Water sorption properties, diffusion and kinetics of zeolite NaX modified by ion-exchange and salt impregnation

Abstract

Zeolite NaX is one of the most popular water sorption materials for dehumidification, thermal energy storage and sorption cooling. In this paper, zeolite NaX is modified by magnesium chloride to achieve ion-exchange (zeolite NaMgX) and salt impregnation (zeolite NaX/MgCl2) with the aim of improving water sorption capacity and thermal performance. The pore structure, desorption heat, water equilibrium sorption property, adsorption kinetics and cycling stability of zeolite NaX, NaMgX and NaX/MgCl2 were studied to evaluate their potential for sorption application. Experimental results show the zeolite NaMgX has stronger hydrophilic and can improve the water sorption capacity from 0.17 g/g to 0.20 g/g at 2500 Pa. The zeolite NaX/MgCl2 with 12.6 wt% salt confined inside the meso-/macro-pore, can improve the water sorption capacity from 0.17 g/g to 0.26 g/g with sorption heat of 842 J/g. The sorption rate of zeolite NaX/MgCl2 is much lower than that of pure zeolite NaX, while zeolite NaMgX has a slight lower sorption rate than pure zeolite NaX. The diffusivity of zeolite NaX is in the range of 8.03 × 10−10–1.97 × 10−9 m2/s at 1000–2500 Pa, while zeolite NaMgX and zeolite NaX/MgCl2 have diffusivities in the range of 5.22 × 10−10–1.73 × 10−9 m2/s and 3.76 × 10−10–9.87 × 10−10 m2/s respectively. The diffusivities of different zeolite samples increase with temperature which can be described by activated energy factor, and the relation between diffusivity and pressure is strongly coupled with the equilibrium equation. The modified zeolites have good thermal stabilities and no obvious uptake loss after ten adsorption-desorption cycles, making them feasible, scalable and economical for sorption thermal energy storage and cooling applications.