Enhanced Incorporation of (alpha)-Olefins in the Fischer-Tropsch Synthesis Chain-Growth Process over an Alumina-Supported Cobalt Catalyst in Near-Critical and Supercritical Hexane Media

Operating Fischer-Tropsch synthesis (FTS) in supercritical fluid (SCF) media offers advantages over conventional gas-phase FTS operation including in situ extraction of heavy hydrocarbons from the catalyst pores coupled with enhanced incorporation of (alpha)-olefins in the chain growth process. In this study, hydrocarbon product distributions in near-critical and supercritical hexane phase FTS (SCH-FTS) was studied over a 15% Co/Al2O3 in a high-pressure fixed-bed reactor system. The critical point of the hexane-syngas-products reaction mixture as collected from the reactor outlet was measured using a variable-volume view cell apparatus. All reactions were carried out in near-critical and supercritical regimes by tuning either the reaction temperature (230-260 (degree)C) or the reaction pressure (30-80 bar). Deviations from the standard AndersonSchultz-Flory (ASF) chain growth model were observed in most cases; however, the degree of deviation depends on the reaction conditions within the near-critical and supercritical regions and varies from gaslike density to liquidlike density within the supercritical region. As an attempt to understand this phenomenon, a modification to the chain growth model in SCHFTS, is presented. The model attributes the deviation from the standard ASF model to enhanced (alpha)-olefin incorporation within the middle distillate hydrocarbon products due to enhanced adsorption/desorption dynamic in the SCF medium. The proposed enhanced olefin incorporation model was found to be in a good agreement with our experimental results.