Modeling of supercritical drying of ethanol-soaked silica aerogels with carbon dioxide

BACKGROUND: Supercritical drying (SCD) of aerogels entails replacement of the primary solvent ethanol in the porous solid network with supercritical carbon dioxide (SCCO2) without a vapor-liquid interface to avoid collapse of the pores by capillary forces. The present paper proposes a mechanistic SCD model for producing crack-free aerogels and elucidates the requirement of such a process by focusing on mass transfer modeling for silica aerogel with parallel cylindrical mesopores having a known pore size distribution. RESULTS: The model incorporates extraction of ethanol by two-way mass transfer of SCCO2 and ethanol to and from the wet gel until it attains 99.995 mole% CO2. Initially the pores are filled with pure ethanol and the liquid volume swells due to dissolution of CO2, rendering spillage of the excess liquid volume. Thus ethanol is removed by spillage as well as by evaporation from the open end of the pores, followed by convective mass transfer. The convective velocity of SCCO2 needs to be kept less than a critical value to obviate vapor-liquid interface within the pores at any time. Simulation results indicate that initially the ratio of spillage to convective transport is very high (16-20) which asymptotically decreases to zero with time as the mixture in the pores attains critical mole fraction (MCM). This indicates that the bulk of the ethanol is removed by spillage caused by diffusion of SCCO2 into the pores, rather than by convective evaporation. Their ratio is higher at lower temperature, higher pressure, lower flow rate and thicker gel, similar to the drying time. CONCLUSIONS: The mechanistic model for SCD of a silica aerogel with SCCO2 illustrates that a vapor-liquid interface within the pores at any time can be avoided by keeping the convective velocity of SCCO2 less than a critical value. Most of the ethanol is removed by spillage as a result of SCCO2 dissolution in the pore liquid, rather than by convective evaporation. Higher pressure, lower temperature, lower SCCO2 velocity and thicker gel slow down the SCD process and are conducive to producing crack-free transparent aerogels. (C) 2008 Society of Chemical Industry.