We all know that air surrounds us on Earth. It is impossible to understand life without air. Now the question arises, is this a mixed substance or a pure substance? Air used to be assumed to be a pure substance, but it was later discovered that air is a gaseous mixture. This gas mixture mainly consists of nitrogen (78%), oxygen (21%) and argon (0.9%), and the remaining 0.1% includes carbon dioxide, neon, helium, krypton and xenon.

Air Separation

Air separation is the most common process used to extract the main components of atmospheric air.

Air Separation Method

In general, air separation is divided into two categories:

Cryogenic systems

Cryogenic systems: Cryogenic air separation technology uses the difference in the boiling points of gases to separate them.

The cryogenic process was first introduced by Carl von Linde in 1895 and developed by George Cloud in the 1900s to produce oxygen on a small scale to meet the needs of various industrial processes such as welding, cutting and as a medical gas. Cryogenic air separation began on an industrial scale at the beginning of the 20th century, making the development of metallurgy and other branches of industry highly dependent on the availability of oxygen, nitrogen, and eventually argon. Air Separation System (ASU) is known for its high quality products, large capacities and reliability. Despite other emerging technologies, cryogenic air separation technology is used as the most common and standard technology to produce high-purity gas-liquid products. In this method, products are produced in liquid and gas form. This separation method includes various processes, which include:

first stage:

Ambient air is compressed using a multi-stage turbo compressor with internal coolers. Dust particles are removed using a mechanical air filter as the air enters the compressor.

second stage:

The second stage includes the removal of impurities, especially the remaining water vapor, carbon dioxide (CO2). These components are removed to meet product quality specifications and before air enters the distillation section of the plant. There are two basic methods for removing water vapor and CO2.

Most new air separation plants use a molecular sieve pretreatment unit to remove water vapor and CO2 from the incoming air. Reverse converters for steam and CO2 removal are more cost-effective for smaller plants.

third stage:

Counterflow heat exchangers cool the process air to a temperature close to the liquid temperature.

fourth stage:

In the distillation process, trays are used to convert air into its components. The main function of the trays is to provide effective contact between the descending liquid and the rising gas.

Hence, the tray provides the basis for cooling and partial condensation of the rising gas, heating and partial vaporization of the descending liquid.


Nitrogen exits the top of the column as a gas and oxygen exits as a liquid at the bottom of the column. A condenser at the top is used to liquefy pure nitrogen gas, and a boiler at the bottom is used to boil oxygen for greater product purity.

Argon can also be separated by withdrawing a stream in the middle of the column at the point where the concentration of argon is higher and transferring it to another column containing almost pure argon.

fifth stage:

The products are usually removed at relatively low pressures, often just over one atmosphere (absolute). In general, the lower the transfer pressure, the higher the efficiency of the separation and purification process.

Production of liquid products:

When liquid products are produced in a cryogenic air separation plant, an additional cooling unit is usually added to the basic air separation plant. This unit is called liquefier.

Non-cryogenic systems:

Air separation is done by non-cryogenic method close to ambient temperature, so the oxygen or nitrogen product is always in gas phase state. The scale of production and purity in non-cryogenic air separation is not the same as the scale and purity that can be achieved with cryogenic air separation. They are designed through different processes such as: surface absorption technology and membrane technology. Non-cryogenic air separation processes use differences in physical properties such as molecular size and mass to produce oxygen and nitrogen of sufficient purity. While argon can be produced only by cryogenically separating air.
Surface Adsorption: Adsorption process technology is based on the ability of some natural and synthetic materials to absorb nitrogen or oxygen. This technology is used to produce nitrogen or oxygen by passing compressed air at several atmospheric pressures through a container containing absorbent materials. Adsorbents are selected based on their absorption characteristics. Special adsorbent materials are used as molecular sieves that preferentially absorb the desired gases.
Membrane: Gas separating membranes are very fine hollow fibers through which clean and dry compressed air is fed. As the gases move through the tubes, a process called selective diffusion (diffusion-adsorption) allows us to separate the gases using Oxygen (O2), water vapor (H2O), and carbon dioxide (CO2) move faster through the membrane tube walls than argon (Ar) and nitrogen (N2). The less diffusible nitrogen and argon gases remain in the fiber tubes longer and can therefore be concentrated as nitrogen gas products.