Diffusional restrictions in the porous network of Fischer‐Tropsch catalysts strongly affect activity and product selectivity. Especially small pores hamper the diffusion of reactants. In order to overcome the diffusion restrictions, the introduction of transport pores into the catalyst and the resulting effect on reaction rate and selectivities were studied. It was shown that transport pores allow for an increase in diffusion length, maintaining high reaction rate and C5+ selectivity. Overall, the productivity can be increased through optimization of the transport pore fraction.
Internal mass transport limitations inside Fischer-Tropsch catalysts due to the slow diffusion of reactants in the liquid-filled pores may significantly alter the selectivity and achievable productivity. In this work, diffusive restrictions for planar catalyst layers were investigated by mathematical modeling and simulation. A one-dimensional model utilizing empirical kinetics, incorporating transport pores as an additional pathway for mass transport and taking into account heat production, allows for calculation of catalyst efficiency and productivity towards C 5+ products. As diffusional mass transport leads to strong concentration gradients that impair selectivity, an optimum layer thickness with maximum C 5+ productivity can be found. Additional transport pores enhance the mass transport but reduce the amount of active phase, which requires a trade-off by optimizing the fraction of transport pores and layer thickness. For reference conditions, the catalyst layer with an ideal amount of transport pores and ideal thickness exhibits a productivity that is about 47% higher than that for the best layer without transport pores. This improvement requires transport pores with diameters not larger than about 60 μm. While the improvement potential significantly depends on the effective diffusivities, the effect of heat generation was found to be negligible.
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