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Behavior of Active Sites in a Changing, Supported Metallocene Catalyst Particle: Modeling Monomer Transport and Kinetics



Summary: Support-catalyst-polymer particles composed of millions of microparticles arranged in cells and having silica nuclei covered with metallocene-methyl alumoxane (MAO) active sites are studied to analyze cell participation during polymerization. Main variables are the changing particle morphology and the kinetic-diffusion effects determining local monomer availability during residence time. The phenomena were studied by means of a mathematical model used to produce a set of predictions for particles polymerizing ethylene in a toluene slurry continuous stirred tank reactor (CSTR) under various operating conditions. This information is employed to predict the micro- and macroparticle behavior in situations designed to explore catalyst activities, monomer availabilities and reactor conditions. Kinetic constants and concentrations range from reference values up to 6 times these figures, with reactor temperatures between 323 and 353 K and particle Reynolds numbers on a 1 to 10 relative scale. Heat transfer and temperature elevation during polymerization are predicted, with no relevant overheating observed. Morphology changes, in the form of density profiles inside the support-catalyst-polymer particle, are monitored with time, and their interaction with transport and reaction phenomena analyzed. Increasing catalyst activity alone may not produce proportional raises in yield; it appears more efficient to improve the monomer availability instead. High catalyst activity may produce monomer depletion at inner cells delaying their fragmentation and decreasing local polymer-production.

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Cell density vs. time for cells located at the exterior, at the center and at half the radius of the macroparticle.