Air separation
Encyclopedia
An air separation plant separates atmospheric air
into its primary components, typically nitrogen
and oxygen
sometimes also argon
and rarely other inert gases
. There are various technologies that are used for the separation process, the most common is via cryogenic distillation. This process was pioneered by Dr. Carl von Linde
in the early 20th century and is still used today to produce high purity gases. In addition to the cryogenic distillation method there are other methods such as Membrane, Pressure Swing Adsorption
(PSA) and Vacuum Pressure Swing Adsorption
(VPSA), which are typically used to separate a single component, such as oxygen
, from ordinary air.
process.
To achieve the low distillation temperatures a modern Air Separation Unit requires a refrigeration cycle, and the cold equipment has to be kept within an insulated
enclosure (commonly called a "cold box").
The cooling of the gases requires a large amount of energy
to drive an air compressor
to make this refrigeration cycle work. The air also has to be "clean" enough for cryogenic distillation, since water and carbon dioxide as well as other minor constituents of air can freeze in the cryogenic equipment.
The process has the following main features:
Since the compressor heats up the air, it is cooled again in a heat exchanger to ambient temperatures.
This can also achieve the removal of some ambient moisture.
bed, which removes any remaining water vapour, as well as carbon dioxide
, which would freeze in the cryogenic equipment. The molecular sieve is often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation.
(usually a plate fin heat exchanger
) and cooled against product (and waste) cryogenic streams. The air is then cool enough to be distilled in a distillation column.
The formation of liquid air
in the cryogenic equipment requires some refrigeration
and liquid is usually formed by Joule Thomson
expansion of air across a valve or through an Expander
, (a reverse compressor).
The air is distilled in at least one and often two distillation columns, depending on the products required.
Cryogenic air separation units are built to provide one or both of nitrogen
and oxygen
although argon
is also often produced. Liquid nitrogen
"LIN", Liquid oxygen
"LOX" and liquid argon can be produced if sufficient refrigeration is provided for in the design. Finally the product gases are warmed against the incoming air to ambient temperatures.
or gas cylinders
for small quantities.
In addition to oxygen, nitrogen and argon; rarely other gas products may be produced - the only viable sources of neon
, krypton
and xenon
are from the distillation of air.
Industrial gas
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
into its primary components, typically nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
sometimes also argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
and rarely other inert gases
Inert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
. There are various technologies that are used for the separation process, the most common is via cryogenic distillation. This process was pioneered by Dr. Carl von Linde
Carl von Linde
Professor Doctor Carl Paul Gottfried von Linde was a German engineer who developed refrigeration and gas separation technologies...
in the early 20th century and is still used today to produce high purity gases. In addition to the cryogenic distillation method there are other methods such as Membrane, Pressure Swing Adsorption
Pressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...
(PSA) and Vacuum Pressure Swing Adsorption
Vacuum swing adsorption
Vacuum swing adsorption is a non-cryogenic gas separation technology.Using special solids, or adsorbents, VSA segregates certain gases from a gaseous mixture under minimal pressure according to the species' molecular characteristics and affinity for the adsorbents...
(VPSA), which are typically used to separate a single component, such as oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, from ordinary air.
The Cryogenic Process
The separation of air into its constituent parts at high purity requires a cryogenic distillationDistillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
process.
To achieve the low distillation temperatures a modern Air Separation Unit requires a refrigeration cycle, and the cold equipment has to be kept within an insulated
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
enclosure (commonly called a "cold box").
The cooling of the gases requires a large amount of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
to drive an air compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
to make this refrigeration cycle work. The air also has to be "clean" enough for cryogenic distillation, since water and carbon dioxide as well as other minor constituents of air can freeze in the cryogenic equipment.
The process has the following main features:
Compression
Atmospheric air is pre-filtered (to remove dust), and compressed to a pressure typically between 5 and 10 bar.Since the compressor heats up the air, it is cooled again in a heat exchanger to ambient temperatures.
