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The influence of morphology on the kinetics of ethylene polymerization from the gas phase
Abstract
This thesis is an analysis of the kinetics of ethylene polymerization from the gas phase. Specifically, the influence of monomer and heat transport, monomer sorption and insertion that occur within the morphologically complex and temporally evolving polymerizing particles was studied. Estimation of kinetic, equilibrium and transport parameters was performed at all stages of polymer yield. A simple kinetic model, which attempts to incorporate these steps and morphological dynamics is proposed and described. The developing void morphology of the catalyst/polymer particles during polymer accumulation and polymerization was characterized by a variety of techniques. Nitrogen adsorption showed that polymer fills the catalyst voids early and completely, rendering the particle inaccessible directly to gas phase monomer. This result was supported by solid-gas chromatographic observations, gravimetric sorption, and mercury porosimetry. Very early in the polymerization, monomer must penetrate to the active surface of the catalyst by first dissolving and diffusing within the polymer phase. A Thiele modulus formulation was used to evaluate the influence of mass transport on the polymerization at all stages of polymer yield. It was found that this formulation, which is based on uniformity of the distribution of active catalyst grains within the polymerizing particle, was inconsistent with experimental kinetic observations. During the nascent period, the polymerization is highly influenced by the slow rate of diffusion of monomer into the catalyst. The estimated Thiele modulus was 2-3, indicating that, at least during the initial stage of the polymerization, only 40-50% v/v of the catalyst is accessible to monomer. This estimate is consistent with the observed highly unstable nascent polymerization. With increasing polymer accumulation and lack of catalyst fragmentation, the accessibility decreases further. At high yields, with friable catalysts in well-cooled or dilute reactors, polymerization is stable and the influence of monomer transport is negligible. However, the Thiele modulus, measured by independent transport and kinetic experiments was found to be 0.3, indicating a still significant retarding influence. Analysis of the polymerizing particles by X-ray imaging techniques, including tomographic analysis, showed that the catalyst fragment distribution is not uniform. The largest fragments are consistently nearest the periphery of the polymer phase. This distribution, which is derived from non-uniform polymerization within fragments of differing sizes, effectively reduces the mean transport length and alters the Thiele modulus formulation, but is consistent with the experimentally observed kinetics.
Subject Area
Chemical engineering
Recommended Citation
Webb, Steven William, "The influence of morphology on the kinetics of ethylene polymerization from the gas phase" (1990). Doctoral Dissertations Available from Proquest. AAI9110230.
https://scholarworks.umass.edu/dissertations/AAI9110230