In the process of distillation in the distillation column of air separation unit, the liquid flows through each layer of packing one by one from bottom to top. When the gas passes through the packing from bottom to top, it contacts with the liquid and produces bubbles, which increases the gas-liquid contact area and makes the heat and mass exchange process efficient. Low boiling point gradually evaporates, high boiling point gradually liquefy, to the top of the column can obtain low boiling point of pure nitrogen, at the bottom of the column to obtain high boiling point oxygen rich liquid air component.