Solvent fractionation is a further form of distillation that takes place in the Light Ends Unit (LEU). The feedstock for solvent fractionation is the residue that is left over from previous Vacuum Distillation, aptly called, vacuum distillation residue (VDR). VDR is a very heavy viscous compound that is solid at ambient temperatures. The reason vacuum distillation residue is so dense, is because of its high aromaticity along with its high asphaltene concentration. Asphaltenes are the highest molecular weight compounds contained in VDR, and appear in solution. What solvent separation is, is when the asphaltenes within VDR are precipitated from the solution using a light paraffinic solvent. The portion of the VDR that is soluble in the paraffin is called maltenes. The first paraffin used is typically n-heptane. The n-heptane soluables can further be separated using n-pentane, which is a lighter an weaker solvent. The result of this step is an insoluable compound called hard resin and n-pentane soluables. Finally, the lightest of the solvents, propane, is used. This final stage yields soft resin and oil products. In refining, this is done by skipping to the final step and only using propane, which yields asphalt and deasphalted oil.

The solubility of compounds in different solvents can be measured. For non-polar solvents, solvents are measured using Hildebrand Solubility Parameters (HSP). What the Hildebrand Solubility Parameters are, are two parameters used to accurately determine the solubility power for non-polar solvents. The first of the two parameters pertains to the surface tension and the molar volume of the solvent, while the second pertains to the energy of vaporization and the molar volume of the solvent. In general, the solubility parameter increases with increasing density of the solvent, as well as increasing surface tension or energy of vaporization of the solvent. With this knowledge we are able to understand why using too large of a volume of a solvent will interfere with the solubility of asphaltenes.