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Reaction Engineering of Heterogeneously Catalysed Polymerisation


Great efforts on drawing up a model of Fluid Bed Reactors of Polyolefin were made by four partners of the CATAPOL consortium. Use was made of data and expertise available within the CATAPOL consortium in different fields: - Know-how on Fluid Bed hydrodynamics available from the open literature; - CFD calculations on specific hydrodynamic aspects; - Polymerisation kinetics supplied by industrial partners; - Plant data supplied by industrial partners. By the end of CATAPOL, very large progress was made and, for the first time, a number of important features of this type of reactors could be explained and predicted in a mechanistic way. The future economic potentials of this work for European polyolefin industry are huge. In Fluid Bed Reactors of Polyolefin, very strong inhomogeneities in temperature and chemical composition can occur. These inhomogeneities are due to the auto-thermic operation of this type of reactors for removing the heat of polymerisation, in combination with monomer conversion and feeding of condensable. Rapid powder mixing inside the reactor came out as the essential feature for keeping these inhomogeneities within limits. By means of CFD (Computational Fluid Dynamics) the powder mixing could be explained and even, to a large extent, predicted. A reactor model was then drawn up that predicts the mixing and the inhomogeneity in both vertical and radial direction. The model was compared with plant data. The model is now very close to being a tool for design and scale-up of this type of reactors.
This result deals with reducing the risk of overheating of fresh catalyst. If overheating occurs, very undesirable lump formation results, sometimes leading to complete plant shut-down. Numerical calculations have now shown that direct heat transfer from the fresh catalyst to the powder bed is the main factor preventing overheating. Immediate and individual mixing of the fresh catalyst through the powder bed is therefore needed. This is in contrast with previous publications, in which heat transfer from the fresh catalyst to the gas phase was given as the main factor preventing overheating. Industrial implementation of these insights is realized in a very specific way for every specific process. This result deals with new insights into ways to feed fresh catalyst into gas-phase reactors of polyolefins. Very rapid, individual mixing of fresh catalyst became necessary in order to prevent strings and lumps and very undesirable agglomeration, eventually leading to plant shut-down. The actual method for achieving this individual mixing will be very process-specific. The method of catalyst injection and the local powder hydrodynamics in the spraying zone play an important role. If the catalyst is injected as a slurry, spraying should be carried out in such a way that every droplet contains not more than one catalyst particle. The method to achieve this, should be developed further by each polyolefin producer in a proprietary way. Local powder hydrodynamics should be such that sufficient refreshment of powder in the spraying zone occurs, in order for the catalyst to stick to the polymer surface as separate particles. The method to achieve this, is proprietary as well.
A methodology was developed for measuring the rate of sorption and de-sorption of alkanes in polyolefin powders. Two different measuring techniques were developed: chromatographic and gravimetric. The role of pore morphology for accelerating sorption rate was explored. The methodology can be used to check if diffusion limitation occurs under polymerisation conditions. Diffusion limitation results into a reduced production rate, as well as a deterioration of product quality. The methodology can also be used to select the optimum purge technology for each specific powder. In addition, important insights were obtained into the relation between the pore morphology of a powder, and the optimum purge technology. Two different methodologies for characterizing Sorption and Desorption of alkanes in polyolefin powders were developed, and the results were compared. These two methodologies were based on chromatographic regarding gravimetric measurements. A good agreement was found between the results of these two methodologies. Typical sorption rates of alkanes in polyolefin powders may vary widely, but the powders were tested often in the order of 1 minute. The occurrence of any diffusion limitation during polymerisation inside the powder particles depends on the ratio of:1. Time scale of sorption 2. Amount of monomer being polymerised during sorption time 3. Amount of monomer adsorbed at equilibrium conditions. If 2 exceeds 3 , diffusion limitation occurs, which is in practice very undesirable. The pore morphology of different powders was studied, and its accelerating effect on Sorption and Desorption rate was assessed. These novel insights will enable the industries to achieve a quantitative relation between, on the one hand, the pore morphology and, on the other hand, the optimum purge technology and the occurrence of diffusion limitation during polymerisation.