Generally, the air separation tower is predivided in the lower tower (pressure Tower), and is rectified into oxygen-rich liquid and pure nitrogen, and then further rectified in the upper tower (low pressure Tower) to obtain oxygen and nitrogen products. The link connecting the upper and lower towers is the condensing evaporator. It uses the pressure nitrogen of the lower tower to heat the liquid oxygen of the upper tower, making the liquid oxygen evaporate, and at the same time, the gas nitrogen is condensed. The advantage of adopting double-stage distillation column is that it can make the product have a higher extraction rate and make it possible to obtain high-purity oxygen and high-purity nitrogen at the same time.
Using single-stage distillation column can also produce oxygen and nitrogen products, but it can only produce one product respectively. As shown in Figure 50, figure 50a is a single-stage distillation column producing pure nitrogen. It is similar to the lower tower of the two-stage distillation column. Pure nitrogen products can be obtained at the top of the tower, and oxygen-enriched liquid is available at the bottom of the tower. Nitrogen condenses in the condenser by the low temperature liquid after the oxygen-enriched liquid flow-saving, which serves as the recirculation liquid needed for rectification. At the same time, oxygen-enriched liquid is heated and evaporated as a byproduct. It also contains a large amount of nitrogen components (about 60%) which cannot be extracted as products. Figure 50b is a single-stage tower for producing pure oxygen. After the air is liquefied, it will flow to the top of the tower as recirculation fluid, and high purity liquid oxygen can be obtained at the bottom of the tower. However, due to the high oxygen content in the liquid sprayed on the top of the tower, the steam discharged from the top cannot be pure nitrogen, which contains about 7% oxygen, that is, one third of the oxygen is lost because it cannot be extracted as a product.
Thus it can be seen that the air separation of single-stage distillation column is not perfect, and it is rarely used in practice. But its structure is simple, and there are also small air separation plants used to produce a single product.
