Effect of presulfiding on the activity and deactivation of hydrotreating catalysts in processing Kuwait vacuum residue

Presulfiding of hydrotreating catalysts plays an important role in creating the essential surface requirements for optimum activity. It transforms the oxides of molybdenum to MoSx crystallites which are the primary active species in this category of catalysts. In this paper, the results of our investigations of the effects of presulfiding on various reactions taking place in residues hydroprocessing are reported. The changes that the catalyst undergoes as a result of pretreatment is also discussed. It has been observed that presulfiding has practically no effect on sulfur and nitrogen removal. In contrast, some improvements were observed for vanadium and nickel removal, asphaltenes reduction, and conversion to distillates. A detailed investigation of the properties of the catalysts with and without presulfiding revealed that the catalysts undergo early deactivation by coke deposition; however, presulfiding reduces the extent of this early deactivation. The unsulfided catalysts were observed to lose over 60% of their initial surface area due to coke deposition and the accessibility to the inner portions of the catalyst pellets appears to be restricted.