Steady-state evaluation of control schemes for binary distillation.

Abstract

Utilizing the Smoker equation, an analytical method equivalent to the McCabe-Thiele diagram method was developed to evaluate some simple control schemes for a binary distillation tower. The basis of evaluation was the steady-state deviations of product purities when the tower was subject to a sustained disturbance in feed composition, rate or enthalpy. The schemes proposed in this work were a simple feedforward and a feedback heat-balance control scheme. These control schemes were simulated on an IBM-360 and IBM-1620 digital computer. The feedback heat-balance control was further verified on a laboratory pilot-scale binary distillation tower separating a methanol-water mixture. These two control schemes handled feed composition and enthalpy disturbances more successfully than the rate disturbance. In general, for one end control, the feedback heat-balance control was better than the simple feedforward one. For feed composition disturbance and two end control the simple feedforward control was the better one. The feedback heat-balance control was more suitable for system with high relative volatility and top composition control. Also, the closer the control tray to the end of the column, the better the product purity.