Supercritical carbon dioxide (s) is a fluid state of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

where it is held at or above its

critical temperature Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine *Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing in ...

and

critical pressure In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. One example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions under which a l ...

. Carbon dioxide usually behaves as a

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

standard temperature and pressure Standard temperature and pressure (STP) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...

(STP), or as a

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

called

dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...

when cooled and/or pressurised sufficiently. If the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

. More specifically, it behaves as a

supercritical fluid A supercritical fluid (SCF) is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous sol ...

above its critical temperature () and critical pressure (), expanding to fill its container like a gas but with a

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

like that of a liquid. Supercritical is becoming an important commercial and industrial

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

due to its role in chemical

extraction Extraction may refer to: Science and technology Biology and medicine * Comedo extraction, a method of acne treatment * Dental extraction, the surgical removal of a tooth from the mouth Computing and information science * Data extraction, the ...

, in addition to its relatively low

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...

and environmental impact. The relatively low temperature of the process and the stability of also allows compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in varies with pressure, permitting selective extractions.

Applications

Solvent

Carbon dioxide is gaining popularity among

coffee Coffee is a beverage brewed from roasted, ground coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content, but decaffeinated coffee is also commercially a ...

manufacturers looking to move away from classic decaffeinating

s. s is forced through green coffee beans which are then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g., to pharmaceutical or beverage manufacturers) by passing the water through activated charcoal filters or by

distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...

crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...

reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane, semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distribu ...

. Supercritical carbon dioxide is used to remove

organochloride Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds that contain one or more carbon–chlorine bonds. The chloroalkane class (alkanes with one or more hydrogens substituted ...

pesticide Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...

s and metals from agricultural crops without adulterating the desired constituents from plant matter in the

herbal supplement Herbal medicine (also called herbalism, phytomedicine or phytotherapy) is the study of pharmacognosy and the use of medicinal plants, which are a basis of traditional medicine. Scientific evidence for the effectiveness of many herbal treatments ...

industry. Supercritical carbon dioxide can be used as a solvent in

dry cleaning Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water. Clothes are instead soaked in a water-free liquid solvent (usually non-polar, as opposed to water which is a Solvent#Solvent classifications, polar ...

. Supercritical carbon dioxide is used as the extraction solvent for creation of

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the ...

s and other

herbal distillate Herbal distillates, also known as floral waters, hydrosols, hydrolates, herbal waters, and essential waters, are aqueous products of hydrodistillation. They are colloidal suspensions of essential oils as well as water-soluble components obtain ...

s. Its main advantages over solvents such as

hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...

and

acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

in this process are that it is non-flammable and does not leave toxic residue. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional

organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

s. The can evaporate into the air or be recycled by condensation into a recovery vessel. Its advantage over

steam distillation Steam distillation is a separation process that consists of distilling water together with other volatile and non-volatile components. The steam from the boiling water carries the vapor of the volatiles to a condenser; both are cooled and retu ...

is that it operates at a lower temperature, which can separate the plant

waxes Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low ...

from the oils. In

laboratories A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which science, scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as s ...

, s is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash, and other media, and determination of

polycyclic aromatic hydrocarbon A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...

s in soil and solid wastes. Supercritical fluid extraction has been used in determining

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

components in water. Processes that use s to produce micro and nano scale particles, often for

pharmaceutical Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the ...

uses, are under development. The gas

antisolvent Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a s ...

process, rapid expansion of supercritical solutions, and supercritical antisolvent

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

(as well as several related methods) process a variety of substances into particles. Due to its ability to selectively dissolve organic compounds and assist enzyme functioning, s has been suggested as a potential solvent to support biological activity on

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

- or

super-Earth A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to t ...

-type planets.

Manufactured products

Environmentally beneficial, low-cost substitutes for rigid

thermoplastic A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains as ...

and fired

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

are made using s as a

chemical reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

. The s in these processes is reacted with the alkaline components of fully hardened

hydraulic cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

plaster Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "re ...

to form various carbonates. The primary byproduct is water. s is used in the foaming of

polymers A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...

. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating, the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e., creating a

foam Foams are two-phase materials science, material systems where a gas is dispersed in a second, non-gaseous material, specifically, in which gas cells are enclosed by a distinct liquid or solid material. Note, this source focuses only on liquid ...

