Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/12/2007
Publication Date: 3/21/2007
Citation: Eller, F.J., Taylor, S.L., Palmquist, D.E. 2007. Enhanced selective extraction of hexane from hexane/soybean oil mixture using binary gas mixtures of carbon dioxide. Journal of Agricultural Food & Chemistry 55(8):2779-2783. Interpretive Summary: Soybean oil is generally extracted from soybeans using the solvent n-hexane which is flammable, toxic and hazardous to the environment. Liquid carbon dioxide (L-CO2) is being studied as an alternative separation method to present energy intensive methods employing heat to separate hexane from soybean oil. Currently, a great deal of energy in the form of heat is required to distill the hexane. Liquid carbon dioxide has been demonstrated to remove the solvent n-hexane from hexane/soybean oil (SBO) mixtures. However, some SBO is carried-over with the hexane during its removal. To avoid this loss of SBO during the removal of hexane, researchers at the National Center for Agricultural Utilization Research, Peoria, Illinois, studied the use of mixtures of L-CO2 in conjunction with gases such as helium, nitrogen and argon. These binary gas mixtures were found to remove essentially all of the solvent hexane from the hexane/SBO mixtures while decreasing the co-extraction of the SBO by 96%. This new technology provides a useful tool for the selective extraction of compounds from complex mixtures and may have broad applications to other extractions.
Technical Abstract: Both supercritical carbon dioxide (SC-CO2) and liquid carbon dioxide (L-CO2) have been shown to effectively separate the solvent hexane from the mixture of soybean oil (SBO) and hexane resulting from the hexane extraction of soybean flakes. However, there is a slight carry-over (ca. 2 wt%) of SBO co-extracted with the hexane as the L-CO2 passes through the mixture. Previous research has shown that SC-CO2 entrained with helium significantly reduced SBO solubility in the SC-CO2. In this study, L-CO2 was mixed with three inert gasses (He, N2 and Ar) to decrease the solubility of SBO in the L-CO2 while maintaining hexane solubility. Various percentages (0.5–30 vol%) of the binary fluids were produced using mass flow controllers and a stirred reaction vessel. The binary mixtures were passed through a 25wt% hexane/SBO mixture held inside a 2.5m long stainless steel fractionation column held at 25°C and 9.31 MPa. A total of 300L (STP) was used at a flow rate of 3-4 L/min (expanded gas). The mass of co-extracted SBO was determined gravimetrically and the residual hexane in the raffinate SBO determined using ISO Method 9832:2002. The mass of SBO co-extracted with the hexane was inversely proportional to the binary gas concentration. Residual hexane in the raffinate was proportional to binary gas concentration. The 10% N2/CO2 and Ar/CO2 binary mixtures appear to optimize both the removal of the hexane while limiting the co-extraction of the SBO. At 10%, the residual hexane level was only 26 ppm while the SBO carry-over was only 40 mg. This carry-over of SBO is only ca. 0.13% of the original amount of SBO (i.e., 31.65 g) placed in the column and represents a 95% reduction in SBO carry-over compared to neat CO2 which gave 741 mg SBO. The binary CO2 mixes had only very limited effects on the fatty acid composition of the SBO.