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Chemical Engineering and Processing 44 (2005) 11081116

Evaluation of three-column distillation system for ternary separation

Young Han Kim

Department of Chemical Engineering, Dong-A University, 840 Hadan-dong, Saha- gu, Pusan 604-714, Republic of Korea

Received 9 March 2004; received in revised form 11 August 2004; accepted 22 February 2005

Available online 10 May 2005

Abstract

A three-column arrangement of ternary distillation system having liquid composition profiles similar to residue curves is investigated

here. The system is expected to have as high column efficiency as a fully thermally coupled distillation column (FTCDC) and improvedoperability. The performance of the system is examined with energy consumption by comparing with a conventional two-column system and

the FTCDC. Two practical processes are utilized for the performance evaluation. From the comparison of energy requirement, it is found

that the three-column system has high column efficiency for BTX process but not for gas concentration process. The collinearity requirement

among the compositions of feed and products of the prefractionator in the three-column system leads to large mixing in feed tray raising

energy consumption for the latter process.

2005 Elsevier B.V. All rights reserved.

Keywords: Process design; Three-column distillation; Ternary separation; Prefractionation

1. Introduction

The three-column system demonstrated in Fig. 1 has a

prefractionator which separates light and heavy components

into two groups. Then, the light component group goes to the

upper column, and the heavy component group goes to the

lower column. Using the prefractionator makes the compo-

sition profile of tray liquid of the system similar to that of a

fully thermally coupled distillation column (FTCDC).

When a multicomponent distillation column is operated

at total reflux, its distillation line is approximated to one of

residue curves[1,2]. In the total reflux operation, thermody-

namic efficiency of the column is ideal to require minimum

energy. This ideal distillation line is also found from the com-position profile of tray liquid of an FTCDC known to be one

of energy efficient distillation systems.

Though the FTCDC consumes less energy than a con-

ventional distillation system in most cases, its operation is

difficult to obstruct wide application of the column. In or-

der to solve the operational problem many studies have been

Tel.: +82 51 200 7723; fax: +82 51 200 7728.

E-mail address: [email protected].

conducted in the search of a good operational strategy of the

column[38].As an alternative structure for the easy oper-ation, modified arrangements of connecting streams in the

FTCDC were introduced and analyzed by Agrawal [911].

Also, the main column of the FTCDC is divided into two sep-

arate columns for easy manipulation of product specification

while the two are thermally coupled, and the multivariable

controllability of the system was examined for improved op-

erability[12].

When the three-column distillation system described in

Fig.1 has liquid compositionprofiles similar to residue curves

like the FTCDC, high distillation column efficiency is ex-

pected to lower the energy demand for a given separation.

Conventionally two distillation columns are utilized for sep-aration of a ternary mixture, but their composition profiles of

tray liquid are quite different from the residue curves. In other

words, the column efficiency is far from the ideal because

feed tray mixing and remixing of intermediate component

require extra utility[3]. Adding a prefractionation column to

the conventional two-column system makes the composition

profiles similar to the FTCDC to result in high distillation

column efficiency. Though the additional column raises the

number of control loops in the operation of the three-column

0255-2701/$ see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cep.2005.02.007

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Y.H. Kim / Chemical Engineering and Processing 44 (2005) 11081116 1109

Fig. 1. Schematic diagram of a three-column distillation system.

system, the specification control of three products is mucheasier than that of the FTCDC due to the elimination of cou-

pling among different control loops.

In this study, a modified distillation system for ternary

mixtures is proposed for high column efficiency and im-

proved operability. The system is composed of three binary

columns having liquid composition profiles similar to those

of the energy efficient FTCDC and binary distillation column

operability. The performance of the three-column system is

investigated by applying it to two practical processes, BTX

and gas concentration processes, and the utilization limita-

tion of the system is examined. In the performance evaluation

HYSYS simulations are conducted.

2. Structural design

The design of a distillation column begins with either the

estimation of operational variables, such as liquid and va-

por flow rates, or the computation of structural information.

In this study, the structural design information of the three-

column system is estimated first because the information can

be adopted from the design result of the FTCDC [13]. Note

that the composition profile of the three-column is similar

to that of the FTCDC. In addition, the commercial design

program, HYSYS utilized in the computation of operational

variables of the three-column system, requires the informa-

tion for the initial formulation of simulation project. While

the tray numbers of prefractionator of both systems are the

same, the upper column of the three-column system is equiv-

alent to the upper section of main column of the FTCDC and

so is the lower column to the lower section. A structural de-

sign technique is used on the design of the FTCDC, and its

details are explained by Kim[8].

