Hydrodemetallization reaction

Most of the metal-organic compounds are found in heavy petroleum fractions, especially residual oils. During the hydrorefining process, all metal-organic compounds undergo hydrogenolysis and the resulting metals deposit on the catalyst surface and deactivate the catalyst, causing the bed pressure drop to rise. The metals deposited on the catalyst surface move deeper into the bed as the reaction cycle extends. A cycle is considered to be complete when the metal in the reactant at the outlet of the plant exceeds the specified requirements. Catalysts contaminated with arsenic or lead can generally be guaranteed to perform in hydrofinishing, when it is the degree of blockage of the catalyst bed that determines the operating cycle.

In naphtha, metals such as arsenic, lead and copper are sometimes present, either from crude oil or from contamination caused by the addition of additives during storage. Naphtha from high-temperature pyrolysis contains organosilicides, which are added as froth-breakers to the equipment prior to hydrofinishing, decompose quickly and cannot be removed by regeneration. Heavy petroleum fractions and residual deasphalted oils contain metallic nickel and vanadium in the form of porphyrin compounds of nickel and porphyrin compounds of vanadium respectively. These macromolecules undergo a degree of hydrogenation and hydrolysis at higher hydrogen pressure, forming deposits of nickel and vanadium on the catalyst surface. In general, the nickel-based compounds are somewhat less reactive than the vanadium complexes, with the latter depositing mostly on the outer surface of the catalyst, while the nickel penetrates more into the interior of the particles.