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A supercritical fluid (SCF) is any substance at a
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
and
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

pressure
above its critical point, where distinct
liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid par ...

liquid
and
gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and c ...

gas
phases do not exist, but below the pressure required to compress it into a
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

solid
. It can
effuse
effuse
through porous solids like a gas, overcoming the
mass transfer Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption Absorption may refer to: Chemistry and biology *Absorptio ...
limitations that slow liquid transport through such materials. SCF are much superior to gases in their ability to
dissolve
dissolve
materials like liquids or solids. In addition, close to the critical point, small changes in pressure or temperature result in large changes in
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
, allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids occur in the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
s of the
gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic t ...
s
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

Jupiter
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
, the
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Su ...
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
, and probably in those of the
ice giant An ice giant is a giant planet composed mainly of elements heavier than hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest el ...
s
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mor ...

Neptune
. Supercritical water is found on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
, such as the water issuing from black smokers, a type of underwater
hydrothermal vent A hydrothermal vent is a fissure A fissure is a long, narrow crack opening along the surface of the Earth. It is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in the Earth's crust, on ice sheets and glaciers, and ...
. They are used as a substitute for
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or ...
solvents A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
in a range of industrial and laboratory processes.
Carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is ...
and
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...

water
are the most commonly used supercritical fluids; they are often used for
decaffeination Decaffeination is the removal of caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is the world's most widely consumed psychoactive drug. Unlike many other psychoactive su ...
and
power generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmis ...
, respectively.


Properties

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 substances that are commonly used as supercritical fluids. †Source: International Association for Properties of Water and Steam
IAPWS
Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids. In addition, there is no
surface tension Surface tension is the tendency of surfaces at rest to shrink into the minimum possible. Surface tension is what allows objects with a higher density than water such as s and insects (e.g. ) to float on a water surface without becoming eve ...

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-like 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.


Mixtures

Typically, supercritical fluids are completely
miscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. ...
with each other, so that a binary mixture forms a single gaseous phase if the critical point of the mixture is exceeded. However, exceptions are known in systems where one component is much more volatile than the other, which in some cases form two immiscible gas phases at high pressure and temperatures above the component critical points. This behavior has been found for example in the systems N2-NH3, NH3-CH4, SO2-N2 and n-butane-H2O. The critical point of a binary mixture can be estimated as the
arithmetic mean In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
of the critical temperatures and pressures of the two components, where ''χ''''i'' denotes the
mole fraction In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
of component ''i''. For greater accuracy, the critical point can be calculated using
equations of state In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through S ...
, such as the Peng-Robinson, or
group-contribution methodA group-contribution method in chemistry is a technique to estimate and predict thermodynamic and other properties from molecular structures. Introduction In today's chemical processes hundreds of thousands of components are used. The Chemical Abs ...
s. Other properties, such as density, can also be calculated using equations of state.


Phase diagram

Figures 1 and 2 show two-dimensional projections of a
phase diagram A phase diagram in physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical ins ...

phase diagram
. In the pressure-temperature phase diagram (Fig. 1) the
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...
curve separates the
gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and c ...

gas
and liquid region and ends in the critical point, where the liquid and gas phases disappear to become a single supercritical phase. The appearance of a single phase can also be observed in the density-pressure phase diagram for carbon dioxide (Fig. 2). At well below the critical temperature, e.g., 280 K, as the pressure increases, the gas compresses and eventually (at just over 40
bar Bar or BAR may refer to: Food *Bar (establishment) A bar is a long raised narrow table or bench designed for dispensing beer or other alcoholic beverage, alcoholic drinks. They were originally chest high, and a bar, often brass, ran the lengt ...
) condenses into a much denser liquid, resulting in the discontinuity in the line (vertical dotted line). The system consists of 2 phases in
equilibrium List of types of equilibrium, the condition of a system in which all competing influences are balanced, in a wide variety of contexts. Equilibrium may also refer to: Film and television * Equilibrium (film), ''Equilibrium'' (film), a 2002 scien ...
, a dense liquid and a low density gas. As the critical temperature is approached (300 K), 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 (310 K), 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 viscosity of a is a measure of its to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, has a higher viscosity than . Viscosity can be conceptualized as quantifying the inter ...

