The interaction of two model asphaltene molecules from the Athabasca sand oil with a water molecule in a toluene solution was studied by means of molecular mechanics calculations. It was found that water forms bridging H bonds between the heteroatoms of asphaltenes with a considerable span in energies. The stronger H bond found has energies higher than those corresponding to the stacking of the aromatic areas of the same asphaltene molecules. This shows that the water molecule may generate additional mechanisms of aggregation of asphaltenes in toluene solution, as found experimentally. The H bond mechanism depends on the heteroatoms involved, the extension of the aromatic regions, and the steric interference present in the asphaltene molecules. The simulation results have been compared with experimental values of enthalpy of association of two different petroleum asphaltenes obtained by titration calorimetry. A simple dimer dissociation model was used to derive the information about the heat and the constant of dissociation from asphaltenes of Mexico and Alaska obtained from the calorimetric data. The association enthalpies calculated were found to be in excellent agreement with those measured, although the simulation only employed the interaction between averaged molecular structures.

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This article collects the work performed by Isothermal Titration Calorimetry (ITC) in the study of the self-association of asphaltenes in toluene solutions. Calorimetric experiments show that asphaltenes start self-associating at very low concentrations and that the existence of a Critical Micellar Concentration is rather improbable. The influence of the water dissolved in the toluene has been studied but the difficulties in keeping the water concentration low hinder the discussion of the results. The calorimetry data have been treated with a simple dimer dissociation model and compared with the results of the titration of three model molecules: nonylphenol, which associates by means of hydrogen bond formation, and coronene and pyrene, which associates by stacking. The results obtained leave open the question about the model-stacking molecules, as it was not possible to elucidate whether they do not associate or the dilution effect does not break the aggregates. The fluorescence spectroscopy results support the results of calorimetry with respect to the self-association at low concentrations. ITC has been applied for the first time to the study of the interaction between asphaltenes and a model resin, namely nonylphenol.

For many years, the concept of critical micellar concentration (CMC) has been projected from surfactant science into asphaltene science. There are several similarities between these two species, such as the stabilization of water-in-oil emulsions and surface activity, which suggested that asphaltenes may also have a concentration at which self-association occurs (CMC). This article presents evidence found by calorimetry and spectroscopic techniques, that suggest that this concept may not be adequate for asphaltene self-association in toluene solutions. Isothermal titration calorimetry has been widely used in surfactant science to determine both the CMC and the enthalpy of micellation of many surfactants. The concentration interval could be divided into three regions: monomer region, micellation region, and micelle region. The absence of the first region (monomer) in the concentration range usually found in the literature as the CMC region of asphaltenes indicates that this concept is not appropriate for asphaltene self-association. Tests were performed down to concentrations of 34 ppm without any sign of a critical micellization or aggregation concentration. Based on the various techniques applied, which also include IR and fluorescence spectroscopy, it is concluded that asphaltenes do not exhibit CMC behavior. Instead, the association of asphaltenes is believed to occur step wise. This is not in disagreement with the fact that the aggregates may end up having a definite size.

A previously developed regular solution model was adapted to predict the onset and amount of asphaltene precipitation from crude oil blends diluted with pure n-alkanes or a mixture of toluene and n-heptane. Tests were conducted on nine different crude oils, a gas oil, and their blends. Oils and blends were characterized in terms of SARA (saturates, aromatics, resins, and asphaltenes) fractions. The mass fraction of each SARA fraction in the blends was confirmed as a weight average of the respective fraction in constituent oils. Asphaltenes were subdivided into fractions based on the gamma function to account for the distribution of aggregates resulting from self-association. To model the asphaltene onset and yield, liquid−liquid equilibrium was assumed between a heavy (asphaltenic) and a light (nonasphaltenic) phase. The distribution of asphaltenes in unblended crude oils was determined by fitting its asphaltene yield data when diluted with n-heptane. The fitting parameter in the model was the average aggregation number of asphaltenes in the source oils. Two approaches were tested to calculate the distribution of asphaltenes in crude oil blends. In the first approach, asphaltenes were assumed to interact, and the final molar mass distribution was determined from gamma function using the average aggregation number of constituent oils. In the second approach, no interaction was assumed, and the final distribution was calculated as a sum of the individual distributions. It was found that the best predictions of the onset and yield data were obtained by using the second approach. The mass fraction of n-heptane required to initiate precipitation was predicted with an average absolute deviation of 0.53% or less in all cases.

In spite of its importance, wax deposition is still not fully understood, and a reliable physical description of wax deposition is still to be agreed upon. Up to now, the focus in wax studies has been mainly on the radial transport of waxes to the cold wall through molecular diffusion, but other mechanisms that have been cited (and so far neglected) may count on the kinetics of deposit formation. An analysis of the experimental evidence is presented here, leading to the conclusion that an important role may be played by the axial convective transport and the oil gelation on the cold surface (i.e., by the bulk liquid-to-gel change driven by the temperature change). Gelation is faster than molecular diffusion and may lead to the formation of a loose solid network that is slowly filled by diffusion (by aging) in a successive step. An experimental apparatus has been employed to measure the gelation kinetics both on a model mixture and on a stock tank oil. Obtained data have been modeled by employing a classic solution of the conductive heat transfer problem. The qualitative agreement obtained indicates that the physics involved in gelation have been well identified.

This paper collects the work performed by isothermal titration calorimetry (ITC) to characterize the interaction between petroleum asphaltenes and resins. The interaction between these two fractions is of great interest in order to understand the mechanism of stabilization ofasphaltenes in crude oil. To simplify the approach, this preliminary study focuses on toluene solutions of both fractions. This paper reports the experimental determination of the average number of sites in asphaltene molecules and the enthalpy of interaction between asphaltenes and resins. Two models have been used to fit the experimental data. The enthalpies calculated by ITC are in the order of -2 to -4 kJ/mol. These values are in the limit of hydrogen bonding and permanent dipole energies. Similar values have been obtained by using the enthalpy as a fitting parameter in the SAFT equation.

This work deals with the study of asphaltene precipitation from a +190 °C residue at different temperatures, using several n-alkanes and n-alkane/oil ratios. Experiments were carried out at the boiling point of the solvent (IP-143 standard) and at other temperatures in order to elucidate if asphaltene yields are conditioned by the nature of the solvent and/or by the boiling temperature of the alkanes. The results show that lower asphaltene yields were obtained when using solvents with higher molecular weights at the same temperature, but also when using higher temperatures with the same solvent. Consequently, both effects that simultaneously influence asphaltene precipitation could be conveniently separated. Preliminary characterization of the asphaltenes obtained at different conditions has been carried out by means of NMR spectroscopy and elemental analysis. Further characterization has focused on the particle size distributions obtained during kinetic tests of asphaltene flocculation by a laser reflectance technique (focused-beam reflectance measurement, FBRM): significant differences have been obtained, all related to the n-alkane and ratio used in the flocculation.