Submitted to: Journal of Chromatography A
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2005
Publication Date: 9/26/2005
Citation: Eller, F.J., Taylor, S.L., Palmquist, D.E. 2005. Supercritical fluid chromatographic analysis for on-line monitoring of hexane removal from soybean oil miscella using liquid carbon dioxide. Journal of Chromatography A. 1094:183-186. Interpretive Summary: Soybean oil is generally extracted from soybeans using the solvent n-hexane which is flammable, toxic and hazardous to the environment. To separate the hexane from soybean oil after the extraction, a great deal of energy in the form of heat is required to distill the solvent. Researchers at the National Center for Agricultural Utilization Research, Peoria, Illinois, studied the use of liquid carbon dioxide (L-CO2) as an alternative to current energy intensive methods employing heat to separate hexane from soybean oil and new methods to monitor the progress of the separation. Liquid carbon dioxide at 25°C and 1350 psi was used to remove the solvents n-hexane and isohexane from hexane/soybean oil (SBO) mixtures contained inside a fractionation column. After passing through the hexane/SBO mixture, the L-CO2 stream was directed through the injector of a supercritical fluid chromatograph (SFC). This configuration provides a useful tool for real time monitoring of the removal of hexane from hexane/SBO mixtures. This new technique will allow a quick determination of the progression of the hexane removal process using L-CO2. Carbon dioxide is inexpensive, non-toxic, non-flammable and environmentally friendly.
Technical Abstract: In previous work, it had been demonstrated that liquid carbon dioxide (L-CO2) could be used to effectively separate n-hexane from hexane/soybean oil (SBO) mixtures using a fractionation column. However, the ISO Method 9832:2002 used for determining residual hexane in the SBO after the extraction (i.e., the raffinate) is tedious and time consuming. This study investigated the use of an on-line monitoring system incorporating a supercritical fluid chromatographic analysis for determining n-hexane and isohexane levels in a (L-CO2) stream after it had passed through hexane/SBO mixtures. L-CO2 at 25 deg C and 9.31 MPa was pumped through a 25% (w/w) hexane in SBO mixture at a flow rate of 3-4 L/min (expanded gas) then the L-CO2 stream was directed through the injector of a supercritical fluid chromatograph (SFC). At timed intervals, the SFC program was initiated and a sample of the L-CO2 stream was injected onto a capillary column and analyzed with a flame ionization detector (FID). Relative FID peak areas of the hexanes were compared to a standard curve of FID counts versus hexane concentration to determine the hexane concentrations in the L-CO2 stream. The FID counts indicated that the hexane concentrations in the L-CO2 stream after 300 liters expanded CO2 were very low, ca. 0.05% and 0.04% for n-hexane and isohexane, respectively. The subsequent headspace GC analysis of residual hexane in the extracted SBO (i.e., raffinate) after 300 liters expanded CO2 gave residual n-hexane and isohexane values of 3.84 and 3.28 ppm, respectively. This technique provided real time on-line monitoring of the solvent extraction process.