Asphaltene Constituents

There has been significant emphasis on determining the nature of the asphaltene-type constituents. As with petroleum, the asphaltene portion of coal-derived liquids are responsible for many of the problems that arise when the liquids are processed. Therefore, a brief notation abont the character of coal-derived asphaltene constituents is necessary, recognizing that more complete reviews are available. Such information aids in understanding of this complex fraction and may also help dispel some prior, but erroneous, notions about the chemical character of coal-derived asphaltene constituents. 

Asphaltene constituents in coal liquids are important for several reasons (1) they contribute to many problems in the processing of crude oil (2) they contribute to instabilities in processing (3) they lead to excessive viscosity, contributing to pumping expense, clogging, and slow processing (4) the presence of asphaltene constituents can lead to incompatibility with some solvents used in petroleum processing and (5) asphaltene constituents lead to coke formation under some circumstances. 

The origin of coal asphaltene constituents has been the subject of much speculation insofar as they have been considered to be not only the initial products of coal liquefaction but also the secondary products of coal liquefaction  

Coal asphaltene constituents oil Coal - oil asphaltene constituents 

Although current concepts tend to favor the former hypothesis, the matter is still not completely resolved. 

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The character of fuel oil generally renders the usual test methods for total petroleum hydrocarbons (Chapters 7 and 8) ineffective since high proportions of the fuel oil (specifically, residual fuel oil) are insoluble in the usual solvents employed for the test. In particular, the asphaltene constituents are insoluble in hydrocarbon solvents and are only soluble in aromatic solvents and chlorinated hydrocarbons (chloroform, methylene dichloride, and the like). Residua and asphalt (Chapter 10) have high proportions of asphaltene constituents, which render any test for total petroleum hydrocarbons meaningless unless a suitable solvent is employed in the test method. 

Many investigations of relationships between composition and properties take into account only the concentration of the asphaltene constituents, independent of quality criterion. However, a distinction should be made between the asphaltene constituents that occur in straight-run resids and those that occur in blown asphalts. Remembering that asphaltene constituents are a solubility class rather than a distinct chemical class means that vast differences occur in the makeup of this fraction when it is produced by different processes. 

The test methods of interest for the analysis of coal extracts include tests that measure chemical composition. The preeminent test methods are those that are applied to measurement of the asphaltene content. The issue with coal extracts is that the presence of asphaltene constituents in the extracts is solvent dependent. But assuming that a test is necessary to determine that whether asphaltene constituents are or are not present, several test are methods available. 

The asphaltene constituents produce the highest amount of coke (relative to the other fractions of the resid). The formation of a coke-like substance during resid upgrading is dependent on several factors 1) the degree of polynuclear condensation in the feedstock 2) the average number of alkyl groups on the polynuclear aromatic systems and 3) the hydrogen-to-carbon atomic ratio of the pentane-insoluble/heptane-soluble fraction. 

In fact, it has been suggested that there exists a somewhat lesser dependence of the molecular chemistry of coal on rank and the chemistry of coal is heavily influenced by its source as well as by the early formation history (Berkowitz, 1988). It is also possible that coals of similar rank may, therefore, be chemically much more diverse than is usually supposed. Indeed, this concept is in agreement with similar conclusion about the formation of petroleum insofar as the chemical nature of the crude oil, with particular reference to the asphaltene constituents, is dependent not only on the types of precursors but also on the relative mix of these precursors that formed the protopetroleum (Speight, 2007) as well as on regional variations in the matnration conditions due to variations in climatic differences between varions geological eras/periods (Bend, 1992 Speight, 2007). 

Determination of the molecular weight (size) of the higher-molecular-weight constituents of petroleum (the asphaltene constituents) is somewhat easier to investigate than the molecular weight of coal constituents by virtue of their solubility in aromatic (and other highly potent) solvents. In contrast, the molecular weight of coal is, because of the nature of the material, unknown. 

However, it is the obvious physical differences between coal and petroleum that can raise questions when similarities are considered. Perhaps the most convenient approach is to consider the differences in dimension and space between the two. The properties of coal are very suggestive of a three-dimensional network. This is much less obvious in petroleum (asphaltene constituents) and may only occur to a very minor extent. Such a difference in spatial arrangement would certainly account for some, if not all, of the differences between the two. Serious consideration of such a proposition would aid physical/chemical/structural studies in both fields and would, hopefully, induce a more constructive thinking in terms of coal/petroleum behavior. 

