette. The end-point is when the first permanent pink colour appears in the solution.
5.2 F R A C T IO N A T IN G
d Use the results to calculate the percentage composition of your distillate sample as follows. Calculate the number of moles and hence the mass of ethanoic acid in your 1.00 g sample. Subtract to determine the mass and hence the number of moles of water present in the sample. Calculate the mole percentage of ethanoic acid present using the expression:
Mole % ethanoic acid moles ethanoic acid X 100 (moles ethanoic acid + moles water)
Q1 At what temperature does a mixture containing 50 mole % ethanoic acid boil? Q2 What is the composition of the vapour obtained when this mixture boils?
= - - - - - - - - - - - - -
Share your classresults to complete figure 5.2 and plot a temperature/composition diagram for mixtures of ethanoic acid and water showing both l iquid and vapour lines. Y our axes should be as shown in figure 5.4, with 100 % ethanoic acid on the lefthand side. u
-.:;.,
o
•. .
Q3 What is the highest quality distillate obtainable from a single simple distillation of a mixture containing 50 mole % ethanoic acid?
Q4 If this first distill ate were placed in a second distillation apparatus and heated until it distilled, what would be the composition of the second distill ate?
III
Q5
Co
If this second distillate were placed in a flask and distilled, what would be the new distillate composition?
t
., IE
Q6
How many such simple distillations would be required to give a mixture containing 90 mole % water (i.e. only 10 mole % ethanoic acid)?
a
50
100
50
100 Mole % water
a
Mole % ethanoic acid
Figure 5.4
COLUMNS
We have seen that simple distillation produces a distillate which is richer in the more volatile component than the original mixture. Successive simple distillations bring about further separation but such a procedure would be very inefficient to operate. A fractionating column is a device which accomplishes in one operation the equivalent of many successive simple distillations. The easiest to understand is probably a bubble-cap column as used in some large-scale distillations (see figure 5.5). Vapour boiled off from the liquid in the kettle (the industrial equivalent of a flask) passes up the column and condenses on the first plate. This condensate is of the same composition as the rising vapour and thus one distillation stage has been completed. The bubble caps force following vapour to bubble through the condensed liquid on the plate and the heat from this vapour causes the liquid to boil. This gives off a new vapour of a composition even richer in the more volatile component which condenses on the second plate, completing a second distillation stage. In this simple treatment each distillation stage or 'step' on the temperature/composition diagram corresponds to one theoretical plate in afractionating column. In practice no plate performs as efficiently as this. Each plate receives liquid from the plate above by means of an overflow weir, and thus the composition of liquid on a plate is 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
What information can temperature/ composition diagrams give? Use your graph to answer the following questions.
liquid
Q7
How many such distillations would berequired to produce pure water asthe distillate? vapour
1
liquid
vapour
Figure 5.5 A plate and bubble cap column.
flowing down it means that a fractionating column gives the same overall effect as a number of successive simple distillations. Bubble caps are expensive on a laboratory scale, and the same effect may be obtained using a column packed with glass beads or rings. This provides a large area of wet surface for vapour and liquid to approach equilibrium at all heights in the column.
In fractional distill ations which you may have conducted previously, almost all of the vapour which reaches the top of the column has passed into a side-arm condenser and been removed from the system. However, it is normal industrial practice to return some of the condensed vapour back down
Ex periment 5.2 To investigate the factors which influence the effectiveness of af ractional distill ation column In this experiment you will investigate the effect of reflux ratio, column height, and column packing on the performance of a fractionating column used for batch distillation. It is possible to use the ethanoic acid/ water system studied in the previous experiment, but more interesting results are obtained using the system: ethanol/propanone. Equilibrium data for this system is given in figure 5.6. Use this data to plot the temperature/composition diagram, drawing both liquid and vapour lines as accurately as you can, with a flexible curve if available.
Mole % propanone in liquid 0.0 10.0 20.0 30.0
Mole % propanone in vapour 0.0 26.2 41.7 52.4
Temperature /oC 78.3 73.0 69.0 65.9
40.0 50.0 60.0 70.0
60.5 67.4 73.9 80.2
63.6 61.8 60.4 59.1
80.0 90.0 100.0
86.5 92.9 100.0
58.0 57.0 56.1
the column. This is known as reflux. The ratio of reflux flow rate to distillate flow rate is known as the reflux ratio. Reflux ratio
=
distillate rate
water condenser
still-head tap (distillate rate) window to observe drop rate (reflux rate)
graduated collecting vessel
fractionating column packed with glass beads
Figure 5.6 Li quid/vapour equilibrium data for mixtures of propanone and ethanol at standard atmospheric pressure. Use your graph to determine the number of theoretical plates (i.e. distillation stages) necessary to obtain a distillate containing 95 mole % propanone starting from a 50 mole % mixture. Carefully draw the appropriate steps on your graph, using a sharp pencil and ruler. Y ou will use this diagram later to interpret your experimental results. Each group in the class should study a different set of conditions. (See figure 5.7.)
