Evaluation and development of improved thermodynamic models for adsorbed phase properties in adsorption cycles

In adsorption processes, gaseous molecules are trapped on the surfaces of the solid adsorbent because of the surface forces. In contrast to bulk liquid or gaseous phases, fluids in adsorbed phase display distinctive characteristics. Understanding of the adsorbed phase and developing accurate models for its thermodynamic properties are crucial in assessing adsorption processes. Most existing models for the adsorbed phase often portend thermodynamic inconsistencies since they invoked numerous assumptions (e.g., ideal gas behavior of the adsorbate and negligible adsorbed phase's specific volume). We thoroughly propose and examine thermodynamically consistent models for adsorbed phase thermodynamics (specific heat, enthalpy, and entropy). A new specific heat capacity expression is derived accounting for the typically neglected adsorbed phase specific volume. Adsorbed phase properties calculated using these new models exhibit behaviors closer to the liquid phase compared to the gaseous phase. In contrast, enthalpy and entropy of the adsorbed phase calculated using the models available in the literature have been found exceeding the corresponding gaseous phase boundaries at higher pressure/coverage. The proposed correlations are applied to the thermodynamic characterization of a newly developed adsorbent material, activated carbon MSF-A30M with ethanol adsorbate, and compared against other activated carbons reported in the literature. The comparison shows consistent values of thermodynamic properties, well within the theoretical boundaries. As a practical application of the new correlations, we applied them to evaluate the performance of the adsorption heat pumps using 30 different working pairs. Accurate evaluations of the entropy, which is a thermodynamic state property, will lead to improved entropy generation calculations using the classical thermodynamic approach. This work will significantly contribute to improved tracking of thermodynamic losses in adsorption processes, from low coverage to near saturation pressures (details on thermodynamics loss evaluations of the cycles not covered in the present work).