The technique of supercritical fluid extraction utilizes the dissolution power of supercritical fluids, i.e. fluids above their critical temperature and pressure. Its advantages include the use of moderate temperatures, and that several cheap and non-toxic fluids are available.
Supercritical fluids are used in the extraction of hop oils, caffeine, vanilla, vegetable oils and /3-carotene. It has also been shown experimentally that the extraction of certain steroids and chemotherapeutic drugs can be achieved using supercritical fluids. Other current and potential uses include the removal of undesirable substances such as pesticide residues, removal of bacteriostatic agents from fermentation broths, the recovery of organic solvents from aqueous solutions, cell disintegration, destruction and treatment of industrial wastes and liposome preparation. There are, however, a number of significant disadvantages in the utilization of this technology:
(i) Phase equilibria of the solvent/solute system is complex, making design of extraction conditions difficult.
(ii) The most popular solvent (carbon dioxide) is non-polar and is therefore most useful in the extraction of non-polar solutes. Though co-solvents can be added for the extraction of polar compounds, they will complicate further downstream processing.
(iii) The use of high pressures leads to capital costs for plant, and operating costs may also be high.
Thus, the number of commercial processes utilizing supercritical fluid extraction is relatively small, due mainly to the existence of more economical processes. However, its use is likely to increase in some sectors, for example the recovery of high value biologicals, when conventional extractions are inappropriate, and in the treatment of toxic wastes (Bruno et al., 1993).
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