Vacuum concentration and freeze drying are related methods used for the gentle drying or preservation of thermally sensitive materials. This often involves the removal of aqueous media. As can be seen from the state diagram of water, which can also serve as an example for other solvents, above a certain process pressure (in this case 6.1 hPa for H2O) drying occurs from the liquid phase. At pressures below this level, the water temperature drops below 0 °C.
Drying occurs directly from the frozen state, bypassing the liquid phase.The triple point (in this case p = 6.1 hPa and T = 0 °C for H2O) can be regarded as a sort of boundary between vacuum concentration (or drying) and freeze drying. Ice, water and vapour can be observed simultaneously in the chamber.