The catalyst material in the commercial FTS fixed bed reactors is called a pellet, which is 1 to 6 mm in size and contains macro, meso and micro pores. These pores are generally assumed to be filled with liquid hydrocarbon products in conventional gas phase FTS process or with supercritical solvent and hydrocarbon products in supercritical fluid FTS (SCF-FTS). As a result of pore filling, transport of reactants and products to and from the metal surfaces becomes slow and is sometimes the rate controlling step [1]. In such a situation, the heavy molecules diffuse at slower rate compared to lighter molecules. This difference in the transport properties alters the reactant and product concentrations near the active sites and creates spatial variations in the H2/CO ratio in the catalyst pellet; it also affects both the FTS conversion and product selectivity [2,3,4]. Therefore, it is very important to understand and to develop an appropriate method to estimate the actual pellet behavior for detailed reactor design. In general the supercritical fluid region is very wide whereby the reaction temperature and pressure are the only two variables to manipulate the reaction environment from liquid-like properties (i.e. diffusivities) to gas-like ones. In the process of tuning transport properties, there is a possibility that the reaction moves from a kinetic controlled regime to diffusion controlled regime. Therefore, analyzing only the variations in transport properties (diffusivity, density and viscosity) with respect to operating conditions is not sufficient for optimal reactor design. The relative change in the diffusion rate with respect to the rate of reaction has to be determined to identify the optimal operating regime [3].
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