Ind. Eng. Chem. Res. 2010, 49, 1333–1350
1333
Externally Heat-Integrated Double Distillation Column (EHIDDiC): Basic
Concept and General Characteristics
Kejin Huang,* Wei Liu, Jiangpeng Ma, and Shaofeng Wang
College of Information Science and Technology, Beijing UniVersity of Chemical Technology, Beijing 100029,
People’s Republic of China
In terms of the thermodynamic characteristics of simple distillation columns, a novel schematic of an externally
heat-integrated double distillation column (EHIDDiC) is proposed and studied in this work. It consists of
high-pressure and low-pressure distillation columns, with external heat integration between the whole rectifying
section of the former and the whole stripping section of the latter. In comparison with conventional distillation
systems (i.e., direct and indirect separation sequences), the EHIDDiC was found to require relatively small
capital investment and low operating costs. It can even outperform conventional distillation systems with the
condenser/reboiler type of heat integration under some favorable operating conditions. In terms of the separation
of a ternary ideal mixture containing hypothetical components A, B, and C, the EHIDDiC was evaluated
through intensive comparisons with its conventional counterparts. Sensitivity analysis was also conducted
with respect to some relevant physical properties and design parameters, including the relative volatilities,
feed composition, external heat-transfer area per stage, product specifications, and utility costs. It was confirmed
that the EHIDDiC can be advantageous over conventional distillation systems with and without condenser/
reboiler-type heat integration, and the results obtained reflect the salient characteristics of the EHIDDiC.
1. Introduction
Internal heat integration between the rectifying section and
the stripping section of a simple distillation column (i.e., with
one feed and two products) leads to the creation of an ideal
heat-integrated distillation column (ideal HIDiC).1-3 A detailed
schematic of an ideal HIDiC is illustrated in Figure 1. Although
both theoretical analyses and experimental evaluations have
demonstrated that the HIDiC can be much more thermodynamically efficient than its conventional counterparts, a large amount
of capital must be invested, among which the necessity for a
compressor constitutes one of the most expensive factors.4-8
Such intensive expenditures also constitute one of the primary
reasons that have prevented the ideal HIDiC from finding wide
application in the chemical and petrochemical process industries
so far. To avoid the use of an expensive compressor, one might
consider external heat integration between the rectifying section
and the stripping section of two individual distillation columns
(i.e., the rectifying section/stripping section type of heat
integration), because the necessary temperature driving forces
can be achieved instead through adjustments of the pressures
of each distillation column through the condensers. (Recall that
the heat duty of the top condenser is generally employed to
control the system pressure.) This modification leads to the
creation of a novel schematic of externally heat-integrated
double distillation columns (EHIDDiCs). For the separation of
close-boiling binary/multicomponent mixtures, a great reduction
in process irreversibility is still likely to be obtained in both
distillation columns, hence giving rise to a considerable
improvement in the efficiency of energy utilization.
Regarding the external heat integration between two individual distillation columns, the most frequently employed
schematic is the condenser/reboiler-type configuration, where
heat is transferred from the condenser of the high-pressure (HP)
distillation column to the reboiler of the low-pressure (LP)
distillation column. There are generally two types of schematics
* To whom correspondence should be addressed. Tel.: +86 10
64434801. Fax: +86 10 64437805. E-mail: huangkj@mail.buct.edu.cn.
for this type of external heat integration. One involves two
individual distillation columns that might or might not have a
direct mass connection. The other accommodates a number of
distillation columns that separate a common mixture (i.e., a
multieffect distillation system), where the feed splitting ratios
are key design variables to maximize the effect of external heat
integration between the distillation columns.9,10 Luyben and coworkers studied this type of external heat integration for a
number of chemical processes and claimed that the reduction
in utility consumption and capital investment could be achieved,
simultaneously.11-13 Annakou and Mizsey claimed that this type
of external heat integration could facilitate a conventional
distillation system to be more thermodynamically efficient than
the Petlyuk distillation system in the separation of ternary
mixtures.14 Emtir et al. found that the composition of the
mixtures separated dominated actually the structure of the
condenser/reboiler type of heat integration for the separation
of ternary mixtures.15 Engelien and Skogestad studied a four
pressure-staged distillation system and indicated that the
condenser/reboiler type of heat integration was advantageous
in that no retrofit had to be made in process design.16 In addition,
a great number of publications have addressed the trade-off
Figure 1. Schematic of an ideal HIDiC.
10.1021/ie901307j  2010 American Chemical Society
Published on Web 12/08/2009
1334
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
between the design and operation of conventional distillation
systems with the condenser/reboiler type of heat integration.17-19
The outcomes demonstrated that it was extremely important to
consider process dynamics and operation at the early stages of
process synthesis and design.
As for the rectifying section/stripping section type of heat
integration, only a few studies have been conducted so far.
Recently, we studied external heat integration in a pressureswing distillation (PSD) system separating a binary azeotropic
mixture of acetonitrile and water.20 We found that this type of
external heat integration might help to improve process design
in terms of not only operating cost but also capital investment.
In pursuit of further improvement in system performance,
simultaneous consideration of the condenser/reboiler type of heat
integration with the rectifying section/stripping section type of
heat integration between the LP and HP distillation columns
(i.e., the EHIDDiC) is strongly recommended for the PSD
process. Kataoka et al. reported a simulation study of external
heat integration in a system producing fuel ethanol from the
fermented mash (termed the compressor-free HIDiC) and found
that more than a 50% reduction of utility consumption could
be obtained.21 These encouraging outcomes gave us further
impetus to perform systematic studies of the energy-saving
potentials of the EHIDDiC.
The main purpose of this work is to explore systematically
the performance and characteristics of the EHIDDiC. After its
principle and configuration are briefly introduced, the EHIDDiC
is evaluated in terms of an example system separating a ternary
ideal mixture of hypothetical components A, B, and C into three
relatively pure substances. Intensive comparisons are made to
conventional distillation systems with and without the condenser/
reboiler type of heat integration. Sensitivity analysis is also
performed with respect to some relevant physical properties and
design parameters, including the relative volatilities of the
mixture components, feed composition, product specifications,
external heat-transfer area per stage, and utility cost. The salient
characteristics of the EHIDDiC are highlighted, and some
concluding remarks are summarized in the last section of the
article.
