Chemical Structure of Asphaltenes

The effect of asphaltenes on the physical properties of heavy oils and bitumen has been studied extensively. It has been demonstrated that the viscosity of petroleum is significantly influenced by the presence and concentration of asphaltenes. Storm et al. demonstrated that when the relative viscosity of heavy oils was plotted versus asphaltenes concentration in both toluene (at room temperature) and vacuum residue (at 93°C), a straight line resulted. Thus, it was concluded that toluene is as good a solvent for asphaltenes as for vacuum resid. However, the amount of solvation is temperature dependent. By analyzing the temperature dependency of solvation. Storm et al. showed that the forces holding asphaltenes in the resid are very weak. Moreover, the fact that the solvation constant is the same for toluene at 25 °C as in a vacuum resid at 93°C implies that the forces between asphaltene colloidal particles and toluene are weaker. 

The dependency of feedstock viscosity on asphaltenes concentration has significant implications, because reducing viscosity could make pipeline transportation of heavy oil less dependent on diluent. Removal of even 100wt% asphaltenes from Athabasca bitumen does not reduce viscosity enough to meet pipeline specifications in Alberta (viscosity of 350 cSt at operating temperature, °API of 19, and BSW of 0.5 vol%). However, removal of approximately 30 wt% of asphaltenes has been shown to reduce diluent requirement by almost 30%. The benefit of partial removal of asphaltenes on thermal processing will be discussed later. 

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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 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... 

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. 

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. 

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. 

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. 

Nuclear magnetic resonance spectroscopy is the use of the NMR phenomenon to study physical and chemical properties of matter. As a consequence, NMR spectroscopy finds applications in several areas of science. NMR spectroscopy is routinely used by chemists to study materials. Solid state NMR spectroscopy is used to determine the molecular structure of solids. In our investigation, NMR spectroscopy was used to determine the molecular structure of asphaltene molecules. 

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. 

The formation of the asphaltenes deposit is one of the most studied phenomena in the production and processing of crude oil. Researches are looking increasingly for the improvement about the chemical structure of molecules present in the asphaltene fraction and, consequently understand its behavior in oil. 

Heavy oils and residua contain hydrocarbons that are characterized by large amounts of heteroatoms and asphaltenes. The nature and chemical structure of these complex components are also other factors that strongly affect the extent of thermal reactions. For instance, it is more feasible to convert asphaltenes with small content of aromatic rings and high number of alkyl side chains are more feasible to convert (Ancheyta et al., 2009). 

Yen, T. F. Structural Differences Between Asphaltenes Isolated from Petroleum and from Coal Liquid, Chemistry of Asphaltenes-, Bunger, J. W. Li, N. C., Eds. Advances in Chemistry Series No. 195 American Chemical Society Washington, D.C., 1981, p 39. 

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. 

Although the residuum is a mixture too complex for isolating chemically pure components, asphaltene investigators in recent years have developed techniques that separate residuum molecules on the basis of compound class rather than solubility class. These studies, discussed next, have greatly modified the concepts of asphaltene structure. 

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. 

Sarret, G., Connan, J., Kasrai, M., Bancroft, G. M., Charrie-Duhaut, A., Lemoine, S., Adam, P., Albrecht, P., and Eybert-Berard, L. (1999). Chemical forms of sulfur in geological and archaeological asphaltenes from Middle East, France, and Spain determined by sulfur K- and L-edge X-ray absorption near-edge structure spectroscopy. Geochim. Cosmochim. Acta 63, 3767-3779. 

Asphaltene Structure by Chemical Methods. The concept of asphaltenes being a sulfur polymer of the type ... 

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. 

Structural information obtained from various methods can usually be represented by a set of structural parameters, S. These parameters are related to the physical and chemical properties of a given substance. A given set of properties, Pjt is unique to a given substance. Therefore, a given set of structural parameters can be used to characterize a given asphaltene ... 

Both structural parameters and properties derived in this manner are in matrix form. Furthermore, the production or genesis of asphaltene must depend on a set of environmental or refinery variables, such as temperature, and pressure. These quantities, tentatively called refinery variables, Rkt can affect the nature of the chemical and physical properties of asphaltenes ... 

Microstructure. By using a group of structural parameters obtained through a number of physical methods (such as x-ray, NMR, MS, IR, VPO, DTA, densimetric methods, EM, and SAS) and chemical methods (such as oxidation, alkylation, and halogenation), the structure of petroleum asphaltene has been gradually revealed. As an example, the structural parameters of an asphaltene derived from a Laquinillas crude oil from Venezuela (API gravity 20°, Conradson carbon number 13.39, Miocene Age) are listed in Table I. An empirical formula can be deduced. 

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