To develop a process for upgrading heavy residual oils, experimental investigations were conducted to study the effect of presulfiding on performance of hydrodesulfurization (HDS) catalyst: activity and life. The authors used straight gas oil from Khafji crude for studied on presulfiding of (Ni)-Co-Mo/Al2O3 type catalysts. The results indicated that presulfiding affected catalyst life, but not catalyst activity, in the residual oil HDS. One of the effects of presulfiding was the suppression of

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... oke deposition. When Co-Mo/Al2O3 type catalysts having smaller pores were employed, presulfiding gave longer catalyst life; while those having larger pores did not improve life because of the greater effectiveness factor. When Ni-Co-Mo/Al2O3 type catalysts were employed, the magnitude of the effectiveness factors did not play a meaningful role. Presulfiding of Ni-Co-Mo/Al2O3 type catalysts also suppressed removal of vanadium. This effect was not observed with the use of Co-Mo/Al2O3 type catalysts.