Comparative Study of an Iron Fischer-Tropsch Catalyst Performance in Stirred Tank Slurry and Fixed-Bed Reactors

Fischer-Tropsch synthesis (FTS) was studied on a precipitated iron catalyst in a fixed-bed reactor under both "supercritical" (propane or n-hexane as supercritical fluids; P = 4.1-7.0 MPa) and conventional operating conditions (P = 1.48 MPa), as well as in a stirred tank slurry reactor (STSR) under conventional conditions. Catalyst activity in the supercritical mode of operation was slightly higher than during conventional FTS in the fixed-bed reactor (FBR). This is attributed to higher diffusivities of reactants in supercritical fluids relative to the conventional mode of operation (pores filled with liquid hydrocarbon wax). Catalyst activity was significantly lower in the STSR than in the FBR, due to incomplete reduction of the catalyst in the STSR. Hydrocarbon selectivities in the FBR were similar in both modes of operation. Methane selectivity in the STSR was lower and C5+ selectivity higher than in the FBR, due to better temperature control in the former and differences in the extent of mixing and 1-olefin readsorption. Total olefin selectivity of high molecular weight olefins was lower and 2-olefin selectivity was higher in the STSR than in the FBR at similar conversions. This indicates that olefin selectivity is not controlled by intraparticle diffusion. It appears that the olefin selectivity in the STSR is largely determined by the residence time of high molecular weight 1-olefins in the liquid phase, 1-olefin solubility dependence on carbon number, and adsorption/desorption phenomena at the catalyst surface.