The reason for the coexistence of gas nitrogen and liquid nitrogen at the top of an air separation tower is that they are in a saturated state where the gaseous and liquid nitrogen can be converted to each other and remain at the same temperature.
In an air separation column, where gas and liquid nitrogen are coexisting in a saturated state, the gaseous and liquid nitrogen have the same temperature, i.e., the saturation temperature. This is because under a certain pressure and temperature, a dynamic balance can be maintained between the liquid and the gas, i.e., the liquid is constantly evaporated into the gas, while the gas is constantly condensed into the liquid, and the amount of both reaches a relatively stable state.
Although the gas nitrogen and liquid nitrogen in saturation state have the same temperature, their energy states are different. To make the temperature has reached the boiling temperature of liquid nitrogen into gas nitrogen, must continue to absorb heat to increase the energy of molecular movement, to overcome the liquid state of the molecules between the larger gravitational force, so that it is transformed into a freer movement of the gas molecules state. Conversely, to turn saturated gas nitrogen into saturated liquid nitrogen at the same temperature, heat must be released to reduce the energy of molecular motion. This energy difference results in the coexistence of gas and liquid nitrogen in the saturated state.
In the actual operation of air separation equipment, the regulation of gas and liquid nitrogen production can be realized by precisely controlling the pressure and temperature in the air separation tower.
