The simplicity of a fixed bed system is attractive, especially for small-scale systems (which utilizing biomass will often necessitate). However, the inability of the gas phase media to effectively remove the liquid products from the catalyst, dissipate produced heat, and inhibit methane production results in a design with a large number of tubes and high pressure drop while giving sub-optimal product distribution. The unique properties of supercritical fluid reaction media (miscibility with gasses coupled with liquid-like density) allow fixed bed reactor designs to be utilized while also providing for excellent product extraction and heat dissipation from the catalyst. The result is a product distribution similar to liquid phase (slurry) FT with the design simplicity of gas phase (fixed bed) FT.
It has been demonstrated that Supercritical Fischer Tropsch (SC-FTS) offers better activity maintenance than Gas-Phase Fischer Tropsch Synthesis (GP-FTS) over a time scale on the order of a week1. This prompted the question of whether a supercritical medium (pure hexanes or SC-FTS) can restore the loss in activity and changes in selectivity seen during GP-FTS operation.
To that end, a series of Fischer Tropsch experiments were performed, alternating between 2 days at GP-FTS conditions and a 1-day washing period using either supercritical hexanes or SC-FTS. This study utilized an industrial cobalt on alumina FT catalyst at 200psi syngas partial pressure and a reaction temperature of 240oC with a total pressure during the washing stages (including SC-FTS) of 900psi.
The results of this investigation demonstrate that the supercritical media can restore some of the lost activity and selectivity during the washing stages. Also, many of the advantages of SC-FTS over GP-FTS often observed (higher olefin selectivity into the middle distillate range, lower methane selectivity, and a shift in oxygen removal from carbon dioxide towards water)2,3 were also seen.
1 N.O. Elbashir, P. Dutta, A. Manivannan, M.S. Seehra, C.B. Roberts: Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer–Tropsch synthesis. Applied Catalysis A: General (2005): V 285, I 1-2, p169-180.
2 Li Fan, Kaoru Fujimoto: Fischer Tropsch Synthesis in Supercritical Fluid: Characteristics and Application. Applied Catalysis A: General (1999): V 186, p 343-354.
3 Gary Jacobs, Karuna Chaudhari, Dennis Sparks, Yongqing Zhang, Buchang Shi, Robert Spicer, Tapan K. Das, Jinlin Li and Burtron H. Davis: Fischer–Tropsch synthesis: supercritical conversion using a Co/Al2O3 catalyst in a fixed bed reactor. Fuel (2003): V82, I 10, p 1251-1260.