Ec

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chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

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

nergy saving and capital cost evaluation in distillationolumn sequences with a divided wall column

assimiliano Erricoa,∗, Giuseppe Tolaa, Ben-Guang Rongb,aniele Demurtasa, Ilkka Turunenc

Università degli Studi di Cagliari, Dipartimento di Ingegneria Chimica e Materiali, P.zza D’Armi sn, I-09123 Cagliari, ItalyUniversity of Southern Denmark, Institute of Chemical Engineering, Biotechnology andnvironmental Technology, DK-5230 Odense M, DenmarkLappeenranta University of Technology, Department of Chemical Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland

a b s t r a c t

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

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

were analyzed. After selecting the best simple column (SC) sequence, the hybrid structures obtained by considering a

configuration with a DWC replacing the first or the last two SCs of the sequence are considered. To simulate the DWCs

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

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

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

capital cost evaluation to select the most convenient configuration. A significant energy reduction was achieved with

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

literature.

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

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

view of the reduction of the thermodynamic losses related

. Introduction

istillation is considered, by far, the prevalent method foreparating fluid mixtures in the chemical and petrochemicalndustry. Despite its huge diffusion this method has the draw-ack of the high energy consumption. After the energy crisisnd with the introduction of more strict environmental regula-ions, the necessity to define more energy efficient structuresecomes the first step in the choice of the design alterna-ives. Many studies in the last decades focused on the problemf the best separation sequence for a given multicomponentixture, from the analysis of the space including all the SC

onfigurations (Thompson and King, 1972) to new distillationolumn arrangements recently proposed (Rong and Turunen,

006). Among all the possibilities, the thermal coupling tech-ique is considered as the most promising strategy to reach

∗ Corresponding author. Tel.: +39 070 675 5061; fax: +39 070 675 5067.E-mail address: m.errico@dicm.unica.it (M. Errico).Received 28 October 2008; Received in revised form 18 May 2009; Acce

263-8762/$ – see front matter © 2009 The Institution of Chemical Engioi:10.1016/j.cherd.2009.05.006

the scope of energy reduction in both design and retrofit 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 configurationor Petlyuk column (Agrawal, 2000; Petlyuk et al., 1965). Com-pletely thermal coupling structures were initially patented inthe first half of the XX century by different authors (Brugma,1942; Wright, 1949) and then reconsidered from the point of

pted 20 May 2009

to the separation technique (Petlyuk et al., 1965). The Pet-lyuk configuration for a 3 component separation is reported

neers. Published by Elsevier B.V. All rights reserved.

1650 chemical engineering research and d

Nomenclature

A heat exchanger area [m2]b bottom component flowrate [kmol/h]C annual incremental unit investment cost

[$/m2 yr]C′ annual incremental unit investment cost in

condenser and reboiler equipment [$/m2 yr]C′′ cost of steam and coolant to vaporize and con-

dense respectively 1 kmole of distillate [$/kmol]d distillate component flowrate [kmol/h]D direct structureDD column diameter [m]DI direct–indirect structureE fractional plate efficiencyF feed flowrate [kmol/h]f feed component flowrate [kmol/h]G allowable vapour velocity [kmol/h m2]G′ vapour-handling capacity of condenser and

reboiler equipment combined [kmol/h m2]I indirect structureID indirect–direct structurem side stream component flowrate [kmol/h]N number of stagesn number of stages above the feed locationP column top pressure [kPa]Q heat exchanger duty [kW]q feed qualityR reflux ratior number of stages below the feed locationS symmetrical structureV rectifying section vapour flowrate [kmol/h]V̄ stripping section vapour flowrate [kmol/h]Y working time [h/yr]

