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CONCEPTUAL DESIGN OF CHEMICAL PROCESSES •

James M. Douglas Un~rsilY

of Massodwsttts

McGraw-Hili Rook Company New York 51 Louis San Frlnciseo !lockland 80101. Cliracu Colorado Springs Ibmburll LISbon London Madrid Muico M,I.n Monrreal New Deihl Oklahoma Cily P.~ rlns Sail Juan Sao Paulo Sinpporc Sydnq' Tokyo Totonlo

CONCEPTUAL DESIGN OF CHEMICAL PROCESSES INTERNATIONAL EOIlION 1988

ABOUT THE AUTHOR

Exclusive rights by McGaw-Hili Boote Co.- Singapore lor manufacture and export. This book cannot be (....xported from the country to which it is consigned

by McGraw-Hill. 10 09 Of! 07 20 09 08 07 06 OS 04 OJ PMP BJE Copyright G1988 by McGraw-Hill, nco All rights reserved. No part atlhis publication may be reproduced or distributed in any form or by any means, Of 510fed In a data base or retrieval system, without the prior written permission of the publisher.

This book was set in Times Roman. The editors .....ere B.J.Oat1l: and James W.Bradley. The production supervisors were Diane Renda and LDuise Karam.

Ubl'ary of Congress Cataloging-in-Publieation Dala Douglas,James M.(James Merrill) CoooeptuaJ design 01 chemical processes.

(McGraw-Hili chemical engineering series) Bibliography:p. Includes index. 1. O'Iemical prooesse5. I. Tille. II. Series.

TPl55.7.D67 1988 ISBN D-OHl1n62·7

660.2'81

87-21359

When ordering this title use ISBN 0-07·100195--6

Printed In Singapore

James M. Douglas, Ph.D.. is currently a professor of chemical engineering at the University of Massachusclls. Previously he taught at the University of Rochester and at the University of [klaware. Before entering leaching, he spent five years at ARea, working on reactor design and control problems. l-Ie has published extensively in areas of reacting engineering, process control (including two books). and conceptual process design. He won the Post-Doctoral FellowshIp Award at ARea, the Faculty Fellowship Award at the University of Massachuselts, and the Computing and ChemIcal EnglOeerlOg Award of AIChE.

DEDICATED TO:

The loves of my life, My lovely wife. Mary E. (Belsy) Douglas, My mOlher, Carolyn K., and the memory of my falher. Merrill H. Douglas, My two wonderful kids, Lynn and Bob, aod to my colleagues, who have taught me so much aboul design and control, Mike Doherty, Mike Malone, Ka Ng, and Erik Ydstie, and to my students, who have suffered so much.

CONTENTS

Preface

..

A Strategy for Process Synthesis and Analysis

Part I

I-I

1-' 1-3

2

,-, '-I

'-3 '-4

,-, ,-. '-7

3 3-1 3-' 3-3 3-4 3-'

The Nature of Process Synthesis and Analysis

I

Creau,·c Aspcc;:u of Proo:;;ess Onlg" A IhcraKhlCal Approach to Conccplual Iks.lgn Summary. Eac:rcue5, and Nomcnclil1urc

3

Engineering Economics Cost Informalioo Requned Estimating Capital and OpelaliDg Costs Toral Capital 'nvestment and TOIII Produ~ Costs Time Value of Money

Measures of Process Profitabllll)' Simphfymg the economIC AnalysIs for Conceptual Designs Summary, Eacrciscs. and Nomenclature

• " " "..." 2J

32 37

.

Economic Decision Making: Design of a Solvent Recovery System

72

Problem Dclimllon and General ConsideratlODS

72

Design ora Gas Absorber. FlowshcCl, Malena! and Energy Balances, and Stream COSIS equipment Design Conslderauons Rules of Thumb Summary, eXerCISeS, and Nomenclature

""

"

90

x;

xii

ro~

~.

Part II 4

.-,

'-1

'-J

5 '-1 ~,

'-J '4

Developing a Conceptual Design and Finding the Best Flowsheet Input Infonnation and Batch versus Continuous Input (nfonnabon u'"el·! DecIsion 8atdl venus Continuous Summary, Elercues., and Nomenclature

99 99 107 111

Input-Output Structure of the Flowsheet

116

Decisions for lhe Input-OulPUI Siructure Deslgn Variables, Overall Malerial Balaooes. and Su"am Costs Process Altc:malives Summary, EJ.erriscs" and Nomc:oclaturc

116 123 IJ2 IJ2

6

Recycle Structure of the Flowsheet

.. 1

DeCIsions that l)etermme the Recycle Structure Recycle Material Balances RuetOf Hut Etreeu EA:juilibnum L!mltalJOns Compressor Design and Costs Reaelor DesIgn Recycle F.oonomlC EvaluatIOn Summary. berClSeS,. and NOOlCnclature

.., ....., .-. .. 3

'-7 "8

7 7-1 7-' 7-J 14 7-' 7-'

8 8-1 8-' 8-J 84 8-' 8-' 8-1 8-8 8-' 8-10 8-11

9 97

137 137

14' 14' 14. 13J 13. 138 13.

Separation System

16J

General Slructure or the SeparatIon Syslem Vapor Reoovery System Uquid Separalion SYSlem AZC'olroptC Systems Rigorous Material BaJances SummaI"}'. ElerCJSCS. and Nomenclature

16J 168 172 189 204

Heat-Exchanger Networks

21' 21'

Minimum Heating and Cooling Requirements MInimum Number of ElchangeD Area Estlm.ates Design of Mlnrmum-Enerl)" Heat-EJ.cbanger Networks Loops and Paths Reducing the Number of Elchan~D A More Complele Design Algorithm Stream Splilting Heal and Power Integration Ileal and DIstillatIOn HDA Proass SummaI"}'. EJlerasc::s, and Nomendature:

'-1 '-2 '-3 '4

.-,

Cost Diagrams and the Qujck Screening of Process Alternatives Cost Diagrams CQ5t Diagraml for Complex Processes QUln>quc for PEllION

.,

Purge Heal

Ccmp....... I

I

IHcat I CooIllDl

Reactor

IHcat I

I

I Hca' I )u

].

Toluene

~

~

Diphenyl

-

Ft""

H 2, CH",

1-

Benune

"

"• "& ~

T

•

Jj :D

•

i

ncURE 1.2-1 HDA process lAfttrJ M DDt.gloJ. AlOE J. JJ JH (l9&.S).]

The homogeneous reactions take place In the range from IISOoF (below this temperature the reactIOn rate 15 too slow) to 1300~F (abo\'e this temperature a significant amount of h}drocrackmg takes place) and at a pressure of about 500 psia. An e:xcxss of h}"drogen (a 5/1 ratio) is oc:c:ded to prevent coklOg. and the reactor effluent gas must Ix rapidly quenched to I 15O"F in order to pre\'ent cokmg m the: heat exchanger following the reactor. One: possIble: fJowshc:c:t for the: process is shown in Fig.. 1.2-1. The toluene and bydrogen raw-material streams are: heated and combined with recycled toluene and hydrogen streams before they are fed to the reactor. The product stream leaving the reactor contains hydroge:n, methane, benzene, toluene, and the unwanted diphenyl. We attempt to separate most of the hydrogen and methane hom the aromatics by wing a partial condenser to coodense the aromatics., and the:n we flash away the light gases We: use the liquid leaviog this flash drum to supply quench cooling of the hot reactor gases (not shown on the 80wshee:t). We would like to recycle the hydrogen leaving in the: flash vapor, but the methane:.. which enters as an impurity io the hydrogen feed stream and is also produced by reactioo 1.2-1, will accumulate in lhe gas-reeycle loop. Heooe, a purge stream is required to remove both the feed and the: product methane .from the proceSS. Note that no rules of thumb (design guidelines) can Ix used to estlma.te the optimum cona:ntration of methane that should be allowed to accum~late m the gas-rec}"C1e loop_ We discuss Ihis design variable in much greater detalll~te~. Not all the hydrogen and methane can beseparated from the aromatics 10 the flash drum, and therefore we remove most of the remainlllg amount in a distIllation column (the stabilizer) to pre\'ent them from contaminating our benzene product

10

5EClION U

~

A HIE.....CHlCAl AI'f'lOACH 10 COI\ICEPTI,IAl OfSlGN

The benzene is then recovered in a second dlstlllation column, and finally, the recycle toluene is separated from the unwanted diphenyl. Other, alternative flowsheets can also be drawn, and we discuss some of these as we go through the analysis.

Energ:r Integration The process f10wsheet shown III Fig 12-1 IS not very reahsllc because: II implies that the heating and coohng requirements for every process stream will take place In separate heat exchangers uSing external utilities (coohng water, steam, fuel. etc.). In the lasl decade, a new design procxdure has been developed that makes II possible 10 find tbe minimum healing and cooLlIlg loads for a proa:ss and the heatexchanger network that gnes the "best" energy integration. This procedure is described In detail in Chap. 8. To apply thIS new design procedure, we must know the flow rate aDd composition of each process stream and the inlet and outlet temperatures of each process stream. One alternative f10wsheetthat results from this energy tIllegratlon analySIS IS shown 10 Fig. 1.2·2.· Now we see that first the: reactor product slream IS used to partially preheat the feed entering the reactor. Tbc:n tbe: hot reactor lases arc: used to dnve the toluene: recycle column reboiler. to preheat some more foc:d, to drive the stabdi7.c=r column reboiler,to supply part of the benzene: product column reboiler load, and to preheat some more feed before the gases enter the partial condenser_ Also the toluene column is prc:ssurized. so that the condenslllg temperature for toluene is higher than the: boiling point of the: bollom Slream in the benzene column With this arrangement, conde::mmg toluene can be used to supply some:: of the benzene rc:bollc::r load. ins-lead of using steam and cooling water from external sources of utllltlcs. II we compare the energy-lIlu:gratc:d f10wsbect (Fig.. 1.2-2) with the flowshec:t indicating only the need for heating and cooling (Fig. 1.2-1), then we see that the energy integration analysis makes the flowshcct morc: complicated (i.e.. there arc many more interconnections). Moreover. to apply the energy integration analysis, we must know the flow rate and composition of every process stream, i.e.• all the process heat loads including those of the separation systc:m as well as all the stream temperatures. Since we need to fix almost aU the Bowshcct before we can design the energy integration system and since it adds the greatcst complication to the process flowsheet, we consider the energy integration analysis as Ihe last step in our proocss design procedure. Distillation Train Let us now consider the train ofdistiUation columns shown in Fig. 1.2-1. Since: the unwanted dlphenyl is rormed by a reversible reaction (Eq. 1.2.2), we could recycle

.l! :t"

1"..-

~~ 2"u -'

u

e~

u " !'.

"

] tl

~

I;

0-

l"

"• 0u

E

0

"

,f

~

u

•

u:•

~~

,

r

,

p

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~

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~"B .ll ~

c1

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'T

1

~: uwnjO:)

t ()

:IU~~

)

.

;)

uwnlOO In!llqllflS :t• U

;..

'"

1,

I uwnlOO

r

,I

,

:)\J~nl°.L

>. e

T

r

u

~

0-

is

I L

+" 0

~

'" u u

"-

"-

0

,

• ThUi wlUllon WH developed by 0 W To..·n!iCnd al lmpenal ChcmlCllllndLl$lf1CS, Runcom., Unlled Kmldom

t

"

• E

•"

~

"

II

12

s£CTKm 1.;1

SECTION 11

Il NIElIllllCHIClll IlPPIIOM'H 10 C'OI'I("(I'Tl'lll DESIGN

Benzene

Toluene (To recycle)

Il HlE....IICHlCIll ""PlIOlleH 10 CONO:I'TlJIll DESIGN

13

might be chealXr than uSing Ihe configuration shown in the original flowsheet (Fig. 1.2-1). The heurisllcs (design guidelines) for separation ~ystems require a knowledge of the feed composition of the Slream entering the distillation tram. Thus. before we conSider the deci~ions associated v.lth the design of the distillation train, we must ~peclfy the remalOder of the llov.shcct and esllmate the process ftows. For thiS reason .... e consider the design of the distIllation train before we consider the design of the heat-d ..... lrnl

la. .~".,

12.000 107.000

"'n.'....

f''''''

gross ulcs

.........

From J R F

InlOnwaetunn, ap"..]

...

Profitabilily Or c)clobexaD~ manur.clure

10,000,000(024)(0 OS)

IS~~

29.000

J It F••• W.. L"".nol) Do:o.lp l:... ~n"), 51 Louu.. Mo_ 1967

Me>... 1"7

Cyclohuanc; I..S.ooo gal @SO.2Jesllmaled ... Olh~l. II S Yo lVlUIi lilies

SlO.ooo

'.600

tln'II011 U....,-

I. Rlw Qlltcnal (SO~~ full) e"lI. 24,jOO pl@SLt23 1. Goods ID prOCCSl UI I1SO pi @ SO.u

10.370

....000

... Y.'orlle d,.poul pbnt. lire p'IcetlOU

(4)

NOIII"IXIU I~nl cornpo$Cd of oltic:c rUl1l/IU~.nd eqwpcnt::ol, ukly .nd medlCll CQUlpn!llll. shop CQwpmcnl, .ulomOfl'-c. c.qwpmc.nl, yard mllC.naJha",lhnl c.qulpmc.nl, 1.00ralory eqUipment, shd~c.s.. bllli. paJkli, hand lrucks. fire nllnlul~hers. bOK$, fire. c.nI'IU:S, Io"dlng c.qlllpme.lll. D,wlb"rron wuJ p"d.Gg,ng !Ddude I'lIw-malmal.nd prodtH;! slorlF and handllnl cqulprnc.nt, plOdUCI pack.llnl equlpmenl. blcndllli r.cihlia;. loadlnl it.UOI\S..

d

'-oJ.• bout I

2~.orFClor4 &~.orPUrdlasc.dequlp'p"nlOOiU_

SuO'c)'i.nd recs PrGpen) ~I$IrulirtCl caw lire. uprMd 110I dJr-ealy lo"'ohul "'IlII matmlll and la.bor 01 aa~llnslall.­ (I) (2)

8_

uoCl,lIboulls.-JO~.orFCI I LtgllWlr"'fJ tJNI ~u_, .bout 4~21 Y. or Fel or S-ISy' or duM ....1$11_ !Aimu,"'fJ COSTS Indude .dmlllJ~ral1~c., process d""'ICl and geucral c.nl'n«nng,

or

2

10"

dmung, CO'it c.n&lnc.c.nng. proocsslng. e~pedlllOl, reproducllon. communlCatlonli. _Ie. modell.. consulI.nl rtts. lra"e1 II ErogutN''''fJ Sf'IN"u_ Qnd IfIJ~/ltMl CQIIJ'fJIC/I"" U/W'UU; IIboUI 4 8 21.0Y. 01. FCI /J TtfPIIIO'/J'Y I«Jllln composed 01 COIlSINCIIOll. O-pcra11Oll., .nd IZAUlleMIlOt' 01 IaDpora.ry fllQbues; ofIiot::s, roads. parkllliiou. raliroacb. clec1nca1. p1p1nJ. commUDOClUOns. kDan... II Coru'TVCI_ ,oob lo Prod + Prod + Prod + 2.18 x lOs Prod (2.5-21) The lenns on lhe nghl-hand Side are the conlnbullons or the \anous quanlllies 10 the total product poce; I e.. the unus of eacb term can be t/lb product. If any of lhe terms on the right-hand Side arc \ery large compared to lhe current product prices. we want 10 consider process alternall\es. For cases where a process produces multiple products. such as a pelroleum refinery. the analysis becomes more complex. In these: situations. we consider both modificalions of the prOClCU thai lead to different product dislributions and processes thai can be used to convert one type of product to another- We contmue in Ihis way unliJ we have developed as many cost expresSions as there are products, and then we look for the optimum process alternative and design condilions.

d(Re\'enue - Tot Prod. Cosl):dl 0'

d(Tot- InvYdx

Revenue - Tot. Prod. Cost Tol. 10v.

(25-22)

However, close to the oplimum design condition, the incremental return on an incremental invcstmenl will become \ery small If this is the case, it ....-ill be more ad\'anlageous to allocate lhat incrementalm\cstment 10 a projcc1 where we would obl3.l0 a 15% DCFROR or CCF = 0.333. Hence, from thLS consideration of incrememal relurn on IOcremenlallO\'cstment, .... e r(:qulre thai d(Re\enue - Tot. Prod_ Cosltdx 0 "'-'-="'T'::~-'-,.;,'--'--'--" ~ .]]] d(Tol In' )fdX

(2.5-23)

In olher words., to find Ihe opumum design condmons for a case Ilohere the mlmmum required produci price IS less than the currenl pritt. first we muimize CCf by solving Eq. 25-22. Then we evaluale CCF at lbe optimum design; and if lhis value is less than 0.3]]. we sohe lhe problem by using Eq. 2.5-23. If the oplimum CCF docs exceed 0.333, we might want to consider the possibllllY of increasing the plant capacHy, since the return on our invcstmenl will then be beller than for most of our other projects. Of course. marketing consideralions may limn this alternative.

