MODELING OF TRANSFER PHENOMENA ON HETEROGENEOUS ZIEGLER CATALYSTS - DIFFERENCES BETWEEN THEORY AND EXPERIMENT IN OLEFIN POLYMERIZATION (AN INTRODUCTION)

This article begins by briefly reviewing the more important contributi ons to the area of modeling heat and mass transfer, and particle growt h during the polymerization of olefins on Ziegler-Natta catalysts. It is shown that these models are capable of identifying the critical are as involved in heat and mass transfer, and of modeling polymerizations where the observed activity is less than approximately 5,000 g of pol ymer per gram of catalyst per hour (g/g/h). However, it is not possibl e to use these models ''as-is'' to model more modern catalysts whose a ctivity levels can surpass the 50,000 g/g/h mark because they predict prohibitively large concentration gradients inside the growing particl es during slurry polymerizations, and temperature gradients outside th e particles during polymerization in the gas phase. An analysis of the mass and heat transfer Peclet numbers (Pe) reveals that certain simpl ifying assumptions may not always be valid. Pe values in the transitio n range suggest that convection inside the particles during polymeriza tion in the liquid phase may help to explain why observed mass transfe r rates are higher than the predicted rates. In an opposite vein, a Pe analysis shows that conductive heat transfer may play an important ro le at length scales characteristic of those in the early stages of pol ymerization. A new mechanism for heat transfer at reduced length scale s is proposed. (C) 1995 John Wiley & Sons, Inc.