Distributed Properties of Asphaltene Nanoaggregates in Crude Oils: A Review

International audience ; The molecules in petroleum asphaltenes follow a molecular structure continuum similar to the rest of the oil but differ in the formation of nanoaggregates in solution. The nature of the aggregation, its size distribution, and dimensions of the nanoaggregates is still debated, and the impact of this aggregation on phase behavior, physical properties, and the processing of heavy petroleum fractions remains unclear. This review first discusses the role of nanoaggregates in the phase separation of asphaltenes, then examines the literature on subfractionation of the asphaltene fraction to define how different molecules partition. Efforts to measure the distribution of nanoaggregate size are summarized, and the roles of size distribution in the behavior of asphaltenes during sedimentation, emulsion formation, and refinery processing are examined. Finally, the molecular basis for formation of a distribution of aggregate sizes/properties is considered. The molecular weight ranges as high as 40,000 Da but the size is below 100 nm. Although few direct experimental data are available for the distribution of aggregate properties, three behaviors indicate the importance of portions of the nanoaggregate population. The highest molecular weight aggregates are the least soluble, driving phase behavior when the asphaltenes are partially precipitated. A portion of these large aggregates are also most responsible for the stabilization of asphaltene films at oil–water interfaces, which stabilize oil-in-water emulsions. The vanadium and nickel species that are most resistant to removal during refining of vacuum residues are also found in the most aggregated fraction, although the stability of the aggregates at process temperatures is not yet proven. Average properties for the asphaltene fraction, including both free molecules and nanoaggregates, are sufficient for prediction of onset of asphaltene phase separation, gravity segregation, and properties such as density and viscosity.Cited ByCitation State