Nonlinear Multivariable Control of Product Properties in an Industrial Gas Phase Polyethylene Reactor

A nonlinear model-based scheme is developed for product property control in industrial gas phase polyethylene reactors. The controller is designed for instantaneous melt index and density regulation over a range of products, and for control during grade changeovers. The feedback controller manipulates the hydrogen and butene feed rates to force the product properties to follow a desired trajectory. During grade changeovers, these trajectories are provided by the results of off-line dynamic optimization studies. From these studies, it was determined that the reactor temperature, bleed stream flow rate, catalyst feed rate and polymer bed level can be used to improve grade changeovers. The open-loop optimal policies for these variables are implemented as part of the changeover strategy.The nonlinear model-based feedback controller design is based upon global input/output linearization methods. Mismatch between the model and the process is removed using an extended Kalman filter. Through simulations on a complex mechanistic model of the process, it is shown that the controller peforms well for both regulatory and servo-control. The simplicity of the control algorithm makes it an excellent candidate for industrial application.