Effect of the thermal conductivity and catalyst layer thickness on the Fischer-Tropsch synthesis selectivity using structured catalysts

Abstract

Metallic monoliths composed of flat and corrugated foils were prepared in-house using two different alloys (Fecralloy® and aluminum) and cell densities (280 and 2300cpsi), and different loadings of a (20% Co–0.5% Re)/Al2O3 catalyst were deposited on their surface by washcoating, producing a wide range of average catalyst layer thicknesses. The obtained structured catalysts were tested in the Fischer-Tropsch synthesis, exhibiting increased selectivity for methane and decreased selectivity for C5+ fraction production with increasing average catalyst layer thickness. This effect was observed for all layer thicknesses in the case of aluminum monoliths with both cell densities and for Fecralloy® ones with a high cell density. Conversely, Fecralloy® monoliths with a low cell density exhibited a critical maximum catalyst layer thickness of ∼50μm, with diffusional and thermal limitations resulting in temperature and selectivity runaways at higher thicknesses. These results indicated the importance of high effective thermal conductivity of substrates for adequate temperature control in structured Fischer-Tropsch synthesis catalysts, which could be achieved by using highly conductive metallic monoliths (e.g., those made of aluminum) or increasing the cell density of less conductive alloys (e.g., Fecralloy®). Finally, the good thermal control observed for aluminum monoliths permitted to enhance the production of C5+ at higher temperatures without significantly increasing methane selectivity.