Distributed Properties of Asphaltene Nanoaggregates in Crude Oils: A Review

Abstract

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.

Physicochemical Characterization of Asphaltenes Using Microfluidic Analysis

“…Asphaltenes are described as colloidal aggregates that may exist as molecules, nanoaggregates, and/or clusters stable in the crude oil phase, schematically shown in Figure B. , The self-association of asphaltenes is affected by concentration, solvent conditions, pressure, and temperature, which has led to controversial discussions regarding asphaltene molecular weight as discussed above. In accordance with the hierarchical structure of asphaltenes described by the Yen model, intermolecular interactions between asphaltene molecules allow a fair number of molecules to self-associate and grow to a small limiting size through a balance of van der Waals interactions, Coulombic, and repulsive exchange interactions through hydrogen bonding and aromatic core interactions, as shown in Figure C. − In the Yen-Mullins model each asphaltene molecule (1.5 nm diameter) participates in a single nanoaggregate (2.0 nm diameter) that forms at low concentration in a good-asphaltene solvent.…”

2021 Pioneers in Energy Research: Alan Marshall

“…■ CHARATERIZATION OF PORPHYRINS, ASPHALTENES, AND AGGREGATES Gray and co-workers 43 reviewed the nature of aggregation and distributed properties of asphaltene nanoaggregates in crude oils. The role of nanoaggregates in sedimentation, emulsion formation, and refinery processing was surveyed and highlighted the need for additional studies.…”

Tracking Changes in Asphaltene Nanoaggregate Size Distributions as a Function of Silver Complexation via Gel Permeation Chromatography Inductively Coupled Plasma Mass Spectrometry

“…The molecular composition, structure, chemical functionalities, and properties of asphaltenes remain the most important issues in petroleum chemistry. − However, molecular characterization is hampered by aggregation, even in “favorable” solvents, which limits molecular characterization to those species that exist as free molecules in solution . Previous studies of the unaggregated fraction of asphaltenes have revealed that the molecular weight of these molecules is not as high as previously suggested and is primarily distributed in the range of 500–1250 Da .…”

“…1−3 However, molecular characterization is hampered by aggregation, even in "favorable" solvents, which limits molecular characterization to those species that exist as free molecules in solution. 4 Previous studies of the unaggregated fraction of asphaltenes have revealed that the molecular weight of these molecules is not as high as previously suggested and is primarily distributed in the range of 500−1250 Da. 5 Past studies were now known to suffer from limitations imposed by the presence of aggregates, because the apparent molecular weight ranged from tens of thousands or even hundreds of thousands of daltons and changed with solvent composition.…”