Modeling diffusional limitations in Fischer Tropsch synthesis catalyst at near critical and supercritical solvent conditions
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Conducting Fischer-Tropsch synthesis (FTS) reaction in near critical and supercritical fluid (SCF) media has been demonstrated to have certain advantages over the traditional routes because of the unique characteristics of the supercritical phase. It combines the desirable properties of gas-like diffusion along with liquid-like heat transfer and solubility to overcome many of the limitations of the current industrial FTS reactors.1 In the process of tuning transport properties, there is a possibility that the reaction moves from a kinetic control regime to diffusion control regime. Therefore, analyzing only the variations in transport properties (diffusivity, density and viscosity) with operating conditions is not a sufficient task for optimal reactor design. The relative change in the diffusion rate with respect to the rate of reaction has to be determined for identifying the controlling regime.2 Very few studies have been reported on diffusional limitations in FTS catalysts at supercritical conditions.3 Those studies were not accounting the details of reaction kinetics, product distribution and density dependency of diffusivities. In most of the cases first order reaction kinetics were assumed, which is not a valid assumption. Even though a large number of kinetic models for FTS have been developed, these models were not used in estimating diffusional effects in FTS catalysts.
