ABSTRACT
In today’s world, composition of petroleum is an important topic in the study of its physical and chemical changes during processing. In this sense, separation by fractionation is a useful tool for understanding more deeply its composition. Asphaltenes are the important and complex component of petroleum, particularly for heavy, extra-heavy crude oil and its residues. It is generally thought that asphaltenes are formed by similar molecules varying mainly in molecular weight, as reported by Yen et al. (1984) and Pelet et al. (1986). However, there is evidence that chemical properties and average molecular structure may vary considerably with molecular size, such as distribution of metals in asphaltenes (Hall and Herron, 1979). In this regard, different fractionation techniques have been reported in the literature, e.g., adsorption and ion exchange chromatography (McKay et al., 1978), differential precipitation (Yang and Eser, 1999), membrane fractionation (Acevedo et al., 1997), and size exclusion chromatography (SEC) (Semple et al., 1990), all reported in Gutiérrez et al. (2001). Liao and Geng (2002) stated that a significant quantity of hydrocarbons (including alkanes) is occluded in the skeleton of asphaltene molecules. The hydrocarbons are probably traces of the “original oil” that had been retained within the asphaltene matrix and protected from the secondary alteration processes, which occurred subsequently in the oil reservoirs. The authors oxidized asphaltenes without releasing signicant amounts of occluded hydrocarbons. These hydrocarbons can be desorbed from asphaltenes by washing with solvents.
