Fischer-Tropsch synthesis: product distribution, operating conditions, iron catalyst deactivation and catalyst speciation

Laboratory experiments conducted for long time on stream (TOS: 14,350h) provide information on Fischer-Tropsch synthesis (FTS) that is representative of time scales of industrial operations. Operation conditions that deliver desirable conversion and product distribution were investigated. Low gas hourly space velocity (GHSV) gave the highest conversion of 20.97% with the highest C5+ selectivity achieved was 59.77%, which was obtained at the highest GHSV level. A one-way ANOVA, followed by a post-hoc Bonferroni correction test, indicated a significant difference in response to GHSV with P(T<=t) two-tail values ranging from 1.5x10(-4) to 2.7x10(-35). The optimum condition for paraffin production is high pressure and low GHSV: in our experiments, this corresponded to 20.85bar (abs): 648h(-1). Conversely, olefins production is favored low pressure and low GHSV [1.85bar (abs): 648h(-1)]. C5+ production was favored at high GHSV (2592h(-1)) and was very sensitive to GHSV, as the sensitivity to C5+ products dropped sharply when the GHSV decreased to low values (from 1296 to 648h(-1)); furthermore, the selectivity to C5+ was found to be independent of pressure. The pressure effect on selectivity is complex and selectivity toward overall gaseous (paraffin+olefin) hydrocarbons and C5+ does not seem to be significantly affected by variations in pressure. Long TOS FTS runs are possible ca. 14,500h though product distribution trends tend to be changed. The catalyst survived long runs, though the selectivity to FTS became comparatively less favored than WGS with increasing TOS. Our findings may have useful implication for the design of a mobile small-scale biomass/waste to liquid process that would last for period similar to that of an industrial plant.