Action of Specific Solvents

In general circumstances, unless solvolysis is involved, the more common organic solvents, such as benzene, alkylbenzenes, methanol, acetone, chloroform, and diethyl ether, dissolve little of the true coal substance and usually extract only that material that is occluded within the coal matrix. 

The effect of pyridine on coal has been known since the late days of the nineteenth century (Bedson, 1899) and extensive follow-up studies were carried out to determine the comparative extractability of pyridine and chloroform (Cockram and Wheeler, 1927,1931 Blayden et al., 1948 Wender et al., 1981). 

These studies, as well as later work (Dryden, 1950,1951a,b Given, 1984), showed that significant yields of extracts, often as high as 35%-40%, can be obtained by using pyridine, certain heterocyclic bases, or primary aliphatic amines (which may, but need not, contain aromatic or hydroxyl substituents). Secondary and tertiary aliphatic amines are often much less effective insofar as more than one alkyl group on the amine appears to present steric problems that interfere with the interaction between the solvent and the coal. 

Other examples of the chemical alteration of coal to improve the extractability yield include lithium/amine reduction (Given et al., 1959 Given, 1984), sodium/alcohol rednction (Ouchi et al., 1981), and sodinm/potassium/glycol ether reduction (Niemann and Hombach, 1979). But it must be remembered that even though the extractability of the coal is enhanced, the chemistry of these reactions is not well understood and is often subject to speculation leaving the precise reasons for solubility enhancement open to speculation also. Reactions that enhance coal solnbility by depolymerization (Heredy et al., 1965) also suffer from the same unknowns. 

Insofar as the ability of solvents to extract material from coal can be correlated with the presence of an unshared pair of electrons on a nitrogen atom or an oxygen atom in a solvent molecule, it is difficult to fully rationalize such a concept. Eor example, some nonsolvents (of which methanol may be cited as an example) have the ability to swell coal almost as much as the more specific solvents (such as pyridine) (Franz, 1979 Szeliga, 1987). Furthermore, the approximate linear relationship between extract yields and the internal pressures of the solvents no longer holds when the solvents are used at temperatures below their normal boiling points. 

Reactive solvents dissolve coal by active interaction. Such solvents are usually hydrogen donors (e.g., tetralin, 1,2,3,4-tetrahydronaphthalene) and their chemical composition is affected appreciably during the process. Again, using tetralin as the example, the solvent is converted to the aromatic counterpart (in this case, naphthalene) and the products from the coal can vary in composition, depending on the reaction severity and the ratio of the solvent to the coal. In addition, the extracts differ markedly in properties from those obtained with degrading solvents. 

Recently, considerable attention has been paid to the use of compressed gases and liquids as solvents for extraction processes (Schneider et al., 1980 Dain-ton and Paul, 1981 Bright and McNally, 1992 Kiran and Brennecke, 1992), although the law of partial pressures indicates that when a gas is in contact with a material of low volatility, the concentration of solute in the gas phase should be minimal and decrease with increased pressure. Nevertheless, deviations from this law occur at temperatures near the critical temperature of the gas, and the concentration of solute in the gas may actually be enhanced as well as increased with pressure. 

The technique of extracting virtually nonvolatile substances is particularly useful for materials that decompose before reaching boiling point and is therefore well suited to the extraction of the liquids formed when coal is heated to about 400°C (750°F). Thus, supercritical gas or fluid extraction affords a means of recovering the liquid products when they are first formed, avoiding undesirable secondary reactions (such as coke formation), and yields of extract up to 25 or 30% have been recorded. 

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