Troubleshooting of Deep Cold Air Separation Equipment

What is the reason why gas nitrogen and liquid nitrogen are in coexistence at the top of an air separation tower?

A user of DEAR air separation reflected that when the air separation plant was put into operation, a large liquid nitrogen storage tank was equipped for emergency backup when the air separation was stopped or export of liquid products. The value of oxygen content in nitrogen product is designed to be less than 3ppm (i.e., < 3 ×10-6) due to the high requirement of users for nitrogen purity. But in fact, the oxygen content of liquid nitrogen in the tanks had been found more than 5ppm. The substandard purity of liquid nitrogen not only affects the export of products, but also directly affects the emergency backup of liquid nitrogen backup system.