The core task of air separation equipment is to separate air into high-purity oxygen and nitrogen products. Modern air separation units generally adopt the structure of two-stage distillation column rather than single-stage distillation column, which is determined by the thermodynamic characteristics of air separation and engineering economy.
The two-stage distillation column is composed of lower column (pressure tower), upper column (low-pressure tower) and condensing evaporator. Compressed air enters the lower tower for pre separation, oxygen enriched liquid air is obtained at the bottom of the tower, and pure nitrogen is obtained at the top of the tower. The pressure nitrogen of the lower tower enters the condensing evaporator as a heat source to heat the liquid oxygen of the upper tower, which is condensed into liquid nitrogen. One part is used as the reflux liquid of the lower tower, and the other part is sent to the top of the upper tower as the reflux liquid after throttling. The oxygen enriched liquid air is throttled and sent to the middle of the upper tower. After deep rectification, high-purity liquid oxygen is produced at the bottom of the tower and pure nitrogen is produced at the top. This structure realizes the cascade utilization of cooling capacity and pressure energy recovery, so that oxygen and nitrogen can reach high purity and high extraction rate at the same time, and the energy consumption per unit product is significantly reduced.
The single-stage distillation column can only produce a single high-purity product - or pure nitrogen at the top of the tower, or pure oxygen at the bottom of the tower. It is impossible to obtain two high-purity products at the same time. Due to the limitation of gas-liquid balance, the single-stage tower has insufficient separation perfection, low product extraction rate, large loss of effective components and high unit energy consumption, which is only suitable for simple devices with single product and small scale.