viscosity
, the
relative permittivity The dielectric constant (or relative permittivity) is the electric permeability of a material expressed as a ratio with the electric permeability of a vacuum. A dielectric In , a dielectric (or dielectric material or dielectric medium) is ...
and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave more like an ideal gas, with a more linear density/pressure relationship, as can be seen in Figure 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure. Many pressurized gases are actually supercritical fluids. For example, nitrogen has a critical point of 126.2 K (−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 nearly ideal gas, similar to CO2 at 400 K above. However, they cannot be liquified by mechanical pressure unless cooled below their critical temperature, requiring gravitational pressure such as within
gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic t ...
s to produce a liquid or solid at high temperatures. Above the critical temperature, elevated pressures can increase the density enough that the SCF exhibits liquid-like density and behaviour. At very high pressures, an SCF can be compressed into a solid because the melting curve extends to the right of the critical point in the P/T phase diagram. While the pressure required to compress supercritical CO2 into a solid can be, depending on the temperature, as low as 570 MPa, that required to solidify supercritical water is 14,000 MPa. The Fisher-Widom line, the Widom line, or the Frenkel line are thermodynamic concepts that allow to distinguish liquid-like and gas-like states within the supercritical fluid. 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 Baron Charles Cagniard de la Tour (31 March 1777 – 5 July 1859) was a France, French engineer and physicist. Charles Cagniard was born in Paris, and after attending the École Polytechnique became one of the ''ingénieurs géographiques''. He exam ...
discovered supercritical fluids while conducting experiments involving the discontinuities of the sound in a sealed gun barrel filled with various fluids at high temperature. More recently, supercritical fluids have found application in a variety of fields, 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.


Natural occurrence


Hydrothermal circulation

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, Beebe, in the Cayman Trough, is thought to display sustained supercriticality at the vent orifice.


Planetary atmospheres

The atmosphere of
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

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 A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic t ...
planets are composed mainly of hydrogen and helium at temperatures well above their critical points. The gaseous outer atmospheres of
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

Jupiter
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
transition smoothly into the dense liquid interior, while the nature of the transition zones of
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mor ...

Neptune
and
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
is unknown. Theoretical models of
extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1992. This was followed by the ...
Gliese 876 d Gliese 876 d is an exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1 ...

Gliese 876 d
have posited an ocean of pressurized, supercritical fluid water with a sheet of solid high pressure water ice at the bottom.


Applications


Supercritical fluid extraction

The advantages of
supercritical fluid extractionSupercritical fluid extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids. ...
(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 Decaffeination is the removal of caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is the world's most widely consumed psychoactive drug. Unlike many other psychoactive su ...
of green coffee beans, the extraction of
hops Hops are the flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plants with a similar ...

hops
for beer production, and the production of
essential oils An essential oil is a concentrated hydrophobic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...
and pharmaceutical products from plants. A few
laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testabl ...

laboratory
test method A test method is a method Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task ...
s include the use of
supercritical fluid extractionSupercritical fluid extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids. ...
as an extraction method instead of using traditional
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

solvent
s.


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.


Dry-cleaning

Supercritical carbon dioxide (SCD) can be used instead of PERC (
perchloroethylene Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon An organochloride, organochlorine compound, chloroc ...
) 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. CO2-based dry cleaning equipment uses liquid CO2, not supercritical CO2, to avoid damage to the buttons.


Supercritical fluid chromatography

Supercritical fluid chromatography Supercritical fluid chromatography (SFC) is a form of normal phase chromatography that uses a supercritical fluid such as carbon dioxide as the mobile phase. It is used for the analysis and purification of low to moderate molecular weight, thermall ...
(SFC) can be used on an analytical scale, where it combines many of the advantages of
high performance liquid chromatography High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry Analytical chemistry studies and uses instruments and methods used to separate, identify, and ...
(HPLC) and
gas chromatography Gas chromatography (GC) is a common type of chromatography In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (g ...
(GC). It can be used with non-volatile and thermally labile analytes (unlike GC) and can be used with the universal
flame ionization detector A flame ionization detector (FID) is a scientific instrument A scientific instrument is a device or tool used for scientific purposes, including the study of both natural phenomena and theoretical research. History Historically, the definitio ...
(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 Chirality is a property of important in several branches of science. The word ''chirality'' is derived from the (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from its ; that is, i ...

