A nonlinear model-based scheme is developed for product property control in industrial gas-phase polyethylene reactors. The controller regulates instantaneous melt index and density, and provides servocontrol during grade changeovers. Hydrogen and butene feed rates are manipulated to force the product properties onto desired trajectories. During grade changeovers, these trajectories are determined from off-line dynamic optimization studies. Optimal open-loop policies for reactor temperature, bleed stream flow, catalyst feed rate, and bed level are implemented as part of the changeover strategy.
The nonlinear feedback controller design is based on global input/output linearization methods. Disturbances are estimated, and plant/model mismatch is removed using an extended Kalman filter. Simulations on a complex mechanistic model of the process reveal that the nonlinear controller performs well for both regulatory and servocontrol. An analogous linear IMC controller is inadequate for disturbance rejection at different operating conditions and for control during grade changeovers. The simplicity of the nonlinear control algorithm makes it an interesting candidate for industrial application. This single controller can be used to control the properties of many grades of polyethylene made in the reactor and to accomplish near optimal changeovers between these grades.