high performance trays and heat exchangers in heat pumped distillation columns

8/18/2019 High Performance Trays and Heat Exchangers in Heat Pumped Distillation Columns

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HIGH PERFORMANCE TRAYS ND HEAT EXCHANGERSIN HEAT PUMPED DISTILLATION COLUMNS

M W Wisz, R Antonel l i , E. G RagiUnion Carbide Corporation

Linde Divis ionTonawanda, New York

ABSTRACT

Vapor recompression of d i s t i l l a t i o n columnsoverheads, followed by subsequent condensation inthe reboi le r r e su l t s in subs t an t i a l operating cos tsavings compared to conventional steam d r i v ; nreboi le r systems. The use of high performance hea texchangers and d i s t i l l a t i o n t rays permits addi t iona lenergy savings by lower reboi le r temperature d i f -ferences, and reduced re f lux requirements for af ixed column height , due to c loser t ray spacings.

This paper surveys the heat pump systems current ly in operation using high performance UCC MDt rays and High Flux tubing. Design cons idera t ionsfor high or low pressure l eve l towers, with s ingleor dual s tage compression equipment are discussed,along with the var ious cont ro l methods. Factorsaffec t ing s tar tup , par t load, and off designopera t ion of the equipment are a l so reviewed.

INTRODUCTION

The philosophy used to design plants 5-10 yearsago i s no longer val id due to the rap id increase inthe cost of energy which has been est imated to haver i s e n , in some cases , by an order of magnitude s incethe ear ly 70 's . (1) The c l a s s i c design approach has

been based on the ava i l ab i l i t y of low cos t fue lswhich caused cap i t a l investment to be the dominantvar iab le in the economic analys is . Although inves tment cos ts cannot be ignored, today ' s economicsfavor higher cap i t a l cos ts in order to reduce energyconsumption and improve overa l l system eff ic iency.This i s usually a t the expense of a more complexsystem, such as a vapor recompression system, whichi s discussed below.

The major consumers of energy in re f in ing orpetrochemical plants are the d i s t i l l a t i o n columns,where crude streams are re f ined to sa lab le f i na lproducts. D i s t i l l a t i o n i s the most widely usedseparation operation due to i t s r e l a t i ve s impl ic i ty. (2)Heat a t a high energy l eve l i s supplied a t the reboi ler and i s re jec ted in the overheads. This,however, a lso makes d i s t i l l a t i o n operations energyi ne ff i c i en t , and i t comes as no surpr i se tha t themajori ty of a p l a n t s operating cost may be a t t r i -buted to i t . Development of an e ff ec t i ve method fo rreducing the energy consumed, pa r t i cu l a r l y for d i f -f i c u l t separa t ions , wil l r e su l t in subs tant ia loperating cost savings.

the ca tegor ies of rev ised opera t ing condi t ions , luseof more e f f i c i e n t equipment, or a di fference i n l t heprocess by which a column i s driven. Among the !former, changes in the pressure and temperatureil eve ls of the solumn may ease the separa t ion an4permit u t i l i z a t i o n o f a lower grade heating m e d ~ u mProduct s p e c i f i c a t i o n s s h o ~ ~ d be relaxed where iposs ib le but , more impor tan t ly, use of an e f f e c ~ i v control system wi l l help to assure tha t no moreienergy than necessary i s used to produce spec i f t-ca t ion products. Addit ional or more e ff i c i en t ~ r y s wi l l reduce re f lux requirements while lower d e l ~

T's in heat exchangers can subs tant ia l ly reduce Ienergy,cos t s . Improvements in system eff ic iencycan a lso be obtained through change in the proc¢sssuch as u t i l i z a t i o n of mult i -effec t d i s t i l l t i o ~ columns, where the overheads of a high pressure lcolumn i s u t i l i z ed to dr ive the r e b o i l e r of a lGwerpressure column. Final ly, a vapor recompressio,cycle (heat pump makes use of the column o v e r h ~ d to bo i l the column bottoms. These are j u s t som¢ ofthe many rou tes tha t can be used to improve thelovera l l energy e f f i c i e n c y of a d i s t i l l a t i o n s y s ~ mUnion Carbide, being both a designer and opera t j ' r,has employed many of these concepts in both gra sroo ts plants and the debott lenecking of ex i s t i nun i t s .

