In the oil and gas industry, the bottom line is the most essential metric of success. We have discussed the complexity of crude oil and the general refinery path which it takes. Certainly, the most energy intensive portion of refining, separation processes, dictates the success or failure of an oil refinery. Along with the increase in light distillates, we observe an increasing need for heavy crude deasphalting capacity. As distillation columns use pressure and temperature gradients to fractionate distillates and bottoms, solvent fractionation is a “carbon rejection” process that uses a “chemical gradient” to separate asphaltenes and resins from deasphalted oil (DAO) in vacuum distillation residue. Additionally, solvent dewaxing involves solvents and temperature gradient to produce wax and lube oil. Ultimately, solvents may enhance the overall refinery profitability while adding product flexibility and utilizing the entire barrel of crude oil!

In order to understand solvent fractionation, we must understand the gradient solubility model. For engineers to able to characterize the seemingly fruitless vacuum distillation residue. Resins in the crude oil dissolve asphaltene molecules in a solution, preventing precipitation. Miscibility, or the “mixabilty” property, must be altered through the use of a solvent. Paul J. Flory, a Standard Oil Development Co. scientist, correlated the difference in molecule sizes (solvent versus VDR) with system entropy; as the difference in molecule size increased, the entropy (or disorder) increased, causing large deviations from ideal miscible behavior. With this understanding, we can attempt to quantify the strength of various solvents to compare.

Fig. 1: Paul J. Flory, 1974 Recipient of Nobel Prize in Chemistry (Source – Nobelprize.org)

At this point in the separation stages, large hydrocarbons such as asphaltenes and waxes can precipitate out of the crude oil solution given a specified paraffinic solvent. The strength of these non-polar solvents are defined by Hildebrand Solubility parameters. While Wikipedia only displays the Hildebrand parameter as a function of vaporization enthalpy, temperature, and molar volume, a similar value may be approximated using surface tension and molar volume. As surface tension increases (or the enthalpy of vaporization increase), the solvent’s strength increases.