Temperature Dependence of Solvent Swelling and Diffusion Processes in Coals

The rates of solvent swelling of the Argonne Premium Sample coals have been measured in various organic solvents at various temperatures. The results show that the extents of swelling, when experiments are carried out in liquid solvents, are independent of the temperature, within the temperature range studied here (10−60 °C). Thermodynamically, this requires that equilibrium swelling should occur with a near-zero enthalpy, as generally required for absence of a temperature effect on equilibrium. This conclusion is consistent with a number of other recently published results. The rates of swelling of the coals do not correlate with rank. The nature of the swelling process varies from relaxation controlled to Fickian diffusion controlled. The activation energies for the kinetics of swelling are consistent with other recently published values, but again, a correlation with rank could not be substantiated. The activation energies all fall in the range from 20 to 60 kJ/mol, suggesting that the activation barrier may be associated with the breakage of internal electron donor−acceptor (e.g., hydrogen bonding) interactions. Thermal pretreatment of some of the coals to 350 °C had significant effects on their swelling behaviors. The effect was generally to increase the rate of swelling, and in some cases, the extent of swelling. The activation energies for swelling were, however, unaffected. This is interpreted as consistent with the hypothesis that the activation energy barrier is determined by donor−acceptor interactions which are unaffected by pretreatment but that other thermally dissociable coal−coal interactions may serve to stiffen its structure.