Pore network effects under the reduction of catalyst size at elevated temperatures

Abstract

To evaluate the pore network effect due to variation in catalyst size, adsorption/desorption isotherms of N2 at 77.3K were measured for γ-Al2O3 particles of 4.0, 1.0, 0.5 and 0.04mm in diameter with ASAP 2020 by Micromeritics. It shows the desorption knee is obvious for the 4.0mm particle, while it is not found for the others. Nevertheless, the loops of isotherm hysteresis are decreasing when the particle size is reduced from 1.0 to 0.04mm. Under elevated temperatures with benzene hydrogenation as the working system, it shows the hysteresis in adsorption and reaction rate are remarkable for the 2.0 and 4.0mm particles, but is much less evident when the particle size is reduced to 1.0 and 0.5mm. With the increase of reaction temperature, the hysteresis loop tends to move to the region of high relative vapor pressures, due to the unstable vapor condensation condition in small pores. To inspect how the reaction rate is affected by the variation of catalyst condition, the internal wetting fraction and the internal effectiveness factor are measured and evaluated over a wide vapor pressure range, and an almost linear relationship is found between them. It shows the effective diffusivity of hydrogen under full vapor condensation in the catalyst is about 13 times that filled with bulk liquid, which suggests the catalyst pore can be considered being filled with vapor–liquid mixture rather than with bulk liquid.