Asphaltene Aggregation and Solubility

• Published in: Energy & fuels 2015-04, Vol.29 (4), p.2120-2133
• Main Authors: Painter, Paul, Veytsman, Boris, Youtcheff, Jack
• Format: Article
• Language: English
• Subjects: Agglomeration; Asphalts; Clusters; Mathematical models; Molecular weight; Nanostructure; Phase boundaries; Thermal expansion
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/ef5024912

An attenuated association model describing the aggregation of asphaltenes in solution is extended to derive an equation for the weight-average degree of association and account for phase behavior. The weight-average molecular weight is calculated to be higher than number average, as it must be for a polydisperse material, but not by enough to explain the very large differences in these quantities reported in the literature. Binodals and spinodals are calculated using expressions derived previously, but modified to account for free volume (thermal expansion) differences. The phase behavior of asphaltene solutions is examined in more detail, particularly in the dilute solution regime. It is shown that the formation of nanoaggregates significantly affects the critical value of the χ interaction parameter. The phase diagram is highly asymmetric and the phase boundary approaches the pure solvent composition limit. This has a number of implications in terms of asphaltene solution characterization and the nature of asphaltene solutions. The results indicate that there are toluene insoluble asphaltene components, but these could exist as microphase-separated clusters stabilized against further aggregation by steric and kinetic factors. This would explain the large difference between observed number and weight-average molecular weights. In addition, because of the shape of the binodal curve at low concentrations, experimental data that have previously been interpreted in terms of a critical cluster or micelle concentration are shown to be consistent with a microphase separation.