Adsorption Isotherm Model for Analyzing the Adsorption Characteristics of Water Vapor to Commercially Available Silica Gel Adsorbents for Adsorption Desalination Applications

In adsorption desalination applications, adsorption isotherms are considered to be a crucial factor, significantly impacting performance. From this perspective, a number of isotherm models have been implemented to describe the adsorption characteristics of an adsorbent. However, conventional isotherm models can produce significant discrepancies in predictions of anomalous adsorption isotherms under specific operating conditions (e.g., in the presence of polar or nonpolar adsorbates). In this study, a hybrid isotherm model consisting of Langmuir and Sips isotherms was proposed. With this model, optimal compatibility is demonstrated with the adsorption isotherms of four different commercial silica gels. First, the thermophysical properties of silica gel were analyzed using nitrogen adsorption and desorption isotherms at 77 K; then, adsorption isotherms of water vapor were evaluated. The results of the hybrid isotherm model were compared with those of the conventional isotherm models (i.e., Toth and Dubinin-Astakhov) at relative pressures ranging from 0 to 0.9 at temperatures ranging from 30 to 70 degrees C. The hybrid isotherm model demonstrated good agreement with the adsorption data measured for all silica gels. The coefficient of determination for the hybrid isotherm model was found to be higher than 0.99 for all silica gels, unlike the coefficients for the conventional isotherm models. The hybrid isotherm model also exhibited the lowest average relative error and maximum relative error.