Group Group Group Group
A B C D
Reflux ratio 1:1 5:1
1:1 1:1
Column height 25 cm 25 cm IOcm 25cm
Packing type Glassbeads Glass beads Glass beads Glassrods
Figure 5. 7 Other variations of these factors may be studied if there are more than four groups in the class.
Caution. Mi xtures of ethanol and propanone are highly flammable and care must be taken to ensure novapours escapeinto the laboratory. a Assemble the apparatus as shown in figure 5.8, but without the insulation at first. Use
reflux rate
liquid running down. Good contact between these two flows is an essential feature of fractionation. b After observing the column in action, insulate i t using cut l engths of domestic pipe insulation. Extend the insulation to the reflux head, but leave a 'window' to allow the reflux drip rate to be counted. Once insulation is complete, allow conditions to become steady under total reflux (do not remove any distillate). The boil-up rate should be brisk but not so fast that 'flooding' occurs in the column. This happens when the condensate is prevented from flowing down the column by excessive vapour flow up the column. Only a small flame is needed under the water bath. When steady conditions have been established for a few minutes, record the kettle and vapour temperature at total reflux.
c Now set the reflux ratio to your desired value by opening the still-head tap slightly and counting the number of drops of distillate and the number of drops of reflux during a 30-second interval. If necessary, adjust the tap and recount the drop rates until you are close to your target ratio (this should not take longer than 5 minutes). Collect the distillate in a graduated container, recording the kettle temperature and the vapour temperature in the still-head when 5,10,15,20, and 25 cm3 of distillate have been collected. Check the reflux ratio periodically. If it has changed, it may be because the kettle is no longer boiling as vigorously asbefore. Before shutting down, turn the reflux head tap off to givetotal reflux again. Observe the effect on the vapour temperature. d Compile your results as in figure 5.9.
Figure 5.8 Fractional distillation column. the thermometer you have previously standardized at the top of the column. In the flask put amixture of 1 mole of ethanol and 1 mole of propanone with a few pieces of fresh porous pot to prevent superheating. Heat the water bath strongly at first, but more gently when it reaches 60 to 70°C. When the bath reaches approximately 80 °c, distillation will begin. Observe the counterflow in the column with vapour going up and
Reflux ratio
Kettle temperature
rC
Kettle composition /mole per cent propanone
Usethe liquid lineon your temperature composition diagram to determine the composition of liquid in the kettle from its boiling temperature. Similarly, use the vapour line to determine the composition of the vapour which is condensed to produce distillate at the top of the column. Construct a graph showing composition (0 to 100 % propanone) on the vertical axis and volume of distill ate (0 to 25 cm3) on the horizontal axis. Useyour results to plot a line showing the variation of distill ate composition with volume of distillate collected. On the same graph plot the results obtained by other groups using different column conditions. Use the results obtained to answer the following questions.
Vapour temperature
rC
Vapour composition /mole per cent propanone
Volume of distil/ate /cm3
Figure 5.9
29
Q8 What effect does column length have on the quality of distillate obtainable from a given kettle liquid composition? Q9
What effect does the surface area of the column packing have on the quality of the distillate obtainable? Q10
What effect does reflux ratio have on the quality of the distillate obtainable? Qll
What conditions would you choose to achieve the most efficient separation of a mixture?
Q12 How does reflux ratio affect the rate of distillation? Assuming all experiments were carried out at approximately the same boil-up rate, you may compare the time each group took to collect 25 cm3 distillate. Q13 In chemical manufacture, a certain quantity of material of a given target quality must be produced in a fixed time. What differences would there be between distillation columns operating at highand low reflux ratios?
Q16 If a product of fixed quality is required, what changes in conditions, made during the course of a batch distillation, would enable a distillate of constant composition to be obtained?
Q14 What is the effect on the composition of the kettle liquid when distillate is removed?
