Air, mixture of gases that composes the atmospheresurrounding Earth. These gases consist primarily of theelements nitrogen, oxygen, argon, and smaller amounts ofhydrogen, carbon dioxide, water vapor, helium, neon,krypton, xenon, and others. The most important attributeof air is its life-sustaining property. Human and animal lifewould not be possible without oxygen in the atmosphere.In addition to providing life-sustaining properties, thevarious atmospheric gases can be isolated from air andused in industrial and scientific applications, ranging fromsteelmaking to the manufacture of semiconductors. Thisarticle discusses how atmospheric gases are isolated andused for industrial and scientific purposes. For moreinformation about air and the atmosphere, seeMeteorology and Atmosphere.
GASES IN THE ATMOSPHERE The atmosphere begins at sea level, and itsfirst layer, the troposphere, extends from 8 to16 km (5 and 10 mi) from Earth’s surface. Theair in the troposphere consists of the followingproportions of gases: 78 percent nitrogen, 21percent oxygen, 0.9 percent argon, 0.03percent carbon dioxide, and the remaining 0.07percent is a mixture of hydrogen, water, ozone,neon, helium, krypton, xenon, and other tracecomponents. Companies that isolate gases fromair use air from the troposphere, so theyproduce gases in these same proportions.
Scientists first isolated oxygen from air in 1774.They did not develop a commercial process forseparating air into its component gases, however,until the turn of the 20th century. German professorCarl von Linde developed a process known ascryogenic (cold-temperature) distillation. Thisprocess purifies and liquefies air at very coldtemperatures. The liquid air is then boiled to isolatethe 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 isincreased, nitrogen vaporizes from the liquid air first,followed by argon, and then oxygen. Modern air-separation plants can isolate samples of thesegases that are up to 99.9999 percent pure.
Scientists first isolated oxygen from air in 1774. They did notdevelop a commercial process for separating air into its componentgases, however, until the turn of the 20th century. Germanprofessor Carl von Linde developed a process known as cryogenic(cold-temperature) distillation. This process purifies and liquefiesair at very cold temperatures. The liquid air is then boiled to isolatethe gases (a process called fractional distillation). Liquid nitrogenboils at –195.79°C (-320.42°F), argon at –185.86°C (-302.55°F), andoxygen at –182.96°C (-297.33°F). As the boiling temperature isincreased, nitrogen vaporizes from the liquid air first, followed byargon, and then oxygen. Modern air-separation plants can isolatesamples of these gases that are up to 99.9999 percent pure. Today many smaller air-separation plants (those thatproduce 200 metric tons or less of oxygen per day) employalternative methods to isolate oxygen and nitrogen from air. Someof these plants use specialized membranes that selectively filtercertain air gases. Others utilize beds of special pellets thatselectively adsorb oxygen and nitrogen from the air.
Most larger air-separation plants continue to usecryogenic distillation to separate air gases. Before pure gasescan be isolated from air, unwanted components such as watervapor, dust, and carbon dioxide must be removed. First, theair is filtered to remove dust and other particles. Next, the airis compressed as the first step in liquefying the air. However,as the air is compressed, the molecules begin striking eachother more frequently, raising the air’s temperature (seeGases; Kinetic Energy). To offset the higher temperatures,water heat exchangers cool the air both during and aftercompression. As the air cools, most of its water vapor contentcondenses into liquid and is removed.
After being compressed, the airpasses through beds of adsorptionbeads that remove carbon dioxide, theremaining water vapor, and moleculesof heavy hydrocarbons, such asacetylene, butane, and propylene.These compounds all freeze at a highertemperature than do the other airgases. They must be removed beforethe air is liquefied or they will freeze inthe column where distillation occurs.
After filtering the air, a portion of the air stream isdecompressed in a device called a centrifugal expander(which is basically a compressor that runs in reverse). Asthe air expands, it loses kinetic energy (energy resultingfrom the motion of the molecules), which lowers itstemperature. The air expands and cools until it liquefies atabout -190°C (about -310°F). After a portion of the air stream is liquefied, the liquidis fed into the top of a distillation column filled withperforated trays (or other structured packing assemblies).These trays or assemblies allow the liquid to trickle downthrough the column. At the same time, the gaseousportion of the air stream (the part that is still compressed)is fed into the bottom of the column. As the gaseous airrises up through the column, it bubbles up through theliquid trickling down through the trays or packing. The gasis slightly warmer than the liquid is, so as it rises, it heatsand eventually boils the surrounding liquid.