2. Principle and Configuration of the EHIDDiC
For a conventional distillation column, the rectifying section
(or the equivalent section in the case of a multiple-feed
distillation column) releases heat during operation and is thus
reasonably considered as a potential heat source. In contrast,
the stripping section (or the equivalent section in the case of a
multiple-feed distillation column) absorbs heat during operation
and is thus reasonably considered as a potential heat sink. Hence,
heat integration can be arranged between the rectifying section
and the stripping section of two individual distillation columns.
With regard to the necessary temperature driving forces, heat
integration can be achieved with the sole adjustment of the
pressure of each distillation column, thus circumventing the use
of an expensive compressor and throttle valve. These factors
represent the primary advantages of the EHIDDiC over the ideal
HIDiC.
Figure 2 shows a schematic of the EHIDDiC. As can be seen,
the whole rectifying section of the HP distillation column is
stage-by-stage heat-integrated with the whole stripping section
of the LP distillation column. Because of the external heat
integration, either the reboiler of the LP distillation column or
the condenser of the HP distillation column can be omitted
(although not simultaneously because of the mismatch between
their heat duties), which is closely dependent on the relative
Figure 2. Schematic of the EHIDDiC.
Table 1. Physical Properties and Design Specification of the
Example System
parameter
value(s)
pressure of the LP distillation column (bar)
heat-transfer area (m2 · stage-1)
heat-transfer coefficient (kW · K-1 · m-2)
feed flow rate (kmol · h-1)
feed concentration (zA/zB/zC, mol %)
feed thermal condition
relative volatility (RA/RB/RC)
molecular weight of the mixture (kg · kmol-1)
latent heat of vaporization (kcal · kmol-1)
product specification (A, B, and C, mol %)
intermediate product specification (A or C, mol %)
vapor-pressure constants
A (Avp/Bvp)
B (Avp/Bvp)
C (Avp/Bvp)
3.0
1, 3, 5
0.6
100
1 1 1
/3: /3: /3
1.0
4:2:1
50
6944
99
0.25
13.0394/4634.4
12.3463/4634.4
11.6531/4634.4
magnitudes of their heat duties. This offers a possibility of
simultaneously reducing capital investment and operating costs.
3. Illustrative Example: Separation of a Ternary Ideal
Mixture with Hypothetical Components
A ternary close-boiling mixture with hypothetical components
A, B, and C is purified into relative pure substances with
composition specifications of 99.0 mol % each. Because two
simple distillation columns are needed in the separation operation, the specifications of the intermediate products are fixed
arbitrarily at 0.25 mol % for component A in the direct
separation sequence (DSS) and 0.25 mol % for component C
in the indirect separation sequence (ISS), although they should
be subjected to a detailed optimization study. Ideal vapor-liquid
equilibrium behavior is assumed, with component A being the
lightest and component C the heaviest. The relative volatilities
between these hypothetical components are independent of the
changes in mixture composition and system pressure. Equal
latent heats are assumed for the three components, and sensible
heat can be ignored as compared with latent heat. Table 1
summarizes the physical properties and design specifications
for the separation problem.
The ideal vapor-liquid equilibrium is expressed as
P)xAPAS+xBPSB+xCPSC
(1)
where P signifies the system pressure and PiS represents the
saturation pressure, which is given by the Antoine equation
ln PSi ) Avp,i - Bvp,i /T
i ) A, B, and C
(2)
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Table 2. Economical Basis for Process Synthesis and Design
parameter
condensers
heat-transfer coefficient (kW · K-1 · m-2)
temperature difference (K)
reboilers
heat-transfer coefficient (kW · K-1 · m-2)
temperature difference (K)
cooling water (US $ · ton-1)
LP steam (US $ · ton-1)
HP steam (US $ · ton-1)
payback period (years)
value
0.852
13.9
0.568
34.8
0.06
25
25 (0.985 + 0.015PHP)
3
where Avp and Bvp represent the vapor-pressure constants.
With the stripping away of the nonlinearity by nonideal
vapor-liquid equilibrium and temperature/composition dependence of heat of vaporization, the merits and demerits of the
EHIDDiC can be analyzed in a more straightforward manner.
In terms of the principles of mass and energy conservation in
conjunction with the given vapor-liquid equilibrium relationship, steady-state models of a conventional distillation system
with and without condenser/reboiler-type heat integration and
of the EHIDDiC have been developed. The external heat
integration between the HP and LP distillation columns is
estimated with a lumped heat-transfer model (cf. eq 3). A
modified Newton-Raphson method is employed as the nonlinear equation solver, and the satisfaction of both the component
mass balance equations and the product specifications is taken
as the convergence criterion. Given the flow rate and composition of the ternary mixture to be separated, the topological
structures of the system flowsheets adopted, and the desired
product specifications, the heat duties of reboilers and condensers can be readily estimated in a robust and yet effective manner.
These steady-state models can be employed to aid process
synthesis and design.
QHI ) US(THP - TLP)
(3)
4. Conceptual Synthesis and Design of Conventional
Distillation Systems with and without Condenser/
Reboiler-Type Heat Integration
The minimization of total annual cost (TAC) is taken as the
objective function for process synthesis and design. The TAC
is the sum of the operating cost (OC) and the annual capital
investment. The annual capital investment is assumed to be the
capital investment (CI) divided by a payback period of 3 years
(cf. eq 4). The cost of equipment is estimated with the formulas
shown in Appendix I, and a recursive procedure is outlined in
Appendix II for the conceptual synthesis and design of
conventional distillation systems (CDSs) with and without
condenser/reboiler-type heat integration [termed HICDS (i.e.,
heat-integrated CDS) and CDS, respectively]. Table 2 lists the
relevant utility costs adopted in the current work. Note that the
cost of HP steam is expressed as 25(0.985 + 0.015PHP),
reflecting its dependence on pressure and temperature.
