The cooling capacity of the air separation column is mainly taken by the expander, but in the air separation column, the lowest temperature is not at the outlet of the expander, but at the top of the upper column. This is because after the normal operation of the air separation column, when the saturated liquid produced by the lower column is throttled to the upper column, the pressure is reduced, part of the gas is gasified, and the temperature is also reduced to the saturation temperature corresponding to the pressure of the upper column. In addition, the liquid oxygen at the bottom of the upper tower vaporizes and exchanges heat and mass with the liquid on the tray. The nitrogen component volatilizes and the gas temperature decreases continuously. When it reaches the top of the upper tower, it is saturated pure nitrogen.
