Oxidation Behavior and Kinetics of Eight C<inf>20</inf>-C<inf>54</inf> n -Alkanes by High Pressure Differential Scanning Calorimetry (HP-DSC)

Abstract

© 2018 American Chemical Society. In this study, the oxidation behavior and kinetics of linear alkanes (C20H42, C24H50, C30H62, C32H66, C36H74, C38H78, C50H102, and C54H110) were investigated by high pressure differential scanning calorimetry (HP-DSC). It turned out that only the exothermic peak of low-temperature oxidation (LTO) was observed during the oxidation process of these linear alkanes, which is different from the oxidation behavior of the crude oil. For the crude oil, two exothermic peaks were observed: LTO and high-temperature oxidation (HTO). This means that the linear alkanes barely contributed in the HTO reaction of crude oils. In addition, the exothermic peaks in the oxidation process of all these linear alkanes overlapped each other. They showed almost the same oxidation behavior in terms of the temperature range of reaction as well as the onset and peak temperatures. It seems that the oxidation behavior of the tested linear alkanes was independent of their carbon number. It was also found that increasing pressure resulted in an increase of the heat release. The kinetics parameters of the oxidation reaction were estimated using three "model-free methods" known as Friedman, Ozawa-Flynn-Wall (OFW), and ASTM E698. The results showed that the activation energy of the LTO process of each linear alkane can be regarded as a constant average value in the range of conversion degree from 0.2 to 0.8, and all the tested linear alkanes had similar activation energy values of 80-120 kJ/mol calculated by the Friedman method and 90-110 kJ/mol calculated by the OFW method. The OFW method showed a lower error than the Friedman method when being applied to the DSC data. The values of activation energy estimated using the ASTM E698 method were 100.41, 95.61, 93.62, 100.55, and 92.47 90-110 kJ/mol for C20H42, C24H50, C30H62, C38H78, and C54H110, respectively, which are also in the same range of the values of the activation energy obtained by the Friedman and OFW methods. Similar activation energy values of different linear alkanes partly explained why they showed almost the same oxidation behavior.