Disturbance-Rejection Capabilities of Distillation Control Systems

In the absence of disturbances, no control action is needed to keep products on specification. Yet, disturbances are so common that control is always required, especially over high-purity products from distillation columns. The cost of poor control, exacted either as low product recovery or as excessive energy consumption, can be related to the integrated error sustained by the composition control loops. Integrated error is a function of the magnitude of the disturbance, the sensitivity of the system to it, and the settings of the feedback controller. For a distillation column, the sensitivity factor changes as a function of the variable manipulated to control composition. Furthermore, the process steady-state gain and, therefore, the composition controller´s proportional band even changes with the choice of structure of the other loops applied to the column. This paper lists the sensitivities of product compositions to disturbances in feed rate and composition and in heat input as a function of the choice of manipulated variable. It also identifies process gains as indicated by relative-gain calculations. The integrated error thus estimated is then verified by nonlinear dynamic simulation. This procedure leads to the choice of the control system most capable of rejecting the type of disturbance anticipated for a given column.