Adsorption/occlusion characteristics of asphaltenes: Some implication for asphaltene structural features
Résumé
In the crude oil, the other fractions such as hydrocarbons could be adsorbed and/or occluded inside asphaltene structures. In this work, the deuterated compound n-C20D42 has been used to simulate the adsorption/occlusion characteristics of asphaltene in toluene solution. Based on the highly polydispersed structures of asphaltenes, the experimental results indicate that the adsorbed hydrocarbons could be exchanged with the outside bulk phase, whereas the occluded ones inside the core of asphaltenes could not be exchanged. These results suggest that substantial microporous units exist inside asphaltene structures, and these structural units are suitable to adsorb and occlude the other fractions in the crude oil. The carbon number of the occluded hydrocarbons could be up to C40, and the weight could be more than 0.5%, based on the initial asphaltene weight. The adsorption should occur at the periphery of asphaltene aggregates (the loosely packed ones), and then they are liable to be exchanged with the outside bulk phase. If the asphaltene molecule is large enough, the occlusion could occur at the asphaltene molecular level. However, this type of occlusion perhaps just occurs inside asphaltene aggregates (the close aggregating ones); if this is the case, this type of asphaltene aggregate should be considered to be the stable unit in toluene solution, and then the occluded hydrocarbons could not be exchanged with the bulk phase. Regarding the adsorption/occlusion properties of asphaltenes, the aforementioned results should be valid, even extrapolated into the crude oil reservoir system.