Kinetics of oxidation and decomposition of soybean biodiesel evaluated by the TTT superposition theory and the Freeman-Carroll method

Abstract

In the present work, the oxidation process of a commercial soybean biodiesel was investigated under oxidative atmosphere in different conditions of temperature (Tox) and time (tox). Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetry analysis (TGA) were used to characterize the oxidation and decomposition processes of biodiesel, respectively. For both experimental techniques we have applied the Time-Temperature-Transformation (TTT) superposition theory in order to explain the oxidation and decomposition phenomena. The FTIR spectra showed that when the oxidation times and temperature was increased the intensity of some characteristic bands suffered a decreasing, proving the oxidation occurrence on soybean biodiesel. The thermal stability studies (TG/DTG curves) have been appointed out that the process for Tox<80°C occurs in a single step which is attributed to the evaporation of the methyl esters. On the other hand, the samples obtained at higher temperatures for Tox>80°C displayed multiple-stage thermal decomposition processes which are ascribed to the evaporation and/or pyrolysis of the methyl esters and their sub products. The kinetic studies realized by the TTT superposition theory were comparable with that obtained by the Freeman-Carroll, Flynn-Wall-Osawa and Vyazovkin methods. The evaluation of kinetic compensation effect (KCE) has been found usefully to predict the isokinetic parameters as lnkiso, which was shown to have a unique value independently of the oxidation conditions.