Optimal Grade Transitions in the High-Impact Polystyrene Polymerization Process

We address the dynamic optimal grade transition problem in large scale high-impact polystyrene (HIPS) plants. The problem of computing the dynamic optimal transition trajectories is cast as a nonlinear programming problem whose solution provides the optimal time dependent values of the manipulated variables. The optimal transition trajectories are computed off-line requiring a feedback control system for their closed-loop tracking and to take care of modeling errors. The optimal dynamic transition trajectories were computed between unstable open-loop operating points. When nonlinear bifurcation diagrams were used, it was found that the simultaneous manipulation of both monomer and initiator flow rates leads to a decrease in monomer conversion to achieve higher monomer conversions. To get smoother grade transition trajectories, it was also found that grade trajectories performed in a step by step manner are advantageous over transitions done with a single overall step.