Elemental compositions of asphaltene

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. 

Table I. Elemental compositions of asphaltenes precipitated by different flocculants from various sources (16)...

Table I. Elemental Compositions of Asphaltene Fractions Precipitated by Different Solvents...

The elemental composition of asphaltenes varies in a ratio of C/H of 1.15 0.05%, however, values outside this range are sometimes found, according to Speight (1999 a). 

Table 2. Range and Typical Values of Elemental Composition of Asphaltenes.

Pyrolysis of kerogens and asphaltenes has demonstrated the nature and logic of this conversion sequence (58.63-64). Relative to elemental composition, initial asphaltenes have a much lower atomic O/C ratio, a slightly lower atomic S/C ratio, and almost the same H/C and N/C ratios as their source kerogens (53). 

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. 

In Table II, the elemental composition of n-heptane asphaltenes from a number of crude sources is shown. The range of composition indicates a range of polarities is quite likely since polarity generally varies largely with the heteroatom content of the fraction. This is particularly true for oxygen and nitrogen content. 

Asphaltenes seem to be relatively constant in composition in residual asphalts, despite the source, as deterrnined by elemental analysis (6). Deterrnination of asphaltenes is relatively standard, and the fractions are termed / -pentane, / -hexane, / -heptane, or naphtha-insoluble, depending upon the precipitant used (5,6,49). After the asphaltenes are removed, resinous fractions are removed from the maltenes-petrolenes usually by adsorption on activated gels or clays. Recovery of the resin fraction by desorbtion is usually nearly quantitative. 

The proton nmr spectrum of fraction 2 of the S02 solubles resembles that of asphaltenes as reported by other workers (1). The elemental composition and the GPC size distribution agrees with the values published for coal derived asphaltenes (1,3). Fractions 3 and 4 of the S02-solubles were separated and identified by GC-MS (see Figures 4 and 5). These fractions contain only a small amount of alkanes. The components are listed in Tables I and II. 

We have recently determined the structural parameters and composition of some asphaltene samples obtained from the Synthoil and Exxon Donor Solvent (EDS) liquefaction processes. The particular EDS sample used was sufficiently volatile for analysis by ultrahigh resolution mass spectrometry, so we could obtain very detailed data on its composition in terms of the distribution of individual carbon-number homologs. Information from this approach, integrated with data from NMR, IR, molecular weight determinations, elemental analyses, and separations furnished us with a novel and detailed insight into the nature of these asphaltenes. The excellent agreement observed between composites calculated from the detailed MS data, where available, and the averages determined by NMR, IR, and elemental analyses reinforces the credibility of the approaches used and allows extrapolations to heavier samples that are not amenable to detailed MS characterization. 

Analytical Methodology. Analytical techniques used in this work included separations, high and low resolution MS, NMR, IR, UV, molecular weight determinations, and elemental analyses. These are discussed in detail in our work on the chemical characterization of Synthoil feeds and products (1,2). Composition of the EDS asphaltenes was determined with the aid of a Kratos-AEI model MS50 ultrahigh resolution mass spectrometer that became available only after we had already completed the work on the Synthoil samples. The roles of the various analytical approaches selected are listed in Table I. The most important techniques are also summarized in the following paragraphs. 

The average elemental compositions for several preparations of crude and residuum asphaltenes are shown in Table II. As can be seen, the two asphaltenes are quite similar with the differences between them being less than the typical errors from analysis to analysis. The H/C ratios are almost identical. 

Elemental Composition. The near constancy of the atomic hy-drogenrcarbon (H/C) ratio of asphaltenes (I) is surprising when the... 

Experience has shown that crudes from adjacent wells can be different in composition. The siuface, physical, and chemical properties of associated emulsions can be expected to be as diverse and complex as the soiuce crudes and water. The variances in the elemental C, H, and N composition of components such as asphaltenes of crudes from various geologic origins as compiled (10, 12) show no apparent pattern emerging from the data as was the case with coal (5, 7). Sharma et al. (13) have shown the use of bitumen asphaltenes as thermal maturation indicators. 

Bitumens are colloid systems, as are crude oils, and consist of the two colloidal components, petroleum resins and asphaltenes, dispersed in a dispersion medium. To investigate the composition of the system, a colloid precipitation according to Neumann [4-10] is carried out. The chemical nature of the bitumen and its components were determined by element analysis, where the atomic ratio H/C includes an indicator of the aromacity. Further characterization is performed by measuring the average relative particle mass (mean of the molecular weight M) by vapor pressure osmometry. 

The behavior of a vacuum residue from a Venezuelan crude was simulated by a distillation bitumen B80 (according to DIN 1995). Further, a vacuum residue of a Middle East crude (VR Kuwait) and its colloid components, i.e. dispersion medium, petroleum resins, and asphaltenes were investigated. Those substances were characterized by element analysis and average relative particle weight (molecular weight) (Table 4-200) and by analysis of their colloid composition according to Neumann [4-10] (Table 4-201). 

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