The effect of start-up solvent on the performance of cobalt-based catalysts and potassium-promoted precipitated iron catalysts was investigated during Fischer–Tropsch (FT) synthesis by using a continuously stirred tank reactor. In this study, four different molecular weight start-up solvents were tested: Polywax-3000 ( $$\overline{MW}$$ MW ¯ (average molecular weight) = 3,000), Polywax-2000 ( $$\overline{MW}$$ MW ¯ = 2,000), Polywax-500 ( $$\overline{MW}$$ MW ¯ = 500) and C-30 oil ( $$\overline{MW}$$ MW ¯ = 420). The conversion and selectivities (methane, C5+ and CO2) were essentially the same for all the start-up solvents tested for the potassium promoted precipitated iron catalysts suggesting that there was no significant effect of startup solvent for the iron catalyst tested. However, with the cobalt catalyst, conversion varied with solvent, with the conversion increasing with decreasing molecular weight of the solvent. This is considered to be due to the particle size of the cobalt alumina catalysts being large relative to that of iron that was used. Under synthesis conditions, the iron catalyst attrits to form a measurable fraction of 1–3 micron sized particles in the lower range of the particle size distribution, whereas the alumina support retains essentially the same, larger size during synthesis. Thus, the decrease in conversion with time is likely to be a result of pore filling with solvent into the interior of the catalyst, which increases with increasing molecular weight of the start-up solvent. In obtaining conversion and aging data for FT catalysts, wax accumulation needs to be considered.
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