retrofit projects through pinch technology

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RETROFIT PROJECTS THROUGH PINCH TECHNOLOGYBodo LlnnhoffDepartment of Chemlcal EnglneerlngUnlverslty of ~ a n c h e s t e r Instltute of Sclence and Technology ( U ~ I S T )

~ a n c h e s t e r ENGLAND

Don R. VredeveldUnlon Carblde CorporatlonTechnlcal Center, South Cnarleston, WV

AbstractThe plnch concept ln heat recovery networkdeslgn has become well-establlshed ln recentyears (1,2). In ICI, the concept was reportedto have saved an average of 30% on energy cost,coupled wlth capltal cost savlngs ln new plantdeslgns (1). Payback tlmes ln retrofltappllcatlons were reported to be typlcally onthe order of 12 months (3).Recent research has extended the concept forbetter conslderat10n of capltal cost trade-offs,of retroflt sltuatlons, and of changes to thechemlcal process ltself .These new pr\nclpleshave been trled and tested ln Unlon Carblde.N\ne projects were completed wlth1n the flrstyear, showlng energy cost savlngs averaglng 5ax\n new plant deslgns and payback tlmes lnretrof1t appllcat\ons typlcally on the order of

INTRODUCTION - ESTABLISHED PRINCIPLESHeat exchanger network deslgn ln the context ofchemlcal process deslgn requlres as a startln!'polnt the process heat and materlal balance w thspec\fled reactors, separators, and other unl,operatlons. The task ls to match the "hot" and"cold" process streams connectlng these unltoperatlons w\th each other or wlth externalut111tles ln a heat exchanger network to redu eutl1\ty consumptlon. Often, there are many idlfferent posslbl1ltles of matchlng such s t r e ~ m s ln a network. The preferred deslgn must be s ~ f e and operable and should exhlblt the lowest [posslble annua11zed cost of energy and caplta'.It ls posslble to construct "Compos\te curvesjlof al l hot streams and cold streams from a g1 enheat and materlal balance(2,4), see Flgure 1. IIf the curves are placed as ln Flgure 1, the hot

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more than corresponds to theIr enthalpybalance. By the same token, there must be acorrespondIng Increase In the cold utIlItyconsumptIon. We conclude that desIgns meetIngthe energy target exhIbIt a zero heat flowacross the pInch. Sub-optImal desIgns exhIbIt aheat flow across the pInch whIch corresponds tothe excess utlllty consumptIon both hot andcold(7).ThIs last conclusIon Is as powerful as It IssImple. It Is now possIble to replace thedIffIcult desIgn objectIve . . . try to mInImIzeenergy consumptIon . . . by the much sImplerobjectIve ' . . . do not transfer heat across theplnch . . . . The latter objectIve Is achIevedeasIly enough by IntegratIng streams above andbelow the pInch separately. The result Is avIrtually foolproof way of desIgnIng mInImumenergy networks.Another conclusIon that can be drawn from FIgure2 Is that excess energy flows tend to Incur hIghcapItal costs due to the extra heat transfercapacIty needed for both utIlIty heatIng andcoolIng. The result Is a capItal cost penalty,not benefIt, for low levels of energy recovery.Or, In other words, Improved energy recovery maylead to capItal savIngs! ThIs Is contrary toexpectatIons, but has been verIfIed In practIceIn ICI(l) and now In UnIon CarbIde.It Is apparent from thIs dIscussIon that the useof the pInch concept does not necessarIlyInvolve the use of systematIc step-by-stepmethods and 'black-box' computer programs.Rather than IntroducIng systematIc rules, thepInch concept Is makIng them superfluous. Theconcept clarIfIes physIcal facts. Just as It IseasIer to launch a satellIte once we understandgravIty, It Is easIer to desIgn a heat recoverynetwork once we understand the pInch. TheperceIved complexIty of the problem Is reduced

The claIms hold valId for desIgns of any degreeof complexIty. Processes such as ethylene,ethylene oxIde, ammonIa, vInyl chlorIde,petroleum reflnerles, etc., are al l analyzedwIth reasonable effort. "ultlple pInches,extended pInches and near-pInches, multIpleutIlIty levels, margInal tarIffs ln utIlItycostlng, safety constralnts, control, etc., canall be taken lnto account. The mostcomprehenslve text descrlblng the establIshedt chnlques publlshed to date Is the "ICHemE'sUser GUlde on Process IntegratIon for theEfflclent Use of Energy(2).

