GEOMETRIC APPROACH TO MULTIVARIABLE CONTROL-SYSTEM DESIGN OF A DISTILLATION COLUMN

A multivariable control problem of a distillation column is considered, where the object is to maintain two output variables, the compositions of the distillate and the bottom product at some desired values by manipulating the reflux flow rate and the boil-up rate. Based on a linearized model, a geometric approach is applied to the design problem of disturbance rejection control. In other words, a feedback control strategy is desired which enables the complete rejection of the effect of disturbances on both output variables. In obtaining the feedback control, the problem of how many and what state variables are to be measured and fed back has been made clear. In this control strategy, only five state variables are fed back. Thus, only five columns of the feedback gain matrix have non-zero values. Furthermore, two out of these five columns are uniquely determined, and the other three columns can be assigned arbitrary values and used for pole assignment of the controlled system. For the disturbances in composition and flow rate of the feed stream, ΔxF and ΔLF, the effect of the disturbance ΔxF is completely rejected by the feedback controller, but the effect of the disturbance ΔLF can only be eliminated from the output ΔxD. A digital simulation of a distillation column composed of nine plates, a condenser and a reboiler was carried out to confirm these results and to show that the linearized model used in this paper is valid for fairly large step changes. © 1979.