and

chemical engineering research and design 8 7 ( 2 0 0 9 ) 16491657

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Chemical Engineering Research and Design

journa l homepage: www.e lsev ier .com/ locate /cherd

Energ acolum al

Massim RonDanielea Universit iali, Pb University logy aEnvironmenc Lappeenra O. Bo

a

The divided wall column (DWC) to separate three components in a single distillation tower is receiving increasing

interest in industrial applications due to the potentiality in energy and capital cost savings. In this work, the DWC

congurations for the separation of a four components mixture was considered, and 5 different composition cases

w

c

a

fo

e

c

D

li

K

1. Int

Distillationseparatingindustry. Dback of theandwith thtions, the nbecomes thtives. Manyof the bestmixture, frconguraticolumn arr2006). Amonique is co

CorresponE-mail aReceived

0263-8762/$doi:10.1016/ere analyzed. After selecting the best simple column (SC) sequence, the hybrid structures obtained by considering a

ongurationwith a DWC replacing the rst or the last two SCs of the sequence are considered. To simulate the DWCs

short-cut code was used to get the input data necessary to initialize the rigorous simulations. The results obtained

r the hybrid structures were compared with the performance of the best SC sequence from which are derived to

valuate energy and capital cost savings. The Petlyuk and the DWC structures were considered independently in the

apital cost evaluation to select themost convenient conguration. A signicant energy reduction was achieved with

WC structures, while the saving in capital costs is lower than the 30% value reported in most of the specialized

terature.

2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

eywords: Divided wall column; Petlyuk column; Multicomponent distillation; Fully thermally coupled structure

roduction

is considered, by far, the prevalent method foruid mixtures in the chemical and petrochemicalespite its huge diffusion this method has the draw-high energy consumption. After the energy crisise introductionofmore strict environmental regula-ecessity to dene more energy efcient structurese rst step in the choice of the design alterna-studies in the last decades focused on the problemseparation sequence for a given multicomponentom the analysis of the space including all the SCons (Thompson and King, 1972) to new distillationangements recently proposed (Rong and Turunen,ng all the possibilities, the thermal coupling tech-nsidered as the most promising strategy to reach

ding author. Tel.: +39 070 675 5061; fax: +39 070 675 5067.ddress: [email protected] (M. Errico).28October 2008; Received in revised form18 May 2009;Accepted20May2009

the scope of energy reduction in both design and retrot cases(Calzon-McConville et al., 2006; Errico et al., 2008). It is wellknown that the thermal coupling is realized by the substitu-tion of a condenser and/or a reboiler with a two-way liquidand vapour interconnecting streams between the distillationcolumns. In the case where all the possible thermal couplingsare introduced at the same time and the separation is car-ried out by employing only one condenser and one reboiler,the structure is called fully thermally coupled congurationor Petlyuk column (Agrawal, 2000; Petlyuk et al., 1965). Com-pletely thermal coupling structures were initially patented inthe rst half of the XX century by different authors (Brugma,1942; Wright, 1949) and then reconsidered from the point ofview of the reduction of the thermodynamic losses relatedto the separation technique (Petlyuk et al., 1965). The Pet-lyuk conguration for a 3 component separation is reported

see front matter 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.j.cherd.2009.05.006y saving and capital cost evalun sequences with a divided w

iliano Erricoa,, Giuseppe Tolaa, Ben-GuangDemurtasa, Ilkka Turunenc

degli Studi di Cagliari, Dipartimento di Ingegneria Chimica e Materof Southern Denmark, Institute of Chemical Engineering, Biotechnotal Technology, DK-5230 Odense M, Denmarknta University of Technology, Department of Chemical Technology, P.

