The air is mainly composed of 78.03% nitrogen and 20.93% oxygen and other gases. Air separation is to cool the air to a certain low temperature firstly and liquefy it into liquid air. Reuse two liquids oxygen and nitrogen with different boiling points (At atmospheric pressure, the boiling point of oxygen is - 183.98°C and that of nitrogen is - 195.8°C), separation is carried out in an air fractionator equipped with sieve plates. The air separation column is also called distillation column. Air distillation column can be divided into single-stage distillation column and double-stage distillation column, and single-stage distillation column can only produce one kind of pure product. Two stage distillation column is used to produce high purity nitrogen and oxygen in common air separation unit.
The so-called distillation is a process in which partial evaporation and partial condensation are used repeatedly. After the compressed and cooled liquid air enters the distillation column, the gasification air in the column passes through each tray from bottom to top and contacts with the liquid on the tray, so the oxygen in the gas gradually condenses into liquid. The liquid evaporates into a gas. The nitrogen concentration in the gas increases once each time it passes through the tray. In this way, it will pass through multiple trays (as long as there are enough trays) and it will be located in the upper part of the column. High purity nitrogen with purity of 99.99% or higher is obtained, and a liquid with higher oxygen purity (30-38%) is obtained at the bottom of the column, which is called oxygen enriched air. Oxygen with purity of 99.2-99.8% can be obtained from oxygen enriched air passing through the distillation column at the bottom of the upper column.