Economic Dedsions among Process Alternalin5 Optimum Design In many silualions we want 10 find Ihe values of design variables, such as reaclor conversion, Ihal maximize the profitability of the process. To do this, first we look for the values of the design variables lhal wiU minimize Ihe produci price thaI guarantees us a 15% DCFROR; ie_I we minimize C... in Eq 25-21 (or the more enCI relalionship given by Eq 2.5-16 divided by Prod). If the minimum product pm~ that we oblam from this analysis exceeds the current producl price, we use a supply-and-dcmand analysis 10 decide whether we should lemnnate the projecl

10 general, we prefer to select the process allernative that salisfies the producllon goal and requires the least capital invcstment, because with a specified CCF this process normally will give lhe smallest producl price. However, if the least cxpensive process involvcs a lot of unproven lcchnology. highly corrosivc or hazardous materials, an uncertalll supply of raw materials, or other similar factors, we must assess the addilional costs that we may encounter in overcoming polential pro~lems

in addition, in some situalions we can decrease lhe losses of either materials or energy from a process by installing addillonal equipment For lhese cases we

agam require that the mcremental return on thiS additionalmveslrnent satisfy our m\e5tment enterion. i e; • CCF of 0.333_

output structure of the ftololisheet, Le., 1e\c1 2 economic potential EP 1 at tbis le\c1 as

In

the hierarchy, we can define an

EP, ,., Re\"enue - Raloli Mati

Economic Decisions for Process Modifications

- (Power

or Replacl'ments If our ne\lo' Idea in1l0h'es the modification or replacement of part of a process by a ne'" technology. we SIll] ... anl to achie\e a 15 e;.. or more, rcturn on our investmenl because this project loliiU be in competition With other projects considered by Ihe company. The m\cstment required is equal to the COSt of the new equipment minus the ac.ual maTur t"alur of the equipment we are replacing.. Note tbat we sbould use the aClual market value in the calculalion rather than the original cost minus the depreciation we ha\c already reco\"ered, becau.$C our original estimates of the equipment life and the depreciation might have been in error. In other words, we always base our economic dcrisions on present conditions. and we ignore our past mistakes. just as we drop out of a poker game if the cards reveal we ha\'e link chance of winning e\'en If we ha\'e a large stake in the pot. The 5a1lmgs ... e expect to gam from the replacement are the old operatlOg costs plus the depreoatlon of the old equipment over Its expected hfe as judged from the present (and nOI the onginal deprecialion calculation) minus the: operating costs for the nev. eqUipment plus the depreciation for this equipment over Its cxpected hre Ifthe~e savings provide a 15% relurn on the nel investment, we might want to conSider the replacement project u~ing more detailed design and costing procedures

2.6 SIMPLIFYING THE ECONOMIC ANALYSIS FOR CONCEPTUAL DESIGNS In Eq. 2.5-17 we presented a \"ery simple economic model thal we can use: for conceptual designs (te.• the screening of a large number of flowsheet alternatives by using order-of-magnitude estimates to determine the best flowshec:t or the best few alternatives): Revenues., Raw MatI. + UtiL + Ann_ Install. EqUiP, Cost + 2.13 )( 10' Opcratof!i

+ ADn Cap. Cost of Feed Compress. if any)

(2.6-2)

Sunilarl)'. when we consider the recycle structure of the f1owsheel, i,e., level 3. and v.e genera Ie cost tstlnlates for the reactor and a recycle gas compressor (if any). we can wnte

EP J

,.

Re"enue - Raw MatI. - (Feed Compress. Cap

+ Op COSt) - Reactor Cost - (Gas-Recycle Compress. Cap

+ Op. Cost)

(2.6-3)

Thus. as we add more detail to the ftowsheet. we merely sublract lhe ncv. utilities costs and the annualized, installed equipment cost of the new equipmenl that is added If the economic polential at any le,'e1 becomes negative. we havc three options: I. Terminate the design stud)'. L Look for a better process altemati\'e.

.l Increase the product pnce so that the economic polentlal IS zcro. and contmue wilh the design. If we follow option 3. we e\entually delermine a \'alue of the product price that would make the process altemau\'e under consideration profitable. If thiS nev. product pnoe were only slightly higher than the current price, we would probably contmue wilh the design. (We need to undenake II supply-and-demand anal)-sis 10 sec how far in the future that .....e might e.tpect to obtain this higher pritt) However, if the product price required to make the alternative profitable Iil;ere much greater than the current price at any of the levels in the hierarchy. we would terminate the work on the current alternative and look for one that was cbeaper. If none of the alternatives were acceptable, we would terminate the project This approach is very efficient because it makes it possible to term mate projects with a minimum amount of design effort

(2.6-1)

The: annualized instalkd equipment costs lire determined by multiplying the installed equIpment costs (sec Sec. 2,2) by a CCF which includes all the investmentrelated costs

Significanf Equipment Items The case study considered in Sec. 21 is somewhat unusual because one piece of equipment (the recycle compressor C-I) comprises almost half of the total purchased (or installed) equipment cost. However. suppose we consider another ca.sc study,· for tbe disproporlionation of toluene to produce benzene and xylene.

Economic Potential In Chap" I we presented a hIerarchical decision procedure that would simplify the development of a conceptual design The appro.timate COSt model presented abO\'C fits lOtO the hICrarchical framework \·ery nittly Thus. when we consider the input-

• R. J Hmplebeck IlDd J T 8.lacheto. Du"optl",_tlOtf t{ T~. WaslunglQII UDJvel~'ly De.p Caw Study No I. ediled by R 0 Snuth. Wash,nston Un,."...ly. St- Lou!$, Mo JII_ 26. t969

TABLE 1.6.. 1

InHslmenl summary, S

•

Pumps (1949) P·I P·2 p.] PA PA p., p.•

.;; ~

"'

~ • .,;

..

0

"'

... M

0 ~

N

~ ~

"'

~

~

,•

tncIudllll spates Pumps (1%9)

•

~

~

~

E·' E·2 E·' EA E·'

"

~ 0

~ ",

E~

%

f ...•

•

...

0

E·' E·'

10

N

U

Ul

"'

.

..

~ ~

• ,•
06

"

..

"'"

2000

'"

1700

Fr..... "- J ll lcb.d and J T Banc:ho,o. WUbJqlOOl U.,~.",,,y ",""lpI Cur Sr""y I>lo .. od,rod by 8 0 W IP_ U.",,",ly. SL Lou... Mo. 1969 • ~. "" .. ! -..d lO_y '" Inod ud ,.ndooaa,.-.lb r-. ,.oducu ......... ' . . .

S,'ever. today most rules of thumb (or design beumtics) are de"e1oped by graduate students who run 5CX) to 1000 case studies on a compuler for a particular problem and then allempl 10 generalize the results Of course:, the: fact That generahzatlons of computer optimization studlCS are possibk Implies That The optImization calculations are very insensithe to changes LD most design and cost parameters If the design and cost equations are Insensitive, then the engineering method mdlcates that we should be able to simplify the: equations. Thus. using the order-of-magnitude arguments to simplify problems, we should be able to derive the rules of thumb. The advantage of a derivation of thiS type is that the assumptions used in the analysis will clearly indicate the potential limitations of the rule of thumb.

Liquid Flow Rate to Gas Absorbfrs For isothermal, dilute gas absorbers, the Kremser equation, Eq. 3.3-1, can be used to calculate the number of trays required for a specified recovery as a function of Lf (mG). A plot of the Kremscrequation is shown in Fig. 3.4-1. From this graph we see thaI if we pick L such that L/(mG) < I, we can never get close to complete recovery of the solute even if we use an mfinlte number of plates (infinite capital cost). If the solule is very valuable or, worse, if It IS toxic (such as HeN), certainly we will want 10 obtain very high recoveries. Thus, we can conclude that we would never choose the liquid flow rale such that L/(mG) < I We also see from the graph that if we choose Lf(mG) = 2, we obtain csscntially complele recovery with ollly five plates. Remember that large solvenl now rates correspond to dllule fec:dsto the dlslillation column-and therefore large

S6

RCTION H

~VLI:S OF TIIV... UCTlO!'l 10

6

~

I plale

I~

TAC (S yr) "" (C, S/moIXGy_ mol/hrX8150 hr yr) + [C", S'(plate'yr»)(N plates) 0.4

0.8

1.2

1.6

20

2.4

2.8

87

COST MODEL It is common practice to r~pofl operating costs on an annual basIs. Thus,to examlllc th~ economic trad~-off......e mU)1 also put the capilal cost on an annualized basIs As discussed In Sec, 2.5, we annualize the capital coSt by InlroduClng a capital charge: faclor (CCF) of i yr, where the CCF mclude:s all capllal-related e"pcn~ (depreciation, repairs and mallltenantt. etc.). A CCF of ~ yr corresponds to aboul a 15~ discounled-cash-f1ow ratc: of return (DCFROR); sec Eq 2513" Suppose we: wrile a IOlal annual cost (TAC) modd as

3 2

,'i,

~VLES Of" TIfVIoU

(3.4-3)

3.2 OPTIMUM DESIGN. No...... if we use our simplified design equation, Eq. 3,).12 we obtain

-L

.c FlGURa,: 3.4-1 Ii: remKr equauol1. (/'-mm T K, SMrwooJ and R. L Ntw YO'"k, /9J1.)

J>iQf~d. AbwrpllOII .1I"t f"ITIICIIOII,

TAC - 8150C,GY,.(Y:,,')

MeG'II....",'/,

+ C.(610&

:i. _

2)

)-,

),.

(34-')

The optimum fractional loss is given by reflux rallOS, high vapor ratcs, large-diameter columns, large condensers and reboilers, and high sleam and cooling-water demands. Hence, if we pick L such thai L/(mG) > 2, we will obtain tin), lIlexpc:nsive absorbers, but very upc:nsive distillation columns. Based on these simple argumen15, we find that .....e want to choose L such that 1
t Ihe economic potential .enus rhe design and

10~) eJlp

.ariablc:s.

6.8-3.. De\-dop the reC)"c1e structure ror the butadtcne sulfone process (see oerCI$IC 5.4-9). As· sume that 6H.. _ -48.000 Btu/mol. K ... _ (6846 J( 10- 1 ' ) e"p (-36.940/(TfR)). k l _(8.172 J( 10") eJlp (-52.200/T(OR)), and lr, - k~IK ...(moINt'.br)): that the reaction rale corresponds to the stoichiometry: and that we use a CSTR for the reactor_ ConSider \'ariable denSlIy effeeu and assume that the annuaJizcd, installed reactor cost lS !Iyen by 315011'" [$I(fl' yrn Plot the economic porential >ersus the significant design \'anables. 6,.1.9. De\'elop the recycle strUCIure ror thc butane alkylation plocesS (see Ellcrase 5 4~ 10) AS5ume that AH, _ -27,440 Btu/mol, AliI _ -25.180 Btu/mol, t, = (9.56)( 10") ell' {- 28.oooI1RT{·R)]} hr I, and k J _ (2.4]9 J( 10") up {-]S.OOOf[RTfR)]} hr - I. Use a CSTR With the cost correlation gi>'en in E"ercise 6.8-8. Nomenclalure

A, 8, R,S. a" h,

A bhp C,

Reactive and product components Order of reaction for component i Ileat-cxchanger area (ft J) Brake horsepower Conoentration of component i

162 ~U.-rlON..

C, £, f(C.. , C.)

1.

F F,

F, Fn

F, F,

F. hp

"K,.

M

MR M &S P,

Pia. PJ

P,,~,

p,.

Q.. Q.

"

•R, T., T_ T•.• U

x Y.. Y,

h. AH.

,

Sl'MMAII.'f.

uunsu,

AN" NOMEtoCLATUl.E

lleat capacity [Btu/(mol "..-)) Activation energy for component i Function of composition Fugacity of component i Rcactor fccd rate (moljhr) Conection factor for a gas compressor Feed rate in excess of reacllon requIrements (mol hr) Fresh f(cd r.lte of toluene (molfhrj Mal-eup gas rate or III (mol/hr) Flow rate of compClnent i (moljhr) Reactor rec:d rate: of toluene (moljhr) Horsc:po....cr Reaction rate: constant Equilibrium constant Molecular weight Molar ratio Marshall and Swift mdex ror inflation ProductIon rate of component; (moljbr) Inlet and outlet pressures for a gas compressor Pressure at stages of a gas compressor Total pressure of reactor Volumetric ftow rate (ft1/min) Reaclor heat load (Btujhr) Reaction ratc Gas constant Recycle gas flow (moljhr) Inlct and outlet temperatures from a gas compressor ("R) Reaelor temperature: (~F) Overall ~eat-transfer coc:ffiClCnt [Btuf(hr. ft 1. CF)] ConversIon Feed mole fraction of H 1 Mole: fraction of component i Purge composition of Hz Heat of reaction at rcaccion tcmperature and pressure (Btu/mol) Fugacity coeffiCient of component Cp/C. - I

Cp/C.

CHAPTER

7 SEPARATIO SYSTEM

Here we consider only the synthesis of a separation system to rc:eo\er gaseous and liquid components Our diSCUSSIOn is broken down mto thrc:c separate: parts. general struelure, vapor reco\'cry system, and hquld separation system. Also keep in mmd that .... e need to detemline the best sc:panuion system as a funelion of the design variabks, I.c., the range or the design vanablc:s In FIg.. 6 7-1 .... here \loe obtain profitable operation Thus. our previous economic stu(hes hc:lp to simpllfy the computauonal effort.

7.1

GE ERAL STRUCTURE OF THE SEPARATIO ' SYSTEM

To determine the geDeral structure or tbe: separation system......e first determmc the phase: of the rc:aelor effluent stream (see Fig. 7.1-1). For vapor-liquid processes, there arc only three posslbilltlCS:

I

I. lIthe reactor effluent is a liquid. we assume that we only need a liquid separation system (sec: Fig. 7.1-2). This system might mclude distillation columns. cxtrac-

tion units, azc:otropic distillation, elc., bUI nonnaJly there will not be any gas absorber, gas adsorption UflitS. etc. 2. If the reactor emuent IS a two-phase mixture, we call usc: the reactor as a phase spllller (or put a flash drum after the reactor). We send the liquids to a hquld separation system If the reactor is operating above cooling-water temperal ure, \loe usually cool the reaclor vapor stream 10 IOOQF and phase-sphl this stream

163

164

SECTIOI'" 11

GENERAl sn.ucrullE OF nlE SEPAIl.. noN SYSTEM

SF.CTlON 1(

Purge

GENERAl STRUCTURE OF TIlE SEPA1lAl10N SYSTEM

165

(see Fig. 7.1-3). If the low-temperature nash liquid we obtain contaills mostly

reaclants (and no product COmponents that are formed as intermediates in a Reactor system

r't'cd streams

/'

consecutive reaction scheme), then we recycle them to the reactor (we ha,'c the

Separation systt'm

/

Products

the reactor emuenl stream contains only a small amount of vapor, we often send the reactor effluent directly to a liquid separatlOll system (i.e, distillation train). 3. If the rcaclor emuellt IS all vapor, we cool the stream to lOO"F (coollng-waler temperature) and we allempt to achieve a phase splil (see Fig. 7.1-4) or 10 completely condense this stream. The condensed liquid is sent to a liquid recovery system, and the vapor is sent to a vapor recovery system. If a phase split IS not obtained, we see whether we can pressurize the reactor system so that a phase split will be obtained. (We see whether a high pressure can be obtained by using only pumps on liquid feed streams, and we check to see that the pressure does not affect the product distribution.) If a phase split is still not oblained, then we consider the possibility of using both high pressure and a refrigerated partial condenser. In case no phase split can be obtained without refrigeration, we also consider the possibility of sending the reactor effluent stream directly to a vapor recovery system.

Focus on reactor exil Slream FtGURF 7.1-1 Pha.sc of the relict". effluent stream

Liquid sepanllion system

Liquid

Reactor system

F=J,

equivalent of a renu~ condenser). However, if the low-temperature flash liquid contains mostly products. we send this stream 10 the liquid recovery system. The low-temperature nash vapor is usually sent to a vapor reco\'cry system. Bu!. if

Products

We need 10 ensure that the: same structure: is obtained for the complete range of design variables under e:onsideration. These: rules are based on the: heuristic that phase splils are the cheapest method or separation and the assumption that some type of distillation separation is possible.

FIGURE 7.I·Z React". cUI is liquid. [F,om J M. DtNgltu, Ale"£: J., 31: JH (l98J).J

I

r-):5- 35"C

Vapor

Ph"" split

Vapor

r--------------

F=J,

Reactor system

Liquid

Purge

I

Gas recycle

Vapor recovery system

Liquid Liquid

Liquid se:paration system

I

Purge

Vapor

co'x""'')' system

-,

Vapor Reactor system

Vapor

35°C ;-

Products

Ph"" split Liquid

I Liquid recycle

Liquid separation system

FIGURE 1.t_3

Reador uJl IS ~apo. alld Jiqutd.lFrom J M Do"9fas. Afe,,£: J~ 31 JJJ (/98J) l

FleURE 7.1-4

RC unattepwble, opllOIl5 are:

1]

UOUlD S£'.......T10S SYSTEM

175

the azeotrope nonnally requIres two columns and therefore is expensi\c. Howcver,

if we recycle the azeotrope, we must oversize all the equipment in the recycle loop 10 handle the incremcntal ftow of the extra components. A gencral design heuristic

SL3blhzcr column Panial

Applicability of Distilllliion

conden~r

Pasleurizatlon section

NOle: Recycle vapor stream

does not seem to be available for mai-Ing thiS deciSIOn, and so we usually nccd to evaluate both alternatives. Azcotropic s)stems are discussed in more detail in the next sccllon.

10

vapor recovery syslem if possible.

FIGURE 1.3-1 AhcrualJVCI lor remoVlDI baht ends

DESTINATION OF UGIIT [:"ODS. For lhe deslinauon of the light ends, we can venl them (possibly to a flare system), send tbe light ends 10 fud, or recyde the 1Ight ends to the vapor rtto\'ery 5~slcm or the ll.ash drum. If the light ends have u:ry little value, we wanl 10 remove tbem from the process through a venl. IfthJs ,-enting causes air pollution problems, we try to vent them through a flare system to bum the offending component. If most of the light ends are flammable, we try to recover the fuel value. However, if the lighl ends arc valuable, we waDt to relain them in the process. If we recycle them to the vapor recovery system, we introduce another recycle stream into the process. SUMMARY FOR L1GIIT [f\,'05. If light ends will not contaminate the product, we merely recycle them to the reactor with a reactant-recycle stream or remove them from the process with a by-product stream that is sent to the fuel supply If light ends will contaminate the product, they must be removed from the process The method of removal and the destination of the light ends depend on the amount of light ends. Hence, we must delermine the amount of light ends as a funcllon of the design variables before we can make a decision.

