Asphaltenes chemical structure

Figure 3 Models of asphaltene chemical structures (a) California (b) Venezuela (c) Athabasca. (From Refs 4,42, and 43.)...

Asphaltene chemical structure was probed by UV-F spectroscopy to determine trends in aromatic cluster sizes. As in the case of SEC results, all asphaltenes showed similar spectra, except the sample with the lowest API gravity (10°), which also showed a larger polyaromatic ring systems. 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

A most striking result from the work described above is that the composition of the bottoms product and residues from the dissolution reaction did not depend on the chemical structure of the original coal material only their relative quantities differed. This supports the view of a mechanism involving the stabilisation of reactive fragments rather than an asphaltene-intermediate mechanism. The formation of a carbon-rich condensed material as a residue of the reaction and the fact that hydrogen transfer occurred largely to specific parts of the coal further supports this view. 

In this paper we have looked firstly at the effect that the catalyst concentration, secondly at the effect that the reactor temperature and finally at the effect that the residence time at temperature have on the chemical structure of the oils (hexane soluble product) produced on hydropyrolysis (dry hydrogenation) of a high volatile bituminous coal. Generally, the hydropyrolysis conditions used in this study resulted in oil yields that were considerably higher than the asphaltene yields and this study has been limited to the effects that the three reaction conditions have on the chemical nature of the oils produced. 

Siskin, M. Kelemen, S. R. Eppig, C. P., et al., Asphaltene Molecular Structure and Chemical Influences on the Morphology of Coke Produced in Delayed Coking. Energy Fuels Published on Web Nov. 4th, 2006, pp. 1-8. 

Michael, G. Al-Siri, M. Khan, Z. H., and Ah, F. A., Differences in Average Chemical Structures of Asphaltene Fractions Separated From Feed and Product Oils of a Mild Thermal Processing Reaction. Energy Fuels, 2005. 19 pp. 1598-1605. 

The early spectroscopic studies using IR (Chelton and Traxler, 1957) and nuclear magnetic resonance (NMR) (Williams, 1959) observed both aromatic and alkyl structures in asphaltenes. Visible-UV spectroscopic studies by Erdman et al. (1958) indicated that asphaltenes and resins from the same petroleum source have a similar chemical structure but that significant variations existed among different petroleums. This is consistent with the observation that the resin-asphaltene interaction is specific in that the resins from one petroleum do not readily solubilize the asphaltene from another petroleum (Koots and Speight, 1975),... 

Mitra-Kirtley, S., Mullins, O. C., Branthauer, J. F., and Cramer, S. P. (1993). Determination of the nitrogen chemical structures in petroleum asphaltenes using XANES spectroscopy. J. Am. Chem. Soc. 115, 252-258. 

The chemical structure of asphaltenes is still not well understood. Ashland Research is currently studying the catalytic cracking of asphaltenes. At the present time these studies have... 

This summary helps to relate our current outlook to the historical prospective outlined earlier. As a whole, deeper study of the chemical character of asphaltenes requires that the concepts of Boussingault advanced 142 years ago be somewhat refined. The earlier work illustrated the large effects that result simply from variation in asphaltene content. The recent work illustrates that not only the asphaltene content but its chemical structure as well are important to the physical properties of the liquids in which they are found. 

Because of the variance in multipolymers, an exact chemical structure is not possible. To differentiate between different asphaltenes, the methodology leading to an average structure is necessary. 

To obtain a more clearly defined picture of these structural features and to establish the relationship between the chemical structure of asphaltene and its reactivity under a variety of conditions, the potential of chemical and thermal degradation reactions as diagnostic tools has been studied. The specific subject of this investigation was the high molecular weight, sulfur rich asphaltene from the Athabasca bitumen. 

The chemistry of asphaltenes is very complicated and it is the least studied field of crude oil chemistry. Because of the complexity of asphaltenes structure, there is no information about the exact chemical structure of an asphaltene molecule. It is natural that only the average asphaletene molecular is possible as given in the literature. The use of such a chemical structure (i.e. average molecular structure) for the asphaltene molecule is warranted because of the wide molecular weight range and the diversity of chemical groups in the structure of asphaltenes. 

Crude petroleum contains complex mixtures of hydrocarbons as well as relatively small amounts of nitrogen-, sulfur-, and oxygen-containing organic compounds, asphaltenes, and various trace metals (uncomplexed and complexed forms). The hydrocarbons can be divided into two classes related to their chemical structure the alkanes (normal, branched, and cyclo) and aromatic compounds (mono-, di-, and poly-, i.e., PAH). 

Nearly all metal contained in crude oil is either chemically bound with asphaltene type structures or associated with asphaltenes (1000-10,000 daltons). It is necessary to remove these metals to prevent contamination of catalysts. 

However, it is the obvious physical differences between coal and petroleum that can raise questions when similarities are considered. Perhaps the most convenient approach is to consider the differences in dimension and space between the two. The properties of coal are very suggestive of a three-dimensional network. This is much less obvious in petroleum (asphaltene constituents) and may only occur to a very minor extent. Such a difference in spatial arrangement would certainly account for some, if not all, of the differences between the two. Serious consideration of such a proposition would aid physical/chemical/structural studies in both fields and would, hopefully, induce a more constructive thinking in terms of coal/petroleum behavior. 

Asphaltenes n. Highly condensed hydrocarbon compounds present in bitumens and asphaltums. Tittle is known of their chemical structure, but they are usually characterized by their insolubility in low boiling aliphatic hydrocarbons. Their physical condition is solid or semisolid. Usmani... 

The chemical structure and physicochemical properties of asphaltenes and resins are not well understood. The operational definitions of asphaltenes and resins are based on their solubility in different diluents. Asphaltenes are defined as the fraction of crude oil insoluble in excess normal alkanes such as n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fraction of crude oil insoluble in excess liquid propane at room temperature. Resins are adsorbed on silica, alumina, or other surface-active material. Figure 5.8 shows the precipitation when a bitumen oil is mixed with various diluents. The normal alkanes used are n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane. According to the operational definition stated above, the asphaltene content of the bitumen... 

Chang, C.-L. and H.S. Fogler Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization, Langmuir, vol. 10, p. 1749,1994a. 

Much effort has been directed at asphaltenes flocculation and precipitation in petroleum crude mixtures that is a significant problem in oil production, transmission and processing facilities. Pressure, temperature, the chemical composition of the oil and the amount of dissolved gases affect this undesired phenomenon. In general, the prediction of asphaltenes precipitation is very difficult and suffers from the definition of an asphaltene. Usually, asphaltenes are defined as the part of the crude oil that is soluble in methylbenzene and benzene but insoluble in pentane or heptane. Asphaltenes consist of many thousands of species, differing in size and chemical structure. The aromatic character of the asphaltenes and their content of heteroatoms influence their solubility in different solvents and the tendency to flocculate. In most of the calculations of the phase equilibria the pseudo-component method has been... 

Peng, P. Morales-Izquierdo, A. Lown, E.M. Strausz, O.P. Chemical structure and biomaiker content of Jinghan asphaltenes and kerogens. Energy Fueb 1999,13,248-265. 

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