The core reason is that the physical properties of impurities will change qualitatively under cryogenic conditions, which will directly threaten the safe and stable operation of the equipment.
The air contains about 0.03% of carbon dioxide, saturated steam and trace hydrocarbons. These impurities seem harmless at room temperature, but they will change rapidly after entering the cryogenic environment from -170 ℃ to -196 ℃. Water will freeze into ice, and carbon dioxide will directly condense into dry ice at -78.5 ℃. Both of them will deposit and block in the narrow channel of plate fin heat exchanger, distillation tower plate and pipeline valves, resulting in a surge in system resistance and a sharp drop in heat exchange efficiency, which will eventually lead to the disorder of distillation conditions and force the emergency shutdown of the unit. More dangerous is acetylene and other hydrocarbons. If they accumulate in liquid oxygen in the condensing evaporator with air, they will easily cause explosion when reaching the critical concentration, which is the most important potential safety hazard of the air separation unit.
The purification system usually adopts the double-layer bed structure of 13X molecular sieve and activated alumina, and uses the principle of temperature and pressure swing adsorption to remove moisture and carbon dioxide in the air below 1ppm, while effectively adsorbing hydrocarbons to stabilize the dew point at -60 ℃ to -75 ℃. The alternate operation design of two towers ensures that when one tower is adsorbed, the other tower is heated, regenerated and cooled to achieve continuous and uninterrupted gas supply.
As the "first line of defense" of the air separation unit, although the purifier is not directly involved in gas separation, it is the prerequisite for the long-term stable operation of the whole equipment, which is directly related to product purity, equipment life and production safety.