This can also achieve the removal of some ambient moisture.
Purification
The process air is generally passed through a molecular sieveMolecular sieve
A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...
bed, which removes any remaining water vapour, as well as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, which would freeze in the cryogenic equipment. The molecular sieve is often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation.
Cooling & Distillation
Process air is passed through an integrated heat exchangerHeat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
(usually a plate fin heat exchanger
Plate fin heat exchanger
A plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between fluids. It is often categorized as a compact heat exchanger to emphasise its relatively high heat transfer surface area to volume ratio....
) and cooled against product (and waste) cryogenic streams. The air is then cool enough to be distilled in a distillation column.
The formation of liquid air
Liquid air
Liquid air is air that has been cooled to very low temperatures so that it has condensed to a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum flask. Liquid air can absorb heat rapidly and revert to its gaseous state...
in the cryogenic equipment requires some refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
and liquid is usually formed by Joule Thomson
Joule-Thomson effect
In thermodynamics, the Joule–Thomson effect or Joule–Kelvin effect or Kelvin–Joule effect describes the temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment. This procedure is called a...
expansion of air across a valve or through an Expander
Turboexpander
A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work that is often used to drive a compressor....
, (a reverse compressor).
The air is distilled in at least one and often two distillation columns, depending on the products required.
Cryogenic air separation units are built to provide one or both of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
although argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
is also often produced. Liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
"LIN", Liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...
"LOX" and liquid argon can be produced if sufficient refrigeration is provided for in the design. Finally the product gases are warmed against the incoming air to ambient temperatures.
Product supply and storage
The air gases are sometimes supplied by pipeline to large industrial users adjacent to or nearby to the production plant or stored as liquid. Unless a viable pipeline system exists, long distance transportation of products is usually done as a liquid product for large quantities or as dewar flasksCryogenic storage dewar
Cryogenic storage dewars are specialised types of vacuum flask used for storing cryogens , whose boiling points are much lower than room temperature. Cryogenic storage dewars may take several different forms including open buckets, flasks with loose-fitting stoppers and self-pressurising tanks...
or gas cylinders
Gas cylinder
A gas cylinder is a pressure vessel used to store gases at above atmospheric pressure. High pressure gas cylinders are also called bottles. Although they are sometimes colloquially called "tanks", this is technically incorrect, as a tank is a vessel used to store liquids at ambient pressure and...
for small quantities.
In addition to oxygen, nitrogen and argon; rarely other gas products may be produced - the only viable sources of neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
, krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
and xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
are from the distillation of air.
See also
- Compressibility factorCompressibility factorThe compressibility factor , also known as the compression factor, is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behavior. In general, deviation from ideal behavior becomes more significant the closer a gas is to a phase change, the lower the...
- Gas separationGas separationGas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
- Gas to liquidsGas to liquidsGas to liquids is a refinery process to convert natural gas or other gaseous hydrocarbons into longer-chain hydrocarbons such as gasoline or diesel fuel...
- Liquid airLiquid airLiquid air is air that has been cooled to very low temperatures so that it has condensed to a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum flask. Liquid air can absorb heat rapidly and revert to its gaseous state...
- Liquefaction of gasesLiquefaction of gasesLiquefaction of gases includes a number of phases used to convert a gas into a liquid state. The processes are used for scientific, industrial and commercial purposes. Many gases can be put into a liquid state at normal atmospheric pressure by simple cooling; a few, such as carbon dioxide, require...
- Industrial gases
- Chemical engineerChemical engineerIn the field of engineering, a chemical engineer is the profession in which one works principally in the chemical industry to convert basic raw materials into a variety of products, and deals with the design and operation of plants and equipment to perform such work...
- TurboexpanderTurboexpanderA turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work that is often used to drive a compressor....
- Louis Paul CailletetLouis Paul CailletetLouis-Paul Cailletet was a French physicist and inventor.- Life and work :Cailletet was born in Châtillon-sur-Seine, Côte-d'Or. Educated in Paris, Cailletet returned to Châtillon to manage his father's ironworks...
