Super Critical Fluid Extraction

Supercritical fluid extraction employs a gas above its critical temperature to extract the material. that increases the extraction efficiency, eliminates the use of solvents and simplifies subsequent concentration procedures. A diagram of the super critical fluid extraction apparatus is shown in figure 22. Most modern super critical fluid extraction instruments use carbon dioxide largely due to its critical temperature and pressure being conveniently, 31.4°C and about 1000 psi respectively and the material is readily available.

Figure 22. Super Critical Fluid Extraction Apparatus

Above the critical temperature carbon dioxide behaves as a gas at all pressures and, thus, transfer rates are fast and rapid. At low pressures liquid carbon dioxide interacts dispersively with a solute much like n–hexane, but at high pressures the interactions increase in strength and are more akin to those of methylene dichloride or chloroform. Carbon dioxide has little or no polar character and, thus, to dissolve polar materials a solvent modifier must be used which is a distinct disadvantage as it involves the use of solvents and introduces a concentration and recovery step. n-hexane, benzene, toluene, methylene dichloride, nitromethane, ethyl acetate, tetrahydrofuran, acetonitrile, propanol, ethanol and methanol have been used as modifiers and range from strongly dispersive to strongly polar. After extraction, the solution passes through a restrictor, where the pressure is reduced and the carbon dioxide bled away. The residual liquid is then concentrated. However, besides needing modifying solvents, the procedure is complicated to develop, the apparatus is expensive and the work-up procedure no simpler than that by normal solvent extraction.