Kinetics liquid slurry polymerization

This raises the question of whether diffusion plays a role in the kinetics of slurry polymerization. Certainly there is no limitation across the gas-liquid interface doubling the catalyst also doubles the polymer yield, but increasing the stirring rate does nothing. Diffusion through the polymer particle is a more troubling issue. There are times when the polymerization clearly becomes diffusion limited, or fouled, due to solvation of the polymer, but this is rarely a problem if the temperature is kept down and the molecular weight up. 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Kinetics and Phase Behavior - Table IV represents a simplified picture of the situation however, some polymerizations go through several phase changes in the course of the reaction. For example, in the bulk polymerization of PVC, the reaction medium begins as a low viscosity liquid, progresses to a slurry (the PVC polymer, which is insoluble in the monomer, precipitates), becomes a paste as the monomer disappears and finishes as a solid powder. As might be expected, modelling the kinetics of the reaction in such a situation is not a simple exercise. 

In a kinetic investigation of the catalytic liquid-phase phenol oxidation carried out in a semibatch slurry reactor [6], it has been found that homogeneous stepwise polymerization reactions are enhanced in the bulk liquid-phase due to the high liquid-to-solid volumetric ratio. The rate of phenol disappearance has been expressed on the basis of power-law kinetics as a sum of heterogeneous and homogeneous (polymerization) contributions, thus... 

For gas-phase or liquid propylene bulk reactors, the bulk monomer concentration in the reactor must be converted to concentration in the polymer phase surrounding the active sites with a thermodynamic relationship. Generally, a simple partition coefficient such as the one used in Equation 2.136a is used. For diluent slurry reactors, where the monomer is introduced in the gas phase, a partition coefficient such as Herny s law constant must also be used to calculate the concentration of monomer in the diluent which, in turn, is used to estimate the concentration of monomer in the polymer phase surrounding the active sites. Evidently, more sophisticated thermodynamic relationships relating the concentration of monomer in the gas phase, diluent and polymer can be used but, from a practical point of view, are only justified when the polymerization kinetic constants are very well known. Similar considerations apply to calculate the concentrations of comonomers, hydrogen and any other reactant in the system. 

Polymerization of ethylene is quite exothermic (3.4 x 10 J/kg) and since the heat capacity of gas is much lower than that of liquid, removal of the heat of polymerization can be problematic compared to solution and slurry processes. This was usually accomplished by lowering the activity of gas-phase catalysts by say 50-75% to reduce the rate of local heat generated. To compensate, the residence time was then extended to several hours. As a result of these differences, gas-phase processes tend to have a much larger polymer inventory in the reactor. The gas-phase approach is also more rigid in its catalyst requirements. The kinetic profile of a catalyst for a gas-phase process should preferably have a steady activity lasting 2-3 h. The particle size for consistent fluidization is also sometimes important, and smaller particles are preferred for heat removal. 

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