The vapor recompression scheme mentioned a!ovemay be one o f the most a t t r a c t i v e means for r e d ~ c i n operating cos ts for d i f f i c u l t s e p ~ r t i o n sThejheatpump has been used in indus t ry fo r many years , putonly recent ly have i t s economic advantages in m ~ n y appl ica t ions become overwhelming, due to i n c r e ~ e energy cos ts . As a r e s u l t , i n t e r e s t in t h i s tYfe ofsystem has increased s ign i f i c an t l y as evidencedlbythe work of many ind iv iduals ( I , 3, 4, 5, 6, 7)lwhohave inves t iga ted and reported on the use of vaforrecompression schemes in the pas t few years .

IHeat pumps are extremely competi t ive when fom-

pared to conventional systems ( i . e . , steam r e b o ~ l e rcooling water condenser) where c lose boi l ing c o ~ -pounds are being separa ted . These compounds inrheren t ly have low r e l a t i v e v o l a t i l i t i e s and r q ~ i r

l a rge amounts of heat to produce each pound of Iproduc t . Expensive heating media provide g r e t ~ r j u s t i f i c a t i o n for the use o f a vapor r e c o m p r e s s ~ cyc le . The separa t ion of components with low r ~ l -t i v e v o l a t i l i t i e s i s idea l ly represented by the! superf r a c t i o n a t i o n of a propylene-propane mixture i n ~ o apolymer grade produc t . This separa t ion can require100-300 t rays depending upon the pressure l eve l ' and

The methods used to decrease the energy usage of product p u r i t i e s requi red . (8) Propylene-propaned i s t i l l a t i o n columns are var ied . These may f a l l in to columns with pressure l eve ls of 40 to 320 ps ia

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Proceedings from the Third Industrial Energy Technology Conference Houston, TX, April 26-29, 1981


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can be found with product pu r i t i e s of 99.5% to 99.7%+,

CW

T I N 8S F

P'27G PSIAT 112 F

p . 7G PSIA

FIGURE I

TYPICAL VAPOR RECOMPRESSION CYCLE

PROCESS FLOW ARRANGEMENT FOR C HEAT PUMPS3

The most common recompression cyc le , ~ concept of which i s i l l u s t r a t e d in Figure 1, 7 d i f f e r sfrom the conventional system by the addi t ion of acompressor, which ra ises the overhead vapor to a

pressure high enough to be used as the condensingheating medium in the reboi ler. In addi t ion , thepressure l eve l of the column i t s e l f can then be lowerthan tha t required for a conventionally-driven system.This has the effec t of making the separa t ion eas ierby s igni f icant ly improving the r e l a t i ve v o l a t i l i t y ofthe components. The overhead vapor, a f t e r being com-pressed, i s desuperheated and pa r t i a l l y condensedaga ins t cooling water in a t r im cooler, removing anamount of heat approximately equivalent to the inputdue to compression. The stream i s then t o t a l l y condensed in the reboiler/condenser. The exi t stream,which i s condensed propylene, i s s p l i t in to productand re f lux f rac t ions . The r e f l u x stream at a higherpressure than column condit ions) i s fed back to thecolumn and w i l l f lash upon ent ry. The f lashed vaporportion of the re f lux stream combines with the vaporfrom the top t ray and comprises the flow a t the com-pressor suc t ion . Depending upon the thermal qual i tyof the feeds and product streams, addi t iona l heatexchange equipment, besides the reboiler-condenserand t r im cooler, may be necessary. For example, inthe severe case of a highly subcooled feed and vaporpropylene product , heat input may actua l ly be neededin the system to maintain a balance. Typically, morethan enough heat i s present , due to the vapor fromthe re f lux stream being recycled. In some cases,addi t iona l exchangers are used to remove heat from

the system to lower compressor horsepower by subcooling the re f lux stream. This lowers the amountof f lash recycled to the compressor.

