Morphological influences in the gas phase polymerization of ethylene by silica supported chromium oxide catalysts

Abstract

Dramatic changes occur during the initial stages of olefin polymerization over heterogenous catalysts. As polymer accumulates, the catalyst fragments and the void space within the growing particle becomes filled with polymer. The changing monomer transport rate to the active sites, dissipation of heat and stress with the particle, and eventually, dispersion of catalyst fragments within the growing particle can control the polymerization. We focus on the changes in, and influence of, polymer, catalyst and void morphology during the nascent polymerization for silica supported chromium oxide catalysts. We find that the course of the polymerization depends on the initial stages yet is seldom considered in polymerization models. We review several years of morphological characterization that identify the important physical phenomena which occur during nascent heterogeneous olefin polymerization. ~~ ~~ ~ ~~~Des changements importants surviennent durant les stades initiaux de la polymerisation de I'olkfine sur des catalyseurs hkttrogknes. Lorsque le polymkre s'accumule, les fragments de catalyseurs et I'espace intersticiel B I'intCrieur de la particule qui grossit se remplit de polymkre. La variation de vitesse de transport de monomkre vers les sites actifs, la dissipation de la chaleur et des contraintes avec la particule, et finalement, la dispersion des fragments de catalyseurs B I'interieur des particules qui croissent contrblent la polymtrisation. Nous nous interessons en particulier aux changements dans le polymkre, le catalyseur et la morphologie du vide et B leur influence durant la polymkrisation naissante pour de la d i c e supportke sur des catalyseurs d'oxyde de chromium. Nous avons trouvt que le cours de la polymerisation dkpendait des stades initiaux bien que cela soit rarement considCrC dans les modkles de polymerisation. Nous avons examine sur plusieurs annkes la caractirisation morphologique qui difinit les phknomknes physiques qui surviennent durant la polymkrisation naissante de l'olkfine hiterogkne.

Influence of support friability and concentration of α‐olefins on gas‐phase ethylene polymerization over polymer‐supported metallocene/methylaluminoxane catalysts

“…McDaniel 9 and Dalla Lana et al 43 observed very low ethylene polymerization activities over silica-supported Phillips catalysts due to mass-transfer limitations. Webb et al 41 noted that a polymer yield of 0.1 g/g of catalyst rendered the active sites on a silica-supported chromium oxide catalyst inaccessible to ethylene. The fragmentation of the outer layer of the catalyst particles was similar to the initiation of shellwise fragmentation of the catalyst particle from surface to the center as proposed by Bonini et al 44 Once the ethylene transport hindrance by the highcrystallinity polymer in the pores of the catalyst cores occurred, manipulation of the polymerization conditions was ineffective in improving activity.…”

“…40 The polymerization rates of such catalysts are very low for want of monomer at the active sites. 41,42 The artichoke structure of the polymer around the catalyst core was probably produced by polymerization on the external surface of the catalyst particle in combination with small fragments of catalyst breaking off the external Figure 14 Proposed fracture mechanism of low-and high-F polymer-supported metallocene/MAO catalyst particles during gas-phase olefin polymerization. (HDPE, high density polyethylene; LLDPE, linear low density polyethylene).…”

Modeling of Particle Fragmentation in Heterogeneous Olefin Polymerization Reactions, 2

“…As discussed by Webb et al, [19] if the catalyst particle can resist the accumulation of mechanical stress, especially at such low reaction rates, the polymer material can clog the catalyst pores and the transport resistance can become the limiting kinetic step. This may eventually lead to depletion of monomer inside the particle and consequently to an overall decrease in the polymerization rate.…”

Packed‐bed reactor for short time gas phase olefin polymerization: Heat transfer study and reactor optimization

“…The extent of the temperature excursions is of course dependent on a number of parameters (catalyst activity and size and gas velocity). [15][16][17][18][19][20][21][22][23] It is possible to avoid (or eliminate) some of the difficulties occasionally encountered during the early stages of polymerization by choosing conditions such that the initial moments of the reaction occur at rates slower than the potential maximum. This can be done by introducing a prepolymerization step, where the catalyst fragmentation step and initial period of the polymerization occur under milder conditions than in the main reaction, 15 or by designing the catalyst so that the activity profile builds up slowly in the main reactor over the course of several tens of seconds or minutes.…”