TEMPERATURE CONTROL OF INDUSTRIAL GAS-PHASE POLYETHYLENE REACTORS

The performance of linear and nonlinear temperature control schemes is assessed for an open-loop unstable gas-phase polyethylene reactor (GP PER), based on speed, damping, robustness and the ability to maintain closed-loop stability in different operating regimes. An existing indu strial GPPER model is improved by modelling the temperature states in the external heat exchanger using linear and nonlinear driving force m odels with varying numbers of heat transfer stages. Differences in hea t exchanger models do not produce gain mismatch but do result in phase mismatch. It is shown that the nonlinear error trajectory controller (ETC) exhibits significantly superior responses in terms of speed, dam ping and robustness compared with an optimally-tuned PID controller. T herefore, substantial benefits could be realized using nonlinear contr ollers because they can provide good disturbance rejection capabilitie s and ensure closed-loop stability over a wide range of operating cond itions. An approach is presented for tuning ETCs for minimum-phase pro cesses of arbitrary relative degree. (C) 1997 Elsevier Science Ltd. Al l rights reserved.