Catalytic olefin polymerization: Modelling of heterogeneous kinetics and single-particle growth

Abstract

As the polymer industry becomes more competitive, polymer manufacturers face increasing pressures for production cost reductions and more stringent "polymer quality" requirements. To achieve these goals one needs to develop comprehensive mathematical models capable of predicting the molecular and morphological properties in terms of reactor configuration and operating conditions. These mathematical representations can be classified into microscale kinetic models, mesoscale physical transport and thermodynamic models and macroscale dynamic reactor ones. In the present work, comprehensive mathematical models on microscale kinetic models and mesoscale transport models (i.e., single particle growth) of heterogeneous catalytic olefins polymerization over Ziegler – Natta (Z-N) catalyst are developed. For micro-scale modelling, a comprehensive kinetic model for ethylene and ethylene – propylene copolymerization reactions is derived. The method of moments is employed to predict the average molecular properties of the produced copolymer chains. Accordingly, a parametric sensitivity analysis is performed to investigate the effect of the operating conditions (i.e., monomer and hydrogen concentrations) and kinetic parameters (i.e. sites multiplicity and reactivity) on the polymerization rate, average molecular properties, molecular weight distribution (MWD) and chemical composition distribution (CCD). Results show that the operating conditions (i.e., monomer(s) and hydrogen partial pressure) and catalyst sites multiplicity and reactivity have a strong effect on the polymerization yield, polymer molecular properties, MWD and CCD. For mesoscale modeling, the random pore polymeric flow model (RPPFM) is developed and modified to describe the dynamic evolution (i.e., particle growth rate, temporal and spatial concentration and temperature profiles, etc.) of supported Z-N catalysts in gasphase ethylene polymerization. A two-site kinetic scheme is considered to describe the monomer polymerization in the presence of a Z-N catalyst. The method of moments is used to obtain simplified rate equations. Henry's law is employed to calculate the equilibrium ethylene concentration in the amorphous polymer phase. Moreover, a comprehensive diffusion model is applied for the calculation of the ethylene transfer rate from the gas phase to the catalyst metal active sites. The model is capable to predict the particle growth by employing three diffusion mechanisms (i.e. molecular, Knudsen and in the amorphous polymer phase) and the dynamic transition of monomer transfer among these three mechanisms. The proposed model is solved using the global collocation method. Based on the above model considerations, a detailed analysis is carried out to assess the effects of catalyst properties (i.e. catalyst size, active sites distribution) operating conditions (i.e. monomer and diluent partial pressure), particle morphology (e.g. porosity and polymer crystallinity) and pre-polymerization conditions on the behavior of temporal-spatial evolution of temperature, monomer concentration, particle overheating and the particle polymerization rate. It is shown that particles with identical size, overall pore volume fraction, metal concentration and reaction conditions but with different pore size distribution exhibit large differences in growth behavior (i.e. polymerization yield and particle overheating). Additionally, it is proved that polymer crystallinity strongly affects the polymerization rate and particle overheating. Furthermore, It is presented that the monomer sorption kinetics and the initial active site distribution greatly affects the polymerization growth behavior (e.g. polymerization yield and particle overheating), independently of the evolution of the internal particle morphology and operating conditions.

Date of Award	2015
Original language	American English
Supervisor	Ali Almansoori (Supervisor)