Soybean oil is an important product of one of the two major agricultural crops grown in the Midwest. Raw soybeans are about 20% oil and the conventional process uses hexane to extract the oil from the beans.
In the conventional process, the hexane is very effective in removing the oil from the beans. Unfortunately, the hexane also extracts the phospholipids, that have to be washed out of the oil in a later processing step called "degumming." In addition, the hexane removes high molecular weight species that cause the unrefined oil to be very dark brown, almost black in color. The environmental problems with the conventional process center around the use of hexane. In the conventional process the dark oil is cleaned up by running it over an adsorbant (such as diatomacious earth). Since it nominally consists of all "natural" products, the spent adsorbant is customarily landfilled. Unfortunately, it inevitably contains some hexane, which is volatilized in the warm summer sun, resulting in pesky fires in the landfill. Also, some hexane is emitted directly from the process to the atmosphere as VOCs. Since hexane is so volatile and flammable, explosions at soybean processing plants are unfortunately common.
The oil extracted from soybeans with supercritical CO2 is of much higher quality than the hexane-extracted oil. It does not contain any phospholipids, thus eliminating the need for the degumming step. Also, higher molecular weight species are left in the beans, yielding a clear, light-colored oil without further processing. The disadvantage of CO2 is that very high pressures (~12,000 psi) are required to achieve sufficiently high oil solubility. Finally, the transport of soybean flakes at high pressure would be quite difficult and the actual implementation of this process would hinge on the successful scale-up of a high pressure screw conveyor.
This project actually involves two designs: the conventional hexane process and the supercritical CO2 process so that comparisons could be made between the two technologies. Either design would be an appropriate project for a one-semester senior design project. Brief reports, PFDs, economic evaluations and full AspenPlus designs are available for both of these processes. Instructors can contact Professor Brennecke for this information.
