Air is a mixture of aeriform substances (gases and vapors) that constitutes the lowest part of the earth’s atmosphere. It is essential for the life of most animal and vegetable organisms, in particular for human life, so its safeguard is fundamental and is regulated by specific legislative norms. This gaseous mixture finds many applications in industry and daily use, particularly in the form of compressed air (i.e. subjected to pressure) and liquid air.

The word air is derived from the Latin āera, nominative āer, from the Greek ἀήρ, aér, of uncertain etymology, probably related by an awer root. The word is associated in the various Indo-European languages with wind, light, and sky.

In the proto-scientific context of the philosophical tradition, air is one of the four primary elements (air, water, earth and fire). In addition to being often personified as a deity in various religions (the Babylonian god Enlil; the god Shu in Egypt), air was also considered, especially in Hellenistic culture, as the seat of souls. For Anaximenes, air is the first principle of the universe and the force that animates the world.

Air composition

The composition of air varies with altitude. For a fixed altitude, the ratio of the amount of nitrogen to the amount of oxygen in the air remains nearly constant due to the balance between the consumption and the continuous supply of these elements associated with the oxygen cycle and the nitrogen cycle; in contrast, the concentrations of water vapor and carbon dioxide are variable. For this reason, the properties of air deprived of water vapor are often referred to as “dry air,” while otherwise it is referred to as “moist air.”

Dry air on the ground is composed of approximately 78.09% nitrogen (N2), 20.9% oxygen (O2), 0.93% argon (Ar) and 0.04% carbon dioxide (CO2), plus other components in smaller quantities, including suspended solid particles, which constitute the so-called “atmospheric dust”.

Humid air is an aeriform mixture that contains dry air and water vapor; the maximum amount of water vapor contained, at equilibrium, is that amount which realizes a partial pressure equal to the vapor pressure: in this case we speak of saturation, which corresponds to a relative humidity of 100%. Absolute humidity is measured in grams (of water vapor) per kilogram (of dry air). Relative humidity, which is often the quantity of interest, is the ratio of the actual absolute humidity to the absolute humidity under saturated conditions at that temperature and pressure.

The rate of carbon dioxide is highly variable depending on the time period considered. In particular, human activities (industry, pollution, combustion and deforestation) have produced in the last century a large increase in this rate, which went from about 280 ppm in 1900 to 315 ppm in 1970 and to over 400 ppm (0.04%) in recent years. The concentration of this component appears to be (together with that of methane and other gases) one of the main causes of the greenhouse effect.

Finally, while the air pressure changes strongly with altitude (it is reduced to 50% at ~5.5 km, 10% at 16 km, and 1% at 32 km), the percentage composition of the main constituents (nitrogen, oxygen, and rare gases) does not change until 80-100 km. This is because there is large-scale turbulence that causes continuous mixing. Above this altitude, solar radiation causes a dissociation of gases, which pass from the molecular to the atomic state, and various chemical reactions vary considerably its composition.

Liquid air

Liquid air is atmospheric air subjected to repeated compression (100÷200 atm), cooling (-194,4 ºC) and expansion, until it is reduced to a liquid state (condensation). It appears as a mobile and colorless liquid similar to water, with very small ice crystals and carbon dioxide in suspension.

Many material bodies (such as plants, rubber, etc.) once immersed in liquid air become rigid and very fragile. It is stored at ordinary pressure in special containers (Dewar vases) and it is mainly used to obtain nitrogen, oxygen and pure noble gases for industrial purposes; it is also used as a refrigerant in high vacuum pumps.

Air liquefaction process

Air liquefaction is an industrial two-column fractional distillation process invented by scientist Carl von Linde (Linde cycle). Liquid air can undergo eventual fractionation in a distillation column to obtain nitrogen, oxygen and pure noble gases for usual industrial purposes. Liquefaction of air can also be achieved at higher temperatures, as long as they are below the critical temperature of -140.6 ºC, operating at higher pressure. Existing industrial processes for the production of liquid air are diverse, but essentially two routes can be followed:

  • valve expansion (isoenthalpic) compressed air is passed through a valve that abruptly lowers its pressure; the air cools and partly condenses; the liquid part is sent to fractionation, the gaseous part goes back into the exchanger and cools the incoming air; it can be subsequently recycled upstream of the compressor.
  • turbine expansion (isoentropic) compressed air is expanded through a turbine recovering work; cooling is higher as well as liquefaction; however dissipative effects of friction and technical complication limit its use; moreover it is not possible to liquefy air inside the turbine to avoid cavitation effects. Turbine expansion is usually used to cool part of the compressed air to be then used in the exchangers (Claude cycle).

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