Process control of polymerization reactors: An industrial perspective

Polymerization reactors usually exhibit complex nonlinear dynamic behavior because of the complexity of the physicochemical interactions and the kinetics of the polymerization reactions. In these reactors many important variables, often related to end-use polymer properties, cannot be measured on-line or can only be measured at very low sampling frequencies. Furthermore, end-use polymer properties are usually related to the molecular weight and composition distributions in the polymerization reactor. Finally, the typical industrial polymer reactor is used to manufacture a variety of grades of the same basic product necessitating frequent startups, on-line transitions, and shutdowns. The approaches used by the authors to meet these challenges are illustrated, within the constraints of protecting proprietary information, by focusing on the modeling and control of a commercial multiproduct continuous emulsion copolymerization reactor. The development of fundamental process understanding captured in a first-principles reactor model was a necessary prerequisite for the design of an efficient and robust system to control polymer properties. The model was verified with laboratory data and was used to represent quantitatively the effect of operating variables on the performance of the industrial reactor. The control system consisted of two levels namely the pressure, temperature, level, and flow regulatory control implemented in the Distributed Control System and the advanced control of polymer properties implemented in the supervisory computer. The structure of the feedforward and feedback control modules, which compute set points for the monomer and catalyst flows to effect on-aim control of reactor composition and inherent viscosity are discussed. The benefits from the application of the advanced control system resulted in a significant improvement of the first-pass, first-quality yield and polymer property uniformity.