On the application of decoupling control to distillation column with time delays

Abstract

Decoupling control scheme, developed recently for a linear systems with time delays by Iwai, Seborg, Fisher and Kobayashi, is applied to a small size distillation column. This control scheme is evaluated using digital computer simulation and experimental application to a distillation column interfaced with on-line digital computer. Simulation study is performed to investigate the effects of parameters on the control performance and to determine the sensitivity of the system to errors in the delays and model. Simulation studies show the insensitivity to variation of time delay and system parameters. Particularly, good results are obtained for disturbances in the dynamic equations. To confirm the results of the simulation and to examine the practicability of decoupling control scheme for industrial application, experimental study is carried out on a pilot plant, packed distillation column, separating a methanol/water mixture at atmospheric pressure. An on-line microcomputer is used for the decoupling control scheme. Experimental results show that the scheme used allows the plant to follow the pre-assigned terminal outputs, when a set point of the top control temperature is chosen to be -1℃ in a deviation from the steady state value. But its performance is somewhat poorer than expected from simulation study. This may be due to the non-linearity of this system and the feedback of state vector obtained from model transfer function (because it is very hard to choose and measure a moderate state vector). When the set point of top temperature is chosen to be -3℃ in a deviation from the steady state, the performance becomes unsatisfactory. The reason may be that the large amount of required inputs violate the linear region of pneumatic control valve and, furthermore, exceed the transport limitation of control valve.