5.3
T H E C O N D I T IO N S F O R C O N T IN U O U S D I S Ti l lA T I O N A fractional distillation column designed for continuous operation is shown diagrammatically in figure 5.10. The mixture to be distilled is fed into the system at a steady rate, and product is continuously removed both at the top and bottom of the column. Heat input is either by pre-heating the feed or re-boiling the bottom residue, using a suitable heat exchanger. The less volatile component is removed as liquid from below the bottom plate and the more volatile component is removed as vapour from the top plate. The boiling liquid on reflux condenser
vapour from top plate
rectification section
feed
each intermediate plate becomes progressively richer in the more volatile component as the column is ascended. Thus, there is a corresponding temperature gradient within the column. The feed must be introduced into the side of the column at a height where its composition corresponds to the composition of the liquid on the plate. In this way, the steady state within the column is not disturbed. The part of the column above the feed point is called the rectification section. In this section the feed is concentrated to the desired distillate quality. Below the fee.d point is the stripping section where the more volatile component is progressively stripped out until an acceptable lower limit of concentration is obtained. The residue is removed as a liquid. In many processes, the liquid residue is as saleable as the distillate, and sufficient plates are present in the stripping section to bring the residue liquid up to the customers' product specification for purity. Batch and continuous fractionating columns operate by the same mechanism but have one major difference. In a batch still, the conditions gradually changewith time because material is being removed as distillate from a fixed quantity of liquid being distilled. In a continuous still, the conditions remain steady because a steady feed of uniform composition compensates for the removal of distillate and residue.
5.4 C O L U M N E F F IC I E N C Y
••
The internal structure of a fractionating column must be designed to bring ascending vapour and descending liquid into intimate contact, so that mass transfer of the components may readily occur. Both packed columns and plate columns are used in industry . Packed columns consist of a hollow shell filled with a large number of specially shaped rings made from ceramic, glass, metal, or plastic. Some common examples are shown in figure 5.12 on the next page. Plate columns contain trays on which liquid rests and through which ascending vapour rises. Traditionally, these have been constructed with bubble caps, but the high cost of
stripping section
steam heilting coil
Q15
What is the effect on distillate composition of continuously removing distillate from the top of the column?
residue
Figure 5.10 Arrangement for continuous fractional distillation. Q17 The relative efficiency of different packings may be compared using a term known as the 'height equivalent to a theoretical plate' (HETP.)
30
HETP = =
Total height of packing Number of theoretical plates
Use this expression to calculate the HETP values for the packings used in your
experiment. The number of theoretical plates should be determined from the steps on your temperature/composition diagram, using the initial vapour composition obtained at total reflux.
bubble-cap tray
) ))
) ) vapour
overflow weir
sieve tray
'downcomer' pipe
valve tray
Figure 5.13 Three types of tray commonly used in distillation
columns. All the trays in one column are normally of the same type.
Figure 5.11 Industrial distillation columns.
Raschig ring
)
Lessing ring
Pall rings
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 (figure 5.13). In their simplest form, sieve trays consist of steel plates drilled with holes. The liquid on each plate isprevented from flowing down through these holes only by the upward flow of vapour from below. Consequently, precise control of conditions within the column is essential. With valve trays these holes are closed off if the vapour flow rate falls. Plate columns have the main advantages that they can cope with awide range of conditions (including liquids which foam), are readily cleaned, and enable side-streams to be removed at intermediate points in the column if desired. Packed columns are generally cheaper to construct for small diameters (less than 1 m) and have superior corrosion resistance since inert ceramic packing materials may be used. The main disadvantages of packed columns are that liquid tends to flow down the walls of the tower instead of through the packing, and for large columns the sheer weight of packing may impose severe structural loads. 5. 5
E C O N O M I C S A N D O P T I M IZ A T I O N The cost of operating a distillation column can be broken down into two principal parts: the capital cost of building the column, and the cost of running it once it has started to function. These depend upon the number of plates, and upon the reflux ratio.
The capital cost is high for very low reflux ratios, since a large number of plates would be required. The capital cost is also high for very high reflux ratios, since a large plate area would berequired 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 the column and the distillation is slower. At total reflux the running costs are infinitely large, since no product at all is obtained. This is shown in figure 5.14. The sum of these two curves gives the total cost, and the value of the reflux ratio which corresponds to the minimum total cost is the optimum reflux ratio.
•. .
total
:s CJ
cost
capital cost
Reflux
ratio
Figure 5.14 The cost of operating a distillation column.
5.6
M U l T I·C O M P O N E N T
D I S T il l A T IO N
The discussion so far has concentrated on the use of distillation to separate binary mixtures, i.e. those containing two components only. Distillation may also be used to separate mixtures containing several components into different boiling ranges, although the theory of multi· component distillation is much more complex. The primary distillation of crude oil is an important example. The oil is pre-heated in a furnace, then fed continuously into a bubble-cap fractionating column. The liquid contents of each plate represent aboiling range of the mixture, and l iquid is removed continuously from several of the plates in the column. Each/raction obtained in this way has its own characteristics and will be further processed and sold as petrol, paraffin (kerosine), fuel oil, and so on.
