Air

I

Introduction

Air, mixture of gases that composes the atmosphere surrounding Earth. These gases consist primarily of the elements nitrogen, oxygen, and 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.

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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.

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