Measurement of Pore Diffusivity in NiMo/Al 2 O 3 Catalyst Pellets by the Zero-Length Column Method

The zero-length column technique was performed on four hydrotreating catalyst pellets with different pore size distributions at a temperature range from 303 to 423 K. In order to analyze the experimental curves of alumina-supported catalysts with meso/macroporous structures, normalized desorption curves were corrected with blank data, and the model expansion method was applied to process data and determine diffusivities at the middle region of the curves, in which the diffusion of the probe molecule was assumed as a desorption-diffusion process. The results showed that diffusional time constants D e / R 2 and pore diffusivities ε D p of these catalyst pellets increased in the presence of macropores. Moreover, a typical mathematical formula for restricted diffusion,, was used to obtain restricted factors (1 – λ) z and tortuosity τ by combining the pore diffusion coefficients and bulk diffusion coefficients. The fitting parameters confirmed the assumption used in the analysis of the desorption curves, which indicated that additional macropores in catalyst pellets enhanced the pore diffusivities by shortening the diffusional path length and decreasing the restricted factor.