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Nature and Characteristics of Asphaltenes and Resins

The investigation of the chemical constitution of petroleum heavy fractions such as resins and asphaltenes is hindered by their complex nature. The first known scientific application of asphaltene is by the French chemist Niepce. According to experts at the Getty Conservation Institute, the first surviving photograph by Niepce (1826-7) currently in Austin, Texas has been made exposing a thin layer of bitumen brushed on pewter plate. When exposed about 8 hours in the camera obscura, the image could be produced by dissolving unexposed bitumen in oil of lavender.

The classic definition of asphaltenes is based upon the solution properties of petroleum residuum in various solvents. The word asphaltene was coined in France by J.B. Boussingault in 1837. Boussingault described the constituents of some bitumens (asphalts) found at that time in Eastern France and in Peru. He named the alcohol insoluble, essence of turpentine soluble solid obtained from the distillation residue "asphaltene", since it resembled the original asphalt.

Marcusson in1945 classified asphaltenes and resins as follows : (i) Neutral resins are defined as the insoluble fraction in alkalies and acids and it is completely miscible with petroleum oils, including light fractions; (ii) Asphaltenes are defined as inso luble fraction in light gasolines and petroleum ether. In contrast to resins, the asphaltenes are precipitated in the presence an excess ether; (iii) Asphaltogenic acid is defined as the soluble fraction in alkaline solutions and in such solvents as benze ne.

Recently, asphaltene is defined by chemists as the part precipitated by addition of a low-boiling paraffin solvent such as normal-pentane and benzene soluble fraction whether it is derived from carbonaceous sources such as petroleum, coal, or oil shale. S ince asphaltogenic compounds are present in petroleum in insignificant quantities, resins and asphaltenes are most important compounds of petroleum. There is a close relationship between asphaltenes, resins, and high molecular weight polycyclic hydrocarbo ns. In nature, asphaltenes are hypothesized to be formed as a result of oxidation of natural resins. On the contrary, the hydrogenation of asphaltic compound products containing neutral resins and asphaltene produces heavy hydrocarbon oils, i.e., neutra l resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ, however, from polycyclic aromatic hydrocarbons by presence of oxygen and sulfur in varied amounts.

On heating above 300-400 oC, asphaltenes are not melted, but decompose, forming carbon and volatile products. They react with sulfuric acid forming sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. The co lor of dissolved asphaltenes is deep red at very low concentration in benzene as 0.0003 % makes the solution distinctly yellowish. The color of crude oils and residues is due to the combined effect of neutral resins and asphaltenes. The black color of som e crude oils and residues is related to the presence of asphaltenes which are not properly peptized.

Structure and Chemistry of Asphaltenes and Resins

Our knowledge of the asphaltenes is very limited. Asphaltenes are not crystallized and cannot be separated into individual components or narrow fractions. Thus, the ultimate analysis is not very significant, particularly taking into consideration that the neutral resins are strongly adsorbed by asphaltenes and probably cannot be quantitatively separated from them. Not much is known of the chemical properties of asphaltenes. Asphaltenes are lyophilic with respect to aromatics, in which they form highly s cattered colloidal solutions. Specifically, asphaltenes of low molecular weight are lyophobic with respect to paraffins like pentanes and petroleum crudes. There have been considerable efforts by analytic chemists to characterize the asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous sources. A number of investigators have attempted to postulate model structures for asphaltenes, resins, and other heavy fractions based on physical and chemical methods.

Molecular structure of asphaltene proposed for Maya crude (Mexico) by Altamirano, et al. [IMP Bulletin, 1986]

Molecular Structure of Asphaltene Proposed for 510C Residue of Venezuelan Crude by Carbognani [INTEVEP S.A. Tech. Rept., 1992]

Physical methods include IR, NMR, ESR, mass spectrometry, x-ray, ultracentrifugation, electron microscopy, VPO, GPC, etc. Chemical methods involve oxidation, hydrogenation, etc.

3D Picture of Carbognani's Model of Venezuelan Crude Asphaltene Molecule (Courtesy of Prof. J. Murgich)

Prof. T.F. Yen in 1974 suggested asphaltene structure which is deduced from microscopic and macroscopic analysis, showing their micro- and macro-molecular bonding. From chemical methods, Yen postulated the micro-structure of asphaltene in which the aroma tic nuclei of petroleum asphaltene are of a peri-condensed nature and various structural parameters such as aromaticity, substitution extent, etc. can specify a given asphaltene structure with hypothetical empirical formula C74H87NS2O. By physical approa ches such as dissociation-association, charge transfer, excitation, defect center, etc., Yen also postulated the macrostructure of asphaltene. One of the properties of peri-condensed polynuclear aromatic systems is the attraction between the p-electron s ystems. For petroleum asphaltenes, in general, the stacking is circa five layers as found by x-ray diffraction.

Nature of flocculation/aggregation of various asphaltene molecules (S. Yaseen and G.A. Mansoori, 2018)

NOTE: Flocs and aggregates of asphaltene which are caused due to the addition on non-polar solvents must not be confused with asphaltene micelles which may be formed by the addition of large amounts of polar (or aromatic) solvents to the crude oil.

