Non-Evaporative Solvent Recovery Step in Deacidification of Used Frying Oil as Biodiesel Feedstock by Methanol Extraction

An alternative deacidification process combining a liquid-liquid extraction with a non-evaporative solvent recovery step was proposed for preparing used frying oil (UFO) as biodiesel feedstock. The liquid-liquid extraction step using methanol was simulated for obtaining refined UFO with a final residual free fatty acids (FFA) content ≤ 1%. Solvent recovery step of the process, which is based on the precipitation of FFA with Ca(OH)₂ as insoluble calcium soaps, was investigated experimentally. With the aim of maximizing the FFA removal from the methanol extract, the influence of process variables such as FFA concentration in the extract phase, Ca(OH)₂ amount, stirring rate and temperature were investigated by using model extract phases. Complete removal of FFA was achieved in 30, 20, and 15 min, from the extract phases containing 3.86, 7.78, and 11.58 wt% FFA, respectively, when the precipitation was carried out at a temperature of 65 °C, stirring rate of 250 rpm and by using 18 times the stoichiometric Ca(OH)₂ amount. The precipitate quickly settled down due to the agglomeration, thereby provided an efficient and easy separation of the methanol from the solids. Provided the final residual content of FFA in methanol was too low, recovered methanol can be recycled for more FFA extraction. Although the suggested process offers a feasible method for preparing UFO as biodiesel feedstock, the effect of other decomposition products in UFO must be investigated in depth for using such a process on an industrial scale.