No abstract

This paper addresses the effect of catalyst particle size distributions on Fischer−Tropsch catalytic performance for cobalt catalysts promoted by rhenium on alumina or modified alumina as catalyst support. The catalyst particle size will affect diffusion of reactants as well as of products. Some key findings are that the C5+ selectivity is rather constant for particle sizes up to ca. 400 μm and that the selectivity reduction thereafter is largely explained by an increase in the methane yield as the effective H2/CO ratio increases. In the whole range of particle sizes investigated, the olefin to paraffin ratio of the light products decreases linearly as the diffusion length becomes longer. Moreover, this ratio is the same for all catalysts in spite of a large variation in selectivity to higher hydrocarbons. It follows that readsorption of olefins and continued chain growth only plays a minor role. The roles of added water and conversion level are also discussed.

Catalysts with cobalt as the Fischer-Tropsch (FT) metal and nickel and/or rhenium as promoters on alumina of different pore sizes, a stabilized alumina, silica, or titania as supports were investigated for activity, selectivity, and deactivation up to 800 h of operation in a fixed-bed reactor. From the observed selectivities, there is no indication that nickel as a promoter with a loading up to 5 wt % influences the selectivities to higher hydrocarbons for low-temperature (<250 °C) cobalt FT synthesis. Nickel is found to have a profound impact on the catalytic activity, and the start of run activity, steady-state level, and deactivation rate are influenced. The freshly reduced catalyst is nearly inactive, but the activity increases rapidly to a stable level which is significantly higher than for catalysts without nickel. It appears that nickel can substitute rhenium as a reduction and activity promoter. The steady-state activity is maintained constant for a prolonged time, but expected deactivation commences after ca. 150 h time on stream.