28 Modeling, Simulation and Control of an Extractive Distillation Column
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E. D. GILLES, Β. RETZBACH, and F. SILBERBERGER Universität Stuttgart, Institut für Systemdynamik und Regelungstechnik, Stuttgart, West Germany
The large scale distillation column shown in Figure 1 is part of an extractive distillation plant employed in the waste water refinement process at HOECHST AG, Frankfurt/Main. The binary azeotrope isopropanol/water F is separated by using the extractant glycol F . Under normal operating conditions, the distillate D has to be of pure isopropanol, whereas the bottom product A must not contain any of it. This column was first investigated by Kunstmann (1), who carried out extensive experiments on the plant. In these experiments a high temperature gradient, a so-called "temperature front" was observed within a small area of the column. It could be shown that a strong dependency exists between the product compositions and the locus of the temperature front. Therefore, three temperatures were measured at the steady-state locus of the front, averaged and fed to a PID-controller which effected the heating steam flow rate u. Because of its high sensitivity to feed disturbances, this control system was not reliable under all operating conditions and the demand for a better configuration arose. The development of this new control concept is treated in the following. A
E
Mathematical
Model for
Simulation
Digital simulation based on an appropriate mathematical model is the best tool in order to understand the dynamic behaviour of the column* We used a set of equations which models every stage as an heterogeneous two-phase system consisting of the two totally mixed homogeneous phases vapour and liquid (Figure 2). Assuming constant molal holdup n , the balance equations of mass and enthalpy h on the k-th stage read I 0 - L (1) k-1 k ik 1
J
η
dx. ik dt - L, 'k-lik-1
ik
+ F. x, k Fik
i=l
, · · ·,1-1
0-8412-0549-a/80/47-124-481$05.00/0 ©
1980
American Chemical Society
In Computer Applications to Chemical Engineering; Squires, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
(2)
C O M P U T E R APPLICATIONS T O C H E M I C A L ENGINEERING
Downloaded by NORTH CAROLINA STATE UNIV on January 14, 2013 | http://pubs.acs.org Publication Date: May 30, 1980 | doi: 10.1021/bk-1980-0124.ch028
482
160.
Figure 1.
L
Q
Extractive distilhtion column and its temperature profile
T
x
k-1' k-1' ik-1
'">J-1
T
