EFFECTS OF OPERATING-CONDITIONS ON STABILITY OF GAS-PHASE POLYETHYLENE REACTORS

Fluidized-bed polyethylene reactors are prone to unstable behavior and temperature oscillations (Choi and Ray, 1985b). Their work is extende d to show the effects of ethylene feed system operation, reactor cooli ng system design, catalyst properties, and gas composition on reactor stability and dynamics. The analysis is performed using a well-mixed m odel, because heat- and mass-transfer resistances between multiple pha ses are small and are not required to account for the observed bifurca tion phenomena. The addition of a gas recycle and hear exchanger syste m to the model significantly affects dynamic performance, including th e formation of limit cycles. The size and dynamics of the heat exchang er, however, have little effect on the overall stability. In contrast, automation of the ethylene feed system to replace the monomer in the reactor as it is consumed leads to substantially different dynamic beh avior than if the ethylene feed is maintained at a constant rate. Cata lyst properties (multiple sites activation energy, and deactivation) s ignificantly affect dynamics and stability whereas comonomer and other gases affect them only mildly. The results confirm that without prope r temperature control, gas-phase polyethylene reactors are prone to in stability, limit cycles, and excursions toward unacceptable high-tempe rature steady states.