In general, dlslllJalion IS the lcast expensive means of separating mixtures of liqUIds.. However, If the relauve volatihucs of two components vo-Ith neighboring boilmg points is less than 1 I or so, distillation bc::comes very expensive; i.e.. a large rcflux ratio IS required which corresponds to a large vapor rate, a large column diametcr,large condensers and reboilers, and large steam and cooling watcr costs. Wheocver we encounler Iwo nelghbonng components having II relatl'-e volauhty orless than 1.1 in a mixture, we group these components together and VO'C treat this group as a single componenl 10 the mixture. In olher words, wc dc\-e!op the best distillation sequence for the group and the other components, and then vo'e separate the lumped components by using other procedures (see Fig.. 7.3-2).

Column

~uencing-Simple

Columns

For sharp splits of a thrcc+componeni muture (with no azeOlropcs) we can cuher reco"cr the hghlest component first or the heaViest componenl first, and then we split the remaining two components (see Fig.. 7.3-3). When the number or components increases, the number of ahernauvC$ increases very rapidl) (see Table 7.3-1). lbe spillS that can be made in the 14 alternatives for a five+component mixture are lisled in Table 7.3-2. It appears as if it will be a major task to decide which distillation column seqUCDCC to sclecl for a panicular prooess, particularly since the best sequencc

It

A

• 3_2

(B,C)

Separate [ B design task _ C

.... --- ......

IS

Lump----l

1.71

I

t~ __ ~~ D

1.0

E

0.4

r--D D,E

Auotropcs with Reactants If a component forms an azcQlrope wilh a reactant, we have the choice ofrecycllflg the azeotrope or spliuing the azeolrope and just recycling the reactant. Splitting

L-_E flGURE 7.3-1 Oul.l1lauQn K~"'"0n)

176

UCTl0N U

lIQUID S[PAUTlDS SYSTflol

SECTION 7J

,-B

,--A

r--A

1I0lllD SEPAUTION S'($11'r.l

177

TABU: 1..]..]

Gt'nrul beutistK::s for column wquencing A

I. ll.emo>'t: 0I>rT0$I>'t: c:omponeal$ all 100II as possobk. 1. Ilemo",", tcao;t,ve componclll$ or ~ n .. _ t i possibk 1. Ilanm'e prodUCIJI as d~hl1ales .. Remo", rcqeloc wurns all datillates. plrtJallarl1 .r they an ~

A B

B C

C

10.

'----c

'--8

Direct

Indirect

FIGURE 1.3-] 0'$1111."')11 IhemllOves ror I lem.ry mlllUtC

TABLE 7..]..1

Number of ahemathes Numoo of comPOIlalIJ Numoo of St:quenc:ell

TABLE 1.3-2

Column Rq~nces fot

I

1 3

,•

• 1

I

•

10

"

11

..

l)

2 I

, •, ., .•, 3

ti,". product sunms

Colo_ ,

C..... ,

A BCDE. A.BCDE. A BCDE. A BCDE. A,BCDE. ABiCDE ABiCDE. ABC/DE. ABC/DE AIlCD/£. A8CD/f. ASCO/f ABeD/£. ABCD/E.

B/eDE. B/eDE. BCDE. BCDE. BCo/E

AlB A'8 DIE D'E A/BCD A/BCD AB/CD ABCiO ABC'D

CoIooo 3

C.......

C/DE.

01'

CDE

CID

BrC

D/E.

BCD

CrD B(C

BClD C/DE. COlE. AIBC AB/C B/CD BClO

A'B A/Be ABIC

o/E.

C/O 8'C A'8 C/O 8'C C,D lliC A'8

J*d:ed bed l'CKIl)r

might change as I,l,e aller Ihe design variables To simplify this effort, we might want to look for heuriSllCS for column sequenCing. There has been a considerable research effort in this area over the past decade or so, and some of the results are given below. GENERAL HEURISTIcs. There are some general heuristics thai can be used to simplify tbe selection procedure for column sequences (see Table 7.3-3). The first lKuristic in this list is based on tlK fact that Ihe malerial of construction of the column is much more expensive than carbon steel if corrosive components are present. Thus, the more columns that a corrosive component passes through, Ihe more expensive will be the distillation train Reacti~·e components will cbange the separation problem and tbus should be removed Il.S soon as possible:. Monomers foul reOOiJers, so it is neoessary to run the columns at vacuum conditions in order to decrease: the column overhead and bollom temperatures, so that the rate of polymerization is decreased. Vacuum columns are more costly tban pressure columns, and we prefer to avoid the increased cleaning costs. We prefer to remove products and recycle streams to paeked bed reactors as a distillate to avoid contamination of the product or recycle stream with heavy materials, rust, etc., which always accumulate in a process. If it is necessary 10 remove a product or recycle stream 8S a bottom stream, it is often taken as a vapor from a ~OOiler and tben condensed again. At tbe same time a small, liquid purge stream may be taken from the reboiler to prevent tbe buildup of contaminants. COLUMN SEQUENCING HEUlUSTlCS FOR SIMPLE COLUMNS- A number of other heuristics for selecting sequences ofsimple columns (i.e., columns wilh ODe top and oDe boltom stream) have heen published; a short lisl is given in Table 7.3-4. TARL£1~

Hauistia for column Sl!:C(UfttCing I. MO!II plentiful lint 1. Lighlcst Iinl ),. H,gh-l'CCXlvcry Jepllt.lIons luI. 4.. DtfficuJI Jepll'It>OllS IIlSI ~ F...or t:qUlmollr SpillS•

" Nest,.."anuon should be cheapesl.

118

Sf:CT1Ot< H

5t:CIl0N JJ

UQtJIIl SI'.'AIUTION SnTDot

However, Ihe firsl and fifth heuristlcs III this list depend on reed eomposltlons, whereas the second and rourth depend on relative volatilities Henee, we ellOpcct that these beunsucs wiJIlead to contradictions; i.e., if the most plentil"ul component Ui tbe heaviest, there is a conOiet between the first and second heunsllcs. A longer hst orheuristic:s has betn published by Tedder and Rudd,· and some Iflvesligatorli ha\e tried 10 order the importance or the heuristics, 10 resolve the conOlcts.' A suney or the literature has been presented by Nishida, Stephanopou. los, and Westenberg, I and a detailed discussion or the limitations or these heuristics has been pubhshed by Malone et all Some additional discussion of the heuristics IS given below. We might also note that as we change the conversion in a process, we expect that Ihe unconverted reactant will go from being the most plentirul component at very low conversions 10 Ihe least plentirul at very high conversions. Hence, the heuristics in Table 7.3-4 imply that the best column sequences will change as we aller the design variables. Similarly, note that tbe studies used to develop the heuristics were limited to sequences of simple columns having a single reed stream that were isolated rrom the remainder or tbe process, so thai different results may be obtained when we eonsider the interacllons betwttn a distillation train and the remainder of the plaol. INTERAcnONS BETWEEN TIfE SEPARATION SVsn:M AND THE PROCESS. For ellOample, suppose we consider the two flowshttt alternatives shown In Fig 7.3-4a and b. We might consider these two configurations to be two of the allematives in a sequencing problem. However, there is a different number of columns in the liquid.recycle loop ror the two systems, and therefore the recycle costs will be different. Hence, the optimum conversion, which usually corresponds to a trade·off between selectivity losses and recycle costs, will be different ror thl: two cases. Of course, we should compare alternatives at the oplimum processing conditions of each alternative, rather than on an identical feed-stream condition for tbe two allernati.. cs. From this simple argument we .see that the problem or selecting the best separation sequence cannol always be isolated from the design or the remainder or the process: i_e.. tbe least expensive sequence for a fixed feed-slream condition might not be the least expensive sequence (becauce the feed-stream condition sboukl be cbanged to correspond to the optimum Oow). In faet, there might be another heuristic:

Select the sequence that minimizes Ihe number of columns in a recycle loop.

· 0 W. Toddcr &.lid D F Rood, AlCIIE J~ 104 )OJ (1918). ' I D Seader &.lid A W WClitcrbcrg, A/ChE J~ 13 951 (1917).

'N NlWda, Ci SlcplwlopoulO1.,.Dd A_ W Wale/berg, AICII£ J~ 19 l26 (1981). 'M F MalllM., K, CiUIIOI., F E. Marquez..Dd I M Doua.W. AICIa£ J .• JI 683 (I91S).

(7.3-1)

lIe,,1

Reactor 1----jCoolant

Reactanl Product

=

LlQl..'10

n .... ATIOI'l

SYSTEIol

179

Flash

Light

,00'

.;;~

J

(oj

F=I

--.:l:....tH~"'~'j ---1 """'0< ~-.jCoolant I----L~~J Reactant Light

end~

.;; 1--_ _--'

Product (bj flGURE 1.3-4 Scqueooe sde>cl1OG cl\.o.lI&CS recyck

oon~

MULTIPLE SEPARATION SEQUENCES. Suppose we consider separalion syslems that correspond to the general flowsheets given in Fig. 7.1-2 or 7.1·3; I.C., .....e need both a vapor and a liquid recovery system. A flash drunl never gives shurp splits, so that some 01 the most volatile. "liquid" components Will leave with the flash vapor, and orten they need to be recovered and sent 10 a hquld separatl?n system 1-l0.... eH·r. the Ilash liqUid nllghl contain a large number or much heaVier

180

5ECTlOfo;

H

1I0UIO SE'UATlOI'I JYS'TDt

components as well as those that are returned hom the vapor recovery system. In silualions such as thIs. It might be better to split the sequencing problem into (Wo parts. That IS, we would split the flash liquid 1010 one portion contammg the oomponenls returne.i born the vapor recovery system and one portion containing the heavier components. Then we would design one separation system having a single feed stream for the heavy components and one separation system having multiple feed streams for the components returned from the vapor recovery system. AN' ALTERNATE API'ROACII TO SELECTING COLU 1N SEQUENCES. The

reason for allemptmg to develop heuristics is Ihat the number of alternati\'e sequences increases very rapidly as the number of components increases (sec Table 7.3--1). However, there are a large number of plants where four or less distillation columns arc needed to accomplish the separalion. Four simple columns (one top and one bollom slream) arc needed to separate a five-componenl mixture into fh'e pure streams. but only four columns arc needed to separate sill: componalts. if two components With neighbonng boiling points leave in the same stream Thus, for five exit streams and using only simple columns, we need to consider the 14 sequences shown m Table 7.3-5. An examination of this table mdlCates Ihat 20 column designs are requIred 10 evaluate all the possibihties. Twenty column designs requires a considerable amount of effort if each of Ihe designs IS rigorous. However, by using shortcut procedures (see Appendix A 4), it is possible to significantly simplify the calculations. Using shortcut techniques. Glinos· demoostrated thai thC' evaluation of the 14 sequC'nces '''..as almost instantaneous on a VAX 11-780; i.e., the results appeared as soon as the program was run. Kirkwood' has shown that the 14 sequC'nces can be evaluated in only a few seconds on an IBM-PC XT. The results of Glinos and Kirkwood indICate that for modest-slZC' sequencing problems it is bellC'r to develop computer codes that evaluate the 00515 of sequence altemalives than it is to use heuristics. Moreover, the running times for these axles .re sufficiently small that the best sequence can be detennined as a function of the design variables.

Complex Columns Rather than consider only sequenoes of simple columns (one overhead and one bouom stream), we can consider the use of sidestream columns, sidestream strippers and reboilers, prefractionators, etc. One set of heuristics for columns of

TABLE 7.J-S

HeuriSlics

rM complex columns-Tedder .00

Rudd

CntuUl I(AI(C

Eue4 IICp"r.uon

IndeX

If ESI < I. the A B Splil

1(.".

0A.

(ESI) "" - - - IS

0.,

harder thall tbe BIC Splll U ESI > I, tbe AlB spbl IS ea$lef than the SiC

_rllt lleunSlIQ lor ESt < I 6 I. H 40 10 80"; IS mtddlc product and nearly eq.... 1 amounts or overhead.nd bot.toms are presenl.

then favOI desll" S 1. If mon lhan ~"IS rmddle produd aftd lao Wtt S"; IS bottoms.. t1aal Ca"O< desl.p 6. l. I f _ than~" Os middle prod'llCl and lao Ibn S" 1S0werheallllClXltllL.,.,U~,,,,z ... I) b U..c tbt ,lId,reet ""'IlICllU II ZA' 'rL.,. ~ le,) e C.Jcub.le lhe .-apor ~IG If (:r d - I) \"... l) > I~,.,'{L, -f la) > 1/("....

2.

-+ I).

Skl~nam ooluJlUU, G AI.... )s wrwda woln,. SlIksueam wtumn .. hen L .. and 0' L,. der llSInJ' PdClitrcam

30

RelIal~ dlSl.iJl.alloD.

c.g., B = HNO)

e

bonom strcam Now, in a third column. wc reco"cr purc .... ater. our second product, as the bottom stream, along wilh lhe onginal bmary azeotrope o\crhead ThIs binary azeotrope IS recycled to the first column. and .....e obtam pure producls from the system of three columns

= H 20 S = H~O~

FIGURE 70U blracll~e distiIIeoc

~

'" DiPhenylT F1GUR[ 1J-12 HDA pl"0Q::5$.

U

•

k\l1:1 4

(7.3-1 )

- 0.947

A"'" H2• CH.

Denuoc

••

~

~

:E

T

T

AZEOTROPIC SYSTEMS

In tbe previous section where we discussed the sequencing of trains of distillation columns, we assumed that it was possible to split the feed mixture between any two components (see Table 7.3-2). However, if azc:otropes are present, it is often impossible to achieve certain splits. This, for azeotropic mixtures it is essential to be able to tdentify when distillation boundaries are present that make ocrtain splits impossible.. Most of the discussion below concerning the behavior of these systems has been taken from the papers Doherty and coworkers..

or

S

~

Distillation Boundaries For ideal, ternary mixtures we can use either the direct 01 the indIrect sequenoc (see FIg. 7.3·3) to oblam three pure products. However. for azeotroptc mixtures, the feaSible separations orten depend OD the feed composition Henoc, It is necessary to

190

SIOCTION 14

S!£TION 14

ALIOOTIlOPIC SUUMS

understand lhe behavior of these prOLCSSCs

In

much greater dC:lail than the IdeJ.1

me For example,. suppose we conSider the ternary mll:turt: of acelone, chloro· form. and benzeoe. We can plot the composItions on a triangular dIagram, and .... e nOle the fact that Ihere is a maJ:lmum boIling azeolrope for acelollC--chloroform blflary mixtures. We abo p101 Ihe boiling temperature; see Fig 7.4--1. ow If .... e suppose Ihal ....e put a blflary mixture having a compo:'lIlon corresponding 10 poinl A 1fI a simple stIli ,md c;onllnue 10 increase the lemperature In the sliII. the composilJon of the malenal remaining In the sull ....·111 move In the direction of the arrow shown on FIg 74·1 (toward the bmary azeolrope). Also mIXtures neh 1fI acetone would be recovered from the top of the slili. In contrast, SHirting with a binary mixture correspondmg to point B on Fig. 7.4-1, as the slilltemperature is increased,lhe malerialleft in lhe still will again be the binary azeotrope, but the overhead will be rich in chloroform. Binary mixtures of acetone and benzene at point C or chloroform and benzellC al point D will both lead to final stili mixtures of pure benzene. Suppose no.... that e conSider lernary mixtures corresponding to points A and B on Fi@ 7.4-2. As e increase Ihe temperature 1fI a simple sUI~ the stIli

191

SO.1 Benzene

I

c

Bmary SO. 1 Benzene

AZEOnO',C SYSTt:~IS

,""".rope 56.2

644

Chloroform

612

flCURE 1.4-2 Terna.,· m""UfCS

D

c A

Acetone 56.2

8

Binary azeolrope 64.4

FIGURE 1.+t AoolOQC-chlorolOl"m bclUl'llC Iyilem-blnary .'ilufd

Chlorofonn

compositions for each mixture will approach Ihat or the binary azeotrope, until at some poin! they will tend to collide. Since benzene has a hIgher boiling pomt than lhe binary azeOtrope, as ....e continue to increase lhe stiU temperature, the trajectories (residue curves) w"l bolh lurn toward benzene. Thus, the final composition In the still pot for both cases will be benzcne. Ternary mixtures corresponding to points C and D on Fig. 7.4-2 will also yield benzene as the final still composition. There arc rigorous proofs for this type of behavior; see Levy. van Dongcn, and Doherly" However, if we merely say Ihat for every source lbere must be a sml, Ihen .... e can de\'elop reasonable pictures of the behavior; i.e., each traJeclory (residue curve) must have a stopping point lhat is eilher a pure component or an azeotrope (these polOtS correspond to the singular polOIS or the sct of differentIal equations describing a simple still).

61.2 • S. G Levy, 0 8 •..,n DoDf;C"D, and M F Doherty, ~ Ocs>&a &nd Syalbc:w of Azcolropoc DuullauOlL II M.lD1IIIUDl lIeftw;. Calcu1allOni,~ /6.£C h",,-,,'j>/s, 24 463 (1915)

192

SEctiON 1.4

Al~OT"Ol'rc IYS'nM5

SFCTIOI'IH

When ",e consider more starling l;:onditions, .....e obtain the results shown in Fig. 7.4-3. Now we see that there is a distillation boundary going from the binary azeotrope to benzene that divides the composition triangle into two distinct regions. Feed mixtures to the left of this boundary produce acetone-rich mixtures overhead and lead to pure benzene in the boltom, whereas feed mixtures to the right of the boundary lead to chloroform-rich mixtures overhead and pure benzene in the boltom_

193

80.1 Benzene

SEPARATION DEI'ENDS ON TIlE FEED COMPOSITION. Suppose thaI we now consider the separation of a feed mixture having a composition of.x in Fig. · . FI 7.4- 4 In Iwo contmuous columns using the indirect sequence. It can be shown that the distillate, feed, and bottoms compositions for a single column must fallon a straight line (this is the material balance expression). Hence, if we remove essentially pure .benzene from the bottom of the first column and recover essentially all. the ~nzene m the ~oltoms,. the ov~rhead ~omposition will correspond to point A In Fig. 7.4-4. Now, If we spill the bmary mIxture corresponding to point A in a second column, we will obtain esscDiially pure a~tone overhead and the binary azeotrope as a bottoms stream_ 80.1 Benzene

UEOnOl'lC S"\>TEMS

Separalrix or simple disllllation boundary

Acetone 56.2

Binary azeotrope

Chlorofonn

64.4

61.2

FlGt:RE 7.4-4 Sequence of 1"·0 ronunuous columns.