HIGH EFFICIENCY PROCESS EQUIPMENT

Union Carbide has developed two speci f ic typesof equipment tha t can reduce operating costs regardl e s s of the schemes used, but which are extremelywell su i ted to a heat pump appl ica t ion . Both arewell proven technologies, with numerous successfulappl ica t ions in the petrochemical indus t ry.

a . High Flux Tubing

High Flux tubing has a porous meta l l ic f i lm whichi s meta l lurg ica l ly bonded to the heat t ransfer s u rface to promote nuclea te boi l ing . 9) Whereas onnon-enhanced tubing, bubbles or ig ina te a t randoms i t e s , the multi tude of pores which function ass i t e s for vapor generation in the High Flux surfacer e su l t in boi l ing heat t r a n s f e r coeff ic ients 10-30fold grea ter than obtained by bare tubes. As ar e su l t of achieving higher boi l ing coeff ic ients , thecont ro l l ing heat t ransfer res i s tance i s shi f ted to

the hot or condensing) s ide , This of ten requi rescomplementary enhancement of the condensing s ide tobalance heat t ransfer res i s tances and to achieveoptimum overa l l coeff ic ients . Sui tab le methods forenhancement of the condensing s ide include the useof extended areas f lu ted tubes) , turbulence promotion, or high tube s ide ve loc i t i e s . Overall hea tt ransfer coeff ic ients of 3 to 8 times the convent ional values are achieved i n t h i s manner. As anexample, Figure 2 shows the performance of High Fluxversus non-enhanced heat exchange equipment for anexterna l ly f lu ted tube condensing propylene and

1000

7

N

..:"-

50 0 i:I:

;; 400

ID

I 300

zwUu: 200-w

oU

II:

"-


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boi l ing propane on the ins ide . At a temperaturedi ffe rence of 100F. , the High Flux tube achievesovera l l coeff ic ien t of approximately 400 BTU/Hr.f t . 2 OF. and r e su l t s in a 4.6 fold improvement overthe bare surface tube. The higher overa l l coeff ic ients achieved by High Flux tubes resul t infewer or smaller uni t s and/or al low operation a tlower temperature di fferences . This i s highlydes i rab le in vapor recompression systems where del ta

T has a major effec t on the compression r a t i o andresu l t ing operat ing cos ts . Studies on the vaporrecompression system ind ica te the optimum High Fluxdesign to be a s ingle she l l uni t with a d e l t a T of100F. This resul t s in minimum horsepower requi rements. Figure 3 shows a High Flux v e r t i c a l thermosiphon reboi le r await ing i n s t a l l a t i o n for use in al a rge propylene-p,ropane heat pump u n i t in Europe,which has been success fu l ly operat ing since ear ly1980.

It PLUX IlIIOILIII POll LAMI C IPLI TTlIIIAT 1ItII

b. Multiple Downcomer Trays

In the d i s t i l l a t i o n column i t s e l f , MultipleDowncomer (MD) t rays a lso permit a more favorablebalance of investment and operat ing cos ts over conventional t rays . The Multiple Downcomer t ray, asshown in Figure 4, makes use of downcomers d i st r ibu ted across the t ray deck. The downcomers,fabr ica ted as elongated troughs, c o l l e c t l i q u i d andd i s t r i b u t e i t to the t ray below and a lso ac t as theprimary mechanical support for the t ray. Each successive t ray i s ro ta ted 90 0 to provideCredis t r ibu t ionof the l iqu id and vapor from t ray to t ray. Thevapor flows up through the per fora ted deck mater ia lwhile the l i q u i d i s sues through spouts in the bottom

of the downcomer d i r ec t l y onto the f ro th of the t raybelow. This e l imina tes the need for the downcomerl iqu id to overcome the head of l iqu id at the t rayi n l e t as on a conventional t ray, and resul t s in t rayspacings c loser than conventional t rays (12-14"versus 18-24"). Thus, lower re f lux r a t i o s areachievable due to the addi t iona l theore t ica l t rayswhich can be supplied within a given column heightl imi ta t ion . In the vapor recompression system, t h i sresul t s in lower horsepower requirements because the

Iamount of vapor being compressed i s reduced. T h ~ a b i l i t y to design a t low t ray spacings wi l l also!al low a d i f f i c u l t separa t ion such as in a propylr ne -propane s p l i t t e r to be performed in one column Iversus two, thereby resul t ing in s igni f icant c a p ~ t a savings. Column diameters with M t rays are alSr't y p i c a l l y smaller because the downcomer arrangement i . e . , downcomers terminating in the vapor Ispace) e l imina tes the need for rece iv ing pans. ~ h i s

maximizes the amount of tower area used as ac t iv j 'a rea for vapor passage. \

I: :

.............. . . .

