In this study, both the standardized Rancimat and thermoanalytical pressure differential scanning calorimetry (PDSC) methods were employed to investigate the effects of antioxidants on the oxidation stability of biodiesel . The differences in the antioxidant effectiveness between these phenolic compounds could be clearly distinguished by both methods. Results revealed that the efficiency of antioxidant was highly dependent on the test conditions used in each method. Among the five antioxidants investigated, tert -butylhydroquinone (TBHQ) showed the highest sensitivity to different conditions between two methods: the highest oxidation stability in Rancimat vs. the lowest efficiency in PDSC. Butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) acted more efficiently in PDSC than in Rancimat due to lower volatilization losses, but not as well as pyrogallol (PY) and propyl gallate (PG) in increasing the oxidation stability of biodiesel. Results from isothermal and nonisothermal PDSC methods were basically consistent with each other with respect to the antioxidant efficiency: PY > PG > BHA, BHT > TBHQ. A positive linear relationship between results obtained from both methods could be observed within each type of antioxidant, but the overall correlation for the entire data set between Rancimat and PDSC was not as strong as that between isothermal and nonisothermal PDSC methods.
Kinetic studies on the oxidation of biodiesel and the consumption of antioxidants were performed in this work, using the standard Rancimat method within the temperature range of 100–150 °C. On the basis of the length of induction period, antioxidant efficiency was ranked in the descending order of tert-butylhydroquinone (TBHQ) > pyrogallol (PY) > propyl gallate (PG) > butylated hydroxyanisole (BHA) > butylated hydroxytoluene (BHT). The natural logarithm of the initial antioxidant concentration varied linearly with respect to the induction period. Kinetic information for the consumption of the antioxidant could be obtained by pseudo-first-order reaction assumptions. TBHQ had the lowest consumption rate, followed by PG, PY, BHA, and BHT; the lower consumption rate seemed to lead to a longer induction period. The critical antioxidant concentration increased in the ascending order of PY < PG < BHA < BHT < TBHQ, basically in accordance with the order of antioxidant efficiency, except TBHQ. The apparent activation energy for the consumption of the antioxidant was determined using the Arrhenius equation: TBHQ (97.85 kJ/mol), PG (96.91 kJ/mol) > BHT (91.74 kJ/mol) > PY (82.76 kJ/mol) > BHA (80.08 kJ/mol), which agreed well with the order of activation energy for the oxidation of biodiesel. TBHQ, PG, and BHT, more temperature sensitive than PY and BHA, showed higher susceptibility to oxidative degradation at a higher temperature. The shelf life of biodiesel at ambient temperature was predicted with the extrapolation method and ranked in the descending order of TBHQ > PG > PY > BHT > BHA, basically in line with the order of the reaction constant of antioxidant consumption.
危险品化学品经营许可证（带存储）