Structural Design and Separation Characteristic of Divided Wall
Column by the Shortcut Method
Main structure of the equivalent conventional column
● What is DWC ?
-The DWC consists of one column in which the prefractionator
is separated from the main column by a divided wall.
No remixing
Reducing feed mismatch
Thermal efficiency benefit
Only one column
Reduced energy
Capital saving
Improved yield/quality
Smaller plot area
Ext. Reflux Ratio
Total Stage No.
Feed Stage No.
Rectify Vap/ Liq (kmol/h)
Stripping Vap /Liq (kmol/h)
1st column 2nd column 3rd column
0.898
1.608
6.86
18
14
14
9
5
10
41.87/19.81 47.75/29.44 36.49/31.85
41.87/64.81 25.69/29.44 36.49/54.79
● Optimal Design of Internal Flow Distribution
The energy consumption by internal flow distribution
Prefractionator
A
ABC
A
Prefractionator
B
ABC
The existence of the
optimal internal flow
distribution can be
explained by the two
competing effects of
internal flow.
B
Main Column
Main Column
C
C
● DWC Structure Design
Fenske-Underwood equation (Shortcut Method) is used for
the structure design of DWC by applying it on three
conventional simple column configuration.
A
II
ABC
I IV
II
ABC
B
I
IV
A
B
III
III
C
C
< The Structural Design by Shortcut Method >
-The reflux flow of 1st column = The internal liquid flow fed
to prefractionator.
- The boil-up flow of 1st column = The internal vapor flow
fed to prefractionator.
* The simple column configuration is not thermally coupled.
And total energy consumption is likely to be different from
DWC’s. However, the relative location of each major section
is not much affected by this structural difference between
the DWC and the equivalent conventional column
configuration.
● DWC Simulation
HYSYS TM simulation program is used with the FenskeUnderwood equation for the design of column structure.
Feed condition of ternary mixture
Feed
Mixture
A: n-Pentane
B: n-Hexane
C: n-Heptane
F1
F2
F3
A : 0.40
B : 0.20
C : 0.40
A : 0.33
B : 0.33
C : 0.33
A : 0.15
B : 0.70
C : 0.15
* 1 atm, 45kmol/h
- The top, side and bottom product purity => 98/ 98.5/ 98%
- The composition of light key component (n-Pentane) at the
bottom and heavy key component (n-Heptane) at the top in
the 1st column is set to less than 0.1mol%.
→ This is based on the fact that the separation efficiency is
highest when the low (A) and high (B) boiling component are
completely separated in the prefractionator section.
In the proposed method, how to select the product purity of the
1st column is very important not only to the optimal structure
but also to the optimal internal flow distribution.
Internal flow & energy consumption of the DWC based on the
shortcut method and optimization.
DWC
DWC
Two sequential
(1st Column) (case study)
column
Rectify Liq Flow/
Internal Liq Flow
Stripping Vap Flow/
Internal Vap Flow
Total Reb Duty (kJ/h)
19.81kmol/h 16.00kmol/h
41.87kmol/h 38.00kmol/h
1.790☓106
1.761☓106
2.630☓106
Although there is some difference in resulting internal flow, the
difference in energy consumption is insignificant just as 1.7%
Internal flow & energy consumption of the DWC based on the
shortcut method and optimization for difference mixtures.
Shortcut Column
DWC
2TOP
Rectify Stripping Liq Internal Vap Internal
Liq Flow Vap Flow
flow
flow
F1
19.81
41.87
16
38
F2
23.18
45.64
15
36
F3
15.48
30.92
14
35
Reboiler
F1 : 1.79
F1 : 1.76
F1 : 2.63
Duty
F2 : 2.02
F2 : 1.92
F2 : 2.85
(106kJ/h)
F3 : 2.19
F3 : 2.05
F3 : 3.33
Improvement of energy efficiency is most significant when the
composition of middle boiling component is high. This result
seem to be authentic because the energy efficiency by the DWC
comes from the prevention of remixing. The difference of energy
consumption between the DWC based on the proposed shortcut
method and that by the optimization method is only 1.7~6.3%. It
indicates the proposed method can be properly used to design
the DWC.
● Conclusion
This study emphasizes a shortcut method using the FenskeUnderwood equation for the design of DWC. The method
utilizes the three conventional column configuration equivalent
to the DWC to find the proper DWC structure in simple manner.
The proposed method shows little difference from that through
optimization study in terms of energy consumption. Extensive
simulation study illustrates the proposed method is suitable to
DWC structure design.