Coal solvent interactions

One of the methods used in the production of liquid fuels from coal is to heat coal in the presence of solvents in order to dissolve and stabilize low molecular weight fragments. Many studies (1-6) have been devoted to elucidating the chemical mechanism of product formation in hydrogen donor and non-donor solvents. In most of these studies, the time dependence of the product yield was used as a measure of the rates of reaction, or the product yield was correlated with the solvent, the rank of the coal or other properties of the coal. In order to gain a better understanding of the nature of the coal-solvent interactions, we... 

Coal-solvent interactions leading to swelling and dissolution of the coal (11, 12). 

However, coal-solvent interactions are very complex and there have been various attempts to correlate the extraction efficiency with the nature of the solvent, nature of the coal, and extraction conditions. 

The solvent extraction of coal is, in essence, a mild form of chemical conversion because in addition to the pnrely solvent action, there may also be molecular alterations that are definite and irreversible. Coal-solvent interactions are complex (Szeliga, 1987) but, in more general terms, extraction is usually enhanced by temperature in addition, the presence of hydrogen will significantly alter the molecular changes. 

When coal is heated in a slurrying vehicle, it is liquefied at 400°C-500°C (750°F-930°F). Though the reaction mechanism involving conversion of coal to oil is very complex, it appears that the interaction of coal with solvent at the initial stage of the reactions plays the vital role to determine the sequential conversion of coal substances—first to a pyridine-soluble solid and thereafter to benzene-soluble liquid hydrocarbons and low-boiling products. Thus the isolation and identification of the products of coal-solvent interactions to yield pyridine-soluble matter may provide information regarding the suitability of the coal for liquefaction. 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

A number of basic studies in the area of donor solvent liquefaction have been reported (2 -9). Franz (10J reported on the interaction of a subbituminous coal with deuterium-labelled tetra-lin, Cronauer, et al. (11) examined the interaction of deuterium-labelled Tetralin with coal model compounds and Benjamin, et al. (12) examined the pyrolysis of Tetralin-l-13C and the formation of tetralin from naphthalene with and without vitrinite and hydrogen. Other related studies have been conducted on the thermal stability of Tetralin, 1,2-dihydronaphthalene, cis-oecalin and 2-methylin-dene (13,14). 

The action of organic solvents on natural polymers combustible minerals (coal and brown coal or peat) is intensively studied for a long time due to following reasons. Firstly, this is one of the successful method of studying the structure of combustible materials and the second is their technological application for obtaining of a so-called montan-wax or low-molecular liquid extracts which can be transformed into synthetic liquid fuel due to hydration process. Moreover, an interaction of a coal with the solvents is a basis of the coals liquation processes and coals transformation into liquid fuel. 

That is why logically to assume the possibility of specific interactions also during the swelling process, since the values of parameters of the coal solubility S2, which are determining accordingly to the Flory Renner s equation are differed. It depends on fact if the data for all solvents are taking into account in calculations or such calculations are performed with the exclusion of results for solvents able to be as acceptors of hydrogen bonds (amines, ketones). Different results have been obtained also under application of other methods for calculations, especially of the Van-Krevelen s method [14],... 

Correctness of the sixth parameter equation (7) and its simplified forms for the generalization of the swelling data was proved for other coals including the Donbas coal [32] at the parameters B and VM- If to apply the equation (7) to the coal extraction data, then the factor of molar volume VM is insignificant, and the connection between quantities of extracted substance (in g/mole of the solvent) and physical-chemical characteristics can be satisfactorily described by fifth parameter equation (6) or by its simplified forms in this case possible acid-base interaction is the decisive factor, that is factor B [33 - 35], This confirmation is in good agreement with the above-said bigger molecules harder introduce... 

The effects of exposure of organic solids to particular solvents such as pyridine on their conformational stability can also be Interpreted In terms of the structural features discussed above. How small nucleophilic molecules disrupt Inter- and Intramolecular polar Interactions In coals thereby relaxing the structural matrix and allowing further solvent penetration has been extensively discussed by Peppas (e.g. 11,12), Larsen (1,13) and Marzec (14-16) and their colleagues. Indeed the extent to which exposure to a polar solvent such as pyridine destabilizes a material s molecular structure Is a measure of the extent to which the stability of the material depends on polar Interactions. 

Finally, due to the extremely heterogeneous nature of coal, x parameters derived from Equation 1 can only represent at best an average interaction between a solvent and coal extract. It is also clear that pyridine extracts contain a substantial amount of low molecular weight material. Thus, an estimate of the degree of polymerization, x, which appears in Equation 1, is required. 

The separation of chemical species by size exclusion chromatography is more reproducible than any other type of chromatography. Once the SEC columns, the mobile phase (most often a pure solvent like THF or toluene), and the flow rate are selected, the retention volume (or retention time assuming the flow rate does not change) is primarily a function of linear molecular size, which can be obtained from the valence bond structure if the compound is known. Some of the chemical species can interact with the solvent forming complexes with an effective linear size greater than that of the molecule. This causes the expected retention volume, based on "free" molecular structure, to shift to a lower but very reproducible retention volume. Phenols in coal liquids form 1 1 complex with THF (9,10) and carry the effective linear molecular size to increase. As a result phenolic species elute sooner than expected from their... 

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