Effect of the addition of antifoulant agents on the deactivation of NiMoP/Al2O3 catalysts for hydrotreating of residuum

The effect of the different antifoulant agents on the performance of a NiMo P/Al2O3 catalyst for hydrotreating of residuum was evaluated in an industri al scale reactor. Two kinds of antifoulant, e.g., P-containing and P-free, were added to the feedstock of the crude oil to prevent scaling in the heat exchanger of a single-stage once-through (SSOT) hydrotreating processor. T he results show that addition of P-containing antifoulant to the feedstock lowers the catalyst activity for hydrodesulfurization (HDS) and accelerates the deactivation of the catalyst, while P-free antifoulant has little infl uence on the catalyst performance. During the reaction some heteroatoms fro m the oil, such as iron sulfide and nickel sulfate, were deposited on the c atalysts, but they appeared to have little effect on the activity. The main factor influencing the activity of catalyst was P-containing antifoulant, which deposited on the outer layer of the trilobe catalyst pellets. During the reaction, the deposited P-containing compounds formed a condensed layer , deactivating the most accessible active sites at the outer layer of the p ellets and depressing diffusion to the inner layer, which is still active. In addition, it appeared that P-containing compounds caused the active cata lyst component, i.e., MoS2 aggregate, resulting in the deactivation of the catalyst. In the case of the P-free antifoulant system, deactivation of the catalyst probably resulted from coking and adsorbed nitrogen compounds. Th erefore, the addition of P-containing antifoulant to oil feedstocks. result s, in faster catalyst deactivation and is not recommended for industrial oi l refining.