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 Chirality is a property of important in several branches of science. The word ''chirality'' is derived from the (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from its ; that is, i ...

chiral
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

isomer
. There are also significant environmental benefits over conventional organic solvents. Industrial syntheses that are performed at supercritical conditions include those of
polyethylene Polyethylene or (incorrectly) polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and container ...

polyethylene
from supercritical
ethene Ethylene (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative ...
,
isopropyl alcohol Isopropyl alcohol (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the aut ...
from supercritical
propene Propene, also known as propylene, is an unsaturated organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The s ...
,
2-butanol 2-Butanol, or ''sec''-butanol, is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...

2-butanol
from supercritical
butene Butene, also known as butylene, is an alkene In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properti ...
, and
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
from a supercritical mix of
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
. Other reactions were, in the past, performed industrially in supercritical conditions, including the synthesis of
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
and thermal (non-catalytic) oil cracking. Because of the development of effective
catalysts that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules ...
, the required temperatures of those two processes have been reduced and are no longer supercritical.


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)
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
s, 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 of a solute by dilution, depressurization or a combination of these. These processes occur faster in supercritical fluids than in liquids, promoting
nucleation Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly File:Iron oxide nanocube.jpg, upright=1.2, Transmission electron microscopy image of an iron oxide nanoparticle. Regularly ar ...

nucleation
or
spinodal decomposition Spinodal decomposition occurs when one thermodynamic phase spontaneously (i.e., without nucleation Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly File:Iron oxide nanocube.jpg ...
over
crystal growth A crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addit ...
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 Supercritical drying, also known as critical point drying, is a process to remove liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...
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 in the manufacturing process of
aerogel Aerogel is a Manufacturing, synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas without significant collapse of the gel structure. The result is a solid with extremely l ...

aerogel
s and drying of delicate materials such as archaeological samples and biological samples for
electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hig ...

electron microscopy
.


Supercritical water electrolysis

Electrolysis of water Electrolysis of water is the process of using electricity to decompose water into oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), g ...

Electrolysis of water
in a supercritical state, reduces the overpotentials found in other electrolysers, thereby improving the electrical efficiency of the production of oxygen and hydrogen. Increased temperature reduces thermodynamic barriers and increases kinetics. No bubbles of oxygen or hydrogen are formed on the electrodes, therefore no insulating layer is formed between catalyst and water, reducing the ohmic losses. The gas-like properties provide rapid mass transfer.


Supercritical water oxidation

Supercritical water oxidation Supercritical water oxidation (SCWO) is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point (thermodynamics), critical point. Under these conditions water becomes a fluid with unique properti ...
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 Hydrogen peroxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...

hydrogen peroxide
) at which point the oxidation reaction occurs.


Supercritical water hydrolysis

Supercritical hydrolysis is a method of converting all biomass polysaccharides as well the associated lignin into low molecular compounds by contacting with water alone under supercritical conditions. The supercritical water, acts as a solvent, a supplier of bond-breaking thermal energy, a heat transfer agent and as a source of hydrogen atoms. All polysaccharides are converted into simple sugars in near-quantitative yield in a second or less. The aliphatic inter-ring linkages of lignin are also readily cleaved into free radicals that are stabilized by hydrogen originating from the water. The aromatic rings of the lignin are unaffected under short reaction times so that the lignin-derived products are low molecular weight mixed phenols. To take advantage of the very short reaction times needed for cleavage a continuous reaction system must be devised. The amount of water heated to a supercritical state is thereby minimized.


Supercritical water gasification

Supercritical water gasification is a process of exploiting the beneficial effect 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 Efficiency is the (often measurable) ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and withou ...
of a
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do work (physics), mechanical work. It does this by bringing a working substance from a higher state temperature to ...

heat engine
is ultimately dependent on the temperature difference between heat source and sink (). To improve efficiency of
power stations A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...
the
operating temperature An operating temperature is the allowable temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow 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.
Supercritical water reactor The supercritical water reactor (SCWR) is a concept , mostly designed as (LWR) that operates at pressure (i.e. greater than 22.1 MPa). The term ''critical'' in this context refers to the of water, and must not be confused with the concep ...
s (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 generator A supercritical steam generator is a type of boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospolita Polska, links=no ), is a country located in Central Europe. It is divided into 16 Voivodeships of ...
s are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.


