Assessment of pore diffusion limitations for the near critical and supercritical Fischer-Tropsch synthesis
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abstract
A study on the diffusional limitations under the gas-phase, as well as in the near critical and supercritical Fischer-Tropsch Synthesis (FTS) conditions, in a fixed bed reactor that contains catalyst particles of few millimeters in diameters was carried out. A conventional theory of diffusion-reaction in porous pellets is employed. The fugacity of CO, H 2/CO ratio, the operating pressure, and the temperature have significant impact on the catalyst effectiveness factor. The CO concentration drops rapidly within the initial layers of the catalyst particle. The higher diffusivity of hydrogen creates an intense hydrogen rich environment inside catalyst pores, which can lead to undesired reactions, e.g., methanation. The catalyst particle effectiveness factor is proportional to the fugacity of CO in the bulk phase. The isothermal catalyst effectiveness factor is sensitive to operating temperature only at low pressures. The catalyst effectiveness factor value in the gas-phase FTS is higher than that of the supercritical phase FTS. After certain reactants conversion, the products start to condense and catalyst effectiveness factor drastically decreases in the gas-phase FTS. However, due to the two phase operation in supercritical fluid-FTS conditions coupled with improved diffusivities of reactants in supercritical solvent, the active sites normally expose to higher CO concentrations relative to gas phase FTS. This phenomenon increases the effectiveness factor of the catalyst particles and suppresses undesired reactions. This is an abstract of a paper presented at the 2011 AIChE Annual Meeting (Minneapolis, MN 10/16-21/2011).
