Effect of hydrogen partioning on homo-propylene polymerization kinetics

Conference Dates

May 20-25, 2018


Despite the intense amount of research to improve the performance, efficiency and costs of polypropylene production, scale up of the scientific results from laboratory scale batch reactors to industrial scale continuous production plants remains still a challenge. In order to illustrate effects of scale-up in terms of kinetics, bulk phase syntheses of homo-polypropylene are presented for two grades with different melt flow rate (MFR) targets (50, 7.5 g/10mins) in different reactor sizes (0.25 and 5l) with calorimetric monitoring of the heat of reaction. In coordinative polymerization of propylene, molecular weight is mainly controlled by addition of hydrogen as chain transfer agent [1]. In laboratory scale, bulk phase polymerization reactions are typically carried out in partially filled reactors. In such partially filled reactors, partitioning of hydrogen between gas phase and liquid phase takes place. In the used 5 liter reactor, for all MFR grades studied, uni-modal molecular weight distributions have been observed. In contrast in the 0.25 liters reactor, it was surprisingly observed, that high MFR grades show a bi-modality in molecular weight. In order to study this phenomenon, hydrogen concentration in liquid phase ( ) has been investigated before and during the polymerization reaction by a combination of gas chromatography measurements and mass balance calculations. It could be shown, that in the 0.25 liter reactor, a drift in hydrogen concentration in the liquid phase is responsible for the observed bimodalities in molecular weight. Due to consumption of propylene, the hydrogen concentration is increasing during the course of reaction. A two-step feeding procedure for the 0.25 liter reactor has been developed in order to minimize these drifts in concentration. The developed procedure allows to produce unimodal product also at high MFR grades. In addition, a correlation between the composition of materials in the reactor ( ) and the concentration of hydrogen in the liquid phase has been established. This correlation has been used to design bimodal reactions in the 5 liter reactor. The experimental MFR results were in agreement with the targeted values.

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