(21d) Diesel Desulfurization to Ulsd – Overcoming Nitrogen Inhibition

The importance of improved desulfurization technologies to the refining industry has been clearly established. As the specification of the final fuel products approaches ?zero sulfur?, traditional hydrodesulfurization economics become increasingly less attractive. The very low sulfur level is dependent on the ability to convert the most refractive sulfur species, at a minimal cost.

The catalytic hydrodesulfurization (HDS) of the most refractive sulfur containing molecules in diesel i.e., dibenzothiophene (DBT) and especially 4,6 dimethyl-dibenzothiophene (DMDBT) is inhibited at different degrees by the components in the reaction mixture such as organic heterocompounds and polyaromatic hydrocarbons. It has been extensively reported that the nitrogen compounds present in the diesel feedstock are the strongest inhibitors in the catalytic HDS. In general, the following order of inhibition has been noticed: saturated and mono-aromatic hydrocarbons A high nitrogen containing diesel blend consisting of straight run diesel (SR), light cycle oil (LCO) and light coker gas oil (LCGO) in a 1:1:1 weight ratio was denitrogenated at room temperature and atmospheric pressure by liquid-liquid extraction using an ionic liquid (IL). The ionic liquid removed 70% total nitrogen, and more than 90% basic N. The IL is highly selective toward the nitrogen compounds and it does not cross-contaminate the diesel blend.

Pilot plant runs aimed at determining the benefit of denitrogenating the feed prior to desulfurization, indicated that it is possible to use 50% less catalyst for a treated feed vs. non-treated feed for the same desulfurization level. This translates into significantly improved economic performance for sulfur removal through a combination of a lower catalyst requirement, longer cycle length, and reduced utility consumption. An economic analysis quantifying these benefits will also be presented.