How is the air in the distillation column separated into oxygen and nitrogen?

The rectification column is a device equipped with a multi-layered tray (the working principle of the sieve tray tower and the packed tower). There is a liquid layer of a certain thickness on the tray. The distillation column is generally a two-stage distillation column, which is divided into two parts, an upper column and a lower column.

The compressed air is purged of water, carbon dioxide, and cooled and expanded in a heat exchanger (for medium pressure flow) and sent to the lower part of the lower tower as the ascending gas of the lower tower. Since it contains 21% oxygen, at 0.6 MPa, the corresponding saturation temperature is 100.05K. The liquid nitrogen condensed in the condensing evaporator flows down from the top of the lower column as a reflux liquid. Since the oxygen content is 0.01% to 1%, the saturation temperature at 0.6 MPa is about 96.3K. It can be seen that the rising vapor temperature in the lower part of the distillation column is high, and the temperature of the liquid flowing down from the top of the column is low. The rising gas of the lower tower encounters a liquid lower than its temperature every time a tray passes, the temperature of the gas itself is lowered, and a part of the vapor is continuously condensed into a liquid. Since oxygen is a less volatile component, nitrogen is a volatile component. During the condensation process, oxygen condenses more than nitrogen, and the concentration of nitrogen in the remaining vapor increases. In this way, once and once, after the top of the tower, most of the oxygen in the vapor has been condensed into the liquid, and its nitrogen concentration is as high as 99% or more. This part of the nitrogen is introduced into the condensing evaporator, and after the heat is released, it is completely condensed into liquid nitrogen, and a part of it flows as the reflux of the lower column from the top to the bottom. In the process of flowing down the liquid, each time a tray passes through the vapor with a higher temperature rising below, a part of the liquid is vaporized after the endothermic. In the gasification process, since nitrogen is a volatile component and oxygen is a less volatile component, nitrogen is more evaporated than oxygen, and the oxygen concentration in the remaining liquid is increased. This time, once and once, reaching the bottom of the tower, an air space having an oxygen content of 38% to 40% can be obtained. Therefore, after the distillation of the lower column, the air can be initially separated into an oxygen-rich liquid space containing 38% to 40% of oxygen and liquid nitrogen containing more than 99% nitrogen.

The liquid is then depressurized by throttling and sent to the middle of the upper column for use as a feed for further rectification. The principle is the same as that of the lower column rectification. When the liquid flows down, the nitrogen is more evaporated by multiple partial evaporation, so the oxygen concentration in the downstream liquid is continuously increased, and the oxygen content is 99.2% to 99.6%. Liquid oxygen. The rectification from the liquid-air feed port to the tray on the bottom of the upper column is to increase the concentration of the less volatile component, called the stripping section. This part of the liquid oxygen absorbs heat in the condensing evaporator and evaporates into gaseous oxygen, and its temperature is about 93.7K at 0.14 MPa. A part of the gas and oxygen is taken out as a product, and most of it is used as the ascending gas of the upper tower. During the ascent, part of the vapor condenses and the nitrogen content in the vapor increases. Since there is more oxygen component in the ascending gas at the inlet of the upper liquid in the middle of the upper tower, if it is released, the loss of oxygen is too large, so rectification should be carried out. A portion of the liquid nitrogen containing more than 99% of nitrogen is withdrawn from the condensing evaporator and sent to the top of the upper column. As a reflux liquid, the vapor is further partially condensed, and the reflux liquid is partially evaporated. Among them, oxygen is mostly left in the liquid phase, and more nitrogen is evaporated into the gas phase. When it reaches the top of the upper column, nitrogen gas containing more than 99% of nitrogen can be obtained. From the liquid nitrogen feed port to the liquid-air feed port is to further increase the concentration of low-boiling components (nitrogen) in the vapor, called the rectification section. If a pure nitrogen product is required, it needs to be re-distilled to obtain a pure nitrogen product containing 99.99% nitrogen. This is the process of separating air into oxygen and nitrogen in a rectification column.