From Wikipedia, the free encyclopedia
A supercritical fluid is any substance at a
above its , where distinct liquid and gas phases do not exist. It can
through solids like a gas, and
materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in , allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for
in a range of industrial and laboratory processes.
and water are the most commonly used supercritical fluids, being used for
and , 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.
Table 1. Critical properties of various solvents (Reid et al., 1987)
Molecular weight
Critical temperature
Critical pressure
Critical density
7.38 (72.8)
(H2O) (acc. )
22.064 (217.755)
4.60 (45.4)
4.87 (48.1)
4.25 (41.9)
5.04 (49.7)
4.60 (45.4)
8.09 (79.8)
6.14 (60.6)
4.70 (46.4)
7.35 (72.5)
Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.
Comparison of Gases, Supercritical Fluids and Liquids
Density (kg/m3)
Viscosity ()
Diffusivity (mm?/s)
Supercritical Fluids
In addition, there is no
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
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
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 , such as the Peng Robinson, or
methods. Other properties, such as density, can also be calculated using equations of state.
Figure 1. Carbon dioxide pressure-temperature phase diagram
Figure 2. Carbon dioxide density-pressure phase diagram
Figures 1 and 2 show projections of a . In the pressure-temperature phase diagram (Fig. 1) the
separates the
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 ) condenses into a much denser liquid, resulting in the discontinuity in the line (vertical dotted line). The system consists of 2 phases in , a dense liquid and a low density gas. As the critical temperature is approached (300K), the density of the gas at equilibrium becomes higher, 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. , the
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.
In recent years, a significant effort has been devoted to investigation of various properties of supercritical fluids. This has been an exciting field with a long history since 1822 when Baron Charles Cagniard de la Tour discovered supercritical fluids while conducting experiments involving the discontinuities of the sound in a sealed cannon barrel filled with various fluids at high temperature. More recently, supercritical fluids have started to be deployed in several important applications, ranging from the extraction of floral fragrance from flowers to applications in food science such as creating decaffeinated coffee, functional food ingredients, pharmaceuticals, cosmetics, polymers, powders, bio- and functional materials, nano-systems, natural products, biotechnology, fossil and bio-fuels, microelectronics, energy and environment. Much of the excitement and interest of the past decade is due to the enormous progress made in increasing the power of relevant experimental tools. The development of new experimental methods and improvement of existing ones continues to play an important role in this field, with recent research focusing on dynamic properties of fluids.
, V. V. Brazhkin and K. Trachenko discovered that
shows a crossover between two different dynamic regimes of the low-temperature rigid-liquid and high temperature non-rigid gas-like fluid. Rigid liquids are rigid like a solid on short time scales, but flow like a liquid while a supercritical gas-like fluid has the dynamic motions of a gas but is able to dissolve materials, like a liquid. The crossover challenges the currently held belief that no difference can be made between a gas and a liquid above the critical point and that the supercritical state is homogeneous in terms of physical properties.
with colleagues formulated a theory of system thermodynamics and heat capacity above the crossover. In that theory, energy and heat capacity are governed by the minimal length of the longitudinal mode in the system only, and do not depend on system-specific structure and interactions.
with colleagues predicted relationship between supercritical exponents of heat capacity and viscosity and derived a
for the supercritical state.
allows to distinguish liquid-like and gas-like states within the supercritical fluid.
A , a type of hydrothermal vent
Hydrothermal circulation occurs within the Earth's crust wherever fluid becomes heated and begins to convect. These fluids are thought to reach supercritical conditions under a number of different settings, such as in the formation of porphyry copper deposits or high temperature circulation of seawater in the sea floor. At mid-ocean ridges, this circulation is most evident by the appearance of hydrothermal vents known as "black smokers". These are large (metres high) chimneys of sulfide and sulfate minerals which vent fluids up to ~400 °C. The fluids appear like great black billowing clouds of smoke due to the precipitation of dissolved metals in the fluid. It is likely that at depth many of these vent sites reach supercritical conditions, but most cool sufficiently by the time they reach the sea floor to be subcritical. One particular vent site, Turtle Pits, has displayed a brief period of supercriticality at the vent site. A further site, , in the Cayman Trough, is thought to display sustained supercriticality at the vent orifice.
The atmosphere of
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
planets are composed mainly of hydrogen and helium at temperatures well above their critical points. The gaseous outer atmospheres of
transition smoothly into the fluid interior, while the nature of the transition zones of
are unknown. Theoretical models of
have posited an ocean of pressurized, supercritical fluid water with a sheet of solid high pressure water ice at the bottom.
The advantages of
(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
of green coffee beans, the extraction of
for beer production, and the production of
and pharmaceutical products from plants. A few
include the use of
as an extraction method instead of using traditional .
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 () or other undesirable solvents for . Supercritical carbon dioxide sometimes
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.
(SFC) can be used on an analytical scale, where it combines many of the advantages of
(HPLC) and
(GC). It can be used with non-volatile and thermally labile analytes (unlike GC) and can be used with the universal
(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
separations and analysis of high-molecular-weight hydrocarbons. For manufacturing, efficient preparative
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.
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
. There are also significant environmental benefits over conventional organic solvents.
