Air separation

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An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes also argon and other rare inert gases.

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The most common method for air separation is fractional distillation. Cryogenic air separation units (ASUs) are built to provide nitrogen or oxygen and often co-produce argon. Other methods such as membrane, pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) are commercially used to separate a single component from ordinary air. High purity oxygen, nitrogen, and argon, used for semiconductor device fabrication, require cryogenic distillation. Similarly, the only viable source of the rare gases neon, krypton, xenon is the distillation of air using at least two distillation columns. Helium is also recovered in advanced air separation processes. [1]

Cryogenic distillation process

Composition of dry atmospheric air Atmosphere3.svg
Composition of dry atmospheric air

Pure gases can be separated from air by first cooling it until it liquefies, then selectively distilling the components at their various boiling temperatures. The process can produce high purity gases but is energy-intensive. This process was pioneered by Carl von Linde in the early 20th century and is still used today to produce high purity gases. He developed it in the year 1895; the process remained purely academic for seven years before it was used in industrial applications for the first time (1902). [3]

Distillation column in a cryogenic air separation plant Coldbox.JPG
Distillation column in a cryogenic air separation plant

The cryogenic separation process [4] [5] [6] requires a very tight integration of heat exchangers and separation columns to obtain a good efficiency and all the energy for refrigeration is provided by the compression of the air at the inlet of the unit.

To achieve the low distillation temperatures, an air separation unit requires a refrigeration cycle that operates by means of the Joule–Thomson effect, 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 make this refrigeration cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines for cooling; the output of the expander helps drive the air compressor, for improved efficiency. The process consists of the following main steps: [7]

  1. Before compression the air is pre-filtered of dust.
  2. Air is compressed where the final delivery pressure is determined by recoveries and the fluid state (gas or liquid) of the products. Typical pressures range between 5 and 10 bar gauge. The air stream may also be compressed to different pressures to enhance the efficiency of the ASU. During compression water is condensed out in inter-stage coolers.
  3. The process air is generally passed through a molecular sieve bed, which removes any remaining water vapour, as well as carbon dioxide, which would freeze and plug the cryogenic equipment. Molecular sieves are often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation that could lead to explosions. [8] The molecular sieves bed must be regenerated. This is done by installing multiple units operating in alternating mode and using the dry co-produced waste gas to desorb the water.
  4. Process air is passed through an integrated heat exchanger (usually a plate fin heat exchanger) and cooled against product (and waste) cryogenic streams. Part of the air liquefies to form a liquid that is enriched in oxygen. The remaining gas is richer in nitrogen and is distilled to almost pure nitrogen (typically < 1ppm) in a high pressure (HP) distillation column. The condenser of this column requires refrigeration which is obtained from expanding the more oxygen rich stream further across a valve or through an expander (a reverse compressor).
  5. Alternatively the condenser may be cooled by interchanging heat with a reboiler in a low pressure (LP) distillation column (operating at 1.2-1.3 bar abs.) when the ASU is producing pure oxygen. To minimize the compression cost the combined condenser/reboiler of the HP/LP columns must operate with a temperature difference of only 1-2 K, requiring plate fin brazed aluminium heat exchangers. Typical oxygen purities range in from 97.5% to 99.5% and influences the maximum recovery of oxygen. The refrigeration required for producing liquid products is obtained using the Joule–Thomson effect in an expander which feeds compressed air directly to the low pressure column. Hence, a certain part of the air is not to be separated and must leave the low pressure column as a waste stream from its upper section.
  6. Because the boiling point of argon (87.3 K at standard conditions) lies between that of oxygen (90.2 K) and nitrogen (77.4 K), argon builds up in the lower section of the low pressure column. When argon is produced, a vapor side draw is taken from the low pressure column where the argon concentration is highest. It is sent to another column rectifying the argon to the desired purity from which liquid is returned to the same location in the LP column. Use of modern structured packings which have very low pressure drops enable argon with less than 1 ppm impurities. Though argon is present in less to 1% of the incoming, the air argon column requires a significant amount of energy due to the high reflux ratio required (about 30) in the argon column. Cooling of the argon column can be supplied from cold expanded rich liquid or by liquid nitrogen.
  7. Finally the products produced in gas form are warmed against the incoming air to ambient temperatures. This requires a carefully crafted heat integration that must allow for robustness against disturbances (due to switch over of the molecular sieve beds). [9] It may also require additional external refrigeration during start-up.

The separated products are sometimes supplied by pipeline to large industrial users near the production plant. Long distance transportation of products is by shipping liquid product for large quantities or as dewar flasks or gas cylinders for small quantities.

Non-cryogenic processes

A nitrogen generator Gas Control Systems, INC PSA.jpg
A nitrogen generator
Bottle of 4A molecular sieves 4A sieves.JPG
Bottle of 4Å molecular sieves

Pressure swing adsorption provides separation of oxygen or nitrogen from air without liquefaction. The process operates around ambient temperature; a zeolite (molecular sponge) is exposed to high pressure air, then the air is released and an adsorbed film of the desired gas is released. The size of compressor is much reduced over a liquefaction plant, and portable oxygen concentrators are made in this manner to provide oxygen-enriched air for medical purposes. Vacuum swing adsorption is a similar process; the product gas is evolved from the zeolite at sub-atmospheric pressure.