Subscript1, 2, 3 identification column numbera above the thermal couplingb below the thermal couplingA, B, C generic componentc condenserf feedL stage withdrawal of the liquid stream from the

main columnm minimumP prefractionatorr reboilerTC thermal couplingV stage withdrawal of the vapour stream from the

main columnw wall

Greek symbols˛ relative volatility� Underwood’s active root

tialize the rigorous simulations performed with the RadFrac

in Fig. 1a and consists of two interconnected columns. Thefirst column, called prefractionator, performs a preliminarynon-sharp separation for the middle boiling component, inthe second column the mixture of the lightest and the mid-

dle components is separated in the upper part of the column,while in the bottom the middle and the highest components

esign 8 7 ( 2 0 0 9 ) 1649–1657

are separated. The two columns are connected by liquid andvapour countercurrent streams. It has been proved that thePetlyuk configuration requires, compared to all the possibleconfigurations, the lowest total boil-up for a given separa-tion for a three components ideal mixture (Fidkowski andKrolikowski, 1987). Often the Petlyuk configuration is associ-ated to the divided wall column (DWC). In fact the DWC isrealized including the prefractionator in the main column. Theresulting configuration, reported in Fig. 1b, consists of a singleshell column with a partitioning wall that separates the feedlocation from the side draw of the middle boiling component.The liquid reflux from the condenser and the vapour from thereboiler are splitted through on the two sides of the wall. ThePetlyuk and the DWC configurations can be considered equiv-alent from the energetic point of view assuming that no heattransfer across the wall occurs (Lestak et al., 1994). Capital costsavings are expected to be considerable due to the single shellcolumn configuration (Becker et al., 2001). The total vapourreduction of these configurations was quantified in the rangeof 10–50% compared to the classical direct and indirect simplecolumn sequences (Agrawal and Fidkowski, 1998).

Notwithstanding the evident benefits of this kind of con-figuration only in the last years it becomes more attractiveand its applicability more realistic (Agrawal, 1999). The prin-cipal limitation in employing this structure was the lack inthe design and the difficulty in the controllability of the sys-tem; anyway with modern control techniques, more suitablemathematical knowledge, high modelling tools and dynamicsimulations, the problem can be easily overcome (Halvorsenand Skogestad, 1997; Parkinson et al., 1999). Recently it wasestimated that more that 100 DWCs are installed world-wide with a trend of 10 columns built each year (Parkinson,2007).

The Petlyuk or its equivalent DWC structure, reported inFig. 1a and b respectively, are considered for a three com-ponents separation, while the methodology was recentlyextended also to a higher number of components in the feedmixture (Rong and Turunen, 2006; Christiansen et al., 1997).However the controllability of the system was proved onlyfor three components separations and up to date no seriousattempts have been made to implement more complex sys-tems. For this reason, in this work, only Petlyuk/DWCs for theseparation of three component mixtures are considered. Inparticular, the possibility to use a Petlyuk/DWC together witha simple column for the separation of a four components mix-ture is evaluated considering different design lay-outs. Theresults obtained are compared in order to identify the mostenergy saving solution, moreover the Petlyuk and the DWCstructures are independently considered for the capital costevaluation.

2. Case study

A mixture of 1000 kmol/h of normal paraffin, from butane toheptane, and the 5 different composition cases reported inTable 1, was considered. For each composition case, accord-ing to Thompson and King (1972), 5 different simple columnsequences are possible. All the sequences were simulatedusing Aspen Plus 13.0 and considering a molar purity of 0.99for each component. The simplified DSTWU model based onthe Winn–Underwood Gilliland method was first used to ini-

model. The top column pressure was chosen as the minimumpressure, above or equal to the atmospheric one, to assure a

chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657 1651

Fig. 1 – (a) Petlyuk configuration, (b) DWC con

Table 1 – Composition feed cases.