The structural design is based on minimum tray column

design. The highest column efficiency is obtained at total

reflux operation requiring the minimum tray of which the

distillation line is similar to one of residue curves. Because

mixing at feed tray lowers the efficiency, it is assumed that the

composition of feed tray is equal to feed composition. Now

two residue curvesone includes feed composition and the

other does the composition of side draware selected for

the distillation lines of the prefractionator and main column

of the FTCDC. The numbers of trays are yielded from thestage-to-stage computation of tray liquid composition begin-

ning with the feed composition for the prefractionator and

the side draw composition for the main column. The loca-

tions of feed and side draw trays are readily found from the

computation by counting tray numbers. The interlinking tray

is determined from matching the tray compositions of con-

nected prefractionator and main column. Notice that this is

the structural information of an ideal minimum tray column.

For a practical column, the tray numbers are taken twice the

minimum while the proportion among feed, side draw and in-

terlinking locations is maintainedfor high distillation column

efficiency. Once the structural information is determined, the

operational variables, such as liquid and vapor flow rates, fora given set of product specifications are found from HYSYS

simulations. More detailed explanation of the design proce-

dure for the FTCDC is given below.

The residue curves of a ternary simple distillation are con-

necting paths between the heaviest component-rich and the

lightest component-rich products. Among the connections,

two paths including feed and side product compositions are

utilized in the structural design. While the composition of

side drawis equal to the liquid composition of the drawstage,

feed composition is different from the feed stage composi-

tion. For the design of minimum tray system, however, the

composition of the feed tray is taken to be equal to the feedcomposition. In the minimum tray structure the thermody-

namic efficiency of the distillation is assumed to be ideal

employing total reflux operation and no feed tray mixing.

In the total reflux operation, the vapor composition of a

stage is same to the liquid composition of one stage above.

Therefore, the liquid composition of the trays above the feed

tray is evaluated from an equilibrium relation in stage-to-

stage manner starting from the feed composition. When a

saturated liquid feed is provided and the feed composition

and tray composition are same, the liquid composition of one

stage above the feed stage is equivalent to the vapor composi-

tion in the feed stage which is computed from the equilibrium

relation. For a tray above the feed tray, liquid composition is

calculated as:

xn+1,i =Kn,ixn,ijKn,jxn,j

(1)

whereKis an equilibrium constant. This computation is suc-

cessively applied up to the top of the prefractionator. The top

composition is determined by comparing with the composi-

tion profile in the main column as explained below. For the

trays below the feed tray, the same procedure is exercised.

The liquid composition of the stages below the feed tray is

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1110 Y.H. Kim / Chemical Engineering and Processing 44 (2005) 11081116

found in the stage-to-stage manner using:

xn1,i =xn,i

Kn1,i

j(xn,j/Kn1,j) (2)

whereas the computation of Eq.(1) is straight forward, that

of Eq. (2) is different because the equilibrium constant has an

implicit information. A simple optimization is necessary tofind the one stagebelow composition satisfyingEq. (2). Inthis

case, the feed composition is equal to the vapor composition

of one stage below the feed stage, and the liquid composition

of the stage is obtained from the vapor composition using

the equilibrium relation. Again, the computation proceeds

successively down to the bottom.

In the design of a main column, computation begins with

the composition of side product, and the whole process is

repeated in the same manner as the design of a prefractiona-

tor. The composition in the top and bottom trays has to meet

the specifications of overhead and bottom products. Note that

these numbers are the minimum numbers of trays of the pre-

fractionator and main column.The composition difference in the interlinking trays of the

main column and the prefractionator produces irreversible

mixing, which lowers the thermodynamic efficiency of the

FTCDC. Therefore, the compositions of the interlinking trays

have to be close. Using the compositions of overhead and

bottom products, the tray number and composition profile

of the main column are found from the stage-to-stage com-

position calculation. Then, the composition of trays above

the feed tray in the prefractionator is computed tray-by-tray

until a close composition with a stage of the main column is

yielded. Thesame procedure is applied to the lowersection of

the prefractionator. The interlinking trays in the main columnare determined from the closest tray to the end compositions

of the prefractionator. The locations of feed tray and side

draw are readily found from the minimum tray structure by

counting from the bottom of column.

