Predicting Properties of Distillation Control Structures

If control properties expressed by process and disturbance gains are known for one control structure of a continuous distillation process, the process and disturbance gains can be calculated for arbitrary control structures by use of so called transformations and consistency relations. The paper reviews these transformations and consistency relations and outlines what information is needed for the calculations. By means of the calculated models, different control structures can be evaluated with respect to such properties as disturbance sensitivity and control loop interaction. As an application, the properties of 15 control structures for four experimental and two simulated distillation columns are investigated analytically. The results indicate that for the separations studied (alcohol-water and alcohol-alcohol separations, moderate or low product purity, liquid feed), the best control structures for rejection of disturbances in the feed flow rate and the feed composition are (L/F, V/F), (D/(L+D), V/F), (L/F, V/B) and (D/(L+D), V/B). However, if setpoint changes are important, (L/F, B/F) or (D/(L+D), B/F) might be preferable.