Molecular coal asphaltene

The average molecular size of coal asphaltenes (linear molecular size is more precise since it is derived from GPC data) is smaller than that of petroleum derived asphaltenes. 

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. 

Figure 7. Comparison of molecular weight distribution off o) coal asphaltenes and Jobo petroleum asphaltenes (-------) vacuum residuum and (%) atmo-...

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. 

The phosphotungstate salts cannot be used for equivalent weight determinations, for their characteristically variable composition leads to inconsistent titration results. However, treatment of the salts with aqueous sodium hydroxide led to recovery of the free bases in reproducible amounts. These results show that one-fifth of the high molecular weight asphaltenes and one-third of the preasphaltenes are precipitated as phosphotungstate salts. Considerably less of the unfractionated H-coal vacuum-still bottoms and none of the low molecular weight asphaltenes and oils and resins were separated in this manner. 

The presence of hydrocarbons and of low molecular weight heterocompounds identical to those found in the cyclohexane- or pentane-soluble fractions of coal liquids was rather unexpected. However, neutral hydrocarbons have been identified in other coal asphaltenes (10), and more recently,... 

FIGURE 10.36 Representation of the petroleum asphaltene fraction and the coal asphaltene fraction on the basis of molecular weight and polarity. 

Coal asphaltene constituents are quite different in nature from petroleum asphaltene constituents (Table 18.7). The molecular weight of asphaltene constituents from coal liquids may be some 8-10 times lower than the observed molecular weight of petroleum asphaltene constituents, although this latter can be revised to lower values for a variety of reasons (Steedman, 1985). 

Varga A complex process for hydrogenating brown coal and high molecular weight asphaltenes. The process uses hydrogen at a high pressure, in the presence of an iron oxide catalyst. Invented by J. Varga in Budapest... 

Figure 3. Molecular weight vs. oxygen content for resins and polar asphaltenes in raw coal liquids ( Z ), regular SRC (O), H-coal (A), SCT-SRC.

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 (. ... 

The two monoaromatic and di- + triaromatic fractions are practically indistinguishable from each other except for a slightly higher molecular weight of the fractions from the Pitt Seam coal liquids. The spectra of the polyaromatic fractions were too weak and unresolved, and no meaningful calculations could be made from them. Similar problems were encountered when it was attempted to analyze the asphaltenes by NMR. Methods have to be developed to analyze polyaromatic and asphaltene fractions. 

Product oil containing 24 percent asphaltenes of 670 molecular weight has a viscosity of 86 (SSF at 180° F, ASTM D88), while product oil containing almost the same amount of asphaltenes (22 percent) but with a molecular weight of 460 has a viscosity of only 14. Benzene insolubles, heretofore regarded as unreacted coal, were found to be soluble in pyridine and to exert a large effect on viscosity. 

Distillations. The upgraded coal liquids were distilled with a metal-mesh-spinning-band still under the conditions shown in Figure 1 to produce cuts at 200°, 325°, and 425° C. Asphaltenes were then precipitated from each >425° C residuum dissolved in benzene by addition of 50 volumes of normal pentane (15). Further distillations on the asphaltene-free materials, at 202° C and 4 micron pressure using a wiped-wall molecular still, produced 425° to 540° C distillate cuts and residua fractions. 

Given, et. al. (2) have reported the molecular parameter analysis of asphaltenes obtained from coals of variable petrographic compositions based on proton NMR analysis. However,... 

The process of hydrogenolysis Is designed to fragment the coal structure into smaller molecular weight units light oil, heavy oil, and asphaltenes. A small amount of gaseous products also result. 

The ratio of aliphatic protons to aromatic protons for the heavy oil was 4.01/1 and of methylene to methyl 1/1.75. For the asphaltenes, the ratio of aliphatic protons to aromatic protons was 3.49/1 and of methylene to methyl 1/1.1. The asphaltenes were observed to melt at HG C. Molecular weights obtained by vapor phase osmometry (o-xylene solvent) were 407 for the heavy oil and 638 for the asphaltenes. There appears to be little difference in the relative yield of heavy oils and asphaltenes from depolymerized coal as compared with the "as received" coals. Acknowledgements... 

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. 

Table I summarizes comparisons between asphaltenes derived from bituminous coal liquefaction and those derived from petroleum crudes. The molecular size and atomic H/C ratios suggest a molecular profile quite different for the two asphaltenes. The ranges represent, as best as could be found, reasonable extremes for each of the properties. We are well aware that the number-average molecular weight of petroleum asphaltenes has been influenced by aggregate formation. To overcome this effect, molecular weight determinations should be made in dilute noninteracting solvents (e.g., methylene chloride), and solutions should be filtered or ultracentrifuged in helium-degassed solvents.

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