Reaction of C2H4 under Lower Temperature Fischer-Tropsch Conditions on a TiO2 Supported Cobalt Catalyst with Co-feeding of H2 and Syngas
Xiaojun Lu*, Diane Hildebrandt, Yusheng Zhang, and Xinying Liu
Material and Process Synthesis; College of Science, Engineering, and Technology, University of South Africa
Co-feeding of C2H4 to the Fischer-Tropsch reaction system has been studied some years back to investigate the effect of secondary reactions of the olefins in the Fischer-Tropsch reaction. Some important phenomena were observed and the conclusions were not consistent due to the complexity of the Fischer-Tropsch reaction. Researchers often found that it is very difficult to investigate certain aspects independently in such complex reaction system. This work tries to investigate one aspect at a time in the complex system. Three groups of experiments were designed. The first group of experiments was designed to investigate the reaction of C2H4 only on the supported Co catalyst. The purpose was to investigate the possible secondary reactions of C2H4 without other reactants. The second experimental design was to add H2 into the reaction system when keeping the feed of C2H4. This was to investigate the reactions of C2H4 with the presence of H2. The C2H4 to H2 ratio was adjusted in a wide range so that excessive amount of both of these two reactants could be achieved. The third experimental design was to co-feed synthesis gas (H2/CO/N2) when keeping the feed of C2H4. The amount of syngas fed into the reaction system was controlled to ensure that the main feed was still C2H4. This is different from the feed ratio chose by other researcher in the past as the amounts of C2H4 co-fed by them was always far less than the syngas feed.
The results we have obtained in these experiments are of big interests. C2H4 was found not to react under the conditions applied when no other feed was introduced into the reactor; while it reacts quickly when H2 was co-fed. The reaction temperature, when H2 was co-fed, could be as low as 100oC and only products of C1 to C5 were found. These results suggest that C2H4 can undertake several reaction pathways to form different products. Fischer-Tropsch synthesis reaction was found to occur, with the presence of C2H4, when the operating temperature was at 140oC, which is much lower than the temperature (higher than 180oC with Co as the catalyst) required for normal Fischer-Tropsch synthesis. The reaction temperature required for reactions to happen changed to around 190oC when the feed of C2H4 was stopped. The presence of C2H4 can promote the Fischer-Tropsch synthesis by lowering the reaction temperature. This has not been reported in any existing literature. The products distribution is quite different when both C2H4 and syngas were fed and this leads to the understanding of the reaction mechanisms for the Fischer-Tropsch Synthesis.
Keywords: Fischer-Tropsch Synthesis, C2H4 reaction, Reaction Mechanism
See more of this Group/Topical: Catalysis and Reaction Engineering Division