Post a blog to comment on how solvent fractionation works and review the parameters to describe the solvent power for non-polar solvents.

Solvent extraction is a process in which compounds may be separated based on their relative solubilities. It is basically the extraction of a substance from one liquid into another liquid phase. As opposed to distillation which exploits the different boiling points of the feedstock to achieve fractionation, deasphalting utilizes a solvent extraction process that factors in solubility or insolubility of compounds in a certain solvent. Vacuum distillation residue (VDR) is completely dissolved in aromatic solvents, like benzene and toluene. A paraffin solvent (n-heptane) is mixed with VDR in toluene, and the soluble portion of VDR in the n-heptane is called maltenes. The n-heptane solubles can then be further separated using a lighter and weaker solvent, such as n-pentane. These solubles can be separated even further utilizing a lighter solvent like propane. Figure 5 of Lesson 5 shows the overall process of solvent fractionation of VDR.

Rather than being considered a suspension of discrete asphaltene particles in VDR, this residue is actually considered a solution (one-phase material). The gradient solubility model is a widely acknowledged hypothesis which explains this observation. This model declares that asphaltene molecules can dissolve in resins which can then be dissolved in oil, ultimately yielding a single phase solution. The asphaltene is able to be forced out of solution in VDR by solvent extraction. The degree of solubility of a compound in a solvent is dependent upon the dissolving power of the solvent which is measured for non-polar solvents by Hildebrand Solubility Parameters (HSP).

There are two different Hildebrand Solubility Parameters. The first relates solubility to the ratio of surface tension to the cubic root of molar volume, in which solubility increases as surface tension increases and as molar volume decreases. The second parameter equates solubility to the square root of the ratio of latent heat of vaporization to molar volume, where solubility increases with an increasing heat of vaporization. It is clear why aromatic solvents are stronger solvents than aliphatic hydrocarbons.

Solvent Extraction: http://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extraction