Supercritical carbon dioxide

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Carbon dioxide pressure-temperature phase diagram

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.

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

[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 perchloroethylene (PCE or Perc) or water. This new approach of cleaning clothes was developed and commercialized by Dr. Joseph DeSimone, professor of chemical engineering at North Carolina State University, in Raleigh. Supramics, environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic, are made using supercritical carbon dioxide as a chemical reagent. The supercritical carbon dioxide in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsum plaster to form various carbonates. The sole by-product is ultra-pure water. Because supramics consume and sequester carbon as stable compounds in useful products, they may serve to reduce carbon that would otherwise be released into the environment.

There is considerable work being done to develop an enhanced version of a gas-turbine power production cycle to operate at temperatures near 550 degrees C. This is a significant usage, which could have large implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at 500+ degrees C and 20 MPa enable very high thermal efficiencies, approaching 45 percent. This could increase the electical power produced per unit of fuel required by 40 percent or more. Given the huge volume of extremely polluting fuels used in producing electricity, the potential environmental impact of such an efficicnet sycle could be very large.

http://web.mit.edu/jessiek/MacData/afs.course.lockers/22/22.33/www/dostal.pdf

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. At the same time, there is the possibility of using the various "clean coal" technologies which are emerging to combine such enhanced recovery methods with carbon sequestation efforts. Using advanced Fischer-Tropsch digesters instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide, and ash. Electrical power is produced by burning the hydrogen in combined-cycle gas turbines, while the CO2 is captured, compressed to the supercritical state, and injected into geological storage, possibly into existing oil fields to improve yields. The unique properties of supercritical CO2 ensure that it will remain out of the atmosphere.

http://books.nap.edu/openbook.php?record_id=10922&page=84 http://www.futuregenalliance.org/technology.stm http://www.zero.no/transport/bio/fischer-tropsch-reactor-fed-by-syngas

Supercritical carbon dioxide is also an important emerging natural refrigerant, being used in new, low carbon solutions for domestic heat pumps. These systems are undergoing continuous development with the first commercial supercritical carbon dioxide heat pumps being now marketed. The "EcoCute" systems from Japan, developed by consortium of companies including Mitsubishi, develop high temerature domestic water at with small inputs of electric power by moving heat into the system from their surroundings.

[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.