Maintaining the minimum tray structure assures the high

thermodynamic efficiency of the minimum tray structure,

which follows the composition profile of total reflux oper-

ation and has the highest efficiency. By increasing the tray

numbers in proportion from the minimum tray structure to a

practical column, the structure for the high thermodynamic

efficiency is maintained. A factor of two in the expansion

from the minimum tray structure to an actual column sys-

tem is common in the industrial application [14]. The total

numbers of trays of the main column and prefractionator are

yielded with the factor, and locations of feed, side draw and

interlinking trays are proportionally computed from the min-

imum tray structure and the expansion factor.

In order to have products of a given specification, one

needs a proper set of operation condition for the distillation

system of which the structure is found from the above pro-

cedure. The condition is obtained from trial simulation until

the computed product composition meets the specification.

Because the commercial process design program HYSYS is

utilized here, the trial computation with the known distilla-

tion structure is relatively simple procedure. In the design of

distillation column with the HYSYS, the information of col-

umn structure has to be determined in the beginning. Then,

operational variables are given in trial basis to simulate the

process and to examine the specification of products. The

variables are adjusted until a predetermined product specifi-

cation is found. The operational variables are found from thesimulation result.

3. Process description

Two practical processes are utilized for the performance

evaluation of the three-column system in this study. One is

a BTX fractionation process[13], which separates benzene,

toluene and xylene from naphtha reformate. The other is a

gas concentration process [15] producing enriched ethane,

propane and butane from gaseous mixtures yielded from

crude distillation, naphtha reformation and naphtha cracking

processes. These processes handle large amount of productswith significant energy consumption, and therefore, the im-

pact of utility reduction obtained from utilizing an energy

efficient distillation system is not negligible. The flow rates

of feed and products of the process in a typical plant are listed

inTables 1 and 2.

4. Results and discussion

Though an FTCDC has high thermodynamic efficiency

requiring reduced energy compared with a conventional two-

column system, its operational difficulty obstructs practicalapplication of the FTCDC. In order to improve its operability

a three-column distillation system is introduced here and its

performance of energy requirement reduction is examined by

comparing utility consumption among the three-column sys-

tem, the FTCDC and the conventional two-column system.

Because the composition profiles of tray liquid of the three-

column system and the FTCDC are similar, their distillation

column efficiency is expected to be higher than that of the

conventional two-column system.

For BTX process the structural design of the three-column

system is derived from the design result of the FTCDC [13]

as listed inTable 3. Though minor modification is accompa-

nied, the structures of the two systems are close. The upper

column of the three-column system is analogous to the upper

section of the main column of the FTCDCthe stage number

of side draw of 28 is comparable to the tray number of the

upper column of 27and so is the lower column to the lower

section. Two prefractionators of the systems have similar tray

numbers.

The composition profile of tray liquid of the three-column

system is shown in Fig. 2, where circles are for the pre-

fractionator, plus symbols are for upper column and times

symbols are for lower column. The circles denoted with U

and L are the compositions of feeds to the upper and lower

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Table 1

Flow rates of feed and products of three-column and conventional systems of BTX process in kmol/h

Component Feed Three-column Conventional first Second

Light Heavy Intermediate Overhead Bottom Overhead Bottom

Light

Benzene 87.85 86.835 0.0000 1.0145 85.686 2.1640 2.1640 0.0000

Dimethylc-pentane 0.0124 0.0113 0.0000 0.0011 0.0088 0.0036 0.0036 0.0000Intermediate

Methyl c-hexane 0.0075 0.0002 0.0000 0.0073 0.0015 0.0060 0.0060 0.0000

Toluene 338.10 0.0028 2.4983 335.60 0.0036 338.10 335.28 2.8180

n-Octane 0.049 0.0000 0.0035 0.0455 0.0000 0.0490 0.0464 0.0026

Heavy

Ethylbenzene 14.975 0.0000 14.747 0.2280 0.0000 14.975 0.0003 14.975

p-Xylene 57.798 0.0000 57.593 0.2051 0.0000 57.798 0.0002 57.798

m-Xylene 128.55 0.0000 128.14 0.4066 0.0000 128.55 0.0003 128.55

o-Xylene 60.160 0.0000 60.107 0.0525 0.0000 60.160 0.0000 60.160

n-Nonane 0.0057 0.0000 0.0057 0.0000 0.0000 0.0057 0.0000 0.0057

n-Pentyl benzene 0.3300 0.0000 0.3300 0.0000 0.0000 0.3300 0.0000 0.3300

Methyl-ethyl benzene 26.010 0.0000 26.010 0.0000 0.0000 26.010 0.0000 26.010

Tri-methyl benzene 75.950 0.0000 75.950 0.0000 0.0000 75.950 0.0000 75.950

Methyl-n-propyl benzene 0.5700 0.0000 0.5700 0.0000 0.0000 0.5700 0.0000 0.5700Di-ethyl benzene 0.3300 0.0000 0.3300 0.0000 0.0000 0.3300 0.0000 0.3300

