(332c) Optimal Design and Operation of Four-Product Dividing-Wall (Kaibel) Distillation Column

Optimal Design and Operation of
Four-Product Dividing-Wall (Kaibel) Distillation

Column

Abdallah
Alshammari* & Farrukh Ilyas Abid

alshammari@kfupm.edu.sa

Due to the increased
market competition, many chemical industries try to optimize their operations
by maximizing productivity and minimizing operational cost, mainly energy
consumption. Distillation operation alone accounts for a significant amount of
the world’s energy consumption since it is widely used in refineries and
petrochemical industries. It has been estimated that approximately 3% of the
world’s energy consumption is utilized by this distillation operation alone
[1]. This energy consumption can be greatly reduced by using an efficient complex
distillation column configuration, such as a fully thermally coupled
distillation column or divided-wall distillation column, over conventional
arrangements. The divided-wall column (DWC) offers potential energy and capital
investment savings. For multi-product separation, conventional distillation
column configurations (direct, indirect and distributed sequences, etc.)
require a greater number of heat exchangers, distillation columns, etc.,
compared with complex distillation column arrangements (side stripper, side
rectifier, fully thermally coupled distillation column and divided-wall
distillation column). Also, compared with conventional column configurations,
complex distillation configurations lead to lower capital investment and energy
consumption as well. However, design and operation remain a challenge due to
the large number of design and operational degrees of freedom. In this study,
it was shown that energy savings of up to 23% can be achieved by using a
four-product divided-wall distillation column (Kaibel) compared with
conventional column arrangements, Table 1. This work demonstrates the
steady-state and dynamic behavior of a Kaibel distillation column using
commercial simulation software i.e., Aspen HYSYS. Three different mixtures
(Alcohols, Hydrocarbons, and Aromatics) are used to conduct the study and the
comparison.

The optimal operation
and control of dividing-wall distillation columns are more difficult than those
of a simple distillation column. Thus, various industries have been reluctant
to adopt this energy-efficient distillation column, as a greater number of
operational degrees of freedom must be specified than for a conventional
column. The distillate (D) and bottom (B) are assumed to be used
as level controls for the condenser and reboiler, respectively. Thus, only six
operational degrees of freedom remain for a Kaibel distillation column i.e.,
liquid split ratio (R_L), vapor split ratio (R_V),
reflux flow rate (L), vapor flow rate (V) and the flow rates of
the two side draw locations (S₁, S₂). The
liquid split ratio (R_L) can be defined as the ratio of the
amount of liquid introduced at the top of the prefractionator section to the
overall amount of liquid sent to the main column as reflux, and the vapor split
ratio (R_V) can be defined as the ratio of the amount of vapor
sent to the bottom of the prefractionator section to the overall amount of
vapor obtained at the bottom of the main column from the reboiler.

Therefore,
it is important to operate the dividing-wall column at optimal values of the
liquid and vapor split ratios, as energy efficiency will be lost if the
distillation column is not operated at the optimum vapor split ratio (Figures
1-3). The main objective of this study is to examine the steady-state and
dynamic behavior of a four-product Kaibel distillation column with the help of
process simulation software; and it discusses the optimal operation parameters
of DWC. Operation of the DWC was studied under two different modes:

     
i.        
To achieve the defined product purities by
providing minimum energy.

   
ii.        
To achieve the maximum product purities with a
fixed available energy value.

The first mode requires specifying
all the required product purities, which should be obtained with minimum
available energy. In the second mode, the available energy should be specified,
and maximum product purities should be obtained with this available energy. The
objective functions of the two modes can be defined as follows, respectively;

 where product purities ≥ 0.95 (Eq.
01)

(Eq. 02)

The optimal operation is studied
using both of the above-described modes. The dividing-wall distillation column is
used to reduce energy consumption and to obtain nearly pure products by
determining liquid and vapor split ratio.

Table 1: Energy comparison between
conventional & complex configurations for alcohol case

Figure 1:
Four-product divided-wall (Kaibel) distillation column.

Figure 2: Effect of vapor split
ratio on product composition

Figure 3: Effect of liquid split
ratio on reboiler duty

 [1] J.
Humphrey and A. Siebert, “Separation technologies; An opportunity for energy
savings,” Chem. Eng. Progress;(United States), 1992.