Fractionation of Used Frying Oil by Supercritical CO2 and Cosolvents

Supercritical extraction with pure and modified CO2 has been used for the fractionation of waste frying oil at different temperature and pressure conditions (25-80 degrees C and 300-400 kg/cm(2)). The cosolvents used to modify the CO2 behavior were ethanol, methanol, acetone, and hexane. They were selected because of their capacity to form hydrogen bonds. Both extraction rate and oil yield were larger under high density conditions (high pressure and low temperature). Further, when cosolvents were used, higher values of both variables were attained at softer operating pressures. Regarding the effect of cosolvents on these variables, it was Found to follow this order: ethanol > methanol > acetone > hexane. The relative separation efficiency (RSE) of triglycerides (TG) from polar compounds with molecular weight higher than triglycerides (HMWC) was not greatly dependent on the operating conditions (pressure, temperature, cosolvent type, and cosolvent concentration). However, better values of the separation efficiency of TG from polar compounds with molecular weight lower than triglycerides (LMWC) were attained under low density conditions and using hexane as cosolvent. On the basis of the results relative to extraction rate, oil yield, and separation efficiency, the best extraction conditions were selected to separate a purified TG fraction from used frying oil. They were P = 300 kg/cm(2), T = 40 degrees C, cosolvent type = hexane, and cosolvent concentration = 0.1 g of hexane/g of solvent. However, when processing the oil at these conditions, the polar content of the lipid fraction recovered still was about twice the fresh frying oil polar content. For this reason, a two-stage supercritical fluid extraction was performed. The composition of the fraction recovered in the two-stage process was very close to that of the fresh oil, a fact that clearly shows that the supercritical extraction with CO2 + hexane is an efficient method for the purification of the TG fraction of waste frying oil.