Chemistry in Chemical Engineering

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-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.

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