Capital Cost Reduction by the Use of Divided Wall Distillation Column

Abstract

The dividing wall distillation column which is a fully thermal integrated system also brings significant capital cost reduction. The analysis of cost reduction is performed in the frame of the commercial simulator HYSYS™ and of a specialized cost evaluator software. A case study for a common hydrocarbon mixture separation in oil refiners demonstrates that the heat transfer units reduction and the use of a single column shell provide a decrease up to 23 % of the cost compared to a classical two column separation sequence. Alike the energy efficiency of the dividing wall distillation column, the capital cost reduction is more important for the case when the middle component is in a great amount in the feed.

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REV. CHIM. (Bucureºti) ♦ 60♦ Nr. 10 ♦ 2009http://www.revistadechimie.ro1056
Capital Cost Reduction by the Use
of Divided Wall Distillation Column
LOREDANA DRAGHICIU, RALUCA ISOPESCU*, ALEXANDRU WOINAROSCHY
University „Politehnica” of Bucharest, Department of Chemical Engineering, 1-5 Polizu Str., 011061, Bucharest, Romania
The dividing wall distillation column which is a fully thermal integrated system also brings significant capital
cost reduction. The analysis of cost reduction is performed in the frame of the commercial simulator HYSYSTM
and of a specialized cost evaluator software. A case study for a common hydrocarbon mixture separation in
oil refiners demonstrates that the heat transfer units reduction and the use of a single column shell provide
a decrease up to 23 % of the cost compared to a classical two column separation sequence. Alike the energy
efficiency of the dividing wall distillation column, the capital cost reduction is more important for the case
when the middle component is in a great amount in the feed.
Keywords: divided wall distillation column, capital cost evaluation, hydrocarbon separation
* email: r_isopescu@chim.upb.ro
The separation of a ternary mixture into three pure
components can be realized by a sequence of two
distillation columns. A single distillation column with side
draw will not provide a high purity of the middle component
due to remixing phenomena. Using thermal coupling
methodology, complex columns can be designed to realize
this separation in a single unit. The main advantage of a
thermally coupled distillation column is the energy savings
up to about 40 % compared to classical simple distillation
sequences [1, 2, 3]. Among thermally complex columns,
the Petlyuk distillation column (figure 1 a) represents the
totally coupled structure and brings the highest energy
reduction. When both the prefractionator and the main
column are built in a single shell, the divided wall distillation
column (DWC) is generated (fig. 1 b). The thermally
coupled systems have several advantages, such as lower
energy consumption, lower capital costs and also reduced
space requirements. The extent of economical advantages
of thermally coupled distillation columns depends on the
nature of the mixture to be separated, mainly on the relative
volatility of the components, on feed composition and on
the throughput considered [4].
The concept of fully thermal coupling distillation column
was introduced in 1965 by Petluyk [5], but practical
applications and industrial use of the DWC were realized
only in recent years. Several theoretical studies concerning
the energy savings in a DWC are presented in literature [6-
9] as well as theoretical aspects regarding the design and
analysis of a DWC [10, 11]. DWC is considered a
challenging solution with both energy and cost reduction
for new design or retrofit practical applications [12, 4].
Despite its advantages, the industrial use of a DWC is still
limited and only a few industrial applications are so far
implemented.The industrial acceptance and
commercialization of DWC by companies as BASF A.G.
and M.W. Kellog is mentioned [13, 14].
The present paper presents a capital cost reduction
analyses of the DWC compared to a classical two-column
distillation sequence used to separate a hydrocarbon
mixture into three products. A case study is presented for
the separation of a common mixture in oil refineries:
benzene, toluene, ethylbenzene and o-xylene aiming to
obtain high purity benzene as light product, toluene as
middle product and ethylbenzene and o-xylene as bottom
product. The commercial simulator HYSYS TM is used to
establish the optimal configuration of the DWC and to
estimate the column main dimensions. Cost evaluations
were performed using specialized software CAPCOST.
Short-cut design and rigorous simulation of the distillation
system
The direct sequence of the simple distillation columns
sequence (fig. 2) was designed by a short cut method,
based on Fenske-Underwood-Gilliand method
implemented in HYSYS simulator, which was further
adjusted by rigorous simulation. A feed of 200 kmol/h was
considered. The feed composition in mol fractions was:
benzene 0.17, toluene 0.60, ethylbenzene 0.06, o-xylene
0.17. In the final separation sequence the number of
theoretical trays was 20 in the first column and 24 in the
second column. The feed tray locations were established
by simulation aiming to obtain a minimum energy
requirement in the reboilers.
The structure of the DWC was estimated by a short cut
method using a thermodynamically equivalent structure
with three columns [13] as presented in figure 3. The
ab
Fig.1. Petlyuk distillation column (a) and dividing wall distillation column (b)

