Solvent fractionation is the process in which solvents, such as n-heptane, n-pentane, and propane, are used to further separate the feedstock that comes from the initial distillation process. The most common application for solvent fractionation is performed on the vacuum distillation residue. The reason that this feedstock is distilled further is because it can create different marketable products such as asphalt (from asphaltene), resins, and oil (used for lubrication and other products). The way that these distillates are separated, from the aromatic solvents of benzene and toluene, occurs in three stages which blend the feedstock with increasingly lighter paraffinic solvents. The first stage is using n-heptane to separate the solid asphaltene product from the VDR feedstock. The portions, such as n-heptane maltenes, which are soluble in the heptane solvent, are introduced to n-pentane. N-pentane allows for hard resin to be separated from the feedstock, and allows for further separation of the soluble products to finally be treated with propane to separate the feedstock into soft resin and oil.
In order to further understand how asphaltenes are extracted from the feedstock, with the use of n-heptane, n-pentane, and propane, one would need to use the Hildebrand Solubility Parameters. These parameters are used to determine the solvent power of these non-polar solvents and how the different carbon numbers define the power of the solvent. The Hildebrand Solubility Parameters use the characteristics of surface tension, molar volume, and energy of vaporization to be able to decide which solvent is best for asphaltene and resid extraction. Ultimately, the goal for this process is to obtain the highly usable deasphalted oils so that they can be further processed to make lube oil base stock and distillate fuels. Most of the deasphalting process is conducted by propane and then is further processed by furfural, phenol, and N-methyl-2-pyrrolidone solvents to separate the high carbon aromatics from the naphthenic and paraffinic ones. [1]
Sources:
1. Gary, J. H., & Handwerk, G. E. (2007). Petroleum refining: technology and economics. New York: M. Dekker.