(4)
TAC ) OC + CI/β
Figure 3I (where I indicates column I) presents three
schematics of the CDS and HICDS for the DDS. For the sake
of a general illustration, a trim-condenser and a trim-reboiler
are drawn at the heat-integrated ends of the HP and LP
distillation columns, respectively, in the two schematics of the
HICDS (cf. Figure 3Ib,Ic). Once the detailed operating conditions are known, either the trim-condenser or the trim-reboiler
should be removed, depending closely on the relative magnitudes of the heat duties of the condenser and reboiler that are
1335
heat-integrated. The two HICDS configurations differ mainly
in the arrangement of external heat integration between the HP
and LP distillation columns, leading to different pressure
elevations, capital investment, and operating costs. Each one
can be an economical option for the separation of the given
ternary mixture, and this is closely dependent on the composition
of the feed to be separated. If the feed composition of
intermediate component B is not too small (i.e., greater than
1
/3), the schematic shown in Figure 3Ib is generally superior to
that shown in Figure 3Ic because of the relatively strong degree
of external heat integration allowed and the small pressure
elevation necessitated between the LP and HP distillation
columns. Therefore, the schematics shown in Figure 3Ia,Ib are
designed and studied here. Figure 4I shows the relationships
between the CI, OC, and TAC and the number of stages, Nc1
and Nc2, for the CDS (DSS). The optimum process design
indicates a distillation system with 39 stages in each the first
and second distillation columns. Table 3 details the comparison
between the CDS (DSS) and HICDS (DSS). For the synthesis
and design of the HICDS (DSS), the minimization of TAC is
still taken as the objective function, and the operating pressure
of the HP distillation column is determined by keeping a
temperature driving force of 34.8 K in the reboiler of the LP
distillation column (cf. eq 5). It is noticed that the condenser/
reboiler type of heat integration leads to simultaneous reductions
of capital investment and utility consumption by 19.21% and
42.97%, respectively.
PHP ) xAPAS(TNc1 + 34.8) + xBPSB(TNc1 + 34.8) +
xCPSC(TNc1 + 34.8)
(5)
Figure 3II (where II represents column II) presents three
schematics of the CDS and HICDS for the ISS. Similarly to
the case of the DSS, if the feed composition of intermediate
component B is not too small (i.e., greater than 1/3), then the
schematic shown in Figure 3IIb is also generally superior to
that shown in Figure 3IIc, and only those shown in Figure
3IIa,IIb are designed and studied here. Figure 4II shows the
relationships between the CI, OC, and TAC and the number of
stages, Nc1 and Nc2, for the CDS (ISS). It is readily seen that
the optimum process design is a distillation system with 39
stages in each the first and second distillation columns. Table 4
details the comparison between the CDS (ISS) and HICDS
(ISS). The condenser/reboiler type of heat integration again
results in simultaneous reductions of capital investment and
utility consumption by 16.64% and 35.28%, respectively.
In Figure 5, the steady-state profiles of temperature, vapor
and liquid flow rates, and liquid composition are depicted for
the DSS (column I) and ISS (column II) of the CDS. It is
stipulated here that the solid lines represent the static behaviors
of the first distillation column, and the dashed lines represent
the static behaviors of the second distillation column. For the
HICDS (DSS) and HICDS (ISS), the steady-state profiles are
similar and are not shown here.
5. Conceptual Synthesis and Design of the EHIDDiC
An iterative procedure is outlined in Appendix III for the
conceptual synthesis and design of the EHIDDiC. Figure 6I
presents two schematics of the EHIDDiC for the DSS, and
again, for the purpose of a general representation, a trimcondenser and a trim-reboiler are drawn at the heat-integrated
ends of the HP and LP distillation columns, respectively.
Similarly to the situations of HICDS (DSS), these two
schematics differ mainly in the arrangement of the external
1336
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 3. Schematics of the CDS and HICDS: (I) DSS, (II) ISS.
heat integration between the HP and LP distillation columns,
exhibiting different pressure elevations, capital investment,
and operating costs. Closely dependent on the composition
of the feed to be separated, each one can be an economical
solution for the separation of the given ternary mixture. If
the feed composition of intermediate component B is not too
small (i.e., greater than 1/3), the schematic shown in Figure
6Ia is also generally superior to that shown in Figure 6Ib
because of the relatively strong degree of external heat
integration allowed and the small pressure elevation necessitated between the LP and HP distillation columns. Therefore, the process schematic shown in Figure 6Ia is designed
and studied in this work. Figure 7I depicts the relationships
between the CI, OC, and TAC and the number of externally
heat-integrated stages, NHI, for the EHIDDiC (DSS). As can
be seen, when NHI is around 23, the TAC approaches its
minimum value. The optimum process design corresponds
to the EHIDDiC with 40 stages in each the LP and HP
distillation columns, and the optimum values of CI, OC, TAC,
and other relevant design and operation variables are also
reported in Table 3. Similarly to the HICDS (DSS), the
EHIDDiC results in simultaneous reductions of capital
investment and utility consumption by 0.62% and 42.51%,
respectively, in comparison with the CDS (DSS). The steady-
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1337
Figure 4. Relationships between (a) CI, (b) OC, and (c) TAC and the number of stages, Nc1 and Nc2, for the CDS: (I) DSS, (II) ISS.
state profiles of temperature, vapor and liquid flow rates, and
liquid composition are shown in Figure 8I. Gradual changes
of the vapor and liquid flow rates take place in the heatintegrated sections of the HP and LP distillation columns.
Analogously, there are two schematics for the ISS, as shown
in Figure 6II. If the feed composition of intermediate
component B is not too small (i.e., greater than 1/3), the
schematic shown in Figure 6IIa is also superior to that shown
in Figure 6IIb, and only the former is designed and studied
here. Figure 7II depicts the relationships between the CI, OC,
and TAC and the number of externally heat-integrated stages,
NHI, for the EHIDDiC (ISS). When NHI is around 22, the
TAC approaches its minimum value, and the optimum process
design consists of an EHIDDiC with 39 stages in each the
HP and LP distillation columns. The optimum values of CI,
OC, TAC, and other relevant design and operating variables
are also included in Table 4. The EHIDDiC results in
simultaneous reductions of capital investment and utility
consumption by 3.29% and 52.33%, respectively, in comparison with the CDS (ISS). The steady-state profiles of
temperature, vapor and liquid flow rates, and liquid composition are shown in Figure 8II. Again, gradual changes of the
vapor and liquid flow rates take place in the heat-integrated
sections of the HP and LP distillation columns.