31
C H E M IC A L E N G IN E E R IN G
C H A P T E R S IX How does a large, complex, expensive industrial plant come into being? In this section we shall consider how a company develops anew idea, from preliminary work in the laboratory, to a pilot plant producing small amounts of product, and finally to the design and construction of a full-scale plant. 6.1
E X A M IN IN G
A NE W P R O J EC T
All new industrial processes begin with an 'idea' which may involve an entirely new product or abetter way of making an existing product. Many ideas originate from pure research carried out in the laboratories of chemical companies, universities, and government establishments. Others stem from experience gained during industrial production and marketing, which can lead to an awareness of the need for a new or modified product. Once a production ide~has been established, it is usually carefully examined in three stages: a preliminary investigation b laboratory tests c pilot plant trials. -------idea
X -----
market research
L _ _ ..
preliminary
laboratory
rough costing from massi:J stilgations and energy balances laboratory scale tests
project committee
j
project committee
I
project committee
cost estilate from first flowsheet
trials of pilot plant product I I
L
pilot plant I
directors
l
I
. I f --.costestlmate rom.•.. _-.J second flowsheet
I directors
r1
directors
directors
I
engineering and construction
sales development
J
full-scale production
~X~ technical services (helping customers to use the product to best advantage)
Figure 6.1 The development of a process.
32
The preliminary investigation involves an examination of the scientific and economic soundness of each of the possible routes by which the product can be manufactured. The requirements of each process, such as raw materials, equipment, heat, and electricity are listed together with the predicted yields of products and by-products. This information is used to prepare a mass balance for the material flow and an energy balance for the energy flow within the proposed plant. Thus an estimate of the capital and running costs of aprocess emerges; this usually enables the best production route to be selected. If the preliminary investigation establishes the existence of a viable production route, laboratory tests are carried out in order to gather as much information as possible about the chemistry of the process. Kinetic and equilibrium data for the reactions will be established, possible by-products will be identified, and the effects of scaling up will be investigated. The consequences of scaling up are of critical importance in process development. Consider the simple case or"a cubic stirred tank reactor 1m wide in which an exothermic reaction is carried out (figure 6.2). The volume of the reactor contents is 1m3 and the surface area of the reactor through which heat may be lost is 6 m2 • Now suppose a similar reactor 2 m wide is to be constructed. The volume of the reactor contents will 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 mixture which is producing heat by a factor of eight. However, the surface area of the reactor through which this heat can escape has only increased by a factor of four. Clearly, the scaled-up reactor will operate at a higher temperature and, since this will increase the rate of reaction andhence the rate of heat evolution, a temperature 'runaway' becomes a real possibility .
metre,
volume 1 m3 surface area 6 m2
I
I---2 metres -----l volume 8 m3 surface area 24 m2
Figure 6.2 The effects of scaling up.
Thus, while a small item of equipment may operate in a satisfactory manner, the effect of. doubling its size may be disastrous unless the consequences of scaling up are anticipated and compensated for. How would you solve the problem in the above example? If a process still appears viable after laboratory tests, the next step is generally the construction and running of asmall· scale pilot plant. Proceeding to the pilot plant stage involves a considerable increase in expenditure on the project. This important decision is often taken by a project committee formed from representatives of all the company departments concerned. The pilot plant is operated on acontinuous basis
information from which detailed engineering drawings are prepared and capital costs are estimated. Considerable thought must be given to the materials of construction, for excessive corrosion not only causes the plant to wear out rapidly but may contaminate or discolour the product. The uses and limitations of some common fabrication materials are given in figure 6.4 below. The pictorial flowsheet also enables an estimate to be made of the day-to-day running costs of the plant, including consumption of steam, electricity, and water, maintenance costs, cost of labour, and so on. The sum of the capital and running costs gives the amount of money needed to make the product. A predicted value of sales is made by the sales department and this, minus the cost of making and marketing the product, represents the potential profit. If the project proves to be a borderline case, the flowsheet can be used to i dentify the main items of cost and further work can be concentrated on trying to reduce these. Ultimately, the decision whether or not to build the plant is made by the company directors who sift the evidence for and against a particular project. They obtain this evidence firstly from the various departments within the company, secondly from their inside information concerning the company's financial position, and thirdly from their knowledge of industry in general, developments in World trade, and political and economic trends at home.
Figure 6.3 A pilot plant.
and usually gives much valuable information on the effects of scale-up, engineering design of equipment, materials of construction, and corrosion problems. The small amounts of material produced may be used by the sales department for a preliminary market evaluation of the product. The information gathered during' pilot plant operation enables a pictorial flowsheet to be drawn up for the full-scale plant. A typical example is shown in figure 6.5. This shows the major items of the plant and gives the principal technical
6.2 B U I L D IN G A P L A N T Once the decision to go ahead has been made, the building programme is planned in great detail so that items of equipment are available when required and costly delays are avoided. It is usual to prepare a schedule for each section of the plant, showing the times of ordering, the expected delivery date, and the dates when erection will be started and completed. This enables the whole of the work to be coordinated.
