Thermogravimetric analysis of petroleum asphaltenes along with estimation of average chemical structure by nuclear magnetic resonance spectroscopy

Combining with the 1H and 13C nuclear magnetic resonance (NMR) determinations, elemental analysis and molecular weight measurement, average molecular formula of the chemical unit for the asphaltenes from Chinese Daqing crude oil were calculated. Thermal pyrolysis kinetics of the asphaltenes had been studied using thermogravimetric analysis (TGA). The distributed activation energy model (DAEM) was used to analyze these complex systems. The results show that the peak activation energy for pyrolysis of the asphaltenes is 245 kJ mol−1 and the pre-exponential factor is 5.88 × 1014 s−1. The DAEM method presented reasonably good results of the prediction of the weight loss curves. A linear relationship can be found from the plots of logarithm of the pre-exponential factor against the activation energy at selected conversion values. This phenomenon known as the compensation effect was explained and it was in agreement with the estimated chemical structure determined by NMR.