Supercritical fluids process concepts

By utilizing the rapid expansion of supercritical solutions, small-size particles can be produced from materials which are soluble in supercritical solvents. In this process, a solid is dissolved in a pressurized supercritical fluid and the solution is rapidly expanded to some lower pressure level which causes the solid to precipitate. This concept has been demonstrated for a wide variety of materials including polymers, dyes, pharmaceuticals and inorganic substances. 

While this patent does report on an improved process for making aerogels, the concept of displacing the reaction solvent with a supercritical fluid has been reported as early as 1983 by Schmitt, Grieger-B lock, and Chapman as described in chapter 12. In the Schmitt process, the aqueous solvent used to form aerogels of silica is displaced by methanol, the system is heated and pressurized above the critical point of the methanol, and the methanol is vented from the system. The benefit is that methanol has a critical temperature that is 135 °C less than that of water. It is not possible to use CO2 to displace the water in the gel without the occunence of a vapor-liquid interface since water and CO2 are not miscible unless the temperature is raised to greater than 300 °C. 

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