A COMPARISON OF 2-PHASE AND WELL-MIXED MODELS FOR FLUIDIZED-BED POLYETHYLENE REACTORS

A steady-state model incorporating interactions between separate bubbl e and emulsion phases in a fluidized-bed polyethylene reactor was deve loped by Choi and Ray [Chem. Engng Sci. 40, 2261-2279 (1985a)]. Correl ations for maximum stable bubble size indicate that bubbles within the bed are considerably smaller than those in their original model. In t he paper, the influence of bubble size and superficial velocity on rea ctor operation are examined. It is shown that bubble size critically i nfluences the rate of heat and mass transfer within the bed, and when the bubbles are as small as those predicted by the maximum stable bubb le size correlations, there is little or no resistance to the transfer of monomer and heat between the phases. A simplified well-mixed model is developed to describe reactor operation in the limiting case where there is no difference between bubble and emulsion gas temperatures a nd concentrations. The differences between the predictions of temperat ure and monomer concentrations of the two-phase and simplified models are less than 2 or 3 K and 2 mol%, respectively, in the operating rang e of industrial interest. Therefore, a simple back-mixed model is appr opriate for predicting temperature and concentration in the gas phase of industrial fluidized-bed polyethylene reactors.