A methodology for on-line setpoint modification for batch reactor control in the presence of modeling error

Abstract

Batch and semi-batch reactors are usually highly nonlinear and involve complex reaction mechanisms. Often, the lack of rapid direct or indirect measurements of the properties to be controlled makes the process control task very difficult. It is the usual practice to follow the prespecified setpoint profiles for process variables for which measurements are available, e.g., temperature, in order to obtain desired product properties. Model error can be the cause of poor performance when these setpoint profiles based on a model are implemented on the actual plant. This paper formulates a state estimation model based algorithm for on-line modification of setpoint profiles utilizing infrequent and delayed measurement information of the properties to be controlled, with the goal of obtaining the desired values of the properties in the minimum batch time. The algorithm modifies the setpoint profile for the remainder of the batch after every such measurement by making one step in the right direction instead of attempting to find a completely new optimal profile. This results in robustness with respect to model error and allows improvement even with infrequent product property measurements. The implementation of the setpoint profiles is made via real-time observer based nonlinear quadratic dynamic matrix control, which has been studied extensively in the literature. The modest additional on-line computational requirements of the proposed method offer promise for the practical on-line implementation. The effectiveness of the algorithm is demonstrated with simulations for bulk polymerization of styrene.