First Evidence of Solvent Spillage under Subcritical Conditions in Aerogel Production

The first evidence of solvent spillage under subcritical conditions during aerogel production is presented. The main objective was to understand the underlying phenomena controlling the solvent extraction kinetics during autoclave pressurization. Alginate, silica, and polyurethane as gels and ethanol, methyl ethyl ketone, and ethanol/water as solvents were investigated. When CO2 diffuses in the gel solvent, there is a volume expansion of the liquid solvent. This expanded liquid mixture spills out of the gel and accumulates as a separate liquid phase at the bottom of the autoclave. A lag time was observed between the start of the autoclave pressurization and the moment in which liquid starts to accumulate in the autoclave. The time needed for the solvent to start accumulating at the bottom of the autoclave is controlled by capillary forces and the saturation of the CO2-gas phase, on which temperature has an important effect. Low operating temperature and thereby low solubility of the solvent in the gas phase is suggested as the factor that enhances the kinetics of solvent removal at subcritical conditions.