Effect of Pressure on the Oxidation Kinetics of Asphaltenes

摘要

This study was carried out with the aim of evaluating the effect of the high pressure on the oxidation kinetics of n-heptane-insoluble asphaltenes, obtained from an extra-heavy crude oil. Thermogravimetric analyses (TGA) were performed under an air atmosphere, at different pressures from 0.084 to 7 MPa, and temperatures from 100 to 600 degrees C at different heating ramps of 5, 10, and 15 degrees C min(-1). The effective activation energy and the kinetic parameters were obtained using a first-order kinetic model, which indicated a pressure-dependent behavior. For a better understanding of the asphaltene oxidation under high-pressure conditions, the temperature range in which the oxidation process was carried out was divided in to four main regions according to the TGA profile, namely: (0 oxygen chemisorption (OC), (ii) decomposition of the chemisorbed oxygen (DCO), (iii) first combustion (FC) region, and (iv) second combustion (SC) region. It was observed that the increase of pressure favors the asphaltene decomposition as the percentages of mass loss in the first combustion region are 20% at 0.084 MPa and 50% at 7 MPa. Furthermore, the temperature at which each thermal event ends is reduced by approximately 35, 23, 13, and 51 degrees C from 0.084 to 7 MPa for OC, DCO, FC, and SC, respectively. Also, by increasing the heating rate, the decomposition of the asphaltene in the second combustion region is increased, indicating that the decomposition follows different mechanisms depending on the exposure time. On the other hand, with the increase in the system pressure, an increase of 53.1 and 71.1% of the effective activation energy values was observed for the thermal events associated to oxygen chemisorption and decomposition of chemisorbed oxygen, respectively, while for the combustion (FC and SC) stages, the activation energy decreases by 61.4 and 75.6%, respectively, indicating that the asphaltene oxidation behavior is controlled by the pressure in the four regions. All of these facts show that the kinetic limiting step for the asphaltene oxidation is the chemisorption of the oxygen, which is favored by the increase of the pressure.