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Supercritical carbon dioxide refers to carbon dioxide with some unique properties. Carbon dioxide usually behaves as a gas in air or as a solid in dry ice. If the temperature and pressure are both increased, it can adopt properties midway between a gas and a liquid. It behaves like a supercritical fluid above its critical temperature (31.1 degrees Celsius) and pressure (73 atm), expanding to fill its container like a gas, but with a density like that of a liquid. Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in compound extraction as well as its low toxicity and environmental impact. As well, the relatively low temperature of the process and the stability of CO2 allow most compounds to be extracted with little damage or denaturing.
Uses
Supercritical carbon dioxide is gaining popularity amongst coffee manufacturers looking to move away from some of the classic decaffinating solvents of the past; many of which have led to public outcry because of real or perceived dangers related to their use in food preparation. Supercritical CO2 is forced through the green coffee beans and then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale to, for example, cola manufacturers by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis.
Supercritical carbon dioxide is also becoming a more common process for extracting volatile oils and fragrance compounds from various raw materials that are used in perfumery. The relatively low supercritical temperature and reactively of CO2 allows the fragrance compounds to be extracted without extensive damage or denaturing, which will alter their odor.
Supercritical carbon dioxide can be used in cleaning clothes, instead of tetrachloroethylene (perchloroethylene or "perc") or water. This new approach of cleaning clothes was developed and commercialized by Dr. Joseph DeSimone, professor of chemistry at the University of North Carolina at Chapel Hill.
Processes which use supercritical carbon dioxide to produce micro and nano scale particles, often for pharmaceutical uses, are currently being developed. The gas antisolvent process, rapid expansion of supercritical solutions, and supercritical antisolvent precipitation (as well as several related methods) have been shown to process a variety of substances into particles (Yeo and Kiran 2005).
Supercritical carbon dioxide can be used in enhanced oil recovery in more mature oil fields.
Environmental impact
Supercritical carbon dioxide is seen as a promising green solvent because it is non-toxic, and a byproduct of other industrial processes. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents.
See also
Further reading
Mukhopadhyay M. Natural extracts using supercritical carbon dioxide. USA: CRC Press, LLC; 2000; ISBN 0849308194
References
S. Yeo, E. Kiran, Formation of polymer particles with supercritical fluids: a review, J. Supercrit. Fluids 34 (2005) 287.
"Supercritical carbon dioxide"