Physical properties and adsorption kinetics of silica-gel/water for adsorption chillers

The choice of a suitable adsorbate/adsorbent pair is critical for an adsorption cooling cycle. The surface characteristics and thermo-physical properties of the adsorbent, and the adsorption rate of adsorbate are key parameters in making the choice. Through literature review, it is found that there are disagreements among the experimental measurements and various equations/models used to describe adsorption isotherms and surface diffusivity of water in silica-gel. In this work, an experimental set-up is built to measure the isotherms and kinetics of vapor adsorption for any working pair. Using the newly measured data, those from the manufacturers and from the literature, these inconsistencies are eliminated by utilizing the Dubinin-Astakhov (D-A) model to fit the entire adsorption isotherm curve. The Brunauer-Emmett-Teller (BET) method is used to calculate the surface area, pore volume and pore diameter of two different types of silica-gel. Based on the adsorption rate and the adsorbent temperature measured simultaneously, a new approach is proposed to measure the surface diffusivity in the temperature and pressure ranges typical of those during the operating conditions of adsorption cooling systems. Analysis of the results indicates that the surface diffusivity follows the Arrhenius-form equation. The calculated activation energy at different adsorption conditions varies from 40.0 to 41.2 kJ/mol and the pre-exponential factor varies from 2.5 × 10−4 to 2.8 × 10−4 m2/s. These values are close to those previously reported in the literature. Thus, the proposed approach can be used to measure the surface diffusivity in nanoporous materials.