Various shapes of asphaltene micelles formed in the presence of large amounts of polar or aromatic solvents

Size of Asphaltene Particles

The physical and physico-chemical properties of asphaltenes are different from those of neutral resins. The molecular weight of asphaltenes is very high ranging from approximately 1,000 to 2,000,000. The reported molecular weight of asphaltene varies cons iderably depending upon the method and conditions of measurement. A major concern in reporting molecular weights is the association / aggregation of asphaltenes which can exist at the conditions of the method of measurement. Vapor pressure osmometry (VP O) has become the prevalent method for determining asphaltene molecular weights. However, the value of the molecular weight from VPO must be weighed carefully since, in general, the measured value of the molecular weight is a function of temperature, the solvent molecular properties. Reported molecular weights from ultracentrifuge and electron microscope studies are high. To the contrary, those from solution viscometry and cryoscopic methods are low.

By careful work of electron micrography with rapid lyophilization, the size of asphaltene is found to be 20-30 by Dickie and co-workers in 1968. According to Dwiggens (1965) and Pollack and Yen (1970) in native oil and solutions, the asphaltene partic le size can be doubled. If the internal structure of a particle is known then, technically, one can predict its size by taking into account the bond lengths.

How to deal with heavy organics depositions from petroleum fluids

Asphaltenes and resins are two of the several, but important, heavy organics present in petroleum fluids. Their exact molecular structures are not generally known in a particular oil field and they could vary from well to well. In developing the know- how to mitigate and prevent heavy organics deposition from petroleum fluids exact knowledge of the molecular structures of all the heavy organics present in a particular oil field are not necessary. What we need to know is their various macroscopically d etectable roles in such depositions as it is discussed in a short course we offere.

We have developed mathematical models using polydisperse statistical thermodynamics, mechanics, kinetics and transport for phase transitions, interactions, aggregations and depositions, ( ASPHRAC), that require to identify these certain macroscopic measurable properties which could then quantify the role of the heavy organics in a petroleum arterial blockage problem.

As we were the first, originally suggesting in 1986: The importance of the molecular structures of asphaltenes and resins to the practicing engineers is similar to the importance of the knowledge of a cardiologist about the structure of cholesterol present in the arteries of a patient.


(I). Behavior prediction of heavy organics (specially asphaltenes) requires application of quantum and statistical mechanics, polydisperse and continuous mixtures thermodynamics of monomer-polymer solution theories, solid, micellar, steric colloidal and fluid phase transition theories, electrokinetic phenomena, and FRACTAL kinetics of aggregation.

(II). Behavior PREDICTION of heavy organics (including asphaltenes) cannot be achieved by any equation of state including van der Waals /perturbation /hard-sphere type equations of state which their prediction-power are limited to vapor-liquid equilibria of light hydrocarbon fractions of petroleum fluids. Any such attempts are purely EMPIRICAL CORRELATIONS which are only valid for interpolation purposes.

(III). Molecular Dynamics (MD) Simulation is a powerful tool to gain certain insights into the microscopic (atomic level) characteristics of materials. The trajectories of atoms are determined by solving Newton's equation of motion along with consideration of intermolecular interaction energy functions. We have used MD simulation for investigating the aggregation onset of limited asphaltene molecular models in various model petroleum fluids due to various effects (1, 2, 3, 4, 5, 6, 7, 8, 9, 10). Such ONSET information has been helping us to improve our predictive software capabilities. However, we are aware of the fact that MD simulation is incapable of predicting the behavior of complex real petroleum fluids and the variety of phase transitions which the heavy organics present in petroleum may go through.

(IV). We have developed this website as a service to science and engineering students so that they learn the more appropriate (sustainable) ways to utilize oil and gas resources. What you read in this page, the LINKED pages (1, 2, 3, 4) and Publications (1, 2, 3) are the result of our 40+ years of research and development work here at UIC. Several individuals who have learned from our work reported here have published their own books. However, what you read here is more extensive than any published book on the subject.

Acknowlesgements: I leaned a great deal from Prof. George V. Chilingarian and late Prof. Teh Fu Yen, who contributed greatly to research on asphaltenes and other subjects related to oil and gas. Please visit Scholar Google to learn of their extensive contributions to these subjects. I was fortunate to interact with Prof's Chilingarian & Yen a great deal during conferences, overseas trips and social events. They were kind to ask me to act as the editor of Energy Sources Journal that they had initiated. I was the co-editor-in-chief of Energy Sources Journal for six years during which we further increased the impact factor of the journal appreciably.

Related References:

"Prediction of the Phase Behavior of Asphaltne Micelle / Aromatic Hydrocarbon Systems" Juan Horacio Pacheco-Sanchez and G.A. Mansoori, Petrol. Sci. & Technology, Vol. 16, No's 3&4, pp. 377-394 (1998).

"Asphaltene micellization and its measurement", Slamet Priyanto and G.A. Mansoori, Proceedings of the 3rd. International symposium on advanced and aerospace science and technology in Indonesia (ISASTI, 98), Jakarta August 31-September 3, 1998, Vol. 2, ISASTI-98-4.8.8, pp.843-860, 1998.

"Structure & Properties of Micelles and Micelle Coacervates of Asphaltene Macromolecule", S. Priyanto, G.A. Mansoori and A. Suwono, Nanotechnology Proceed. of 2001 AIChE Annual Meet., 2001. (Preprint PDF).

"Measurement of property relationships of nano-structure micelles and coacervates of asphaltene in a pure solvent" S. Priyanto, G.A. Mansoori and A. Suwono, Chem. Eng. Science, Vol.56, pp.6933-6939, 2001.