Separatrix or simple distillation boundary

Acetone

Binary

Chloroform

",""ope

56.2

64.4

61.2

FIGURE 7.4-3 Relidue CUnall Doagm. ll1td JtI F o.4«'y, 1&.£( F~,,'tUJ, U 46J (/9Ij), ""i,}, ".,mU5,Ott!'om 'M A..."..cmI CMm...ol S«v'J)

Heptane

1.0

08

0.6

R = 2.15

X, 04

0.2

oLL--'of.2'---'0!-,"'-all

~ ... DNJ M F DoIwr'1, IdEC FuruJonw"ftW, 2-1 46J

(lSISj}. ..,A ".nJft1M_!IDM llot A..-K'1JI'I CJwm.cgJ Soocotly]

'99

200

SECTION H

AZF.oTJIO!'lC nJTQ.lS

Heptane

F Using our shortcut calculations, we found thai the purge ftow rate was 496 molfhr and that the gas·recycle flow was 3371 rnol/ltr for a CllSC where x _ 0.7j and Y,.H - 04. Hence, the fraction of tbe flash vapor tbat is purgc

nOION I '

10

each temperature

",000 110.000

"',000 ""000

These results also are pIoued on Fig. 8.1-7. From Fig. 8.1-7 we Dote that the enthalpy of the hot streams tbat must be rejected 10 a cold utility is Qc "" 60 X 10] Btufhr, and the arnount or heat that must be supplied from a hot utility is QB _ 70 X 10] Btu,lhr. Moreover, wben TN _ 1400 and Tc =- 130", we see that the minimum approach temperature exists, i.e., the heating and cooling curves are closest together. Thus, this temperature-enthalpy diagram gives us eUelly the same information as ~'e ~nerated previously. Suppose now that ~·e set the base entbalpy ortbe cold curve equal to 110.000. instead of 60.000, and ~e repeat the calculations for the cold curve. This shifts the composite cold curve 10 the rigbt (see Fig. 8.1-8). We nOle from the figure that the beat we must supply from a hot utility increases by SO x 103 to 120 x 103 Btu,lhr. Thus, the increase in the heating load is exactly equal to the increase in the cooling load. Also at the point of closest approach between the curves (T..... = 150°F and Tcekl = nO°F) the temperature difference is 20"F. Thus, if the minimum approach temperature had been Specified as 2O"F. tben the minimum heating and cooling requirements would have been 120 x lOJ and 110 )C IO J Btufhr. respectively. and the pincb temperature .....ould change from T. = 140 and T_ = 130 to T. = 150 and T..w = 13O"F. By sliding the curve for the cold streams to the right. we can chan8e the minimum approach temperature, Q"._ and Olllhlp 10 fint law

• D Boland IDdt:: Illlldffilllh, Hal u.chanlC'r NClworllmp.o~cmcnls..~ (/0,,,. E.NJ 1',~, 11Il(1): 47 (19&4), H uMhotf and 0 k V.edcveld,· P,nch TochPook>f,y COIUQ or Age. C/o,m. £IOfI 1',~, 110(7). H (198-4) M

M

228

5f:CT10J'i'I

~F("lION.l

MlJ'iINUM IlUTING ANI) coot.lNG "EQUIIlf.MErO"S

".

MINIMl'W !lUTING AND C'OOlI'0-;1.. '20 + Q£(("')--+~'-=.='----:::;-:--;::{J T = 130

120 + Q£

-+__

T ~..1'211:0,,+_--1'---_---,;;-::-;;;_ _ "

-T-90

c-

H ~ 130 )10 - Q£

\

T"" ISO

- - T .. 145 Q -

-----..x)

130

_ 168.4

220

Q : Q - \20

T - 120

so

-..-..A ...... r

1 N e , which is not Stream data al the pinch

I

I NH S N c?

y"

I Fi~PH ~ FcCl'C for every pinch match

y"

I

No Spilt a hot stream

INo

Place pinch matches

L

Split a stream (usually cold)

It

IIUl "''''D

P'OW'[a

INlWUlIO",

261

allowahle. Ho",·e:\e:r. If \loe: spill a cold stream, We can make: a match between stre:ams I and 4 away from the pinch and thc:reby &\'OId the: F u e,,11 < FcC"r constraint (see Fig. 8.7-6)

General Design Pl"ocedurl' A gc:neral dc:sign algorithm for conditions abov'e: and belo", the: plllch is sho",n In Figs 8.7-7 and 8.7-8 Afte:r we put 10 the' plOch matches corrc:ctl). It u~ually I~ a sImple: task to complc:te: the: deSign.

8.8

HEAT A 0 POWER INTEGRATION

According to the first law. heat and power are related Thus, it should not be surprising that the energy integration procedure can be extended 10 giv'e results for heat and power integration. A detailed analysis of heat and power mtegration can be found m Towsc:nd and Linn hoff.· lIere:, we only outline the basic ideas. Heal Engines Every lex I on thennodynamlCS diSCUSses the perfonnance and effiClCncy of heat engines as mdividual entitic:s Howevc:r, if we lake a total systc:ms viewpoint. we obtain a lolally different perspecrive. This systems approach was previously discussed for gas absorber/distillation processes in Chap, 3. Suppose we conside:1 a cascade diagrnm for the process sho"'n in Fig, 8-8·1 We pu! heat into the network, no heat is a1l0l01'c:d to cross the plOch. and we remove heat below the: pinch. Now suppose that we inslaU a heal engine above the he:al input to the cascade diagram (Fig. 8.8-1 shows this arrangemenl at the hlghcst possible temperature, although from Fig. 8.1·5 we sec: that it could be a lower temperature). U we add an incremental amount of heat IYto this engine, recover an amount of work W, and reject the remainin8 heal QIoo 10 the network (which is the amount of beat that we: were required to add in any case), lhen the efficiency of the heat engine based on lhe inCI"eme:ntal amount of eoc:rgy input is lOlly" ThermodynamiCS tals imply that the efficiency of a heat engine is always less than 100Y. because some of the heat output must be wasted. However. wilh Ihe arrangemc:nt shown in Fig. 8.8-1, this waste heat is just whal is required for anolher task.. Thus., again. a systems viewpoint leads to different conclusions from the consideration of a particular unit in isolation. From Fig. 8.8-2 we sec: that if we install a heal e:o:,:ne below the: pinch. we can also oblain an incremental efficiency of lOO~,.. We convc:rt some of the heat that would be discarded to a cold utility in any event into useful power, and then we

FlGUR.- 1.,-1 DeslP ('fcaadUIIe

!odmtr the- pnch_ (F~_ B LmnItoff n aI~ J9I])

·0 W To-'nKnc! and B llnllholl. -IIe111 .1Id Po-n Nc:twcade

- -

U

'-r nGUR£ 8.9-1 Cucadc dl&g"'m Iud dl~lIl1ill.on. (F,om B LuuJ.off, H Dw./onl. und R. S"mh, C4cm. Clog S£-I.l] /17S (/98j))

Q, Q~,

'----

y '----

Dlphen)"l FIGURE 8.9.... 1I0A prlloCC:u

I Q,

Design Procedure

Energy

A design procedure for the energy integration of a 'rain or distillation columns WIth a proocss has been presented by Hindmarsh and Townsend" H we consider ,he HDA process' (sec: Fig. 8.9-4) and develop the composlle enthalpy-temperalure diagram for a partIcular set of flows, we obtain the resuhs shown in Fig. 8.9-5. ThiS diagram shows that the column condensers and reboilt:rs fall across the pinch, which ~'e said was nol a desirable situation. To sec how to pressure-shift the columns and to best integrate the columns wllb the remamder or Ihe proces:s. il IS simpler first to remove the columns from lhe: process (sec Fig 89-6) and to consider Just the energy IDtegralion of the process With no columns (see Fig. 8.9-1). The correspondmg T-II curves for the columns are shown ID Fig 8.9-8, and we can move these curves around by pressure-shIfting

a>cade


;11 rapidly increase. Of course, we could a ...oid this difficully enlirely simpl)' by eliminatmg the steam preheater Hentt. our inspection of the cost diagram indicates that an energy mtegration analysis should be undertaken.

Use or Cost Diagrams to Identify the Significant Design Variables Numerous optimizatIon variables exist for the f10wsheet shown in Fig.. 9.1-2. indudmg the con...ersion.. the reflux ratios in both distillation columns, the fractIOnal reco\·ery of atttone overhead in the product column. the fractional reco\'C'ry of azeotrope o\"erhead and II> ater in the bottoms of the rce)'c1e column. the fractional recovery of acetone in the compressor, the approach temperature ~tween the Dowtherm fluid leaving the furnace and the gases leaving the reactor, and the approach temperature between the steam preheater outlet and the reactor products leaving the feed-effiuent heat exchanger. Almost all of Ihese optimization problems involve only local trade-offs. That is, the reflux ratio in either of the columns affects only the cost of that column, and the approach temperature for the feed-effluent exchanger affects only the cost of the feed-effiuent exchangers, the steam preheater, and the partial condenser. Howe...er, changes m the conversion caUS(: the recycle flow rate to change.. and therefore changing the con ...ersion affects the cOst of every piece of equipment shown on the f1owsheet. Thus, if the design com'ersion is not dose to its optimum value, we can pay significant economic penalties, whereas errors in tbe reflux ratios are much less important; i.e., the total separation cosl of either column is a relati...e1y small fraction of the lotal cost of Ihe plant.

COSt diagram helps us understand the interactIons among ...anous pIeces of equipment. Thus. it is \'ery helpful in screening alternati ...es. although the final design of the Mbe!iC" alternati...e should be reported by using the convenl1onal procedure_ With a cost dlagram .....e can also break costs into gas-reC)'c1e effects. fresh f~d elrcetS-. and hquid-recycle effects. We consider this approach as y,:e look at a more complex plant in the nut ~eclion.

9.2

cosr

DIAGRA:\IS FOR COMPLEX

PROCESSES The new energy integratIon procedure dcscnbed in Chap 8 lDtroduces a signifK:3nt amount of additional coupling and complexity inlo a flowsheet. This addItIOnal complexity makes it more dilflCult to visualil.e the IOteractions in a floll>sheet Hentt. we oeed to find a way of simplif)ing the cost dIagram

Allocalion Procedures Suppose we consider our process for the hydrodealk)'lalion of tolu~ne (see Fig 9.2-1). When we use the procedure described in Chap. 8 to energy·mtegrate Ihe flowsheel, one of the alternati...e solutions we obtain IS shown in Fig. 9.2-2. ThIS f10wsheet has so many interconncetions that it is ...er)' difficult to gain an o\erall perspective of the process. However. this additional complexit)' is primarily caused b)' the addition of the heat exchangers. Hence, our first task is to remme thIS coupling Simply b)' allocating the heat-elchanger costs 10 the indi\;dual process streams passing through each exchanger.

AUneating Heat-Exchanger Costs Following T ownscnd and linnhoft,· we allocate the annualized capit~1 cost of the exchanger to each stream proportional to the indi...idual film coefficient of that stream (set' Eq. 8.3-7). These allocations are listed in Table 9.2-1 for the HDA process. Now the flo"'sheet again appears to be the same as Fig.. 9.2-1 except that we have established a cost for each of the exchangen. Lumping We can simplify the cost diagram further by lumping costs that go togetber. In other words. for the purpose of e...aluating process alternati...es. there is no ad...antage to treating Ihe annualized capital cost and the annual power cost of the

A Systems Vieft'poinl The usc of cost diagrams enables us to look altbe total system...... herca~ in Table 91-2 we are constrained 10 look at mdividual pieces of equipment. Similarly. the

• D W TOWTlsccnd and 8 llnnhnlJ. wSurf• ..,. Arr. Tilrl!rlJ fOl Ileal Eachanger NrlwOIh: Annllal m«lInl! of tIM: tnsUllmon ofCbc:llltC'l1 fnpnccrs. Balh, Umled Klnldom. AI'1'11 1984

£!

Purge

H

1.

311K I

"''''

g89SK I

328 K

~

To!uene 298 K, ~

r""

I

89.5 K .

I

:I 89.5 K

ReaClOr

L

895 K.

r.I-

311 K

391 K _ _.J rl

Toluene recyde

~f--,

§

•

8~

~

i2

Benzene product

H, . CH.

•

~

~I 466 K

J 395 K

FlGURE U·l HDA prooeN. [hom J. M. DINg/tIS and D, C Wlll'dcock, IliEC Pro.:. IRJ. Df:lI.].4 470 UMS}.]

H, (eed

Toulenc feed

PtIrge

(8 & T)

600

Po"", 60

- 01

Toluene recycle

FIGURE 9.2.2 Cost d'JITIm rOt tnetlY.,nlerl,alecl HDA rrooeu

!8

rF."", J

M "/lIN/I", mId D C Wt>Ddrn{'/r.

Idee Proc Dn

Df'V 1r.4Pu.x P.~ES

TABLE 9..2-2

TA8U: 9.1-1

AllocaCion of

he.c~xchangtT COSfS

Onllnal CO:It, SIO'/yr 5lream

..,

.. [kwJ(IlI K)) ' II1t. allocaled «ar

'"

fk_l{m' K»)

.69

. . _D

reboilcr

1S7

•

•

....,

",

R~~

R~~

k UdQf

emuent

diluent

efttuenl

•.69

.69

I.

10

F.... J Iol 00qIu ud 0 C w"""""d, '.f;C I'rwc 0... o.c-..al Soernr

o-~ 14

kl·mot/1lr

C~.

Gas

3J71

'"~

fer;yck

LiquId recycle Toluene fttd. 271} U , feed.. 496

"

RuctQf

.69

SttBm

SIO'/rr

ReactQr

cmuenl 10

J"

" ",

Ik=~

Stabili2u n:boiler

•",

keacto. diluent

II., a1Iocaled COlt

"

Toluene n:boilcr

0.69

Strum Jr,

"

6J Ructor

RelicCor cOSI allocalion

•.69

•

"

'69

l6

4231

198

29

9JU(I9ISl. _h pel

......

F..... J M 00Dp.-.I 0 C "'oodcod./.t:(",,_ THo14 91'0 flQlSl. w1'lll p : _ « lhoe A_10m.

n.-r~

a..-icaI_r

compressor separaldy. Hence. we combioc: lbese values into a total annual compression cost. Similarl)'. we lump the capital and opemting costs for boch che furnace and partial condenser. and we combitx the column, reboiler. condenser. sccam. and cooling-water cosls for each distillation column inco a single separalor cost value. Wllh this approach we obtain the values shown in Fig.. 9.2-3 AUOCIItion to Process Streams We can gam e\'en more mslght inlo the nature of the interactions in the proce:ss 11' we nov. allocate the: costs to the processing of the fresh feed. the ~as-recyde stream. and tlK hquld-reqcle stream We base lbe allocation on the 60w rales and the

Recycle compc-essor

fractional heat loads or the various sCreams. The reaC10r allocations are given In Table 9.1-2. and the healing and cooling allocations are 8"ial In Table 9.2-3. The resull:§ of Ihis allocation procedure are shown in Fig.. 9.2-4. The raw-material COSI listed is the value in ellcess of the sloJChlometric requirements for lhe reaction. ie_. the cosl lI,e might be able 10 reduce by looking for a Dew process allemali'·e. Now we can dearly sec: that lhe: most apensive costs are lhe exoess h)'drogen and toluene that we fc:cd 10 the process. Thus. any alternative that will reduce the §t:lttlivit~, losses or the purge losses should be considered 1ne nexl most expensi\e COSIS are associated with the: gas-recycle ftow, which are considerably larger than the liquid-reC)'clc or fresh fc:cd COSts. Thus. Ihe COSt diagram provides a useful tool for rank-orderinJ, an evaluallon of process alternati,'es It is also useful for the gross screening of allernath'cs, as we show in tbe next section.

109

TolueneL. Feed and recycle heating

H,

....

371

L. Toluene column

76

I

Diphenyl

I-

f-

React..

198

TABLE '..2-]

P"",oct

I-

Benzene column

174

I

Purge

cooling 153

I-

,

Srabilizcr

28

~

Beru.ene

FIGURE '.2.J A Simplified COSI dl.",Rm '01 lhe IIDA l'fooe:l:S IF,om J, M Dougl"l aruI D C WfI'O1

F...... J M. DonaJ.u.ncl D C Wooderxl. UlFC P.«, THs ..... h,.............", al' .... Amer_n ('1onDooo1 ~r

n..,. u

172 J9

97l)tl"S~

SECTION U

QUICK SCIUNTNG

or

'10CESli ALTU"'An~ES

303

93 QUICK SCREENING OF PROCESS ALTERNATlVFS 1be systematic procedure for developing a process design thai .....e discussed nrher

,•

.

-" g

"

i!:0O

~

IS 8

NOO

· . "' .... '" "' ~

" " ·~ ••

"•

~§L,

.l!

~~

-~

"-

'""' "-Z '...l"

~-

.!!

