Supercritical drying
From Wikipedia, the free encyclopedia
Supercritical drying is a process to remove liquid in a precisely controlled way, similar to freeze drying. It is useful in the production of MEMS and the drying of spices, and is commonly used in the production of aerogel.
As a substance crosses the boundary from liquid to gas (see phase diagram), the size of the liquid decreases. As this happens, surface tension at the solid-liquid interface pulls against any structures that the liquid is attached to. Delicate structures, like cell walls, the dendrites in silica gel, and the tiny machinery of MEMS devices, tend to be broken apart by this surface tension as the interface moves by.
To avoid this, the sample can be brought from the liquid phase to the gas phase without crossing the liquid-gas boundary on the phase diagram; in freeze-drying, this means going around to the left (low temperature, low pressure). However, some structures are disrupted even by the solid-liquid boundary. Supercritical drying, on the other hand, goes around the line to the right, on the high-temperature, high-pressure side. This route from liquid to gas does not cross any phase boundary, instead passing through the supercritical region, where the distinction between gas and liquid ceases to apply.
Fluids suitable for supercritical drying include carbon dioxide (critical point 31.1 degrees Celsius 1072 psi) and freon (25 to 30 C, 500-600 psi) . Nitrous oxide has similar physical behavior to carbon dioxide, but is a powerful oxidizer in its supercritical state. Supercritical water is also a powerful oxidizer, partly because its critical point occurs at such a high temperature (374 C) and pressure (3212 psi)[1].
In most such processes, acetone is first used to wash away all water, exploiting the complete miscibility of these two fluids. The acetone is then washed away with high pressure carbon dioxide, the industry standard now that freon is unavailable. Again, acetone and liquid carbon dioxide are completely miscible. The carbon dioxide is then heated until its pressure goes beyond the critical point, at which time the pressure can be gradually released, allowing the gas to escape and leaving a dried product.
