Deacidification of Soybean Oil Using Membrane Processing and Subcritical Carbon Dioxide

Vegetable oils have been deacidified using supercritical carbon dioxide and membrane processing. However, the pressures required are substantially greater than those used in industry. Therefore, the feasibility of using subcritical carbon dioxide (at much lower pressures) and membrane processing to separate free fatty acids (FFA) from triacylglycerols (TAGs) was examined. First, FFA/TAG solubility tests were completed (10–25 °C and 68–136 atm). The oil samples were separated using a FilmTec NF90 or a FilmTec BW30 membrane in a dead-end type cell. Within the range examined, the greatest solubility for oleic acid was at 25 °C and 136 atm. For soybean oil TAGs, the greatest solubility was at 20 °C and 136 atm. However, for the separation of the two components, 20 °C and 68 atm was best among the condition combinations examined. The solubility of oleic acid ranged from 0.294 to 0.455 mg/mL in subcritical carbon dioxide, while the solubility of triacylglycerols ranged from 0.066 to 0.139 mg/mL. The FilmTec BW30 membrane provided significantly better separation of FFAs from TAGs than did the NF90 membrane. Both membranes were selective for oleic acid, although the BW30 had greater selectivity for oleic acid (β _{oleic acid} = 2.12, β _TAGs = 0.24) than the NF90 membrane (β _{oleic acid} = 1.26, β _TAGs = 0.81).