-

i!:

Design Decisions

H

N

-

~

also be used to gc::nerate a lisl of process alternatives. All that we need 10 do is 10 leep a list of each decision thai .....c make as we proceed through the base-case design. Then as we: change: these decisions, we: generate: process altc:rnati\'cs. We .... ant to male some estimate of the economic Imporlance of each alternative, rather than repeal the: deSign for each case, In order 10 minimize our design draft. We: use the COSI diagram as a 1001 in making these estimates. caD

" .;;;:~ 6'. ~] S £

!I,~

e. . • E

~u

g:z;~.2 ~~Cor'\

~ u ,

"''''~ ~~

.. "' .... '" '8 •

~ ~

-

•

o-~

N

" ~

",Z...l H

",Z.J H

"

~"'~ ~~

-

.. '"

'" "' Z...l I !i

~

II

~

~

~

",

H

",• ,•E "

i§8

°Olo iii

-

.. "' .. '"

",Z...l

~'" ~:e "0 • E ,~

;2:::.

~

'" N N

:t

-

O~~ ~

-

N

. "' .... '" , •" 0 " ~

•

f-

"-

•" •E

§

The decisions we made 10 generate the base-case design for tbe HDA process are listed in Table 9.3·1. We can proceed through this list decision by decision and try to evaluate the savings associated with changing any oftbe decisions. An alternate procedure: would be to use the cost diagram to identify the largest COSlS and lben 10 identify the decisions thai have the greatest impact on these costs.

N

• "' .. '"

:E

~

"'Z ...l H

•

~

N

-

~

Process .altcrn.ath·es for the HDA pro«:ss

I--

",Z...l H

OO~

'" ~

.... -

"' '"...l ",Z

H

.

I

~

•

•

OO~

~ ~

" "

- ~~ .. e"' . '" .~

H

TABU '.)...1

",z

...l

H

Ln-d·2 doc:wons: Inpul-DUlpUI iUlIClIlre I Do nOI punlr Ihe bydrogen feed Ufam. 2. Recover, ralher than recydo:. dlphenyl &0 tbal there arc thn:c pwduct ilrelllli (pur,e. benzene: product, dlpbenrl by-product) 3 UIoC a p i recycle and puree: Slfam level.) dceu.IOM' Ra::ycle S1rvaufC I UK a $IuP readnr. 2. Ute; a p.J (H. and at.) and I bquid (Ioluene) ra::)'Clc 11ram.. l. Ute; a 5/1 ",.10-...010"'" 11100 10 pteW:nt cokul,-.-unun& this 10 be: a dcsip CQIlStratnl (althouJb 01 eoWd be rorm..lalllCl as In opc.im.ization problem). 4 A PJ'-f'eCJ'dc wmprasor IS -sed. S. Operate the reador adiaba~r 6. Do DOl ClOtlI>da c:quilibnwa e&cta. Levd ...... ~. Vapor I'a::1OftI)' l)'Itea I If a vapor ra::overy 1)'IteP! • UIOd., plaa: II 011 tbe: lluh Yllpor (II reqdr: ~ II loI1 10 byproduct) or the pUlJC: IIrel1ll (rllbcre IS 110 loA caused by bcm:ene rccydc) 2. Do DOl. l&1li: a vapor recovery l)'SteP! Levd-4b dcoIiom Uqwd 1oC~11I1IOlI 1)"Ilem I Mal.e alIlep11l11tions by daltiliallon. 2. Direct lCq noe oJ wnpk QOlumnl IS uscd-prnblblr UK of complu. columns Ihould be. ~"

.........

3. Remove the bght eDds in a Ilabilizer 4, Sc:od the blbl ends 10 rue! DO vapor recovery l)'Skrn. Lem-S dea»oas. EncrC lnlegrallOA. There arc oumerow allernatlves. F..- J M Doql.aIIDd D C WoodoDo.l, '40£C /'roc Va:

"'--' J02

""""

Dft~

14 910 It9tS). wItII per.......... 01 tile ..... nc:u

SlCTlO,,", U

For our I-IDA process (Fig. 9.2-4), the largest cost items correspond to the use of the excess raw materials The decisions that affect these costs are made in level 2. the Input·output structure of the flowsheet, so we want to start at the beginning of our decision list in any event. The next most important COSLS are those associated with the gas-recycle stream, and these correspond to the leyel 3 decisions. So 3g3in we find that we want to proceed sequentially through the decision list Purification of thl.' 1I)'drogen F'ud Stream

QUICK JClfENlNQ

or

paocf.SS ALna"'AnVEll

305

increased flow rate. From Hougen and Watson- Il.e find that the equilibnum constant is K ",,0.248t685 C

The stream flows for our base-case Diphenyl

0

H,

1547 2320 91 27J

CH. Toluene Benzene

de~lgn

(9.3-3)

are as follows'

The hydrogen feed stream containS methane as an Impurity, so that we mIght be able to cut some: costs by removing the methane from the feed; i_e.. we decrease: the amount ofineru thai pass through the pr~s. However. methane is also produced as a by-product in the reactor. Heoce, there seems to be Iittk iocentiye for purifying the methane feed stream On the other hand. if we should decide to purify the hydrogen-recyck stream. then .... e could feed the process through this recycle separation s)'stem We defer consideration of this alternative until later in this section.

and

Req-cling of Oiphl.'n)1

Assummg that the costs ate proportional to the Increased tlo'" and using the cost dIagram to estimate the Iiquld-reg-cle costs. .....e find that

Since diphen)'1 is produced by a reversible reaction, we can recycle aU the diphen)I and let the diphenyl bUild up in the process until it reaches iLS equilibrium Ie",::! If we adopt this approach, we chromate our selectivity 10S5eS, i.e:... the 8 molfhr or toluene that geLS converted to dlphenyl (the purge losses of toluene and benzene are oot affected). Of course. we lose the fud credit of the dipheoyl. We can use the cost diagram to estimate the raw-material savings; Raw Matt Savings = 1691(1\) - 200

= 840 (x SIOl/YT)

(9.3-1)

which is a significant value compared to the other costs. In addition......e save the cost of the toluene column .....hicb was used to separate the recycle toluene from the diphenyl. Again from the cost diagram we see that Savings for Toluene Column = 76 (x SlOl/yr)

(9.3.2)

which is also fairly large. We should recogoile that this calculation is only qualitativel)' correct because the toluene column reboiler was integrated with the reactor effluent stream (see Fig. 9.2-2). Thu~, the flows in the heat-cxchange and quench systems will change. 1I0we...er, this "gross" screening of alternatives will help u:o/f,.S

• W JI r,sher. M F Doherty, and J DousJas, AIC"£ J~ 31: 15J8119SS).

(10.3-9) (10.3-10)

I) mol/hrJ(8ISO hr/yr) (IOJ-II)

334

SECflON 10 J

A COST l.tOlJEI. fOX A SIlo'lPlE

I'.oa:.ss

SEClION 10 I

Stlecli"ily and Reaclor Model

A JI.· - (I - x)] S = -I ( - x 1. 1 - 1. 1

II : (I.2R.

+

I)D

= (I.2R.

+

1{(I-:)Fr . A ]

(10.3-14)

(103·22)

1lIc recycle column condenser and reboiler caplIal COSIS arc given by (10.3.15) I' ) ' " Co = Co _- { ~ LK

From a recycle balance .... e obtam

F,

F~=­

x

(I0.3-23)

(10.3·24)

(10.3·16) The associated opera ling cosls are, for cooling water and steam.

and .....e assume Ihat the reaClor COSI IS gIVen by (103.25) (10.3-17) (10.3-26) Recycle Column We arc not mlcresled jll opllllll;ring the design of the recycle column (the rCnllX ratio and fractIonal recoveries) in this case study, but we want to include its cost in tile economic model We as~ume that the design reflux ratio is 1.2 tllllCS lhe minimum and lhat lhe theoretical number of trays is about twice the l111nimUI11

No-=2N= ..

2 lD SF EolnaAI'

Proclucl Colunlll. The dcsllcd ceononuc model for the product column shell is also of the foml

C.~2 ...

C.hJ.Jt< (

NN )""( I'V] )' '" lie

l,K

(103-18) • K OlmoS:llnd M ,.

M~;)n~,/dt"C

P'oc /Hs

()ft).2J

764 (l91l4)

(10.3-27)

SfCT1Ofery In recycle coJumn DJphcnyl rceo>cry in rcqclc column Recycle r;uio m rocydc coJumn

'"428 K 31 J K 099

"

0 986 0801 10

FrOID J. J MeK ...., £""fC/ortll", ofC~m>cwIY)

"'

~ 0

u

•

."0

0

68

o

20

40

60

100

80

Exchanger overdesign.

U

12

~

nCURE 11.1-7 Estl.al*II01l olrbl: opUrnl,Ull R:lrofil polJr.:J rOllbe 1Ccd-dlllM:DI beal c..o.cM.n&t=r (F,_ W Il Fulln. AI F ~ty. Q/ld J M [)o"gw, - Sc'U"UItJ P,oass Rcuofil AIIUNl'fW,- IcfEC /UNa,"..... pten, 1911, ..·ilil fH""UJIOft from ,~ "IP.or,le"" Clwmit'Q/ $Dc""y.)

368

g;cn()",

IIJ

SU........... y ... ~D UUCJSES

considered the reC}'c1e of dlphenyl or the rcco...ery of any of the hydrogen In the purge stream Once \l;e dcci~e on a retrofit poliC)" \Ooe must also refine our !iCrccllllIg calculallons. The UK of computer-aided design tools is discussed In the next chapter

11.3 SUMMAR' ANI) EXERCISES Summary We ha\'e described a s)'stematic procedure: for retrofitting processes that uses many of the met hods descnbcd carher. The retrofit analysis proettds through thiS series of steps: 1. Estimate an upper bound on the incentive for reuofitting-prepare an operating cost diagram 2. Estlnlate the lIIcenll ...e of replacing the exlstlllg plant with an Identical s)~tem­ uSC: shortcut calculations and generate a list of process alternatives, J. EstImate the inccnti\'e of replacing the existing plant with the best process ahematl\·e. 4. Estimate the lIIcremental Investment cost and the savings In operatlllg costs associated with changing lhe existing process. Q_ Elimlllate the prooess heal exchangcrs_ b_ Identify the Significant operating variables. c. Identify the equipment that constrains changes in the significant opcratlllg variables. d RemO\'e the equipment constraints by adding excess capacity until the Incremental. annualized capllal cost balaooes the incremental savings in operating costs. ~. Energy-integrate the process. f Retrofit the heat-cJ(changer network... S. Refine the retrofit calculations. if justified. 6. Find the best retrofit alternative.

Exe:rc:ises 11.3-1. Retrofit the HDA process by recycling the dipbenylto extinction. 11.3-2. For any process that you have designed. look up the original case study and compare ,he nt._material aJ$1s, utlhties COS!s.and M&:S indu "'ltb current values How would you apecllhe optimum design values to chaoge n the economic ractors chao!e1 Calculate the ranle·order runctions and the proximity paramelers. using current pricc:s bUI the ori"n01.I ca.. D Spcofic:o tbe flo", fllcsol cadi componcnlln lhe Inpul S(.cams and mUlal esumlln ul

Spcofic:o the des'JIl r«yck

TEMP PRESS

NOFLSII END CASto END JOB

Sflams.

Spcofic:s lbe lempelllllle of uc:b ked sr.ram "ncIlJ1lll.lll CSllmll"S ollhe lempc:uI...u or r«ydc u.ea,," Spcafics lbe preu"re 01 cadl feed scream:tn(( Imllal CSllm:ttcs ntlhe l«")Ck ouc...Da USCIlto .... pp.CS& a !Losh calarLllUon _ Pllamclnc casc .ludlCli

a Ia:~k Slream

f ...... J D $tIde

'U'lU'l

IOr-Oce

o(r-NNMO

.",

0

N CO 10

P;:'Ol"OcDO\rO\OChNNCO Po • 'Qf"'l""

0",

~

•

lnN\oo\"" • alNN\DO\""

N

~

o

O ....,....,. '\0

p:;Q)\O\O ....

:C,...

'.VlCQ

FLO\VfRAN Program for Material Balaoce CalcullitiOl13 The: computer IOformallon diagram we Ulie for the material balance calculations IS given m Fig. 12.2-5 However, since It IS easy to make nlistakes in developmg any CAD code, it is always a good Idea to bUIld the program in small pieces and to debug each of these smaller porlions. Thus, we might develop separate programs for the liquid separation system, i c.., from the flash drum through the pump shown in Fig. 12.2-5, and another program for most of the gas-recycle loop. i.e.. from the flash vapor stream back through the reactor. We can use our approximate stream flows gh'en in Table 12.2-1 or 12.2-2 as staning values for the two slTeams. A program for the calculation of the material balances is given in Table 12.2-5. We use SEPR blocks inltlally for the dlsu.llallon columns. because they provide the simplcst way of estimating the component flows for specified fractlonal recoveries, ane we use the conventional rules of thumb to fix these fractional recovenes. (If necessary, \lie adjust these spill fractions to satisfy our product purity requirement) The feed rate to the flash drum is taken from Table 12.2·2. The PARAM statement for the SPLIT block on the purge stream requires that \lie specif)' the spilt fracllon. We can Ulie our approximate material balances (see Table 12.2-1 to esllmate this value) 199.7 + 298.0 (12.1-1) Spht Fraction for Purge Stream :co 1548.7 + 2323 = 0.1283 The PARAM statement for the recycle compressor requires lhat we specify the exit pressure and the efficiencies., while the PARAM statement for the REACT block requires that we specify the conversion of each reaction. These conversions are related to one another by our selectivity correlation. Again, we use our approximate solution to estimate the appropriate values

"

-OW::> ::t:U)QO

..'" " .. ="'''''' .'"..... .. ~It

HDA p r _ WIth colwnn modds.

By-producl

AnOlher approach that we could usc: is to develop SImulation programs that avoid including rigorous models for dislillalion columns as part of any matenal balance recycle loop. That is, any time we have a distillation column in a material balance recycle loop, we can double the flow rate of Ihis stream (using a MULPY block in FLOWTRAN), IheD spht the stream in half (using a SPLfT block in FLOWTRAN), and lhen usc a SEPR block to connect one of the exil Sireams from the spliller to the recycle portion of the f10wshcet and a rigorous model on the other half of the stream. Since we can COntrollhe order in which lhe simulalor calls the subroullnes, we can usc thIS approach to complelely converge Ihe material balances before we call the rigorous dislillation routine. A Simple example of tillS type is shown in Fig. 12.3-2.

FleURE 12.J..2 A .wmpl( planl

Energy B.IIl.IInces and I-Iut Exch.llngers

In the material balance computer information diagram in Fig. 12.2.7, we neglected the quench stream thai uscs flash liquid to reduce: the temperature of Ihe reactor c effluent to II50 F. An lOspcctioll of the flowshcet wilh the quench stream included indicales thai the quench sHearn merely provides a recycle loop arouod the flash drum. Thus, If we mal.e a material balance from the reactor effluent 10 the flash vapor and the pressure reduction valve before the stabilizer, i.e., if we IOciude lhe quench-recycle loop completely wlthm this balance, Ihen the process flow rales will

400

Reno,",

I1J

COM .... nE PL",n SlMUUT)()fol

SECTlO!'I IlJ

S02

Re3CIOT

S06

(SI7)

Q miJ. ADD

S07

CONVG

TE~

SCVW

/QCONT

SOl

Flash IFLSH

S03

~ontTOI

SIl5

Q split SPLIT

COIolPL£TE

PlANT SINUL\T'I()l'I

401

Complete Plant Simulation AfieT we have used simple studies to make certain that OUT simulator subroutines WIll converge and will give the correct predictions. we can put these subroutines together to generate a plant simulation. Table 12.3-1 gi'o'es a program for the HDA process that contains a fced-effluent heat eJ.changer{see Fig. 123-4). Since we ha\e already calculated all the process flows. we can teaT as man) recycle streams as we desire in this flo ....-shcct, and "e do Dot need to mclude any controllers. With this approach, "e can calculate the equipment SIZCS and the required utility flows with a mmimum of computational cost. Of course, if we want to change the processing conditions (i.e.. the values of the design variables), we must include the controllers in the program.

Cosl Models, Process Profitability, and Optimization

S04 flGURE Il.3-J QurrdI C2kulal>oa

not change. However, the heal dUI} of the parllal condenser will depend on the quench now rate. To calculale the qUl'nch flov.. rate, and the load on the panial condenser, we must adjust the flo" r3te of the quench stream to decrease the reactor uit temperature to 1150~F. Thus, we need to lDstall another controller that \\'111 soh'e the problem Ilerativel). sec Fig, 12.3-3. Again. a beginner is advised to solve this problem separalcly and to make certain that convergence is obtained before attempting to add all iteration loop to a large program. We use our shortcut calculations as a stanin~ point to con\'erge the calculations. CAD programs also make il fairly easy to geocrale the temperaturc:-enthalpy curvcs for each process S!ream that are needed for the energy integration aDalysis. In particular, whell there is a phase change in a stream containing a miJ.ture, the temperalure-enlhatpy calculations are tedious to undertaJcc by hand. In the initial simulation of a complete plant, we would illclude only heaters and coolers on the streams and then design the heat-exchanger network using the procedure described in Chap. 8. However, I proocdure for incorporating the heal-cJ.changer design procedure into a sequential modular simulator has been presented by Lang. Biegler, and Grossmann·

• \' D. Lang. L T B~lIn, end t F GrOWllollrta, ·Stm\lh.eneow OphmwotlOQ end IIul Inl~&retion W'nh PJoa:ss Sim\llation.- Pern no 12b. t986 AIIIIUlI AtCbE Medin" M\f.m. Bcadl. No'lTfll~' I98&, $\lbmJU~

10

e-,..'OUll prOClCSKS Inpul-Otllpul SH1Klllr~ 0I11o"'$~1 I -Sh~uld we purify .be r~ ...-m;al"nal $!reams be(ore lhey ar" rcd 10 Ih~ rOllS due 10 a da::lioc ill producl ql....lIt)' or poor propertleS.1 Iu&b IOblb cooo;arlrauon 5. Plaol and Slle dala D. COSI cl uhlJl~-fut:1. steam Inek, ooobn, .... Ia. rd'riIF"'UOn. dc. " Wasle dISposal f..cilllles .nd OOSlS

no-

moderate- to hlgh-tonnag.=, conllnuous, morgamc processes that produce solId products rrom liquid and/or solids feeds. The Input information required is presented In Table: 13.2-1, and the decisions required are listed In Table: 13.2-2 An example: or the application or the procedure to a design problem has been published by Rossiter! and an application to a retrofit study was given by Rossiter. Woodcock. and Douglas.!

