Effective diffusivity under inert and reaction conditions

Abstract

Effective diffusivity in a commercial catalyst of silica—alumina has been determined. The measurements have been carried out using the chromatographic technique in a packed column (under inert conditions) and two different reacting systems (under reaction conditions) using the cyclohexanol dehydration and the 1-butene isomerization reactions. The experiments under inert conditions have been carried out at 298 K, using different catalyst particle sizes and employing nitrogen and argon as tracers into a helium carrier stream. The study under reaction conditions has been made in a differential fixed-bed reactor at 573 K in the case of cyclohexanol dehydration and in a BSTR at 523 K for the 1-butene isomerization, also changing the catalyst particle size. Both measurement techniques, under inert and reaction conditions, have also been carried out with catalyst partially deactivated by coke deposition, with a maximum coke content of 4.5% (w/w) for which the catalyst activity is around 0.1. Different porous structural models have considered obtaining the best results when the experimental pore size distribution was taken into account. The results given by both techniques, under inert and reaction conditions, are comparable using the cross-linking pore model. The structural parameter (tortuosity factor) values obtained are similar, but some different, being smaller under inert conditions (τ = 4.5), and greater under reaction conditions (τ = 5.7, for cyclohexanol dehydration and τ = 6.5, for 1 − butene isomerization). These discrepancies can be due to the different role played by the microporosity in the different cases, under different conditions. Coke content and catalyst deactivation do not affect the D e values, which must be due to a monolayer coke deposition and a deactivation by active site coverage.