b s t r a c ttion in distillationl column

gb,

.zza DArmi sn, I-09123 Cagliari, Italynd

x 20, FIN-53851 Lappeenranta, Finland

1650 chemical engineering research and design 8 7 ( 2 0 0 9 ) 16491657

Nomenc

AbC

C

C

dDDDDIEFfGG

IIDmNnPQqRrSVV

Y

Subscript1, 2, 3abA, B, CcfL

mPrTCV

w

Greek sym

in Fig. 1a arst columnon-sharpthe seconddle componwhile in th

arated. The two columns are connected by liquid andr couk conuratir awsklature

heat exchanger area [m2]bottom component owrate [kmol/h]annual incremental unit investment cost[$/m2 yr]

are sepvapouPetlyucongtion foKrolikoannual incremental unit investment cost incondenser and reboiler equipment [$/m2 yr]cost of steam and coolant to vaporize and con-dense respectively 1kmole of distillate [$/kmol]distillate component owrate [kmol/h]direct structurecolumn diameter [m]directindirect structurefractional plate efciencyfeed owrate [kmol/h]feed component owrate [kmol/h]allowable vapour velocity [kmol/hm2]vapour-handling capacity of condenser andreboiler equipment combined [kmol/hm2]indirect structureindirectdirect structureside stream component owrate [kmol/h]number of stagesnumber of stages above the feed locationcolumn top pressure [kPa]heat exchanger duty [kW]feed qualityreux rationumber of stages below the feed locationsymmetrical structurerectifying section vapour owrate [kmol/h]stripping section vapour owrate [kmol/h]working time [h/yr]

identication column numberabove the thermal couplingbelow the thermal couplinggeneric componentcondenserfeedstage withdrawal of the liquid stream from themain columnminimumprefractionatorreboilerthermal couplingstagewithdrawal of the vapour stream from themain columnwall

bolsrelative volatilityUnderwoods active root

nd consists of two interconnected columns. Then, called prefractionator, performs a preliminaryseparation for the middle boiling component, incolumn the mixture of the lightest and the mid-ents is separated in the upper part of the column,e bottom the middle and the highest components

ated to therealized incresulting coshell columlocation froThe liquidreboiler arePetlyuk andalent fromtransfer acrsavings arecolumn coreduction oof 1050% ccolumn seq

Notwithguration oand its appcipal limitathe designtem; anywamathematisimulationand Skogesestimatedwide with2007).

The PetFig. 1a andponents seextended amixture (RHowever thfor three coattempts htems. For thseparationparticular,a simple coture is evaresults obtenergy savstructuresevaluation.

2. Ca

A mixtureheptane, aTable 1, waing to Thomsequencesusing Aspefor each cothe WinnUtialize themodel. Thepressure, antercurrent streams. It has been proved that theguration requires, compared to all the possible

ons, the lowest total boil-up for a given separa-three components ideal mixture (Fidkowski andi, 1987). Often the Petlyuk conguration is associ-divided wall column (DWC). In fact the DWC isluding theprefractionator in themain column.Thenguration, reported in Fig. 1b, consists of a singlen with a partitioning wall that separates the feedm the side draw of the middle boiling component.reux from the condenser and the vapour from thesplitted through on the two sides of the wall. Thethe DWC congurations can be considered equiv-

the energetic point of view assuming that no heatoss thewall occurs (Lestak et al., 1994). Capital costexpected to be considerable due to the single shellnguration (Becker et al., 2001). The total vapourf these congurations was quantied in the rangeompared to the classical direct and indirect simpleuences (Agrawal and Fidkowski, 1998).standing the evident benets of this kind of con-nly in the last years it becomes more attractivelicability more realistic (Agrawal, 1999). The prin-tion in employing this structure was the lack inand the difculty in the controllability of the sys-y with modern control techniques, more suitablecal knowledge, high modelling tools and dynamics, the problem can be easily overcome (Halvorsentad, 1997; Parkinson et al., 1999). Recently it wasthat more that 100 DWCs are installed world-a trend of 10 columns built each year (Parkinson,

lyuk or its equivalent DWC structure, reported inb respectively, are considered for a three com-paration, while the methodology was recentlylso to a higher number of components in the feedong and Turunen, 2006; Christiansen et al., 1997).e controllability of the system was proved onlymponents separations and up to date no seriousave been made to implement more complex sys-is reason, in this work, only Petlyuk/DWCs for theof three component mixtures are considered. Inthe possibility to use a Petlyuk/DWC together withlumn for the separation of a four componentsmix-luated considering different design lay-outs. Theained are compared in order to identify the mosting solution, moreover the Petlyuk and the DWCare independently considered for the capital cost

se study

of 1000kmol/h of normal parafn, from butane tond the 5 different composition cases reported ins considered. For each composition case, accord-pson and King (1972), 5 different simple column

are possible. All the sequences were simulatedn Plus 13.0 and considering a molar purity of 0.99mponent. The simplied DSTWU model based onnderwood Gilliland method was rst used to ini-

rigorous simulations performed with the RadFractop column pressure was chosen as the minimumbove or equal to the atmospheric one, to assure a

chemical engineering research and design 8 7 ( 2 0 0 9 ) 16491657 1651

Fig. 1 (a) Petlyuk conguration, (b) DWC conguration and (c) three columns model.

