Asphaltene solubility in common solvents: A molecular dynamics simulation study

Effect of Temperature on Asphaltene Precipitation in Crude Oils from Xinjiang Oilfield

During the production of crude oil, asphaltenes are prone to precipitate due to the changes of external conditions (temperature, pressure, etc.). Therefore, a series of research studies were designed to investigate the effect of temperature on asphaltene precipitation for two Xinjiang crude oils (S1, S2) so as to reveal the mechanism of asphaltene dissolution. First, the changes of asphaltene precipitation were intuitively observed by using a microscope. The results demonstrated that the asphaltene solubility increased with the increase of temperature and the dispersion rate of asphaltene particles increased with the decrease of particle size. Second, the variation of asphaltene precipitation with temperature was quantified by a gravimetric method. The results suggested that the different asphaltenes showed different sensitivity to temperature within the temperature range 25-120 °C. Third, a hypothesis was proposed to explain these results and proved that the asphaltene aggregate structure was an important factor for asphaltene stability. The crystallite parameters of asphaltenes were obtained by X-ray diffraction (XRD) to describe the structural characteristics. The results revealed that the layer distance between aromatic sheets (d _m ) of asphaltenes derived from S1 oil and S2 oil were 0.378 and 0.408 nm, respectively, which implied that the asphaltene aggregates derived from S2 oil were looser than those of S1 oil. Therefore, high temperature could facilitate the penetration of resins into asphaltene aggregates and ultimately improve the dispersion of asphaltenes. Finally, molecular dynamics (MD) simulation was used to verify the conclusions. Based on the molecular dynamics method, asphaltene aggregate models were developed. The compactness and internal energy of each model were calculated. The results showed that the asphaltene dispersion capability was proportional to the porosity and internal energy.

Investigating the Compatibility of Various Components in Marine Low-Sulfur Fuel Oil by Molecular Dynamics Simulations

Asphaltene aggregation and precipitation are one of the major issues for marine low-sulfur fuel oil used on board. Many research studies have been carried out to investigate the aggregation behavior of asphaltene under different conditions, but the mechanism of asphaltene aggregation in low-sulfur fuel oil at the molecular level is still unclear. In this work, molecular dynamics (MD) simulations were performed to calculate the solubility parameters, intermolecular interaction energies, and radial distribution function (RDF) curves of each component in marine low-sulfur fuel oil to examine their mutual compatibility. Simulation results indicate that the solubility parameter of resin gains the highest value and it is close to asphaltene. The solubility parameters of aromatic, hexadecane, and saturate decrease successively. The interaction energy between resin and asphaltene molecules is higher than that between the same kind of molecules, which means that resin can inhibit the aggregation of asphaltene molecules. Typically, a light distillate component (hexadecane) is added to heavy fuel oil to yield low-sulfur oil, and our calculations reveal that this has a negative effect on asphaltene aggregation. Specifically, asphaltene is more likely to self-aggregate, as shown by the increase in peak height in the radial distribution function of the asphaltene-asphaltene pair. The findings of this study will provide theoretical support for the production of marine low-sulfur fuel.

Molecular Characterization of Nonvolatile Fractions of Algerian Petroleum with High-Resolution Mass Spectrometry

Algerian crude oil displays a marked propensity for asphaltene precipitation, leading to solid deposits during extraction, transportation, and storage. The relationship between precipitation and chemical composition is unclear; in fact, Algerian crude oil actually features a low asphaltene concentration, despite its relatively large rate of deposit formation. The rationalization of the precipitation process and its remediation should benefit from a molecular characterization of the crude oil. In this study, two unstable asphaltene fractions (A1 and A2) from two different deposits, and two resin crude oil fractions (R1 and R2) from the Hassi-Messaoud Algerian field have been characterized at the molecular level by means of high-resolution mass spectrometry with an Atmospheric Pressure Chemical Ionization (APCI) source. Positively and negatively charged compounds with molecular weights 200-1200 m/z were detected. Several thousand molecular stoichiometries were identified and classified for each sample, in terms of heteroatom content and aromaticity, searching for trends characteristic of the two asphaltenes and of the associated resins. The A2 asphaltene, from a downstream storage tank, displays a higher aromaticity and O-heteroatom content, which correlates with an enhanced aggregation propensity, in comparison to the A1 fraction, collected at the well bore. The resin fractions are found to be abundant in aliphatic hydrocarbons and heteroatomic compounds of moderate aromaticity. The more polar resin fraction, R2, is enriched in N-containing species, with respect to the less polar resin fraction R1, which correlates with the stabilizing function observed in previous works. The results stress the view of crude oil fractions as complex mixtures, rather than in terms of average prototypical compounds, when facing the understanding of asphaltene deposition conditions.