HEAT AND MASS TRANSFER DURING THE POLYMERIZATION OF ALPHA-OLEFINS FROM THE GAS PHASE

A model for the growth of a catalyst-polymer particle during gas-phase polymerization of (alpha)-olefins is developed. In order to design the polymerization reactor, the behavior of a pellet is analyzed through the model. Low-pressure, gas-phase polymerization processes use various heterogeneous catalysts, supported and unsupported. Initial catalyst particles break into small fragments, kept in contact through polymer entanglements. This phenomenon creates a pellet formed by the aggregation of thousands of microparticles, each with a catalyst micronucleus surrounded by a growing polymer layer. Monomer diffuses from the gas phase to the active sites on the surface of the micronucleus. This process include transport within the pellet along the voids among microparticles, and through their growing polymer layer. Heat of polymerization must be transferred from the active sites to the bulk phase. Approximate analytical solutions are developed to model the heat and mass transfer in the microparticle. A multigrain model combines these solutions to predict transport in the pellet. The analytical solutions make the separate treatment of the microparticle and pellet scales tractable. Consequently, fragmentation steps are included in the model. Polymerization and pellet growth rates are predicted. Results show energy transfer effects are critical during initial steps of the pellet growth. Thermal runaways leading to polymer melting are possible if heat transfer to the bulk phase is inadequate. Catalyst fragmentation and overheating phenomena occur at the same time scale, and their interaction is important. Diffusion effects in the pellet are critical at the initial growth steps and during thermal excursions. Time scales for these processes are much shorter than the pellet total reaction times. Thus, transport through the pellet has little effect on the molecular weight distribution of the polymer. The pellet model is applied to the base-case design of a fluidized-bed reactor. The fluid velocity and reactor dimensions are predicted. The model is used to analyze the effects of the reactor environment on the polymerization process