II

V POR ND LIQUID now PATHS fO IltO TltAYS

I

A number of engineering cont rac tors and 1operat ing companies have employed Union Carbide'\sHigh Flux and Multiple Downcomer t ray t e c h n o l o g ~ e to improve performance, decrease u t i l i t i e s and Ilower opera t ing cos t s in t he i r designs. Howevetexperience i s not l imi ted to supplying heat e x c ~ a n and d i s t i l l a t i o n t r a y equipment, and r e s p o n s i i ~ i has been undertaken for the equipment, process ~ n d cont ro l system design when required. IHEAT PUMP APPLICATIONS

IThe vapor recompression cycle can u t i l i z e 4i ther

one or two stages of compression. The two s t g ~ system wil l be more energy e f f i c i e n t , but p r o c e ~ s complexity wi l l be increased . Questions o n r ~ i n g system r e l i a b i l i t y, balancing of f lows, and easd ofcont ro l wi l l need to be addressed. Although l aJges ize p lan ts can j u s t i f y the addi t iona l c o m p l e x i ~ of a two stage heat pump system, the optimum f r ~ m the standpoint of s impl ic i ty and r e l i a b i l i t y i s !the

.l '

Is1ng e s tage un1t. II. Single Stage of Compression iI

Union Carbide has designed two energy e ff i d i en theat pumps s ince 1976, u t i l i z i n g a s ingle s tage!ofcompression a t the process condit ions discussed!below. Both u n i t s were designed to produce p o l ~ e r grade propylene with p u r i t i e s in excess of 9 9 ~One of these uni t s has been success fu l ly o p e r t ~ n g in the Gulf Coast s ince 1979, while s ta r tup of ~ h e second u n i t i s expected in l a t e 1981.

The pressure l eve l of the s i n g l e s tage systemi s dependent upon the cooling water temperature :and

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the temperature differences on the heat exchangeequipment. With an assumed cooling water temperatureof 850F and an allowable r i se of 200F. , the com-pressor discharge conditions are a pressure ofapproximately 275 psia with a dew point of l1 2 0F.The actual conditions are somewhat higher since thedischarge i s superheated. The tr im cooler removesthe superheat and par t i a l ly condenses the propylenestream. This two phase stream then enters thereboiler/condenser. Using High Flux tubing toachieve a 100F del ta T and minimize the compressionra t io resul ts in a bottoms temperature of 102 0F.,which corresponds to approximately 190 psia , thebottoms being mainly propane. Accounting for thepressure drop through the maximum number of M t raysthat could be supplied in one column resul ts in ahead pressure of 175 psia.

The lower pressure level of the heat pump system along with the use of high performance equipmentresul ts in s ignif icant ly lower heat and u t i l i t yrequirements than can be achieved in a high pressureconventional system. The heat required to performthe same separa t ion a t a head pressure of 175 psiai s less than 85 of that needed by the conventional

system which operates a t 280 psia , as set by therequirement for condensation against cooling water.Lower operating costs are obtained due to eliminationof the majority of steam and cooling water requirements. Table I compares u t i l i t y requirements forthese two systems and shows the advantage of theheat pump system. The estimated energy savings areapproximately $1,340,000 per year for th is s izeplant . Larger s ize plants wi l l exhibi t greaterbenef i ts . For example, in one plant producing70,000 lbs /hr. of propylene, the savings in switching to a heat pump system using High Flux wasestimated to be in excess of $2,300,000 per year.