1338
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Table 3. CDS and HICDS versus EHIDDiC for the DSS
parameter
CDS
HICDS
EHIDDiC
pressure of the first distillation column (bar)
pressure of the second distillation column (bar)
number of stages in the first distillation column
number of stages in the second distillation column
feed location of stages in the first distillation column
feed location of stages in the second distillation column
heat-transfer areas in condensers (m2)
heat-transfer areas in reboilers (m2)
side heat-transfer areas (m2)
reflux flow rate in the first distillation column (kmol · s-1)
reflux flow rate in the second distillation column (kmol · s-1)
distillate flow rate in the first distillation column (kmol · s-1)
distillate flow rate in the second distillation column (kmol · s-1)
condenser heat duty in the first distillation column (MW)
condenser heat duty in the second distillation column (MW)
trim-condenser heat duty in the second distillation column (MW)
reboiler heat duty in the first distillation column (MW)
reboiler heat duty in the second distillation column (MW)
trim-reboiler heat duty in the first distillation column (MW)
bottom flow rate in the first distillation column (kmol · s-1)
bottom flow rate in the second distillation column (kmol · s-1)
diameter of the first distillation column (m)
diameter of the second distillation column (m)
height of the first distillation column (m)
height of the second distillation column (m)
annual capital investment (US$ × 106 · year-1)
operating cost (US$ × 106 · year-1)
TAC (US$ × 106 · year-1)
3.0
3.0
39
39
19
22
143.346
85.8841
0.0210
0.0190
0.0093
0.0092
0.8800
0.8176
0.8800
0.8176
0.0185
0.0093
0.8184
0.8006
28.5293
28.5293
0.2764
0.4957
0.7721
3.0
10.6641
39
39
19
22
74.3065
85.8841
0.0210
0.0190
0.0093
0.0092
0.8800
0.8176
0
0.8176
0.8176
0.0624
0.0185
0.0093
0.8183
0.6007
28.5293
28.5293
0.2233
0.2827
0.5060
3.0
4.2471
40
40
17
24
81.5021
62.3126
115
0.0239
0
0.0093
0.0092
0.9652
0
0.9372
0.2945
0.0185
0.0093
0.8571
0.6564
29.2608
29.2608
0.2747
0.2850
0.5596
parameter
CDS
HICDS
EHIDDiC
pressure of the first distillation column (bar)
pressure of the second distillation column (bar)
number of stages in the first distillation column
number of stages in the second distillation column
feed location of stages in the first distillation column
feed location of stages in the second distillation column
heat-transfer areas in condensers (m2)
heat-transfer areas in reboilers (m2)
side heat-transfer areas (m2)
reflux flow rate in the first distillation column (kmol · s-1)
reflux flow rate in the second distillation column (kmol · s-1)
distillate flow rate in the first distillation column (kmol · s-1)
distillate flow rate in the second distillation column (kmol · s-1)
condenser heat duty in the first distillation column (MW)
condenser heat duty in the second distillation column (MW)
trim-condenser heat duty in the first distillation column (MW)
reboiler heat duty in the first distillation column (MW)
reboiler heat duty in the second distillation column (MW)
trim-reboiler heat duty in the second distillation column (MW)
bottom flow rate in the first distillation column (kmol · s-1)
bottom flow rate in the second distillation column (kmol · s-1)
diameter of the first distillation column (m)
diameter of the second distillation column (m)
height of the first distillation column (m)
height of the second distillation column (m)
annual capital investment (US$ × 106 · year-1)
operating cost (US$ × 106 · year-1)
TAC (US$ × 106 · year-1)
3.0
3.0
39
39
24
20
163.870
98.1808
0.0202
0.0188
0.0185
0.0093
1.1243
0.8164
1.1243
0.8164
0.0093
0.0092
0.9329
0.7882
28.5293
28.5293
0.2980
0.5658
0.8638
11.2101
3.0
39
39
24
20
94.9358
98.1808
0.0202
0.0188
0.0185
0.0093
0.8164
0.8164
0.3079
1.1243
0.8164
0
0.0093
0.0092
0.6916
0.7882
28.5293
28.5293
0.2484
0.3662
0.6146
4.8637
3.0
39
39
23
17
121.729
72.9322
110
0
0.0218
0.0185
0.0093
0
0.9045
1.3485
0.0931
0.0093
0.0092
0.7847
0.8297
28.5293
28.5293
0.2882
0.2697
0.5579
Table 4. CDS and HICDS versus EHIDDiC for the ISS
Notice that, for the conventional distillation systems, the DSS
outperforms the ISS in terms of both capital investment and
utility consumption. With the inclusion of the condenser/reboiler
type of heat integration in the HICDS, no variations occur in
this tendency because this type of heat integration presents
hardly any influences on the separation operation itself. For the
rectifying section/stripping section type of heat integration, the
tendency has been altered because this type of heat integration
can have a strong impact on the synthesis and design of the
EHIDDiC.
