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
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 Earthâ€™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 â€“195.79Â°C (-320.42Â°F), argon at â€“185.86Â°C (-302.55Â°F), and oxygen at â€“182.96Â°C (-297.33Â°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
Divisions of the Atmosphere
A relatively thin envelope, the atmosphere consists of four layers of gases. Companies that isolate air gases use air from the troposphere, the densest of these four layers. 02 December 2010Comment