Asphaltenes coal-derived

Sternberg et al. (7) showed that the presence of asphaltenes in coal-derived oils caused a marked increase in the viscosity. This group also showed that these asphaltenes were acid-base complexes and that hydrogen bonding occurs between the acidic and basic components of asphaltenes (9, 10). Recent work (J3, 11) on coal liquefaction bottoms has shown the importance of hydrogen bonding on the viscosity of coal liquids. 

The separation of coal liquids by gel permeation chromatography using lOOA Styragel columns and solvents such as THF and toluene has been reported elsewhere (7.8.9.13.14). Coal liquids and petroleum crude are similar in their physical appearance as well as the complexity in composition. The major difference between the two is that petroleum crude does not contain oxygenated compounds, such as alkylated phenols, in substantial quantity. In addition, the average linear molecular size of petroleum derived asphaltenes (15.16) is much larger than that of coal derived asphaltenes (. ... 

Proton NMR spectra of coal derivatives generally give rise to either broad peaks or complicated multiplets which can be easily divided into band envelopes. For example, the H1 spectrum of a coal-hydrogenation asphaltene (4) consists of three peaks, two of which overlap. A broad peak at lowest field is caused by protons in aromatic and phenolic systems, whereas two higher field peaks are caused by protons bonded to carbons situated o to aromatic rings and those bonded to other nonaromatic carbons, respectively. The ratios of these spectral areas are the same as the ratios of the hydrogens in each of these three hydrogen classes. This accounts for one of the most important characteristics of proton NMR spectra—namely, no calibration data are necessary. 

Application of gas chromatographic/mass spectrometric analysis to acidic/basic subfractions of coal-derived asphaltenes has led to the conclusion that the asphaltenes are made up of one-ring and/or two-ring aromatic units that are linked by methylene chains as well as by functional groups (Koplick et al., 1984). Projection of this finding to coal itself is of interest only if it can be assumed that the intemuclear bonds withstood the high temperatures and were not formed as a result of secondary and tertiary (etc.) reaction. In short, the question relates to the relationship of the structural types in the asphaltenes to those in the original coal. 

Mima, M. J. Schultz, H. McKinstry, W. E. Method for the Determination of Benzene Insolubles, Asphaltenes, and Oils In Coal Derived Liquids. ERDA/PERC/RI-76/6, 1976, 15 pp. 

The stability of the products from coal-derived syncrudes must be examined carefully. Many unique compounds are present in these syncrudes peri-condensed aromatics and naphthenes, oxygen compounds, and asphaltene-like hydrocabons. Traces of these compounds may remain in the hydrotreated product and their effect on jet, thermal, and oxidation stabilities cannot be predicted from the behavior of petroleum products. 

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. 

The extracts were fractionated by a Preparative Liquid Chromatography method - PLC-8 [2], in eight distinct chemical classes FI-saturated hydrocarbons (HC), F2-monoaromatics, F3-diaromatics, F4-triaromatics, F5-polynuclear aromatics, F6-resins, F7-asphaltenes and F8-asphaltols. This method, proposed by Karam et al. as an extension of SARA method [4], was especially developed for coal-derived liquids. It combines solubility and chromatographic fractionation, affording discrete, well-defined classes of compounds which are readable for direct chromatographic and spectroscopic analysis. 

A Co-Mo-AlgOs catalyst (Harshaw CoMo 0402T, 3% CoO - 15% M0O3) was used In some experiments either with or without potassium carbonate. Pyrlte Isolated from coal was also used as a catalyst with potassium carbonate In some experiments. Anthracene oil obtained from Crowley Tar Products Company was used as the start-up solvent. In the recycle runs with Sheridan Field Coal (W-74-45), 80% of the anthracene oil was gradually replaced by coal-derived oil after nine recycles. The benzene and pentane used for separation of oil, and asphaltene were Fisher solvent grade. 

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 Figure 1, the classic definition of asphaltenes is illustrated. This definition is an operational one that is, asphaltenes are soluble in benzene and insoluble in pentane. Usually, for virgin petroleum samples, the residuum is completely soluble in benzene. However, with heat-soaked samples or coal-derived liquids, the benzene insolubles can be appreciable. Therefore, further... 

An additional subtlety in solvent choice is shown by cyclohexane and methylcyclohexane. In these cases, petroleum asphaltenes are completely soluble, while coal-derived asphaltenes are only sparingly soluble. This would imply that the precipitation line for cyclohexane runs completely outside the petroleum field of Figures 5 and 6 but passes through the coal-liquids field in Figure 6. Thus, some care must be shown when considering the equivalence or nonequivalence of various solvents. 

We believe it is important, at this time, to establish a documented historical review of asphaltenes and the separation/characterization procedures used by early petroleum and coal chemists to profile crude oils and the products of coal hydrogenation. We shall explore the most important differences between petroleum- and coal-derived asphaltenes. 

