Adsorption and Desorption Isotherms Of Desiccants for Dehumidification Applications: Silica Aerogels and Silica Aerogel Coatings on Metal Foams

Silica aerogels are frequently employed as solid desiccants in enthalpy wheels for dehumidifying the supply stream in air-conditioning systems. These desiccant materials possess good moisture adsorption and desorption characteristics due to their porous structure. Analysis of adsorption and desorption isotherms is critical for performance characterization and is often performed to evaluate the capacity and transient performance of desiccantbased dehumidification systems. The current study is focused on the adsorption and desorption isotherms of solid silica aerogels and silica aerogel coatings on open-cell metal-foam substrates. The sol-gel process is adopted to synthesize silica aerogels using different basic (ammonium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts, with the same precipitator (tetra methyl orthosilicateTMOS) and solvent (methanol). Scanning electron microscopy is used to characterize the microstructure of supercritically dried aerogels and adsorption/desorption isotherms for the different samples are obtained by the dynamic vapor sorption method. The steady-state moisture adsorption and desorption capacity of silica aerogels is affected by their porous structure, which depends on the synthesis technique used to prepare the silica aerogels. For the silica aerogel coatings on metal foams, the substrate structure and surface area also play an important role. The effect of the substrate surface area on adsorption/desorption capacity is analyzed by comparing the isotherms for solid silica aerogel samples, and silica aerogels coatings on flat plates and on metal foams with different pore sizes.