HEAT-EFFECTS FOR TOLUENE ADSORPTION ON ACTIVATED CARBON FROM SUPERCRITICAL CARBON-DIOXIDE

The isochoric adsorption isotherms of toluene from supercritical carbon dioxide on activated carbon were provided in this study. The experimental data were obtained by a recycle technique. The activated carbon pellets were loaded in a spinning basket autoclave. After injection of a certain amount of toluene into autoclave, the equilibrium loadings of toluene on activated carbon at various temperatures were measured in a path of gradually decreasing or increasing temperatures. The equilibrium adsorption data for temperatures ranging from 308 to 338 K at CO2 densities of 0.32, 0.45, and 0.69 g cm(-3) were correlated accurately by the Toth isotherm expression. A thermodynamic model was proposed herein related the mole fraction of toluene in supercritical CO2 phase with the changes in partial molar enthalpies, partial molar volumes, and partial molar internal energies of toluene. The experimental data obtained were analyzed by the proposed model revealing that the changes in partial molar enthalpies and partial molar volumes were always positive for the conditions studied and were significantly large near the CO2 critical condition. This enthalpy change represents the energy transfer of one mole of toluene from the nonideal, supercritical solution onto the adsorbent surface at isobaric and isothermal operations. This quantity, however, is not the heat of adsorption as conventionally defined which represents the energy transfer of one mole of solute from an ideal gas solution onto the adsorbent surface. But on the other hand, the change in partial molar internal energies of toluene was found to be always negative. This indicated that the energy effects associated with an adsorption process at supercritical conditions should be clearly defined so as to elucidate the adsorption is endothermic or exothermic.