Supercritical carbon dioxide

Carbon dioxide pressure-temperature phase diagram

Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.

Carbon dioxide usually behaves as a gas in air at STP or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (31.1 °C) and critical pressure (72.9 atm/7.39 MPa), 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 chemical extraction in addition to 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. In addition, the solubility of many extracted compounds in CO₂ vary with pressure,^[1] permitting selective extractions.

Uses

Solvent

Main article: Supercritical fluid extraction

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 lead 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 they are sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g. to the pharmaceutical industry or to beverage manufacturers) by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis.

Supercritical carbon dioxide can also be used as a more environmentally friendly solvent for dry cleaning as compared to more traditional solvents such as hydrocarbons and perchloroethylene.^[2]

Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates. Its main advantages over solvents such as hexane and acetone in this process are that it is non toxic and non-flammable. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents, merely by allowing it to evaporate into the air recycling it by condensation into a cold recovery vessel. Its advantage over steam distillation is that it is used at a lower temperature, which can separate the plant waxes from the oils.

In laboratories, supercritical carbon dioxide is used as an extraction solvent, e.g., in determination of Total Recoverable Hydrocarbons from soils, sediments, fly-ash, and other media,^[3] and determination of PAHs in soil and solid wastes.^[4] Supercritical fluid extraction has also been used in determination of hydrocarbon components in water.^[5]

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.^[6]

Manufactured products

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.^[7] The primary byproduct is water.

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.

Working fluid

There is considerable work being done to develop a supercritical carbon dioxide closed-Brayton-cycle gas turbine to operate at temperatures near 550 °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 above 500 °C and 20 MPa enable very high thermal efficiencies, approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the volume of polluting fuels used in producing electricity, the environmental impact of cycle efficiency increases would be significant.^[8]

Supercritical carbon dioxide is also an important emerging natural refrigerant, being used in new, low carbon solutions for domestic heat pumps.^[9] These systems are undergoing continuous development with supercritical carbon dioxide heat pumps already being successfully marketed in Asia. The EcoCute systems from Japan, developed by Mayekawa, develop high temperature domestic water with small inputs of electric power by moving heat into the system from their surroundings. Their success makes a future use in other world regions possible.^[10]

Supercritical carbon dioxide has been used for more than 30 years 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 sequestration efforts. Using gasifiers instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide, and ash. This hydrogen gas can be used to produce electrical power 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.^[11][12][13]

It has been suggested that supercritical carbon dioxide could be used as a working fluid in Enhanced Geothermal Systems. Possible advantages compared to water include higher energy yield resulting from its lower viscosity, better chemical interaction, CO2 storage through fluid loss and higher temperature limit. As of 2011, the concept has never been tested on the field.^{[14] [15] [16] [17] [18]}

Sterilization of biomedical materials

Recent studies have proved SC-CO₂ is an effective alternative for terminal sterilization of biological materials and medical devices. Moreover, this process is gentle, as the morphology, ultrastructure, and protein profiles of inactivated microbes are maintained.^[19]

See also

• Alliance for CO2 Solutions
• Caffeine
• Dry cleaning
• Perfume
• Supercritical fluid
• The Cool War

Further reading

Mukhopadhyay M. Natural extracts using supercritical carbon dioxide. USA: CRC Press, LLC; 2000 Free preview at Google Books

References

1. ^ Discovery - Can Chemistry Save The World? - BBC World Service
2. ^ Stewart, Gina (2003), Joseph M. DeSimone and William Tumas, ed., "Dry Cleaning with Liquid Carbon Dioxide", Green chemistry using liquid and supercritical carbon dioxide (USA: Oxford University Press): 215–227 
3. ^ U.S.EPA Method 3560 Supercritical Fluid Extraction of Total Recoverable Hydrocarbons. http://www.epa.gov/SW-846/pdfs/3560.pdf
4. ^ U.S.EPA Method 3561 Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbons. http://www.epa.gov/SW-846/pdfs/3561.pdf
5. ^ Use of Ozone Depleting Substances in Laboratories. TemaNord 2003:516. http://www.norden.org/pub/ebook/2003-516.pdf
6. ^ S. Yeo and E. Kiran, "Formation of polymer particles with supercritical fluids: A review", J. Supercrit. Fluids, 34 (2005), 287
7. ^ Rubin, James B.; Taylor, Craig M. V.; Hartmann, Thomas; Paviet-Hartmann, Patricia (2003), Joseph M. DeSimone and William Tumas, ed., "Enhancing the Properties of Portland Cements Using Supercritical Carbon Dioxide", Green chemistry using liquid and supercritical carbon dioxide (USA: Oxford University Press): 241–255 
8. ^ V. Dostal, M.J. Driscoll, P. Hejzlar, "A Supercritical Carbon Dioxide Cycle for Next Generation Nuclear Reactors", MIT-ANP-Series, MIT-ANP-TR-100 (2004)
9. ^ FAQs - Supercritical CO₂ in heat pumps and other applications
10. ^ Eco Cute hot water heat pumps in Japan
11. ^ "The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs", p. 84 (2004)
12. ^ FutureGen Technology
13. ^ Øyvind Vessia: "Fischer- Tropsch reactor fed by syngas"
14. ^ K Pruess(2006), "A hot dry rock geothermal energy concept utilizing supercritical CO2 instead of water" http://www.geothermal-energy.org/pdf/IGAstandard/SGW/2000/Brown.pdf
15. ^ Donald W. Brown(2000), "On the feasibility of using supercritical CO2 as heat transmission fluid in an engineered hot dry rock geothermal system" http://geothermal.stanford.edu/pdf/SGW/2006/Pruess.pdf
16. ^ K Pruess(2007)Enhanced Geothermal Systems (EGS) comparing water with CO2 as heat transmission fluids"
17. ^ J Apps(2011), "Modeling geochemical processes in enhanced geothermal systems with CO2 as heat transfert fluid" http://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2011/apps.pdf
18. ^ http://earthsciences.typepad.com/blog/2011/06/achieving-carbon-sequestration-and-geothermal-energy-production-a-win-win.html ESD News and Events "Achieving Carbon Sequestration and Geothermal Energy Production: A Win-Win!"
19. ^ A.White, D. Burns, T.W. Christensen, "Effective terminal sterilization using supercritical carbon dioxide", J. of Biotechnology http://www.novasterilis.com/BIOTEC_4178.pdf