Table 1

Compone

A: n-C4H10B: n-C5H12C: n-C6H14D: n-C7H16

distillate teto use coldA single tratray efcienrated liquidwas evaluaan average2100kJ/(m2

minimum ters, an annof 10 yearsfor the capconsideringble to morewere updat(Chemical Ehot and col

8). Ting ve oy Faias ashellsumny utand tratia and in

GeuraComposition feed cases.

nt Case 1 Case 2 Case 3 Case 4 Case 5

Molar fraction

0.70 0.10 0.25 0.10 0.400.10 0.10 0.25 0.40 0.100.10 0.10 0.25 0.40 0.100.10 0.70 0.25 0.10 0.40

mperature at least of 50 C. In this way is it possiblewater as auxiliary uid in the overhead condensers.y pressure drop of 0.1 psi was considered and thecy was neglected. The feed was assumed as satu-at the pressure of 1 atm. The heat exchanger area

al., 200assum0.8. Thgiven barea wsteel sthe coltied bindexconguers arereporte

3.congted with the usual design formula by consideringheat exchange coefcient of 1800kJ/(m2 h C) andh C) for condensers and reboilers respectively. Aemperature approach of 10 C in the heat exchang-ual running time of 8000h/yr and a plant life timewere also assumed. The Douglas correlations

ital cost estimation were utilized (Douglas, 1988)that are simple to use and the results compara-recent data (Taal et al., 2003). The capital costs

ed to year 2008 by the Marshall and Swift indexngineering, 2008). The operational costs related tod utilities are referred to European prices (Errico et

Once that tit is possibstituting 2of 4 compoand from eathe TAC indThe formersequence frst 2 simplowed by ththe structu

Fig. 2 TAC values for the SC seqhe column diameter calculations were performedapour velocities with a ooding fraction equal tooding velocity was estimated using the correlationr (1961), available in the simulator. The downcomersumed equal to 10% of the total tray area. Carbonand sieve trays 0.6m spaced are considered for alls. The best simple column conguration was iden-ilizing the total annual cost (TAC) as the economiche results are reported in Fig. 2. Details about theon parameters, column diameters, heat exchang-d energy consumption for the selected best SCs areTable 2.

neration of the alternativetions

he best simple column conguration is identied,le to dene the new structures obtained by sub-simple columns with a Petlyuk/DWC. In the casenent separation, 3 simple columns are necessarych best simple column sequence selected by usingex, it is possible to generate two new structures.is obtained utilizing the rst simple column of theollowed by a Petlyuk/DWC. In the latter case thele columns are substituted by a Petlyuk/DWC fol-

e last simple column of the sequence. Fig. 3 showsre considered for each composition case obtained

uences.


Table 2 Design parameters and energy consumption of the best SC congurations selected.

Best SC Case 1 Case 2 Case 3 Case 4 Case 5

C1 C2 C3 C1 C2 C3 C1 C2 C3 C1 C2 C3 C1 C2 C3

N 40 33 30 35 60 28 40 40 30 40 27 33 32 45 42Nf 16 14 15 18 27 14 20 20 15 18 14 17 16 20 21R 0.83 2.50 2.85 0.19 1.00 1.42 1.33 2.41 2.05 3.00 1.95 1.15 0.96 3.20 2.00P [kPa] 480 170 110 220 310 480 480 160 110 480 160 110 480 110 160DD [m] 3.10 1.48 1.86 3.77 1.63 1.12 2.76 2.43 2.73 2.62 2.87 2.83 2.97 2.73 1.54Ac [m2] 749 211 160 790 286 141 338 606 289 233 812 326 458 656 215Ar [m2] 857 76 108 355 183 53 593 175 276 345 212 320 365 259 78

Qc [kW] 12339.71 10820.07 15077.12 17182.39 13037.44Qr [kW] 14687.50 15073.76 17412.86 18609.00 16409.57

from the best simple column sequences already selected inthe previous paragraph.

