Liquid nitrogen is nitrogen in a liquid state
at an extremely low temperature. It is produced
industrially by fractional distillation of
liquid air. Liquid nitrogen is a colorless
clear liquid with density of 0.807 g/ml at
its boiling point and a dielectric constant
of 1.43. Liquid nitrogen is often referred
to by the abbreviation, LN2 or "LIN" or "LN"
and has the UN number 1977. Liquid nitrogen
is a diatomic liquid meaning the diatomic
character of the covalent N bonding in N2
gas is retained even after liquefaction.
At atmospheric pressure, liquid nitrogen boils
at −196 °C and is a cryogenic fluid that
can cause rapid freezing on contact with living
tissue. 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 77 K 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. The development of pressurised
super-insulated vacuum vessels have enabled
liquefied nitrogen to be stored and transported
over longer time periods with losses reduced
to 2% per day or less.
Liquid nitrogen can easily be converted to
a solid by placing it in a vacuum chamber
pumped by a rotary vacuum pump. Liquid nitrogen
freezes at 63 K. Despite its reputation,
liquid nitrogen's efficiency as a coolant
is limited by the fact that it boils immediately
on contact with a warmer object, enveloping
the object in insulating nitrogen gas. This
effect, known as the Leidenfrost effect, 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
rather than liquid nitrogen alone.
Nitrogen was first liquefied at the Jagiellonian
University on 15 April 1883 by Polish physicists,
Zygmunt Wróblewski and Karol Olszewski.
Uses
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 cryotherapy for removing unsightly or potentially
malignant skin lesions such as warts and actinic
keratosis
to store cells at low temperature for laboratory
work
in cryogenics
in a Cryophorus to demonstrate rapid freezing
by evaporation
as a backup nitrogen source in hypoxic air
fire prevention systems
as a source of very dry nitrogen gas
for the immersion, freezing, and transportation
of food products
for the cryopreservation of blood, reproductive
cells, and other biological samples and materials
to preserve tissue samples from surgical excisions
for future studies
as a method of freezing water and oil pipes
in order to work on them in situations where
a valve is not available to block fluid flow
to the work area, method known as "ice plug"
– nowadays replaced by electrical heat pumps
in the process of promession, a way to dispose
of the dead
for cryonic preservation in hopes of future
reanimation.
to shrink-weld machinery parts together
as a coolant
for CCD cameras in astronomy
for a high-temperature superconductor to a
temperature sufficient to achieve superconductivity
for vacuum pump traps and in controlled-evaporation
processes in chemistry.
to increase the sensitivity of infrared homing
seeker heads of missiles such as the Strela
3
to temporarily shrink mechanical components
during machine assembly and allow improved
interference fits
for computers and extreme overclocking
for simulation of space background in vacuum
chamber during spacecraft thermal testing
in food preparation, such as for making ultra-smooth
ice cream. See also molecular gastronomy.
in container inerting and pressurisation by
injecting a controlled amount of liquid nitrogen
just prior to sealing or capping.
as a cosmetic novelty giving a smoky, bubbling
"cauldron effect" to drinks. See liquid nitrogen
cocktail.
as an energy storage medium.
branding cattle.
Culinary use of liquid nitrogen
The culinary use of liquid nitrogen is mentioned
in an 1890 recipe book titled Fancy Ices authored
by Mrs. Agnes Marshall, but has been employed
in more recent times by restaurants in the
preparation of frozen desserts, such as ice
cream, which can be created within moments
at the table because of the speed at which
it cools food. The rapidity of chilling also
leads to the formation of smaller ice crystals,
which provides the dessert with a smoother
texture. The technique is employed by chef
Heston Blumenthal who has used it at his restaurant,
The Fat Duck to create frozen dishes such
as egg and bacon ice cream. Liquid nitrogen
has also become popular in the preparation
of cocktails because it can be used to quickly
chill glasses or freeze ingredients. It is
also added to drinks to create a smoky effect,
which occurs as tiny droplets of the liquid
nitrogen come into contact with the surrounding
air, condensing the vapour that is naturally
present.
Safety
Because the liquid-to-gas expansion ratio
of nitrogen is 1:694 at 20 °C, a tremendous
amount of force can be generated if liquid
nitrogen is rapidly vaporized. In an incident
in 2006 at Texas A&M University, the pressure-relief
devices of a tank of liquid nitrogen were
malfunctioning and later sealed. As a result
of the subsequent pressure buildup, the tank
failed catastrophically. The force of the
explosion was sufficient to propel the tank
through the ceiling immediately above it,
shatter a reinforced concrete beam immediately
below it, and blow the walls of the lab 0.1
-0.2m off their foundations.
Because of its extremely low temperature,
careless handling of liquid nitrogen may result
in cold burns.
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 odorless, colorless, and
tasteless and may produce asphyxia without
any sensation or prior warning. A laboratory
assistant died in Scotland in 1999, apparently
from asphyxiation caused by liquid nitrogen
spilled in a basement storage room. In 2012,
a young woman in England had her stomach removed
after ingesting a cocktail made with liquid
nitrogen.
Oxygen sensors are sometimes used as a safety
precaution when working with liquid nitrogen
to alert workers of gas spills into a confined
space.
Vessels containing liquid nitrogen can condense
oxygen from air. The liquid in such a vessel
becomes increasingly enriched in oxygen as
the nitrogen evaporates, and can cause violent
oxidation of organic material.
Production
Liquid nitrogen is produced commercially from
the cryogenic distillation of liquified air
or from the liquefication of pure nitrogen
derived from air using pressure swing adsorption.
An air compressor is used to compress filtered
air to high pressure; the high-pressure gas
is cooled back to ambient temperature, and
allowed to expand to a low pressure. The expanding
air cools greatly, and oxygen, nitrogen, and
argon are separated by further stages of expansion
and distillation. Small-scale production of
liquid nitrogen is easily achieved using this
principle. Liquid nitrogen may be produced
for direct sale, or as a byproduct of manufacture
of liquid oxygen used for industrial processes
such as steelmaking. Liquid-air plants producing
on the order of tons per day of product started
to be built in the 1930s but became very common
after the Second World War; a large modern
plant may produce 3000 tons/day of liquid
air products.
See also
Industrial gas
Computer cooling
Cryogenic nitrogen plant
Liquid nitrogen vehicle
References
