In reality, distillation can never achieve absolute separation of the different hydrocarbons liquids that enter the Light Ends Unit (LEU) as feedstock. Theoretically, there is a way to achieve a degree of separation that proves suitable for the refinery’s needs by distilling small fractions of the liquid using a number of ‘theoretical plates’. The feedstock that enters the LEU has components of low molecular weight and high volatility. These characteristics allow refineries to analyze the feedstock on a molecular level and define the degree of separation in terms of specific hydrocarbon concentrations. Once a stage of distillation is complete, the new condensate, enriched in the more volatile component, is taken past the next plate and redistilled in a different section. This process proceeds for however many theoretical plates are calculated until almost-pure components are left. For the Light Ends Unit, the feed is separated into two fractions, the distillate and the bottoms.

Solvent extraction out of the vacuum distilled residue pulls the asphaltenes out of the one-phase material. The strength of a specific compound, determined by the two Hildebrande Parameters, indicates the solubility of said compound in a solvent. The first parameter depends upon the surface tension and molar volume of the non-polar solvent while the second parameter uses the solvent’s energy of vaporization and molar volume. With these different characteristics in mind, it is easy to see how each component of the vacuum distilled residue has their own solubility properties. Aromatic hyrdocarbons have higher density than aliphatic hyrdrocarbons, which means that they also have a lower molar volume. This results in the increase of the second Hildebrande Paramenter (and consequently the increase of compound strength) since molar volume is the denominator in the equation. On the same line, higher carbon number paraffins have a larger molar volume, resulting in a lower strength value. Using this information, refineries add a large amount of low strength paraffin solvent to the vacuum dissolved residue to basically ‘cancel out’ the asphaltene’s solubility and yield solid particles that can be filtered out.