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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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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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Y.H. Kim / Chemical Engineering and Processing 44 (2005) 11081116 1111
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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Y.H. Kim / Chemical Engineering and Processing 44 (2005) 11081116 1113
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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Recommended