Study on the polarity, solubility, and stacking characteristics of asphaltenes

doi:10.1016/j.fuel.2014.03.015

Fuel

Volume 128, 15 July 2014, Pages 366-372

https://doi.org/10.1016/j.fuel.2014.03.015 Get rights and content

Highlights

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The sub-fractions tending to precipitate have larger polarity and less solubility.
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The TEM can reflect the largest fused aromatic ring system in asphaltene.
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The largest fused aromatic ring system influences the stacking characteristics.

Abstract

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. Polarity, solubility and structural characteristics of asphaltenes derived from Tahe atmospheric residue (THAR) and Tuo-826 heavy crude oil (Tuo-826) were analyzed for study of their internal relationship. A fractionation method was used to separate the asphaltenes into four sub-fractions, based on their solubility in the mixed solvent, for the study of different structural and physical–chemical properties, such as polarity, solubility, morphology, stacking characteristics, and mean structural parameters. Transmission electron microscope (TEM) observation can present the intuitive morphology of asphaltene molecules, and shows that the structure of asphaltenes is in local order as well as long range disorder. The analysis results showed that n-heptane asphaltenes of THAR and Tuo-826 had larger dipole moment values, larger fused aromatic ring systems, larger mean number of stacking layers, and less interlayer spacing between stacking layers than the corresponding n-pentane asphaltenes. The sub-fractions that were inclined to precipitate from the mixture of n-heptane and tetrahydrofuran had larger polarity and less solubility. From the first sub-fraction to the fourth sub-fraction, polarity, mean stacking numbers, and average layer size from the TEM images follow a gradual decrease. The structural parameters derived from TEM images could reflect the largest fused aromatic ring system in asphaltene molecule, yet the parameters derived from ¹H NMR data reflected the mean message of poly-aromatic ring systems. The structural parameters derived from TEM images were more consistent with the polarity variation of sub-fractions than those derived from ¹H NMR data, which indicates that the largest fused aromatic ring system will play a more important role in the stacking characteristics of asphaltene molecules than the mean parameters of poly-aromatic ring systems.

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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Citation Excerpt :
The ordering result of the average dipole moment of the three asphaltenes is consistent with that of δp in Table 6, which showed that the polar component δp of the HSPs can indicate the strength of polar interaction of asphaltenes. Meanwhile, Zhang[29,30] and Xu[29,30] found that polarity is positively correlated with the condensation degree of asphaltenes by studying asphaltenes. However, it is worth noting that both of them only characterized the condensation degree of asphaltenes through the H/C, which was unilateral and required various characterizations to deeply analyze the relationship between the condensation degree and polarity.
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View full text

Study on the polarity, solubility, and stacking characteristics of asphaltenes

Highlights

Abstract

Introduction

Section snippets

Experimental section

Polarity of asphaltenes derived from different heavy oil

Conclusion

Acknowledgements

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J Fuel Chem Technol

J Colloid Interface Sci

J Colloid Interface Sci

Macrostructures of the asphaltic fractions by various instrumental methods

Anal Chem

The colloidal aspect of a macrostructure of petroleum asphalt

Fuel Sci Technol Int

Petroleum asphaltene properties, characterization, and issues

Energy Fuels

A phase-separation kinetic model for coke formation

Ind Eng Chem Res