Steady-state equation of water vapor sorption for CaCl₂-based chemical sorbents and its application

Green CaCl₂-based chemical sorbent has been widely used in sorption refrigeration, air purification and air desiccation. Methods to improve the sorption rate have been extensively investigated, but the corresponding theoretical formulations have not been reported. In this paper, a sorption system of solid-liquid coexistence is established based on the hypothesis of steady-state sorption. The combination of theoretical analysis and experimental results indicates that the system can be described by steady-state sorption process. The steady-state sorption equation, μ = (η - γ_T) , was obtained in consideration of humidity, temperature and the surface area. Based on engineering applications and this equation, two methods including an increase of specific surface area and adjustment of the critical relative humidity (γ) for chemical sorbents, have been proposed to increase the sorption rate. The results indicate that the CaCl₂/CNTs composite with a large specific surface area can be obtained by coating CaCl₂ powder on the surface of carbon nanotubes (CNTs). The composite reached sorption equilibrium within only 4 h, and the sorption capacity was improved by 75% compared with pure CaCl₂ powder. Furthermore, the addition of NaCl powder to saturated CaCl₂ solution could significantly lower the solution's γ. The sorption rate was improved by 30% under the same environment.