Addition of Zeolite ZSM-5 to an Iron-based Fischer-Tropsch Catalyst supported on Activated Carbon: Effect of Reactor Conditions.

摘要

An iron-based catalyst containing 6 wt% Mo, 16 wt% Fe, 0.8 wt% Cu, and 0.9 wt% K supported on Norit SX Ultra activated carbon is used as a FTS catalyst. Zeolite ZSM-5 (of medium acidity and Si/Al = 50) is used to check its effect on FT base catalyst to form aromatic compounds and to reduce the size of long-chain hydrocarbons. The influence of ZSM-5 amounts, bed arrangements, and the temperature on the product distribution were investigated.;The performance of ZSM-5 along with the base catalyst was analyzed in two catalyst bed configurations: separate-bed and mixed-bed. Both separate-bed and mixed-bed catalyst arrangements show conversion of alcohols to hydrocarbons and water. The separate-bed arrangement gives higher conversions of CO and H2, and higher yields of liquid hydrocarbons. The decline in FT activity in the mixed bed is probably due to lowering of the alkali content of the iron catalyst because of alkali migration from FT catalyst to ZSM-5, as confirmed by energy-dispersive X-ray spectroscopy of the spent ZSM-5.;Product profiles were analyzed for changing ZSM-5 weight from 50 to 200% of the FTS amount in the separate bed and the mixed bed catalyst arrangements. Oligomerization, aromatization, isomerization, and cracking reactions are enhanced with increasing ZSM-5 weight in both the catalyst arrangements. The base catalyst shows no selectivity towards aromatic compounds, but addition of ZSM-5 in both arrangements forms C7-10 aromatic compounds. Long-chain hydrocarbon molecules (C13+) formed by the base catalyst are effectively cracked, and isomerized on the ZSM-5 catalyst surface in both type of catalyst arrangements.;Product profiles were analyzed for change in the temperature from 280 to 320°C in the base catalyst as well as in the separate-bed catalyst arrangement. Amounts of small-chain hydrocarbons (C1-5) increase with increase in temperature because of higher FTS rates. Liquid hydrocarbon production rate (in both the base catalyst and the separate bed) is increased at 300°C due to increase in FTS rate. Further increase in the temperature causes these rates to drop to give small-chain hydrocarbon (C1-4) species. Aromatic yields are found to reach a maximum at 300°C in the separate bed. The alcohol rate for the base catalyst increases with increase in temperatures probably because of higher FTS rates. 280°C is sufficient to convert all the alcohols formed by the base catalyst in the separate bed.;The base catalyst and ZSM-5 together were tested for 240 h time on stream (TOS) to check the stability of the combination of the catalysts in the separate-bed arrangement. The base catalyst shows stability up to 120 h and slow decline in activity up to 240 h. A progressive loss of the ZSM-5 activity occurred with TOS. Coke deposition on ZSM-5, confirmed by energy-dispersive X-ray spectroscopy, decreases aromatic compound yield, and increases olefin selectivity with time on stream due to reduction in oligomerization reactions.