Effect of time on stream and temperature on upgraded products from Fischer–Tropsch synthesis when zeolite is added to iron-based activated-carbon-supported catalyst

Abstract

In a continuation of previously reported work, the effect of temperature on products obtained when zeolite ZSM-5 is added to a supported iron-based Fischer–Tropsch (FT) catalyst was studied. Further, the stability of the combination was investigated for up to 240h time on stream at the optimum temperature. From 280°C to 320°C, amounts of small-chain hydrocarbons (C1–5) increase with increase in temperature because of higher FT rates. Liquid hydrocarbon production rates increase at 300°C due to an increase in the FT rate. A further increase in the temperature causes these rates to drop, to give small-chain hydrocarbon (C1–4) species. Aromatic yields are also found to reach a maximum at 300°C in the separate bed. Alcohol formation for the FT catalyst increases with increase in temperatures. The lowest temperature used, 280°C, is sufficient for the ZSM-5 to dehydrate all the alcohols formed by the FT catalyst to hydrocarbons. Further, the FT catalyst and ZSM-5 were tested for 240h time on stream (TOS) at 300°C. The catalyst shows FT stability up to 120h and a slow decline in activity up to 240h. A progressive loss of the ZSM-5 activity occurs with TOS. Deposition of coke and/or coke precursors on ZSM-5, confirmed by energy-dispersive X-ray spectroscopy, decreases the aromatic compound yield, and increases olefin selectivity with time on stream due to reduction in oligomerization reactions.