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Summary: Sorption experiments of ethylene and propylene in different polypropylene powder samples, both homopolymer and heterophasic copolymers with different rubber content, have been carried out in a high-pressure magnetic suspension balance at 10 bars pressure and 70 8C. The gross solubilities measured can be well correlated with the rubber content of the polymer samples. Solubility of ethylene and propylene in the rubber phase differ from solubility in the amorphous fraction of the homopolymer, especially the concentration ratio of propylene to ethylene differs significantly between rubber phase and amorphous fraction of the homopolymer. From the slope of monomer uptake, information on kinetics of mass-transfer can be gained. No significant differences were observed in terms of mass-transfer for ethylene and propylene. With increasing rubber content, effective diffusion coefficients increased slightly. By combined sorption studies with powder samples and compressed films, information about both effective diffusion coefficients and the effective length scale of diffusion could be gained. It could be shown, that the particle radius is not the characteristic length of diffusion in the studied powder samples. Mass transfer of nearly all samples could be described by a constant diffusion length of 120 to 130 mm, independently on particle size. This indicates that the effective scale of diffusion in polymer particles is in between microparticle and macroparticle scale used in classical particle modeling.

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The model-based optimization of the cooling system of an industrial tubular reactor for the production of lowdensity polyethylene (LDPE) is studied in this paper. First a detailed reactor simulator is presented. Second, a series of wellposed optimization problems are formulated and solved. To this end, the optimization problems are presented with an increasing number of degrees of freedom. Moreover, economic cost functions consisting of conversion and energy terms are derived and constraints due to operational and safety reasons are added. The degrees of freedom involve parameters such as the initiator feed rates, the cooling water temperatures, and the switching position between hot and cold cooling water. The optimization results indicate that a dual water circuit operating at a low and a high temperature allows significant improvements in conversion with respect to the reference case of a single temperature circuit, while maintaining similar molecular characteristics.