Thermodynamic and thermokinetic study on pyrolysis process of heavy oils

摘要

Enthalpy of pyrolysis and its variation in the pyrolysis process of four heavy oils: Daqing vacuum residue (DQVR), Karamay vacuum residue (KRVR), Liaohe vacuum residue (LHVR), and Venezuela vacuum residue (VNVR), have been quantitatively studied by differential scanning calorimetry associated with thermogravimetry. The results indicate that overall enthalpies at different heating rates show a linear trend with respect to the final coke yields in the thermal analysis. Classical kinetic method (Friedman method) is used to further analyze pyrolysis enthalpy variation in the pyrolysis process and determine the thermokinetic parameters. The main stage of thermal reaction (conversion ranges from 0.1 to 0.9) could be described by 1.5 order reaction model for four heavy oils. The mean activation energies determined by Friedman method are 216.3, 194.9, 173.9, and 168.7 kJ mpl~(-1) for DQVR, KRVR, LHVR, and VNVR, respectively. It means that endothermic enthalpy of pyrolysis in the thermal process of VNVR is easier to change compared with other oil sample cases. For the sake of simplification of kinetic treatment, Sharp method is tentatively used to perform kinetic analysis. The comparison between results from two methods indicates that activation energies from Sharp method are valid to a certain degree under the condition that the mechanism of thermal process is properly chosen although isoconversional method (Friedman method) is recommended and thought to be the better way.