Membrane nitrogen generator Membrane nitrogen generator.jpg
Membrane nitrogen generator

Membrane technologies can provide alternate, lower-energy approaches to air separation. For example, a number of approaches are being explored for oxygen generation. Polymeric membranes operating at ambient or warm temperatures, for example, may be able to produce oxygen-enriched air (25-50% oxygen). Ceramic membranes can provide high-purity oxygen (90% or more) but require higher temperatures (800-900 deg C) to operate. These ceramic membranes include ion transport membranes (ITM) and oxygen transport membranes (OTM). Air Products and Chemicals Inc and Praxair are developing flat ITM and tubular OTM systems.[ citation needed ]

Membrane gas separation is used to provide oxygen-poor and nitrogen-rich gases instead of air to fill the fuel tanks of jet liners, thus greatly reducing the chances of accidental fires and explosions. Conversely, membrane gas separation is currently used to provide oxygen-enriched air to pilots flying at great altitudes in aircraft without pressurized cabins.

Oxygen-enriched air can be obtained exploiting the different solubility of oxygen and nitrogen. Oxygen is more soluble than nitrogen in water, so if air is degassed from water, a stream of 35% oxygen can be obtained. [10]

Applications

Rocketry

Liquid oxygen for companies such as SpaceX. [11]

Medical

Pure oxygen is delivered to large hospitals for use with patients.

Steel

In steelmaking, oxygen is required for the basic oxygen steelmaking process. Modern basic oxygen steelmaking uses almost two tons of oxygen per ton of steel. [12]

Ammonia

Nitrogen used in the Haber process to make ammonia. [13]

Coal gas

Large amounts of oxygen are required for coal gasification projects; cryogenic plants producing 3000 tons/day are found in some projects. [14]

Inert gas

Inerting with nitrogen storage tanks of ships and tanks for petroleum products, or for protecting edible oil products from oxidation.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Argon</span> Chemical element, symbol Ar and atomic number 18

Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934%. It is more than twice as abundant as water vapor, 23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

<span class="mw-page-title-main">Distillation</span> Method of separating mixtures

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 mixture and the condensation of the vapors in a still.

Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation to fractionate. Generally the component parts have boiling points that differ by less than 25 °C (45 °F) from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is typically used.

<span class="mw-page-title-main">Liquid nitrogen</span> Liquid state of nitrogen

Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about −196 °C (−321 °F; 77 K). It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose viscosity is about one tenth that of acetone (i.e. roughly one thirtieth that of room temperature water). Liquid nitrogen is widely used as a coolant.

Cryogenic fuels are fuels that require storage at extremely low temperatures in order to maintain them in a liquid state. These fuels are used in machinery that operates in space where ordinary fuel cannot be used, due to the very low temperatures often encountered in space, and the absence of an environment that supports combustion. Cryogenic fuels most often constitute liquefied gases such as liquid hydrogen.

Liquid air is air that has been cooled to very low temperatures, so that it has condensed into a pale blue mobile liquid. It is stored in specialized containers, such as vacuum flasks, to insulate it from room temperature. Liquid air can absorb heat rapidly and revert to its gaseous state. It is often used for condensing other substances into liquid and/or solidifying them, and as an industrial source of nitrogen, oxygen, argon, and other inert gases through a process called air separation.

<span class="mw-page-title-main">Carl von Linde</span> German engineer and scientist

Carl Paul Gottfried von Linde was a German scientist, engineer, and businessman. He discovered a refrigeration cycle and invented the first industrial-scale air separation and gas liquefaction processes, which led to the first reliable and efficient compressed-ammonia refrigerator in 1876.

<span class="mw-page-title-main">Membrane gas separation</span> Technology for splitting specific gases out of mixtures

Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.

An oxygen concentrator is a device that concentrates the oxygen from a gas supply by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are used industrially, to provide supplemental oxygen at high altitudes, and as medical devices for oxygen therapy.

<span class="mw-page-title-main">Molecular sieve</span> Filter material with homogeneously sized pores in the nanometer range

A molecular sieve is a material with pores of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrates through the stationary bed of porous, semi-solid substance referred to as a sieve, the components of the highest molecular weight leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size. Another important use is as a desiccant. Most of molecular sieves are aluminosilicate zeolites with Si/Al molar ratio less than 2, but there are also examples of activated charcoal and silica gel.

<span class="mw-page-title-main">Pressure swing adsorption</span> Method of gases separation using selective adsorption under pressure

Pressure swing adsorption (PSA) is a technique 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 temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas.

<span class="mw-page-title-main">Liquefaction of gases</span>

Liquefaction of gases is physical conversion of a gas into a liquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures, using, for example, turboexpanders.

<span class="mw-page-title-main">Industrial gas</span> Gaseous materials produced for use in industry

Industrial gases are the gaseous materials that are manufactured for use in industry. The principal gases provided are nitrogen, oxygen, carbon dioxide, argon, hydrogen, helium and acetylene, although many other gases and mixtures are also available in gas cylinders. The industry producing these gases is also known as industrial gas, which is seen as also encompassing the supply of equipment and technology to produce and use the gases. Their production is a part of the wider chemical Industry.

