Sulfur Species in Middle-Distillate Oils

Composition of Sulfur Species in Middle-Distillate Oils 

As discussed in the previous section, gas oils are complicated mixtures of several different hydrocarbon classes contaminated with small amounts of sulfur-containing species that lower product quality and limit marketability of the whole gas oil (see Fig. 1). The concentration of these sulfur species must be lowered significantly to meet present standards and, in the future, they must be nearly completely removed. To understand the difficulties in such conversions, it is helpful to consider the detailed composition of gas oils, from both the viewpoint of the desirable components of gas oils which are not to be converted and that of the sulfur-containing species which must be treated to extinction. 

With the analytical procedures available today, it is now possible to determine in fine detail the composition of all of the hydrocarbon components as well as the offending sulfur species. In this section we discuss the general features of gas oils and address the present state of knowledge of 

Double and Triple Rings Single Ring Double and Triple Rings 

4-Methyldibenzothiophene 4,6-Dimethyldibenzothiophene Fig. 4. Typical polyaromatics in gas oil. Reproduced from Ref. 12, with permission. 

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III. Composition of Sulfur Species in Middle-Distillate Oils... 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

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