6. Comparison between the HICDS and EHIDDiC
The advantages of the EHIDDiC over the HICDS are twofold. One is the reduction in process irreversibility. The relatively
large reflux ratios of conventional distillation columns (e.g., in
the separation of close-boiling mixtures) is the main source of
process irreversibility, but the condenser/reboiler type of heat
integration gives no correction at all in this respect. External
heat integration between the rectifying section and the stripping
section of the HP and LP distillation columns can serve to reduce
the reflux ratios and lead to a relatively small utility consump-
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1339
Figure 5. Steady-state profiles of (a) temperature, (b) vapor and liquid flow rates, and (c) liquid composition for the CDS: (I) DSS, (II) ISS.
tion. The other advantage is the relatively low pressure elevation
from the LP to the HP distillation column. For the HICDS,
because heat transfer in the heat-integrated condenser/reboiler
needs to not only overcome the inherent negative temperature
difference but also maintain a relatively large temperature
driving force, a high pressure elevation is necessitated between
the LP and HP distillation columns. For the EHIDDiC, owing
to the structure modification for the rectifying section/stripping
section type of heat integration, it is no longer necessary to keep
the pressure elevation from the LP to the HP distillation column
as high as in the HICDS. If the relative volatility of the mixture
components appears to be sensitive to pressure variations, then
the EHIDDiC could be more thermodynamically efficient than
the HICDS. With reference to Tables 3 and 4, one can readily
find that the two EHIDDiCs for the DSS and ISS allow a much
smaller pressure elevation from the LP to the HP distillation
column than the two HICDSs. In terms of operating cost,
although the EHIDDiC appears to be comparable to the HICDS
for the DSS, the former appears to be more advantageous than
the latter for the ISS. These facts indicate that whether the
EHIDDiC can be more thermodynamically efficient than the
HICDS depends heavily on the process synthesis and design.
1340
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 6. Schematic of the EHIDDiC: (I) DSS, (II) ISS.
In terms of capital investment, the EHIDDiC always needs a
higher CI than the HICDS, implying that the rectifying section/
stripping section type of heat integration must be accompanied
by additional capital expenditures.
To search for the operating conditions that favor the application of the EHIDDiC over the CDS and HICDS, sensitivity
analysis should be conducted with respect to the relevant
physical properties of the mixture components, the given design
specifications, and the utility costs.
7. Influences of Feed Composition
To simplify the analysis, the feed composition of component
B is varied independently, and the ratio between the feed
compositions of components A and C is kept unchanged in all
situations. For the DSS, Figure 9I displays the influences of
feed composition on the conceptual synthesis and design of the
CDS, HICDS, and EHIDDiC. The CIs and OCs of the three
process designs increase monotonically with increasing feed
composition of component B, and the HICDS and EHIDDiC
appear to be much more thermodynamically efficient than the
CDS despite the great changes in operating conditions. Regarding the HICDS and EHIDDiC, the former is advantageous in
terms of operating cost when the feed composition of component
B is less than 40 mol %. When the feed composition of
component B is greater than 40 mol %, the reverse becomes
true. This phenomenon is, in fact, caused by the reduction in
process irreversibility with the rectifying section/stripping
section type of heat integration, indicating that the EHIDDiC
can be a competitive alternative to the HICDS, although its being
so closely dependent on the composition of the feed to be
separated.
For the ISS, Figure 9II displays the influences of feed
composition on the conceptual synthesis and design of the CDS,
HICDS, and EHIDDiC. In terms of capital investment, the
HICDS needs the smallest CI, with the EHIDDiC and CDS
ranked second and third, respectively. With reference to
operating costs, the EHIDDiC is most thermodynamically
efficient, with the HICDS and CDS ranked second and third,
respectively.
8. Influences of External Heat-Transfer Area
Because the external heat-transfer area between the HP and
LP distillation columns is a key design variable for the
EHIDDiC, it is therefore imperative to examine the influences
of this area on process synthesis and design. Figure 10I shows
the effect of the external heat-transfer area per stage on the
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1341
Figure 7. Relationships between (a) CI, (b) OC, and (c) TAC and the number of externally heat-integrated stages, NHI, for the EHIDDiC: (I) DSS, (II) ISS.
conceptual synthesis and design of the EHIDDiC (DSS). It is
interesting to note that the CI and OC increase with increasing
external heat-transfer area, indicating that the intensification of
external heat integration is not always beneficial to the
performance of the separation operation. This phenomenon is
due to the complicated interplay between the integrated part
and the nonintegrated part of the HP and LP distillation columns.
In process synthesis and design, the assignment of external heattransfer area per stage should therefore be cautiously determined.
Likewise, Figure 10II shows the effect of the external heattransfer area per stage on the conceptual synthesis and design
of the EHIDDiC (ISS). As can be readily seen, a more
complicated relationship than in the case of DSS is found
between the CI and OC and the external heat-transfer area per
stage. With an increase in the external heat-transfer area, the
CI and OC show first a decrease in magnitude, implying that
the extent of external heat integration is appropriate in these
situations and has a positive influence on the system performance. Beyond their minimum points, the CI and OC exhibit
an increase in magnitude, implying that the degree of external
heat integration is stronger than necessary and has an adverse
influence on the system performance. The outcomes again
1342
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 8. Steady-state profiles of (a) temperature, (b) vapor and liquid flow rates, and (c) liquid composition for the EHIDDiC: (I) DSS, (II) ISS.
indicate the great importance of determining the distribution of
external heat-transfer area cautiously in process synthesis and
design.
9. Influences of the Top- and Bottom-Product Qualities
For the DSS, Figure 11I displays the influences of product
qualities on the conceptual synthesis and design of the CDS,
HICDS, and EHIDDiC. Here, to simplify the analysis, the
compositions of the three products are assumed to be equal to
each other, and the bottom composition of component A in the
first distillation column is kept unchanged (i.e., 0.25 mol %) in
any cases. Whereas the CI increases with an enhancement of
the product qualities, the OC shows almost no variation,
implying that the enhancement of product purities is mainly
accomplished by the establishment of two increasingly tall and
large distillation columns.
For the ISS, Figure 11II displays the influences of product
qualities on the conceptual synthesis and design of the CDS,
HICDS, and EHIDDiC. Quite similar tendencies as in the case
of the DSS can be observed.
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1343
Figure 9. Feed composition versus conceptual process synthesis and design: (I) DSS, (II) ISS.
10. Influences of the Relative Volatilities of the Ternary
Mixture Components
R ) PAS /PSB )
To simplify the analysis, the relative volatilities between
components A/B and B/C are assumed to be equal in this
situation, that is, RAB ) RBC ) R. When the relative volatilities
undergo changes, the vapor-pressure constants of components
A and B have to be modified accordingly as follows
Figure 12I displays the influences of the relative volatilities
of the ternary mixture components on the conceptual synthesis
and design of the CDS, HICDS, and EHIDDiC for the DSS.
