Air - chemistry. 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 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 transistors. 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-thannormal 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 steampowered 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. Contributed By: Dennis L. Derr Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. 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