4. Modelling

Develop a designmodel to describe the steady state behaviourof a divided wall column is not an easy task. In the most usedprocess simulation packages, like Aspen Plus, Petlyuk/DWCcongurations are not available as a standard unit opera-tion already implemented in the simulator libraries. For thisreason DWCs must be considered as a combination of sim-ple columns connected by thermal couplings (Becker et al.,2001). Usuacolumn statialize mormodus optraditionaletc.) is act1992; MuraThe appliction of themost usedcoupled sy

and Westerberg, 1989). Instead different approaches are pro-posed for the evaluation of the minimum and the theoreticalnumber of stages (Kim, 2001; Sotudeh and Shahraki, 2008).In this work the procedure followed by Muralikrishna et al.(2002), here briey resumed, has been considered. This pro-cedure was chosen because allows to identify the completespace that includes all the possible operational points for thePetlyuk/DWC conguration, then the identied space can beexplored to get the best solution by using a specic objectivefunction.

The Petlyuk/DWC conguration can be modelled using thethree columns model reported in Fig. 1c. In the rst column itis possible to apply the Underwood equations in their original

Unde

i fi j

d1i j

consng cofalslly the design procedure for a simple distillationrts with the choice of a short-cut method to ini-e rigorous calculations. It is possible to extend thiserandi also to DWCs, even if the application of theshort-cut methods (Underwood, Fenske, Gilliland,ually under discussion (Triantafyllou and Smith,likrishna et al., 2002; Sotudeh and Shahraki, 2007).ation of the Underwood method for the evalua-minimum vapour owrate in the column is themethodology and its modication for thermallystems today is a well known procedure (Carlberg

form (

i

i

i

ii

BytributiregulaFig. 3 Structures derived from the best SCrwood, 1948):

= F (1 q) (1)

= (d1A + d1B + d1C) (1 + R1m) (2)

idering that in the prefractionator there is one dis-mponent (B), Eq. (1) can be solved with a commoni to obtain the two active roots. For a separationcongurations.


that takes place at innite reux ratio, the Fenske equations(Fenske, 19ber of stagelaborationratio aboveretical num1940). The dtion of thefeed locatio

The metto the othewood equa(1989). Bothtor realizekey compononly one Un

Obviouslyuk/DWC csatised:

1. Consideumn ofowrateowratethe twowhen a

V2 = V3

2. The secoical platstages oconnectmal courespecti

N1 = r2 +

This conpractical obto have a cassure a go

The debounds sum

1. the prefhigher tcolumn

d1A d2

2. the prefhigher tcolumn

d1B d2B

3. the maxator distfeed;

d1A fA

3

para

2 yr]2 yr]

kmol]r]

ol/hmol/h

B queast

= (fB fB

owuld bpon

= (fC fC

feasuxnden

R1,m

numn thndind3 ration

n3)

e theC repsignl an

E Y

houlor sk/DWly forstn conf evwassee

eters. Table 3 summarizes the parameters values used(12).

Simulation and results

the short-cut methodology for the Petlyuk/DWC cong-a Fortran code was compiled to initialize the rigorous

tions performed by means of Aspen Plus 13.0. All theeters obtained, like the number of trays, the feed loca-the thermal coupling owrates are checked and then32) can be utilized to calculate the minimum num-es for the separation. With some mathematicals (Treybal, 1980) and dening an effective reuxthe minimum, it is possible to calculate the theo-ber of stages by the Gilliland correlation (Gilliland,esign of the prefractionator endswith the applica-Kirkbride equation (Kirkbride, 1944) to identify then.hodology sequence described can be applied alsor two columns considering the modied Under-tions so as proposed by Carlberg and Westerberg

the columns downstream to the prefractiona-a sharp separation between the light and heavyent. Since no distributing components are presentderwoods root is active.

ly to adapt the three columns model to the Pet-onguration the following two conditionsmust be

ring that columns 2 and 3 substitute the main col-the Petlyuk conguration, the rectifying vapourfor column 3must be equal to the stripping vapourof the column 2. In this way it is possible to mergecolumns in an only one. This consideration is validliquid withdrawal is considered.