NEW PRINCIPLESThere used to be three major short-comIngs Inthe establIshed technIques. FIrst, the capItalcost consIderatIon was rather crude, beIng basedon lnltlal guesses for 6Tm n (the mInImumpermIssIble temperature dlfference). Second, It15 usually easy to avoId cross-plnch heattransfer In the deslgn of a new plant, but Itcan be a dIffIcult task In retrofIt sItuatIons.ThIrd, the heat IntegratIon has to be carrIedout around a 'frozen" process Interface. Inother words, dIstIllatIon column pressures,recycle flowrates, flash sequences, etc., areall accepted as beIng fIxed. Research Is nowco pleted addressIng all these problems.SpecIfIc dlscusslons are glv n In separateresearch publlcatlons(9,14,15>' The presentpaper presents an outlIne summary of thesedevelopments.CapItal Cost Targets for Correct CapItal EnergyTradeoffPrevlously, the englneer had to guess an InItIalvalue for 6T In. For example, In lowtemperature processes he would choose a lowervalue than for an above ambIent desIgn. Insltuatlons where expensIve materIals of

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Cap1tal costs depends largely on the "number ofunHs" (-'.e., 1nstalled Hems of equIpment) andthe total heat transf r surface area. Targetsfor the number of un1ts are well estab11shed(5J.Targets for the total surface area are alsowell-estab11shed, as long as U, the overall heattransfer coeffIcIent, Is constant throughout thenetwork. From any compos1te curve, we canestablIsh the temperature d1fference between hotstreams and cold streams for all heat exchangedQ". Thus. the overall area requ1rement A Is:

A (1 )

ThIs Is a well-known procedure(4,8). Theproble Is that U = constant Is too rough anapproxImatIon to g1ve mean'ngful results. AconsIderatIon of Indlv1dual stream f11m transfercoeffIcIents Is requIred.To re-wrlte equatIon (1) In terms of fIlmtransfer coefflc1ents for IndIvIdual streams IsdIffIcult as , In many segments of the composItecurve. there w111 be several hot streams andseveral cold streams mak1ng a multItude ofmatches possIble. However, thIs proble has nowbeen overcome(9). We are able to pred1cturface area and capItal costs typically wIthInthe same bounds of accuracy wh1ch are 1ntr1nslcIn the technIcal parameters (f11m transfercoefflc1ents. v1scos1t1es, pressure droppredlct1ons, etc.) and economIc parameters(capital cost correlat10ns) wh1ch aff ct capitalcost calcul tlons anyway. In br ' f, 1t 1s nowpossIble to pred1ct. prIor to desIgn. on thebasIs of any gIven composIte curves, materIalsof construction. utIlIty and capItal cost data;etc., a value for t>Tml n whIch Implies awell-optlm1zed comblnat10n of energy and capItal

wIll do so because they necessItate lower velsewhere.ConsIder now FIgures 4b and 4c. The proposmatch 1n fIgure 4b roughly falls In line wItthe "Ideal" t>Ts. Th1s match would thereformake good use of drIvIng forces 1n the cont tof the speclf1c desIgn task at hand. Theproposed tches 1n f1gure 4c do not fall 1nHne wHh the "Idea" !lTs and would thereformake bad use of drIvIng forces. They would otwaste energy but surface area.Process Mod'flcat10nsIt has been accepted up to date that heatexchanger network desIgn r quIres, as a star IngpoInt, the underlyIng process heat and mater 1b lance, an that much could be gaIned byIdentIfyIng process changes to complementnetwork desIgn changes. However"t has lsbeen accepted that open1ng up the questIon 0process changes Is lIke openIng up Pandor '5box. There are InfInItely many "settIngs' f rreactor conversIon, evaporator stages,dlst',1at'on column pressures and reflux ra t os.feed vapor1zatlon pressures, pump-aroundflowrates. etc. The multItude of choIces Is solarge that It seems an ImpossIble goal toconf'dently predIct wh'ch three or four suchparameters could be changed to advantage 1n heover 11 context. Aga'nst thIs background,now have sImple results whIch allow us tod1scuss wIth great conf1dence many dlfferenprocess p rameters from the poInt of vl&W ofthelr utllmate Impact on the overall desIgn. Ad'scusslon follows.ConsIder once aga1n the composite curves InfIgure 1. What are the changes to theunderly1ng process heat and mater'al balancethat w1l1 change the hot and/or cold energy