The fact that coals are heterogeneous as a group and, indeed, heterogeneous individually does not mean that there cannot be a concept of a macromolecular structure. But such a concept should include a variety of molecular types, perhaps in a manner analogous to the formulation of the structural types in petroleum asphaltene constituents (Figure 10.36) (Long, 1979 Speight, 2007). 

Two properties that have found some relevance in defining solvent behavior with coal (as well as with other complex carbonaceous materials such as petroleum asphaltene constituents) are the surface tension and the internal pressure. 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). 

In summary, it has been assumed, on the basis of the behavior of the thermal decomposition of polynuclear aromatic systems, that coal must also consist of large polynuclear aromatic systems (Chapter 10). Be that as it may, such assumptions are highly speculative and, to say the least, somewhat lacking in caution. As an example, similar lines of thought have been applied to structural assumptions about petroleum asphaltene constituents when it is known from other pyrolysis studies that smaller, but polar, systems can produce as much thermal coke as the larger nonpolar highly condensed systems (Speight, 2007). 

Asphalts are also regarded as colloidal suspensions, in which the oily constituents are the dispersant and the asphaltene constituents, principally, the dispersoid. Oily constituents of an aromatic nature lead to sol-like dispersions of lower temperature susceptibility and non-Newtonian behavior that is, viscosity changes less with temperature but is affected by rate of shear for the velocity gradient. 

In general, asphaltene constituents provide the major element of viscosity, and viscosity-temperature susceptibility increases with viscosity. These effects are contradictory as some statements are found to the effect that asphaltene constituents decrease temperature susceptibility and give high fluidity flow viscosity). In fact, it seems to be a predominant opinion that asphaltene constituents lower the temperature susceptibility and that by increasing the asphaltene content (at least up to a point) the temperature susceptibility will be improved. This depends on a lot of ifs and such qualifying phrases as other things being equal. ... 

While it is conceded that synthetic bitumen obtained from the hydrogenation of coal is not the same as a petroleum asphalt, still there are similarities that may permit characterization in similar terms. The same element of immiscibility exists between oils and asphaltene constituents or tars (oxygenated materials). It has been noted that these raw synthetic bitumen contain volatile oils and hard, asphaltene-like constituents. The oils are predominantly aromatic, giving stable sol-like dispersions with high temperature susceptibility and exhibiting Newtonian behavior. 

Such synthetic bitumen lends itself to a variety of modifications and uses. Steam distillation under vacuum, a universal practice in asphalt processing, would remove volatiles and the nonvolatile residue could be air-blown to asphalt. The residue from distillation, if pitch-like, could be fluted with a nonvolatile oil and excessive asphaltene constituents could be precipitated by solvent refining. 

Nevertheless, structural studies of the asphaltene constituents frran coal liquefaction processes have progressed to the ptmt where various molecular types have been identified. It has also been pointed out that the character of the asphaltene constituents is process dependent and the structural character of the asphaltene may even bear s( ne relationship to the structural types in the parent coal (Snape et al., 1984). 

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). 

Coal asphaltene constituents have frequently been defined in terms of an acid-base complex with the asphaltene existing as a composite of the two systems, acid and base ... 

However, a more recent report indicates that pentane insolubility of the bulk of the coal asphaltene constituents cannot be ascribed to hydrogen-bonding effects between the acidic and basic components. 

Current evidence indicates that coal-derived asphaltene constituents are a collection of predominantly one-to-four ring condensed aromatic systems that contain basic and nonbasic nitrogen constituents as well as oxygen (acidic and etheric) functions. These functionalities play a role in intramolecular relationships within the asphaltene fraction and also with the other constituents of the coal-derived liquid. This latter effect influences the viscosity of the liquid. Thus, coal-derived asphaltene constituents are an extremely complex solubility class by virtue of their thermal derivation from coal. 

As with petroleum asphaltene (Speight, 2007), it is difficult (if not impossible) to accurately depict the structure of coal-derived asphaltene constituents. A simple assessment should include the statement small-ring polynuclear aromatic systems, basic and nonbasic nitrogen as well as phenolic and etheric oxygen. How these types enjoy an intramolecular existence is another matter which is difficult and whose definition is left to future research. 

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