Supercritical

Last updated June 13, 2019

Supercritical may refer to:

Physics and technology

Condensed matter physics

Critical temperature, T_C, a temperature above which distinct liquid and gas phases do not exist for a given material
- Supercritical drying, a process used to remove liquid in a precisely controlled way, similar to freeze drying
- Supercritical fluid, a substance at a temperature and pressure above its thermodynamic critical point:
  - Supercritical carbon dioxide:
    - Supercritical fluid chromatography, a form of liquid chromatography using supercritical carbon dioxide as the mobile phase
  - Supercritical water:
    - Supercritical steam generator, a steam generator operating above the critical point of water, hence having no water–steam separation
    - Supercritical water oxidation or SCWO, a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point
    - Supercritical water reactor (SCWR), a Generation IV nuclear reactor concept that uses supercritical water as the working fluid
  The supercritical water reactor (SCWR) is a concept Generation IV reactor, mostly designed as light water reactor (LWR) that operates at supercritical pressure. The term critical in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor.
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, thermally labile molecules and can also be used for the separation of chiral compounds. Principles are similar to those of high performance liquid chromatography (HPLC), however SFC typically utilizes carbon dioxide as the mobile phase; therefore the entire chromatographic flow path must be pressurized. Because the supercritical phase represents a state in which liquid and gas properties converge, supercritical fluid chromatography is sometimes called convergence chromatography.
A supercritical steam generator is a type of boiler that operates at supercritical pressure, frequently used in the production of electric power.
Supercritical water oxidation or SCWO is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantage in the destruction of hazardous wastes such as PCBs. The fluid has a density between that of water vapor and liquid at standard conditions, and exhibits high gas-like diffusion rates along with high liquid-like collision rates. In addition, the behavior of water as a solvent is altered - it behaves much less like a polar solvent. As a result, the solubility behavior is "reversed" so that chlorinated hydrocarbons become soluble in the water, allowing single-phase reaction of aqueous waste with a dissolved oxidizer. The reversed solubility also causes salts to precipitate out of solution, meaning they can be treated using conventional methods for solid-waste residuals. Efficient oxidation reactions occur at low temperature with reduced NOx production.

Supercritical drying, also known as critical point drying, is a process to remove liquid in a precise and controlled way. It is useful in the production of microelectromechanical systems (MEMS), the drying of spices, the production of aerogel, the decaffeination of coffee and in the preparation of biological specimens for scanning electron microscopy.

A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned".

Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.

Flows

Supercritical flow, where flow velocity is larger than wave velocity
- Supercritical airfoil, an airfoil designed to delay the onset of wave drag in the transonic speed regime

A supercritical flow is a flow whose velocity is larger than the wave velocity. The analogous condition in gas dynamics is supersonic.

A supercritical airfoil is an airfoil designed primarily to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered ("downward-curved") aft section, and larger leading-edge radius compared with NACA 6-series laminar airfoil shapes. Standard wing shapes are designed to create lower pressure over the top of the wing. The camber of the wing determines how much the air accelerates around the wing. As the speed of the aircraft approaches the speed of sound, the air accelerating around the wing reaches Mach 1 and shockwaves begin to form. The formation of these shockwaves causes wave drag. Supercritical airfoils are designed to minimize this effect by flattening the upper surface of the wing.

Nuclear physics

Supercritical mass, an amount of fissile material that will undergo a sustained nuclear chain reaction

Mathematics

Hopf bifurcation, in mathematics, a local bifurcation; when the first Lyapunov coefficient is negative, the bifurcation is called supercritical
Pitchfork bifurcation, in mathematics, similar to Hopf bifurcation

Other uses

Super Critical , an album by British pop/rock group The Ting Tings

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In thermodynamics and engineering, a heat engine is a system that converts heat or thermal energy—and chemical energy—to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the working body of the engine while transferring heat to the colder sink until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. During this process, a lot of heat is lost to the surroundings and so cannot be converted to work.

A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases occur and coexist at equilibrium.

Pressurized water reactors (PWRs) constitute the large majority of the world's nuclear power plants and are one of three types of light water reactor (LWR), the other types being boiling water reactors (BWRs) and supercritical water reactors (SCWRs). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary water as both coolant and neutron moderator.

