(55f) Advancement in Iron-Based Low Temperature Fischer-Tropsch Synthesis with Integrated Product Upgrading Via Utilization of Supercritical Fluid Reaction Media

Fischer-Tropsch (FT) synthesis consists of a set of surface catalyzed polymerization reactions that convert synthesis gas (a mixture of CO and H₂ that can be derived from various carbonaceous feedstocks) into various hydrocarbons that can be further used in the production of liquid fuels and high value-added chemicals. FT synthesis inherently yields a high selectivity towards methane and a broad product distribution of gaseous light products, naphtha (C₅-C₉), middle distillates (C₁₂-C₂₂), heavy wax (C₂₂+), as well as CO₂ and H₂O. Subsequent product upgrading via catalytic oligomerization (O) can be employed to convert light FT olefins into middle distillate range hydrocarbons, while the heavy waxes can be cracked (C) into fuel range products with enhanced branching through successive hydrocracking/isomerization reactions.

In an effort to direct the FT product distribution towards transportation fuel range products (C₅-C₂₂), we have conducted a series of experiments where we have sequentially incorporated catalytic oligomerization and hydrocracking/isomerization stages subsequent to FT synthesis in a single pass operation. A multiple fixed bed reactor system was designed and utilized in this investigation. Different reaction temperatures were maintained in each bed to provide optimal reaction performance, while the system pressure was kept uniform in all three reaction beds.  The effect of each upgrading bed on the product distribution was investigated by combining the FT bed with the oligomerization bed (FTO) and combining the FT bed with the hydrocracking bed (FTC), as well as the combination of the FT bed with both sequential oligomerization and hydrocracking/isomerization stages (FTOC). The effects of reaction parameters, such as operation temperature, pressure and syngas flow rate, were investigated as well. Our experimental results indicate that the FT synthesis process with product upgrading results in a reduction in olefin content, generation of aromatics, suppression of heavy hydrocarbon (C₂₂₊) selectivity and an enhancement of branched hydrocarbon production.

Furthermore, we have utilized a supercritical hexanes reaction medium in the FT stage and the subsequent upgrading stages due to its superior heat transfer properties. This resulted in a reduced selectivity towards CH₄ and CO₂ compared to gas phase operation. Moreover, the use of a supercritical solvent provides enhanced product solubility in the bulk supercritical media, thus enhancing the product extraction rate from the catalyst pores relative to gas phase operation. As a result of this enhanced extraction, we observed various intermediates which are not noticeably present in the products stream under traditional gas phase operation. Enhancement of the carbon chain growth probability (α) was also observed under the supercritical phase operation. Fresh catalysts were characterized and compared with used catalysts for both gas phase operation and supercritical phase operation. In particular, this study involves a systematic investigation into the influence of the supercritical fluid reaction medium on the overall product distribution and selectivity towards fuel range products.