Organosulfur compounds in petroleum

During the early 1990s IGT collaborated with EBC to develop the BDS process. The combined results from the two companies produced a series of patents describing the process for desulfurization of fossil fuels [237-240], Compared with the patent described in the earlier paragraph [235], these later patents propose a more realistic approach to desulfurization. The basis of these patents is the fact that some of the organosulfur compounds in petroleum are not desulfurized during hydrodesulfurization (HDS), but may be susceptible to BDS. 

Some of the first studies on the biodegradation of sulfur compounds focused on their fate and removal from oil-contaminated environments. Another major area of study is the microbial process of "biodesulfurization" which has been suggested as a means of selectively removing sulfur compounds from petroleum prior to refining. Information gathered from these two areas of research provide the basis of the present knowledge of metabolism of organosulfur compounds in petroleum. This information is reviewed with emphasis on the metabolism of dibenzothiophenes and n-alkyl tetrahydrothiophenes (n-alkyl thiolanes). 

FEDORAK Microbial Metabolism of Organosulfur Compounds in Petroleum 97... 

S-selective mode it is useful to characterize the organosulfur compounds in complex petroleum mixtures, as the much higher levels of coeluting hydrocarbon peaks give minimal response. Operated in P-selective mode it is a sensitive detector for organophosphorus pesticide trace residues in complex environmental mixtures. Only the sulfur mode response range is illustrated in Fig. 12.13. 

Sulfur for commercial purposes is derived mainly from elemental sulfur mined by the Frasch process or from the sulfur by-products of purified sour natural gas and petroleum. (The term sour is generally associated with high-sulfur petroleum products.) During World War n, sulfur was first produced commercially from sour natural gas by the early 1970s, sulfur from sour natural gas and organosulfur compounds in crude oil already exceeded that produced by mining elemental sulfur. 

Sulfur in crude oils is mainly present in the form of organosulfur compounds. Hydrogen sulfide is the only important inorganic sulfur compound found in crude oil. Its presence, however, is harmful because of its corrosive nature. Organosulfur compounds may generally be classified as acidic and non-acidic. Acidic sulfur compounds are the thiols (mercap-tans). Thiophene, sulfides, and disulfides are examples of non-acidic sulfur compounds found in crude fractions. Extensive research has been carried out to identify some sulfur compounds in a narrow light petroleum fraction.Examples of some sulfur compounds from the two types are ... 

Sulfur can be found in most all petroleum fractions. In naphtha, distillates and some lube fractions, sulfur is a component of carbon and hydrogen containing molecules. In heavier fractions, sulfur may be bound in asphaltene and resin matrices in combination with nitrogen and oxygen. The organosulfur compounds present in most fuels are either thiols, sulfides or thiophenes. 

The microbial metabolism of a large number of hydrocarbons has been thoroughly studied and described in the literature. In contrast, much less is known about the metabolism of organosulfur compounds found in petroleum. These investigations have been hampered by the commercial unavailability of most of the sulfur compounds of interest. 

Dibenzothiophenes. Because of its commercial availability, dibenzothiophene is the most extensively used compound in studies of organosulfur metabolism. It has been used as a model compound in the studies of petroleum and coal biodesulfurization. These topics have been reviewed by Foght, J.M. Fedorak, P.M. Gray, M.R. Westlake, D.W.S. (Microbial desulfurization of liquid fossil fuels. In Microbial Mineral Recovery, in press) and Monticello and Finnertv (501. 

The types of organosulfur compounds found in petroleum feedstocks are shown in Table Of them, the alkyl and aryl thiols (RSH), sulfides (RSR ), and disulfides (RSSR ) are the most rapidly desulfurized (equation 1). It is the broad class of thiophenes, stabilized by their aromatic character, that are most difficult to desulfurize and require relatively severe temperatures ( 400 °C) and H2 pressures ( 100 atm). Thus, it is the HDS of thiophenes that has been of greatest interest to inorganic and organometallic chemists. There are fewer model studies of the HDS of thiols, sulfides, and disulfides. Organometallic aspects of HDS and HDN have been summarized most comprehensively in a recent book by Sanchez-Delgado. A briefer overview has also been published. More specific reviews are cited in later sections of this chapter. 

A patent for desulfurization of petroleum streams containing condensed ring heterocyclic organosulfur compounds on the basis of the similar principle was filed by Research and Engineering Company in 2001.58... 

The most abundant heteroatom is invariably sulfur, appearing in concentrations from below 0.1 wt. % in North African or Indonesian light crudes to over 5 wt. % in Venezuelan heavy crudes (Boscan) or Canadian tar sands. A wide variety of sulfur containing compounds are present in petroleum and refinery fractions, ranging from thiols to thiophenes the most important classes of organosulfur compounds of interest for our purposes are represented in Fig. 1.1. 

One of the largest uses for molybdenum catalysts is in the desulfurization of petroleum, petrochemicals, and coal-derived liquids, in which organosulfur compounds react with hydrogen at the catalyst surface, and sulfide ions are removed as hydrogen... 

The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR ), disulfides (RSSR ), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene > benzothiophene > dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiophene is, for example, very difficult to desulfurize. 

Figure 1. Organosulfur compounds present in petroleum feedstocks are thiols, thioethers, disulfides, thiophenes, benzothiophenes and dibenzothiophenes.

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