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

Diffusivities of a series of hydrocarbons with different configurations in two zeolites (ZSM-5 and SBA-15) and a hydrotreating catalyst (NiMo/Al2O3) were determined by the zero-length column technique. The results revealed that the molecular configuration of hydrocarbons has a significant effect on the diffusion property in zeolites. In addition, the aromatics show surface diffusion effect in the mesoporous SBA-15 and NiMo/Al2O3. The diffusion characteristic of 1,3,5-triisopropylbenzene (TiPB) was applied to evaluate the change of diffusion properties of a mesoporous-scale hydrotreating catalyst with 2000 h of deactivation. Arrhenius plots of the diffusional time constants show two distinct linear fitting regions for both the fresh and aged catalysts. The reduction degree in diffusivity over the aged catalyst shows an increasing tendency at low temperatures but gradually decreases at high temperatures. A correlation between pore size and diffusivities was established to reveal the influence of structure on its diffusion property.