Balch Processes The design of batch prooc:sscs was discussed in Sec. 4.2 The: design of batch plants rc:quires not only that we select the units to be used in the process and the interconnections between these units, but also thaI we decide whether we want to merge adja~nt batch operations into a single vessel and/or to replace some batch units by continuous units. Hence, the: design of batch prooesses is more dirrlCult than the: design or a continuous process. To simpliry tbe understandirtg of the: design of a batch plant, we start by designing a continuous process, using the techniques presented in Chaps. 4 through

r ROSSIter. (",.,., btg /',oe. ~J. 6'1 191 (1986) tAP ROSlUIa. 0 C. Wdcod:. and} 1-4 Dou~ ·U"'" aI .. lh.,.....dual ~ Procedure fC>! Rroolil SlI.La or Sahib P!oor:sses,- Paprr preS.'llltel .. I lilt 1916 Anmtal AtCbE M""'hns. MIolIIDI Beadt. Ncnembc:•• I~

• II

410

SfCTION III

I>f.SION 0,. SOLIDS l'aOCE.';SU "''''0 IlATCll PROCUSU

SECTION 111

OUIGN

or SOUDS I'R()(;USU

AND ....TCH PROCUSI!S

411

TABLE 1J,,2..2

TABLE 1).2..-3

Hierarchiul desision procedure for solid processes

A hienlrchical procedure for Ihe cooccpluill design of dNicaled balch JWocesses

I. Balch v~nus conIlIlUOU' prClCal we Q)MJdcr only CODllnL>QUS proc:c:sKS 1. Illpul-oc, SIlD1 oIlbc U1Duahza;! C&pw aDd lbe opcrall.lll C05I 0I1hc c:rysWlua)7 .. xparaUOll ~YSlcm 5Jl'C'QficabOll-lCvcraJ sobd-bqwd 5oCpar.lUODS rmghl be ~ G. flow cao the pnmary Pfoduet be rcal.erall " What lypc:I 01 sobds recovuy S)'Slems e.n: reqwr-cd? c. How dIould lbe wule-lOIod scparaUOOI be aa:omploshcdl ~ Arc aoy bquod-bqwd lC~raOODS rcq... nd7 • LocaUOol of ..,,.nbOll UDIU (pur. or r-«yclc IUItlUI>S Of boc.b}l f Whal lithe cconomM" poIclllAl (1.c..1hc CCODOIDX poIe.mal1.1 levd 3 mlllltS Ihc KpRnUoa S)'Ileal (I.olluakud) captaland opcntrng """ minus bqUOf 1oI.s~1 DlInw. .."Ulunl aODllI.lrud Coplll aod opa;tlwi c:ow)7 So Produci dtyull G. Whal 1)'pC 01 dfyc:r sbouJd be 1>ICd1 " What Ioucs call be c.peclcd~ C Whal. tbc polC!lIW (i.e.. lbe level 4 cc;onolWC pol~uaJ OlJnu.s the aoDuaJu:ed capolal and opctl.1l1l1 CUIi of the df)'er)1 .. EDell)' s)'SIems G. Wball.t~ lhe llUllltIl,ull hcaUOI and cooblllllWl(h~ b HDw lll.IJly helll ucban&en 01 whal JU.C ate rcqwtcd? c Whal J:S the ccollOfllic polc:oual {i.e~ Ihc kvel S CCOIlOImC porcaualllllDus lbe alUluahud (:lpnal lind DflUlIUlll 0061) of lbe bu.t..,achan&nle - t a l 10 schedule lhoc planl and opumw: lbe dc:i.J.pL Opuouzc the bett fto",st-t aJlcmallVC: IllChlodtDJ IIcraFChcd Ihc opcrabilllY 0I1bc PfDCall, lISlnla balch SIDlulalor

F.1lm M F Maloot. penoa.al

9. Then we use the systemauc approach developed by Malone and coworkers- that is given in Table 13.2-3. This procedure is also hierarchical, so Ihat a series ofsmall problems can be considered that eventually lead to the best design. OTHER STUDIES IN Tl-IE DESIGN OF RATOI PROCESSES. The design of balch

processes is expected to take on growing importance in the future. and for Ihis

""""."IIQUOIl,

reason we are including a survey of some of lhe previous work. Most of these studies consider fixed cycle tImes for the batch units, which makes them different from Malone's approach. Kelner- developed a procedure for mmimizrng the capilal cost (by using linear cost correlations) for single-produci plants with a fixed flowsheet that contain bolh balch and sc:mlconlinuous units. The cycle limes and size faclOrs of the balch unilS were held constant, and then Ihe lrade-olf lhal balances lhe balch

·0 lnbanell and M F Malone, -A Syslemallc Procedure for Barch p/ocr:as Synthes,s," Papel p.-escnted 1.1 the 1985 Annllal AICbE Meetmg, ChlClgo.1Il ; C M Mynllbcas, MFlcalb,ltry and TallCU fOI Balch PrClCal Dulgns.M M S "T'IIQ\I~. Umvcnuy of MusachrucrUi, Amherst. 1986.

• S. Kelner.

eM,.,

EnfJ~

121 (AUI 12, 1960)

412

UCTJOt< 03

SEcnot


,

Ii

E

~

g

'i

~

1 1 ,~~~ --

-,,

~I';:;:~I'" I, : + + -

c, ,,

i I'; i'1",

+

~

IE-

~~} ;-

:;.;;; _I " .. r..

,

~,::~~,:::

~

•• ,, -,>< E

i?

-':::-:; I

-

~;~

€j~-

';~n of solute: in gas In ~qullibnum with liquid/mole fraction of solUie in liquid 1'£ .,

I

~

I'll

""

3\

= At average: column lcmpere ""here: the: rc:lali ... c: volatlllly b reasonably
nc.ld.tCPNK ~J Oft, D 7641'98 = XI> = 1.0 (that is, we set XI> = 1.0 in Eq. A.2-28 to simplify this expression). Thus, the operating lme is moved closer to the equilibrium line, and more trays will be required. For expensive columns wllh numerous trays, wc expect that most of the trays will be located ncar the ends of a McCabe·Thlcle diagram. In these regions bolh

N =

where

fJ

=

In {[xnf(l - Xl>)l((l - xw)/xwl!Jl In [al(1 + lIRx,.)1I 2 ] (RIll. .. - x,.XRIll.. - I (RIR", 1)2

+ aX,.)

(A 2-47)

(A.2-48)

We can use this result 10 estimate Ihe sensilivity of the change in the number of plates as we decrease the reflux ratio below RIll... = 1.2. This result is expected to be conservative. We can "tunc" the approximation by using Eq. A.2-47to calculate N when RIll.", = 12, and Ihen we compare the result to Eq. A.2-23. If we introduce a correction factor mto Eq 2-47 to make the resulls agree when RIR. = 1.2, we WIll probably obtam a more accurate estimate.

448

SFM10"l.u

UNDERWOOD'S EQUATIO:,,/ FOR MULTICOMPONEf"o'TSYSfEM5. Underwood ha5 .1150 proposed an expre$5lon for calculating the number of tnaY5 for mullicomponent mlJ:tuTeS. At the operating vapor rate of the column. we sol.... for the values or 8 and iT satisfying the expressions·

.

,1I,DlC,D V- L

Rcclifpng section

",

_''''''''1:

Stoppmg

2

"'u....n. or n.AYS

449

Since the arilhmetlc mean is (I,

+

Ct,

(A 2-S1)

2

1;[•• ""

we sec that Eq. A 2-56 will be within IO~~ of the arithmetiC mean if

'(J sO.I(ld

(A.2-49)

(A.2-S8) (A 2-59)

(s 04

0'

,8>:,.

Hence, a criterion for the IWO kinds of mean~ 10 be approximately the same is

a, - 0'

Then \Ooe use these ..alues to calculate Nit. and N s from the ClIpresslons for Ihe rectIfying section

a,-ar SO.4

(A.2-60)

",

(A_2-SI)

EFFECT OF VARIATIONS TN a ON TilE COLUMN DESIGN, Gilliland's correlatIon predicts Ihat the number or trays required to ach~ve a particular separation i5 approximalely equal to twice the number of Irays at total reflux:

(A.2-S2)

(A 2·61)

and the slripping seclion

If \Ooe let

A Critl'rion for Constant Rl'lalh·l' VolatililY

(I

Most of the short-cul procedules for eSlJmallng the number of theorcllcal trays reqUIre Ihat the relallve \'olaUhty be constant. To de\'e1op a criterion for constant a, firsl we evaluate the relative \olatihties of the hght key with respect 10 the heavy key at the top and the bottom of the column (I" ==

DISlllt,AnON COLU .... NS

stenON AJ

I>lSTILl.AllON COI.U .... "'S NU .... RFl OF nAYS

(K,)

then

N

= (1:.,(1

+ ,p)

A.2-62)

21nSF .., In (1.,(1 + ¢) - In

Q'••

2lnSF + In (I

+ ¢) --

2N. I

+ In (I + t;)j1n Ct•• (A.2-63)

However, for small changes in

Q'

we can use Taylor series expansions to write

(A.2-S3)

K l "."om

(A.2-64)

'"(1+';)-';

Then it is common practIce: to estimate an a\'erage volatility as the geometric mean

,

.;

+ t;j1n Ct..

in a••

~.;.,;-- _ I - -

I

(A.2-54)

(A.2-65)

so that Eq, A.2-68 becomes RELATIONSHIP BE1WEE."'l TIlE GEO;\fETRIC lEA .... AND TilE ARITliMETIC MEAN. To Simplify the analysis, ....-e write the: a\·eragc volatility 10 terms of the arithmellc mean rather than the geometric mean. The relationship bet....·een th~ quantilles can be estabhshed by leuing

(A.2-SS)

and then usmg a Taylor

~nes

expansion of the: geometric mean to obtain

(A.2-SG)

N _ 2N (1 ""\

_~)

(A_2 66) 0

In a ••

Thus, variations in a will introduce less than a lOY. enor in the column design if we require that

q,

(A.2.67)

S 0.1

In a., Artcr substItuting FQ A.2-62 with a "" (1:, and the definition of becomes

Ct•••

Eq. A.2-G1

(A.2-68)

450

StC"l'ON 4J

DlnlLLArlUN COLUMNS NUMUJIl ()~

ncrioN 4J

TUts

which provides a simple cnlenon for eValUJlJng the efft:CI of vanalJons column design.

McCabe-Thiele Binary

Diagram~-Ideal lind

In a

on the

IHSTILLAriON COLUMNS "'UIoI8U

or

T....n

451

ratio untd we reach a pomt where we oblaln a sharp AB/Be split. The value of the mInimum reflux rallo for Ihi) AB/BC split is

,

R• - LIl,X,.,-

Nonide:.lol

~plilTations

..

At thIS condition the fracllon of component

The McCabe·Thiele procedure often bUsed 10 calculale Ihe number of theoretlCd.l Crays (or bmaf) mlxlures., particularly when Ihe relalne \'olallhly IS nOl conSlant In addlllon, by lnlroducing fklluous molecular .... elghts, the McCabe-llue1e melhod can be used 10 describe heat clfeus caused by laTJ!:e dlfferenas In Ihe heals of vaporization of the l..eys. To account (or heal of mixing effects, II IS necessary to use the Ponchon-Savant techOlque, a method deeribed b) Kmgln and Doherty,· or a computer program that sohcs the tray.by-lray malenal and energy balan~s_ The details of the Ponchon·Savant procedure or compuler roultnes arc available In texts on distillation.

Sloppy Splits MOSI of the research on column desl@n and shorlcul procedures has been limned 10 sharp splits in simple columns. lIo\\,ever. Ihe optimum fraellonal recovery for any stream leaying a column, except for a product stream, represents an economic trade-off between adding more tra)s In the end of lhe column (I.e., the top OJ the boll om) where the Slream of mlereSI lea\cs the column balanced against ellher a decrease m the loss of malerials In ",aSle or fuel streams or a decreased cost 10 recyclc the material through the reaClOr s~slem Shor!cul procedures for cstimating the optimum rccoyenes ha\e been propo~d b) Fisher. Dohert). and Douglas' In addition, a case study Ihal IlIu~lrales the dcslrabillty of ma},;Ing a sloppy spill (i.e., nOI a high fractional ra:u"ery) halO been prescnted b) Ingleby, R0lOSner, and DougJas. 1 A procedure for cstlmatmg the O1lfllmum reflux ratlO for sloppy splits of ternary mixlures has been published b) Glmos and Malone' and IS prcscnted below They noted thai Underwood's equalloD (Eq. A.1.21) for the mlmmum reflux ratio can be wnllen as a linear funcllon of Ihe o\erhead composilion of Ihe light key x"p. providing thai only A and B Iea\c o\crhead_ Ifwe merely flash Ihe feed, we obtain a lower bound on the dIstillate composition X"p, which corresponds to R.-O. Then when we add a recufYlng secuon aDd contlnue to increase the reflux ratio, we will increase the purity of lhe O\erhead as a !lnear funcllon of the reflux

n m the fttd that IS recovered o\erhead OIK -

I

I- radlon of B Rcco\'ered O\'erhcad ... !. = - - I

(A 2·70)

"~-

SO .....e can calculate the composiuon of A o\'erhead X AD • Then if .....e merely draw a straight hne connectmg these points., we can estimate the yalue of R. for any value of x AU thai falls between the values calculated abO\'e; see Fig. A 2-2 If we continue to increase the reflux rallO, .....e conllnue to increase the pumy oflhe overhead as a linear function oflhe reflux ratio unlil .....e oblain a sharp A/Be splil. We can estunale tbe minimum reflux ratio for this case where l"p = I, usmg Ihe approximale results given in Table A.2-1. Then we merely draw a straIght hne connecting the yalues for the AB/BC and A/BC splits (see Fig. A.2-1), and \\,c can estimate Ihe values of R. correspondmg 10 any Olher value of lAIl. The figure also ghes a plol of lhe ratio of the fraclional recoyenes of 8, !'h to the fractional recovery of A,!", overhead, but these results arc nonlinear. GlInos and Malone also considered the cost of sloppy spirts in lhe 0011001 of a column. Agam, the results for a SImple flash, tbe ABIBC splil, Eqs. A_2-69 and A.2470, and an ABle spill, Table A_2-1, prOVide Ihree points that define twO slTalghtlincs. The graph IS SImpler to construct If lhe reboil rallO S IS us.cd Instead of the reflux rallo (see FIg. A 2~2), bUI the Ideas are exactly lhe same. Actual Tnl)'s-Plate EfficieoC)' A simple, but not \'ery accurate, techmque for estimating o\'eraU plale effiaeDOCS 1$ use O'Connell's correlation for fractionators; sec Fig. A.I-3. If l\e wnte an equation for the curve in the range from 30 to 90 ~~ efficienClCS, we.obtalD 10

0.4983

0.5

(A 2-71)

Eo = (cr:IlF)O lU - (cr:Il,)II.

Also, if we conSIder columns haying saturated.liquid feeds, and If we usc the result that the viscosity of most liquids at their nonnal boiling points IS OJ cp,· Ihen (A 2-72)

'J R Knll'" and M F Doheny. IdtC '''''''.25 271

This should be a reasonable esllmate for large: lo.....ers .....uh many trays, bUI II probably will be 100 low for smaller lo.... ers. We use Eqs A.3-1 and A.J-2 for bolh gas absorbers and dlSldlallon columns. Flooding VelocilY and To"cr Diaml.'ler The lower diameter usuall) is selecled so Ihat Ihe upor ~'e1ociIY is belween 60 and 80% ofille f100dlRg \'e1OCllY. The early sludy of Souders and Brown- assumed Ihal f100dlllg .....as caused by the eDlrammenl of droplets carned along wllh the gas; see Fig. A.J-I When a small droplet IR Ihe lo....·er IS stationary, 115 .... eJghl will be balanced b)' Ihe drag force el.erled by the flUid tIfR)(PL - PG)g = Compla

X,II

Column A/lrtIfQIll'CJ In DUlIlIll"lI'I S,"c ......- Jub ","'rd'll Cltr", Eng RCJ Dr.'. /981,)

t: .. s,

....~ = (O:lK X .,.

- I)

+ xc,-)(O: ...

1)

(A 5-5)

and the minimum vapor ralc is given by

(A.5-6) The value of R 1 . ..,;_ can be estimated by usmg Undcr.....ood·s equations or the apprOXImate ellpressions ofGhnos and Malone- (see Table A.2-1) for the: ABle split R1

. . . ,...

.

t:.. r!(7- AC -

(x..,.

I)

+ (x" + XO)!(ClIIC -

+ x.,.)(1 + x..,.x c ,)

-

I) -

(A.5--4)

The approlllmale ellpression is usually Wlthm 4 ~ or Ihe euet result, which is adequate for screemng purposes DESIGN OF" COI.UMNS wnll SIDESTRf..AMS ABOVE TIlE FEEl>, If ....·eha e

a

ca~

where a high pUrity of lhe sidestrcam IS nol required (c g.• suppose thal

e

From Eq. A_5-5 .....e sec thai if CI... J> I and/or if x ..,. is small. then we can obtain a product with high purity as a sK1eslream The correspondmg results for sidestreams below the feed are a.,u{x..,. ,t:cs, •• ~ "'"

:I... X .. ,.

R """ x..,.

+ X.,-) +

x.,.

+ x.,.

..

xAf(a.c - J)

s

x.,. +- x c ,-

F

(A 5·7) (A 5-8)

"c,-/x c ...-

xcs/x c..'

