A parametric study on mass diffusion coefficient of desiccants for dehumidification applications: Silica aerogels and silica aerogel coatings on metal foams

Abstract

A separate sensible and latent cooling air-conditioning system integrated with a solid desiccant-based dehumidification device provides potential energy savings as compared to a conventional vapor compression system. Silica aerogels are hygroscopic materials that are deployed as solid desiccants for dehumidifying devices, such as an enthalpy wheel. These hygroscopic materials are highly porous and have good moisture adsorption and desorption characteristics due to the microstructure. Mass diffusivity is an important parameter that relates to the rate of the adsorption or desorption process in silica aerogels. The current study is focused on the evaluation of the mass diffusivity of solid silica aerogels and silica aerogel coatings on metal foam substrates. The porous structure of silica aerogels affects the mass diffusion coefficients. The synthesis process to manufacture the silica aerogels causes the porous structure to differ and thus impacts the mass diffusivity. The sol-gel process is used to prepare silica aerogels using various basic (ammonium hydroxide, sodium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts with the same precipitator (tetramethyl orthosilicate) and solvent (methanol). The wet samples have been dried using a super-critical process to avoid internal cracking due to the capillary force. Scanning electron microscopy is used to analyze the microstructure of super-critically dried silica aerogels. The dynamic vapor sorption method is used to determine the effective mass diffusivity for the different silica aerogels. It is found that the mass diffusivity is related to the microstructure of silica aerogels, which depends on the catalysts used in the sol-gel process; however, the values for mass diffusivities for solid desiccants and desiccant coatings are similar. In addition, a parametric study is conducted to determine the effect of relative humidity and temperature on the adsorption and desorption mass diffusivity. Furthermore, the impact of the drying process and aging on the mass diffusivity of aerogels has been investigated.