Petroleum asphaltene

The present authors studied the solvolytic liquefaction process ( ,7) from chemical viewpoints on the solvents and the coals in previous paper ( 5). The basic idea of this process is that coals can be liquefied under atmospheric pressure when a suitable solvent of high boiling point assures the ability of coal extraction or solvolytic reactivity. The solvent may be hopefully derived from the petroleum asphaltene because of its effective utilization. Fig. 1 of a previous paper (8) may indicate an essential nature of this process. The liquefaction activity of a solvent was revealed to depend not only on its dissolving ability but also on its reactivity for the liquefying reaction according to the nature of the coal. Fusible coals were liquefied at high yield by the aid of aromatic solvents. However, coals which are non-fusible at liquefaction temperature are scarcely... 

The road asphalt used in this study was obtained from the road as a fresh sample. The road asphalt is composed of asphaltenes (GPC peak at lOOA and petroleum residual oils (15) (GPC peak at n-C QHgo). The GPC of road asphalt is shown in Figure 9. Since petroleum asphaltenes cannot be separated by a lOOA pore size gel column, the asphaltene appears without any separation at the total size exclusion limit of the column. But the nonasphaltene components are separated showing a peak at n-C QHg2. The performance of the road asphalt depends on the asphaltene content as well as on the molecular size distribution of the nonasphaltenic fraction. 

Yen, Teh Fu, Wen Hui Wu, and George V. Chilingar. 1984. A study of the structure of petroleum asphaltenes and related substances by infrared spectroscopy. Energy Sources, 7(3) 203-235. 

As a result of structural studies, it is evident that petroleum asphaltenes are agglomerations of compounds of a condensed aromatic type (I, 2). Thus, it is not surprising that asphaltenes undergo a wide range of chemical and physical interactions based not only on their condensed aromatic structure but also on the attending alkyl and naphthenic moieties. In the following discussion, we again rely heavily on the evidence... 

Asphaltenes are thought to be the most complex, high-molecular-weight, high-boiling components in petroleum. Asphaltenes constitute a solubility class of materials consisting primarily of highly polar and... 

Yen et al. (1961) examined the structure of isolated petroleum asphaltenes by using XRD. From the diffraction pattern they were able to calculate the aromaticity, defined as the number of aromatic carbon atoms over the total carbon atoms. The aromaticity ranged from 0.26 to 0.53 for petroleum asphaltenes. In addition, the characteristic dimensions of an asphaltene were obtained by the XRD method. The asphaltene model developed by Yen et al. (1961) from these observations is presented in Fig. 6 with characteristic molecular dimensions. The model consists of... 

Asphaltenes may contain both porphyrin and nonporphyrin metals, depending upon the origin of the crude oil. Yen et al. (1969) characterized the vanadium complexes in a petroleum asphaltene by mass spectroscopy, optical spectroscopy, and ESR. Porphyrins (Etio and DPEP), acid-resistant porphyrin macrocycles of increased aromaticity (Rhodo), and nonporphyrins with mixed donor complexes were identified. Baker (1966) and Baker et al. (1967) extracted porphyrins from Boscan crude oil asphaltenes and also found Etio and DPEP as the two major porphyrin series. These homologous series range in molecular weight by 7 to 18 methylene groups. Gallegos (1967) observed by mass spectroscopy that asphaltenes and maltenes from a Boscan crude oil had nearly identical porphyrins in terms of mass distribution. 

There have been many attempts to define solvent behavior in terms of one or more physical properties of the solvent, and not without some degree of success. However, it is essential to note that the properties of the coal also play an important role in defining behavior of a solvent, and it has been reported that the relative solvent powers of two solvents may be reversed from one coal type to another. Thus, two properties that have found some relevance in defining solvent behavior with coal (as well as with other complex carbonaceous materials, such as petroleum asphaltenes) are the surface tension and the internal pressure (Speight, 1994, p. 201). However, the solvent power of primary aliphatic amines (and similar compounds) for the lower-rank coals has been attributed to the presence of an unshared pair of electrons (on the nitrogen atom). 

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. 

Felix, G., Bertrand, C., and Van Gastel, F., A new caffeine bonded phase for separation of polyaromatic hydrocarbons and petroleum asphaltenes by high-performance liquid chromatography, Chro-matographia, 20, 155, 1985. 

Sulfur K-Edge X-ray Absorption Spectroscopy of Petroleum Asphaltenes and Model Compounds... 

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

This work has demonstrated for the first time that organically bound sulfide and thiophenic sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures and in petroleum asphaltenes and coal. The use of the third derivative XANES spectra was the critical factor in allowing this analysis. 

Although the evidence available in the literature appears to indicate that the hydrocarbon structures and some features, such as the various condensed ring systems, in different petroleums are similar (from the asphaltenes and resins to the constituents of the oil fraction), the variety of source materials involved in petroleum genesis implies that, on an individual molecular scale, there may be substantial structural differences among the constituents of the various crude oils and bitumens. As well, the difficulty with which resins from one crude oil peptize (As in a colloid, the terms peptization, dispersion, and solubilization are often used interchangeably to describe the means by which asphaltenes exist within petroleum.) asphaltenes from a different crude oil, and the instability of the blend (5) are evidence for significant structural differences among the asphaltenes and resins of various crude oils. 

Speight, J. G. The Structure of Petroleum Asphaltenes, In Information Series ... 

The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. This generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale. 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. This effort is summarized by Speight and Moschope-dis (i) in their chapter in this volume along with a good summary of the current thinking. Thus, there are petroleum asphaltenes, coal tar asphaltenes, shale oil asphaltenes, tar sands bitumen asphaltenes, and so on. In this chapter I will attempt to show how these materials are special cases of an overall concept based directly on the physical chemistry of solutions and that the idea that they have a specific chemical composition and molecular weight is incorrect even for different crude oil sources. 

In Table I, a comparison is made of the elemental composition of typical asphaltenes from petroleum and coal liquids. This table shows the typical lower H/C ratio and higher oxygen content for the coal asphaltenes. Furthermore, the GPC molecular-weight distributions shown in Figure 7 illustrate the higher molecular-weight of petroleum asphaltenes as well as the wider molecular-weight distribution. 

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