Abstract
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 − γT) , 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.
Highlights
Green CaCl2-based chemical sorbent has been widely used in sorption refrigeration, air purification and air desiccation
Fresh water that can be directly used has been exhausting rapidly due to gradual natural environmental degradation, especially the serious pollution of lakes and rivers[1,2,3,4]
Among the various chemical sorbents, green CaCl2 has been widely applied to air desiccation, thermal energy storage, and gas purification, because of its large water adsorption capacity, excellent chemical and thermal stability, excellent cycle performance, low cost, and absence of environmental pollution
Summary
Green CaCl2-based chemical sorbent has been widely used in sorption refrigeration, air purification and air desiccation. Among the various chemical sorbents, green CaCl2 has been widely applied to air desiccation, thermal energy storage, and gas purification, because of its large water adsorption capacity, excellent chemical and thermal stability, excellent cycle performance, low cost, and absence of environmental pollution. Its equilibrium water adsorption capacity exceeds 100 g/100 g, more than three times larger than that of the silica gel physical sorbent. To improve the adsorption rate, CaCl2 has typically been added to porous media such as silica gel and activated alumina so that composite sorbents with large specific surface areas can be obtained[20]. Because CaCl2 blocked the mass transport channels in MCM-41 for water vapour, the adsorption time for CaCl2 to reach sorption equilibrium exceeded 30 h. The initial adsorption rate of the composite sorbent was 0.0275 kg/m3/s, and the time was approximately 2 h to reach adsorption equilibrium
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