(92b) Study of the Effect of Water On Fischer-Tropsch Synthesis Over Iron Based Catalysts

Water is produced during Fischer-Tropsch synthesis and will always be present in varying quantities during reaction. Most of the water, formed as a primary product, is converted on an iron catalyst to carbon dioxide by the simultaneous occurrence of the water gas shift reaction. The present work aims at studying the effect of water on the performance of alkali (Li, and K) promoted precipitated Fe catalysts during Fischer-Tropsch synthesis (FTS) in a continuously stirred tank reactor (CSTR) at 270 °C. We also carried out a run at 230 °C for K promoted iron catalyst. In these studies, the added water (5 to 25 volume %) replaced an equivalent amount of inert gas so that all other reaction conditions remained the same before, during and after water addition.

For iron catalysts, external addition of water to the FT synthesis affected the CO and hydrogen partial pressures inside the reactor and increased the rate of the water-gas shift reaction. The externally added water had a positive effect on CO conversion for all the alkali promoted precipitated iron catalysts at 270 °C, whereas the added water decreased the CO conversion at 230 °C for the same catalyst. The rate of FT is not inhibited by the addition of water with the syngas feed. The impact of the added water is to increase the rate of the WGS reaction up to a certain extent, leveling off at 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; subsequently, water-gas shift (WGS) activity and the oxygenate selectivity increases. Common for all the catalysts was a decrease in C5+ selectivity and an increase in the methane selectivity occurs with increasing that amount of water up to 10 - 15 volume %; with further increases of water, the C5+ selectivity increases and methane selectivity decreases for all the alkali promoted catalysts.