o-Cymen 4.1200 0.0000 4.1200 0.0000 0.0000 4.1200 0.0000 4.1200

Tetra-methyl benzene 4.7500 0.0000 4.7500 0.0000 0.0000 4.7500 0.0000 4.7500

Penta-methyl benzene 2.2389 0.0000 2.2389 0.0000 0.0000 2.2389 0.0000 2.2389

Total 801.81 86.850 377.40 337.56 85.700 716.11 337.50 378.61

columns, respectively. Note that the concentrations of main

feed described with F and feeds with U and L are collinear. In

case of the FTCDC the profile is illustrated inFig. 3, where

L2 is the composition of liquid from the main column to the

prefractionator and LB is that from the bottom of the prefrac-

tionator. The three compositions are arbitrary in this system

unlike the three-column system, and the availability of wide

selection of composition of interlinking streams makes the

structural design of the FTCDC flexible. For the conventional

two-column system, the profile shown in Fig. 4is quite dif-

ferent from the previous two and is far from residue curves.

In other words, the column efficiency of the system is low.

The results of the structural design and HYSYS simu-

lations for the BTX process are listed in Table 3. Though

the three different systems produce the same products of a

given set of composition specifications, utility consumption

varies in the three. The three-column system and the FTCDC

requires 12 and 13% less energy than the conventional two-

column system, respectively. The two-column system is in

direct sequence. This indicates that the column efficiency of

the former two systems is higher than the last system. It is ex-

plained with thecompositionprofileof tray liquid(Figs.57).

Whereas the former two have profiles (Figs. 2 and 3) similar

to residue curves, the two-column system has quite different

Table 2

Flow rates of feed and products of three-column and conventional systems of gas concentration process in kmol/h

Component Feed Three-column Conventional first Second

Light Heavy Intermediate Overhead Bottom Overhead Bottom

LightMethane 0.9879 0.9879 0.0000 0.0000 0.9879 0.0000 0.0000 0.0000

Ethane 22.966 22.922 0.0000 0.0444 22.901 0.0650 0.0650 0.0000

Propene 0.5729 0.0034 0.0079 0.5616 0.0036 0.5693 0.5623 0.0070

Intermediate

Propane 92.553 0.0866 3.2068 89.260 0.1072 92.446 89.319 3.1273

Heavy

I-Butane 75.866 0.0000 73.695 2.1710 0.0000 75.866 2.1047 73.761

1-Butene 3.2506 0.0000 3.2163 0.0343 0.0000 3.2506 0.0285 3.2221

n-Butane 71.740 0.0000 71.586 0.1537 0.0000 71.740 0.0792 71.660

I-Pentane 1.1720 0.0000 1.1719 0.0009 0.0000 1.1720 0.0000 1.1720

n-Pentane 0.0171 0.0000 0.0171 0.0000 0.0000 0.0171 0.0000 0.0171

Total 269.13 24.000 152.90 92.225 24.000 245.13 92.158 152.97

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Table 3

Tray numbers from structural design and operating conditions for three-column, the FTCDC and two-column systems of BTX process

Name Three-column FTCDC Two-column

Prefractionator Upper Lower Prefractionator Main First Second

Structural

Number of trays 22 27 61 21 89 60 50

Feed/side product 9 15 33 7 28 31 26Interlinking stages 6

74

Operating

Feed (kmol/h) 801.8 190.9 610.9 801.8 801.8 716.1

Overhead (kmol/h) 190.9 86.85 233.5 86.8 85.70 337.5

Bottom (kmol/h) 610.9 104.1 377.4 377.4 716.1 378.6

Side (kmol/h) 337.8

Reflux (kmol/h) 362.8 390.8 549.0 290.1 1792 401.3 1106

Vapor boilup (kmol/h) 533.0 458.4 730.7 492.9 1634 595.2 1315

Heat duty (GJ/h) 18.89 14.80 26.54 59.35 20.71 47.77

Tray numbers are counted from top.

Fig. 2. Liquid composition profile of BTX process in three-column system.

