(95c) Middle Distillates Production Via Fischer Tropsch Synthesis with Product Upgrading Under Supercritical Phase Conditions

Fischer-Tropsch (FT) synthesis involves surface catalyzed polymerization reactions that inherently yield a high selectivity towards methane and a broad product distribution of gaseous light products, middle distillate hydrocarbons (olefins + paraffins), and heavy wax, as well as CO₂ and water. Subsequent product upgrading via catalytic oligomerization and hydrocracking/isomerization can be employed to convert light olefin FT hydrocarbons into middle distillate range liquids, while also converting the long-chain hydrocarbons of heavy wax into fuel range products. This results in a reduction in olefin selectivity, generation of aromatics, suppression of heavy hydrocarbons selectivity and an enhancement of cold-flow properties.

In an effort to further direct the FT product distribution towards middle distillate transportation fuel range products, we have undertaken a series of experiments where we have sequentially incorporated catalytic oligomerization and hydrocracking/isomerization stages subsequent to FT in single pass operation. A multiple fixed bed reactor system was designed and utilized in this investigation. A precipitated iron-based FT catalyst was applied due to its low price and high tendency to yield olefins, which can be upgraded in the oligomerization, hydrogenation and isomerization reactions within the sequential reaction beds. The reaction temperatures were maintained differently in each bed to provide optimal reaction performance, while the system pressure was kept uniform in all three reaction beds. 

Moreover, we have utilized the enhanced heat transfer properties of a supercritical fluid reaction medium in the FT and subsequent upgrading stages in order to reduce the selectivity towards CH₄ and CO₂. The use of the supercritical solvent provides prolonged catalyst life and activity maintenance. Distinguishable enhancement of propagation probability was observed under the 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.