Effects of Temperature, Antioxidants, and High‐Vacuum Distillation on the Oxidation of Biodiesel Derived from Waste Vegetable Oil

Abstract

AbstractBiodiesel offers several environmental benefits and improvements to some fuel performance properties, but its poor oxidative stability has been a major concern. Currently, the accepted practice to improve biodiesel oxidative stability is the addition of antioxidants; numerous antioxidants have been studied but their effectiveness in inhibiting biodiesel oxidation is difficult to predict due to variation with resonance stability, solubility, reactivity, and volatility. To improve prediction efforts, this study explored the Rapid Small‐Scale Oxidation Test (RSSOT) as a means to investigate how biodiesel oxidation is affected by antioxidant concentration and temperature, and compared its results with the oxidative stability index test. A weak correlation was identified due to antioxidant variation. A kinetic model expressed in temperature and induction period was developed for biodiesel before high‐vacuum distillation (HVD), after HVD and also after HVD with three concentrations of propyl gallate (PG) and tert‐butylhydroquinone (TBHQ) antioxidants. The approach was validated by comparing collected data on the oxidation of methyl oleate with kinetic parameters found in the literature. Antioxidant concentrations from 130–930 ppm were tested, and the results revealed that the apparent activation energy of biodiesel oxidation increases with increasing concentration of primary antioxidants and decreases during vacuum distillation. When treated with an increasing concentration (130–930 ppm) of PG and TBHQ, the apparent activation energies of a vacuum distilled biodiesel changed from 108.46 ± 4.45 to 112.72 ± 1.46 kJ·mol−1 and from 77.14 ± 2.25 to 89.91 ± 2.29 kJ·mol−1, respectively. These observed trends agree with both the accepted mechanism of primary oxidation of fuels and mode of action of primary antioxidants.