Utilization of Supercritical Fluids in the Fischer-tropsch Synthesis over Cobalt-Based Catalytic Systems

Abstract

Fischer-Tropsch synthesis (FTS) holds great potential for the production of ultra-clean transportation fuels, chemicals, and other hydrocarbon products through the conversion of readily available syngas (CO/H2) from abundant resources (coal, natural gas, and biomass). Utilization of supercritical phase in FTS as a medium that has superior properties (liquid-like density and heat capacity, and gas-like diffusivity) represents a new challenge to the 80-years old FTS technology. The objective of this research is to establish optimum operating conditions for FTS within the supercritical region that would maximize the production of value added chemicals and middle distillate hydrocarbons (gasoline fuel, jet fuel, and diesel fuel fractions) and at the same time minimize the production of methane and carbon monoxide. Chapters 3-5 of this dissertation are designed to examine the effects of supercritical fluid (SCF) (n-pentane or hexane) on FTS over an alumina supported cobalt catalyst in a fixed-bed-reactor. The influence of reaction conditions (such as temperature (210-260 ?C), pressure (20-80 bar), syngas feed ratio (H2/CO ratio of 0.5-2), contact time and space velocity (50-150 sccm/gcat)) on the FTS activity, selectivity, and hydrocarbon product distributions in the supercritical fluids (SCF) media was studied. Our results show that the adjustable thermophysical properties of the SCF significantly impact the FTS reaction performance and in most cases the SCF-FTS operations yield higher activity and better selectivity towards the most desired products compared to conventional gas-phase FTS operations. An excellent opportunity to maximize the production of desired fuel fractions, through a simple tuning process of the reaction environment from liquid-like properties to vapor-like properties, can be achieved in the SCF-FTS conditions as discussed in Chapter 4. An approach to understand the enhanced chain growth probability in SCF-FTS conditions is reported in Chapter 5. This phenomenon was attributed to the enhanced ?-olefins incorporation in the chain growth process. Chapter 6 covers a preliminary examination of the kinetics of the FTS reactions under high-pressure high-temperature conditions in both conventional gas-phase FTS and supercritical hexanes FTS (SCH-FTS). Our findings illustrate that the classical surface reaction kinetics model fails to predict the rates in the SCH-FTS. Our findings also show that the cobalt-based catalytic systems show excellent stability in terms of activity and selectivity as well as their structure under the SCF-FTS conditions for relatively long time-on-stream (up to 13 days). The influence of the cobalt-based catalyst characteristics on the FTS performance in both SCH-FTS and conventional gas-phase FTS is addressed in Chapters 7 and 8.