Kinetic parameters of the oxidation reaction of commercial biodiesel with natural antioxidant additives

Biodiesel in Brazil is obtained through the mixture of vegetable oils and animal fats. However, some of those raw material sources have characteristics that are more susceptible to oxidation. As an alternative to reduce the speed of the onset of the oxidation reaction, spices are added to biodiesel. The oxidative stability was analyzed through the period of induction using the Rancimat® method. This study evaluated the effects of antioxidant extracts from senna leaves, blackberry fruits and hibiscus flowers mixed to commercial biodiesel through the determination of kinetic parameters, as well as investigating possible linearity deviations from the Arrhneius equation. It was possible to verify that the rate constant values were smaller when compared to the control sample; lower values were observed for mixtures with higher proportions of hibiscus flower extract. The activation energy values did not present linear behavior, and some tests presented values above the control sample, showing that the addition of antioxidants can significantly alter this parameter. Sub-Arrhenius behavior was observed for the sample containing the binary mixture of senna leaves and hibiscus flowers and for the control; however, sample containing only senna leaf extract presented super-Arrhenius behavior.