Purification of used frying oil by supercritical carbon dioxide extraction

Supercritical fluid extraction (SFE) was used for the purification of used frying oil at different temperatures (40 and 60 degrees C) and pressures (1 50, 225 and 300 bar). The calculated solubilities of the oil ranged from 0. 3 x 10(-3) to 7.4 x 10(-3) g oil/g CO2 depending on the extraction conditio ns. The solubility and the selectivity of triglycerides (TG) were between t hose of the low molecular weight compounds (LMWC) and the polymers. The cal culated relative separation efficiency (R) of low molecular weight compound s (LMWC) from TG was highest under the low pressure and high temperature ex traction conditions, but the extraction rate was minimal under this conditi on. However. the separation efficiency (R) of polymers from TG was not grea tly dependent on the pressure and temperature. Based on the relative separa tion efficiency result, two-stage supercritical carbon dioxide extraction w as performed for the effective purification of TG from used frying oil. LMW C were removed under the low pressure extraction conditions (150 bar, 60 de grees C) of the first stage extraction, and then TG were concentrated under the high pressure extraction conditions of the second stage extraction (30 0 bar, 60 degrees C). The extracts obtained from the second to the fifth fr action of the second stage included less than 1% polymer, more than 92% TG, and less than 8% LMWC. The conjugated diene contents of these extracts ran ged from 0.202 to 0.231%, showing a similar conjugated diene content to the fresh frying oil. The present data clearly showed that two-stage supercrit ical CO2 extraction was an efficient method for the purification of TG from used frying oil. (C) 2000 Elsevier Science Ltd. All rights reserved.