The distillation column is equipped with multi-layer tower plates (the working principles of the sieve Tower and the packing tower are the same. There is a liquid layer with a certain thickness on the tower plate. The distillation column is usually a two-stage distillation column, which is divided into two parts: the upper tower and the lower tower.
Compressed air is sent to the lower part of the lower tower as the rising air after the moisture and carbon dioxide are cleared and cooled and expanded in the heat exchanger (the medium pressure process. Because it contains 21% oxygen, the corresponding saturation temperature is 100.05 K at 0.6MPa. The liquid nitrogen condensing in the condensing evaporator flows down from the top of the lower tower as the recirculation liquid. Because the oxygen content is 0.01% ~ 1%, the saturation temperature at 0.6MPa is about 96.3 K. Thus it can be seen that the rising steam temperature at the bottom of the distillation tower is high, while the liquid temperature from the top and bottom of the tower is low. When the rising gas of the lower tower passes through a tower plate, it will encounter the liquid whose temperature is lower than it, and the temperature of the gas itself will decrease, and some of the vapors will continuously condense into liquid. Since oxygen is a volatile component and nitrogen is a volatile component, in the process of condensation, oxygen condenses more than nitrogen, so the concentration of nitrogen in the remaining vapor increases. After going on like this once and again, most of the oxygen in the steam has been condensed into the liquid after reaching the top of the tower, and the concentration of nitrogen is as high as 99%. This part of nitrogen is led to the condensing evaporator, and all of it condenses into liquid nitrogen after releasing heat, and part of it flows from top to bottom as the reflux liquid of the lower tower. In the process of liquid flowing down, each tower plate meets the higher-temperature vapour below, and after heat absorption, a part of the liquid will be gasification. In the process of gasification, since nitrogen is volatile and oxygen is volatile, nitrogen evaporates more than oxygen, and the oxygen concentration in the remaining liquid increases to some extent. If we continue like this once and again, we can get the oxygen content of 38% to the bottom of the tower ~ 40% liquid. Therefore, through the rectification of the lower tower, the air can be preliminarily separated into oxygen-containing 38% ~ 40% oxygen-enriched liquid and liquid nitrogen containing more than 99% nitrogen.
Then the liquid air is sent to the middle of the upper tower after throttling and depressurizing, as the raw material for further rectification. Same with the principle of rectifying in the lower tower, when the liquid is downstream, after several partial evaporation, more nitrogen evaporates, so the concentration of oxygen in the downstream liquid increases continuously, reaching the bottom of the upper tower can get oxygen 99.2% ~ 99.6% liquid oxygen. The rectification from the liquid inlet to the tower plate at the bottom of the tower is to improve the concentration of volatile components, which is called distillation section. This part of liquid oxygen absorbs heat in the condensing evaporator and evaporates into oxygen, and its temperature is about 93.7 K at 0.14MPa. Part of the gas and oxygen are introduced as products, and most of them are the rising gas of the upper tower. In the process of rising, part of the steam condenses and the nitrogen content in the steam increases continuously. Since there are more oxygen components in the rising air at the liquid inlet in the middle of the upper tower, if it is released, the loss of oxygen will be too big, so rectification should be carried out again. Part of liquid nitrogen, which contains more than 99% nitrogen, is extracted from the condensing evaporator to reduce flow and then sent to the top of the tower as recirculation fluid. The vapour performs partial condensation for many times, and the recirculation fluid evaporates for many times at the same time. Among them, more oxygen is left in the liquid phase, and more nitrogen is evaporated into the gas phase. When it reaches the top of the tower, more than 99% nitrogen can be obtained. From the liquid nitrogen inlet to the liquid-air inlet is to further improve the concentration of low boiling point component (nitrogen) in the steam, which is called the distillation section. If pure nitrogen products are needed, rectifying again is needed to get pure nitrogen products with 99.99% nitrogen content. This is the process of separating air into oxygen and nitrogen in the distillation column.
