The principle of separating air into oxygen and nitrogen in an air separation distillation column

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A distillation tower is a device that uses distillation to separate various components. Thereby obtaining equipment for high-purity components. After being cooled to a temperature close to the liquefaction point, the air is fed into the lower section of the distillation tower. There, it comes into full contact with the cooler reflux liquid from bottom to top, allowing heat to be transferred and causing part of the air to condense into a liquid. Since oxygen is a non-volatile component while nitrogen is volatile, during the condensation process, more oxygen condenses than nitrogen, thereby increasing the purity of nitrogen in the gas. At the same time, when the gas condenses, latent heat of condensation is released, causing part of the returning liquid to vaporize. Because nitrogen is a volatile component. Therefore, more nitrogen than oxygen evaporates, increasing the oxygen purity in the liquid. In this way, the gas exchanges heat and mass with the reflux liquid on each tray, moving from bottom to top; with each tray passed, the purity of nitrogen in the gas increases. By the time the gas reaches the top of the tower, most of the oxygen has been condensed into the liquid, resulting in a nitrogen purity of 99.999% in the gas phase. A portion of the nitrogen enters the condensation evaporator, where it is condensed into liquid nitrogen, which serves as the reflux liquid. At the same time, the liquid oxygen at the bottom of the upper column vaporizes and, as rising gas in the upper column, participates in the distillation process there. The oxygen-enriched liquid with an oxygen content of 38–40% obtained at the bottom of the lower column is throttled and then fed into the upper column, where it serves as part of the reflux stream that comes into contact with the rising gas for heat exchange; part of this oxygen-enriched liquid vaporizes. Since oxygen is a non-volatile component while nitrogen is a volatile one, more nitrogen evaporates than oxygen, thereby increasing the purity of liquid oxygen. The liquid undergoes multiple heat and mass transfer interactions with the rising gas from top to bottom, resulting in an increasing oxygen purity in the liquid phase; by the time the liquid reaches the bottom of the upper tower, 99.6% liquid oxygen can be obtained.