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F\gure 5 that temperature changes wh\ch areconf\ned to e\ther s\de of the p\nch can have noeffect on the energy targets. It \s also clearthat temperature changes across the p\nch w\llsh\ft heat dut\es from one part of the processto the other. Thus, the pattern \n F\gure 6emerges:

sh\ft hot streams from belowthe p\nch to above sh\ft cold streams from abovethe pInch to below.

For example, \f the pressure for a feedvapor\zer can be chosen, \t should be set so asto fall below the p\nch.Th\s pr\nc\ple \s \n l\ne w\th the general \deathat \t ought to be benef\c\al to \ncrease thetemperature of hot streams (th\s must make \teas\er to extract heat from them) and thatl\kew\se cold streams should be cold. We sImplenow real\ze that chang\ng the temperatures ofstreams \n th\s fash\on w\ll \mprove dr\v\ngforces, but cannot poss\bly decrease the energytargets unless the temperature changes extendacross the p\nch. Aga\n, we obta\n a defln\tereference. We can pred\ct wh\ch mod\f\cat\onswould be benef\c\al, detr\mental, or \nconsequent\al.Another \nterest\ng po\nt to note \s that F\gure6 represents the general pr\nc\ple beh\ndprev\ously recogn\zed spec\al cases \n the areaof heat and power Integrat\on and d\st\llat\oncolumn \ntegrat\on. The "appropr\ate placement"concept for heat eng\nes, heat pumps, andd\st\llat\on columns as publ\shed prev\ously(lO.ll) turns out to \mply sh\fts of streamsthrough the p\nch exactly \n l\ne w\th thegu\del\nes expressed \n F\gure 6. Furthermore,stream sh\fts through the p\nch have recently

There are two elements of cap\tal cost to becons\dered. F\rst, cap\tal costs w\ll change \nthe un\t operat\ons of the process (\ .e. , thed\st\llat\on columns, reactors, etc.). Second,cap\tal costs wIll change \n the heat exchangernetwork wh\ch has as yet to be des\gned.CapItal cost changes In the unIt operatIons haveto be est\mated by short-cut calculat\ons.Cap\tal cost changes \n the network can bepredIcted by us\ng cap\tal cost targets (seebelow).W\th a l \ t t le pract\ce, the above pr\nc\plesenable the eng\neer to qu\ckly screen fromamongst, say, f\fty poss\ble mod\f\cat\ons thosethree or four that w\ll lead to benef\c\aloverall cost effects. A s\mple computer programwh\ch allows the eng\neer to repeatedly changeh\s stream data and study the consequent\alchanges \n the shape of the compos\te curves andthe targets (see F\gure 7) \s a useful a\d(13).Retrof1t ProjectsIn the past, the p\nch technIques had to beappl\ed \n retrof\t projects w\th a great dealof ad-hoc \mprov\zat\on, lead\ng to stud\eswh\ch were both cumbersome and prone to \dent\fysecond-best projects.A d\fferent approach has now been developed(14).As a f\rst step, the surface area target \s usedto establ\sh how much energy can be recovered byus\ng the \nstalled surface area. (Note the"\nverse" appl\cat\on of the tool.) Then, theex\st\ng des\gn \s analyzed \n terms of wh\chexchangers transfer heat across the p\nch. Therema\n\ng exchangers are analyzed as to the usethey make of dr\v\ng forces (compare F\gure 4).Next, those un\ts that are transferr\ng heatacross the p\nch and/or make \nappropr\ate useof dr\v\ngforces are cons\dered to be "re-p\ped"or ore-used". Somet\mes th\s \s all that \s

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confIguratIon are shown In Figure 9. The curvesIndicate a mInImum energy target of 101 mIllIonBTU's per hour for the hot utIlity, which Issteam. The orIgInal process consuned 133mIllIon BTU's per hour of steam. Thus, the"scope" for Improvement Is 26 mIllIon BTU's perhour or 20 percent of orIgInal demand.A brIef analysIs of the exIstIng flowsheetuncovered the specIfic reasons for currentutilIty requIrements being greater than thetarget: these reasons Included:

Two major process-to-processheat transfers across thepinch, plus several otherrelatively small pinchviolations.