Component Case 1 Case 2 Case 3 Case 4 Case 5

Molar fraction

A: n-C4H10 0.70 0.10 0.25 0.10 0.40B: n-C5H12 0.10 0.10 0.25 0.40 0.10C: n-C6H14 0.10 0.10 0.25 0.40 0.10D: n-C H 0.10 0.70 0.25 0.10 0.40

dtAtrwa2meofcbw(h

7 16

istillate temperature at least of 50 ◦C. In this way is it possibleo use cold water as auxiliary fluid in the overhead condensers.

single tray pressure drop of 0.1 psi was considered and theray efficiency was neglected. The feed was assumed as satu-ated liquid at the pressure of 1 atm. The heat exchanger areaas evaluated with the usual design formula by considering

n average heat exchange coefficient of 1800 kJ/(m2 h ◦C) and100 kJ/(m2 h ◦C) for condensers and reboilers respectively. Ainimum temperature approach of 10 ◦C in the heat exchang-

rs, an annual running time of 8000 h/yr and a plant life timef 10 years were also assumed. The Douglas’ correlationsor the capital cost estimation were utilized (Douglas, 1988)onsidering that are simple to use and the results compara-le to more recent data (Taal et al., 2003). The capital costs

ere updated to year 2008 by the Marshall and Swift index

Chemical Engineering, 2008). The operational costs related toot and cold utilities are referred to European prices (Errico et

Fig. 2 – TAC values for

figuration and (c) three columns model.

al., 2008). The column diameter calculations were performedassuming vapour velocities with a flooding fraction equal to0.8. The flooding velocity was estimated using the correlationgiven by Fair (1961), available in the simulator. The downcomerarea was assumed equal to 10% of the total tray area. Carbonsteel shells and sieve trays 0.6 m spaced are considered for allthe columns. The best simple column configuration was iden-tified by utilizing the total annual cost (TAC) as the economicindex and the results are reported in Fig. 2. Details about theconfiguration parameters, column diameters, heat exchang-ers area and energy consumption for the selected best SCs arereported in Table 2.

3. Generation of the alternativeconfigurations

Once that the best simple column configuration is identified,it is possible to define the new structures obtained by sub-stituting 2 simple columns with a Petlyuk/DWC. In the caseof 4 component separation, 3 simple columns are necessaryand from each best simple column sequence selected by usingthe TAC index, it is possible to generate two new structures.The former is obtained utilizing the first simple column of thesequence followed by a Petlyuk/DWC. In the latter case the

first 2 simple columns are substituted by a Petlyuk/DWC fol-lowed by the last simple column of the sequence. Fig. 3 showsthe structure considered for each composition case obtained

the SC sequences.

1652 chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657

Table 2 – Design parameters and energy consumption of the best SC configurations 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

�Q [kW] 12339.71 10820.07 15077.12 17182.39 13037.44

c

�Qr [kW] 14687.50 15073.76

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

4. Modelling

Develop a design model 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/DWCconfigurations 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). Usually the design procedure for a simple distillationcolumn starts with the choice of a short-cut method to ini-tialize more rigorous calculations. It is possible to extend this“modus operandi” also to DWCs, even if the application of thetraditional short-cut methods (Underwood, Fenske, Gilliland,etc.) is actually under discussion (Triantafyllou and Smith,1992; Muralikrishna et al., 2002; Sotudeh and Shahraki, 2007).The application of the Underwood method for the evalua-

tion of the minimum vapour flowrate in the column is themost used methodology and its modification for thermallycoupled systems today is a well known procedure (Carlberg

Fig. 3 – Structures derived from

17412.86 18609.00 16409.57

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 briefly 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 configuration, then the identified space can beexplored to get the best solution by using a specific objectivefunction.

The Petlyuk/DWC configuration can be modelled using thethree columns model reported in Fig. 1c. In the first column itis possible to apply the Underwood equations in their originalform (Underwood, 1948):

∑i

˛i · fi

˛i − �j= F · (1 − q) (1)

∑i

˛i · d1i

˛i − �j= (d1A + d1B + d1C) · (1 + R1m) (2)

By considering that in the prefractionator there is one dis-tributing component (B), Eq. (1) can be solved with a common“regula falsi” to obtain the two active roots. For a separation

the best SC configurations.

design 8 7 ( 2 0 0 9 ) 1649–1657 1653

t(berr1tf

tw(tko

ls

1

2

pta

b

1

2

3

Table 3 – Cost parameters used in Eq. (12).

