Impact of process variables on the selectivity to higher hydrocarbons in Fischer-Tropsch synthesis (FTS) on Co catalysts are summarized and discussed. Discussion is based on mechanistic understanding, in particular kinetics based on hydrogen assisted Co dissociation, the linear relationship between CH4 and C5+ and surface coverages based on SSITKA data.The most striking observation is the negative correlation between selectivity to C5+ hydrocarbons and the hydrogenation activity. Increasing the H2/CO ratio, introducing pore diffusion restrictions, higher temperatures and hydrogen activation promoters all results in a lower selectivity to higher hydrocarbons. There is a clear positive correlation between C5+ and the partial pressure of water either added or produced by the FTS reaction. There seems to be a common pool of monomer units on the closed packed cobalt surfaces that is responsible for generation of all hydrocarbons in FTS independent of chain length and including methane and ethylene. Process conditions, catalyst formulations and gas feed composition that suppress hydrogen coverage favor generation of long hydrocarbon chains. It is particular challenging that the nature of the support apparently strongly afftects the pool of reaction intermediates on cobalt surfaces.

Selectivity of C5+ in Co-based Fischer-Tropsch synthesis

251st National Meeting (2016)

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