One Instance of utility heatingbelow the pInch. One Instance of utility coolingabove the pinch.

These vIolations totalled 26 million BTU's perhour. I f elhnlnated, they would bring theprocess utIlity requirements to target. Theproblem was, It would take six new exchangersand a substantial new Investment to accomplIshth15 .At this stage, one should always look forprocess modIficatIons. We chose to re-examInethe composIte curves to look for clues on howthe process mIght be modIfied to both Increasethe scope for Improvement and reduce the needfor heat exchangers. ThIs revIew suggested thatthe process could be Improved sIgnIfIcantly bymaking several process modificatIons IncludIngthe following:

(1) Decrease the pressure of column

savIngs. The problem was, It would still takesIx new exchangers and a substantial newInvestment to accomplIsh thIs, see Figure 11.At thIs stage, one should look for "secondorder" process modIfIcatIons aImed atsImplIfying the necessary hardware changes.Instead of retrofittIng the exchangers so thattheIr sIzes fIt the process, why not modIfy thprocess slightly so that It s enthalpy changesfIt as many of the exIstIng exchangers aspossible? These mInor adjustments led to thesacrIfIce of 4.5 mIllIon BTU's per hour, buthelped to save four exchangers, see Figure 11.The flowsheet for the process finallyrecomnended 15 shown In FIgure 12. I t 15InterestIng to note that the two new exchangerare required only because of consIderatIons Iregarding pressure and materials of constructl nand that three exIstIng exchangers reelIminated (I.e. Investment avoidance In a newdesIgn). The project resulted In reducing theoverall requirement for hot utIlIties by 36.5mIllIon BTU's per hour. ThIs represents 89percent of the maxImum scope for Improvement a d140 percent of the scope wIthout processmodIficatIon, see FIgure 11. The other realadvantage of going beyond "sImple heat exchangQrnetworkIng" and Into "process modifIcatIon" w a the Improvement In the sImplicity of the isuggested hardware arrangements. The estimatedInvestment payback for the final retrofitproject was approximately six months.SUMMARY or APPLICATIONS IN UNION CARBIDEThIs new technology was first Introduced InUnIon CarbIde In 1982, when Dr. Llnnhoffpresented hIs course on Heat Exchanger NetworDesIgn" to a number of our engineers. In lessthan a year, the technology was applIed to nlndIfferent projects: an overall summary Is shown

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Carb\de that the new technology was matureenough for general \ndustr\al appl\cat\on, andthat more concentrated efforts were requ\red tomax\m\ze the benef\ts. To accompl\sh th\s, atechn\cal funct\on was establ\shed \n January1983 w\th\n the Chem\cal Eng\neer\ng Technologysect\on of the Central Eng\neer\ng Department.It s purpose was to prov\de the focal po\nt forcont\nued development, appl\cat\on, and eventualtransfer of the technology to the organ\zat\onat large.S\nce \t s format\on, members of the new ProcessDes\gn Methodology funct\on have been \nvolved\n several add\t\onal projects not yet l\sted \nTable 1, both retrof\ts of ex\st\ngmanufactur\ng un\ts and new des\gns. As webecame more fam\l\ar w\th the spec\f1ctechn\ques and ga\ned exper1ence, the qua11ty ofour results has cont\nued to \mprove. Certa\nconclus10ns have been reached:

We are conv\nced from ourresults that th\s newtechnology can cons\stentlylead to s\multaneous energycost reduct\ons and \nvestmentsav\ngs \n new des\gns.