Material Mild steel
Advantages cheap; good mechanical strength; easily fabricated; resistant to most organic liquids and dilute alkalis; widely used for general purpose construction
Disadvantages readily corroded by dilute acids and moisture in atmosphere
Stainless steel
good corrosion resistance under oxidizing conditions; withstands nitric and organic acids
several times more expensive than mil d steel; corroded by acids under reducing conditions, and by solutions containing chloride ions
Aluminium
significantly lighter than steel; easily fabricated; good corrosion resistance to organic acids, nitric acid, and nitrates
relatively expensive; corroded by alkalis and halogen acids
Copper
easily fabricated; resistant to acids and alkalis
very expensive; corroded by ammonia and amines; may discolour certain products
Glass
relatively cheap; transparent; resists corrosion by almost all chemicals including bromine; may be used to line vessels and pipes
susceptible to mechanical and thermal shock; corroded by hydrofluoric acid
Synthetic polymers
very light; often inexpensive and easily fabricated good resistance to corrosion by inorganic chemicals; may be used to line steel vessels
may soften and melt at moderate temperatures; often corroded by organic solvents
Figure 6.4 Some common materials of construction.
33
~---!
0 ;;--
:
CD
I
~
Output
running
24
hours/day
330
days/year
at 120
T Iday avg
&
130
max
i.e. 40 000 T /year avg of HNO, as 60 % Raw materials
air
liquid ammonia
, + + +
Item no.
34 tonnes/day 11 200 ton nes/year 96 % plant efficiency
1
2
3
27
28
29
30
Item no.
Ammonia trip valve
Ammonia pressure controller
Air filter
Feed water (to absorber) cooler
Feed water injection pump
Condensate tank
Acid tanks
Description
1
1
1
2
no. off
1.6 m high 1.7 m dia.
7.5 m dia. X9m
dimensions
1W1S
280 m
12 m surface
2
PICTORIAL
2
mild steel
mild steel
cloth bags m.s. case
mild steel
cast iron
mild steel
18/8/titanium stainless steel
material of construction
2000 m3/hr
2000 m3/hr
16000 m3/hr
336000 kJ/hr
1 Y, to 4 m' /hr max.
3 m'
25 tonnes 60 % acid
rate or capacity
atmos
atmos
atmos
80-30°C C.W. 26°C
80°C
80 °c
25°C
temperature
1 atm
1 atm
1 atm
150 m head
1 atm
1 atm
pressure
NH 3 gas
NH, gas
air
condensate
60 % HNO,
analysis
-
-
-
-
-
-
-
-
-
solenoid operated
to maintain constant NH3 gas pressure
condensate
steam 4kW
power cooling water
15m3 /hr
capacity 2 X 15 tonnes HNO; as 60%
FLOWSHEET
drawn by: A. Smith date: 1.4.84
Figure 6.5 Part of a pictorial fJ owsheet for design data.
Progress is constantly monitored, and any delays minimized by the rescheduling of other operations. The work can then proceed as smoothly as possible: first the site work and drainage, then the foundations, then the steelwork, buildings, vessels, machines, and pipes, and finally the motors and instruments. A few months' delay in production may cost hundreds of thousands of pounds, for while a plant is under construction interest charges are being paid on the capital investment without any return being made. Once construction is complete, the plant is ready for 'start-up'. Whenever possible, items of equipment are first run under minimum load conditions: the pumps and tanks are tested with water and the blowers and fans are tested with air. This is to ensure that the plant is correctly assembled and will run as intended when the chemical materials, which may 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
34
or another by scaling up, and it may take weeks or even months to commission a complex new plant. Even when the plant is running continuously it will be kept under constant review, both to improve the efficiency of the process and to make any product modification necessary to suit customer requirements. Meanwhile, research continues to develop new products and processes in order to ensure the future survival of the company.
6.3 MASS BALANCES The preparation of mass and energy balances is an essential feature of almost all chemical engineering design where flows of material are involved. Mass balances are based on the principle (sometimes called the Law of Conservation of Matter) that during physical and chemical changes matter is neither created nor destroyed.
Ethanol /kg h(l
Water /kg hr-1
Stream totals
Ethanol column
Ethanol /kg hr-1 Stream 2
.7 5
50
Stream 1
125
I I
l I
J
Stream 3
Water /kg h(l
Stream totals
40
5
45
10
70
80
50
75
125
,
50
75
125
Component
totals/kg
hr-1
Figure 6.6 Massbalance of distillation column seRarating ethanol
+
and water.
