Modelling the catalyst fragmentation pattern in relation to molecular properties and particle overheating in olefin polymerization

A two-dimensional single particle finite element model was used to examine the effects of particle fragmental pattern on the average molecular weights, polymerization rate and particle overheating in heterogeneous Ziegler-Natta olefin polymerization. A two-site catalyst kinetic mechanism was employed together with a dynamic two-dimensional molecular species in diffusion-reaction equation. The initial catalyst active sites distribution was assumed to be uniform, while the monomer diffusion coefficient was considered to be different inside the fragments and cracks. In other words, the cracks were distinguished from fragments with higher monomer diffusion coefficient. To model the particle temperature a lumped heat transfer model was used. The fragmentation pattern was considered to remain unchanged during the polymerization. A Galerkin finite element method was used to solve the resulting two-dimensional (2-D) moving boundary value, diffusion-reaction problem. A two-dimensional polymeric flow model (PFM) was implemented on the finite element meshes. The simulation results showed that the fragmentation pattern had effects on the molecular properties, reaction rate and the particle temperature at early stages of polymerization.