Table I. Comparisons of Properties Between Petroleum- and Coal-Derived Asphaltenes...

Figure 1 shows the atomic H/C vs. O/C ratios for a series of fossil fuels. It is evident from the figure that coal-derived asphaltenes stand alone, with H/C atomic ratio about 60% of those derived from other fossil fuels. The asphaltenes from coal liquefaction products have experienced hydrotreatment at high pressures (2000-4000 psi) and temperatures near 450° C, yet they are much more aromatic than petroleum asphaltenes. Asphaltenes from coal have an aromaticity, /a, determined from 13C nuclear magnetic resonance, of 0.60-0.70, while petroleum values are listed as 0.40-0.55. 

To illustrate these points, we take up in more detail the relationship between the chemical processing history of coal-derived liquids and the properties of the asphaltenes. Variation of their properties is related in turn to the viscosity of the liquids containing the asphaltenes. 

Asphaltene content bears directly on the physical properties of the liquid product. Viscosity is of particular interest because of the importance of this parameter to operation of liquefaction plants and as a measure of the extent of liquefaction. The correlation between asphaltene content and the viscosity of the liquid has been a subject of a number of investigations (23-27). The logarithm of the viscosity ratio, In 7j/rj0 (where i and y0 are the viscosities of the solution and solvent, respectively) was found to be a linear function of concentration when asphaltene was redissolved in the pentane-soluble oil isolated from a coal-derived liquid (24). The slopes of these lines, termed the logarithmic viscosity numbers, are a measure of the contribution to the viscosity of a solution attributable to asphaltene. By comparison of logarithmic viscosity numbers of asphaltenes and their acidic and basic subfractions, it was determined that intermolecular association, which is especially strong between the acid and base subfractions, is responsible for a significant portion of the viscosity of these solutions. 

Benzene Insolubles, Asphaltenes, and Oils in Coal-Derived Liquids, PERC/ RI-76/6, NTIS, U. S. Department of Commerce Springfield, VA, 1976. 

In this chapter, I emphasize the origin or source from which the asphaltenes came, rather than the environment or conditions to which they were exposed. For example, I will group asphaltenes from refinery bottoms with petroleum-derived asphaltenes in the discussion. Similarly, asphaltenes from the solvent extracts of raw coal are also classified as coal-derived asphaltenes. 

Microstructure. The characterization of coal-derived asphaltene is quite similar to that of petroleum-derived asphaltene. Since it is anticipated that coal-derived asphaltene will have acid/neutral and base characteristics (26, 36), the average structure of both must be considered. In Table III, Structure I is amphoteric (or slightly basic), and Structure II is an acid/neutral representation. A mixture of both may be typical of the average structure of a coal-derived asphaltene. At present, we will illustrate this by an asphaltene obtained from coal liquid of the Synthoil process. (The coal is hvAb, West Kentucky, Homestead Seam the coal liquid is obtained by catalytic hydrogenation at 450° C and 4000 psig having %C, 86.7 %H, 8.38 %N, 0.93 %S, 0.09 %Q, 3.2 and %Ash, 0.7.)... 

Macrostructure. A very distinctive feature of coal-derived asphaltenes is that they appear to be associated both in concentrated and diluted solutions, and, particularly, in nonpolar solvents. This fact is supported by the viscosity measurement (40). The fitting of a model of monomer-dimer-trimer for the coal-derived asphaltenes is almost perfect (34). The hydrogen-bonding nature of the interaction within the coal-derived asphaltenes is important (35, 41, 42). The other interaction is still the tt-tt association. X-ray data give a low value of... 

Table III. Examples of the Microstructure of a Coal-Derived Asphaltene...

Table IV. Parameters for the Characterization of the Macrostructure of Coal-Derived Asphaltenes and Related Derivatives...

The aromaticity of petroleum-derived asphaltene (/a = 0.2-0.5) is lower than that of coal-derived asphaltene (/a — 0.6-0.7). 

The aromatic ring systems within petroleum-derived asphaltene are much more condensed (Haru/Car = 0.3-0.5, which is peri) than those of coal-derived asphaltene (Haru/Car = 0.5-0.7, which is kata). 

The substituents of the petroleum-derived asphaltenes are longer (n = 4-6) than those of coal-derived asphaltenes (n =... 

The aromatic system of petroleum-derived asphaltene is extensively substituted (50%-70%) whereas the coal-derived asphaltene is sparingly substituted (30%-50%). 

The molecular weight of petroleum-derived asphaltene is about ten times higher than that of the coal-derived asphaltene. Unit molecular weight of coal asphaltene is 400-600 whereas that of petroleum asphaltene is 800-2500. 

Petroleum-derived asphaltene is less reactive to physical or chemical agents than is coal-derived asphaltene. 

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