THE PENETRATION OF SOLVENT VAPOR INTO COAL PARTICLES (SOLUBILITY, DIFFUSION, RELAXATION)

The investigation of the sorption of solvent vapor into high volatile bituminour coal (PSOC-375) at temperatures up to 175(DEGREES)C indicates that the solvent weight gain by a coal particle involves a complex coupling of several phenomena including adsorption, sorption into the coal matrix, capillary condensation and extraction into the condensed vapor phase. The acceptor number (AN), which indicates the electrophilic property of the solvent, and the donor number (DN), which indicates the nucleophilic property of the solvent, correlate the sorption behavior quite well. A similar correlation has been used for coal extraction measurements. It appears that the sorption in untreated coal is dominated by the capillary condensation induced by solvent extraction. As a result, an equilibrium state is not attainable for the sorption in untreated coal. This extraction mechanism can be eliminated by the preextraction of the coal particles with pyridine. Vapor sorption experiments conducted on pyridine-extracted coal can be used to obtain information concerning the adsorption process and the process associated with the diffusion of the solvent molecules into the coal matrix.;Thermodynamic equilibrium data as a function of solvent activity have been employed to evaluate the Flory model. This model was derived to describe the sorption of solvent vapors into crosslinked polymers. Although the Flory model fits the coal sorption data quite well, the physical significance of the resulting parameters in this model is open to question. The thermodynamics of solvent sorption into coal particles is further complicated by the presence of severe hysteresis effects. The amount of solvent sorbed by a coal particle is not only a function of solvent activity but depends upon the past history of the sorption process which influences the structure of coal.;Vapor sorption studies conducted on pyridine-extracted coal particles indicate that the sorption process involves a coupling of adsorption, molecular diffusion and a relaxation of the coal structure to a new state. Similar behavior is commonly observed for vapor sorption into glassy polymers. The coal sorption measurements have been accurately described by use of two models, one by Berens and Hopfenberg, the other by Long and Richman. Both models were derived to describe coupling of molecular diffusion and polymer chain relaxation in glassy polymers.