Deep hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene: Kinetics, effects of aromatics and hydrogen sulfide

The effects of different aromatics, H2S and processing parameters on deep hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) over a commercial NiMo/gamma-Al2O3 hydrotreating catalyst were investigated in a fix-bed multiphase microreactor under designed conditions. The sulfur-containing compounds and aromatics in both feeds and hydrotreated products were identified and quantified by GC-MS and GC-AED. Catalyst deactivation was investigated and modeled with a simple model. The kinetic behavior of these model compounds was studied by assuming pseudo-first-order reaction kinetics. The rate constants and the corresponding activation energies were determined by fitting experimental data at different temperatures and liquid hourly space velocity (LHSV). Adsorption heat for each compound on the catalyst surface was computed by using the Material Studio RTM software.; As expected, the HDS rate for 4,6-DMDBT was much lower than that for DBT, which is attributed to the steric hindrance of the methyl groups at 4 and 6 positions and lower adsorption ability of 4,6-DMDBT on catalyst surface. Under the similar conditions, the apparent activation energy for 4,6-DMDBT was higher than that for DBT. H2S inhibited the conversion of both DBT and 4,6-DMDBT. The HDS reaction rate significantly decreased with the increase in the concentration of aromatics in the feed. The aromatics with two or more aromatic rings were found having stronger retarding effect on HDS than those with single aromatic ring. This adverse effect was more pronounced for 4,6-DMDBT than for DBT. The competitive adsorption between sulfur compounds and aromatics on the catalyst surface was the main reason for the decreased HDS efficiency as qualitatively verified by the data of heat of adsorption.