Cost parameters Value

C [$/m2 yr] 296.01C′ [$/m2 yr] 17.76C′′ [$/kmol] 20.61 × 10−3

Y [h/yr] 8000E [%] 90

2

chemical engineering research and

hat takes place at infinite reflux ratio, the Fenske equationsFenske, 1932) can be utilized to calculate the minimum num-er of stages for the separation. With some mathematicallaborations (Treybal, 1980) and defining an effective refluxatio above the minimum, it is possible to calculate the theo-etical number of stages by the Gilliland correlation (Gilliland,940). The design of the prefractionator ends with the applica-ion of the Kirkbride equation (Kirkbride, 1944) to identify theeed location.

The methodology sequence described can be applied alsoo the other two columns considering the modified Under-ood equations so as proposed by Carlberg and Westerberg

1989). Both the columns downstream to the prefractiona-or realize a sharp separation between the light and heavyey component. Since no distributing components are presentnly one Underwood’s root is active.

Obviously to adapt the three columns model to the Pet-yuk/DWC configuration the following two conditions must beatisfied:

. Considering that columns 2 and 3 substitute the main col-umn of the Petlyuk configuration, the rectifying vapourflowrate for column 3 must be equal to the stripping vapourflowrate of the column 2. In this way it is possible to mergethe two columns in an only one. This consideration is validwhen a liquid withdrawal is considered.

V2 = V3 (3)

. The second condition requires that the number of theoret-ical plates in the prefractionator is equal to the sum of thestages of the stripping section (below the thermal couplingconnection) and of the rectifying section (above the ther-mal coupling connection) of the second and third columnrespectively.

N1 = r2 + n3 (4)

This condition is not a real bond, but is derived from theractical observation that if a DWC is considered, it is bettero have a close number of plates on both sides of the wall tossure a good column stability.

The definition of the design space is related to sevenounds summarized as follows:

. the prefractionator distillate flow rate of component A ishigher than or equal to the same component flow rate incolumn 2;

d1A ≥ d2A (5)

. the prefractionator distillate flowrate of component B ishigher than or equal to the same component flowrate incolumn 2;

d1B ≥ d2B (6)

. the maximum quantity of component A in the prefraction-ator distillate flowrate is limited from the A quantity in the

feed;

d1A ≤ fA (7)

G [kmol/h m ] 219.71G′ [kmol/h m2] 0.49

4. the B quantity in the bottom of the prefractionator must beat least equal to the B quantity in the bottom of column 3;

b1B = (fB − d1B) ≥ b3B

d1B ≤ fB − b3B

(8)

5. the flowrate of C from the bottom of the prefractionatorshould be equal to or higher than the quantity of the samecomponent in the residue of column 3:

b1C = (fC − d1C) ≥ b3C

d1C ≤ fC − b3C

(9)

6. the feasible design space must include all the cases witha reflux ratio of the prefractionator higher than the corre-spondent minimum value:

R1 ≥ R1,m (10)

7. the number of stages for the prefractionator must be higherthan the sum of the minimum number of stages corre-sponding to the stripping and rectifying sections of column2 and 3 respectively obtained from the Fenske and Kirkbrideequations:

(r2 + n3)m ≤ N1 (11)

Once the design space is defined the simplified function forthe TAC reported in Eq. (12) was chosen to select the attrac-tive design options for the subsequent rigorous simulations(Happel and Jordan, 1975):

TAC = C

E × Y × GN(1 + R) + C′

Y × G′ (1 + R) + C′′(1 + R) (12)

It should be noted that this relation was originally devel-oped for simple column configurations but, in the case ofPetlyuk/DWCs, no simplified expressions are available andprobably for these systems the internal column costs, includedin the first term of Eq. (12), are higher compared to the simplecolumn configurations due to the internal wall. Anyway at thisstage of evaluation, and taking into account that the designspace was mapped using a short-cut method, this approxi-mation seems reasonable for the first selection of trial designparameters. Table 3 summarizes the parameters values usedin Eq. (12).