Consider a system which is exchanging matter surroundings: mass in
~ ) '~
with its
+
(CH3COhO ethanoic anhydride
[6.2]
Propanone at atmospheric pressure is vaporized and fed into a tubular reactor which is heated to between 650 and 800°C; thermal cracking occurs as shown in equation [6.1] . Unfortunately, as is often the case, other undesirable reactions also occur such as the two competing reactions:
mass out
~
CH3C02H ethanoic acid
)
Wecan say that during a given time interval: [6.3] mass in = mass out + accumulation
within the system
If the system is operating in the 'steady state', then there is no accumulation of material within the system, and this expression simplifies it to: mass in per unit time = mass out per unit time
If the operations carried out within the system do not involve chemical change but are of a physical nature only, such as distillation or solvent extraction, then a simpfe mass balance may also be drawn up for each component within the system. A mass balance for a continuous distillation column separating ethanol and water is shown diagrammatically 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 change while within the system (e.g. in a reactor) then the masses of the individual components will change as they pass through the system. Thus a component balance will need to take into account the kinetics of the reaction (see page 18). However, there will still be no change in the total mass of the materials and an overall mass balance may still be applied. (Note that whilst the total mass always remains constant, the total number of moles may not. Why?) To illustrate these ideas, let us consider a mass balance for a proposed chemical plant designed to produce 2600 kg of ethanoic anhydride per hour. A partially completed flowsheet for such a process is shown in figure 6.7. This represents an accounting system for all the flows within the plant. In this section all material flows are measured in kg per hour. The starting materials for this production route are propanone and ethanoic acid, and the main reactions involved are: (CH3hCO propanone
thermal) cracking
CH4 + methane
[6.1]
(CH3hCO propanone
3H2
+ CO
+
2C
[6.4]
This gives rise to a variety of substances in the process stream which must be separated from the anhydride product. The gases which leave the reactor are cooled very quickly in a quench unit by mixing them with a mixture of ethailoic anhydride and ethanoic acid (see the flowsheet in figure 6.7). The gaseous mixture then passes to a packed tower in which the same mixture of acid and anhydride is used to cool the hot gases further. The vapour stream then passes to a shell and tube condenser, where 90 % of the ketene in the reactor effluent reacts with ethanoic acid to form ethanoic anhydride as in equation [6.2] . The remainder of the gas passes to an absorption unit where most of the residual ketene is absorbed in (and reacts with) recycled ethanoic acid. The liquid streams from the condenser and the absorption unit are both mixed in a crude product storage vessel. The crude product is then fed to a distillation column where essentially pure propanone is recovered as an 'overheads' product. The 'bottoms' product from the propanone still then passes to another still where ethanoic acid is recovered overhead and recycled to the absorber and quench unit. The ethanoic anhydride product, as 'bottoms' from the anhydride still, then passes to storage through a cooler. Details of streams 12 and 15 are not yet shown on the flowsheet and must be calculated from mass balances. The composition of stream 12 may be obtained from a mass balance on the propanone still. mass in per hour
= mass out per hour
stream 10 = stream 11 + stream 12 + stream 13 or
stream 12 = stream 10 -
stream 11 -
stream 13
Since no chemical change occurs, this is true not only for the total streams but also for each component present.
35
For ethanoic anhydride: stream 12 = 3786 ~ () -
Thus:
For propanone:
stream 12 = 6641
-
6621
-
= 14 kg hr-1
6
Check total: stream 12 = 14213 - 6622
For ethanoic acid: stream 12 = 3786
-
1 -
= 2635 kg hr-1
1151
5285 kg hr-1
- 2306
= 2636 kg hr-1
1149
Figure 6.7 Flowsheet for the manufacture of enthanoic anhydride. 2
16 14
propan one
obsorber
food
5
quench unit
15
crude product
13
1
2
3
4
Component
Propanone feed
Ethanoic acid make-up
Feed to cracker
Cracker products
Propanone
2245
8866
6650
S Total product ex qup.nch
1
1842
Ethanoic acid
onhydride
----,1'~
storage
Anhydride
I
product
6
7
8
Liquid product from condenser
Feed to absorber
Off-gas from absorber
6650
5670
980
9
1600
32
142
142
1152
3515
71
71
1120
1120
112
1
Methane
599
599
600
600
Unsatu rates
176
176
176
176
Carbon monoxide
296
296
296
296
22
22
22
22
4
4
4
4
8867
11619
2403
1321
Ketene
Carbon dioxide Hydrogen Total kg hr-1
Component
Propanone Ethanoic acid Anhydride
8867
9217
2245
1842
9
10
11
12
13
14
15
Li quor ex absorber
Feed to propanone still
Propanone recycle
Anhydride still feed
Anhydride recycle
Ethanoic acid recycle
Anhydride product
971
6641
6621
6
14
3753
3786
1
1149
2495
269
3786
1151
9
2306
2518
Ketene Methane Unsaturates Carbon monoxide Carbon dioxide Hydrogen Total kg h('
36
4993
14213
6622
"
Q1 Using the composition for stream 12just derived, calculate the composition of stream 15 by means of a mass balance on the anhydride still. State: a whether the plant will achieve its production target of 2600 kg hr-I of ethanoic anhydride
b the percentage purity of the product by
a the new composition of stream 15
mass.
