Enhanced incorporation of α-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 degreesC) or the reaction pressure (30-80 bar). Deviations from the standard Anderson-Schultz-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 supercrifical 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 SCH-FTS, 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.