(1\5-9)

470

UcnON" I

roMI't..U IllrnU-"lION COLU/>tN~

5EcnON ,"-I

471

o

Thus, we have falfly simple expr~siom a\'allable for lhe preliminary d~signs of pasteurization columns Sidesfream Strippers and SidClltrcam

COMI'l.EJr IlISlIU....TIOJ' COlUMJ'S

X"D

R(:Clilicr~

If lhe pUrity reqUIrements of a siJestrcam column are not sallsfactory, we can add a column secllon, as ellhe:r a sidestream stripper or a Sidestream rectifier (!iCe Figs. A 5-] and A.5-4) Now we agalll have fOUf column sectIOns, so that any spc 0. > el!IC_ Gimes and Malone found thai for tbe case where q = I, the 10lal vapor gencralcd fOf the two rcboilcrs in a sidcsueam stripper is cuetly Ihe same as the \'apor requirement for the singk rcboikr io a sideslream rectifier The operating costs may differ. ho.....ever, Sllla: Ilk- reboiler of lhe sidestream stripping section operates at a different temperature and tlk-rerore may usc: a less expensive utility.

2

POTENTIAL SAVINGS. GllIlos and Malone' proved that these sideslream columns require less total vapor than either the direct or indirect sequence. In addition, they showed that the vapor savings are alll-'ays large when x. r is small, whIch agrees with tbe results of Tedder and Rudd (see Sec. 7.3). The maximum savmgs possible is 50%, independent of the \olalilities. and tbe maximum savings is obtallled when X.tr" ( 8 > aBC' BUI for euher a sharp -18 Be Spill or a sloppy ABC split in the pre(raCllOnlHOr (whIch IS operal mg al 1IIIIIIJng conditIOns). and lhe lower feed point controls, lhe result IS a AC

v·6 . _ ~(~)F 8 __ I ,

(A 5-21)

I

(lAC

and lhe amounl of B fed 10 Ihe downstream column al lhe upper fced Io;;a\lon is Column sections 3 and 4 operate al a condlllon aoo\e the 1n1l1l1TlUm. so thaI Ihe)' can handle larger amounts or H. Thus, we could have deSIgned for a sloppy AB'C splil and talen more B overhead. 1I0wel'er, there is an upper bound on Ihe fraClional recovery of B overhead In the prefraetlOnalor. say 1.,_.. I'thlch corresponds to lhe snuallon where the minimum vapor rate III column secuons 3 and 4 becomes equal 10 1'.. _. We wnte thes O2 > I. Slmilarl)-, if lhe upper column controls, lhen column sections 5 and 6 can handle more B lhan corresponds to OJ sharp split III lhe prefraclionator ThaI is, lAC can perfonn a sloppy AIBe split 10 the prefracllonalOr, bUI now we encounter a I".... er bound on the fracllon of B tale.n overhead I. _. lIence, ~e can wnte

where. appro),:Imatel)'.

(A 5-24) t': _

R ••

-

a..C.prCSSlon for this bound on lhe fractional recovery of B:

f

(A.5-19) and

475

I'",.a;.. == {max(xF1(R~,~

+ I), (x.. r + x.,-)(R::.ra c + I)J}F (A,5-20)

Thus, Simple approximale expressions that are useful for conceplUal designs are available. FLEXIBILITY IN Till DESIGf'\. E\en though the vapor rate II: . eslablishes lhe ffilOlmum reboiler duty of Ihe separallon syslem, II is sull to choose a value for VI independenlly (providlOg Ihal lhe chOice for VI docs nOI change which feed to the downslream column is controillng)_ To understand thIS flexibility associaled Wllh tbe deSign, we conSider OJ case where Ihe lower Feed IS controlling,

_ ....... -

a.dVj,aun - (lAcX~r I)I'J .... I.. - (aACXAr + xllf)F

(iliAC (iliAC -

(A.5-25)

The results abO\e indicate that a Petlyuk column can handle a range of feed composillons wilhout changing the reboiler dUly, but merely b) changing lhe flows 10 column sections 1,2, 4, and 5. This flexiblhty both for the hmllmgconE J, J2

lNu, U

10111 (I93S)

H7 (1986)

'W J Stupln and F J t.oclhan. -Thermally Co"pled Dtii',lIallon Columns A Ca"" Sludy,~ 64lh Ann"at AIChl: Mccnng. San haocliCO, 1971, and T L Wayburn and J D Seader, In I',oc !d '"" FOCAPD C-I, A W W~,C'.bo:'& and II ChIC". eds., CACI E Corp c/o I'rof H Carnahan. Dtpaflmcnl of C""fIUC1IJ lo"p'-U"L Un'¥U$'lly of "'hclupn, An.. Arbor. MkJI~ 48t09, P 7bS, J ....

"81

Sf.t..IOI'l

.-, I

-

I

,

J., Lz

-

V1

I( we add a condenERFORMANCE OF PRt:VRACTtONATORS. TIle maximum possible savings with a prdractionalor arc never as large as with a Pctlyuk column. n the ~PlXr feed contlols lhe maximum fractional savmgs arc (O:AC 1l.r)/(a Ac I), wluch occurs when .x~ ----mallesl valuc of

(Q AT),•.• ~

L (Q AT),

(A.6-12)

Ihen the minlillum uullly bound is estlmatcd by USlllg the expn:sSlOn Q ""n

~ (Q AT),•.••• 11 1;,•• ;1

(A 6-13)

On a T-Q dlagram, .....e diVide the Q l!.Tfor each separallon task into widlhs equal to QmiD' and then we Slack lhe various plea:S that we obtain between the ulllity levels. An illustration of this procedure- is given in Fig. A 6-1 Thus, there IS an easy .....ay of eSlimaung the utilily bounds. Of course, as we add effects in a multieffect column, we are increasing the capital cost, even though the energy costs are decreasing. Nevertheless, the pressure shifting 10 allow the stacking of columns will provide a lower bound on the utility requlCcments, even if we forbId the use of any multicffect columns For e"amplc, III Fig A.6-4a ~e consider three columns_ By pressure shifting and stacllllg the columns, we can reduce the utility consumpllon. The lowest uIlIllY bound then becomes equal to the "alue of Q for the column with the largest heat load (see Fig A 6-4b)

We can reduce this load by replacing the column with the largest heat load QI...... by a multieffect column (see Fig. A.6-4c). Normally, we prefer to pressure-shifl as little as possible when we stack the columns.

Integralion of Column Sequences wilh a Process If heat is available from process streams that muSI be ejected to a cold utility, we prefer to use this heat rather than a hot utility to supply the energy 10 a distillation system, In this situation l!. T"••il depends on the nature of the composile curves (sec Chap. 8). We never want to have a column straddle the process pinch, and our goal is to fit the columns in between the hot and cold composite curves (see Fig. A.6-5) or below the grand composite curve (see Chap. 8).

TCONO

TCW

Q FIGURE: A.6-2 Mmlmlzlna Ullhl1C5 by uSing a mullldfccl W~lnbu'l/. A{CItE J~ 3t 163 (/985) 1

co"fi~u'~I'"''

11,_, If J

A"d'u-Olld, .",.1

~

II

• M J ... "d,eo:o.och dud A W Wo,'erbcrg. ,HCltt. J. 31 36] (1985)

T (e)

(b)

(a)

7

CD Gl I~

0

I

I

ilAI

I---Q_~

Q FICURI: "'.&4

II Vlro'tnl ul,hll~" (..) Three columns, (b) s.adtd confi&urallOn~te) ",,,Iudkel. [from M J A,w'rro"'c

Chlallo_led bydrQC:;lfbons Slum Bo"er ... ~ICI

CoollOlilower Wiler

hUI

nchangers Boiliolc liot-

oon

000
n. imphes that the heats of vaporization of the 1.....0 compounds are drlTen:nl, whIch In turn implies that the common assumptIon of equal molal o\erflow in the column .....iII nOI be correct We could correct for Ihis difference 10 latent heats b) introducing a fictilious molecular weight for one of the components and then uSlOg the McCabe-Thiele procedure to design the column. However. for our prelimmary calculations .....e Ignore thiS potential difficulty. From the vapor-pressure data.. .... e find that

d"

LIQ\IID

25 - I =

[0.9997 0,956

h (1.0456 -

25(1_ - 0,9997)J I - 0.956

0227) = 0,0347

(8-72)

(B-7])

which is \'ery low: the feed composition is very high. and III is \'ery large With a ~'cry low ~'alue of renul such as th,s. .....e should also consider the usc of only a strippmg column as an alternatIve. Ho"'e\er, w-e contlllue wilh Ihe design, and we let R:::::: J.5R .. = 1.5(0,0]47) = 0.05

(B·74)

According to fenske's equallon, the mlOlrnum number o[thc:orellcaltra)'s at total reflul needed for the separation is about • ..

,~

In [xaJ(1 - XD)]((1 - x.J/x..J In[(0.9996/O·()()()4XO.9051O,095)] -= =31] loa In 25 . (8-75)

We can obtam an estimate of the number of theoretical trays requ,ired at the operating renuJ! ratio by using Gilliland's approlimation

The o\erall plate efficiency is given b)' O'Connell's correlation, Eq A2-72 For a quicl C5tlmate we assume thallJr = 0.3, and we write thai 0.5 Eo

r- "'0

,

'" "'> "... co "'" ">Z

"[i. ~

C

"~

.. ~

""

~

" "•_.

-, ~

2

•• :

,

•• -,~, >

,•

"'f:. ~

T .... BLE: 1).1-1

APPENDIX

[mpi..... l I",mlli.

D FORTRAN INPUT FORMS

N,

r-llf"~C"

NiH,,'" " ..de

The tables in this appendix are taken from J. D. Seader, W. D. Seider, and A. C. Pauls, ~Flo .....trall Simulation~An IntroductIOn:' 2d ed., CAChE Corp., Cambridge. Mass.• 1977. CopIes arc: available from Ulrich's Book Swre 549 E University Ave" Ann Arbor, Mich., 48104. • .

CliO,

Chloroform Ill'd",~n c~an,,'c

CH,O

Fonnaldch}dc MeThyl chlonde Melhyl IOdIde

ell,

MeThane

CH,a

MnhanoJ Melh)'1 amHl"

C.I/. C.II.

C,H.S C,H_N

I chlonde A10..... MaiM oonve:rgencc: nC'galJ\l: valu~ cnn con,sidnobf)' Jfl('Cd up conYer~cnce bul int'fcuc lbe likelihuod "f in~abiliIY. T t,l4,.'~I:tlflll r~vllr1lblc IrluJ(o-off t.ell''ftfl ~petd n"d ,,'..hllil)'. e¥h q. 1~ Im\llcd co Ihe l'anllP

•

Pfo~rllts

Used

rnlllalpi~ ud .... pof-equilibtl3 dala :m' needed II 1I.e uUlput SIJCllm

I!C

n..,hC'd.

Rdeft'f1Cf'S

~._I

y.riilblc 1!4 c-.kut:fle:!J rtom

If q. :'

There' is none.

!l.r._,)

,.,'here ~"_I l~ the: y..ll1~ .",en to the pr{)(~s IOF • ptlrticul"r !>lR'... m Yamlblc.lblaule

100 '00

FlGUR.E £.2...

Tra) slack helghl, Ii (2-1"10 spacing)

DwdbuOClcoIWllGUlIY'LA '" G"I/r .~. C"- £ng~ 76(6) 114 (M....,. U 1~9).)

TABLE U--6

COrTeclion fMCIOD for column Ir.. ,.!>

V

v--- Honzonl'll labricatlon

T n.y ~paan.. In F, Tra) Iypc

"10

""

God (00 do.... n· cOIner)

F, TUlltUlenal F.

00

"

22 !'Lile

..

00

Ci

SS

00

"

MoDel

SIC\"e

00

TrOUllh

K~h

o. valve

Bubble ap

KQCllde

"

"

"

576

Sl'M'loomlCS ror OlCrmetl EngHlecrs.M~ ~ MeG,... Ihl~ Ne... YOlk, I9'lQ P 562

4PI'£NI)IJr

r

Force

APPENDIX

I Ibf = 4 4482 (kg· m,o'sl) _ 32.174 Jbm·ft/s J _ 4 4482 x 10' dyn (I' cnu 2 )

F

Heal Load-Also see PO""er

CONVERSIO FACTORS

1 Btulhr _ 029307 w

Heal-Transfer Coefficient I Btu/(hr (tl·"F)=5.6782w/(ml."q =

1.3571 x IQ-·ca!f(cm J ·s·"C)

Length Ift=03048m

Mass I Ibm "" 045359 kg I ton (shorl) = 2000 Ibm

Pressure Area

I atm .. 14.7 psi I (II

= 0.0929 m J = 144 in J

I psi _ 6894.76 N/m J (dyn/cm l )

POl\er-AIso see Heal Load

Density Ilb/fl) ""' 16018kg/m) .. 1/62.4 g/cm) I Ib mole of an ideal gas, 0"negen, 0 8., 191, 19" 203.

21 ..

Vrede\tdd, D R, 227

Walson, K. M.. 305, 526 W8ybum. T L,475 Wcnner, R W., 113, 1]4 Wessel, Ii E.. 45 Wel>lerberg, A W., 178, 204. 272, 475, 481 485 Wilke, C. R., 4]1 Winn, F W.,443 Woodcock, D. C. 29] 294,298·303, 306, 409 Woods. J M, 412

Yorl, R_, 526

Zimmerman, C C.. 526

SUBJECT INDEX

Ab~I.l(bcl.

Il. 73, 1611 adldb;itk;., 88 90 ahernatnc:s.74 II') b....... -e..f-the-en ..dopc model, IB, 427 Colbllrn'~ melhoJ, 43 I 434 cos!. 457 design equ3UOnlt., M2 M4,425 430 design problem, 74 90 de~lgn procedure, 514 diameter, 454 457 energy balance~ 80 81 flooding, 454 457 fractlonal recovery, 77, 86 89 heat effects, 429 430 height. 453 Krem~r equation for Ira}:>. 83. 417 430 matenal balaoces., 76-79 mtnlmUm sohenl now. 88 90 mulllcompooent.431 lIumbc:r of plales., 83. 425 436 pdcled w.... ers, 431 435,460,514 plale efficiency, 435 436 pressure. 82 rule~ of thumb, 76 77,85 89 shoncul design for plates., 83. 4~6 436 !>Ohent. ~3 sohent flo .... rate, 77, 85 86 schent los5., 78 80 lempcrature, 74 76, 83 AcetiC aCid, I' 2, 134, 138, 140, 160 ActllC anb}dnde, 112. 134, 138, 140, 160,213 Acclone. 20. 112. 134, 138, 140, 147 148,152.160 161,213 absorplloo, 72-81 condensation, 490 507 Acr~ hc aCid, 152 Adlaballc temperature change, 141 149 Ad~orpllon, 13,73. 168 AII.xallon of costs. 297 302

Alternatnes cost diagrams, 2li9 315 economIC evaluation of. 63. 289315 flo .... shecIS, 74, 116. 303 heat-elllhangcr networllt.. 140, 3{)9-)15 IdentlfYIng. 119, 30J number of, 4, lb retrofit. 354 368 scrc:c:mng: companson b) desJglllng each,S cost diagrams, 289 315 elimlllallon uSlllg beunSllcs, 5 oplimlzatlon, 319-349 solvent recovery. 73, 81 (Ste 1l1so Hydrodealk)lallOn of loluene proo:ss) Ammoma s)othesls- 153 Annuity, 52 Approach temperature, 222, 196, 324 Area of heat exchangers, 233-236 ArtIst's approach (set' Englllcrnng method) Azeotrope, 21, 143, 174 AzeotropJc dJslJllallon. 185 1&6 dlstillallon boundanes, 189 194 feed compositiOn, 192 194 mlllimum rcflux ratio, 194 ~04

Back-of-the-envelopc calculations (see Shorlcul designs) BasJC research, 8 Batch proo:.c:sses, 16, 107 110. 11 4 115,409-412 Benzene, 8 15.20 21.23. 113. 126 1l2, 134 135, 138, 140. 142,147 148. ISO, 153. 160 161,207 208,213, 518 542 Benzoic aCid, 22, 213 Blower co:.t, 516

591

392

SUBJf.n I"D~;X

SUIJECT INDEX

Boundmg solullons, 90 93,464 465 Bubble points, 428 Butadiene (see Butadiene sulfone) Butadiene sulfone, 135, 161, 213 Butane alkylation, 135, 140. 161, 213.

m

Butene (set' Butane alkylation)

Capital charge factor (CCF), 60 Capital costs, 23. 29, 32 contingencies, 41 ~42 correlations, 33-37, 569~577 cost diagrams, 289-315 depreciation. 46 direct cost. 41-42 fixed capital, 37. 41-42 Guthrie's correlations, 34. 569 577 indirect costs, 41 42 inflation factor, 36 inside battery limits (ISOl). 41 42 installation factors. 29, 33 35 offsite C05t, 41-42 onsite cost. 41-42 \·s, operating cost. 48 54,354-368 outside battery limits (OSBl), 41-42 owner's cost, 41 42 start-up cost, 40-42 tolal capital investment, 37-42 vendor's quote. 33 working capital, 29. 37, 41-42, 70 (See a/50 Cost: Profit) Cascade dia8-ram, 220-222 Cash flow. 48 Catalyst deactivation. 103 Centrifuge.408-410 Complex distillation columns, ISO 182,466 478 Compressor cost, 155 156,329,573 Compre~~or de~ign. 153 155,490. 513 computer-aided de~lgll. 557 refrigeration processes, 490 507

Computer-aided design (CAD), 369 404 computer inFormation diagram. 381 controllers, 389 convergence, 378, 559 equipment subroutines, 373-375, 550-567 roxecutive. 370 flash calculations, 382-387, 550-553 Iterations, 389 physical property data, 371, 379 sequential modular, 377 starting values, 381 stream tearing. 377 thennodynamics, 371 Computer infonnation diagram. 381 Conceptual design (see Process synthesis) Condensation, 13,73,168 Condenser, 458, 487-488 Consecuti\'e reactions, 113 Conservation of money, 49 Constraints. 105.210,357-368 Continuous interest, 49-54 Control,414-416 Convergence, 378, 559 Conversion, 17, 94, 124, 145, 150. 157,209,296,320,324-327, 341~349