The symbols U and L indicate feed compositions of upper and lower main

columns, respectively.

profiles (Fig. 4). The energy requirement comparison be-

tween direct and indirect sequences of conventional two-

column system is tabulated inTable 4. The outcome is ob-

tained from the HYSYS simulation with the distillation sys-

tems of equal number of trays. In both processes the direct

split consumes less energy, and the split is practically em-ployed in industrial applications.

Table 4

Energy requirements of direct and indirect distillation systems

System First column Second column Total

BTX process

Direct 20.71 47.77 68.48

Indirect 69.63 16.23 85.86

Gas concentration process

Direct 2.97 5.96 8.93

Indirect 5.56 3.48 9.04

Units are in GJ/h.

The high efficiency and low energy requirement of the

FTCDC are known to field engineers, but its operationaldifficulty induces reluctance on the practical application of

the column. The three-column system solves the operational

problem associated with the FTCDC while the high column

efficiency is maintained. Though one more column than the

conventional two-column system is employed in the three-

column system requiring more control loops, the specifica-

tion control of three products is much easier than that of the

FTCDC. The control of three product specifications is exam-

ined with the HYSYS dynamic simulation. For BTX process

step changes of overhead product and vapor boilup rates of

the upper column and vapor boilup rate of the lower column

are applied in the three-column system, and the responses ofkey component specification of overhead, side draw and bot-

tom products are demonstrated inFig. 8. Though the change

Fig. 3. Liquid composition profile of BTX process in FTCDC system. The

symbols L2 and LB indicate liquid compositions of top and bottom flows in

prefractionator, respectively.

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Fig. 4. Liquid composition profile of BTX process in conventional two-

column system. The symbols D1 and B1 indicate the compositions of over-head and bottom products in the first column, and D2 and B2 are of the

second column.

of overhead product flow rate results in a coupled variation of

overhead and side draw specifications as shown in top three

figures, side draw and bottom product specifications are read-

ily adjusted with vapor boilup rates of the upper and lower

columns, respectively. No coupled response is observed in

middle and bottom rows of figures. Because the specifica-

tion of side draw product is controlled with the vapor boilup

rate of upper column in a single loop control, the coupled

changes from the manipulation of overhead product flow rate

can be separately controlled with the combined adjustment ofthe overhead product flow and the vapor boilup rate of upper

column.

Fig. 5. Liquid composition profile of gas concentration process in three-

column system. The symbols U and L indicate feed compositions of upper

and lower main columns, respectively.

Fig. 6. Liquid composition profile of gas concentration process in FTCDC

system. The symbols L2 and LB indicate liquid compositions of top andbottom flows in prefractionator, respectively.

The upper and lower columns of the three-column sys-

tem yield two products each, while the FTCDC does three

products in a single column. As a result, the control of the

upper or lower column is similar to binary column opera-

tion requiring control of two product specifications with two

manipulated variables. As illustrated in Fig. 8, there is no

step response of simultaneous specification variation of all

three products.However, theresponseof step changeof reflux

flow rate gives the simultaneous variation of three products as

demonstrated in the top three figures ofFig. 9.Also, the step

responses of vapor boilup and side draw rates exhibit the cou-

pled variation of side and bottom product specifications. The

Fig. 7. Liquid composition profile of gas concentration process in conven-

tional two-columnsystem.The symbolsD1 andB1 indicatethe compositions

of overhead and bottom products in the first column, and D2 and B2 are of

the second column.

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Fig. 8. The responses of overhead, side draw and bottom product specifications with step changes of overhead product and vapor boilup rates in upper column

and vapor boilup rate in lower column of three-column system for BTX process. Top three figures are of overhead product flow, middle three are of vapor

boilup rate in upper column and the bottom three are of vapor boilup rate in lower column.

Fig. 9. The responses of overhead, side draw and bottom product specifications with step changes of reflux, vapor boilup and side draw rates in the FTCDC

for BTX process. Top three figures are of reflux flow rate, middle three are of vapor boilup rate and the bottom three are of side draw rate.