TABLE I

COMPARISON BETWEEN CONVENTIONALPROPYLENE PROPANE SPLITTER

ANDHEAT PUMP WITH HIGhEFFICIENT PROCESS F.QUIPMF NT

VaporConvent i o n a ] Recompress ion

System C y c l e

Tra.y Type Vnlve M u l t i p l e Downcomer

NumlJcr o f Tr:lYR- 140 180

Tray Spacinl{, In. 18 14

Reflux Ra t io I R . 3 12 .6

Rplloj lC'Ts a re Tube lIh :h Flu,," T t l h e ~ 6

I P

C o n d C n ~ i l l ~ Tt1mp. OF 270 112

Rebo i l e r De l l a T, OF ]35 109 , 1 0 0llebn11 (>r } \ r t ~ aFt 2 3 ,325

Overa l l C O f ~ ~ ~ / : [ ~ ~ \ t 2 OF 100 noo m p r e s ~ o rKW 1 , 2 70

St.eam F'lnw, L b s / l i r . 46,21,:)

EnerKY COF;t 5 / l 0 0 0 I b s . RteQ.m $.O;'/KW6

Opcrllt.inr, Cust , $ p c r y p a r 1 . 8 5 x lOfi .51 x 106O p e r a l i n ~ Savjnr;l':, / Y I ~ a r 1 .3'1 x 10

• F i , « ' d as nll\ximum w i t h i n h f l ~ h t n' :-;tl'ie-tlon.

The control scheme for a heat pump can be designed to be no more complex than a conventionalsteam/cooling water system which r e l i e s on flow andleve l con t ro l l e r s to set the various column flowr a t e s . This type of scheme has the advantage ofr e l a t ive s impl ic i ty but must be somewhat overdrivento account for minor upsets in product puri ty andrecovery. Although th is may be t yp ica l of cornmonoperating pract ices , i t does waste some of thebenef i ts to the heat pump system. Additional instrumentation for feedback and feedforward controlcan be added to the system to make i t operate in amanner so that design pur i t i e s and recoveries areobtained a t the minimum energy requirement. UnionCarbide's design of the control scheme for two C3heat pump uni ts provided the degree of s impl ic i ty /complexity that was required to produce a highgrade propylene product.

The star tup of these uni ts i s f a i r ly s t r a igh tforward, with only the normal level of di ff i cu l t i e sexpected a t the s t a r t of any new system. Themajority of the problems can be reduced by properpurging of the system and subsequent re ject ion ofa l l ine r t components. These compounds can accu

mulate in the system and prevent the design condi t ions from being met. Most cornmon i s the buildupof non-condensibles in the reboiler/condenserwhich impairs effect ive heat t ransfer. Afterpurging and re ject ion of any iner ts in the system,the column should be pressurized to prevent thermalshock upon addi t ion of the propylene feed. Thecompressor i s run on to t a l recycle with the tr imcooler keeping the system from bui lding up excessenergy. Liquid level i s bui l t up in the columnbottoms and vapor i s allowed to be fed to the reboiler/condenser. Vaporization of the bottomsl iquid s t a r t s to bring the un i t on l ine . As thelevel in the bottoms drops, more feed i s added.The condensed vapors from the reboiler-condensergo to the column as ref lux. After pressur izat ion,s tar tup of th is type uni t can be completed in 3-8hours. In bringing the system down for shor tperiods of t ime, no specif ied techniques areneeded, as much of the equipment i s kept runningas long as poss ib le . Short shutdowns (I-day) wi l lnot require removal of the l iquid from the columni f i t i s insula ted, since a la rge volume of l iquidi s involved and heat leak wil l be slow. Forlonger per iods , process l iquid should be drainedfrom the system.

b. Two Stages of Compression

The heat pump descr ip t ion up to th i s point intime has been for a system with a single stage ofcompression. We wil l now describe how a customerused the Union Carbide products somewhat different ly

in a heat pump design. The propylenec ~ l u m n

wasdesigned with a single stage of compression, ahead pressure of 130 psia , and produced a polymergrade product. A single column w ~ t h approximately200 M t rays spaced a t 13 inches was used toperform the s p l i t T h ~ 13.0' diameter column wasshop fabr ica ted and erected with the t rays inplace. High Flux tubing was not original ly specif ied for th is uni t . Conventional heat exchangeequipment was used and the temperature differencein the reboiler/condenser was 40 0F. The potent ia l

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for s ign i f i can t improvement exis ted with t h i s hightemperature difference . I t required t ha t the columnbe operated a t a pressure of 130 psia with theen t i r e overhead stream compressed to 290 ps ia , suchthat the heat of compression could be removed withcool ing water. This r e su l t ed i n a compression r a t i oof 2.23 and an associa ted horsepower of 6200.

