Hydrolysis of Waste Frying Oils in Subcritical Water for Biodiesel Production by Esterification Using a Heterogeneous Catalyst

Waste frying oils have become a material of great interest for various routes of biodiesel production. This study investigates waste frying oils (WFO) under subcritical hydrolysis to generate free fatty acids for biodiesel production using zinc aluminate as heterogeneous catalysist. WFO were pre- treated to reduce particulate material and saponified compounds present in the raw material. Their kinematic viscosity, fatty acid composition and proton Nuclear Magnetic Resonance (NMR+) were determined, which identified strong similarities between WFO and refined soybean oil. Through the fatty acid composition analysis of WFO, a molecular weight of 873 g/mol was obtained and linoleic acid was identified as the main component in the tested oil. The hydrolysis experimental runs were conducted over a range of temperatures between 200-250 degrees C. Once hydrolysis reactions were completed, a significant increase in the acid value was observed for all samples. After phase separation, a subsequent ethyl esterification of free fatty acids obtained from hydrolysis was carried out at 100 degrees C and 150 degrees C, using 10:1 and 20:1 as ethanol: oil molar ratio. The biodiesel produced by subcritical water hydrolysis and esterification was analysed by Gas Chromatography (GC). The results showed that this route provides an effective contribution towards the feasibility of alkyl ester production by esterification of free fatty acids using a zinc aluminate catalyst.