Location: Bio-oils Research Unit
Title: Does the “Rancimat method” really measure the oxidative stability of biodiesel? Author
Submitted to: Annual Meeting and Expo of the American Oil Chemists' Society
Publication Type: Abstract Only
Publication Acceptance Date: February 28, 2013
Publication Date: April 28, 2013
Citation: Dunn, R.O. 2013. Does the “Rancimat method” really measure the oxidative stability of biodiesel? (Poster). Annual Meeting and Expo of the American Oil Chemists' Society. p. 63. Technical Abstract: Biodiesel is composed of a mixture of saturated and unsaturated mono-alkyl esters of fatty acids derived from vegetable oil or animal fat. While the saturated esters are relatively stable, the monounsaturated and especially polyunsaturated esters are susceptible to oxidative degradation. If biodiesel is contacted with air during storage, then oxidation of the unsaturated esters may lead to increases in kinematic viscosity and acid value and affect other important fuel properties. Maintaining good fuel quality during storage is of utmost concern to fuel producers. Therefore, biodiesel fuel standards such as ASTM D 6751 and CEN EN 14214 require it to pass an oxidation induction period (IP) test. The test (EN 15751), often referred to as the “Rancimat method,” measures the IP under accelerated conditions by bubbling air through the oil sample and heating it at constant T = 110°C. The experimental conditions necessary to initiate oxidation in method EN 15751 as well as provide a timely result are known to promote reaction pathways that differ from the pathways experienced by the same fatty oil sample under realistic conditions. The present study is an investigation into the reaction kinetics of oxidation at different temperatures. Biodiesel made from soybean, canola, and palm oils as well as pure methyl oleate and linoleate were analyzed by pressurized-differential scanning calorimetry (P-DSC) and a Rancimat instrument. P-DSC data were analyzed by the Ozawa-Flynn-Wall method to infer reaction rate constants (k) for the oxidation reaction. The k-values were then utilized to predict IP data for comparison with measured IP data from the Rancimat instrument. Results suggested that moderate to very high degradation rates were necessary to achieve the measured IP, suggesting that running the Rancimat instrument at 110°C may overestimate the actual IP of biodiesel.