Exploiting the effects of mass transfer to boost the performances of Co/γ-Al2O3 eggshell catalysts for the Fischer–Tropsch synthesis

• The FTS performances of a small Co/γ-Al2O3 eggshell catalyst are investigated.
• Eggshell reactivity is compared to that of a powdered catalyst.
• Eggshell catalyst outperforms the powdered catalyst in terms of C15+ yield.
• Higher yield depends on the presence of weak mass transport restrictions.
• Eggshell catalyst prevents strong transport limitations, granting low ΔP.

In this work, the performances of a spherical Co/γ-Al2O3 eggshell catalyst characterized by a diameter of 600 μm and an active shell region 75 μm thick are assessed in a lab-scale fixed bed reactor operated at industrially relevant Fischer–Tropsch process conditions. Such performances are compared to those of a powder catalyst obtained by grinding and sieving a fraction of the eggshell catalyst so to obtain powders with a particle size distribution centered in the range 75–100 μm. It is shown that, thanks to the presence of modest mass transport restrictions, the eggshell catalyst outperforms the corresponding powder in terms of C15+ specific yield. We explain these results by considering the effect of mass transport on the olefin re-adsorption probability and on the H2/CO ratio in proximity to the active sites. The former is increased, and results in greater selectivity to heavy hydrocarbons. The latter is also increased, due to the negative order of CO conversion kinetics with respect to CO, and results in augmented reaction rate.

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