Supercritical carbon dioxide

From Wikipedia, the free encyclopedia

1 Uses
2 Environmental impact
3 See also
4 Further reading
5 References

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. The relatively low temperature of the process and the stability of CO₂ also allows most compounds to be extracted with little damage or denaturing.

[edit] Uses

Supercritical carbon dioxide is gaining popularity amongst coffee manufacturers looking to move away from some of the classic decaffeinating 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 critical temperature and reactivity 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 is also used in the foaming of polymers. Many corporations utilize supercritical carbon dioxide to saturate the polymer with solvent (carbon dioxide). Upon depressurization and heating the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e. creating a foam. Research is also ongoing at many universities in the production of microcellular foams using supercritical carbon dioxide.

Supercritical carbon dioxide is beginning to be used to enhance oil recovery in mature oil fields.

[edit] 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.