Deep hydrodesulfurization of light gas oil (Part 2) effect of hydrogen sulfide on hydrodesulfurization of dibenzothiophenes included in hydrotreated light gas oil

Sulfided Co-Mo/Al2O3 catalyst was used to investigate the hydrodesulfurizat ion (HDS) of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) included in hydrotreated light ga s oil (H-LGO) of less than 0.168 wt% of sulfur-content. The behavior of DBT , 4-MDBT, and 4,6-DMDBT in HDS was traced under the following conditions: t emperature 290-370 degrees C, LHSV 2-10 h(-1) and Gas/Oil 125 Nl/l. The HDS of these compounds was treated as a pseudo-first-order reaction with respe ct to their concentrations. The ratio between HDS rate constants of various DBTs in H-LGO was substantially larger than that between the comparable co nstants in straight-run LGO (SR-LGO). Activation energies of the HDS of DBT s increased in the order DBT < 4-MDBT < 4,6-DMDBT, while the differences be tween the activation energies were larger for H-LGO HDS than for SR-LGO HDS . It thus appeared that H-LGO HDS formed a lesser quantity of H2S than did SR-LGO HDS. The retarding effect of H2S on HDS was investigated by using po lysulfide, which easily formed H2S under HDS conditions. The retarding effe ct of H2S on HDS decreased in the order DBT > 4-MDBT, while the HDS of 4,6- DMDBT was not retarded. These results could be explained by assuming a comp etitive adsorption between DBTs and H2S.