Revisiting the adsorption equilibrium equations of silica-gel/water for adsorption cooling applications

This paper addresses the discrepancies among the common adsorption isotherms of silica-gel/water provided in the literature. It is reported that the Freundlich model and Tòth equation cannot be used to estimate the uptake at relative pressure less than 0.15. In addition, inconsistencies are found among the various models and equations used to describe the uptake of water vapor onto silica-gel. New coefficients for the Dubinin-Astakhov (D-A) model are proposed to eliminate these disagreements. Due to the limited experimental measurements, an experimental setup is designed and built to measure the sorption kinetics and equilibrium uptake of any working pairs. Experimental measurements show that the maximum uptakes of silica gel RD-2060 and Type-RD are 0.38 kg kg−1 and 0.48 kg kg−1, respectively. Apparent capillary condensation is observed at a relative pressure of 0.4 and 0.35 for silica-gel RD and RD-2060, respectively. Also, it is found that the D-A model can fit the adsorption isotherms of silica-gels appropriately for the entire range of relative pressure when the characteristic energy is set as a function of relative pressure instead of assuming constant values.