Isothermal low-temperature oxidation kinetics of heavy crude oil and its oxidized oils

Abstract

A large amount of heavy oil inevitably encountered different degrees of isothermal low-temperature oxidation (LTO) reactions in the processes of in-situ combustion, storage, and transportation, so it was of vital importance to study the isothermal LTO kinetics of heavy oil that was still less well-understood. In this study, a thermogravimetry was used to study the isothermal LTO characteristics of heavy crude oil and its oils oxidized at 120, 160, and 200 °C, and four LTO temperatures of 125, 200, 275, and 350 °C with 1 h heating treatment were taken into account. The isothermal kinetic models were then established based on the n-order reactions. Subsequently, the comparisons of conversion degree between experiments and predictions were conducted to ensure the reliability of the established kinetics. The results showed that the mass losses for all the oil samples were almost linearly proportional to the LTO duration at 125 °C, indicating that the LTO time generated a significant effect on the thermal deposition. The shift rightward of the LTO peak with the elevated LTO temperature verified that the structures of all the oil samples were affected by LTO to a certain extent. After thermal degradation at 350 °C, 49.5% of LTO residue remained for the oxidized at 200 °C, obviously more than that for the other three oil samples. This gave a hint that the oils oxidized at higher LTO temperatures could be transformed into more fuel serving for combustion reactions. The appropriate values of reaction order of crude oil and its oils oxidized at 120, 160, and 200 °C were determined to be 4, 5, 5, and 7, respectively. There was no apparent difference in the values of activation energy, indicating that all the oil samples had close mass loss rates that determined the entire mass loss during isothermal LTO. The oil oxidized at 200 °C had fairly higher frequency factor than the other three oil samples, due primarily to its stronger polar molecular structure that resulted in higher molecular collision frequency. The prediction lines matched well with the experimental ones for all the oil samples, demonstrating that the developed isothermal kinetics was a useful tool to predict the LTO behavior of heavy oils.