| I | INTRODUCTION |
Air, mixture of gases that composes the atmosphere
surrounding Earth. These gases consist primarily of the elements nitrogen,
oxygen, argon, and smaller amounts of hydrogen, carbon dioxide, water vapor,
helium, neon, krypton, xenon, and others. The most important attribute of air is
its life-sustaining property. Human and animal life would not be possible
without oxygen in the atmosphere. In addition to providing life-sustaining
properties, the various atmospheric gases can be isolated from air and used in
industrial and scientific applications, ranging from steelmaking to the
manufacture of semiconductors. This article discusses how atmospheric gases are
isolated and used for industrial and scientific purposes. For more information
about air and the atmosphere, see Meteorology and Atmosphere.
| II | GASES IN THE ATMOSPHERE |
The atmosphere begins at sea level, and its
first layer, the troposphere, extends from 8 to 16 km (5 and 10 mi) from Earth’s
surface. The air in the troposphere consists of the following proportions of
gases: 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent
carbon dioxide, and the remaining 0.07 percent is a mixture of hydrogen, water,
ozone, neon, helium, krypton, xenon, and other trace components. Companies that
isolate gases from air use air from the troposphere, so they produce gases in
these same proportions.
The various atmospheric gases have many
industrial and scientific uses. By far the most commercially important air gases
are nitrogen, oxygen, and argon, each of which has valuable industrial
applications. For example, fertilizers are manufactured from compounds made from
nitrogen gas, steelmaking furnaces are heated with oxygen, and incandescent
light bulbs are filled with argon.
Scientists first isolated oxygen from air in
1774. They did not develop a commercial process for separating air into its
component gases, however, until the turn of the 20th century. German professor
Carl von Linde developed a process known as cryogenic (cold-temperature)
distillation. This process purifies and liquefies air at very cold temperatures.
The liquid air is then boiled to isolate the gases (a process called fractional
distillation). Liquid nitrogen boils at –195.79° C (-320.42° F), argon at
–185.86° C (-302.55° F), and oxygen at –182.96° C (-297.33° F). As the boiling
temperature is increased, nitrogen vaporizes from the liquid air first, followed
by argon, and then oxygen. Modern air-separation plants can isolate samples of
these gases that are up to 99.9999 percent pure.
Today many smaller air-separation plants
(those that produce 200 metric tons or less of oxygen per day) employ
alternative methods to isolate oxygen and nitrogen from air. Some of these
plants use specialized membranes that selectively filter certain air gases.
Others utilize beds of special pellets that selectively adsorb oxygen and
nitrogen from the air (see adsorption).
| III | PURIFYING AIR |
Most larger air-separation plants continue
to use cryogenic distillation to separate air gases. Before pure gases can be
isolated from air, unwanted components such as water vapor, dust, and carbon
dioxide must be removed. First, the air is filtered to remove dust and other
particles. Next, the air is compressed as the first step in liquefying the air.
However, as the air is compressed, the molecules begin striking each other more
frequently, raising the air’s temperature (see see Gases; Kinetic
Energy). To offset the higher temperatures, water heat exchangers cool the air
both during and after compression. As the air cools, most of its water vapor
content condenses into liquid and is removed.
After being compressed, the air passes
through beds of adsorption beads that remove carbon dioxide, the remaining water
vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and
propylene. These compounds all freeze at a higher temperature than do the other
air gases. They must be removed before the air is liquefied or they will freeze
in the column where distillation occurs.
| IV | LIQUEFYING AND SEPARATING AIR |
After filtering the air, a portion of the air
stream is decompressed in a device called a centrifugal expander (which is
basically a compressor that runs in reverse). As the air expands, it loses
kinetic energy (energy resulting from the motion of the molecules), which lowers
its temperature. The air expands and cools until it liquefies at about -190° C
(about -310° F).
After a portion of the air stream is
liquefied, the liquid is fed into the top of a distillation column filled with
perforated trays (or other structured packing assemblies). These trays or
assemblies allow the liquid to trickle down through the column. At the same
time, the gaseous portion of the air stream (the part that is still compressed)
is fed into the bottom of the column. As the gaseous air rises up through the
column, it bubbles up through the liquid trickling down through the trays or
packing. The gas is slightly warmer than the liquid is, so as it rises, it heats
and eventually boils the surrounding liquid.
The gaseous air also cools as it rises up
through the column. The cooling of the gas as it rises creates a temperature
difference along the column. The gas heats the liquid at the bottom of the
column the most, raising it to a temperature higher than that of the liquid at
the top of the column. As the liquid trickles down, it heats up and reaches the
boiling point of nitrogen first. The nitrogen boils off near the top of the
column and quickly rises to the top. Argon has a boiling point between that of
nitrogen and oxygen, so it boils off near the middle of the column. Oxygen has a
higher boiling point than that of argon or nitrogen, so it remains a liquid
until it reaches the bottom of the column, where the temperature is highest,
before boiling away. See also Fractional Distillation.
