A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation, respectively.
In general terms, supercritical fluids have properties between those of a gas and a liquid. In Table 1, the critical properties are shown for some components, which are commonly used as supercritical fluids.
|Solvent||Molecular weight||Critical temperature||Critical pressure||Critical density|
|Carbon dioxide (CO2)||44.01||304.1||7.38 (72.8)||0.469|
|Water (H2O) (acc. IAPWS)||18.015||647.096||22.064 (217.755)||0.322|
|Methane (CH4)||16.04||190.4||4.60 (45.4)||0.162|
|Ethane (C2H6)||30.07||305.3||4.87 (48.1)||0.203|
|Propane (C3H8)||44.09||369.8||4.25 (41.9)||0.217|
|Ethylene (C2H4)||28.05||282.4||5.04 (49.7)||0.215|
|Propylene (C3H6)||42.08||364.9||4.60 (45.4)||0.232|
|Methanol (CH3OH)||32.04||512.6||8.09 (79.8)||0.272|
|Ethanol (C2H5OH)||46.07||513.9||6.14 (60.6)||0.276|
|Acetone (C3H6O)||58.08||508.1||4.70 (46.4)||0.278|
Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.
|Density (kg/m3)||Viscosity (µPa∙s)||Diffusivity (mm²/s)|
In addition, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary. By changing the pressure and temperature of the fluid, the properties can be "tuned" to be more liquid- or more gas-like. One of the most important properties is the solubility of material in the fluid. Solubility in a supercritical fluid tends to increase with density of the fluid (at constant temperature). Since density increases with pressure, solubility tends to increase with pressure. The relationship with temperature is a little more complicated. At constant density, solubility will increase with temperature. However, close to the critical point, the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again.
All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
- Tc(mix) = (mole fraction A) x TcA + (mole fraction B) x TcB.
For greater accuracy, the critical point can be calculated using equations of state, such as the Peng Robinson, or group contribution methods. Other properties, such as density, can also be calculated using equations of state.
Phase diagram 
Figures 1 and 2 show projections of a phase diagram. In the pressure-temperature phase diagram (Fig. 1) the boiling separates the gas and liquid region and ends in the critical point, where the liquid and gas phases disappear to become a single supercritical phase. This can be observed in the density-pressure phase diagram for carbon dioxide, as shown in Figure 2. At well below the critical temperature, e.g., 280K, as the pressure increases, the gas compresses and eventually (at just over 40 bar) condenses into a much denser liquid, resulting in the discontinuity in the line (vertical dotted line). The system consists of 2 phases in equilibrium, a dense liquid and a low density gas. As the critical temperature is approached (300K), the density of the gas at equilibrium becomes denser, and that of the liquid lower. At the critical point, (304.1 K and 7.38 MPa (73.8 bar)). there is no difference in density, and the 2 phases become one fluid phase. Thus, above the critical temperature a gas cannot be liquefied by pressure. At slightly above the critical temperature (310K), in the vicinity of the critical pressure, the line is almost vertical. A small increase in pressure causes a large increase in the density of the supercritical phase. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave like a gas, as can be seen in Figure 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure.
Many pressurised gases are actually supercritical fluids. For example, nitrogen has a critical point of 126.2K (- 147 °C) and 3.4 MPa (34 bar). Therefore, nitrogen (or compressed air) in a gas cylinder above this pressure is actually a supercritical fluid. These are more often known as permanent gases. At room temperature, they are well above their critical temperature, and therefore behave as a gas, similar to CO2 at 400K above. However, they cannot be liquified by pressure unless cooled below their critical temperature.
Natural occurrence 
Submarine volcanoes 
Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous pressure from the weight of the water above them prevents the explosive release of steam and gases. This causes the water to be heated to over 375 degrees C, turning the water in the hottest parts of the vents into a supercritical fluid since the pressure at this depth of over 3 km is over 300 atmospheres, well above the 218 atmospheres required.
Planetary atmospheres 
The atmosphere of Venus is 96.5% carbon dioxide and 3.5% nitrogen. The surface pressure is 9.3 MPa (93 bar) and the surface temperature is 735 K, above the critical points of both major constituents and making the surface atmosphere a supercritical fluid.
