Measurement of Pore Diffusivity in NiMo/Al₂O₃ Catalyst Pellets by the Zero-Length Column Method

Abstract

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 εD 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.

Diffusivities of Hydrocarbons in the Fresh and Aged Hydrotreating Catalysts Studied by Zero-Length Column Method

“…To verify the rate-limiting step of desorption processes, the ZLC experiment is performed on the fresh catalyst with four different flow rates (30, 60, 90, 120 mL/min) at 50 °C. The desorption curves shown in Figure S3 in the Supporting Information reveal that the diffusional time constants are almost unchanged as the purge flow rate exceeds 60 mL/min, in good agreement with our previous study . Therefore, the purge flow rate of 120 mL/min (STP) is chosen in the subsequent ZLC measurements to ensure a desorption process of kinetic control.…”

“…Diffusion studies were performed in the ZLC experimental apparatus, as shown in Figure . Details of the experimental procedure are described in our previous study . In addition, a brief summary of the ZLC experiments is given below.…”

“…Therefore, the value of L and D / R 2 can be determined by the nonlinear fitting of c / c 0 and t . The detailed procedures of the data analysis using the model expansion method have been given previously . Diffusional time constants of alumina-supported catalyst particles and crystalline zeolites can be measured in the same model.…”

“…The zero-length column (ZLC) technique was initially developed for the measurement of intracrystalline diffusivity, which shows potential as a rapid and efficient method to characterize diffusion properties of hydrocarbons and other simple molecules in microporous materials. − Recently, the ZLC method was extended to the diffusion study of NiMo/Al 2 O 3 catalyst in our previous study . Based on the modified mathematical model, the diffusivities of benzene in four NiMo/Al 2 O 3 catalysts with different pore size distributions were determined, which indicated that additional macropore in the catalyst pellets enhanced the pore diffusivity by shortening the diffusional path length.…”

Core‐shell structured HZSM‐5@mesoSiO2 catalysts with tunable shell thickness for efficient n‐butane catalytic cracking

“…The apparatus and procedures of ZLC experiment was described in detail elsewhere. 24 And a brief summary of ZLC experiment is given below. A short sample layer was packed between two sintered discs within a 1/8 in.…”