. Research is ongoing on microcellular foams. An

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...

carboxylation Carboxylation is a chemical reaction in which a carboxylic acid is produced by treating a substrate with carbon dioxide. The opposite reaction is decarboxylation. In chemistry, the term carbonation is sometimes used synonymously with carboxylation ...

of a para-

isobutyl In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula , derived from either of the two isomers (''n''-butane and isobutane) of butane. The isomer ''n''-butane can connect in two ways, giv ...

benzyl chloride Benzyl chloride, or α-chlorotoluene, is an organic compound with the formula . This colorless liquid is a reactive organochlorine compound that is a widely used chemical building block. Preparation Benzyl chloride is prepared industrially ...

ibuprofen Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain, fever, and inflammation. This includes dysmenorrhea, painful menstrual periods, migraines, and rheumatoid arthritis. It can be taken oral administration, ...

is promoted under s.

Working fluid

s is chemically stable, reliable, low-cost, non-flammable and readily available, making it a desirable candidate

working fluid For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...

for

transcritical cycle A transcritical cycle is a closed thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. In particular, for power cycles the working fluid is kept in the liquid region during the compression phase an ...

s. Supercritical is used as the working fluid in domestic water

heat pump A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm s ...

s. Manufactured and widely used, heat pumps are available for domestic and business heating and cooling. While some of the more common domestic water heat pumps remove heat from the space in which they are located, such as a basement or garage, heat pump water heaters are typically located outside, where they remove heat from the outside air.

Power generation

The unique properties of s present advantages for closed-loop power generation and can be applied to power generation applications. Power generation systems that use traditional air Brayton and steam

Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sour ...

s can use s to increase efficiency and power output. The relatively new Allam power cycle uses s as the working fluid in combination with fuel and pure oxygen. The produced by combustion mixes with the s working fluid. A corresponding amount of pure must be removed from the process (for industrial use or sequestration). This process reduces atmospheric emissions to zero. s promises substantial efficiency improvements. Due to its high fluid density, s enables compact and efficient

turbomachinery Turbomachinery, in mechanical engineering, describes machines that transfer energy between a Rotor (electric), rotor and a fluid, including both turbines and gas compressor, compressors. While a turbine transfers energy from a fluid to a rotor, ...

. It can use simpler, single casing body designs while

steam turbine A steam turbine or steam turbine engine is a machine or heat engine that extracts thermal energy from pressurized steam and uses it to do mechanical work utilising a rotating output shaft. Its modern manifestation was invented by Sir Charles Par ...

s require multiple turbine stages and associated casings, as well as additional inlet and outlet piping. The high density allows more compact, microchannel-based heat exchanger technology. For

concentrated solar power Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated whe ...

, carbon dioxide

is not high enough to obtain the maximum energy conversion efficiency. Solar thermal plants are usually located in arid areas, so it is impossible to cool down the heat sink to sub-critical temperatures. Therefore,

supercritical carbon dioxide blend Supercritical carbon dioxide blend (sCO2 blend) is an homogeneous mixture of Carbon dioxide, CO2 with one or more fluids (dopant fluid) where it is held at or above its critical temperature and critical pressure. Carbon dioxide behaves as a sup ...

s, with higher critical temperatures, are in development to improve concentrated solar power electricity production. Further, due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperatures and therefore higher cycle efficiency. Unlike

two-phase flow In fluid mechanics, two-phase flow is a flow of gas and liquid — a particular example of multiphase flow. Two-phase flow can occur in various forms, such as flows transitioning from pure liquid to vapor as a result of external heating, sep ...

, the single-phase nature of s eliminates the necessity of a heat input for phase change that is required for the water to steam conversion, thereby also eliminating associated

thermal fatigue A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

and corrosion. The use of s presents

corrosion engineering Corrosion engineering is an engineering specialty that applies scientific, technical, engineering skills, and knowledge of natural laws and physical resources to design and implement materials, structures, devices, systems, and procedures to mana ...

, material selection and design issues. Materials in power generation components must display resistance to damage caused by high-temperature,

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

and

creep The Committee for the Re-election of the President (or the Committee to Re-elect the President, CRP, but often mocked by the acronym CREEP) was, officially, a fundraising organization of United States President Richard Nixon's 1972 re-election ...