The interior atmospheres of the solar system's gas giant planets are composed mainly of hydrogen and helium at temperatures well above their critical points. The gaseous outer atmospheres of Jupiter and Saturn transition smoothly into the fluid interior, while the nature of the transition zones of Neptune and Uranus are unknown. Theoretical models of extrasolar planets 55 Cancri e and Gliese 876 d have posited an ocean of pressurized, supercritical fluid water with a sheet of solid high pressure water ice at the bottom.
Supercritical fluid extraction 
The advantages of supercritical fluid extraction (compared with liquid extraction) are that it is relatively rapid because of the low viscosities and high diffusivities associated with supercritical fluids. The extraction can be selective to some extent by controlling the density of the medium and the extracted material is easily recovered by simply depressurizing, allowing the supercritical fluid to return to gas phase and evaporate leaving little or no solvent residues. Carbon dioxide is the most common supercritical solvent. It is used on a large scale for the decaffeination of green coffee beans, the extraction of hops for beer production, and the production of essential oils and pharmaceutical products from plants. A few laboratory test methods include the use of supercritical fluid extraction as an extraction method instead of using traditional solvents.
Supercritical fluid decomposition 
Supercritical water can be used to decompose biomass via supercritical water gasification of biomass This type of biomass gasification can be used to produce hydrocarbon fuels for use in an efficient combustion device or to produce hydrogen for use in a fuel cell. In the latter case, hydrogen yield can be much higher than the hydrogen content of the biomass due to steam reforming where water is a hydrogen-providing participant in the overall reaction.
Supercritical carbon dioxide (SCD) can be used instead of PERC (perchloroethylene) or other undesirable solvents for dry-cleaning. Supercritical carbon dioxide sometimes intercalates into buttons, and, when the SCD is depressurized, the buttons pop, or break apart. Detergents that are soluble in carbon dioxide improve the solvating power of the solvent.
Supercritical fluid chromatography 
Supercritical fluid chromatography (SFC) can be used on an analytical scale, where it combines many of the advantages of high performance liquid chromatography (HPLC) and gas chromatography (GC). It can be used with non-volatile and thermally labile analytes (unlike GC) and can be used with the universal flame ionization detector (unlike HPLC), as well as producing narrower peaks due to rapid diffusion. In practice, the advantages offered by SFC have not been sufficient to displace the widely used HPLC and GC, except in a few cases such as chiral separations and analysis of high-molecular-weight hydrocarbons. For manufacturing, efficient preparative simulated moving bed units are available. The purity of the final products is very high, but the cost makes it suitable only for very high-value materials such as pharmaceuticals.
Chemical reactions 
Changing the conditions of the reaction solvent can allow separation of phases for product removal, or single phase for reaction. Rapid diffusion accelerates diffusion controlled reactions. Temperature and pressure can tune the reaction down preferred pathways, e.g., to improve yield of a particular chiral isomer. There are also significant environmental benefits over conventional organic solvents.
Impregnation and dyeing 
Impregnation is, in essence, the converse of extraction. A substance is dissolved in the supercritical fluid, the solution flowed past a solid substrate, and is deposited on or dissolves in the substrate. Dyeing, which is readily carried out on polymer fibres such as polyester using disperse (non-ionic) dyes, is a special case of this. Carbon dioxide also dissolves in many polymers, considerably swelling and plasticising them and further accelerating the diffusion process.
Nano and micro particle formation 
The formation of small particles of a substance with a narrow size distribution is an important process in the pharmaceutical and other industries. Supercritical fluids provide a number of ways of achieving this by rapidly exceeding the saturation point of a solute by dilution, depressurization or a combination of these. These processes occur faster in supercritical fluids than in liquids, promoting nucleation or spinodal decomposition over crystal growth and yielding very small and regularly sized particles. Recent supercritical fluids have shown the capability to reduce particles up to a range of 5-2000 nm.
Generation of pharmaceutical cocrystals 
Supercritical fluids act as a new media for the generation of novel crystalline forms of APIs (Active Pharmaceutical Ingredients) named as pharmaceutical cocrystals. Supercritical fluid technology offers a new platform that allows a single-step generation of particles that are difficult or even impossible to obtain by traditional techniques. The generation of pure and dried new cocrystals (crystalline molecular complexes comprising the API and one or more conformers in the crystal lattice) can be achieved due to unique properties of SCFs by using different supercritical fluid properties: supercritical CO2 solvent power, anti-solvent effect and its atomization enhancement.