(3)

nd condition requires that the number of theoret-es in the prefractionator is equal to the sum of thef the stripping section (below the thermal couplingion) and of the rectifying section (above the ther-pling connection) of the second and third columnvely.

n3 (4)

dition is not a real bond, but is derived from theservation that if a DWC is considered, it is betterlose number of plates on both sides of the wall tood column stability.nition of the design space is related to sevenmarized as follows:

ractionator distillate ow rate of component A ishan or equal to the same component ow rate in2;

A (5)

ractionator distillate owrate of component B ishan or equal to the same component owrate in2;

(6)

imum quantity of component A in the prefraction-illate owrate is limited from the A quantity in the

(7)

Table

Cost

C [$/mC [$/mC [$/Y [h/yE [%]G [kmG [km

4. theat l

b1B

d1B

5. theshocom

b1C

d1C

6. thea respo

R1

7. thethaspo2anequ

(r2 +

Oncthe TAtive de(Happe

TAC =

It soped fPetlyuprobabin thecolumstage ospacemationparamin Eq.

5.

Usingurationsimulaparamtions,Cost parameters used in Eq. (12).

meters Value

296.0117.7620.61103800090

2] 219.71m2] 0.49

antity in the bottom of the prefractionator must beequal to the B quantity in the bottom of column 3;

d1B) b3B b3B

(8)

rate of C from the bottom of the prefractionatore equal to or higher than the quantity of the sameent in the residue of column 3:

d1C) b3C b3C

(9)

ible design space must include all the cases withratio of the prefractionator higher than the corre-t minimum value:

(10)

ber of stages for the prefractionatormust be highere sum of the minimum number of stages corre-g to the stripping and rectifying sections of columnespectively obtained fromtheFenske andKirkbrides:

m N1 (11)

design space is dened the simplied function fororted in Eq. (12) was chosen to select the attrac-options for the subsequent rigorous simulations

d Jordan, 1975):

C

GN(1 + R) +C

Y G (1 + R) + C(1 + R) (12)

d be noted that this relation was originally devel-imple column congurations but, in the case ofCs, no simplied expressions are available andr these systems the internal columncosts, includedterm of Eq. (12), are higher compared to the simplegurations due to the internalwall. Anyway at thisaluation, and taking into account that the designmapped using a short-cut method, this approxi-ms reasonable for the rst selection of trial design


Table 4 Design parameters and energy consumption of the SC+DWC sequences.

SC+DWC Case 1 Case 2 Case 3 Case 4 Case 5

SC DWC SC DWC SC DWC SC DWC SC DWC

N 40 36 35 32 40 35 40 35 32 35Np 20 19 20 17 23Nf 16 13 18 10 20 13 18 6 16 16NL 7 6 8 7 7NV 25 23 26 23 29Ns 20 15 21 13 23LTC [kmol/h] 165.60 86.40 331.20 306.00 396.00VTC [kmol/h] 133.20 280.80 406.80 540.00 306.00R 0.83 4.75 0.19 5.90 1.33 5.05 3.00 3.75 0.96 14.5P [kPa] 480 160 220 480 480 160 480 160 480 160DD [m] 3.10 1.37/1.97a 3.77 1.26/1.96a 2.76 2.18/3.37a 2.62 2.40/3.80a 2.97 2.01/3.50a

Ac [m2] 749 388 790 402 338 1077 233 1312 458 1106Ar [m2] 857 201 355 140 593 549 345 692 365 234

Qc [kW] 10838.51 10657.22 13617.24 15346.34 14998.78Qr [kW] 13399.35 14989.28 16297.27 17138.62 18464.84

a Prefractionator/main column.

optimized by sensitive analysis to reach the minimum energyconsumptition is densubstitutedmal couplienergy con

5.1. En

The maintions is theThis aspecreboiler duSC sequencwith a Petlysequence iing rst ththe rst SCof the lightPetlyuk/DWis quantie6 and 8%

tion case 2, where the best SC sequence is the indirect one,bstitto pthenaryneny forn cahose4 it icon

aluathe

mpaibuthe Pd in ced frpos

hteste beion t

Table 5

DWC+SC

NNpNfNLNVNsLTC [kmol/hVTC [kmol/RP [kPa]DD [m]Ac [m2]Ar [m2]

