CSTR PERFORMANCE LIMITATIONS DUE TO COOLING JACKET DYNAMICS

Abstract

The classical exothermic CSTR is known to exhibit complex static and dynamic behavior, although only a two-state (reactor concentration and temperature) model is used. Adding a third state to account for the cooling jacket temperature dynamics has an important influence on the open and closed-loop behavior of exothermic CSTR's. In this paper we show that a stabilizing inner-loop cascade controller is implicitly assumed in the two-state model, because the three-state model may be open-loop unstable in regions where the two-state model is open-loop stable. The affect of process parameters on the open-loop stability of the two-state and three-state CSTR models is examined. We show that when the CSTR is operated at an open-loop unstable operating point, there are right-half-plane zeros in the secondary process transfer function, causing an inherent performance limitation in the closed-loop system.