MATHEMATICAL SIMULATION OF THE PYROLYSIS OF A MODEL ASPHALTENE.

The model predictions were consistent with the available experimental data on a qualitative basis without exception and on a quantitative basis in several instances. This agreement between model and experimental results is striking because the model deals with a simplified asphaltene structure, includes only model compound-deduced reaction pathways and kinetics, and contains no kinetics parameters regressed from experiments with actual asphaltenes. The overall consistency of the experimental and simulated asphaltene pyrolyses suggest that the model included many key features of asphaltene structure and its thermal reactivity and that the pyrolysis kinetics of the model compounds mimicked those of the related moieties in asphaltene. The model results show that dealkylation of the asphaltene unit sheets caused the particles to become increasingly hydrogen deficient and more aromatic thereby suggesting an attendant change in their toluene solubility. Thus, as reactant asphaltenes, toluene soluble because of their aliphaticity, were cleaved of their peripheral substituents their toluene solubility diminished and they eventually appeared as coke in the pyrolysis simulation. The modeling results thus demonstrate that severe overreaction of primary products is not necessary to predict high yields of coke. The model results also permit speculation into the role of pyrolysis in nominally catalytic asphaltene hydroprocessing reactions. The simulations showed that asphaltene depolymerization occurred even at short reaction times and that many particles existed as single unit sheets rather than covalently bonded oligomers thereof. These individual asphaltene unit sheets, which are smaller than the macromolecular particles, will be major participants in catalytic reactions because they can more readily diffuse within the porous catalyst. This suggests that catalytic hydroprocessing at high temperatures will be of thermally derived asphaltene fragments and not the asphaltene particle itself.