b the composition of stream 14 from the
new still. Q2
If the product specification demands 99 % pure ethanoic anhydride, then a more efficient anhydride still will berequired. Assuming no change in the number of kg hr- of ethanoic anhydride in the product stream, calculate: 1
Q3 By performing a mass balance over the absorber, calculate the flow rate of stream 16, assuming it to be pure ethanoic acid.
\
6.4 E N ER G Y B A L A N C ES Just as mass balances provide an accounting system for the flow rate of material through a chemical plant, so energy balances enable chemical engineers to predict the energy transfer requirements at each stage. The high cost of energy demands great efficiency in this area, and every effort must be made to ensure that surplus energy from one section of the plant will be used elsewhere with a minimum of waste.
Heat content of input cooling water = 8000 x 4.2 x 15 = 504 000 kJ hr-1
For a given system during a fixed time interval: energy input
For example, if the specific heat capacity of ethanoic anhydride is taken as 2.0 kJ kg-1 K- 1, then the heat content ofthe input stream is 2600 X 2.0 x 140 =728 000 kJ hr-1 • This ethanoic anhydride is to be cooled using river water at 15 DC, with a maximum discharge temperature of 30 DC. Suppose the optimum water flow rate through the heat exchanger is 8000 kg hr-1 • Specific heat capacity of water =4.2 kJ kfl K-1
energy output
energy input = energy output + energy accumulation
As for mass balances, during steady state operation the accumulation term is zero. Energy may enter and leave a chemical system in many forms, such as kinetic, potential, electrical, and heat energy. However, in the simple example which follows, only heat energy is involved.
Example: Heat balance over the product cooler Consider a heat exchanger designed to cool 2600 kg hr-1 of ethanoic anhydride, initially at 140 DC. It is common practice to calculate the heat content of a process stream relative to a datum temperature at which the materials are said to possess zero heat content. (273 K is often used by chemical engineers.) The heat content of a stream above this temperature may be calculated using the expression: Heat content = mass flow X specifi c heat capacity X temperature above datum
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, it is possible to calculate the exit temperature of the ethanoic anhydride. This may be summarized on a diagram such as figure 6.8. Total heat input
728 000 + 504 000 = 1 008 000 + heat content of anhydride exit stream heat content of anhydride exit stream 224 000 kJ hr-1 Let the temperature of the anhydride exit stream be t°c 224 000 = 2600 .'. t
Figure 6.8 Heat balance over anhydride product cooler •
= total heat output
x 2.0
x t
43 D C
.
Anhydride product cooler
IN
Material
Cooling water Ethanoic anhydride
Flow /kg hr-I
Temperature
I"c
Heat content /kJ hr-I
8000
15
504000
2600
140
728 000
Material
~
Q4 Calculate the ethanoic anhydride exit temperature in the above example if:
1 232 000
Ethanoic I - anhydride
Flow Temperature /kg hr-1 /0 C
Heat content /kJ hr-I
2600
ICooling water
I
Input
OUT
Total
Output
a the maximum water exit temperature is only 25°C (assuming unchanged flow rates) b the cooling water flow rate is 9000 kg hr-' (assuming unchanged water temperatures).
8000
30
1 008 000
1 232 000
Q5 Construct a heat balance table similar to figure 6.8, using your results from the heat exchanger experiment (4.2).
37
C H E M IC A L E N G IN E E R IN G
A CASE STUDY In this Special Study we have examined some of the problems encountered when large quantities of materials have to be chemically reacted or physically separated on a continuous basis, and we have looked at the ways in which chemical engineers have tackled these problems. The object of the Case Study is to give you an opportunity to investigate further the applications of these ideas. Y ou are expected to write a report on a chemical manufacturing process. This report may be based upon a visit to a chemical plant in your vicinity or can be prepared solely from the material produced by several of the larger companies about the chemical processes which they operate. Whatever your sources of information, you should try to treat the chemical process which you choose to study as a series of 'unit operations' , and use the principles developed in this book to describe the scientific basis underlying each step in the process. The style and presentation of your report will obviously depend upon the nature of the process which you choose to investigate. However, a typical report should contain some consideration of most of the following points.