Qc [kW]Qr [kW]

a Prefractioon. The pressure of the Petlyuk/DWC congura-ed according to the highest pressure value of theSCs. The results of conguration parameters, ther-

ngs owrate, diameters, heat exchanger area andsumption, are summarized in Tables 4 and 5.

ergy comparison

advantage expected for these types of congura-possibility to achieve an energy load reduction.

t is considered rst using the total condenser andty to compare, for each composition case, the bestewith those derived substituting 2 simple columnsuk/DWC. For composition cases 1, 3 and4 thedirects the best simple conguration. Thus, by consider-e combination SC+DWC, so as reported in Fig. 3,remains unchanged and performs the separationest component at the highest pressure, then theC column completes the separation. The savingd in about 12, 10 and 11% for condenser and 9,for the reboiler duties respectively. For composi-

the suforcesassurein ordicompopenaltpositiowith tTablecan bethe evwhereing cobe attrand 3 tinstearemovIn comthe ligthat thguratDesign parameters and energy consumption of the DWC+SC se

Case 1 Case 2 Case 3

DWC SC DWC SC DWC

34 30 34 28 3224 24 1917 15 12 14 94 4 528 24 2314 14 13

] 162.00 252.00 248.40h] 1144.80 936.00 975.60

1.91 2.85 8.98 1.42 5.55480 110 310 480 4002.67/3.89a 1.86 3.01/4.92a 1.12 2.69/3.87a

1188 159 1428 141 1694613 87 728 53 471

14197.68 14256.28 15419.5316735.29 19015.70 18114.52

nator/main column.ution of the last two columns with a Petlyuk/DWCerform the separation at the highest pressure topossibility to condensate the lightest componentwater cooled condenser. In this way all the three

ts (A, B,C) are separated at the samepressurewith athe separation efciency. Comparing, for this com-se, the results for the best SC reported in Table 2obtained for the SC+DWC sequence included ins possible to notice that the energy consumptionsidered similar, or equal to the approximation oftion method. Composition case 5 is the only oneSC+DWC conguration is more energy demand-red to the corresponding best SC. The reason caned to the feed component distribution: in cases 1etlyuk/DWC column is fed by an equimolar stream,ase 4 there is an equal excess of component B andCom the distillate and the side stream respectively.ition case 5 the feed contains an equal excess ofand heaviest components and taking into accountst SC is the direct-indirect, in the SC+DWC con-he rst simple column removes the excess of thequences.

Case 4 Case 5

SC DWC SC DWC SC

30 39 33 33 42 17 22 15 10 20 19 21 9 5 22 27 13 15 288.00 756.00 864.00 1692.00 2.17 17.40 1.02 5.80 2.00110 420 110 480 1602.68 2.57/3.94a 2.73 3.80/5.17a 1.37299 1536 335 1586 206231 1538 187 1586 55

16506.61 16845.2318489.55 21780.62


lightest component, so the feed to the Petlyuk/DWC is a streamunbalanced

The eneare summaters. For allappears toThere is thesaving of 4duty of thebest SC seqthe direct olyuk/DWCpenalizatiocase 2 the his fed withsuitable to ethat the premain paramPetlyuk/DWtwo SCs.

5.2. Ca

Moreover tconguratimost of thethe capitaland Collinsobtained. Itof a three cand one rebno indicatithat is the mings in theevidenced humn and hregard the ison is thereboilers. Bbest SCs anconguratithe DWCalexchangerple columnfor the capciated to thconsideringconguratiit was chosequences(i.e. conguTo this regacongurati

5.2.1. PetThe Petlyucombinatiocolumn concost can blations (Dothe stagestor and thethe prefracand liquid

6 Normalized capital cost of the selectedgurations for the Petlyuk and the DWC design.

WC Petlyuk DWC

1 0.89 0.863 0.99 0.994 1.09 0.98

cost, normalized with respect to the capital cost of theponding best SC, are reported in Table 6. From theseit is possible to notice that the Petlyuk congurationicularly convenient in composition case 1 where theolumn is fed at the lowest owrate compared to theases considered. The main column diameter increasesfeed owrate increases, thus reducing the convenienceloy the Petlyuk conguration as evidenced in cases 3