<span class="mw-page-title-main">Turboexpander</span> Type of turbine for high-pressure gas

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 or generator.

<span class="mw-page-title-main">Natural-gas processing</span> Industrial processes designed to purify raw natural gas

Natural-gas processing is a range of industrial processes designed to purify raw natural gas by removing contaminants such as solids, water, carbon dioxide (CO2), hydrogen sulfide (H2S), mercury and higher molecular mass hydrocarbons (condensate) to produce pipeline quality dry natural gas for pipeline distribution and final use. Some of the substances which contaminate natural gas have economic value and are further processed or sold. Hydrocarbons that are liquid at ambient conditions: temperature and pressure (i.e., pentane and heavier) are called natural-gas condensate (sometimes also called natural gasoline or simply condensate).

<span class="mw-page-title-main">Nitrogen generator</span>

Nitrogen generators and stations are stationary or mobile air-to-nitrogen production complexes.

Oxygen plants are industrial systems designed to generate oxygen. They typically use air as a feedstock and separate it from other components of air using pressure swing adsorption or membrane separation techniques. Such plants are distinct from cryogenic separation plants which separate and capture all the components of air.

<span class="mw-page-title-main">Cryogenic gas plant</span> Industrial facility that creates cryogenic liquid at relatively high purity

A cryogenic gas plant is an industrial facility that creates molecular oxygen, molecular nitrogen, argon, krypton, helium, and xenon at relatively high purity. As air is made up of nitrogen, the most common gas in the atmosphere, at 78%, with oxygen at 19%, and argon at 1%, with trace gasses making up the rest, cryogenic gas plants separate air inside a distillation column at cryogenic temperatures to produce high purity gasses such as argon, nitrogen, oxygen, and many more with 1 ppm or less impurities. The process is based on the general theory of the Hampson-Linde cycle of air separation, which was invented by Carl von Linde in 1895.

Gas separation can refer to any of a number of techniques used to separate gases, either to give multiple products or to purify a single product.

The Liquid Nitrogen Wash is mainly used for the production of ammonia synthesis gas within fertilizer production plants. It is usually the last purification step in the ammonia production process sequence upstream of the actual ammonia production.

References

  1. ↑ Chrz, Vaclav. "Helium Recovery" (PDF). CERN. CERN. Retrieved 30 November 2022.
  2. ↑ NASA Earth Fact Sheet, (updated November 2007)
  3. ↑ "Cool Inventions" (PDF). Institution of Chemical Engineers. September 2010. Archived from the original (PDF) on 2014-01-13. Retrieved 2014-01-12.
  4. ↑ Latimer, R. E. (1967). "Distillation of Air". Chemical Engineering Progress. 63 (2): 35–59.
  5. ↑ Agrawal, R. (1996). "Synthesis of Distillation Column Configurations for a Multicomponent Separation". Industrial & Engineering Chemistry Research. 35 (4): 1059–1071. doi:10.1021/ie950323h.
  6. ↑ Castle, W. F. (2002). "Air separation and liquefaction: Recent developments and prospects for the beginning of the new millennium". International Journal of Refrigeration. 25: 158–172. doi:10.1016/S0140-7007(01)00003-2.
  7. ↑ "How air separation works". Messer. Retrieved 9 November 2022.
  8. ↑ Particulate matter from forest fires caused an explosion in the air separation unit of a Gas to Liquid plant, see Fainshtein, V. I. (2007). "Provision of explosion proof air separation units under contemporary conditions". Chemical and Petroleum Engineering. 43 (1–2): 96–101. doi:10.1007/s10556-007-0018-8. S2CID   110001679.
  9. ↑ Vinson, D. R. (2006). "Air separation control technology". Computers & Chemical Engineering. 30 (10–12): 1436–1446. doi:10.1016/j.compchemeng.2006.05.038.
  10. ↑ Galli, F; Comazzi, A; Previtali, D; Manenti, F; Bozzano, G; Bianchi, C. L.; Pirola, C (2017). "Production of oxygen-enriched air via desorption from water: Experimental data, simulations and economic assessment". Computers & Chemical Engineering. 102: 11–16. doi:10.1016/j.compchemeng.2016.07.031.
  11. ↑ Copeland, Mike. "Messer to build $50 million gas plant in McGregor". Waco Tribune-Herald. Waco Tribune-Herald. Retrieved 30 November 2022.
  12. ↑ Flank, William H.; Abraham, Martin A.; Matthews, Michael A. (2009). Innovations in Industrial and Engineering Chemistry: A Century of Achievements and Prospects for the New Millennium. American Chemical Society. ISBN   9780841269637.
  13. ↑ Wingate, Philippa; Gifford, Clive; Treays, Rebecca (1992). Essential Science . Usborne. ISBN   9780746010112. liquid Nitrogen used in the Haber process to make ammonia.
  14. ↑ Higman, Christopher; van der Burgt, Maarten (2008). Gasification (2nd ed.). Elsevier. p. 324.
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