5. 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.
6. 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.
7. 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.
8. 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.
9. 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
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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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).
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.
16. 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.
17. In contrast to nitrogen, which reacts with certain
metallic elements at higher temperatures, argon is
completely unreactive . 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 transistors.
18. 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.
19. 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.
