How is air 'separated'? A brief discussion on the principles of cryogenic air separation technology

The air that surrounds us everywhere consists primarily of nitrogen (approximately 78%) and oxygen (approximately 21%), with trace amounts of other gases such as argon. In industrial and medical fields, we frequently require pure oxygen for steelmaking, medical applications, or chemical processes; nitrogen as a protective atmosphere; and argon for welding. So how do we separate these tightly mixed gases from the air? The most fundamental and widely applied method is "cryogenic air separation".
This process can be simply understood as "distillation at low temperatures". First, air passes through filters to remove impurities such as dust, then undergoes significant compression by compressors. The compressed air heats up and must be cooled by a refrigeration system. Subsequently, the high-pressure air enters a critical piece of equipment—the main heat exchanger—where it exchanges heat with the separated cryogenic product gases, cooling itself to an extremely low temperature (close to -170°C or below). At this point, the air has transformed into a liquid.
The liquid air is then fed into a device called a "distillation column". Utilising the differing boiling points of nitrogen, oxygen, and argon (oxygen: -183°C, nitrogen: -196°C, argon: -186°C), the lower-boiling nitrogen readily vaporises and rises through the multiple trays of the column. Conversely, the higher-boiling oxygen predominantly flows downward in liquid form. By positioning outlets at different heights, highly pure nitrogen and oxygen, along with argon fractions, can be extracted separately. This vast and intricate system serves as one of the "power sources" for modern heavy chemical industries.