The advantages of the HICDS and EHIDDiC over the CDS in
terms of operating costs diminish gradually with increasing
PSB /PSC ) exp(Avp,A - Avp,B) ) exp(Avp,B - Avp,C)
(6)
1344
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 10. Effect of external heat-transfer area on the synthesis and design of the EHIDDiC: (I) DSS, (II) ISS.
relative volatilities, implying that the HICDS and EHIDDiC are
used favorably for the separation of close-boiling multicomponent mixtures. The HICDS outperforms the EHIDDiC in terms
of operating costs when the relative volatilities of the ternary
mixture components are below 2.3, and above that value, the
reverse becomes true. This finding indicates that the latter could
be more thermodynamically efficient than the former when the
relative volatilities of the ternary mixture components are
comparatively high.
Analogously, Figure 12II displays the influences of the
relative volatilities of the ternary mixture components on the
conceptual synthesis and design of the CDS, HICDS, and
EHIDDiC for the ISS. Quite similar conclusions as in the case
of the DSS are obtained.
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1345
Figure 11. Product qualities versus conceptual process synthesis and design: (I) DSS, (II) ISS.
11. Influences of Utility Cost
Utility cost varies with the pressure and temperature requirements (cf. Table 2), and their relationship can confine the
elevation of system pressures in the HICDS and EHIDDiC.
Furthermore, the relative volatilities of the mixture components
might decrease dramatically with increasing operating pressure,
and this enhances the operating cost correspondingly. From a
purely economic point of view, the phenomenon can be viewed
as a net increase in utility cost if the relationship between the
relative volatilities and the system pressure is not known.
Therefore, it is worth examining here the impact of utility cost
on the feasibility of external heat integration in conventional
distillation columns. Figure 13I displays the influences of the
utility cost on the conceptual synthesis and design of the CDS,
HICDS, and EHIDDiC for the DSS. It is noted that, when the
utility cost coefficient is less than 0.015 US $ · ton-1 · bar-1, the
HICDS appears to be superior to the EHIDDiC in operating
cost, and it loses the advantage beyond that value. When the
1346
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 12. Relative volatility versus conceptual process synthesis and design: (I) DSS, (II) ISS.
utility cost coefficient is greater than 0.18 US $ · ton-1 · bar-1,
the HICDS cannot even compete with the CDS, demonstrating
that the HICDS is most sensitive to the utility cost among the
three processes studied. The EHIDDiC requires the least
operating cost ever because the utility cost coefficient is beyond
0.015 US $ · ton-1 · bar-1, and the more expensive the utility is,
the greater the advantage of the EHIDDiC becomes. However,
it should be borne in mind that the distinct advantage of the
EHIDDiC in operating cost is actually achieved at the expense
of additional capital investment. Figure 13II displays the
influences of the utility cost on the conceptual synthesis and
design of the CDS, HICDS, and EHIDDiC for the ISS. Quite
similar tendencies as in the case of the DSS can be observed.
The above results indicate that the EHIDDiC can be more
thermodynamically efficient than the HICDS when the utility
cost is relatively sensitive to the elevation of system pressure.
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
1347
Figure 13. Utility cost versus conceptual process synthesis and design: (I) DSS, (II) ISS.
12. Discussion
It is worth stressing here that the illustrative example studied
in this work represents a general circumstance of a threecomponent mixture separation. Under some favorable operating
conditions, such as a relatively high feed composition of
intermediate component B and high utility costs, the EHIDDiC
can be more thermodynamically efficient than the CDS and
HICDS for both the DSS and ISS. This outcome lends definite
support to the rationale of considering the EHIDDiC as a
valuable alternative for the separation of multicomponent
mixtures. Although the EHIDDiC considered in this work has
the same number of stages in the heat-integrated sections of
the HP and LP distillation columns (and is therefore called a
symmetrical EHIDDiC), different numbers of stages can be
involved according the physical properties of the mixture
components, which would lead to an asymmetrical EHIDDiC.
Because an asymmetrical EHIDDiC might display a higher
energy efficiency with an even lower capital investment than a
1348
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
external heat exchangers to approximate external heat integration
between the HP and LP distillation columns.22,23 Through the
careful arrangement and design of these external heat exchangers, a good approximation to the EHIDDiC can be achieved,
offering an alternative means of practical implementation.
13. Conclusions
Figure 14. Schematic of the EHIDDiC separating a single binary/
multicomponent mixture.
In light of the principle of the rectifying section/stripping
section type of heat integration, a novel schematic for the
EHIDDiC has been proposed and studied in this work. In terms
of the separation of an ideal ternary mixture of hypothetical
components A, B, and C, the EHIDDiC was evaluated through
intensive comparisons with the CDS and HICDS. It was found
that the EHIDDiC is advantageous over the CDS in terms of
operating costs and capital investment. Under appropriate
operating conditions, the EHIDDiC can be made more thermodynamically efficient than the HICDS, demonstrating the fact
that the former can be a valuable alternative for the separation
of binary and multicomponent mixtures.
Future work will be focused on the development of an
effective and yet practical way to implement the EHIDDiC.
There are two alternatives in this respect. One is to accommodate
the EHIDDiC within one shell through the deliberate arrangement of the external heat exchangers, and the other is to use
several external heat exchangers to approximate the external
heat integration between the LP and HP distillation columns.
Owing to the strong coupling between the LP and HP distillation
columns, it is quite likely that complicated process dynamics
and potential control difficulties would occur, just as in the case
of the ideal HIDiC.24-26 Therefore, this represents another
important issue to be studied in the near future.
Acknowledgment
Figure 15. Simplified schematic of the EHIDDiC with three external heat
exchangers.
symmetrical EHIDDiC, it is not difficult to understand that the
conclusions obtained in the current work are all based on a
relatively conservative stance.
Similarly to the HICDS, the EHIDDiC can be used in two
alternative ways. One is for the accomplishment of two
separation tasks simultaneously, and the example studied in this
work is of this type. The other is for the accomplishment of
only one separation task, and Figure 14 presents a schematic
for this situation. Notice that the latter case is quite similar to
a double-effect distillation system, but with a high degree of
external heat integration between the HP and LP distillation
columns. The feed splitting ratio still represents a key variable
for process synthesis, design, and operation.
