Fischer-Tropsch synthesis: Attempt to tune FTS and WGS by alkali promoting of iron catalysts

The effect of water on the performance of alkali-promoted, precipitated Fe catalysts was investigated during Fischer-Tropsch (FT) synthesis in a continuously stirred tank reactor. In these studies, the added water (5-25 mol%) replaced an equivalent amount of inert gas so that all other reaction conditions remained the same before, during and after water addition. The externally added water had a positive effect on the CO conversion for all the alkali-promoted, precipitated iron catalysts. The impact of the added water is to increase the rate of the water-gas shift (WGS) reaction with up to 10-15 mol% added water, and then leveling off for further increases in the amount of water added to the feed gas. Increasing the water amount in the feed gas caused the fraction of CO converted into hydrocarbons to decrease and both the WGS activity and the oxygenate selectivity to increase. Common for all the alkali promoted iron catalysts was a decrease in C₅₊ selectivity and an increase in the methane selectivity with increasing amounts of water up to 10-15 mol%; with further increases of water, the C₅₊ selectivity increases and methane selectivity decreases. Methane selectivity is directly correlated to the CO conversion and H₂/CO ratio inside the reactor.