PrintDeasphalting
While distillation achieves separation/fractionation of the feed components with respect to differences in their boiling point, deasphalting (as a solvent extraction process) fractionates the feedstock, atmospheric, or more often vacuum distillation residue (VDR), with respect to solubility/insolubility of molecular components in a given solvent. Figure 5.5 shows a solvent fractionation scheme used in laboratories to separate VDR into fractions that are defined based on the solubility behavior. VDR, which could be a solid at ambient temperature, is completely dissolved in aromatic solvents such as benzene and toluene. The highest molecular weight components of VDR classified as “asphaltene” can be separated by precipitation (as solid material) when a light paraffin solvent is mixed with VDR solubilized in an aromatic solvent (e.g., toluene). The paraffin solvent used (e.g., n-heptane) defines the identity of the separated asphaltenes (e.g., toluene soluble and n-heptane insolubles). The portion of VDR that is soluble in the paraffin solvent is called maltenes, but also labeled with respect to the solvent used in the separation, i.e., n-heptane maltenes. As can be seen in Figure 5.5, the n-heptane solubles can further be separated using n-pentane (a lighter and a weaker solvent) into insoluble (hard resin) and soluble (n-pentane solubles) fractions. The n-pentane solubles can also be separated using a lighter solvent yet (propane) to soft resin and oil products. In refinery practice, only one stage of separation is used, using the lightest solvent (e.g., propane) to produce asphalt and deasphalted oil (DAO) fractions, as discussed later in this section.
Asphaltenes consist of high-molecular weight compounds with strong aromatic character and contain the highest concentrations of heteroatom (S, N, and metal) species. Because of the low H/C ratio of asphaltenes and resins, the deasphalting process can be considered as a “carbon rejection” process, yielding a high H/C ratio product (i.e., oil) after removing the asphaltenes and resins, as, typically, lower value byproducts.
