Study on the polarity, solubility, and stacking characteristics of asphaltenes
Introduction
Asphaltenes, the highest polar fraction in the residue, are believed to comprise the cores of the colloidal systems of petroleum residues [1], [2], [3]. They play a key role in the processing of heavy oil [4]. Asphaltene precipitation has brought a series of negative effects on the production, transformation, and processing of heavy oil. The fraction compositions of residue have the different polarity, which influences the colloidal stability, reaction characteristics, and the emulsification performances greatly [5], [6], [7], [8], [9], [10], [11]. The structure and transformation of fused aromatic ring system in asphaltenes play an important role in the character of asphaltenes, and in step affect the properties of heavy oils. Strausz et al. [12] studied asphaltenes by chemical and thermal degradation methods and found that each asphaltene molecules contained several aromatic units connected by alkyl or sulfur bridges. The recent study has implied that the asphaltene molecule may have several internal polycyclic structures joined by flexible aliphatic chains [13]. The connection between aliphatic chains and aromatic units is weak and can be broken by the absorbed energy from the laser.
The properties of asphaltenes are difficult to be achieved because of its complexity. One of the proposed resolvent is to separate asphaltene into sub-fractions, and the results showed that the low soluble fraction can be dispersed into the soluble part [14].
In the present work, n-heptane and n-pentane asphaltenes derived from two kinds of heavy oils were studied. The n-heptane asphaltenes were then fractioned into four sub-fractions in terms of solubility difference in the mixed solvent. The physical–chemical properties as well as structural properties, such as polarity, solubility, aggregation characteristics, and elemental composition were analyzed. The stacking characteristics were studied both by transmission electron microscope (TEM) and proton nuclear magnetic resonance spectroscopy (1H NMR). Compared with 1H NMR that can only provide the average message of asphaltene molecule, TEM images can reveal the direct verification of asphaltenes, and reflect the largest fused aromatic unit in asphaltene molecules.
Section snippets
Experimental section
Tuo-826 heavy crude oil (Tuo-826) of Shengli Oilfield and Tahe atmospheric residue (THAR), an atmospheric residue with boiling point higher than 350 °C from the Tahe Branch of China Petroleum and Chemical Corporation, were used as the research samples. Asphaltenes were separated by adding certain amount of n-pentane or n-heptane (40 cm3 in 1 g oil) into the heavy oil. Precipitates were filtrated and extracted using a Soxhlet Apparatus by boiling n-pentane or n-heptane until the filtrate was
Polarity of asphaltenes derived from different heavy oil
The dipole moment can be calculated by the mean molecular weight, the variation rate of dielectric permittivity, and the square of refractive index to the weight concentration of the sample in toluene. The polarity variation relates to the elemental difference. The dipole moments data and the elemental composition are shown in Table 1.
It can be seen from Table 1 that C7-asphaltenes had larger molecular weight, and dipole moment values than corresponding C5-asphaltenes. This is consistent with
Conclusion
Firstly, the C7-asphaltenes derived from THAR and Tuo-826 had larger polarity than corresponding C5-asphaltenes, and had smaller solubility in the same solvent. This phenomenon indicates that there were relationships between the polarity and solubility of asphaltenes.
Secondly, the sub-fractions of C7-asphaltenes derived from THAR and Tuo-826 had different polarity and solubility. The sub-fractions with larger polarity had smaller solubility in mixture of toluene and n-heptane. There was good
Acknowledgements
This work was partial supported by the National Science Fund Committee of China (20776160), the Special Feature of China National Petroleum Corporation Major Scientific and Technological Project: Research and Development of the Catalyst for Petroleum Refining.
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