The Selection of Catalyst for Ultra Deep Desulfurization

These compounds react quite differently over CoMo and NiMo catalysts, and the response of the two catalysts to the hydrogen sulfide produced during reaction and to inhibitors in the feed is dissimilar. To appreciate the criteria governing the choice of catalyst for ultra deep desulfurization and the consequences for product properties and hydrogen consumption, it is important to imderstand how the most refractive sulfur compounds react as well as the role played by inhibitors. 

Desulfurization of DBT progresses preferentially via the direct extraction route. When alkyl substituents are attached to the carbon atoms adjacent to the sulfur atom, the rate of direct sulfur extraction is diminished, whereas the rate of sulfur removal via the prehydrogenation route is relatively unaffected. 

For DBT-molecules with alkyl substituents in the 4- and/or the 6-position, both routes are important, but for all other sulfur-containing molecules in the diesel boiling range, only the direct route is important for HDS. 

CoMo catalysts desulfurize primarily via the direct desulfurization route. NiMo catalysts, which exhibit a higher hydrogenation activity, have a relatively higher selectivity for desulfurization via the hydrogenation route. The extent to which a given catalyst desulfurizes via one route or the other determines the effect of H2 partial pressure, H2S partial pressure and feed properties on the catalyst activity. 

The effect on catalyst activity of process variables such as LHSV, temperature, hydrogen partial pressure, hydrogen sulfide partial pressure and gas/oil ratio can be predicted by a suitable kinetic expression. It was found that the expression shown in equation 1 could be used to describe the kinetics of CoMo and NiMo catalysts for very deep desulfurization of diesel.  

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