Design of large air separation unit

When the amount of processing air is reduced, the rising steam and reflux liquid in the upper column will be reduced, but the reflux ratio will not change. Therefore, the reduction of processing air in a small range will not affect the purity of oxygen and nitrogen, and the reduction of processing air volume will help to reduce the pressure of the upper and lower columns. However, according to the material balance, the production of oxygen and nitrogen will naturally decrease when the amount of processing air is reduced. If the amount of processing air is too small, there will be leakage on the tray. If the heat and mass exchange of the leaked liquid is not sufficient, the purity of oxygen and nitrogen will be greatly affected, and even the normal production can not be realized. Therefore, the minimum allowable load of distillation column is generally specified.

When the air separation unit is in normal operation, the liquid flows down and can only flow on the tray and flow to the lower tray through the overflow bucket because the supporting force of the rising steam prevents the liquid from flowing down from the small hole of the tray. After shutdown, due to the interruption of rising gas flow, the liquid on the tray can flow from the overflow bucket to the lower tray, and also can flow through the small hole on the tray to the lower tray. This will cause the surface of liquid oxygen and liquid air rise, which may exceed the position of the expansion air outlet pipe, resulting in liquid carrying accident of the expander during restart. Therefore, pay attention to the liquid level position when stopping.