The Air

 

 

What's in the air

The air around us is a mixture of gases, mainly nitrogen and oxygen, but containing much smaller amounts of water vapour, argon, and carbon dioxide, and very small amounts of other gases. Air also contains suspended dust, spores, and bacteria. Because of the action of wind, the percent composition of air varies only slightly with altitude and location. The table indicates the composition of a typical sample of air after all water vapor and suspended particles have been removed.

Manufacture of atmospheric gases

All the gases present in the atmosphere, except carbon dioxide and radon, are manufactured by separating them from air. Carbon dioxide is obtained as a by-product of brewing and by other industrial processes, while radon is collected from the radioactive decay products of certain substances.

If we cool carbon dioxide at atmospheric pressure it turns into not a liquid but a solid. All the other gases in the atmosphere turn into liquids if cooled enough. Their boiling points (in oC at atmospheric pressure) are about


Radon -62
Xenon -108
Krypton -154
Oxygen -182
Argon -186
Nitrogen -196
Neon -246
Helium -269

These gases (except radon) are manufactured by cooling the air until it liquefies and then separating the gases in the liquid air by fractional distillation. Liquid nitrogen is not expensive because industry needs very large quantities of liquid (or gaseous) oxygen, but for every tonne of liquid oxygen produced four tonnes of liquid nitrogen are produced.

First the air is filtered to remove dust and pollen, and treated to remove water vapour and carbon dioxide (which would solidify and block the pipes when the air was cooled). Then the air is compressed to a very high pressure. This makes it very hot and it is cooled with water back to normal temperature. Then the pressure is released and as the air expands it cools to about -200oC. At this temperature the nitrogen, oxygen, argon, krypton, radon and xenon liquefy. This mixture is called liquid air, and we can separate the gases in it by fractional distillation. To make helium and neon we take the gas that remains after all the other gases have liquified and compress it to a very high pressure. We then cool it, first with water and then with liquid nitrogen, and then release the pressure to cool and liquefy it and then fractionally distil it as before.

Helium is present in large quantities in rocks in natural gas deposits.Iit is more economic to extract it than to obtain it by fractional distillation from liquid air.

Water vapour

The amount of water in the air varies tremendously with location, temperature, and time. In deserts and at low temperatures, the content of water vapour can be less than 0.1% by volume. In warm, humid zones, the air may contain over 6% water vapour.

Air contents

Air is the commercial source for many of the gases it contains. It is separated into its components by fractional distillation of liquefied air. Before air is liquefied, water vapor and carbon dioxide are removed, because these substances solidify when cooled and would clog the pipes of the air liquefaction plant. The dry, CO2-free air is compressed to about 200 atmospheres. This compression causes the air to become warm, and the heat is removed by passing the compressed air through radiators. The cooled, compressed air is then allowed to expand rapidly. The rapid expansion causes the air to become cold, so cold that some of it condenses. By the alternate compressing and expanding of air, most of it can be liquefied.

Carbon dioxide

Removed by filtering off the solid CO2 from the liquid air. Carbon dioxide is used in fire extinguishers and fizzy drinks and for many other purposes.

Nitrogen

Nitrogen is widely used to replace air where there might be a risk of explosion, for example when a supertanker is unloading, as the petroleum is taken out of the tanks they are filled with nitrogen. It is also used in food storage units to prevent food going bad. Liquid nitrogen (­196oC ) is widely used to keep things very cold. Liquid nitrogen is very cheap because it is a by-product of the production of liquid oxygen. Nitrogen is obtained from liquid air by distillation at -196°C. The gas obtained by this process is actually a mixture of nitrogen and about 1.25% noble (or "inert") gases, argon, neon, krypton, and xenon. Nitrogen is second only to sulphuric acid in the volume produced by the U.S. chemical industry. Its major uses are as an inert blanketing atmosphere in chemical processing (14%), electronics (15%), and, in liquid form, as a freezing agent (21%). Liquefied nitrogen, because it is very cold, is used extensively to chill materials for preservation, as in freeze-drying of foods, and in manufacturing processes that require low temperatures, such as machining of aluminum.

Noble gases

The lighter noble gas neon is obtained from air. Its boiling point (-246°C) is too low for neon to condense during the liquefaction of air, and neon concentrates in the gas that remains after air is liquefied. (This remaining gas also contains helium, but the major commercial source of helium is natural gas, in which its concentration is much higher than it is in air.) The heavier noble gases argon, krypton, and xenon are obtained by the fractional distillation of liquid air. Under the proper conditions, a fraction containing roughly 60% noble gases, 30% oxygen, and 10% nitrogen can be obtained from liquid air.

Nobel gases purification

Oxygen is removed from the mixture by passing it over hot copper. Oxygen reacts with hot copper to form copper(II) oxide, CuO. The remaining gas is a mixture of noble gases and nitrogen. This mixture is the gas used to fill incandescent light bulbs. Nitrogen is removed from the mixture by passing the mixture over hot magnesium, which reacts with nitrogen to form magnesium nitride, Mg3N2. The remaining gas is a mixture of argon, neon, krypton, and xenon. Because these elements are chemically very unreactive, chemical means cannot be used to separate them. They are separated by adsorbing the liquid mixture onto activated charcoal at very low temperature. As the activated charcoal is warmed slowly, each gas desorbs individually in a particular temperature range. When the temperature is raised to -80°C, the gas that escapes is nearly pure argon. As the temperature is raised to higher temperatures, nearly pure krypton, and then xenon, are released.

Nobel Gases uses

Argon is the most abundant and most used of the noble gases. Its chief uses are in metallurgy, where it provides an inert atmosphere in which hot metals can be worked. Because argon is so very unreactive, it prevents chemical reactions of the very hot metal being welded or forged. Perhaps the most familiar use of the noble gases is in "neon" signs. These are lamps constructed usually from clear, colorless glass tubes filled with a gas which emits light when an electric current passes through it. Pure neon in such a tube produces orange-red light, argon produces blue-green. (Other gases are also used; for example, mercury vapor produces blue light. Other colors are produced by using colored tubes, mixtures of gases, and fluorescent coatings inside the tubes.)

Helium

Helium is much lighter than air and is used in airships because although it is very much more expensive than hydrogen it is non-flammable. Deep sea divers also often breathe a mixture of oxygen and helium to avoid decompression sickness ("the bends"). Such a mixture makes your voice go squeaky. A mixture of helium and air or oxygen is harmless but if you breathed pure helium, for example from a balloon, for more than one short puff you would harm yourself because you need oxygen to live. Liquid helium (-269oC or 4K) is used in very low temperature experiments

Oxygen

Nearly all commercial oxygen (over 95%) is produced by fractional distillation of liquid air. It boils at -183°C. Oxygen is the third highest-volume chemical produced in the U.S., and most of this product is more than 99.5% pure. Oxygen is paramagnetic, that is, it is attracted to a magnet. Liquid oxygen is pale blue. The major commercial uses of oxygen are in metal manufacturing (30%), metal fabricating (33%), and in health services (13%). In the steel industry, oxygen is passed through impure molten iron in a blast furnace to oxidize and remove impurities such as carbon, sulfur, phosphorus, and silicon. Oxygen is also used as the oxidant in torch cutting of steel. In this process, the steel is heated by an oxygen-acetylene flame, and a stream of hot oxygen is directed at the hot steel. The hot steel is oxidized by the hot oxygen and erodes away, severing the steel. Oxygen is also used extensively in the chemical industry, such as in the production of nitric acid, HNO3, from ammonia, NH3.