DWhodly noce oal eqer h

ain p. Aal coC c

indiczonemm4 sh

tlyukses sinc

ion oht sitionf theolumd foulatabovhe rom tobtspecle 6 t

Diameter stage distribution in the Petlyuk mainn for the selected cases.in the heaviest component.rgy performances of the DWC+SC congurationsrized in Table 5 together with the design parame-the composition cases the DWC+SC congurationbe not convenient from the energetic point of view.only exception of the composition case 4 where a

% in the condenser duty was achieved, while thereboiler is similar to that of the correspondinguence. In the other congurations, derived fromr from the direct indirect (cases 1, 3 and 5), the Pet-is forced to operate at the highest pressure with an in the separation efciency. For the compositionighest pressure column is the SC, anyway the DWCan excess of heaviest component that makes notmploy this conguration. It is possible to concludessure and the feed component distribution are theeters that affect the energy consumption of theC congurations when are used in substitution of

pital cost comparison

he possibility of energy saving, the Petlyuk/DWCon has the potential of a capital cost reduction. Inspecialized literature a reduction of about 30% of

costwas estimated (Kolbe andWenzel, 2004; Lestak, 1997). Anyway it is not clear how this value wasis known that employing a DWC for the separationomponents mixture allows saving one condenseroiler, compared to the traditional SC sequence, butons were provided about the total exchanger areaain parameter related to the cost. Also some sav-column shell cost are achievable but it was notow to allocate the prefractionator in the main col-

ow to evaluate the corresponding diameter. To thisrst parameter that we considered for the compar-

heat exchanger area requested for condensers andy comparing the results reported in Table 2 for thed in Tables 4 and 5 for the SC+DWC and DWC+SCon respectively, it is possible to notice that even iflows to reduce the number of equipments, the totalarea, inmost of the cases, is higher than of the sim-congurations. The second parameter consideredital cost evaluation is the sum of the costs asso-e distillation column internals and shell. Anyway,that the main scope of the alternative proposed

ons is to reduce the energy demand of the plant,sen to limit the capital cost analysis only to thewhere an appreciable energy saving was achievedrations SC+DWC for composition cases 1, 3 and 4).rd the column capital cost of the Petlyuk and DWCon must be considered separately.

lyuk capital cost evaluationk conguration, so as previously described, is thenof two columns, theprefractionator and themainnected by thermal couplings. Both the columns

e evaluated using the classical simplied corre-uglas, 1988), utilizing the columns diameter andnumber reported in Table 4 for the prefractiona-main column respectively. The column height fortionator is calculated without considering vapourdisengagement. The results obtained for the total

Tablecon

SC+D

CaseCaseCase

capitalcorresresultsis partmain cother cas theto empand 4.

5.2.2.A metactualpresenclassicthe oththe mmationclassicthe DWbut nofor the1995; A

Fig.the Petion casectionreductthe rigprefracarea omain crequireter calcstagesFrom ttage frresultswith rein Tab

Fig. 4 columC capital cost evaluationfor the capital cost evaluation of this column ist well dened. First of all in the case of DWC thef the internal dividing wall makes unreliable theuations used for the plate cost calculations, but onand simplied correlations are not available androducers are reluctant to give this type of infor-few researchers made the assumption to use thest evaluation formulas considering in the case ofonguration only the stages of the main column,ations are provided about the diameter consideredof the columnwith thewall (Wolff and Skogestad,

inudin et al., 2001).ows the diameter prole in the main column ofobtained from the simulations for the composi-elected. It is possible to notice that in the columnluded between the thermal couplings there is af the diameter value. This zone corresponds tode of the wall section in the DWC. To allocate theator in the main column is necessary to add theprefractionator to the corresponding area of then; in this way it is possible to dene the diameter

r the DWC. Table 7 reports the results of the diame-ions considering the three sections of the DWC: thee, below the wall and in the divided wall section.esults it is evident the possibility to take advan-he construction of a single diameter column. Theained for the capital cost evaluation, normalizedt to the corresponding best SC cases, are reportedogether with those of the Petlyuk congurations.