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Table 5

Tray numbers from structural design and operating conditions for three-column, the FTCDC and two-column systems of gas concentration process

Name Three-column FTCDC Two-column

Prefractionator Upper Lower Prefractionator Main First Second

Structural

Number of trays 21 14 30 10 55 30 35

Feed/side product 6 5 14 5 23 13 17Interlinking stages 5, 43

Operating

Feed (kmol/h) 269.1 39.8 229.3 269.1 269.1 245.1

Overhead (kmol/h) 39.8 24.0 76.4 24.0 24.0 92.2

Bottom (kmol/h) 229.3 15.8 152.9 152.9 245.1 153.0

Side (kmol/h) 92.1

Reflux (kmol/h) 151.2 72.0 290.4 200.0 589.8 319.2 249.5

Vapor boilup (kmol/h) 204.2 87.9 347.6 275.9 511.9 202.1 413.0

Heat duty (GJ/h) 3.020 1.142 4.997 7.361 2.969 5.962

Tray numbers are counted from top.

manipulated variables of reflux, vapor boilup and side draw

rates are known to be the best for the control of the three prod-

uct specifications[16]. This comparison of step responsesgiven inFigs. 8 and 9 indicates that the operability of the

three-column system is much better than that of the FTCDC.

However, the application of the three-column system is

limited due to the loss of selection flexibility of interlink-

ing stream compositions. The compositions of feed and two

productsfeeds of upper and lower columnsof the pre-

fractionator of the three-column system are collinear, and

thus the determination of the product compositions of the

prefractionaror is restricted and affects the product compo-

sitions of the upper and lower columns. The following gas

concentration process is one of the cases.

The structural design of the three-column system in gasconcentration process begins with the design result of the

FTCDC listed inTable 5. In this process the tray number of a

prefractionator of the FTCDC is too small to eliminate heavy

components in the feed stream of upper column when the

prefractionator of the three-column system has the number of

trays. The FTCDC handles the heavy component in the main

column because the upper and lower columns of the three-

column system are combined as the main column. Hence, the

tray number of the prefractionator is adjusted for the separa-

tion of heavy components by transferring 11 trays from the

upper section of the main column of the FTCDC. Also, minor

alterations of tray numbers are accompanied from the result

of the HYSYS simulation for better performance.

For the gas concentration process, the composition pro-

files of tray liquid of the three-column system, FTCDC and

two-column system are illustrated inFigs. 57,respectively.

The symbol notation in the figuresis the same as in the figures

of the BTX process. Again, the profiles of the three-column

system and FTCDC are similar. However, the comparison

of utility consumption among the three systems is quite dif-

ferent from that of the BTX process. The design results of

the gas concentration process are summarized in Table 5,

which shows that the FTCDC requires 18% less utility than

the conventional two-column system while the three-column

system needs 3% more. The high energy requirement of the

three-column system is explained with the liquid composi-

tion profile,Fig. 5. Unlike the FTCDC, the compositions offeed and two products of the prefractionator in the three-

column system have to be collinear. This condition induces

more mixing in feed tray, which is indicated with the distance

between feed composition marked with F and feed tray com-

position, the closest circle to the F. It is also noticed from the

prefractionator tray number which is more than twice of that

in the FTCDC. The prefractionator processes the feed. The

difference of the mixing is found from the liquid composition

profiles ofFigs. 2 and 5, and equilibrium relation of a system

determines the level of the mixing. The mixing is an irre-

versible process incurring extra utility for separation [8]. The

outcome of the three-column system of the gas concentrationprocess indicates that the system is not always applicable to

any process for operability improvement.

Though the three-column system needs additional control

loops compared with the conventional two-column system,

the manipulation of product specification is much easier than

the FTCDC. Also, while the column operation pressures in

the three columns of the three-column system are arbitrarily

adjusted, setting different pressures in an FTCDC is diffi-

cult due to the two-way stream connections between the pre-

fractionator and main column. The three-column system has

another advantage in the utilization of distillation columns.

An existing two-column system is readily converted to the

three-column system by adding one distillation column and

changing stream connections.

5. Conclusion

A modified distillation structure from a fully thermally

coupled distillation system is examined for the possible im-

provement of operability. The system is composed of three

binary columns for the easy control of product specifications.

A brief procedure of structural design and operational vari-

able computation is explained, and the performance of the

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system is investigated with two practical systems, BTX and

gas concentration processes. The investigation results show

that thethree-column systemis usefulfor theBTX process but

it is not for the gas concentration process. Unlike the FTCDC

the prefractionator of the three-column system requires the

compositions of feed and two products to be collinear, which

results in large mixing in feed tray to raise energyrequirementin the gas concentration process.

Acknowledgments

Financial support from the Korea Science and Engineering

Foundation (Grant no. R01-2003-000-10218-0) and partially

through the CANSMC is gratefully acknowledged.

Appendix A. Nomenclature

K equilibrium constant

x liquid composition

y vapor composition

Subscripts

i componenti

j componentj

n tray number

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