TRANSPORT-ENHANCED ALPHA-OLEFIN READSORPTION PATHWAYS IN RU-CATALYZED HYDROCARBON SYNTHESIS

Residence time and. cofeed studies show that olefins and paraffins are primary products in Rucatalyzed hydrocarbon synthesis. Olefins readsorb and initiate surface chains that are indistinguishable from those formed directly from CO/H2 and that continue to grow and ultimately desorb as higher molecular weight hydrocarbons. Transport-enhanced α-olefin readsorption leads to an increase in chain growth probability (a) and in paraffin content with increasing pore and bed residence time. Deviations from conventional (Flory) polymerization kinetics and the increasing paraffinic content of higher hydrocarbons are quantitatively described by transport effects on the residence time of intermediate olefins, without requiring the presence of several types of chain growth sites. Our transport-reaction model combines a description of diffusive and convective transport with a mechanistic kinetic model of olefin readsorption and of CO hydrogenation and chain growth. It quantitatively describes carbon number, site density, pellet size, and space velocity effects on hydrocarbon synthesis rate and product distribution. The model is consistent with the experimentally observed maximum C5+ selectivities at intermediate values of site density and pellet size. These intermediate values permit extensive readsorption of α-olefins without significant CO arrival transport limitations. © 1991.