Effect of fragment size distribution on reaction rate and molecular weight distribution in heterogeneous olefin polymerization

Abstract

A one-dimensional polymeric flow model is used to study the effect of particle fragment size and size distribution on molecular weight distribution and particularly on reaction yield in heterogeneous olefin polymerization. The broadness of molecular weight distribution is explained by a multi-active site assumption, while the high rate of reaction for active catalysts due to diffusion limitations in the particle is questionable. In this study, the modeling is shifted from particle to fragment level. The fragments are assumed to be spherical and homogeneous with regular physical properties, separated from each other by interconnected cracks. There is only monomer diffusion taking place inside the particle, while in comparison the diffusion inside the cracks is much higher. Diffusion coefficient is assumed to be similar in all cracks. The polymer particle is taken to be made up of seven different size fragments from 0.5 to 3.5 µm, distributed in five different patterns. The results show that by introducing the fragments and fragment size distribution in the particle model higher yields of polymerization can be achieved. In other words, the fragments and cracks inside the particle can make strong impact on the reaction yield while the results show that the broadness of molecular weight and consequently a higher PDI can be justified by the multiple sites assumption.