Krypton, xenon, helium, and neon also
separate from the other gases in the column but remain a mixture because the
temperature of the column is not cold enough to liquefy these gases. If
operators decide to recover these rare gases in the air-separation process and
save them for future use, they withdraw the mixture of these gases from the
column. They can then separate and purify the krypton, xenon, helium, and neon
from the mixture. With the exception of helium, there is little commercial
demand for these gases, so operators usually do not recover them. The majority
of the world’s helium supply is recovered from natural gas by a similar
distillation process.
| V | SHIPMENT AND STORAGE |
Oxygen, nitrogen, and argon are shipped and
stored either as liquids or as compressed gases. As liquids, they are stored in
insulated containers; as compressed gases, they are held in steel cylinders that
are pressurized up to 170 kg/cm2 (2,400 lb/in2). When
recovered, neon, krypton, and xenon are packaged as gases in steel cylinders or
glass flasks. Because industries can obtain helium at lower costs from other
sources, it is generally returned to the atmosphere after the separation
process.
| VI | INDUSTRIAL USES OF THE GASES IN AIR |
Oxygen, nitrogen, argon, neon, krypton, and
xenon are used in making industrial products essential to modern living. These
products include steel, petrochemicals, lighting systems, fertilizers, and
semiconductors (substances used to make the chips in computers, calculators,
televisions, microwave ovens, and many other electronic devices).
| A | Oxygen |
More than half of the oxygen produced in
the United States is used by the steel industry, which injects the gas into
basic oxygen furnaces to heat and produce steel (see Iron and Steel
Manufacture: Basic Oxygen Process). Metalworkers also combine oxygen with
acetylene to produce high-temperature torch flames that cut and weld steel.
Oxygen is also important in the aerospace
industry. Oxygen reacts with fuel, such as hydrogen, burning the fuel and
supplying energy for launching and powering rockets. The oxygen is stored aboard
the rocket as a liquid and converted to gas before reacting with the propellant
fuel ( See also Combustion).
| B | Nitrogen |
About 36 million metric tons of nitrogen
are produced each year in the United States, and about 4 million metric tons are
produced in Canada each year. Nearly a third of the nitrogen produced in the
United States is used as a cryogenic liquid to instantly freeze and preserve the
flavor and moisture content of a wide range of foods, including hamburger and
shrimp. Nitrogen is also used extensively in the chemical industry to produce
ammonia (NH3), which in turn is used to produce urea fertilizers,
nitric acid, and many other important chemical products. During oil drilling,
nitrogen is used to help force petroleum up from underground deposits. Due to
its chemical stability, nitrogen is added to various manufacturing processes to
prevent fires and explosions. For example, manufacturers often blanket highly
flammable petroleum, chemicals, and paint in a protective layer of nitrogen
during processing.
Nitrogen is used in the electronics
industry to flush air from vacuum tubes before the tubes are sealed.
Incandescent lamp bulbs are flushed with nitrogen gas before being filled with a
nitrogen-argon gas mixture. In metalworking operations, nitrogen is used to
control furnace atmospheres during annealing (heating and slowly cooling metal
for strengthening). Metalworkers also use nitrogen to remove dissolved hydrogen
from molten aluminum and to refine scrap aluminum.
| C | Argon |
In contrast to nitrogen, which reacts with
certain metallic elements at higher temperatures, argon is completely unreactive
(see Noble Gases). In addition to being extremely stable, argon is a good
insulator and does not conduct heat well. Because of these properties, argon gas
(in combination with less expensive nitrogen gas) is used to fill incandescent
lamp bulbs. The stable, insulating gas allows bulb filaments to reach higher
temperatures and therefore produce more light without overheating the bulb.
Argon has the unusual ability to ionize, or
become electrically conductive, at much lower voltages than most other gases
can. When ionized, argon emits brightly colored light. As a result, argon is
also used to make brightly colored “neon” display signs and fluorescent tubes
used to light building interiors. Argon is also used in the electronics industry
to produce the highly purified semiconductor metals silicon and germanium, both
of which are used to make transistor (see also Metalloids).
| D | Neon, Krypton, and Xenon |
Like argon, the noble gases neon, krypton,
and xenon have the ability to ionize at relatively low voltages. As a result,
these gases are also used to light “neon” display signs. In addition, the atomic
industry uses neon, krypton, and xenon as the “fill gas” for ionization
chambers. Ionization chambers are containers filled with gas and grids of wires
that scientists use for measuring radiation and for studying subatomic
particles.
| VII | COMPRESSED AIR |
Not all industrial uses of air require it
to be separated into its component gases. Compressed air—plain air that has been
pressurized by squeezing it into a smaller-than-normal volume—is used in many
industrial applications. When air is compressed, the gas molecules collide with
each other more frequently and with more force, producing higher kinetic energy.
The kinetic energy in compressed air can be converted into mechanical energy or
it can be used to produce a powerful air flow or an air cushion. Compressed air
is easily transmitted through pipes and hoses with little loss of energy, so it
can be utilized at a considerable distance from the compressor or pressure
tank.
The first large-scale application of
compressed-air energy occurred in 1871, during the excavation of the Mont Cenis
railroad tunnel through the Alps. Engineers developed a water-wheel-driven air
compressor that powered the rock drills used to dig the tunnel. Before the
invention of air compressors, miners used steam-powered rock drills, but exhaust
steam made working conditions in underground mines unbearable. After the
development of the air compressor, the mining industry began using
compressed-air energy to drill mines. Soon other industries were utilizing
compressed-air energy for a variety of uses.
Modern compressors can pressurize air up to
1,025 kg/cm2 (15,000 lbs/in2). Modern pneumatic
(compressed-air-driven) tools include nail guns, grinders, rotary drills, and
jackhammers. Compressed air drives conveyers that transport grain, powdered
coal, and other materials. Compressed air also powers pneumatic cylinders that
apply the brakes on railroad trains. It is used to furnish the forced draft for
blast furnaces and other combustion processes, to ventilate mines and buildings,
and to operate control equipment in processing plants.
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