Con\'ersion factors, 578-580 Cooling water, 75. 458 cost, 328-332 Cost: capital (see Capital costs) cooling water. 328-332 data, 106, 569-577 diagrams. 289 315 distillation, 330. 457, 460. 461, 574 577 equipment (see Capital costs) estimates. 23 71 opcratlllg (see Operating cosl~) product. total. 37. 43 revenue. 44 48

Cost diagrams, 289 315 cost allocatiOltS, 297-302 heat exchangers. 297 process streams. 300 heuristics, 295 operaling costs. 289-315. 354, 358 360 significant design variables. 296 structural modifications, 296 Cost models, 327-332 Creative activity, 4-5 Crystallization, 187 -1 88, 408--411 C)"c1ohexane, 23. 134, 142, 150. 161, 213

Decomposition, 8, 17 Design: costs of developing. 7 optimum (see Optimum design) types, 6 \ariables (see Conversion; Molar ratio of reactants; Pressure; Temperature) De..... points, 438 Diethylbenz.ene (see Styrene) Diethylcther (see Ethanol) Diluents,153 Diminishing returns, 6 Dipbenyl (see Hydrodealkylation of toluene process) Discounted-cash-flow rate of return (DCFROR), 56-59. 70 (See 0150 Profit) Discrete compounding, 50-54 Disproportion of toluene, 66, 213, 290-293 Distillation: alternatives, 182 188 applicability, 175 azeotropes, 189-204 bubble point. 438 complex column~, 180 182. 466 478 compoSItions, 436. 450, 509

593

DistillatIon (COTII.): computer-aided design, 397. 553, 562 564 condenser, 458 cooling water requirement, 458 costs, 330. 457. 460. 461, 574 577 dCl'lgn equalJOlls, 436 453 deSign procedure. 508 511 dew point, 438 diameter. 454-457, 510 energy integration, 264 272, 478 485 FellSk e- U nderwood-Gillila nd procedure, 439-444 Fenske's equation for minimum plates. 441 flooding, 454-457 Gilliland's correlation. 439 441 heat integration, 264 272 height, 453. 510 heuristics for sequencing, 462 McCabe-Thiele method. 450 material balances, 436, 450, 509 minimum reflux ratio, 194-204, 441-444, 447 number or plates, 439-453 packed tower. 460 pasteurization columns. t 73,469 Petlyuk columns, 472-476 plate efficiency. 451-453 prdractionator, 476--478 pressure, 436-437. 509 reboiler, 459, 512 reflux ratio. 197-204,296,321-325, 341-349,441-444 relative volatility. 438, 448-450 sequencing. 10, 175-182,461 465, 511 sidestream columns, 12, 466 sidestream rectifiers and strippers. 470 471 simple column~, 175 180 Smoker's equation. 444 447 splits, 436, 450, 509 stream requirement, 459

594 SUIJECT

II'lDVl

$UIJECT II'lOU

Distillalion (ConI.): Underwood's equations: minimum reflux, 441 444 plales, 448 vapor rate, 463 (Su also Azeolropic dJslilJation) Dominant design variables, 319 350

Feasible matches, 237

Drums:

Feed distribution, 157 158

COSt, 67 ftash (set' Flash) Drying. 408 41 I

Economic pOienlial, 61, 64, 73 )evel 2. I 3O~ I 32 level 3, 158-159 )e~1 4, 188-189 EconomIC trade-offs, 5, 319-350, 356-357 Economics, englnccnng. 23-71 Economy of scale, 104 EffCClive interest rate, 51-54 Electricity COSI, 32 Energy conservation (Slit Heatexchanger networJu) Energy inlegralion. 10 distiJlalion, 264-284.478-485 heat and power, 261-264 (SI!I! also Heat-exchangcr networJu) Engineering method, 5-g, 20 Environmental constraints, 5, 143 Equilibrium limitations, 15, 141, 149-153 Equipment COSt (UI! Capital costs) Equipment sizes, 23 Equipment subroutines, 373-375, 550-567 Ethane cracking, 112, 135, 161, 286-287 Ethanol, 21,134,143-144,152,161 Ethyl acrylale, 152 Ethylbenzene (sel! Styrene) Ethylene, 21, 112, 134 135, 143 Extent of reaction, 128 129

Extraction, 181~ I85 Extraclive: dislillalion, 184

Fenske's equal Ion for mimmum plates. 441 Filter, 408-41 1 Finite differenct: calculus, SO-54 First law, 218, 225, 230 FiJl:cd capital investment, 37, 41-42 Flasb: cakulations, 166 168 computer-aided design, 382-387, 5SO-553 splits, 9 Aowshccl: absorber (stripper), 75 decomposition. 8, 17 input-output structure of lYe Input-oUlput Slructure of llowsheet) processes: atttone from isopropanol dehydrol!:enation, 18,294 benzene from toluene hydrodealkylation, 8-17, 216~284, 297~315

benzoic acid £rom loluene oxidation, 21 cycioheJ.ane from benzene hydrogenation, 24 disproportionation or toluene 10 give benzene and xylene, 66, 213,289-293 ethane cracking to ethylene, 286 ethanol from ethylene and water,

20 ethyl benzene from benzene and ethylene, 19 hydrodc:sulfurization, 285 recycle struClUre of (see Recycle structure of ftowshcet) FLOWTRAN, 369 4O-t, 548 567

FLOWTRAN (Com.); equipmenl subroulines, 373 375, 550-567 IIlput data, 371 physical propeny data, 371 373 thermodynamICS opllons, 372 Fractional recoveries, 296, 325, 341-349 Fuel cost, 32 Furnace, 329. 489, 513 cost, 570-571

Gas absorber (su Absorber) Gas compressor, (sa Compressor desIgn) Gas rec)cJe and purge, 9,126-128, 209, 324, 520 522 Gasolll)e, 135 (Sel! abo Butane alkylauon) Gl\hland's correlauon for dIStillatiOn, 439---+41 Grand composile cune, 224 Grass·roots plant, 41, 70 Guthnc's correlatIons, 34, 569 577

Hazards, 417-420 Heat: and dlsuJlation integration, 264-284, 478-485 and power integration, 261 264 Heat carrier, 149 Heat effecls, 142, 146-149 Heat engines, 261 Heat-exchanger design, 486 489, 511-514 COSI, 572 Heat-exchanger networks: area eSllmates, 233~236 heaHransfer coefficienls, 234, 486-487 computer-aided desIgn, 399 cost model. 327

59S

Heat-exchanger networks (COrll.): design of minimum-energy networ~s, 236-261 algonlhm, 257-261 alternatives, 240 capital vi. operatlOg cost tradeoff,251-256 complete design. 244 design above the pinch, 236-241 dC'S1gn below the pinch, 241-244 distillation columns. 264-271, 478-485 eliminating CJ,changen, 251 energy relaxation, 251 feasible matches, 237 heat engines, 261 heat pumps, 263 heurisucs, 238, 248, 251, 260 loops, 248, 251-256 optimum approach lemperalUre. 24. paths, 248 plOch matches, 239 stream splilling. 257-261 mmimum healing and cooling rcq uired, 216-230 approach temperature, 222 cascade diagram, 220 first-Ia ....· analysis, 218, 225 grand composite curve, 224 limitations, 229 minimum approach temperature, 222

minimum utility loads, 222 multiple utilities, 227 phase changes, 228 pinch temperature, 222 lemperature-c:nthalpy diagram, 222-224 temperature intervals, 218-220 minimum number of exchangers, 230-233 first law, 230 independent problems, 231 loops, 231 second law, 232 retrofit, 354-368

596

SU.J~CT II"D£X

SUBJECT INDFJC

Heat pumps. 263 HeaHransfcr coefficiellts, 234, 486 487 Heuristics,S, 9, 85. 90 91 approach temperature In heat exchangers, 92 93 batch processes, lOS compressIOn ratio, 155 con~-ersion, 94. 145 cost diagrams, 295 dlSllllalion column scquencll1g. 91 92.177 178.180 184, 461-466.511 fraclJonal recovery. 77, 86~88 hcal-exchangcr fKtworks, 227. 231. 238,248,251.260.284 heal loads, 14g II1pul-output s!ruClUre. 13] hmllallOM, 88~89 minimum trays In a dlsullallon column, 91 numtX'r of product streams, 121~123

optimization, 345 pipe: H:IOCIty, 91 reactor con~erslon (smglc: reaction), 94. 145 reactor dcsign. 157 recycle structure, 160 separallon system. 163 165, 21 1~212 solvent flow In gas absorber. 77, 85~86

Hierarchical design proccdure batch processes, 409 410 pertochemical processes, 8 16, 407 solids processes. 408 410 Hierarchical plannmg. 17 Hydrodealkylalion of toluene (I"IDA) process, 518 542 alternativcs: feed purificatIon, 304 punfy gas-recycle Slream, 306 recycle of dlphenyl, 304 dlphenyl recycled, 20, III, 1J3, 161. 213, JOJ JI5 dlphenyl removed ad13balle

Hydrodealkylallon of toluene (IIDA) process. diphenyl removed (Conl.):

temperature risco 14 7~ 148, 524 alternativcs. 132,522.541 benzene oolumn, 535 -537 case study, 518-542 compressor, 524 computer-aided design, 375 40] .:onstramts, 307 cost diagram. 297-303 decisions. 520, 530-541 decomposition of fIowshcct.

.

"

distillation, 397. 530 541 distillation column sequencln@. 531 economic pOienual. I JO 1]2. ISS 159,188-189.522. 527,

54. ecooomic trade-offs. 12J~ 124, 158-159,18g 189 energy integralion. 216 284 flash calculations. 167.528 heat-ellchanger net\l.ork. 216 284.399,542 heat load. 147, 524 hydrogen purification. 134, 52'0 mput data. 107.518-520 input-output structurt:, 520·523 levels of detail, 8 15 number of reactor systems. 138 operating cost diagram, 360 overall material balances., 126 130,521-522 reaclor size and cost, 526 rt:CycJe and purge, 120, 520 rcqde compressor, 524 526 recycle material balanCt:S, 142-14],145,523 retrofil. 358 368 revcrsible by-products, 149,520 rigorous material balances. 204 211 sclt:Ctlvlty data. 'i19 ~tabihzcr, 5]7 541

Ilydrodealkylation of toluene (IIDA) process, diphenyl removed (Com.): loluene column. 532-535 vapor recovery system, 169 170, 519 H)'drodesuUuriz.ation, 285, 287 Hydrogen. 8-15, 20. 23

Independent plOblems. 231 Input information: constraints. 105 COSI data. 106 ph)"slcal property data. 105 plant and site data, 105 product purity, 104 production rale. 104 ray, matenals, 104 reaction information. 99 103 Input-output structure of ftoVl:shttt. 15.116-136 dt:51!D ~-ariabks. 124 c:xccssrcactants,l20 121 gas reqde and purge. 120 DumtX'r of product streams. 121-123 o~erall material balances. 123 130 purification of reed streams. 118-119 reco\-er or recycle re\'ersiblc byproducts, 119 120 Inside ballet')' limits (ISBl), 41 42 InvCSInlt:Dt (st'e Capital COSIS) lsobutane (see Butant: alkylation) lsooctane (Sl'e Butane alkylation) lsopropano~ 20, 134, 147-148, 152. 213 Ileralion, 389

K ~'alucs" 543 -546 Ketene (set' Acetic anh)'dride)

591

Levels of designs (St'l! Design, types) Limitations: heat-ellchanger nety,ork dcsign, 229 opliml:lation procedure, 327 synthesis procedure. 15 llmitmg reactant, 142 liquid separation system. 10 13. 172-204 alternath'CS 10 dIstillation azc:otroplc diSlillation. 185 crystallization, Ig7~188 ClItractlOn, 182-185 ClItraCtlH: distillation. 184 rcacthe distillation, 187 ;azco(ropes with reactants, 174 distillalion: applicabitit). 175 complex oolumns. 180 182. 466-478 sequencing of simple columns. 175-182. 461-465. 511 mteraClJon with process. 178 hght ends. 173-174 multiple- dlstlltalloD sequences, 179 Loo~, 2]1. 248-256

McCabe-ThICk method for distillation. 450 Marshall and Swift (M&S) Index.

3" Material balances: absorber (stripper). 76-79 approximate, 78 computer-aided design, ]75~]96 equilibrium conversion. 150 linear, 204-211 overall (reeds and products). 123130,521522 recycle, 142 145,523 reversible by-products, 146 rigorous,2Q4 211. 375 396 solvent loss from absorber, 78 vapor leco~'c:ry system, 169 170

598

suaJ~cr INlJlX

SUlIJlocr INOD:

Malerial lemperature ranges, 547 Membrane separator~, 13,73, 168 Melhane.8 15,20 Methyl isobutyl kelone (MIBK), 79 Muumum coohng required, 216 230 Minimum-energy heat-cxchanger networks, 236 251 Minimum healing n:qulrcd, 216230 Molar ratio of reactants, 17, 124, 140 142, 157,324 327 Monomers, 152 MOrlgage payments, 69 Multielfecl distillalion, 48 Muhipass heal exchangers, 49

Nominal interest rate, 50 54

Olfsile cost, 41 ~42 On site cost, 41-42 Open~ended design problems, 4 Operating cost diagram, 354, 358-360 Operating costs, 23-25, 32 vs. capital cost, 48-54, 354 368 cooling water, 328-332 (See afso Utilities) cost diagrams, 289-315, 354, 358-360 direct product cost, 43-45 insurance, 43-45 interest, 43-45 labor, 43-45 laboratory charges, 43 45 manufacturing costs, 43 45 opera lor. 45 overhead, 44 raw malerials, 15.43 45 rent, 43 45 repairs and maintenance, 4]-45 royalty. 43 45

Operaling COSts (COnt): sales, research, admmistration and engmeering (SARE) cost, 43 45 start-up cost, 40 steam. 328 332 (See also [JI1I!IY COSIS)

superVtSlOn, 43-45 supphes,43 45 laxes., 4] 45 utilities. 25, 43-45, 328-]]2, 568-569 (See a/so Cost) Operating time, 73 Oplimum design, 62, 319 350 3pproach temperatures, 246 retrofit, 356-368 \ariables. 124, 209 Order of magnitude, design estimales, 7 Outside ballery limits (OSBL), 41-42 Oxygen. 2, 120

Packed absorber (see Ab~orber; Distillation) Parallel reaclions, II] Partial condenser, 173 Pasleurization dislillation column, 173,469 Paths. 248-256 Payout lime, 55 (See also Profil) Pellyuk distillation columns, 472 476 Phase changes, 229 Phosgene, 141 Physical property dala, 105,371.379, SOH, 543- 546 Plllch: dIstillation, 264 heat engines, 261 heat pumps, 263 Pinch matches, 239 Plllch lemperature. 222 Planl and sile data, 105

Planl design, ddinitiun of. 3 Plale ellklency: absorbers, 435 4]6 distillation columns, 451-45] Plate gas absorber (see Absorber) Power and heat inh:gration, 261 264 Power cost, 32 PrefractlOnator di~lJlIation column~, 476 478 Pre~nt \alue, 53-54 Pressure, 124, 157. 324 absorber. 82 distillalion columns, 436-437, 509 Problem definition (see Input infonnation) Process ahernatives (see Alternatives) Process constraints, 105 Process control, 414-4 J 6 Process design definition of, 3 (See also Process synthesis) Process flowsheet (see Flowsheel processes) Process retrofits, 64. ]53-368 Process synthesis. batch \s. conllllUOUS processes, 110 creat1\e activity, 5 definillon of, 4, 110 hierarchical procedure, 8 heat·cxchanger network, 216-218 Input infonnation, 99-107 input-output structure, 116-136 recycle structure, 137 -162 separation system, 163-215 Product purity, 104 Production rate, 104 Profit, 23, 47-48 measures: discountcd-cash-f1ow rate of return, 56 payout time, 55 return 00 investment, 31, 55 model,61 after tales, 47-48 before tales, 45 46 Proximity parameter. 344, 356 Pumps, 67 Purge (see Gas recycle and purge)

.599

Purge composition (see Gas recycle and purge)

Rank-order parameter, 343, 356 Raw materials, 104, ]20 CO~b, 15,32 (See (llslJ Operating costs, raw malerials) Reaction information, 99-103 Reaclions: consecutive (see Consecutive reactions) parallel (see Parallel reactions) Reactive distillation, 187 Reactor: computer+aided design, 564-567 configuration, 157-158 cost model, 329, 334, 574 design, 156-158, 507 design guidelines. 157 design variables: conversion (see Conversion) molar ralio of reactanls (see Molar ratio of reactants) pressure (see Pressure) temperature (see Temperature) equilibrium limilalions (see Equilibrium Iimilations) heat effects, I, 142. 146-149 heat load, 146 separator reaclOrs, 152 Reboiler, 459, 512 Recycle and purge (see Gas recycle and purge) Recycle S!fucture of nowsheet, 14, 137-162 case study, 523-528 compressor design and costs, 153-156 economic evaluation, 158 159 equilibrium limitations, 149-153 excess reactants, 140 material balances, 142 145,523 number of reactor systems, 138

600

SU~jf("T l~[}fl(

Recyele structure of nowshcct (Co"r l' nUlllhcr of rcC}'CIe streams. Il8 140 reactor d«ign. 156 158. 507 reactor Mat elTttt~. 146 149 Renux lallO. 197 204.296.321-325. 341 ."\49. 441 444 Rdnger3110n s~'stcm dcslgn_ 490 "'07 Rdatl\c H,lallht). 438. 448 450 Relrofit (.su Process retrofits) Return on m\t'slmenl (ROI). 31. 55 Reversible by·products, 10. 119. 146. 520 Rule of thumb (Mf' Heunsllcs)

SaFety. 5, 417 421 Salvage \