It \s extremely \mportant to gobeyond s\mple heat exchangernetwork analys\s. The key toh\gh qua11ty results \s processmod\f\cat\on sett1ng theprocess up to prov\deadd\t\onal opportun\t\es forenergy reduct\on and tofac\l\tate effect\ve des\gnsw\th the m\n\mum of\nvestment. In a sense,process mod\f\cat\ons lendretrof\t projects much the samecharacter as new des\gn

Understand \t Fully. It \s notposs\ble to work effect\vely \nth\s new f\eld of technologyw\thout a good understand\ng ofthe fundamental pr\nc\ples. It\s therefore necessary thatpotent\al pract\t\oners are\dent\f\ed at the outset andproperly educated. In Un\onCarb\de's judgement, th\s\n\t\al educat\on \s bestprov\ded d\rectly by therelevant lead\ng author\t\es \nthe f\eld.

Spec\al\sts are Essent\al. It\s \mportant to havespec\al\sts \n the organ\zat10nto concentrate on theass\m\lat\on and development ofthe technology, and to prov\decont\nu\ty 1n appl\cat\ons.These spec\al\sts need to actas consultants and educators.They need to be act\velyengaged \n appl\cat\ons work. Background Illum\nat\on \sEssent\al. A technology wh\chsaves m\ll\ons of dollars \stoo good to be true. A veryreal problem delay\ngw\de-spread appl\cat\on \severyday cyn1c\sm andd\sbel\ef. Eng\neers andmanagers \n product\on, 1ndes\gn and development, and \nthe bus\ness areas have to be

1nformed about the \mpact and\mpl\cat\ons of the newtechnology. Processeng\neer1ng general\sts have tobe educated to that they can

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desIgn and analysIs technlqu s.Many practl al decIsIons needto be made. Computer programsare useful as long as they donot InhIbIt the engIneer'sclear grasp of the Issues athand. They can easIly becomecounter- productIve If they areallowed to "take-over".

EmphasIze Prolect Work. TheultImate educatIon In thIsfIeld comes throughapplIcatIons. ThIs takes placenaturally and effectIvely In ateam envIronment where thetechnology specIalIst, processengIneerIng generalIst, plantrepresentatIve, and otherapproprIate support personnelwork together to solve a commonproblem.CONCLUSIONS

Ten years ago, heat exchanger network desIgn wasa fIeld of Interest In academIc research only,of lIttle relevance to IndustrIal practIce.FIve years ago, the pInch technIques resulted InpromIsIng savIngs In the real IndustrIalenvIronment In ICI(l,3). Today, thetechnology has come of age. From an Isolatedenergy savIng tool, It has developed lnto ageneral chemical engIneerIng desIgn methodology,embracIng conslderatlons of heat and materIalbalance, unIt operatIon deslgn, retrofItequIpment constraInts and capItal costs.savlngs today average at 50% energy costs In newdesIgns and at slx months payback In retroflts.UntIl recently, It seemed unllkely that exIstIngprocesses could offer quIte so much scope forImprovement. The authors believe that the

5) Llnnhoff, 8. , Mason, D. R. and wardle, l.Understandlng Heat Exchanger Networks",Computers & ChemIcal EngIneerIng 3, p. 295(1979).

6) Umeda, T., Itoh, J. and Shlroko, K., "HeatExchange Syste SynthesIs", C.E.P., Vol 74(7) , p. 70 (1978).

7) Llnnhoff, B. "New Concepts lnThermodynamIcs for Better Chemlcal ProcessDesIgn", ProceedIngs of the Royal SocIety,386, No. 1790, p. 1 March (19B3).8) NIshIda, N., Llu, Y. A. and LapIdus, L.Studles In ChemIcal Process DesIgn andSynthesIs: II I A SImple and PractIcalApproach to the OptImal SynthesIs of HeatExchanger Networks", AIChE Journal, 23,p.77 (1971). -9) Townsend, D. W. and Llnnhoff, B. "SurfaoeArea Targets for Heat Exchanger N e t w o r k ~ , Paper presented at IChemE Annual ResearqhMeetlng, Bath, UK, AprIl (1984).