Table 7 selected c

SC+DWC

Da [m]Dw [m]Db [m]

Thus fromlayout outpdisposition

6. Co

The DWC cnents mixtstudied. Thfor the comintroducingstructuresconsumptiPetlyuk/DWlized to repthe simplethe pressursequences)form the laThe mostwith equimponent mix

With reguation wasthe DWC csidering thinternals amated condesign of tvery impormust propestructure. Fthe DWC olyuk structPetlyuk/DWsaving whicosts.

Reference

Agrawal, R.,distillatidistillati

Agrawal, R.,distillatiTrans IC

Agrawal, R.distillatifor terna

AmminudinDesign adistillatioptimiza

Becker, H., Gdistillati6874.

A.J. Brugma

-McConville, C.J., Rosales-Zamora, M.B.,ovia-6, Deillatituresg, N.ram

lyukChemanseillatim Ens, J.MGrawM., Rnsiillati): 197., 196ding, M.Rolineski, ZuiremhE J,d, E.lticom.sen, Iyuk davioul, J. anion)..H., 2pled0246de, ClticomB. an-shel, F. anapita, F., Sml of d644.krishelopma spe.on, Gded ion, Gm En, F.B.rmodlticom.-G.temsgratih, N.ded w1.h, N.desigDiameter values in the DWC sections for theases.

Case 1 Case 3 Case 4

1.97 3.37 3.801.93 3.27 3.261.91 3.35 3.75

the results it is possible to conclude that the DWCerforms or equalizes at least (case 3) the Petlyuk.

nclusions

ongurations for the separation of a four compo-ure with 5 different feed composition cases aree best SC conguration is chosen as the basisparison with the new congurations obtained bya Petlyuk/DWC structure. The most promising

are rst selected on the basis of the lowest energyon. From the calculations it was evidenced that theC is not convenient when this conguration is uti-lace two SCswith a high pressure difference. For allcongurations where there is a decreasing value ofe through the columns (i.e. direct, direct-indirectit is convenient to use rst a SC and then to per-

st separation by using a Petlyuk/DWCarrangement.promising cases correspond to Petlyuk/DWC fedolar of equal excess of lightest and middle com-tures.ard to the capital costs evaluation, a detailed eval-carried out considering separately the Petlyuk andongurations. The analysis was performed con-e total exchanger area together with the columnnd shell. In particular the DWC diameter was esti-sidering the prefractionator and the main columnhe corresponding Petlyuk structure. This aspect istant because the real application of DWC columnsrly consider such design indications for the DWCrom the obtained results it can be concluded thatutperforms both the SC conguration and the Pet-ure. It should be noted that the main advantage ofC arrangements remains the potential in energyle a minor prot can be obtained for the capital

s

2000, A method to draw fully thermally coupledon column congurations for multicomponenton. Trans IChemE, Part A, 78: 454464.1999, More operable fully thermally coupledon congurations for multicomponent distillation.hemE, Part A, 77: 543553.and Fidkowski, Z.T., 1998, Are thermally coupled

CalzonSeg200distmix

Carlberdiagpet

(2008).Christi

distChe

Dougla(Mc

Errico,intedist47(6

Fair, J.Roo

Fenskegas

FidkowreqAIC

Gillilanmu920

HalvorPetlbeh

Happeedit

Kim, Ycou246

Kirkbrimu

Kolbe,one

Lestak& c

Lestakwal639

MuraliDevfor166

Parkinsdivi

ParkinsChe

PetlyukThemu

Rong, Bsysinte

Sotudedivi129

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., Kamiya, T., DAquino, R. and Ondrey, G., 1999, Then distillation. Chem Eng, 106(4): 3235.., 2007, Dividing-wall columns nd greater appeal.g Prog, 103(5): 811., Platonov, V.M. and Slavinskii, D.M., 1965,ynamically optimal method for separatingponent mixtures. Int Chem Eng, 5(3): 555561.

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R.O. Wright, U.S. Patent No. 2,471,134, May 24, 1949.

Energy saving and capital cost evaluation in distillation column sequences with a divided wall columnIntroductionCase studyGeneration of the alternative configurationsModellingSimulation and resultsEnergy comparisonCapital cost comparisonPetlyuk capital cost evaluationDWC capital cost evaluation

ConclusionsReferences

and

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