Air separation units

Lean alloy steel is often used in special pressure-bearing equipment. The alloying elements often added include manganese, silicon, chromium, nickel, molybdenum, vanadium, boron and rare earth elements. Most of silicon and manganese melt into ferrite, only a small amount into cementite, which are commonly used strengthening elements in lean alloy steel. The amount of chromium and nickel is generally small, and the main role of chromium and nickel is to increase the undercooling degree of austenite and refine the microstructure. Part of chromium is dissolved in ferrite and some in cementite, which can improve the stability of cementite. Molybdenum is dissolved in ferrite slightly, and has the effect of solid solution strengthening. It is a strong carbide forming element, which can improve the high temperature strength of steel and improve the temper brittleness.

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.