5.4.3. Extraction with supercritical CO₂

A fluid is said to be supercritical when the temperature and pressure exceed the critical temperature and pressure for the substance considered.

Due largely to the higher pressure and temperatures involved, the solvent power of supercritical CO₂ under typical extraction conditions is higher than that of liquid CO₂ though the general features shown in the table continues to apply.

The part of the supercritical region actually used in present day supercritical extraction processes is restricted to a comparatively limited range of reduced temperature between unity and about 1.4 and reduced density between about 1.3 and 2.1 (see figure). Within this range density changes rapidly with pressure at given temperature or with temperature at given pressure. This fact is quite important for the development of a process, since the solvent power of supercritical fluids is strongly dependent on density.

Gerard et al report supercritical carbon dioxide extraction at a pressure of 500 bar and a temperature of 60ºC with ground and pelletised Rosemary (Rosmarinus Officinalis) and Sage (Salvia Officinalis) leaves that gives CO₂ extracts composed by essential oil, phenolic diterpenes, wax, carotenoids and other plants. The effectiveness of the natural CO₂ extract of Rosemary and Sage as antioxidant have been tested.

Several plants are in operation worldwide for extracting aroma and cosmetic ingredients using supercritical carbon dioxide. In France, CAL Pfizer employ supercritical CO₂ at about 40°C and between 200 and 250 bar to obtain extracts of rosemary and other herbs and spices (Parkinson and Johnson, 1989, Moyler, 1993). The above extraction conditions are shown as point 4 on the figure and it is seen that they lie on the boundary of the supercritical and the subcritical regions. In Germany, also Flavex operate a supercritical carbon dioxide extraction plant to extract aroma and cosmetic ingredients.