Continuous extraction rate measurements during supercritical CO2 drying of silica alcogel

•A new apparatus has been developed which continuously measures the alcohol extraction rate during supercritical CO2 =  based drying of silica alcogels.•Extraction is shown to be largely diffusion limited, except at low CO2 flow rates.•A model of the process which considers a composition-dependent diffusivity is shown to be in good agreement with experimental results.•Use of mixture density as a metric by which to measure drying progress is shown to have great potential for integrating into industrial aerogel production systems.

We investigated the kinetics of supercritical CO2 (SCCO2)-based drying of silica aerogels, a common, but time consuming and energy intensive step in their manufacture. An apparatus was developed to continuously measure alcohol extraction rates from alcogels as a function of key process variables by two redundant techniques. Kinetics data are reported for the drying of 2.5 mm, 5 mm, and 7.5 mm thick annular alcogels by pumping SCCO2 through a 10 mm-thick concentric annulus surrounding their exterior. The SCCO2 was at a temperature of 323 K and a pressure of 12.4 MPa and its mass flow rate varied from 1 kg/h to 5 kg/h. Gel thickness and SCCO2 flow rate were both shown to significantly effect drying rate and required drying time. The results of a conjugate mass transfer model assuming pure diffusion in the alcogel compared favorably with the data when the composition dependence of molecular diffusivity was captured utilizing available correlations.

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