I ST. STAGE 2 NO. STAGE

TRIM

. COOLER

~ - . . I PRODUCT

REBOILER - CONDENSER

DISTILL TION

TOWER

FIGURE 5

CONVERSION OF SINGLE STAGE HEAT PUMP WITH LIN DE j

ProeC!'ilj Type Ileut Pump.

S t a j i ; f ~ o f Comprf ' :"f i ion

H{'ud r r ~ s s u r , p ~ j a l ~ O

29 0P r o p y l e ne C o n d e n se r. p s t a

Prury lcn( ' C o n d p n ~ w r . o r 120

Battaml- ' Temper.atuTf" , Or' RO

40llt 'boilf 'T n e l t o . T, o f

R 6 . 0 9 6 . 6I lpbui If 'T Duty, MMnTU/Hr

RnTf' Tu b e JliJ ,"h F l u xrtf-hoi 1( 1 '{'YPC

2 12 , RRn 1H, 313H c h o i PT Arc, l Ft

Ov('ra 11 U ' nTO/IIT. H 2 o fo

fiR" x_ f ,H " x lf1 r

H p b o i l t ' r Si7. 2 - 1 0 '

Numher n f 1 ' ray! ' " ,

: , ( ; ,000Pt 'odl lct Flow, Ib:-- /hr. 47,($,\0

6 , 2 0 0 '1 ,700

I'OWPT S : t v i n ~ ~ s , a t $0.05/KW a n rt R . 000 I h · ~ . "!iO.O(I$

SUMM RYji

Although not un ive r sa ll y app l icab le due to It heava i l ab i l i t y of waste heat and o the r sepa ra t i on Icons t r a in t s , the heat pump process can be a verY,useful energy saver under co r r ect cond it i ons.

the current worsening energy s i tua t ion , lowc o s ~

heat i s scarce and methods for in.proving system ie ff i c i enc ies , such as the heat pump, are requirdd.Although Union Carbide ' s vapor rec'ompression idesigns have been with systems separa t ing C3 c o ~ -pounds, t h e hea t pump scheme should be equally !advantageous for un i t s separa t ing close boi l ing icompounds such as C4 ' s ~ i s o / n o r m a l ) , xylenes, e ~ c . The continuing r i s e in e n ~ r g y costs may, i n facti,make t h e hea t pump scheme \economical in the f u t ~ r e for systems which today would not seem to be ifavored by i t . i

REFERENCES

(1) Kenney, N.F., "Reducing the Energy Demand elfSeparation Processes," CEP, March 1979, Ipp. 68-71.

(2) Mix, T. J . , Dweck, J . S . , Weinberg, M.,Armstrong, R.C., "Energy Conservation in D i s t i l l a t i o n , CEP, April 1978 , pp . 49-55.

(3) Wolf, C.W., Weiler, C.W., Ragi, E.G.,"Energy Savings Prompt Improved D i s t i l l a t i o n ,Oil and Gas Journal , 9/1/75, pp. 85-88.

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(4) Wolf, C.W., High Flux Tubing Conserves Energy, (7) Quadri, G.P., Use Heat Pump for P-P S p l i t t e r ,July 1976. Hydrocarbon Processing, Vol. 60, No.2 , Feb.

1981.(5) Null, H.R., Heat Pumps in Dis t i l l a t i on ,

CEP, July 1976, pp. 58-64. (8) Kirkpatrick, R.D., MD Trays Can Provide Savingsin Propylene Pur i f ica t ion , Oil and Gas Journal,

(6) Null, H.R., Heat Pumps Reduce Dis t i l l a t i on April 3, 1978, pp. 72-83.Energy Requirements, Oil and Gas Journal,2/9/76, pp. 96-98. (9) Gottzmann, C.F. , O 'Nei l l , P.S. , Minton, P.E. ,

High Efficiency Heat Exchangers, CEP .£2.N o . 7 , July 1973.

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high performance trays and heat exchangers in heat pumped distillation columns

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