REV. CHIM. (Bucureºti) ♦ 60♦ Nr. 10 ♦ 2009 http://www.revistadechimie.ro 1057
number of theoretical trays and feed locations for this
equivalent structure were estimated in the frame of HYSYS
using the short-cut distillation module and are indicated in
figure 3. As HYSYSTM has not a specific module for a DWC,
the rigorous simulation was performed using four tray
sections interlinked by liquid and vapor streams (fig. 4).
This is a thermodynamic equivalent structure as well,
which can be easily built from the three-column model
used in short-cut design. The four columns model is more
adequate for equipment sizing stage as each tray section
has a well defined cross section. For rigorous simulation,
the same number of trays was considered on both sides of
the dividing wall, as this is solution that can be easily built
and is therefore recommended when the DWC is a tray
column.
In figure 4, the „Top” section represents the trays above
the dividing wall, the „Prefractionator” and „Side-Draw”
stand for the left and right side of the dividing wall, while
by „Bottom” is denoted the section below the dividing wall.
The number of trays in each section, the feed tray, side-
draw location and the position of thermal coupling streams,
as identified by short-cut design, were slightly modified by
repeated rigorous simulations, aiming to realize at a low
reboiler duty the purity requirements (more than 0.95 mol
fraction) in the three products.
Antoine model was used to calculate the
thermodynamic properties of the mixture.
Main equipment sizing
In order to evaluate the capital costs, the equipment
implied in the separation flowsheets were sized using the
HYSYS TM options and databases. The two distillation
columns in the direct sequence (fig. 2) and the tray sections
in the DWC model were sized considering sieve trays with
overall efficiency of 0.75. The distance between the trays
was set to 0.4 m. The other data, such as the overflow, free
weight area, the flooding conditions were taken according
to standard simulator options. Under these conditions, for
the direct separation sequence (fig. 2) the two columns
diameters were 1.22 m and 1.68 m. The corresponding
heights were 15.85 m and 17.07 m. As can be noticed, the
second distillation column is larger in size as the internal
vapor flow was also higher of about 330 kmol/h compared
to 180 kmol/h in the first distillation column.
The two reboilers were considered of U tube-kettle type
and the condensers as tubular heat exchangers with fixed
tubular plate and bonnet round cap. The calculation of main
geometric dimensions for heat transfer equipment as
implemented in HYSYSTM simulator is based on an
automatic selection of the utility (cooling water or steam)
according to the temperatures of technological fluids. A
heat balance provides the flow rate of utility and,
considering the heat transfer equipment type selected, an
overall heat transfer coefficient is evaluated. The main
geometric dimension that is required for further cost
evaluation is the heat transfer area. In the two columns
sequence, the heat transfer area of the condensers were
21 m2 for the first column and 28.9 m2 for the second
column. The heat transfer area of reboiler of the first column
was 16.2 m2, while the reboiler of the second column had
a surface of 39.6 m2.
For the DWC, the sizing step referred to each tray section
according to the structure presented in figure 4. The same
options concerning the tray type and distance between
the trays were selected. Figure 5 presents the tray section
sizing step in the frame of HYSYS simulator, while figure 6
contains the reboiler sizing results. All characteristic
dimensions for the tray section in the DWC structure and
associated heat transfer equipment are presented in table
1.
Fig.3. Three columns model for DWC short-cut design:
Benzene (B), Toluene (T), Ethylbenzene (E), o-Xilene (X)
Fig.4. DWC structure in the frame of HYSYTM simulator
Fig.2. Two columns separation
sequence

REV. CHIM. (Bucureºti) ♦ 60♦ Nr. 10 ♦ 2009http://www.revistadechimie.ro1058
Taking into account that the DWC was simulated by
four interlinked tray sections that are included in a single
column, the cost evaluation was performed by formulating
a single column shell, with constant diameter and having
the total height defined as the sum of Top height,
Prefractionator height and Bottom height, which represents
21.6 m. The diameter of the DWC in the region of the
dividing wall was calculated as the diameter corresponding
to the sum of cross section area in the prefractionator and
side-draw region. This equivalent diameter is 1.77 m. In
the cost evaluation step this diameter was considered for
the entire DWC.
Cost evaluation
Cost evaluation was realized in the frame of a specialize
software, respectively CAPCOST. The database of this
software allows the estimation of a large variety of process
equipment costs if the characteristic dimensions are
known.
The capital cost of the DWC was estimated using the
simple distillation column option (fig. 7) and increases with
Fig.6. Results in sizing step for the
DWC reboiler
Fig.5. Sizing step results for the “Top”
region in the DWC
10 % to include the costs of special internal adjustments
required by the existence of the dividing wall.
A single reboiler and a single condenser are associated
to the DWC. Their cost evaluation is presented in figure 8.
The total capital cost of the DWC was 484 441 $ while
the cost of the two column sequence was 625 684 $. As
can be noticed a significant cost reduction of about 23%
can be realized by using the DWC.
All calculation above mentioned were repeated for two
other feed composition, corresponding to 0.5 and,
respectively, 0.3 toluene mol fraction. The results are
synthesized in table 2. As table 2 shows, the capital cost
reduction increases with the concentration of intermediate
component in the same sense in which increases the
energy efficiency, as it was presented in a previous work
[8]. The saving resides not only in the reducing of heat
transfer equipment cost, but also in a smaller column cost,
although it is well known that DWC is a large size
equipment.

REV. CHIM. (Bucureºti) ♦ 60♦ Nr. 10 ♦ 2009 http://www.revistadechimie.ro
Table 1
DWC SIZING RESULTS
Table 2
COST EVALUATION FOR DWC
AND TWO COLUMNS
SEPARATION SEQUENCE
Fig.7. DWC cost evaluation
Fig.8. Condenser (a) and
reboiler (b) cost evaluation
Conclusions
The present work investigated some possibilities to
design complex distillation columns and to evaluate the
capital cost. The DWC proved to bring important
economical benefits in term of capital cost reduction. A
case study demonstrated the effectiveness of
interconnection between different software environments
for the design engineer.
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REV. CHIM. (Bucureºti) ♦ 60♦ Nr. 10 ♦ 2009http://www.revistadechimie.ro
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