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An Investigation into Supercritical Fluids as a Reaction Media for Fischer Tropsch Synthesis

J. Ed Durham1, Nimir. O. Elbashir2, and Christopher B. Roberts1. (1) Department of Chemical Engineering, Auburn University, 230 Ross Hall, Auburn, AL 36849, (2) Engelhard Corp., 101 Wood Avenue, Iselin, NJ 08830

Fischer Tropsch Synthesis is traditionally performed in either gas phase or liquid phase reaction media. The gas phase media provides for excellent transport of the gaseous reactants to the catalyst active sites. However, the low media density results in poor transport of heavy hydrocarbon products away from the active sites, resulting in physical deactivation of the catalyst. The low density coupled with the highly exothermic nature of the reaction produces hot spots within the catalyst bed contributing to high methane and other light hydrocarbon selectivity. Conversely, the high density of the liquid phase reaction media alleviates the product transport and heat dissipation problems, but the reduced solubility of syngas in the liquid media leads to severely limited transport of reactants to the active site. Consequently, the liquid phase reaction media gives lower reaction rates than the gas phase. Use of a supercritical phase reaction media in FTS couples the high reactivity of the gas phase with the low methane selectivity and high activity maintenance of the liquid phase.

The influence of temperature (210o C to 260oC), pressure (20 bar to 80 bar), H2:CO ratio (0.5 to 2.1), syngas rate (50 SCCM/g to 150 SCCM/g), and supercritical fluid media (pentanes and hexanes) on conversion and product distribution was studied. The supercritical fluid media was shown to offer comparable conversion to the gas phase, higher selectivity in the middle distillate fraction (gasoline and diesel), and lower methane and other light hydrocarbon selectivity. To explain this increase in selectivity an enhanced olefin reincorporation model has been proposed.

The influence of catalyst surface characteristics on the FTS reaction has also been studied. Three cobalt catalysts were investigated: 15% cobalt on high surface area silica, 15% cobalt on low surface area silica, and 15% cobalt on alumina. The cobalt on silica catalysts were synthesized in our laboratory while the cobalt on alumina catalyst was purchased. Each was subjected to a set of surface characterization tests (nitrogen surface area, room temperature X-ray diffraction, low temperature (5 K) electron magnetic resonance, and temperature and magnetic field variation of the magnetization both before and after being used in gas phase and supercritical hexane FTS. The three cobalt catalysts performed differently in the gas phase, but nearly identically in the supercritical phase. These results indicate that the supercritical hexane reaction media alleviates intra-particle transport limitations.