Finally, we have to consider seriously the implementation
issues of the EHIDDiC. Although it is advantageous to
implement the EHIDDiC in one shell, which could lead to a
further reduction in capital investment, a number of challenging
issues, including the internal structure design for heat transfer,
the arrangement of sufficient heat-transfer area, and the possible
influences on mass transfer, have to be resolved. To avoid these
challenging issues, one can still adopt the two-column structure
and use a number of external heat exchangers (notice that the
number should be much smaller than the number of heatintegrated pairs of stages) to approximate the heat transfer
between the HP and LP distillation columns. Recently, we
devised a simplified scheme for the EHIDDiC as shown in
Figure 15, which is characterized by the use of only three
This project was financially supported by the National High
Technology Research and Development Program of China (i.e.,
863 Program under Grant 2007AA05Z210) and the Scientific
Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry.
Notation
A ) component
Avp ) vapor-pressure constant (Pa)
b ) bottom-product flow rate (kmol · s-1)
B ) component
Bvp ) vapor-pressure constant (Pa · K)
c ) utility cost (US$ · ton-1)
C ) component
CDS ) conventional distillation system
CI ) capital investment (US$)
d ) diameter (m)
D ) distillate flow rate (kmol · s-1)
DSS ) direct separation sequence
EHIDDiC ) externally heat-integrated double distillation columns
F ) feed flow rate (kmol · s-1)
H ) height (m)
HICDS ) heat-integrated conventional distillation system
∆HV ) heat of vaporization (kJ · kmol-1)
HIDiC ) heat-integrated distillation column
ISS ) indirect separation sequence
L ) liquid flow rate (kmol · s-1)
m ) number of stages
MW ) molecular weight of a mixture (kg · kmol-1)
n ) number of stages
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
NF ) feed stage
OC ) operating cost (US$ · year-1)
P ) pressure (Pa)
Q ) heat duty (kW)
S ) heat-transfer area (m2 · stage-1)
T ) temperature (K)
TAC ) total annual cost (US$ · year-1)
U ) overall heat-transfer coefficient (kW · K-1 · m-2)
V ) vapor flow rate (kmol · s-1)
x ) liquid composition
y ) vapor composition
z ) feed composition
Greek Letters
R ) relative volatility
β ) payback time period (years)
∆ ) perturbation
Φ ) splitting ratio
Subscripts
A ) component
B ) component
C ) component
c ) conventional distillation column
CON ) condenser
F ) feed
HI ) heat integration
HP ) high pressure
LP ) low pressure
REB ) reboiler
vp ) vapor pressure (Pa)
Superscript
S ) saturation
employed for the synthesis and design of conventional
distillation systems with and without the condenser/reboiler
type of heat integration.
(i) Given the flow rate and composition of the feed to be
separated, the topological structure of the distillation system
adopted, and the desired product specifications, do the following:
(ii) Determine the number of stages for each distillation
column, Nc1 and Nc2.
(iii) Determine the feed locations for each distillation column,
NF1 and NF2.
(iv) Conduct the steady-state calculation and determine the
operating conditions.
(v) Calculate the TAC using the formulas in Appendix I.
(vi) Check whether the TAC is minimal with respect to the
feed locations, NF1 and NF2. If yes, go to the next step;
otherwise, go to step iii.
(vii) Check whether the TAC is minimal with respect to the
number of stages, Nc1 and Nc2. If yes, go to the next step;
otherwise, go to step ii.
(viii) Summarize the design results, and stop.
Appendix III: Procedure for the Synthesis and Design of
the EHIDDiC
Appendix I: Sizing and Economic Basis of Distillation
Columns
Assuming an F factor of 1 in engineering units, the diameter
of a distillation column is calculated with the equation
d ) 173.5(MW × T/P)0.25VN0.5
1349
(A1)
The height of a distillation column is calculated assuming a
tray spacing of 0.61 m and allowing 20% more height for the
base-level volume
H ) 0.73152N
(A2)
The heat-transfer areas of the reboiler and condenser are
calculated using the steady-state vapor flow rates and the heat
of vaporization
With the application of the steady-state models introduced
in section 3, the following grid-search philosophy can be
employed for the synthesis and design of the EHIDDiC
separating a given mixture.
(i) Given the flow rate and composition of the feed to be
separated, the topological structure of the EHIDDiC adopted,
and the desired product specifications, do the following:
(ii) Determine the number of stages for external heat
integration, NHI.
(iii) Determine the number of stages for the HP and LP
distillation columns, NLP and NHP, respectively.
(iv) Conduct the steady-state calculation and determine the
corresponding operating conditions.
(v) Calculate the TAC using the formulas in Appendix I.
(vi) Check whether the TAC is minimal with respect to the
number of stages, NLP and NHP. If yes, go to the next step;
otherwise, go to step iii.
(vii) Check whether the TAC is minimal with respect to the
numbers of the externally heat-integrated stages, NHI. If yes,
go to the next step; otherwise, go to step ii.
(viii) Summarize the design results, and stop.
SREB ) VN∆HV /(UREB∆TREB)
(A3)
Literature Cited
SCON)V2∆HV /(UCON∆TCON)
(A4)
(1) Nakaiwa, M.; Huang, K.; Endo, T.; Ohmori, T.; Akiya, T.;
Takamatsu, T. Internally Heat-Integrated Distillation Column: A Review.
Chem. Eng. Res. Des. 2003, 81, 162.
(2) Olujic, Z.; Fakhri, F.; Rijke, A.; Graauw, J.; Jansens, P. J. Internal
Heat Integration: The Key to an Energy-Conserving Distillation Column.
J. Chem. Technol. Biotechnol. 2003, 78, 241.
(3) Huang, K.; Matsuda, K.; Iwakabe, K.; Takamatsu, T.; Nakaiwa, M.
Graphical Synthesis of an Internally Heat-Integrated Distillation Column.
