Modelling of propylene polymerization in an isothermal slurry reactor

Abstract

A new model, the polymeric multigrain model, has been developed to explain the broad molecular weight distribution of polypropylene obtained from a slurry reactor using Ziegler-Natta catalysts. In this model, catalyst subparticles are assumed to be in a continuum of polymer with only a single level of diffusional resistance for the monomer. The catalyst subparticles move outward as the macroparticle expands during polymerization. Thus, this model incorporates some physical aspects of the detailed multigrain model, and some of the simpler continuum models. It has been shown that the present model can predict higher values of the polydispersity index (PDI about 4-10), than the multigrain model, using a single-site, non-deactivating catalyst. It has been observed that only a few parameters, namely size of catalyst subparticles, active-site concentration, etc., are most important in terms of their effect on the broadening of the molecular weight distribution. However, to predict still higher values of the PDIs, one needs to incorporate multisite activity and deactivation of catalyst in the model.