Complex Nonlinear Behavior in the Full-Scale High-Impact Polystyrene Process

In this work, the open-loop nonlinear bifurcation analysis of the high-impact polystyrene process is carried out and used for optimal design, operation, and control analysis. The process can be described by a set of seven continuous stirred tank reactors (CSTRs) connected in series. Process design was carried out using an optimization approach; the nominal optimal operating point is open-loop unstable and agrees with typical industrial operating conditions. For the seven CSTRs cascade, as much as 398 steady states were found, but only 11 are considered as industrially feasible; 9 of them are open-loop unstable. Both input and output multiplicities were found, and implications on optimal grade transitions and process control are discussed. A simple cascade control system was used to demonstrate that linear PI controllers add patterns of nonlinear behavior previously not present in the process.