J. Chem. Eng. Jpn. 2006, 39, 703.
(4) Naito, K.; Nakaiwa, M.; Huang, K.; Endo, A.; Aso, K.; Nakanishi,
T.; Nakamura, T.; Noda, H.; Takamatsu, T. Operation of a Bench-Scale
Ideal Heat Integrated Distillation Column (HIDiC): An Experimental Study.
Comput. Chem. Eng. 2000, S24, 495.
(5) Huang, K.; Matsuda, K.; Iwakabe, K.; Takamatsu, T.; Nakaiwa, M.
Interpreting Design of an Ideal Heat-Integrated Distillation Column through
Exergy Analysis. J. Chem. Eng. Jpn. 2006, 39, 963.
(6) Iwakabe, K.; Nakaiwa, M.; Huang, K.; Nakanishi, T.; Rosjorde, A.;
Ohmori, T.; Endo, A.; Yamamoto, T. Energy Saving in Multi-Component
Separation Using an Internally Heat-Integrated Distillation Column (HIDiC).
Appl. Therm. Eng. 2006, 26, 1362.
In terms of the above sizes, the capital and operating costs
of a distillation column can be roughly estimated using the
following equations
column shell cost ) 17640d1.066H0.802
stage cost ) 229d
1.55
heat exchanger cost )
7296SREB0.65
(A5)
(A6)
N
+ 7296SCON
0.65
utility cost ) 3600 × 24 × 300 × (cCONQCON /∆HV +
cREBQREB /∆HV)
(A7)
(A8)
Appendix II: Procedure for the Synthesis and Design of
the CDS and HICDS
With the application of the steady-state models introduced
in section 3, the following grid-search philosophy can be
1350
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
(7) Huang, K.; Shan, L.; Zhu, Q.; Qian, J. A Total Heat-Integrated
Distillation Column (THIDiC)sThe Effect of Feed Pre-Heating by Distillate.
Appl. Therm. Eng. 2008, 28, 856.
(8) Huang, K.; Iwakabe, K.; Nakaiwa, M.; Matsuda, K.; Horiuchi, K.;
Nakanishi, T. Considering Heat Integration to Improve Separation Performance. Hydrocarbon Process. 2008, 87, 101.
(9) Wankat, P. C. Multi-Effect Distillation Processes. Ind. Eng. Chem.
Res. 1993, 32, 894.
(10) King, C. J. Separation Processes; McGraw-Hill: New York, 1980.
(11) Chiang, T. P.; Luyben, W. L. Comparison of Energy Consumption
in Five Heat-Integrated Distillation Configurations. Ind. Eng. Chem. Res.
1983, 22, 175.
(12) Luyben, W. L. Distillation Design and Control Using Aspen
Simulation; John Wiley & Sons: New York, 2006.
(13) Luyben, W. L. Design and Control of a Fully Heat-Integrated
Pressure-Swing Azeotropic Distillation System. Ind. Eng. Chem. Res. 2008,
47, 2681.
(14) Annakou, O.; Mizsey, P. Rigorous Comparative Study of EnergyIntegrated Distillation Schemes. Ind. Eng. Chem. Res. 1996, 35, 1877.
(15) Emtir, M.; Rev, E.; Fonyo, Z. Rigorous Simulation of Energy
Integrated and Thermally Coupled Distillation Schemes for Ternary Mixture.
Appl. Therm. Eng. 2001, 21, 1299.
(16) Engelien, H. K.; Skogestad, S. Multi-Effect Distillation Applied
to an Industrial Case Study. Chem. Eng Process. 2005, 44, 819.
(17) Bildea, C. S.; Dimian, A. C. Interaction between Design and Control
of a Heat-Integrated Distillation System with Pre-Fractionators. Chem. Eng.
Res. Des. 1999, 77, 597.
(18) Rix, A.; Gelbe, H. On the Impact of Mass and Heat Integration on
Design and Control of Distillation Column Systems. Chem. Eng. Res. Des.
2000, 78, 542.
(19) Tsai, W. H.; Huang, X. P.; Yu, C. C. Control Structure Design for
Parallel Process. Application to Heat-Integrated Column. Chem. Eng. Res.
Des. 2005, 83, 153.
(20) Huang, K.; Shan, L.; Zhu, Q.; Qian, J. Adding Heat Integration to a
Pressure-Swing Distillation (PSD) Process. Appl. Therm. Eng. 2008, 28, 923.
(21) Kataoka, K.; Yamaji, H., Noda, H.; Mukaida, T.; Kaneda, M.; Nakaiwa,
M. Compressor-Free Heat-Integrated Distillation Column System for Dehydration of Fermented Mash In Production of Fuel Bio-Ethanol. Presented at the
AIChE 2008 Annual Meeting, Philadelphia, PA, Nov 16-21, 2008.
(22) Huang, K.; Wang, Y.; Zhang, X.; Chen, H.; Wu, G.; Wang, S.
Apparatus of Externally Heat-Integrated Double Distillation Columns
(EHIDDiC) and its Control Technology. Chinese Patent 200910084908.4,
2009.
(23) Wang, Y.; Huang, K.; Wang, S. A Simplified Scheme of Externally
Heat-Integrated Double Distillation Columns with Three Heat Exchangers.
Ind. Eng. Chem. Res., manuscript submitted.
(24) Huang, K.; Nakaiwa, M.; Owa, M.; Akiya, T.; Nakane, T.; Sato,
M.; Takamatsu, T. Identification and Internal Model Control of an Ideal
Heat-Integrated Distillation Column (HIDiC). J. Chem. Eng. Jpn. 1998,
31, 218.
(25) Zhu, Y.; Liu, X. Dynamics and Control of High Purity HeatIntegrated Distillation Columns. Ind. Eng. Chem. Res. 2005, 44, 8806.
(26) Huang, K.; Wang, S.; Iwakabe, K.; Shan, L.; Zhu, Q. Temperature
Control of an Ideal Heat-Integrated Distillation Column (HIDiC). Chem.
Eng. Sci. 2007, 63, 6486.
ReceiVed for reView August 20, 2009
ReVised manuscript receiVed November 4, 2009
Accepted November 23, 2009
IE901307J