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Thermal characterization and kinetics of crude oils by tga and dsc methods

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2010
Gündoğar, Satı Aslı
In recent years, the application of thermal analysis to study the combustion and pyrolysis behavior of fossil fuels has gained a wide acceptance because of its significance for industry and economy. In this thesis, the thermal and kinetic analysis of different origin crude oil samples are performed by two well-known thermal analysis techniques: Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG/DTG). The investigation of combustion and pyrolysis behaviors, kinetic analysis of oil samples and the determination of heating rate effect are the main objectives of this study. Six different crude oils from the Southeastern region of Turkey are analyzed throughout the study. All experiments are performed at different heating rates (5, 10 and 15ºC/min) and air is used for combustion and nitrogen for pyrolysis experiments. In combustion experiments, TGA and DSC techniques indicate that the combustion process of crude oils studied is composed of two main reaction regions. These are low-temperature (LTO) and high-temperature oxidation (HTO) regions. In LTO, huge mass loss occurs (from 69 to 87 %) due to high amount of free moisture and volatile hydrocarbons contained in oil samples. Combustion reactions continue up to 900 K. On DSC curves, two exothermic regions of oxidation regimes are detected. Comparing TG/DTG and DSC curves, it can be understood that the mass loss under combustion is accompanied by exothermic peaks because of the oxidative degradation of crude oil components. As in combustion, two distinct reaction regions are revealed under pyrolysis for all samples. The first region indicates distillation and the second one is due to thermal cracking reactions occur at high temperatures and completed up to 840 K. As expected, lighter crude oils have relatively higher amounts of mass loss in distillation region as compared to heavier ones. Besides, residue amount and burn-out temperatures are higher for heavier oils with higher asphaltene content in cracking region. DSC curves for both reactions show endothermic effects. In combustion and pyrolysis experiments, it is noticed that higher heating rates are resulted in higher reaction regions. Distinguishing peaks of samples shift to higher temperatures with an increase in heating rate. Heat of reaction amount under DSC curves is related to asphaltene content and ˚API gravity of crude oils. It is deduced that, when ˚API gravity of crude oils decreases, the heat value of this reaction increases. The kinetic parameters are evaluated by different kinetic models and mean activation energies (Em) of samples are obtained. At the end, a correlation is established between Em and ˚API gravity of oil samples. It is concluded that heavier oils have higher activation energy and Arrhenius constant values for each reaction region. Besides, it is proved that the activation energy is mostly insensitive to the heating rate.