In lesson 4, we learned that distillation is a separation process which depends on the boiling point of compounds. Compare to distillation, solvent fractionation fractionate the feedstock or vacuum distillation residue (VDR) depend on the solubility of the compound in the given solution. Use graph from lesson 5 on class website as an example, it showed us a simple flow of the process. Asphaltenes, which are compounds that have highest molecular weight of VDR, is soluble in aromatic solution such as benzene and toluene. They can be separated by using precipitation under the condition that paraffin solvent was mixed in. Maltenes is the part of VDR which is soluble in paraffin solvent and also known as a solvent in the separation process. For further separation of n-heptane soluble, a lighter and weaker solvent n-pentane is used. That gives us an insoluble fraction (hard resin) and soluble fraction (n-pentane). Even lighter solvent propane is used to separate soluble fraction (n-pentane) to soft resin and oil products.
Since there is a large difference in structure of asphaltenes and oil fractions in VDR, it is normal for us to see a suspension of discrete asphaltene particles rather than a solution in VDR. However, VDR normally appear as a solution (one-phase material). The gradient solubility model is a general acceptable hypothesis that explains what we observed. In this model, the strength of solvent influences the solubility of a compound in given solvent. Hildebrand Solubility Parameters (HSP) measures the strength of solvent in this model. There two types of parameters used in the measurement and they correlate well to each other. 1st Hildebrand Parameter depends on surface tension and molar volume of the given solvent. 2nd Hildebrand Parameter depends on energy of evaporation and molar volume of given solvent. From the equations of two parameters, we can see that solubility increase with increasing surface tension, increasing energy evaporation and decreasing molar volume. As we know, aromatic solvent are stronger solvent than aliphatic hydrocarbon and it is explained by two types of Hildebrand parameters. That also explains why increasing carbon number make the solvent power of paraffin solvent increase.