. Candidate materials that meet these property and performance goals include incumbent alloys in power generation, such as nickel-based

superalloys A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Key characteristics of a superalloy include mechanical strength, Creep (deformation), thermal creep deformation resistance, ...

for turbomachinery components and

austenitic stainless steel Austenitic stainless steel is one of the five families of stainless steel (along with ferritic, martensitic, duplex and precipitation hardened). Its primary crystalline structure is austenite (face-centered cubic). Such steels are not hardena ...

s for piping. Components within s Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components and

intergranular corrosion In materials science, intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides. (''Cf.'' transgranu ...

and pitting in the piping. Testing has been conducted on candidate Ni-based alloys, austenitic steels, ferritic steels and ceramics for corrosion resistance in s cycles. The interest in these materials derive from their formation of protective surface oxide layers in the presence of carbon dioxide, however in most cases further evaluation of the reaction mechanics and corrosion/erosion kinetics and mechanisms is required, as none of the materials meet the necessary goals. In 2016, General Electric announced a s-based turbine that enabled a 50% efficiency of converting heat energy to electrical energy. In it the is heated to 700 °C. It requires less compression and allows heat transfer. It reaches full power in 2 minutes, whereas steam turbines need at least 30 minutes. The prototype generated 10 MW and is approximately 10% the size of a comparable steam turbine. The 10 MW US$155-million Supercritical Transformational Electric Power (STEP) pilot plant was completed in 2023 in San Antonio. It is the size of a desk and can power around 10,000 homes.

Other

Work is underway to develop a s

closed-cycle gas turbine A closed-cycle gas turbine is a turbine that uses a gas (e.g. air, nitrogen, helium, argon, etc.) for the working fluid as part of a closed thermodynamic system. Heat is supplied from an external source. Note: front matter (including preface a ...

to operate at temperatures near 550 °C. This would have implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500 °C and 20 MPa enable thermal efficiencies approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the volume of carbon fuels used in producing electricity, the environmental impact of cycle efficiency increases would be significant. Supercritical is an emerging natural refrigerant, used in new, low carbon solutions for domestic

s. Supercritical heat pumps are commercially marketed in Asia. EcoCute systems from Japan, developed by Mayekawa, develop high temperature domestic water with small inputs of electric power by moving heat into the system from the surroundings. Supercritical has been used since the 1980s to enhance recovery in mature oil fields. "

Clean coal Coal pollution mitigation is a series of systems and technologies that seek to mitigate health and environmental impact of burning coal for energy. Burning coal releases harmful substances that contribute to air pollution, acid rain, and greenho ...

" technologies are emerging that could combine such enhanced recovery methods with

carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. It plays a crucial role in Climate change mitigation, limiting climate change by reducing the amount of Carbon dioxide in Earth's atmosphere, carbon dioxide in the atmosphe ...

. Using

gasifier Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (). This is achieved by reacting ...

s instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide and ash. This hydrogen gas can be used to produce electrical power In

combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turb ...

gas turbines, is captured, compressed to the supercritical state and injected into geological storage, possibly into existing oil fields to improve yields. Supercritical can be used as a working fluid for geothermal electricity generation in both

enhanced geothermal systems An enhanced geothermal system (EGS) generates geothermal electricity without natural convective hydrothermal resources. Traditionally, geothermal power systems operated only where naturally occurring heat, water, and rock permeability are suffici ...

and sedimentary geothermal systems (so-called Plume Geothermal). EGS systems utilize an artificially fractured reservoir in basement rock while CPG systems utilize shallower naturally-permeable sedimentary reservoirs. Possible advantages of using in a geologic reservoir, compared to water, include higher energy yield resulting from its lower viscosity, better chemical interaction, and permanent storage as the reservoir must be filled with large masses of . As of 2011, the concept had not been tested in the field.

Aerogel production

Supercritical carbon dioxide is used in the production of silica, carbon and metal based

aerogels Aerogels are a class of 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 ...

. For example, silicon dioxide gel is formed and then exposed to s. When the goes supercritical, all surface tension is removed, allowing the liquid to leave the aerogel and produce nanometer sized pores.

Sterilization of biomedical materials

Supercritical is an alternative for thermal sterilization of biological materials and medical devices with combination of the additive

peracetic acid Peracetic acid (also known as peroxyacetic acid, or Percidine) is an organic compound with the formula CH3CO3H. This peroxy acid is a colorless liquid with a characteristic acrid odor reminiscent of acetic acid. It can be highly corrosive. Perac ...

(PAA). Supercritical does not sterilize the media, because it does not kill the spores of microorganisms. Moreover, this process is gentle, as the morphology, ultrastructure and protein profiles of inactivated microbes are preserved.

Cleaning

Supercritical is used in certain industrial cleaning processes.

References

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