Super-Critical Fluids as Reaction Media for Fischer Tropsch Synthesis
Advancing the chemical engineering fundamentals
Chemical Reaction Engineering: Advanced Concepts (T2-2b)
Keywords: Fischer Tropsch Synthesis, Supercritical Fluids, Catalysis
Fischer Tropsch Synthesis (FTS) is a mature technology that utilizes gasified non-petroleum carbonaceous feedstocks (coal, natural gas) to meet the demand for liquid transportation fuels and chemicals. Fischer Tropsch Synthesis has received renewed interest in recent years for several reasons including both economic (the price and availability of crude oil) and environmental (the drive to utilize renewable energy resources such as biomass). FTS converts syngas (CO and H2) to a spectrum of hydrocarbon products (alkanes, alkenes, oxygenates, and branched compounds) with various chain lengths (C1 to C30+) through surface catalyzed polymerization reactions. As the reactants are gasses and a large fraction of the product stream is liquid, the ideal reaction media would be able to readily transport gasses to the catalyst active sites and liquid products from the active site to the bulk reaction media. As the reaction is strongly exothermic, the media would also be able to efficiently dissipate the produced heat to avoid catalyst deactivation and unwanted methane formation.
The process was originally carried out in the gas phase with a simple fixed bed reactor design. With a gaseous media, there is excellent transport of reactants the catalyst active sites, resulting high activity. However, the low density of the media results in poor extraction and transport of liquid products from the active site to the bulk media and poor heat dissipation from the catalyst active site. This results in poor activity maintenance and high methane selectivity. To counter this, liquid phase techniques were developed for FTS, utilizing slurry phase reactors where the reactant gasses are bubbled through a heavy hydrocarbon liquid media. This media, being an excellent solvent for liquid hydrocarbons, aleviating buildup of liquid products on the active sites allowing for improved activity maintenance. Moreover, the high media density also provides for efficient heat dissipation, resulting in lower methane selectivity. While liquid phase FTS addresses the methane selectivity and activity maintenance issues in gas phase FTS, it does so at the expense of losing the simplicity of the gas phase, fixed bed reactor design couple with lowering the catalyst activity due to decreased reactant transport rates.
Supercritical fluids (SCF) offer properties intermediate between gasses and liquids and show promise for FTS, being miscible with gasses, having intermediate diffusivities, being good solvents for liquids and solids, and providing for rapid heat dissipation. This paper presents the use of supercritical hexane as a reaction media for Fischer Tropsch Synthesis over conventional cobalt catalysts (e.g. 15% Co on alumina, 15% Co on silica). The influence of temperature (210oC 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 (e.g. supercritical hexane) on conversion and product distribution was studied.
Supercritical phase Fischer Tropsch synthesis exhibits interesting behavior relative to gas phase operation, including comparable conversion with excellent activity maintenance, decreased methane selectivity, increased olefin selectivity, and higher product selectivity in the middle distillate fractions (gasoline and diesel). Deviations from the standard ASF product distribution have been observed in the near-critical region, with the degree of deviation being highly dependent upon the operating conditions (i.e. temperature, pressure, density, phase behavior). It was also observed that the use of supercritical hexane enhances the extraction of heavy products from the catalyst, restoring some catalytic activity lost during gas phase operation. Surface characterization studies indicate that the crystalline structure of the catalyst is less affected by the reaction when performed in the supercritical phase than the gas phase. Supercritical FTS shows a shift from gas phase FTS in terms of oxygen removal, favoring H2O over CO2 far more than gas phase operation.
Presented Monday 17, 16:00 to 16:20, in session Chemical Reaction Engineering: Advanced Concepts (T2-2b).
