Asphaltenes heteroelements

Asphaltenes have high concentrations of heteroelements sulfur, nitrogen, nickel and vanadium. Their content varies widely in petroleum oils (Table 1.5). They cause a number of problems throughout the petroleum industry. 

The refining industry generally seeks either to eliminate asphaltenes or to convert them to lighter materials because the presence of heteroelements cause pollution problems, e.g., sulfur and nitrogen, catalyst poisoning, and corrosion (formation of metal vanadates during combustion). 

The amount of asphaltene precipitate from one crude oil is dependent on the carbon number of the alkane solvent. A decrease in the solvent carbon number results in an increase in the asphaltene precipitate. This observation would suggest that asphaltenes and resins are not greatly different materials. Rather, a continuum exists in the solubility behavior. An increase in boiling point in heavy oil is generally accompanied by an increase in both aromaticity and in the concentration of heteroelements or polar aromatic molecules (Corbett, 1969). Similarly, there is a gradual... 

Tynan and Yen (1969) have suggested that the association of aromatic sheets in the asphaltene macrostructure may occur through coordination of heterocycles. The aromatic sheets may have defect centers with heteroelements providing coordination centers for metals (Yen, 1974). 

Other basic generalizations also have been noted with increasing molecular weight of the asphaltene fraction, both aromaticity and the proportion of heteroelements increase (3, 4, 5). In addition, the proportion of asphaltenes in... 

In addition, the use of different hydrocarbon liquids for the separation yields asphaltene fractions that are substantially different from each other (I, 25). For example, the H/C ratios of the heptane precipitate are lower than those of the pentane precipitate, indicating a higher aromaticity in the heptane precipitate. The N/C, O/C, and S/C atomic ratios are usually higher in the heptane precipitate, indicating higher proportions of the heteroelements in this material (26). 

The data from these studies support the hypothesis that asphaltenes, viewed structurally, contain condensed polynuclear aromatic ring systems bearing alkyl side chains. These systems carry alkyl and alicyclic systems with heteroelements (i.e., nitrogen, oxygen, and sulfur) scattered... 

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