Effective diffusion coefficient in the low temperature process of silica aerogel production

In low-temperature aerogel production, replacement of an original solvent with liquid or supercritical carbon dioxide is the crucial stage, and it is one fully controlled by a diffusional mechanism. The diffusion of ethanol as the primary pore fluid through an alcogel structure to the surrounding near-critical CO2 was investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with an online gas chromatographic analysis. The experiments covered the range of temperatures from 292 to 315 K and pressure from 6.9 to 9.1 MPa. On the basis of the concentration histories during the drying, the diffusion coefficient in the ethanol–CO2 mixture inside the silica gel was identified for each experiment. A comparison of measured concentrations with values derived from a mass transfer model is presented and discussed. The method for identifying an effective diffusion coefficient is described and the values of ethanol–CO2 effective diffusivity in a silica network are presented. The correlation of effective diffusion coefficient in the porous structure of silica gel with the Schmidt number ratio covering the entire investigated range of temperature and pressure is proposed.