Supercritical carbon dioxide

Overview
Supercritical carbon dioxide refers to carbon dioxide that is in a fluid state while also being at or above both its critical temperature and pressure, yielding rather uncommon properties. 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 (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 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.

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 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 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 is also becoming a more common solvent 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 would alter their odor.

Supercritical carbon dioxide can be used in cleaning clothes, instead of perchloroethylene (PCE or Perc) or water. This new approach to cleaning clothes was developed and commercialized by Dr. Joseph DeSimone, joint professor of chemical engineering at North Carolina State University and chemistry at the University of North Carolina, Chapel Hill.

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°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 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 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 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 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.

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.

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.