Technology Development for Iron and Cobalt Fischer-Tropsch Catalysts

A cobalt catalyst (25%Co/γ-Al2O3, prepared by slurry phase impregnation) was used in a fixed bed reactor under a pressure/density tuned supercritical fluid mixture of n-pentane/nhexane. By using an inert gas as a balancing gas to maintain a constant pressure, the density of the supercritical fluid could be tuned near the supercritical point while maintaining constant space velocity within the reactor. The benefits of the mixture allowed for optimization of transport and solubility properties at an appropriate reaction temperature for Fischer Tropsch synthesis with a cobalt catalyst. There was an important increase in conversion due to greater accessibility to active sites after extraction of heavy wax from the catalyst, and additional benefits included decreased methane and carbon dioxide selectivities. Decreased paraffin/(olefin + paraffin) selectivities with increasing carbon number were also observed, in line with extraction of the hydrocarbons from the pore. Removal of the wax products resulted in lower residence times in the catalyst pores and, therefore, decreased probability for readsorption and reaction to the hydrogenated product. Even so, there was not an increase in the alpha value over that obtained with just the inert gas.