Equilibrium and Transient Pyridine Poisoning of a Hydrotreating Catalyst

Abstract

An investigation was conducted on the adsorption of pyridine on a NiMo/Al203 (Shell 324) hydrotreating catalyst. Equilibrium and transient adsorption data were collected at 250�C, 350�C, and 450�C in the pyridine partial pressure range of 284 N/m2-2130 N/m2 (11 wt%-30 wt%). The equilibrium adsorption isotherms demonstrated idealized Langmuir characteristics (Type I isotherm behavior). The partial pressure independent equilibrium values (at 250�C, 350�C, and 450�C) demonstrated a linear temperature dependence. No systematic trend was found between the transient adsorption data and pyridine partial pressure. However, as the adsorption temperature was decreased the rate of adsorption increased. Pyridine adsorption at 450�C demonstrated a 33% irreversibility which is attributed to coke formation. Two mathematical models were developed which accurately predicted the transient adsorption data. The first model predicted the period when reversible pyridine adsorption and irreversible coke formation were simultaneously poisoning the fresh catalyst. The second model predicted the period when coke formation had reached a steady-state level and only reversible pyridine adsorption was occurring on the catalyst surface. The model parameters for these exponential expressions are functions of pyridine partial pressure and rate constants for adsorption, desorption, and coke formation. Additional experimentation was completed to study catalyst reduction. The oxidic catalyst was reduced when exposed to hydrogen or ammonia (in the same temperature and partial pressure range as that of pyridine).