10) Llnnhoff, B. and Townsend, D. W. "DesIgnIngTotal Energy Systems", C.E.P., Vol. 78 (17Lp. 12 (1982).11) Llnnhoff, B., Dunford, H. and SmIth, R."Heat Integra Ion of Dlstl11at\on Colu nsInto Overall Processes", Che . Eng. ScI.,Vol. 38 (B), p. 1175 (19B3).12) Shlroko, K. and Umeda, T. A PractIcalApproach to the OptImal Deslgn of HeatExchange Systems", Process EconomIcsInternatIonal Vol. II I (4), p. 44 (1983).13) Llnnhoff, B. and SenIor, P. R. TARGET - acomputer program to set energy targets.AvaIlable from the authors of: The

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FIGURE 2 Heat flow acrossexcess heat flow the pInch equalsIn and out.FIGURE 3 The optImum choIce for Tml ndepends on a varIety of factorsIncludIng the "shape" for thecomposIte curves.FIGURE 4 Good matches utIlIse the temperaturedlference IndIcated by the composItecurves. no less and no more.FIGURE 5 The energy targets can only bemodIfIed If the process streamenthalpy changes are modIfIed.FIGURE 6 Temperature changes can affect theenergy targets only If streams areshIfted from one part of the processto the other.FIGURE 7 Computer graphIcs are convenIent tostudy the effect of process changeson the energy target.

FIGURE 8FIGURE 9

FIGURE 10

FIGURE 11

FIGURE 12

The process prIor to retrofIt.The compos1te curves before processmodIfIcatIons. (Target: 107.106Btu/hr).The composIte curves after processmodIfIcatIons. (Target: 92.106Btu/hr)"FIrst order" process modlflcat'onsreduce the energy target from 107 NMBtu/hr to 92 mm Btu/hr. "Secondorder" process modIfIcatIons reducethe number of new exchangers from sIxto two.The process after retrofIt. ProcessmodIfIcatIons and two new exchangersgIve 28% energy savIngs at sIx monthspayback.

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TMLE 1FIRST RESULTS CF APPLYlto.C THE PINCH TECHN


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TemperatureHot Utility Target

~ r:,\ROc P PINCH ~ o \ . 1:1Tmin

ProcessAbove thePinch

ProcessBelow thePinch

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Temperature

ExcessEnergyFlow

Cold Target

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Temperature

Enthalpy(a) "Narrow" Composite Curves

Temperature

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bT

a)

A Good Match

Two Badb)

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Temperature Hot Target

ffi

Cold Target EnthalpyFIGURE 5 - The energy targets can only be modified i f theprocess stream enthalpy changes are modified.

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Temperature

Enthalpy

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Initial Stream Data

Easy Data Editing

Graphical Output

Computer ProgramTarget Setting

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17,000 Ib/hr SteamBASE CASEFeedandRecycle Recycles Recycle

wU1\D

Preheater

i

FlashStripper

11

FIGURE 8 - The process prior to retrofit.

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Proceedings from the Sixth Annual Industrial Energy Technology Conference Volume I, Houston, TX, April 15-18, 1984


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180 T(OC)160140

Hot120 Composite1008060 BASE CASE

\Cold Composite\

107.106 Btu1- -I11

Column 5

40 H(106 Btu)

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180160 '(

Column 5140Hot120 Composite

100 )80 Cold Composite60 MODIFIED PROCESS

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Base CaseProcess and Network(Figure 8)

133 MM BTU/HR

TWOExchangers

SIXExchangers

SIXExchangers

Base CaseComposite Curve(Figure 9)

107 MM BTU/HR

FinalProcess and Network(Figure 11)

96.5 MM BTU/HR

ModifiedComposite Curve(Figure 10)

92 MM BTU/HR

DESIGN TARGETEXISTINGPROCESS

W0 \N

MODIFIEDPROCESS

FIGURE 11 - "First order" process modifications reduce the energy target from 107 MM Btu/hr to. 92 MM Btu/hr. "Second order" process modifications reduce the number of new exchangersfrom si x to two.

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FINAL PROCESS AND NETWORK 11,000 Ib/hr SteamFeed

w0)w

FlashStripper

andRecycles

h_ ' ~ 1

Recycles

FIGURE 12 - The process after retrofit. Process modifications and two new exchangers give28% energy savings at six months payback.

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retrofit projects through pinch technology

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