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) , is a special case of this. Carbon dioxide also dissolves in many polymers, considerably swelling and plasticising them and further accelerating the diffusion process.
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
of a solute by dilution, depressurization or a combination of these. These processes occur faster in supercritical fluids than in liquids, promoting
and yielding very small and regularly sized particles. Recent supercritical fluids have shown the capability to reduce particles up to a range of 5-;nm.
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.
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
and drying of delicate materials such as archeological samples and biological samples for .
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. ) at which point the oxidation reaction occurs.
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.
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
is a process of exploiting the beneficial effect of supercritical water to convert aqueous biomass streams into clean water and gases like H2, CH3, CO2, CO etc.
is ultimately dependent on the temperature difference between heat source and sink (). To improve efficiency of
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.
(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
are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.
Conversion of vegetable oil to
reaction, where the
is converted to the methyl ester plus . This is usually done using
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.
Supercritical carbon dioxide is used to
recovery in mature oil fields. At the same time, there is the possibility of using "" to combine enhanced recovery methods with . The CO2 is separated from other , 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., ), 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
to generate power and sequestering the CO2 produced.
Main article:
The use of supercritical carbon dioxide, instead of water, has been examined as a geothermal working fluid.
Supercritical carbon dioxide is also an important emerging , 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
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 fluids can be used to deposit functional nanostructured films and nanometer-size particles of metals onto surfaces. The high diffusivities and concentrations of precursor in the fluid as compared to the vacuum systems used in
allow deposition to occur in a surface reaction rate limited regime, providing stable and uniform interfacial growth. 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. Additionally, due to the high rates of precursor transport in solution, it is possible to coat high surface area particles which under
would exhibit depletion near the outlet of the system and also be likely to result in unstable interfacial growth features such as . The result is very thin and uniform films deposited at rates much faster than , the best other tool for particle coating at this size scale.
CO2 at high pressures has
properties. While its effectiveness has been shown for various applications, the mechanisms of inactivation have not been fully understood although they have been investigated for more than 60 years.
In 1822, Baron
discovered the critical point of a substance in his famous
barrel experiments. Listening to discontinuities in the sound of a rolling
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
become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.
Edit Székely. . Budapest University of Technology and Economics.
A.A. Clifford (). . Critical Processes Ltd.
Bolmatov, D.; Brazhkin, V. V.; Trachenko, K. (2013). "Thermodynamic behaviour of supercritical matter". Nature Communications 4. :. :. :.
Bolmatov, D.; Brazhkin, V. V.; Trachenko, K. (2012). "The phonon theory of liquid thermodynamics". Scientific Reports 2. :. :. :.
Bolmatov, D.; Brazhkin, V. V.; Trachenko, K. (2013). "Thermodynamic behaviour of supercritical matter". Nature Communications 4. :. :. :.
Webber, A.P.; Murton, B.; Roberts, S.; Hodgkinson, M. . Goldschmidt Conference Abstracts 2014. Geochemical Society 2014.
U.S.EPA Method 3560 Supercritical Fluid Extraction of Total Recoverable Hydrocarbons.
U.S.EPA Method 3561 Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbons.
Use of Ozone Depleting Substances in Laboratories. TemaNord .
Bart, C. J. (2005). "Chapter 4: Separation Techniques". Additives in Polymers: industrial analysis and applications. John Wiley and Sons. p. 212. :.  .
R. Scott O Anthony A. C Keith D. B Mark Thornton Pett & Christopher M. Rayner (1999). "Sulfur oxidation in supercritical carbon dioxide: dramatic pressure dependent enhancement of diastereoselectivity for sulphoxidation of cysteine derivatives".
(3): 247–248. :.
Sang-Do Yeo & Erdogan Kiran (2005). "Formation of polymer particles with supercritical fluids: A review".
34 (3): 287–308. :.
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. :
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. :
. Renmatix 2013.
and Satyakam,R, 2000,Influence of climatic parameters on optimal design of supercritical power plants,IECEC, , pp. ,
V. D P. Hejzlar.
(PDF). MIT-ANP-TR-100. MIT-ANP-Series.
Kunchana B Sukunya M Ruengwit Sawangkeaw & Somkiat Ngamprasertsith (2006). "Continuous Production of Biodiesel via Transesterification from Vegetable Oils in Supercritical Methanol".
20 (2): 812–817. :.
IPCC Special Report on Carbon Dioxide Capture and Storage.
Ye, Xiang-R Lin, YH & Wai, CM (2003). "Supercritical fluid fabrication of metal nanowires and nanorods templated by multiwalled carbon nanotubes". Advanced Materials 15 (4): 316–319. :.
. <. Navolta 2014.
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.
Fraser, D. (1951). "Bursting bacteria by release of gas pressure." Nature 167: 33–34.
Berche, B Henkel, M Kenna, Ralph (2009). "Critical phenomena: 150 years since Cagniard de la Tour". Journal of Physical Studies 13 (3): 01–4. :. :.
Brunner, G. (2010). "Applications of Supercritical Fluids". Annual Review of Chemical and Biomolecular Engineering 1: 321–342. :. &#160;.
for density, enthalpy, entropy and other thermodynamic data of supercritical CO2
(broken link)
(28 April 2008). . Test Tube.
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