A Comparative Study of the HDS Kinetics of Straight Run and Coker Gas Oils Under Deep Desulfurization Conditions

Abstract

Deep desulfurization of diesel blending streams has gained considerable importance in the petroleum refining industry in recent years due to environmental regulations which limit the sulfur content of diesel fuel to very low levels. Among the various factors influencing deep desulfurization of diesel to lower and lower sulfur levels, the quality of the feedstock plays an important role. In the present work, the degree of desulfurization of two different feedstocks, namely, coker gas oil (CGO) and straight run gas oil (SRGO) and their blends was investigated under different operating severities. The distributions of different types of sulfur compounds in both feeds were examined by GC-SCD. Kinetics of desulfurization of both feeds under deep desulfurization conditions was also studied. The results revealed that SRGO, in spite of having a significantly larger amount of total sulfur, desulfurized more easily and produced diesel fuel with low sulfur levels in the desired range (< 350 ppm) at lower temperatures compared with CGO. More severe operating conditions (higher temperature, lower LHSV etc.) were found necessary to bring about deep desulfurization of CGO and its blends with SRGO to the same sulfur level. Kinetic experiments showed a reaction order of 1.5 for the overall HDS of both types of feeds, but the activation energy was higher for CGO (30 kcal/mole) than SRGO (24 kcal/mole). The results are explained on the basis of the differences in the concentrations of nitrogen and refractory sulfur compounds present in the two feeds.