The role of diffusion in propylene polymerization

Abstract

SynopsisThe role of monomer diffusion in the polymerization of propylene by organometallic catalysis was examined by use of mathematical models which couple the rate of diffusion through the polymer film surrounding the catalyst with the rate of surface reaction. An approximate form of a second-order, integrated rate equation was used to describe the disappearance of active sites on the surface. For the most conservative model conceivable, it was estimated that the particle size would have to be 10-100 times the size for the catalysts presently in use before diffusion time would be significant. The size of the catalyst was determined by photomicrographs and nitrogen adsorption surface areas. The surface areas for three different cat,alysts were 7, 20-21 and 35 m.2/g., respectively. The kinetic model wit,hout the diffusion term was used satisfactorily to correlate productivity data. The characteristic decline in reaction rate was examined in terms of the decay of active sites on the surface of the catalyst. The rate of decay was determined to be second order with respect to the site concentration. The kinetic model indicates that the total polymerization time for a specified productivity is the sum of the monomer diffusion time and the surface reaction time. The model derived by use of an approximate second-order decay function is unique because of the additivity of diffusion and reaction times, which is not the case when the second-order function is used rigorously.

On the size of the primary particles in ziegler catalysts

“…Furthermore, as estimated by the method of Salame (8), the rates of diffusion of the monomers through the corresponding polymers under the ambient circumstances were nearly equal. This suggests that in propylene polymerization, it is not the diffusion through the polymer, but the reaction rate at the catalyst surface that constitutes the rate-determining step as was also found by Begley (9). As under these conditions, there will be sufficient supply of monomer throughout the catalyst conglomerate, one may expect all the primary particles to grow polymer spheres at the same rate.…”

Prediction of Molecular Weight Distributions for High-Density Polyolefins

“…Monomer diffusion through the polymer shell to active sites on the TiC13 catalyst is the central theme of these models. Begley (1966) proposes such a model and concludes that diffusion is not a factor. The monomer gradient across the polymer shell is calculated analytically after invoking the steady-state assumption for the diffusion equation.…”

Advances in Ziegler‐Natta polymerization ‐ unique polyolefin copolymers, alloys and blends made directly in the reactor

“…A detailed knowledge and understanding of these complex processes is essential in order to rationalize the polymerization behavior of the catalytic systems, and fully utilize their potential for novel polyolefin production (15)(16)(17)(18)(19).…”