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Sorption models

As part of the PhD work of Y. Banat (IPP University of Twente Enschede), Sorption models must be used in the particle growth and morphology models. The expected result is a review of the existing models and the selection of the best option for usage in the final model.

This includes literature survey, model testing vs. existing data and data generation where some data are missing. Collaboration with other POLYPROP partners will allow minimising experimental work for the benefit of all partners.

Detailed Result:
A review of the equilibrium sorption of ethylene and a-olefins in semi crystalline polyethylene is done. The influence of crystallinity, type of polyethylene, and type of catalyst used, branching, and polyethylene structure on penetrant sorption is discussed. The Sanchez- Lacombe equation of state with temperature-dependent binary interaction parameters has been used for quantitative comparison of the studied systems. The predicted polymer swelling from S-L EOS using the optimised binary interaction parameters is compared to the measured swelling isotherms for the polyethylene samples studied. Moreover, the concentration of sorbed ethylene/a-olefins in the amorphous part of polyethylene is also predicted. An excellent match between the swelling isotherms obtained from the dilation isotherms and the values predicted by S-L EOS are obtained at all temperatures studied and for all the systems reviewed. It is found that no general correlation can be used for a precise description of the concentration or solubility of ethylene/a-olefins, even in the simplest form of polyethylene represented by HDPE. It is concluded that several structural factors, such as crystallinity, branching, type of comonomer, comonomer composition, molecular weight, molecular weight distribution and comonomer composition distribution, contribute significantly to the sorption process.

Furthermore, the equilibrium sorption of gaseous propylene in semi crystalline polypropylene particles of different diameters of 100-425, 600-800 micron, and 2.4 mm was measured. The sorption measurements were performed at temperatures of 320.9 and 344.5 K and pressures up to the vapour pressure of propylene using an electromagnetic thermal balance. The swelling of polypropylene caused by the sorption of propylene was estimated from the corresponding sorption isotherms and the specific volumes of the crystalline and the amorphous polypropylene fractions, assuming ideal mixing. The measured solubilities were correlated with the Sanchez-Lacombe equation of state (S-L EOS) with temperature-dependent binary interaction parameters. The predicted polymer swelling from S-L EOS coupled with the optimised binary interaction parameters was compared to the estimated swelling based on ideal mixing. The concentration of sorbed propylene in the amorphous part of polypropylene was estimated. Unexpectedly, Henry’s law with a temperature-dependent constant could describe propylene concentration.

Furthermore, the results were extrapolated to represent liquid propylene sorption in polypropylene at different temperatures using Flory-Huggins theory (FH) with temperature-dependent F-H binary interaction parameters. The results obtained were compared with well-known trends found in the literature. The measured sorption data were used for the comparison of gas and liquid phase kinetics of propylene polymerisation with a highly active 4th generation Ziegler-Natta catalyst. It was found that the same catalyst system at the same temperature and hydrogen concentration has the same relative activity in liquid pool and gas phase polymerisation.

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