Solvent Deasphalting in Petroleum Refineries

Solvent deasphalting is a refinery process for extracting asphaltenes and resins from heavy vacuum gas oil, atmospheric residue or vacuum residue to produce valuable, deasphalted oil that otherwise can not be recovered from the residue by conventional distillation. The deasphalted oil can be used to make lubricants or as feed to the fluid catalytic cracker or the hydrocracker. The process consists of contacting the feedstock with a solvent in a countercurrent extractor at elevated temperature to precipitate the asphaltene and resin fractions that are not soluble in the solvent. Paraffins on the other hand are soluble in the solvent at lower temperatures, but their solubility decreases with increasing temperature. Therefore, the yield of the deasphalted oil decreases with increasing extractor temperature. Operation is typically well below the critical temperature to avoid flooding. The pressure is mainly selected to keep the solvent in the liquid phase. Typically, the quality of the deasphalted oil decreases with increasing deasphalted oil yield. Consequently, the temperature can be selected to yield deasphalted oil with the desired properties [1, 2, 3, 4]. For more information on Intertek PARC’s capabilities, please visit http://www.intertek.com/testing/ pilot-plant/thermal-heavy-oil/.


The solvents used include propane and the isomers of butanes and pentanes either alone or as mixtures. As the molecular weight of the solvent increases the deasphalted oil yield increases and the asphalt yield decreases [3]. However, with the higher molecular weight solvent the quality of the deasphalted oil, as measured, for example, by Conradson Carbon residue or metals content, also decreases [4]. Furthermore, the asphalt may become more difficult to handle as higher temperatures may be required to make it flow. The solvent in the deasphalted oil and in the pitch must be recovered by flash vaporization or supercritical means. The solvent is then collected, condensed and returned to the solvent tank for re-use.

Care must be taking when selecting the solvent. In particular, attention must be paid to the solvent’s auto-ignition temperature (cf. Table 1) and the temperature required to keep the asphalt flowable.

A potentially dangerous situation in the pilot plant setting can develop during a unit upset in which solvent is released above its auto-ignition point into the process area. The temperature required to make the asphalt flow depends on the feedstock processed, the solvent selected and the operating conditions. Based on these factors, Intertek PARC may recommend the use of iso-pentane over n-pentane as solvent or the use of diluents to improve the flowability of the solvent-free asphalt so that the temperature required to make the asphalt flow is well below the solvent’s auto-ignition temperature.

References:

1. Speight, J.G, and Ozum, B., Petroleum Refining Processes, Marcel Dekker, New York, 2002, pp. 572-579.

2. Gary, J.H., Handwerk, G.E, and Kaiser, M.J., Petroleum Refining: Technology and Economics, Fifth Ed., C RC Press, New York, 2007, pp.311-313.

3. Sequira, Jr. A., Lubricant Base Oil and Wax Processing, Marcel Dekker, New York, 1994, pp. 53-80.

4. Sprague, S.B., “How Solvent Selection Affects Extraction Performance”, Paper AM-86-36, presented at the 1986 NPRA Annual Meeting, Los Angeles, March 23-25, 1986.