The gaseous air also cools as it rises up through thecolumn. The cooling of the gas as it rises creates atemperature difference along the column. The gasheats the liquid at the bottom of the column the most,raising it to a temperature higher than that of the liquidat the top of the column. As the liquid trickles down, itheats up and reaches the boiling point of nitrogen first.The nitrogen boils off near the top of the column andquickly rises to the top. Argon has a boiling pointbetween that of nitrogen and oxygen, so it boils offnear the middle of the column. Oxygen has a higherboiling point than that of argon or nitrogen, so itremains a liquid until it reaches the bottom of thecolumn, where the temperature is highest, beforeboiling away.
Krypton, xenon, helium, and neon also separatefrom the other gases in the column but remain amixture because the temperature of the column isnot cold enough to liquefy these gases. If operatorsdecide to recover these rare gases in the air-separation process and save them for future use,they withdraw the mixture of these gases from thecolumn. They can then separate and purify thekrypton, xenon, helium, and neon from the mixture.With the exception of helium, there is littlecommercial demand for these gases, so operatorsusually do not recover them. The majority of theworld’s helium supply is recovered from naturalgas by a similar distillation process.
Oxygen, nitrogen, and argon are shipped and storedeither as liquids or as compressed gases. As liquids, theyare stored in insulated containers; as compressed gases,they are held in steel cylinders that are pressurized up to170 kg/cm2 (2,400 lb/in2). When recovered, neon,krypton, and xenon are packaged as gases in steelcylinders or glass flasks. Because industries can obtainhelium at lower costs from other sources, it is generallyreturned to the atmosphere after the separation process.
Oxygen, nitrogen, argon, neon, krypton, and xenon areused in making industrial products essential to modern living.These products include steel, petrochemicals, lightingsystems, fertilizers, and semiconductors (substances used tomake the chips in computers, calculators, televisions,microwave ovens, and many other electronic devices). More than half of the oxygen produced in the UnitedStates is used by the steel industry, which injects the gas intobasic oxygen furnaces to heat and produce steel (see Iron andSteel Manufacture: Basic Oxygen Process). Metalworkers alsocombine oxygen with acetylene to produce high-temperaturetorch flames that cut and weld steel.
About 36 million metric tons of nitrogen areproduced each year in the United States, and about 4million metric tons are produced in Canada each year.Nearly a third of the nitrogen produced in the UnitedStates is used as a cryogenic liquid to instantly freezeand preserve the flavor and moisture content of a widerange of foods, including hamburger and shrimp.Nitrogen is also used extensively in the chemicalindustry to produce ammonia (NH3), which in turn isused to produce urea fertilizers, nitric acid, and manyother important chemical products. During oil drilling,nitrogen is used to help force petroleum up fromunderground deposits. Due to its chemical stability,nitrogen is added to various manufacturing processes toprevent fires and explosions. For example,manufacturers often blanket highly flammablepetroleum, chemicals, and paint in a protective layer ofnitrogen during processing.
In contrast to nitrogen, which reacts with certainmetallic elements at higher temperatures, argon iscompletely unreactive . In addition to being extremelystable, argon is a good insulator and does not conductheat well. Because of these properties, argon gas (incombination with less expensive nitrogen gas) is used tofill incandescent lamp bulbs. The stable, insulating gasallows bulb filaments to reach higher temperatures andtherefore produce more light without overheating thebulb. Argon has the unusual ability to ionize, or becomeelectrically conductive, at much lower voltages than mostother gases can. When ionized, argon emits brightlycolored light. As a result, argon is also used to makebrightly colored “neon” display signs and fluorescent tubesused to light building interiors. Argon is also used in theelectronics industry to produce the highly purifiedsemiconductor metals silicon and germanium, both ofwhich are used to make transistors.
Like argon, the noble gases neon,krypton, and xenon have the ability to ionizeat relatively low voltages. As a result, thesegases are also used to light “neon” displaysigns. In addition, the atomic industry usesneon, krypton, and xenon as the “fill gas” forionization chambers. Ionization chambersare containers filled with gas and grids ofwires that scientists use for measuringradiation and for studying subatomicparticles.
Not all industrial uses of air require it to be separatedinto its component gases. Compressed air—plain air that hasbeen pressurized by squeezing it into a smaller-than-normalvolume—is used in many industrial applications. When air iscompressed, the gas molecules collide with each other morefrequently and with more force, producing higher kineticenergy. The kinetic energy in compressed air can beconverted into mechanical energy or it can be used toproduce a powerful air flow or an air cushion. Compressedair is easily transmitted through pipes and hoses with littleloss of energy, so it can be utilized at a considerable distancefrom the compressor or pressure tank.