- Hampson–Linde cycle
- Siemens cycleSiemens cycleThe Siemens cycle is a technique used to cool or liquefy gases.. A gas is compressed, leading to an increase in its temperature . The compressed gas is then cooled by a heat exchanger, then the cool, compressed gas is allowed to decompress, further cooling it...
Industrial gasIndustrial gasIndustrial gas is a group of gases that are commercially manufactured and sold for uses in other applications. These gases are mainly used in an industrial processes, such as steelmaking, oil refining, medical applications, fertilizer, semiconductors, etc.,...
companies
- CRYOTEC Anlagenbau GmbH
- AGA ABAGA ABAGA AB, previously AB Gasaccumulator and AB Svenska Gasaccumulator, was a Swedish industrial gas company founded in 1904. Nobel Prize laureate Gustaf Dalén was instrumental in the success of the company. Important inventions included the AGA cooker and the Dalén light. In the 1990's AGA conceived...
(part of The Linde GroupThe Linde GroupThe Linde Group, registered as Linde AG is an international industrial gases and engineering company founded in Germany in 1879. Linde shares are traded on all the German stock exchanges and also in Zürich, and the Linde share price is included in the DAX 30 index...
) - AirgasAirgasAirgas, Inc. , headquartered in Radnor Township, Pennsylvania, through its subsidiaries, is the largest U.S. distributor of industrial, medical and specialty gases , and hardgoods ....
- Air LiquideAir LiquideL'Air Liquide S.A., or Air Liquide , is a major French company supplying industrial gases and services to various industries including medical, chemical and electronic manufacturers. Founded in 1902, it is first in the world market in its field, now operating in over 80 countries. It is...
- Air Products & ChemicalsAir Products & ChemicalsAir Products and Chemicals, Inc. is an international corporation whose principal business is selling gases and chemicals for industrial uses. Air Products' headquarters is in Allentown, Pennsylvania, in the Lehigh Valley region of Pennsylvania, in the United States. Air Products is the Lehigh...
- BASFBASFBASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
- BOCThe BOC GroupThe BOC Group plc was the official name of the multinational industrial gas and British based company more commonly known as BOC, and now a part of The Linde Group. In September 2004, BOC had over 30,000 employees on six continents, with sales of over £4.6 billion. BOC was a constituent of the...
(part of The Linde GroupThe Linde GroupThe Linde Group, registered as Linde AG is an international industrial gases and engineering company founded in Germany in 1879. Linde shares are traded on all the German stock exchanges and also in Zürich, and the Linde share price is included in the DAX 30 index...
) - The Linde GroupThe Linde GroupThe Linde Group, registered as Linde AG is an international industrial gases and engineering company founded in Germany in 1879. Linde shares are traded on all the German stock exchanges and also in Zürich, and the Linde share price is included in the DAX 30 index...
(formerly Linde AGThe Linde GroupThe Linde Group, registered as Linde AG is an international industrial gases and engineering company founded in Germany in 1879. Linde shares are traded on all the German stock exchanges and also in Zürich, and the Linde share price is included in the DAX 30 index...
) - PraxairPraxairPraxair, Inc. is the largest industrial gases company in North and South America and one of the largest worldwide. The company supplies atmospheric, process and specialty gases as well as high-performance coatings and related services to a wide diversity of customers around the world...
- MESSER Group
- Matheson Tri-GasMatheson Tri-GasMatheson is a supplier of atmospheric gases, industrial gases, specialty gases, electronic materials, helium, propane, gas handling equipment, high performance purification systems, engineering and gas management services, and on-site gas generation, to various industries including semiconductor...
External links
- Linde CryoPlants Ltd. Manufacturer for small and medium sized air separation Plants
- TNSC Air Separation Air Separation Plant Design, Engineering, Manufacturing, Installation.