Biodiesel production

Conversion of vegetable oil to
biodiesel Biodiesel is a form of diesel fuel Diesel fuel in general is any liquid fuel Liquid fuels are combustible or energy-generating molecules that can be harnessed to create mechanical energy In physical sciences Physical science is a ...
is via a
transesterification In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . Stu ...

transesterification
reaction, where the
triglyceride A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a an ...

triglyceride
is converted to the methyl ester plus
glycerol Glycerol (; also called glycerine in British English and glycerin in American English) is a simple polyol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
. This is usually done using
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
and
caustic Caustic most commonly refers to: * Causticity, a property of various corrosive substances ** Sodium hydroxide, sometimes called ''caustic soda'' ** Potassium hydroxide, sometimes called ''caustic potash'' ** Calcium oxide, sometimes called ''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.


Enhanced oil recovery and carbon capture and storage

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 Carbon sequestration or carbon dioxide removal File:Tree planting closeup.jpg, Planting trees is a means of carbon dioxide removal. Carbon dioxide removal (CDR), also known as greenhouse gas removal, is a process in which carbon dioxide g ...

carbon sequestration
. The CO2 is separated from other
flue gases Flue gas is the gas exiting to the atmosphere via a flue A flue is a duct, pipe, or opening in a chimney Smokestacks in Manchester, England c. 1858 watercolor by William Wyld power plant in Ekibastuz, Kazakhstan Kazakhstan,, * r ...
, 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 Biomass is plant or animal material used as fuel to produce electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position ...

biomass
to generate power and sequestering the CO2 produced.


Enhanced geothermal system

The use of supercritical carbon dioxide, instead of water, has been examined as a geothermal working fluid.


Refrigeration

Supercritical carbon dioxide is also emerging as a useful high-temperature
refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an app ...

refrigerant
, being used in new,
CFC CFC, cfc, or Cfc may stand for: Science and technology * Chlorofluorocarbon, a class of chemical compounds * Cardiofaciocutaneous Syndrome, a rare and serious genetic disorder * Subpolar oceanic climate (''Cfc'' in the Köppen climate classifica ...
/ HFC-free domestic
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the Heat pump and refrigeration cycle, refrigeration cycle, moving heat in the opposite ...

heat pump
s making use of the
transcritical cycle A transcritical cycle is a thermodynamic cycle where the working fluid goes through both Superheated_water, Subcritical and Supercritical fluid, supercritical states. This is often the case when carbon dioxide, CO2, is the refrigerant. The modern t ...
. These systems are undergoing continuous development with supercritical carbon dioxide heat pumps already being successfully marketed in Asia. The EcoCute systems from Japan are some of the first commercially successful high-temperature domestic water heat pumps.


Supercritical fluid deposition

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 chemical vapour deposition 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 chemical vapour deposition would exhibit depletion near the outlet of the system and also be likely to result in unstable interfacial growth features such as dendrites. The result is very thin and uniform films deposited at rates much faster than atomic layer deposition, the best other tool for particle coating at this size scale.


Antimicrobial properties

CO2 at high pressures has antimicrobial 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.


History

In 1822, Baron
Charles Cagniard de la Tour Baron Charles Cagniard de la Tour (31 March 1777 – 5 July 1859) was a France, French engineer and physicist. Charles Cagniard was born in Paris, and after attending the École Polytechnique became one of the ''ingénieurs géographiques''. He exam ...
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 phase (matter), phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase.


See also

* Supercritical adsorption * Transcritical cycle * Critical point (thermodynamics) * Iceland Deep Drilling Project


References


Further reading

*


External links


Handy calculator
for density, enthalpy, entropy and other thermodynamic data of supercritical / water and others
videos to present supercritical fluid critical point and solubility in supercritical fluid

NewScientist Environment FOUND:The hottest water on Earth
* {{Authority control Condensed matter physics Phases of matter Gases