Solvent extraction is a process whereby the feed stock is fractionates the vacuum distillation residue (VDR) according to the solubility/insolubility of the molecular components in a given solvent. The heaviest molecular components of the VDR, asphaltenes, can be separated by precipitation by dissolving into a light paraffin after with VDR is solubilized in an aromatic solvent such as benzene or toluene. The paraffin solvent that is used determines the type of asphaltenes that are seperated. The VDR that is dissolved in the paraffin solvent are known as maltenes with respect to the solvent used. The VDR can theoretically undergo several dissolutions using different paraffin solvents for each process. Each process produces different insoluble products such as asphaltenes, hard resin, soft resin and deasphalted oil (DAO) fractions depending on the paraffin solvent used. However, only one stage of separation is usually implemented in industry processes whereby the lightest solvent, usually propane, to produce asphalt and DAO fractions.

There are several parameters that determine the power of such non-polar solvents. The gradient solubility model provides some justification regarding how the asphaltenes in VDR can be removed through solvent extraction. The Hildebrand Solubility Parameters (HSP) determines the power of a non-polar solvent. There are two HSP definitions; the first HSP depends on the surface tension and molar volume values of the solvent. Solubility increases as surface tension increases and molar volume decreases (increasing density). The second HSP depends on the energy of vaporization and molar volume. Solubility according to this parameter increases as the heat of vaporization increases. Thus, the HSP conveniently explains why paraffins with more carbons are more powerful solvents. It also explains why aromatic solvents, such as toluene, are more powerful solvents than aliphatic hydrocarbons, such as propane. Therefore, the Hildebrand Solubility Parameters prove to be very useful when deciding which solvent to use to yield a specific product for a solvent extraction process.

Sources:

Course Webpage

Wikipedia: Solvent Extraction, Hildebrand Solubility Parameters