Supercritical Fluids for Fischer Tropsch Synthesis and Related Reactions

Abstract

Fischer Tropsch Synthesis (FTS) is a process for the hydrogenation of carbon monoxide. FTS can be used to synthesize hydrocarbon and oxygenated hydrocarbon fuels and chemicals from any carbonaceous feedstock. The process dates back to Germany between WW1 and WW2 and has been continuously utilized industrially in South Africa for over 50 years. Industrially, FTS is operated in two modes: High Temperature (HTFT: 300oC – 350oC) for the production of light olefins and gasoline and Low Temperature (LTFT: 210oC – 250oC) for the production of diesel and wax. The experimental work in this dissertation will focus exclusively on LTFT. Supercritical fluids offer a number of interesting properties that can be useful in FTS. Supercritical fluids are miscible with gasses and can be excellent solvents for liquids and solids, with the Fischer Tropsch reaction requiring both the transport of the gaseous reactants to the active sites and the transport of the liquid products away from the active sites. Supercritical fluids, having liquid-like densities, have high thermal mass, giving them an enhanced capacity for supplying and dissipating heat. The Fischer Tropsch reaction is highly exothermic, requiring rapid heat removal. FTS utilizing a supercritical medium (SC-FTS) has been shown to provide a number of catalytic performance advantages (such as improved selectivity and activity maintenance) and is believed to offer a number of process advantages as well. Of particular interest in this work is the opportunity that supercritical fluids offer in terms of studying reaction fundamentals by suppressing thermal gradients and stabilizing primary products. The objective of this work is to (1) identify and demonstrate the benefits of SC-FTS operation, (2) use the supercritical reaction media to probe the fundamentals of the FTS reaction, and (3) identify a SC-FTS reactor design to improve the economics of industrial SC-FTS utilization. In the second chapter (the first experimental chapter) of this dissertation, efforts to close the material balance for SC-FTS on a cobalt catalyst will be discussed. In the third chapter, work in which a supercritical environment was used to successfully reactivate a cobalt catalyst partially deactivated by traditional fixed-bed FTS operation will be discussed. The fourth chapter will focus on work done on an iron-based catalyst in which SC-FTS operation gave a high selectivity to heavy liquid methyl ketones and aldehydes, with the aldehydes being shown to be one of the primary products for FTS. Secondary reactions appeared to convert these aldehydes to the corresponding olefin. Appreciable amounts of aldehydes and methyl-ketones were not seen in traditional FTS operation (fixed-bed and slurry-phase) using the same catalysts at comparable conditions. The fifth chapter will focus on the use of a novel reactor design principle for SC-FTS.