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

In low-temperature aerogel production, replacement of an original solvent w ith liquid or supercritical carbon dioxide is the crucial stage, and it is one fully controlled by a diffusional mechanism. The diffusion of ethanol a s the primary pore fluid through an alcogel structure to the: surrounding n ear-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 chromatogra phic analysis. The experiments covered the range of temperatures from 292 t o 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 fur each experiment. A compari son of measured concentrations with values derived from a mass transfer mod el is presented and discussed. The method for identifying an effective diff usion coefficient is described and the values of ethanol-CO2 effective diff usivity in a silica network are presented. The correlation of effective dif fusion coefficient in the porous structure of silica gel with the Schmidt n umber ratio covering the entire investigated range of temperature and press ure is proposed (C) 2001 Elsevier Science B.V. All rights reserved.