Some challenges in ethylene polymerization: Particle overheating in gas phase reactors, and modeling ethylene polymerization over nickel-diimine catalysts.

摘要

The first part of this research deals with particle overheating, a severe process issue in heterogeneous olefin polymerization reactors. Insufficient heat removal in heterogeneous polymerizations can lead to melting of polymer particles, potentially causing temperature runaways or reactor downtime. Particle overheating is most severe in gas phase polyethylene reactors due to high activity catalysts and ineffective heat transfer across the particle boundary layer. The research herein combines modeling and experimental work to evaluate addition of inert species to the gas mixture while increasing the total pressure as a way to mitigate particle overheating without sacrificing productivity or product quality. Heat transfer from growing polymer particles is improved through increased gas density, and particle swelling through enhanced sorption of inert species into the polymer. Ethylene homopolymerization experiments are conducted at constant monomer partial pressure, both in the presence and absence of inert species. Without inert species, the rate profiles exhibit increased decay, most notably at higher temperatures. When inert species are present, the rate profiles are comparatively flat, indicating a reduction in particle overheating.; The second part of this work focuses on modeling ethylene polymerization over nickel-diimine catalysts. The development of these new catalysts is important because of their potential to polymerize polar monomers and their ability produce a broad range of product densities from homopolymerization of ethylene. The polyethylene produced from these catalysts can exhibit a unique range of properties. Branching is obtained in the absence of comonomer. Furthermore, by varying the reaction temperature and pressure, the degree of branching can be radically affected such that from atactic to semi-crystalline polymer can be produced. The "chain walking" branching mechanism, describes the migration of the active site from the chain terminus to interior carbons in the chain leading to branch formation upon monomer insertion. A deterministic model for ethylene polymerization is developed to describe the chain walking mechanism and the overall kinetic behavior exhibited by these catalysts. The model is incorporated into existing polymerization reactor process models to predict end polymer properties from reactor operating conditions, and to determine the important process issues associated with using these new catalysts.