Effects of Natural Convection Mass Transfer on Drying Aerogels With Supercritical Carbon Dioxide

In manufacturing silica aerogels, the alcohol solvent is often extracted from a wet gel via supercritical carbon dioxide (SCCO₂) drying to avoid, during the majority of the drying process, crossing the liquid-vapor phase boundary and thus prevent capillary forces from collapsing the delicate mesoporous structure. Our understanding of the relevant physics is limited due to the complex phenomena involved in this transport process. Therefore, a rigorous model that captures the underlying physics is essential to understand the transport phenomena for scaling up the drying process and reducing overall manufacturing costs. Several studies that model the SCCO₂ drying of aerogel have ignored the effect of natural convection on the extraction rate. Here, we developed a 3D model to capture the natural convective mass transfer due to variation in fluid density as the concentration of carbon dioxide CO₂ changes in an ethanol-carbon dioxide mixture. The geometry is an annular gel concentric with an open region where SCCO₂ flows. The aerogel is modeled as a porous medium where the flow is modeled using Darcy’s law. We solve the compressible forms of the momentum, continuity, and species equations that govern and couple the transport in the open region and the porous medium. Also, we account for the variation in molecular diffusivity in the species transport equation using a mass-fraction-dependent diffusion coefficient. We show the changes in the mass transfer rates relative to the case where natural convection is ignored to be substantive.