Editing Liquid air

{{Short description|Air that has been condensed into a liquid}}
{{other uses}}
{{more citations needed|date=June 2012}}
'''Liquid air''' is [[Atmosphere of Earth|air]] that has been cooled to very low temperatures ([[cryogenics|cryogenic temperatures]]), so that it has condensed into a pale blue mobile liquid.<ref>{{cite news|last=Babbage|title=Difference Engine: End of the electric car?|url=https://www.economist.com/blogs/babbage/2012/10/nitrogen-cycle|newspaper=The Economist|accessdate=Oct 21, 2012|date=Oct 15, 2012}}</ref> It is stored in specialized containers, such as [[vacuum flask]]s, 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 gas]]es through a process called [[air separation]] (industrially referred to as air rectification.).

== Properties ==
Liquid air has a density of approximately {{convert|870|kg/m3|g/L g/cm3|abbr=on|lk=on}}. The density of a given air sample varies depending on the composition of that sample (e.g. humidity & {{CO2}} concentration). Since dry gaseous air contains approximately 78% nitrogen, 21% oxygen, and 1% [[argon]], the density of liquid air at standard composition is calculated by the percentage of the components and their respective liquid densities (see [[liquid nitrogen]] and [[liquid oxygen]]). Although air contains trace amounts of [[carbon dioxide]] (about 0.03%), carbon dioxide solidifies from the gas phase without passing through the intermediate liquid phase, and hence will not be present in liquid air at pressures less than {{convert|5.1|atm|abbr=on|lk=on}}.

The boiling point of air is {{convert|-194.35|C|K F|abbr=on|lk=on}}, intermediate between the boiling points of [[liquid nitrogen]] and [[liquid oxygen]]. However, it can be difficult to keep at a stable temperature as the liquid boils, since the nitrogen will boil off first, leaving the mixture oxygen-rich and changing the boiling point. This may also occur in some circumstances due to the liquid air condensing oxygen out of the atmosphere.<ref>{{cite book | last1 = Armarego | first1 = W. L. F. | last2 = Perrin | first2 = D. D. | title = Purification of Laboratory Chemicals | edition = 4th | publisher = [[Butterworth-Heinemann]] | date = 1996-10-16 | isbn = 978-0750628396 | oclc = 762966259 | ol = OL722457M | lccn = 97109714 | df= dmy-all}}</ref>{{rp|page=36}}

Liquid air starts to freeze at approximately {{convert|60|K|C F|abbr=on}}, precipitating nitrogen-rich solid (but with appreciable amount of oxygen in solid solution). Unless the oxygen is previously accommodated in the solid solution, the [[eutectic]] freezes at 50 K.<ref>{{cite journal |last1=Kochenburger |first1=Thomas M. |last2=Grohmann |first2=Steffen |last3=Oellrich |first3=Lothar R. |title=Evaluation of a Two-stage Mixed Refrigerant Cascade for HTS Cooling Below 60 K |journal=Physics Procedia |date=2015 |volume=67 |pages=227–232 |doi=10.1016/j.phpro.2015.06.039 |bibcode=2015PhPro..67..227K |doi-access=free }}</ref>

== Preparation ==
=== Principle of production ===
The constituents of air were once known as "permanent gases", as they could not be liquified solely by compression at room temperature. A compression process will raise the temperature of the gas. This heat is removed by cooling to the ambient temperature in a heat exchanger, and then expanding by venting into a chamber. The expansion causes a lowering of the temperature, and by [[Countercurrent exchange|counter-flow heat exchange]] of the expanded air, the pressurized air entering the expander is further cooled. With sufficient compression, flow, and heat removal, eventually droplets of liquid air will form, which may then be employed directly for low temperature demonstrations.

The main constituents of air were liquefied for the first time by Polish scientists [[Karol Olszewski]] and [[Zygmunt Florenty Wróblewski|Zygmunt Wróblewski]] in 1883.

Devices for the production of liquid air are not commercially available, and not easily fabricated.

=== Process of production===
The most common process for the preparation of liquid air is the two-column [[Hampson–Linde cycle]] using the [[Joule–Thomson effect]]. Air is fed at high pressure (>{{convert|75|atm|kPa psi|abbr=on|lk=on}}) into the lower column, in which it is separated into pure nitrogen and oxygen-rich liquid. The rich liquid and some of the nitrogen are fed as reflux into the upper column, which operates at low pressure (<{{convert|25|atm|kPa psi|abbr=on}}), where the final separation into pure nitrogen and oxygen occurs. A raw argon product can be removed from the middle of the upper column for further purification.<ref>{{Cite web| url =http://www.linde.com/international/web/linde/like35lindecom.nsf/docbyalias/page_ch_chronicle_18911934details_1| title =Air liquefaction, "Linde Air", rectification: into new markets with new research findings| publisher =[[Linde plc|The Linde Group]] | access-date =2007-08-09| url-status =dead| archive-url =https://web.archive.org/web/20070927213521/http://www.linde.com/international/web/linde/like35lindecom.nsf/docbyalias/page_ch_chronicle_18911934details_1| archive-date =2007-09-27 | df=dmy-all}}</ref>

Air can also be liquefied by [[Liquefaction of gases#Claude's process|Claude's process]], which combines cooling by [[Joule–Thomson effect]], isentropic expansion and regenerative cooling.<ref>https://uspas.fnal.gov/materials/10MIT/Lecture_2.1.pdf {{Bare URL PDF|date=March 2022}}</ref>

== Application ==
In manufacturing processes, the liquid air product is typically fractionated into its constituent gases in either liquid or gaseous form, as the oxygen is especially useful for [[Oxy-fuel welding and cutting|fuel gas welding and cutting]] and for medical use, and the argon is useful as an oxygen-excluding [[shielding gas]] in [[gas tungsten arc welding]]. [[Liquid nitrogen]] is useful in various low-temperature applications, being nonreactive at normal temperatures (unlike oxygen), and boiling at {{convert|77|K|0}}.

=== Transport and energy storage ===
{{Main|Cryogenic energy storage}}
Between 1899 and 1902, the automobile [[Liquid Air]] was produced and demonstrated by a joint American/English company, with the claim that they could construct a car that would run a hundred miles on liquid air.

On 2 October 2012, the [[Institution of Mechanical Engineers]] said liquid air could be used as a means of storing energy. This was based on a technology that was developed by Peter Dearman, a garage inventor in [[Hertfordshire]], England to power vehicles.<ref>{{cite news| url=https://www.bbc.co.uk/news/science-environment-19785689 | work=BBC News | title=Liquid air 'offers energy storage hope' | date=2012-10-02}}</ref>

== See also ==
{{Portal|Chemistry|Science}}
* [[Liquid nitrogen]]
* [[Liquid oxygen]]
* [[Cryogenic energy storage]]
* [[Industrial gas]]
* [[Liquefaction of gases]]
* [[Liquid nitrogen vehicle]]

== References ==
{{reflist|30em}}

== External links ==
* 2013-05-20 [[MIT Technology Review]] article [https://www.technologyreview.com/news/514936/liquefied-air-could-power-cars-and-store-energy-from-sun-and-wind/ on liquid air developments for transportation and grid energy storage]

[[Category:Atmosphere]]
[[Category:Coolants]]
[[Category:Cryogenics]]
[[Category:Energy storage]]
[[Category:Energy technology]]
[[Category:Engineering thermodynamics]]
[[Category:Industrial gases]]
[[Category:Industrial processes]]
[[Category:Phases of matter]]