Process control Special safety precautions necessary Disposal of waste products Geographical location of plant, with reasons Market value of product Principal uses of product. It is unlikely that you will be able to obtain information about every one of these aspects of your chosen chemical process. Alternatively, additional information may be available which is not included in the list. The important point is that you should take this opportunity to conduct your own investigation into the chemical engineering principles on which amodern manufacturing process is based. Y our teacher should be able to suggest sources of information to you and give some additional guidance on the form your written report is to take.
REVIEW QUESTIONS
Q1 Overall Process outline including flowscheme Mode of operation: batch or continuous Alternative processes available to manufacture product Advantages of chosen manufacturing route, including economic considerations.
Raw materials Sources of raw materials and approximate cost per tonne Method of transport of raw materials to site Storage capacity for raw materials Separation/purifi cation operations before reaction Heat transfer operations before reaction.
Ethanoic anhydride reacts with water as follows: (CH3C010(l) ethanoic anhydride
If the reaction is carried out with a large excess of water, the kinetics are pseudo first order with respect to ethanoic anhydride.
At 20°C the value of the rate constant k is 0.11 min-1 • If the initial concentration of ethanoic anhydride is 1 mol dm-3 , calculate the residence time required to achieve 75 % conversion at 20°C in abatch reactor vessel of capacity IOdm3• The reaction in part a is to be carried out in acontinuous b stirred tank reactor, also of 10 dm3 capacity. Use the design equation 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 i n the CSTR than in the batch reactor. What are the advantages of the CSTR which make it a more attractive proposition for large·scale production? d Calculate the flow rate of reactants through the CSTR in part b. e When estimating the throughput of a batch reactor, allowance must be made for the 'shut·down time' between batches. Calculate the maximum shut-down time which would still allow the batch reactor in part a to equal the throughput of the CSTR in part b.
a
Synthesis stage Chemical changes to be performed Kinetic and equilibrium considerations Possible side reactions and by-products Reactor type and conditions used, with reasons Use of catalysts Reactor design, dimensions, flow rates, special features Materials of construction.
Separation stage Substances present after reaction Types of separation operations used, with reasons Separation equipment details, including materials of cons· truction Heat transfer operations during separation.
General Process energy requirements Types of pumps and other ancillary equipment necessary Instrumentation and measuring devices used 38
2CH3COzH(l) ethanoic acid
Q2 Ethene may be manufactured by 'the thennal 'cracking' of ethane gas at temperatures in the region of 900 DC.
Studies have shown that this reaction is first order with respect to ethane. Rate = k[C 2 H6 ]. The value of the rate 1 constant k is 770 min- - at 900 DC.The reaction is carried out in a tubular reactor and, because such cracking is highly endothermic, a constant temperature of 900 DCis maintained by heating the outside of the reactor tube strongly (isothermal operation). Ignoring any changes in volume, use the design equation for a continuous 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 a reactor tube of length 65 m and internal radius 0.1 m, for 50 % conversion. Q3 Ethane-l,2-diol (ethylene glycol) is an important industrial chemical 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) """0/ epoxyethane
-+
CH20HCH20H(I) ethane-l,2-diol
However, the ethane-l,2-diol formed may itself react with epoxyethane to produce an undesired by-product commonly known as diethylene glycol. CH20HCH20H ethane-l,2-diol
(1) + CH2 -CH2 """0/
(1)
epoxyethane -+
CH20HCH20CH2CH20H(1) diethylene glycol
Using this information, and by considering the characteristics of batch, CSTR, and tubular reactors, decide which type of reactor you would recommend for the large-scale manufacture of ethane·l,2-diol. Explain how you arrive at your answer.
Q4 Temperature/composition data for mixtures of methanol and water at standard atmospheric pressure is given in the table below. Mole per cent methanol in liquid
Mole per cent methanol in vapour
Temperature J OC
0.0 10.0 20.0
0.0 41.8 57.9
100.0 87.7
30.0 40.0 50.0 60.0
66.5
78.0 75.3 73.1
70.0 80.0 90.0 100.0
87.0 91.5 95.8 100.0
72. 9
77.9 82.5
81. 7
71. 2
69.3 67.6 66.0 64.5
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 is heated, what is the composition of the vapour obtained from the liquid as it boils? c If this vapour is condensed andredistilled, what is the composition of the liquid obtained after two simple distillations? d How many simple distillation stages are required to produce a liquid containing 95 % methanol? e If a packed column of heightl5 cm is required to bring about the degree of separation in part d, what is the 'height equivalent to a theoretical plate', (HETP) of the column packing? f During a batch distillation, the percentage of methanol in the distillate tends to fall as the distillation proceeds. What causes this effect? What adjustments can be made to maintain the quality of product during such a batch distillation? a
Both of these reactions may be regarded as first order with respect to epoxyethane and have similar rate constants at a given temperature.
39