5. Simulation and results

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

simulations performed by means of Aspen Plus 13.0. All theparameters obtained, like the number of trays, the feed loca-tions, the thermal coupling flowrates are checked and then

1654 chemical engineering research and design 8 7 ( 2 0 0 9 ) 1649–1657

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 14998.78�Qr [kW] 13399.35 14989.28 16297.27 17138.62 18464.84

a Prefractionator/main column.

optimized by sensitive analysis to reach the minimum energyconsumption. The pressure of the Petlyuk/DWC configura-tion is defined according to the highest pressure value of thesubstituted SCs. The results of configuration parameters, ther-mal couplings flowrate, diameters, heat exchanger area andenergy consumption, are summarized in Tables 4 and 5.

5.1. Energy comparison

The main advantage expected for these types of configura-tions is the possibility to achieve an energy load reduction.This aspect is considered first using the total condenser andreboiler duty to compare, for each composition case, the bestSC sequence with those derived substituting 2 simple columnswith a Petlyuk/DWC. For composition cases 1, 3 and 4 the directsequence is the best simple configuration. Thus, by consider-ing first the combination SC + DWC, so as reported in Fig. 3,the first SC remains unchanged and performs the separationof the lightest component at the highest pressure, then the

Petlyuk/DWC column completes the separation. The savingis quantified in about 12, 10 and 11% for condenser and 9,6 and 8% for the reboiler duties respectively. For composi-

Table 5 – Design parameters and energy consumption of the DW

DWC + SC Case 1 Case 2

DWC SC DWC SC D

N 34 30 34 28 32Np 24 – 24 – 19Nf 17 15 12 14 9NL 4 – 4 – 5NV 28 – 24 – 23Ns 14 – 14 13LTC [kmol/h] 162.00 – 252.00 – 24VTC [kmol/h] 1144.80 – 936.00 – 97R 1.91 2.85 8.98 1.42 5.5P [kPa] 480 110 310 480 40DD [m] 2.67/3.89a 1.86 3.01/4.92a 1.12 2.6Ac [m2] 1188 159 1428 141 16Ar [m2] 613 87 728 53 47

�Qc [kW] 14197.68 14256.28�Qr [kW] 16735.29 19015.70

a Prefractionator/main column.

tion case 2, where the best SC sequence is the indirect one,the substitution of the last two columns with a Petlyuk/DWCforces to perform the separation at the highest pressure toassure the possibility to condensate the lightest componentin ordinary water cooled condenser. In this way all the threecomponents (A, B, C) are separated at the same pressure with apenalty for the separation efficiency. Comparing, for this com-position case, the results for the best SC reported in Table 2with those obtained for the SC + DWC sequence included inTable 4 it is possible to notice that the energy consumptioncan be considered similar, or equal to the approximation ofthe evaluation method. Composition case 5 is the only onewhere the SC + DWC configuration is more energy demand-ing compared to the corresponding best SC. The reason canbe attributed to the feed component distribution: in cases 1and 3 the Petlyuk/DWC column is fed by an equimolar stream,instead in case 4 there is an equal excess of component B and Cremoved from the distillate and the side stream respectively.In composition case 5 the feed contains an equal excess of

the lightest and heaviest components and taking into accountthat the best SC is the direct-indirect, in the SC + DWC con-figuration the first simple column removes the excess of the

C + SC sequences.

