Small air separation unit instrument grounding

Why do gas nitrogen and liquid nitrogen coexist at the top of the distillation tower of an air separation plant

The amount of liquid nitrogen removed from the lower column of the air separation unit will affect the purity of the liquid nitrogen in the lower column and the oxygen enriched liquid air at the bottom of the column. When the liquid nitrogen removal amount of the lower column increases, the reflux liquid participating in the heat and mass exchange of the lower column will be reduced, the temperature rise of the rising steam on the column plate will be correspondingly reduced, the condensation of the rising steam will not be sufficient, the oxygen component will be condensed less, the purity of nitrogen at the column top will be reduced, and the purity of liquid nitrogen will be naturally reduced. At the same time, the temperature rise of the downflow liquid will increase, the liquid evaporation will be more sufficient, more nitrogen components will be evaporated to the top of the column, and the purity of the liquid air at the bottom of the column will be improved accordingly.