Experimental evaluation of catalyst layers with bimodal pore structure for Fischer–Tropsch synthesis

Abstract

As catalyst pores are typically filled with liquid, long-chained hydrocarbons during low-temperature Fischer–Tropsch synthesis internal mass transport limitations can impede conversion rate and C5+ selectivity. Utilization of transport pores may improve reactant diffusion and thereby reduce these negative effects. In this work preparation, characterization and experimental testing of CoRe/γ-Al2O3 catalyst layers, with and without transport pores, of different thickness is presented. All prepared layers with thicknesses in the range from 50 to 600μm exhibit similar cobalt crystallite sizes of 8.7nm irrespective of the presence of transport pores. Experimental results showed an increase in methane selectivity from 10 to 40% and a drop of C5+ selectivity from 80 to 42% with increasing diffusion length. These negative effects could be retarded by use of transport pores. The highest CO conversion was achieved for layers of about 140μm thickness exhibiting 50–70% higher values than obtained with the thinnest layers (50–60μm). This trend holds also for layers with transport pores but use of additional pores did not result in a significant improvement of conversion and space time yield. Nonetheless, on a catalyst mass basis transport pores lead to a benefit in productivity of 25–50%.