Case 3 Case 4 Case 5

WC SC DWC SC DWC SC

30 39 33 33 42– 17 – 22 –15 10 20 19 21– 9 – 5 –– 22 – 27 –– 13 – 15 –

8.40 – 288.00 – 756.00 –5.60 – 864.00 – 1692.00 –5 2.17 17.40 1.02 5.80 2.000 110 420 110 480 1609/3.87a 2.68 2.57/3.94a 2.73 3.80/5.17a 1.37

94 299 1536 335 1586 2061 231 1538 187 1586 55

15419.53 16506.61 16845.2318114.52 18489.55 21780.62

design 8 7 ( 2 0 0 9 ) 1649–1657 1655

lu

ataTsdbtlpcistmPt

5

Mcmtaooantieurirbctepfcccis(Tc

5Tccclttta

Table 6 – Normalized capital cost of the selectedconfigurations for the Petlyuk and the DWC design.

SC + DWC Petlyuk DWC

Case 1 0.89 0.86Case 3 0.99 0.99

with respect to the corresponding best SC cases, are reportedin Table 6 together with those of the Petlyuk configurations.

chemical engineering research and

ightest component, so the feed to the Petlyuk/DWC is a streamnbalanced in the heaviest component.

The energy performances of the DWC + SC configurationsre summarized in Table 5 together with the design parame-ers. For all the composition cases the DWC + SC configurationppears to be not convenient from the energetic point of view.here is the only exception of the composition case 4 where aaving of 4% in the condenser duty was achieved, while theuty of the reboiler is similar to that of the correspondingest SC sequence. In the other configurations, derived fromhe direct or from the direct indirect (cases 1, 3 and 5), the Pet-yuk/DWC is forced to operate at the highest pressure with aenalization in the separation efficiency. For the compositionase 2 the highest pressure column is the SC, anyway the DWCs fed with an excess of heaviest component that makes notuitable to employ this configuration. It is possible to concludehat the pressure and the feed component distribution are the

ain parameters that affect the energy consumption of theetlyuk/DWC configurations when are used in substitution ofwo SCs.

.2. Capital cost comparison

oreover the possibility of energy saving, the Petlyuk/DWConfiguration has the potential of a capital cost reduction. Inost of the specialized literature a reduction of about 30% of

he capital cost was estimated (Kolbe and Wenzel, 2004; Lestaknd Collins, 1997). Anyway it is not clear how this value wasbtained. It is known that employing a DWC for the separationf a three components mixture allows saving one condensernd one reboiler, compared to the traditional SC sequence, buto indications were provided about the total exchanger areahat is the main parameter related to the cost. Also some sav-ngs in the column shell cost are achievable but it was notvidenced how to allocate the prefractionator in the main col-mn and how to evaluate the corresponding diameter. To thisegard the first parameter that we considered for the compar-son is the heat exchanger area requested for condensers andeboilers. By comparing the results reported in Table 2 for theest SCs and in Tables 4 and 5 for the SC + DWC and DWC + SConfiguration respectively, it is possible to notice that even ifhe DWC allows to reduce the number of equipments, the totalxchanger area, in most of the cases, is higher than of the sim-le column configurations. The second parameter consideredor the capital cost evaluation is the sum of the costs asso-iated to the distillation column internals and shell. Anyway,onsidering that the main scope of the alternative proposedonfigurations is to reduce the energy demand of the plant,t was chosen to limit the capital cost analysis only to theequences where an appreciable energy saving was achievedi.e. configurations SC + DWC for composition cases 1, 3 and 4).o this regard the column capital cost of the Petlyuk and DWConfiguration must be considered separately.

