Supercritical carbon dioxide
Uses 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.
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 byproduct is ultra-pure water. 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 huge volume of extremely polluting fuels used in producing electricity, the potential environmental impact of such an efficient cycle could be very large. 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.
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 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. 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 supercritical carbon dioxide heat pumps already being successfully marketed in Asia. The EcoCute systems from Japan, developed by consortium of companies including Mitsubishi, 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.
Supercritical carbon dioxide has also been used as an extraction solvent in the creation of highly-potent hash oil, known as honey oil. 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, and determination of PAHs in soil and solid wastes. Supercritical fluid extraction has also been used in determination of hydrocarbon components in water. 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. 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 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; ISBN 0-8493-0819-4 References ^ 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) ^ S. Yeo and E. Kiran, “Formation of polymer particles with supercritical fluids: A review”, J. Supercrit. Fluids, 34 (2005), 287 ^ “The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs”, p. 84 (2004) ^ FutureGen Technology ^ yvind Vessia: “Fischer- Tropsch reactor fed by syngas” ^ FAQs – Supercritical CO2 in heat pumps and other applications ^ Eco Cute hot water heat pumps in Japan ^ U.S.EPA Method 3560 Supercritical Fluid Extraction of Total Recoverable Hydrocarbons. http://www.epa.gov/SW-846/pdfs/3560.pdf ^ U.S.EPA Method 3561 Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbons. http://www.epa.gov/SW-846/pdfs/3561.pdf ^ Use of Ozone Depleting Substances in Laboratories. TemaNord 2003:516. http://www.norden.org/pub/ebook/2003-516.pdf ^ A.White, D. Burns, T.W. Christensen, “Effective terminal sterilization using supercritical carbon dioxide”, J. of Biotechnology Categories: Carbon dioxide | Inorganic solvents