Why is the pressure in the lower tower of the air separation equipment higher than that in the upper tower?

What is the operating principle of purification system of air separation equipment?

This project we use nitrogen booster cycle in the design process of oxygen compression fast variable load process. The process in small air separation unit before the air leaves the molecular sieve is the same as the conventional process, and the air after leaving the molecular sieve purification unit is cooled in the main heat exchanger and then enters the pressure tower. The flow of the nitrogen booster cycle is ...