.2.1. Petlyuk capital cost evaluationhe Petlyuk configuration, so as previously described, is theombination of two columns, the prefractionator and the mainolumn connected by thermal couplings. Both the columns’ost can be evaluated using the classical simplified corre-ations (Douglas, 1988), utilizing the columns diameter andhe stages number reported in Table 4 for the prefractiona-

or and the main column respectively. The column height forhe prefractionator is calculated without considering vapournd liquid disengagement. The results obtained for the total

Case 4 1.09 0.98

capital cost, normalized with respect to the capital cost of thecorresponding best SC, are reported in Table 6. From theseresults it is possible to notice that the Petlyuk configurationis particularly convenient in composition case 1 where themain column is fed at the lowest flowrate compared to theother cases considered. The main column diameter increasesas the feed flowrate increases, thus reducing the convenienceto employ the Petlyuk configuration as evidenced in cases 3and 4.

5.2.2. DWC capital cost evaluationA method for the capital cost evaluation of this column isactually not well defined. First of all in the case of DWC thepresence of the internal dividing wall makes unreliable theclassical equations used for the plate cost calculations, but onthe other hand simplified correlations are not available andthe main producers are reluctant to give this type of infor-mation. A few researchers made the assumption to use theclassical cost evaluation formulas considering in the case ofthe DWC configuration only the stages of the main column,but no indications are provided about the diameter consideredfor the zone of the column with the wall (Wolff and Skogestad,1995; Amminudin et al., 2001).

Fig. 4 shows the diameter profile in the main column ofthe Petlyuk obtained from the simulations for the composi-tion cases selected. It is possible to notice that in the columnsection included between the thermal couplings there is areduction of the diameter value. This zone corresponds tothe right side of the wall section in the DWC. To allocate theprefractionator in the main column is necessary to add thearea of the prefractionator to the corresponding area of themain column; in this way it is possible to define the diameterrequired for the DWC. Table 7 reports the results of the diame-ter calculations considering the three sections of the DWC: thestages above, below the wall and in the divided wall section.From the results it is evident the possibility to take advan-tage from the construction of a single diameter column. Theresults obtained for the capital cost evaluation, normalized

Fig. 4 – Diameter stage distribution in the Petlyuk maincolumn for the selected cases.

1656 chemical engineering research and d

Table 7 – Diameter values in the DWC sections for theselected cases.

SC + DWC Case 1 Case 3 Case 4

Da [m] 1.97 3.37 3.80Dw [m] 1.93 3.27 3.26

separation schemes. AIChE J, 18(5): 941–948.

Db [m] 1.91 3.35 3.75

Thus from the results it is possible to conclude that the DWClayout outperforms or equalizes at least (case 3) the Petlyukdisposition.

6. Conclusions

The DWC configurations for the separation of a four compo-nents mixture with 5 different feed composition cases arestudied. The best SC configuration is chosen as the basisfor the comparison with the new configurations obtained byintroducing a Petlyuk/DWC structure. The most promisingstructures are first selected on the basis of the lowest energyconsumption. From the calculations it was evidenced that thePetlyuk/DWC is not convenient when this configuration is uti-lized to replace two SCs with a high pressure difference. For allthe simple configurations where there is a decreasing value ofthe pressure through the columns (i.e. direct, direct-indirectsequences) it is convenient to use first a SC and then to per-form the last separation by using a Petlyuk/DWC arrangement.The most promising cases correspond to Petlyuk/DWC fedwith equimolar of equal excess of lightest and middle com-ponent mixtures.

With regard to the capital costs evaluation, a detailed eval-uation was carried out considering separately the Petlyuk andthe DWC configurations. The analysis was performed con-sidering the total exchanger area together with the columninternals and shell. In particular the DWC diameter was esti-mated considering the prefractionator and the main columndesign of the corresponding Petlyuk structure. This aspect isvery important because the real application of DWC columnsmust properly consider such design indications for the DWCstructure. From the obtained results it can be concluded thatthe DWC outperforms both the SC configuration and the Pet-lyuk structure. It should be noted that the main advantage ofPetlyuk/DWC arrangements remains the potential in energysaving while a minor profit can be obtained for the capitalcosts.

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