Continuous drying of alginate aerogel particles: Residence time measurement and process optimization under high pressure conditions

This study advances continuous supercritical carbon dioxide (scCO₂) drying of aerogel particles by introducing a non-invasive optical method to determine particle residence time in a countercurrent extraction column. In countercurrent operation, scCO₂ flows upward while the particle suspension in ethanol enters from the top. The method enables precise, real-time residence time measurement under high pressure conditions without disturbing the process. The effects of pressure (100–150 bar), temperature (40–80 °C), CO₂ flow rate (30–80 g/min), and suspension flow rate (10–45 g/min) on residence time and drying efficiency were accordingly analyzed. Experiments were performed in a 1.25 m high extraction column, with an internal diameter of 20.5 mm, using highly spherical alginate beads with a diameter of ~ 400 µm as a model system. Evidence of effective solvent removal throughout the whole operation range was provided by determination of the residual ethanol content in the intact aerogel beads after the drying process (0.0053–0.0341 g ethanol /g aerogel ). The dried products featured a specific surface area of 363 ± 27 m²/g, a mesopore volume of 3.2 ± 0.7 cm³/g, consistent with the typical range of alginate aerogels. The combined insights provide a comprehensive picture of the countercurrent column’s operational response and allow the definition of practical operating windows. Elevated temperature and high pressure provide the most favorable trade-off between short residence time and minimized residual ethanol, maximizing the time-specific yield. Overall, the approach establishes a robust, transferable framework for optimizing continuous scCO₂ drying of aerogel particles and supports extension to other particle sizes and formulations. • Continuous scCO₂ drying of freely settling aerogel beads in a countercurrent column • Non-invasive optical residence time distribution measurement in situ under high-pressure operation • Statistical model links P, T, CO₂ and suspension flow to residence time and drying efficiency • ~400 µm alginate beads dried to residual ethanol level of 0.0053–0.0341 g/g • Aerogel quality preserved: 363 m²/g surface area and 3.2 cm³/g pore volume