Sulfur in asphaltene

When sulfur conversion levels are pushed above 90%, the unique refractoriness of asphaltenes becomes dominant. The tendency of a fine pore catalyst to partially exclude asphaltenes and the complete steric hindrance or "burying" of sulfur in asphaltenes contribute to this refractoriness. 

Vacuum distillation of the atmospheric residue complements primary distillation, enabli r.ecoyery of heavy distillate cuts from atmospheric residue that will un r o further conversion or will serve as lube oil bases. The vacuum residue containing most of the crude contaminants (metals, salts, sediments, sulfur, nitrogen, asphaltenes, Conradson carbon, etc.) is used in asphalt manufacture, for heavy fuel-oil, or for feed for others conversion processes. 

Aromaticity is the most important property of a carbon black feedstock. It is generally measured by the Bureau of Mines Correlation Index (BMCI) and is an indication of the carbon-to-hydrogen ratio. The sulfur content is limited to reduce corrosion, loss of yield, and sulfur in the product. It may be limited in certain locations for environmental reasons. The boiling range must be low enough so that it will be completely volatilized under furnace time—temperature conditions. Alkane insolubles or asphaltenes must be kept below critical levels in order to maintain product quaUty. Excessive asphaltene content results in a loss of reinforcement and poor treadwear in tire appHcations. 

In sample 1, it is clear that the predominant form of sulfur in the residuum is thiophenic, while the sulfide forms seem to predominate in residua 2 and 3. In asphaltene samples 1 and 3, the thiophenic sulfur content increases relative to the respective residua, indicating that the molecules containing sulfide sulfur may be more soluble in heptane than those containing thiophenes. For sample 2, there appears to be no such discrimination within the 10% accuracy limits currently established for the XANES analysis. 

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. 

Consideration of Catalyst Pore Structure and Asphaltenic Sulfur in the Desulfurization of Resids... 

Table II compares two atmospheric resids, West Coast and Kuwait, in a traditional manner. The obvious differences include sulfur, nitrogen, asphaltenes, total metals and mid-boiling point. Apart from sulfur content, one might surmise a greater catalyst demand by the West Coast feedstock in that the boxed values suggest heavy coke laydown and metals deposition. Neither of the sulfur values is boxed because there is no indication as to (1) what fraction of the sulfur is refractory or "hard" sulfur, nor (2) the degree of desulfurization to be achieved.

Whereas the relative amount of aromatics remained fairly constant as sulfur conversion level was increased to 92-94%, the relative amount of sulfur in the aromatic fraction decreased markedly. This also is depicted in Figure 3. Polar aromatics are intermediate to the aromatics and asphaltenes in regard to this behavior. 

Where S is the weight fraction of sulfur in the liquid phase, t is the residence time, PH is the partial pressure of hydrogen, A is the weight fraction of asphaltenes in the liquid phase, PH2s, is the partial pressure of hydrogen sulfide, S is the weight fraction of sulfur associated with component i in the range of i to j, ka is the... 

High amounts of asphaltenes and resins require high hydrogen partial pressures and may actually limit the maximum level of hydrodesulfurization, or final traces of sulfur in the residuum may only be eliminated under extremely severe reaction conditions where hydrocracking is the predominant reaction in the process. High asphaltene and resin contents are also responsible for high viscosity (Figure 6-7) which may increase the resistance to mass transfer of the reactants... 

Chevron RDS and VRDS processes processes designed to remove sulfur, nitrogen, asphaltene, and metal contaminants from heavy feedstocks consisting of a once-through operation of the feedstock coming into contact with hydrogen and the catalyst in a downflow reactor (q.v.). 

Sarret, G., Connan, J., Kasrai, M., Bancroft, G. M., Charrie-Duhaut, A., Lemoine, S., Adam, P., Albrecht, P., and Eybert-Berard, L. (1999). Chemical forms of sulfur in geological and archaeological asphaltenes from Middle East, France, and Spain determined by sulfur K- and L-edge X-ray absorption near-edge structure spectroscopy. Geochim. Cosmochim. Acta 63, 3767-3779. 

Waldo, G. S., Mullins, O. C., Penner-Hahn, J. E., and Cramer, S. E. (1991b). Determination of the chemical environment of sulfur in petroleum asphaltenes by x-ray absorption spectroscopy. Fuel 70,549-559. 

Oil sand asphaltenes pyrolysis oil also contain n-alkane-derived cyclic sulfides along with /z-alkane-derived thiophenes, benzo- and dibenzothiophenes, showing that the precursor oil was of n-alkanoic origin. Approximately 25% of the sulfur in these asphaltenes is in the form of sulfides. 

The rest of the sulfur in oil sand asphaltenes, which is not in the form of thiane and thiolane, is present as thiophenes. Indeed, the asphaltene pyrolysis oil contains homologous series of a and a,a n-alkyl substituted thiophenes, 2-, 4- and 2,4-... 

In this paper organically-bound sulfur in three types of high-molecular-weight organic matter (kerogen, asphaltenes and resins) obtained from three organic sulfur-rich sedimentary rock samples has been studied. Kerogen, asphaltene and resin fractions were isolated and characterised by the two described techniques. 

One of the main reasons for the recent interest in organic sulfur in sedimentary organic matter is the growing evidence that it plays a role in the petroleum generation process. Organically-bound sulfur has been implicated in the formation of low API gravity, asphaltene-rich petroleums at a low level of thermal exposure (1). Recent studies indicate that... 

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