Liquid nitrogen

Liquid nitrogen (liquid density at the triple point is 0.707 g/mL) is the liquid produced industrially in large quantities by fractional distillation of liquid air and is often referred to by the abbreviation, LN₂. It is pure nitrogen, in a liquid state. Liquid nitrogen has the UN number 1977.

At atmospheric pressure, liquid nitrogen boils at convert|77|K|0, and is a cryogenic fluid which can cause rapid frostbite on contact with living tissue. It has a dielectric constant of 1.4. [cite web | title = Dielectric Constants | url = http://www.apgsensors.com/ltr2/access.php?file=pdf/dielectric-constants.pdf] When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at 77K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks.

Liquid nitrogen can easily be converted to the solid by placing it in a vacuum chamber pumped by a rotary vacuum pump. [Umrath, W. (1974) Cooling bath for rapid freezing in electron microscopy. Journal of Microscopy 101, 103–105.] Liquid nitrogen freezes at convert|63|K|0. Despite its reputation, liquid nitrogen's efficiency as a coolant is reduced by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect is known as the Leidenfrost effect and applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen than into liquid nitrogen alone.

Applications

Liquid nitrogen is a compact and readily transported source of nitrogen gas without pressurization. Further, its ability to maintain temperatures far below the freezing point of water makes it extremely useful in a wide range of applications, primarily as an open-cycle refrigerant, including:
* in cryogenics
* as a source of very dry nitrogen gas
* the immersion freezing and transportation of food products
* the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials
* as a coolant for overclocking a central processing unit, a graphics processing unit, or another type of computer hardware [cite book | title = The Book of Overclocking: Tweak Your PC to Unleash Its Power | first = Scott | last = Wainner | coauthors = Robert Richmond | pages = p. 44 | isbn = 188641176X | publisher = No Starch Press | year = 2003]
* as a method of freezing water pipes in order to work on them in situations where a valve is not available to block water flow to the work area.
* in cryotherapy for removing unsightly or potentially malignant skin lesions such as warts and actinic keratosis.
* in the process of promession, a way to dispose of the dead.
* cooling a high-temperature superconductor to a temperature sufficient to achieve superconductivity.
* the cryonic preservation of humans and pets in the hope of future reanimation.
* as a coolant for vacuum pump traps and in controlled-evaporation processes in chemistry.
* as a coolant to increase the sensitivity of infrared homing seeker heads of missiles such as the Strela 3.

afety

Since the liquid to gas expansion ratio of this substance is 1:694, [cite web | url = http://www-safety.deas.harvard.edu/services/nitrogen.html | title = Information Specific to Liquid Nitrogen |date=30 July 2003 | publisher = Harvard University] a tremendous amount of force can be generated when liquid nitrogen boils off for whatever reasons. In a well-known accident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically, and exploded. The force of the explosion was sufficient to propel the tank through the floor/ceiling immediately above it. [cite web | title = Investigative Report on Chemistry 301A Cylinder Explosion | author = Brent S. Mattox | publisher = Texas A&M University | format = reprint | url = http://ucih.ucdavis.edu/docs/chemistry_301a.pdf]

Due to its extremely low temperature, careless handling of liquid nitrogen may result in cold burns. It should be also borne in mind, that as liquid nitrogen evaporates it will reduce the oxygen concentration in the air and might act as an asphyxiant, especially in confined spaces. Nitrogen is odourless, colourless and tasteless, and may produce asphyxia without any sensation or prior warning.British Compressed Gases Association (2000) BCGA Code of Practice CP30. The Safe Use of Liquid nitrogen dewars up to 50 litres. ISSN 0260-4809. [http://www.bcga.co.uk] ]

Vessels containing liquid nitrogen can condense oxygen from air. The liquid in such a vessel becomes increasingly enriched in oxygen (boiling point = 90 K) as the nitrogen evaporates, and can cause violent oxidation of organic material.

Cost

The cost of liquid nitrogen depends on the distance from related facilities and the price of energy; the actual cost tends to range between 0.10 and 0.50 USD/L. [cite web |url=http://hypertextbook.com/facts/2007/KarenFan.shtml |title=Price of Liquid Nitrogen |accessdate=2008-08-29 |work=The Physics Factbook |publisher= |date=2007 ]

ee also

* Computer cooling
* Liquid nitrogen economy

References

External links

* [http://www.younghotelier.com/interviews/video-cooking-with-liquid-nitrogen-in-the-real-world-tang-restaurant-dubai/ Behind the scenes video] - How liquid nitrogen is used in restaurants for cooking and cocktails