There are three ways of air separation technology: absorption, membrane separation and cryogenic way. So far, the most widely used in industrial filed is the third method. 

Absorption: this way is to take advantage of molecular seive’s adsorption performance to different element and then to reach the goal of separation; this technology is easy in operation, low in cost operation; however, it is difficult to get high purity products with this way and the capacity is limited; 
Membrane: it is to use the membrane penetration techonology. The speed of oxygen and nitrogen passing through the membrane is different, so these two products could be separated. The plant under this way is simple and easy for operation, the investment is small but only 28%~35% rich oxygen air could be reached. Also, the scale for such a plant is suitable for small or middle-size, it is only reasonable for rich oxygen combustion and medical health care industries. 
Cryogenic: this plant is to apply different boiling points in the different contents in air. After a series of process, the air is liquefied. Then, rectification way is adopted. By using this way, it could make the air separates completely, the products are completely purified. The plant could be designed in large size, and the products could be both liquids or gases. Therefore, it has a leading position in industry. Comparing with the tradiditonal separation technology, this seaprated gases could only be realized under the 100K cryogenic temperature. That’s why it is called cryogenic way. 

Steps: There are altogether five main steps for the air separation plant, they are: air compression, air purification, heat exchange, refrigeration and rectification. 

The main equipment are below four parts: 
Air compression, expansion refrigeration; removal of water and impurities in air; air cooling and liquefaction by heat exchange; air rectification, and separation; cold energy recovery and compression of cryogenic products; 

Processes: 
1.Classifcation in working pressure: 
A.According to working pressure, it has three processes: high pressure, middle pressure and low pressure processes; 
B.According to expander types, it has three ways: piston type, turbine type and booster turbine type;
C.According to expansion gases: it includes: air expansion and nitrogen expansion processes. 

2.Classification in purification way: 
A.Freezing way to remove water and CO2;
B.Molecular sieve adorption for puriciation

3.Classification through separation way:

Main Features of cryogenic way air separation technology: 
1.Cryogenic vessels such as heat exchanger, rectifying tower and pipelines are installed in the cold-insulated box and filled with perfect insulation materials to stop heating from surrounds and reduce cold energy loss; Or, the equipment will not able to run;
2.The materials for cryogenic equipment fabrication should be strong enough with good welding and process performance. The three general materials are alluminium alloy, copper alloy and stainless steel. 
3.The impurities with high boling points in air such as water and CO2 etc. Should be removed ahead under normal temperature. Otherwise, they will block the passage ways in the equipment and the equipment are not able to work.
4.The acetylene and hydrocarbons in the air enter into air separation columns and accumulate to some extent. As a result, this will affect safety operation or even cause explosion accidents. Therefore, we must set purification equipment and remove them accordingly. 
5.The closed vessels for storing cryogenic liquids: when there’s heating source from outside, partial cryogenic liquids will absorb heat and vapors, and the pressure will rise up. To avoid over pressure, we must set reliable safety devices. 
6.Nitrogen and argon gases are suffocating gases, we need to set liquids outlet pipelines to lead them outside the room. Gas discharge pipelines should have a certain angle, the discharge nozzle shall not towards platform ladders. 
7.Oxygen gas is a strong accelerant, its discharge pipelines shall not in the plant without ventilation. 

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