Submitted to: Energy and Fuels
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/3/2007
Publication Date: 1/15/2008
Citation: Dunn, R.O. 2008. Effect of temperature on oil stability index (OSI) of biodiesel. Energy and Fuels. 22:657-662. Interpretive Summary: Biodiesel is an alternative fuel derived from vegetable oil or animal fat. The main components of biodiesel from soybean oil and other feedstocks are very reactive to oxygen when placed in contact with air. Extensive oxidation can cause fuel quality to degrade during storage. The ability of biodiesel to resist oxidation can be determined by measuring the oil stability index (OSI). Temperature is known to accelerate the oxidation reaction and can influence individual steps in the reaction. This work investigates effects of temperature on OSI of biodiesel from two feedstocks, soybean and used cooking oils. Results were compared with those for a pure fatty acid methyl ester standard. Two mathematical models were developed for predicting OSI of biodiesel as a function of temperature. Activation energy data was also calculated from these results. This work shows that while increasing temperature accelerates oxidation, the reaction mechanism (steps to complete the reaction) was not significantly altered. It provides a means to estimate the maximum storage time before resistance to oxidation breaks down. It reports valuable data that may be used to characterize oxidation under various reaction conditions. Finally, results will directly benefit scientists and engineers seeking to further understand biodiesel fuel quality issues.
Technical Abstract: Biodiesel is primarily composed of saturated and unsaturated fatty acid alkyl esters. Fuel suppliers, terminal operators and users are becoming more concerned with monitoring and maintaining good biodiesel fuel quality with respect to oxidative degradation during storage. Oil stability index (OSI), a parameter that measures the ability of fatty material to resist oxidation, is typically measured at high temperature to accelerate oxidation. The present work investigates the effects of temperature (T) on OSI of biodiesel from soybean oil fatty acid methyl esters (SME), used cooking oil fatty acid methyl esters (UCOME) and pure methyl oleate (MO). Increasing T accelerated the oxidation reaction causing a decrease in OSI. Response factors determined using MO as a reference showed little variation with respect to T in the ranges studied. At constant T, SME yielded lower OSI values than either UCOME or MO. However, despite having comparable iodine values UCOME yielded significantly higher OSI than MO. Two models for predicting maximum storage time as a function of T demonstrated linear correlations for all three methyl esters. Results from the second model, ln(OSI) versus T-1, were employed to calculate the activation energy of first-order oxidation reactions. Although both models showed relatively small deviations between calculated and measured OSI, the models were not reliable at temperatures below 50 deg C.