Supervisory control of high pressure LDPE reactors

A comprehensive mathematical model was employed to simulate the dynamic behaviour of ethylene polymerization in high-pressure autoclave reactors. The model was capable of describing the complex mixing patterns occuring in multizone, multifeed low density polyethylene (LDPE) autoclaves. To represent the kinetics of ethylene polymerization a general reaction mechanism was considered and the method of moments was used to calculate the molecular properties. A two-zone autoclave reactor model was considered for our control studies. The reactor operated at an unstable steady-state, thus, the temperature in each zone was controlled by manipulating the corresponding initiator now rate. A Quadratic Dynamic Matrix Controller (QDMC) was designed for controlling the polymerization temperature in a two-zone autoclave and its performance was compared to that of two SISO PI controllers, usually employed to control the polymerization temperature. Finally, a supervisory QDMC controller was implemented to a simulated reactor model for the optimal control of polymer quality (e.g., melt index) during reactor start-up and grade transitions. Copyright (C) 1998 IFAC.