The coexistence of liquid nitrogen and gaseous nitrogen in the fractionator of air separation equipment is caused by the saturated state of dynamic balance between the two. In the overhead condenser, after the rising nitrogen is cooled to saturation temperature, part of the gas is condensed into liquid nitrogen, while the remaining gas remains in gaseous state, forming a saturated system with coexistence of gas and liquid.
The boiling point of nitrogen is - 196 ℃. Under the low temperature and high pressure environment at the top of the fractionator, the rising nitrogen drops to the saturation point due to the temperature, the kinetic energy of some molecules is weakened and mutually gathered to form liquid nitrogen; While the other molecule still maintains enough kinetic energy and exists in the form of gas, so as to form gas-liquid two phases in the same space.
The fractionator provides a large surface area for gas-liquid contact through a multilayer tray or packing structure. The liquid nitrogen flows downward along the tray under the action of gravity, while the gaseous nitrogen rises counter-currently. During the contact process, the two continuously exchange heat and mass: the liquid nitrogen absorbs heat and evaporates into gaseous nitrogen, and the gaseous nitrogen releases heat and condenses into liquid nitrogen. When the amount of liquid nitrogen evaporated per unit time is equal to the amount of condensed gas nitrogen, the system reaches dynamic equilibrium, and liquid nitrogen and gas nitrogen coexist stably.
The gas-liquid ratio can be adjusted by accurately controlling the overhead temperature and pressure, so as to produce liquid nitrogen and gas nitrogen products of different purity and meet the requirements of industrial diversification.