The Rankine cycle is a model used to predict the performance of steam turbine systems. It was also used to study the performance of reciprocating steam engines. The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work while undergoing phase change. It is an idealized cycle in which friction losses in each of the four components are neglected. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. It is named after William John Macquorn Rankine, a Scottish polymath and Glasgow University professor.

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, separated flows, and dispersed two-phase flows where one phase is present in the form of particles, droplets, or bubbles in a continuous carrier phase.

A coolant is a substance, typically liquid or gas, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

Critical point (thermodynamics) Temperature and pressure point where phase boundaries disappear

In thermodynamics, a critical point is the end point of a phase equilibrium curve. The most prominent example is the liquid-vapor critical point, the end point of the pressure-temperature curve that designates conditions under which a liquid and its vapor can coexist. At higher temperatures, the gas cannot be liquefied by pressure alone. At the critical point, defined by a critical temperatureT_c and a critical pressurep_c, phase boundaries vanish. Other examples include the liquid–liquid critical points in mixtures.

Supercritical 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. SFE can be used as a sample preparation step for analytical purposes, or on a larger scale to either strip unwanted material from a product (e.g. decaffeination) or collect a desired product (e.g. essential oils). These essential oils can include limonene and other straight solvents. Carbon dioxide (CO₂) is the most used supercritical fluid, sometimes modified by co-solvents such as ethanol or methanol. Extraction conditions for supercritical carbon dioxide are above the critical temperature of 31 °C and critical pressure of 74 bar. Addition of modifiers may slightly alter this. The discussion below will mainly refer to extraction with CO₂, except where specified.

Superheated water is liquid water under pressure at temperatures between the usual boiling point, 100 °C (212 °F) and the critical temperature, 374 °C (705 °F). It is also known as "subcritical water" or "pressurized hot water." Superheated water is stable because of overpressure that raises the boiling point, or by heating it in a sealed vessel with a headspace, where the liquid water is in equilibrium with vapour at the saturated vapor pressure. This is distinct from the use of the term superheating to refer to water at atmospheric pressure above its normal boiling point, which has not boiled due to a lack of nucleation sites.

Supercritical adsorption also referred to as the adsorption of supercritical fluids, is the adsorption at above-critical temperatures. There are different tacit understandings of supercritical fluids. For example, “a fluid is considered to be ‘supercritical’ when its temperature and pressure exceed the temperature and pressure at the critical point”. In the studies of supercritical extraction, however, “supercritical fluid” is applied for a narrow temperature region of 1-1.2 $or to +10 K, which is called the supercritical region.$

A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.

Supercritical liquid–gas boundaries are lines in the p–T diagram that delimit more liquid-like and more gas-like states of a supercritical fluid. They comprise the Fisher–Widom line, the Widom line, and the Frenkel line.

Carbon dioxide cleaning (CO₂ cleaning) comprises a family of methods for parts cleaning and sterilization, using carbon dioxide in its various phases. It is often preferred for use on delicate surfaces. CO₂ cleaning has found application in the aerospace, automotive, electronics, medical, and other industries. Carbon dioxide snow cleaning has been used to remove particles and organic residues from metals, polymers, ceramics, glasses, and other materials, and from surfaces including hard drives and optical surfaces.

Non ideal compressible fluid dynamics, abbreviated as NICFD, is a branch of fluid mechanics studying the actual characteristics of dense vapors, supercritical flows and compressible two-phase flows, namely whereby the thermodynamic behavior of the fluid differs considerably from that of a perfect gas. At high reduced pressure and temperature, close to the saturation curve the speed of sound is largely sensitive to density variations along isentropes. Consequently, the fluid flow departs from the ideality assumption and under particular conditions may even exhibit non classical gas dynamic phenomena, whose nature is governed by the value of the fundamental derivative of gas-dynamics Γ. A non-monotonic Mach number trend along an expansion is typical for 0 < Γ < 1, while for Γ < 0 values admit the occurrence of inverse gas-dynamics phenomena such as rarefaction shock waves, splitting waves or even composite waves. Inverse gas-dynamics behavior has been theoretically predicted for heavy complex molecules in the vapor region,and a recent study discovered that two-phase rarefaction shock waves are physically realizable close to the critical point.

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