A supercritical fluid is obtained when a component is handled above a certain pressure (critical pressure) and above a certain temperature (critical temperature). These two parameters determine the so-called critical point of a component which is a component-specific property. Below this critical point a component can be transferred from the solid phase to the liquid phase and subsequently to the gaseous phase by increasing the temperature at constant pressure. The phase changes in this case are all connected to a certain energy which is needed to change the state of the component. Above the critical point this behavior of phase change cannot be observed.
Supercritical fluids can replace conventional solvents such as methanol, ethanol, methylene chloride etc. Since some of these solvents are toxic, efforts are now being made to avoid their use. An alternative to these solvents is provided especially by supercritical CO2. Substances dissolve at a much higher rate in supercritical fluids than would normally be expected at a given vapour pressure. CO2 has proven to be particularly well suited for the extraction of natural substances.
The low critical temperature of 31 °C allows gentle treatment of natural substances. Moreover, CO2 is not only readily available, but may also be obtained at relatively low prices.