Usually, the air in the lower column is pre separated by the air separation column, which is rectified into liquid air which is rich in oxygen and pure nitrogen, and then further rectified in the upper column to obtain oxygen and nitrogen products. The connection between the upper and lower columns is the condensing evaporator. It uses the pressured nitrogen of the lower column to heat the liquid oxygen in the upper column, so as to evaporate the liquid oxygen while condensing the liquid nitrogen. The advantage of using two-stage distillation column is that the product has high extraction rate, and high purity oxygen and nitrogen can be obtained at the same time.
Oxygen and nitrogen products can also be produced in a single-stage distillation column, but only one product can be produced separately. It is similar to the lower column of two-stage distillation column, in which pure nitrogen product can be obtained at the top of the column and oxygen rich liquid air can be obtained at the bottom of the column. The cryogenic liquid throttled through the oxygen enriched liquid space condenses nitrogen in the condenser as the reflux liquid for distillation. At the same time, oxygen rich liquid air absorbs heat and evaporates, and inhales it as a by-product. It also contains a large amount of nitrogen (about 60%) that cannot be extracted as a product. After the air is liquefied, it is throttled to the top of the column as reflux liquid, and high purity liquid oxygen can be obtained at the bottom of the column. However, due to the high oxygen content in the liquid sprayed on the top of the column, the steam discharged from the top of the column can not be pure nitrogen, which contains about 7% oxygen, that is, one third of the oxygen will not be extracted. Products are lost. It can be seen that the air separation of single-stage distillation column is not perfect, and it is rarely used in practice.
