CONTROL OF AN UNSTABLE BATCH CHEMICAL REACTOR

As often experienced in industrial practice, a fixed-parameter PID can do a good job, even for potentially challenging problems such as open-loop unstable processes. However in such cases, considerable a priori process knowledge may be required in order to adequately tune the controller and make the control performance robust to changes in operating conditions. Adaptive schemes, on the other hand, require less prior plant information but they should not be regarded as magic solutions to control problems. In this study, alternative adaptive control schemes are presented for the temperature control of an open-loop unstable batch chemical reactor in which the sequential exothermic reactions A --> B --> C are carried out. The performance of such control systems are compared with that of a PID controller, designed using IMC-based rules, and detuned to ensure robustness to process parameter changes along the temperature trajectory. Since the required detuning results in poor disturbance rejection, one would expect that the adoption of adaptive strategies should improve performance. However, the fact that a fixed-parameter PID controller can be designed to perform reasonably well does not imply that a self-tuning version will do at least as well. A self-tuning scheme combined with a parametric control approach can successfully deal with the reactor start-up and the regulatory problem, provided that the adaptive scheme's process model order is adequately selected. Thus, a self-tuning PID controller, which is based on a second-order model, is liable to failure if the true process is effectively of higher order.