Supercritical drying 
Supercritical drying is a method of removing solvent without surface tension effects. As a liquid dries, the surface tension drags on small structures within a solid, causing distortion and shrinkage. Under supercritical conditions there is no surface tension, and the supercritical fluid can be removed without distortion. Supercritical drying is used for manufacture of aerogels and drying of delicate materials such as archeological samples and biological samples for electron microscopy.
Supercritical water oxidation 
Supercritical water oxidation uses supercritical water as a medium in which to oxidize hazardous waste, eliminating production of toxic combustion products that burning can produce.
The waste product to be oxidised is dissolved in the supercritical water along with molecular oxygen (or an oxidising agent that gives up oxygen upon decomposition, e.g. Hydrogen Peroxide) at which point the oxidation reaction occurs.
Supercritical water hydrolysis 
Supercritical hydrolysis is a method of converting biomass into cellulosic sugar by employing water under supercritical conditions. The water, acting as a solvent, extracts C6 sugar (glucose) from cellulose plant matter. Lignin remains as a solid particle. Users of the process, including Renmatix, claim advantages in reaction time and reactor size compared to alternative biological and chemical processes, including acid hydrolysis and enzymatic hydrolysis.
Supercritical water gasification 
Supercritical water gasification is a process of exploiting the beneficial effects of supercritical water to convert aqueous biomass streams into clean water and gases like H2, CH4, CO2, CO etc.
Supercritical fluid in power generation 
The efficiency of a heat engine is ultimately dependent on the temperature difference between heat source and sink (Carnot cycle). To improve efficiency of power stations the operating temperature must be raised. Using water as the working fluid, this takes it into supercritical conditions. Efficiencies can be raised from about 39% for subcritical operation to about 45% using current technology. Supercritical water reactors (SCWRs) are promising advanced nuclear systems that offer similar thermal efficiency gains. Carbon dioxide can also be used in supercritical cycle nuclear power plants, with similar efficiency gains. Many coal-fired supercritical steam generators are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.
Biodiesel production 
Conversion of vegetable oil to biodiesel is via a transesterification reaction, where the triglyceride is converted to the methyl ester plus glycerol. This is usually done using methanol and caustic or acid catalysts, but can be achieved using supercritical methanol without a catalyst. The method of using supercritical methanol for biodiesel production was first studied by Saka and his coworkers. This has the advantage of allowing a greater range and water content of feedstocks (in particular, used cooking oil), the product does not need to be washed to remove catalyst, and is easier to design as a continuous process.
Carbon capture and storage and enhanced oil recovery 
Supercritical carbon dioxide is used to enhance oil recovery in mature oil fields. At the same time, there is the possibility of using "clean coal technology" to combine enhanced recovery methods with carbon sequestration. The CO2 is separated from other flue gases either pre- or post-combustion, compressed to the supercritical state, and injected into geological storage, possibly into existing oil fields to improve yields.
At present, only schemes isolating fossil CO2 from natural gas actually use carbon storage, (e.g., Sleipner gas field), but there are many plans for future CCS schemes involving pre- or post- combustion CO2. There is also the possibility to reduce the amount of CO2 in the atmosphere by using biomass to generate power and sequestering the CO2 produced.
Supercritical carbon dioxide is also an important emerging 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 fluid deposition 
Supercritical fluids can be used to deposit functional nanostructured films and nanometer-size particles of metals onto surfaces. The gas-like surface tension, diffusivities, and viscosities allows access to nano pores much smaller than can be accessed by liquids, and the liquid-like solubilities allow much higher precursor concentrations than are typical in chemical vapour deposition. This is crucial in developing more powerful electronic components, and metal particles deposited in this way are also powerful catalysts for chemical synthesis and electrochemical reactions.
Antimicrobial properties of highly compressed fluids 
CO2 at high pressures has antimicrobial properties. While its effectiveness has been shown for various applications, the mechanism of inactivation have not been fully understood although they have been investigated for more than 60 years.
