PID temperature control of an unstable gas-phase polyolefin reactor

Temperature control of a gas-phase polyolefin reactor is prone to be unstable when operating conditions and/or catalyst are changed. Using a simple first principles model, it is shown that the dominant dynamics of the open-loop can be expressed by a second order transfer function, with which the number of unstable poles varies from zero to two depending on operating conditions. The frequency domain analysis shows that these changes in the process dynamics primarily affect the process phase and cause the closed-loop instability. Thus, PID temperature controller is retuned so as to increase phase margin. A practical retuning method of a PID temperature controller, which can be performed at the onset of closed-loop instability, is developed and its effectiveness is demonstrated through simulations.