In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.
See also 
- Edit Székely. "Supercritical Fluid Extraction". Budapest University of Technology and Economics. Retrieved 2007-11-20.
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- 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
- "Supercritical water gasification of biomas". Retrieved 201-11-17.
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- Bart, C. J. (2005). "Chapter 4: Separation Techniques". Additives in Polymers: industrial analysis and applications. John Wiley and Sons. p. 212. doi:10.1002/0470012064.ch4. ISBN 978-0-470-01206-2.
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- R. Scott Oakes, Anthony A. Clifford, Keith D. Bartle, Mark Thornton Pett and Christopher M. Rayner (1999). "Sulfur oxidation in supercritical carbon dioxide: dramatic pressure dependent enhancement of diastereoselectivity for sulphoxidation of cysteine derivatives". Chemical Communications (3): 247–248. doi:10.1039/a809434i.
- Sang-Do Yeob and Erdogan Kirana (2005). "Formation of polymer particles with supercritical fluids: A review". The Journal of Supercritical Fluids 34 (3): 287–308. doi:10.1016/j.supflu.2004.10.006.
- L. Padrela, M.A. Rodrigues, S.P. Velaga, H.A. Matos and E.G. Azevedo (2009). "Formation of indomethacin–saccharin cocrystals using supercritical fluid technology". European Journal of Pharmaceutical Sciences. 38, pp. 9–17. doi:10.1016/j.ejps.2009.05.010
- L. Padrela, M.A. Rodrigues, S.P. Velaga, H.A. Matos and E.G. Azevedo (2009). "Screening for pharmaceutical cocrystals using the supercritical fluid enhanced atomization process". Journal of Supercritical Fluids. article in press, corrected proof. doi:10.1016/j.supflu.2010.01.010
- "Our Technology". Renmatix. Retrieved 12 February 2013.
- Malhotra, Ashok and Satyakam,R, 2000,Influence of climatic parameters on optimal design of supercritical power plants,IECEC, Energy Conversion Engineering Conference, pp. 1053–1058,
- "Supercritical steam cycles for power generation applications". Retrieved 2007-11-20.[dead link]
- Dostal, M.J. Driscoll, P. Hejzlar. "A Supercritical Carbon Dioxide Cycle for Next Generation Nuclear Reactors" (PDF). MIT-ANP-TR-100. MIT-ANP-Series. Retrieved 2007-11-20.
- Kunchana Bunyakiat, Sukunya Makmee, Ruengwit Sawangkeaw, and Somkiat Ngamprasertsith (2006). "Continuous Production of Biodiesel via Transesterification from Vegetable Oils in Supercritical Methanol". Energy and Fuels 20 (2): 812–817. doi:10.1021/ef050329b.
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- "The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs", p. 84 (2004)
- FutureGen Technology
- Øyvind Vessia: "Fischer- Tropsch reactor fed by syngas"
- Intergovernmental Panel on Climate Change IPCC Special Report on Carbon Dioxide Capture and Storage.
- FAQs – Supercritical CO2 in heat pumps and other applications
- Eco Cute hot water heat pumps in Japan
- Ye, Xiang-Rong; Lin, YH and Wai, CM (2003). "Supercritical fluid fabrication of metal nanowires and nanorods templated by multiwalled carbon nanotubes". Advanced Materials 15 (4): 316–319. doi:10.1002/adma.200390077.
- Cinquemani, Boyle, et al. (2007). "Inactivation of microbes using compressed carbon dioxide – An environmentally sound disinfection process of medical fabrics." Journal of Supercritical fluids 42(3): 1–6.
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Further reading 
- Brunner, G. (2010). "Applications of Supercritical Fluids". Annual Review of Chemical and Biomolecular Engineering 1: 321–342. doi:10.1146/annurev-chembioeng-073009-101311. PMID 22432584.
- Handy calculator for density, enthalpy, entropy and other thermodynamic data of supercritical CO2
- animated presentation describing what a supercritical fluid is
- NewScientist Environment FOUND:The hottest water on Earth
- Poliakoff, Martyn (28 April 2008). "Supercritical fluids". Test Tube. Brady Haran for the University of Nottingham.