Cryogenic air separation equipment can be used in many fields such as chemical industry, new energy, metallurgy, etc., which require a large amount of high-purity nitrogen for industrial production. Cryogenic air separation equipment uses low-temperature condensation to convert air into liquid. Due to the different evaporation temperatures of different gases, they can be separated and purified from the air to produce high-purity nitrogen, oxygen and other gases required by a large number of industries.

▎Cryogenic air separation process

The raw air is compressed to 0.7~0.85MPa by the air compressor, and then the air is precooled to 5~10℃ in the precooling unit to separate most of the water. The remaining water, carbon dioxide and hydrocarbons are filtered out by adsorption in the deep cold purifier. The air is then expanded and refrigerated in the expander to provide the required cooling capacity of the device. The air exchanges heat with the reflux oxygen, nitrogen and dirty nitrogen in the main heat exchanger of the fractionation tower, is cooled to a temperature close to the liquefaction temperature and the reflux oxygen, nitrogen and dirty nitrogen are reheated to the ambient temperature. Nitrogen is subcooled in the subcooler to the liquid air and liquid nitrogen before throttling. The air is distilled and separated in the distillation tower, and the product nitrogen is obtained at the top of the upper tower and the product oxygen is obtained at the bottom of the upper tower.

▎Schematic diagram of nitrogen production by positive expansion of cryogenic air separation

The air positive flow expansion process is generally suitable for occasions where the pressure level of the nitrogen product is not very high (such as less than 0.2MPa). The raw air is divided into two streams after dust removal, compression, precooling and purification: one stream of air enters the main heat exchanger, is cooled to a certain temperature by the refluxed dirty nitrogen, and is extracted from the middle of the main heat exchanger for expansion. After expansion, it enters the bottom of the nitrogen tower to participate in distillation; the other stream of air is cooled to saturation temperature in the main heat exchanger and directly throttled into the nitrogen tower to participate in distillation. In this way, the product nitrogen is obtained at the top of the nitrogen tower, and the oxygen-rich liquid air at the bottom of the nitrogen tower enters the evaporation side of the condenser evaporator after throttling to condense the gas nitrogen at the top of the nitrogen tower. The oxygen-rich air extracted from the top of the condenser evaporator directly enters the cold end of the main heat exchanger, exchanges heat with the positive flow air, and is discharged from the cold box after being reheated to room temperature. Part of it is used for the regeneration of the molecular sieve adsorber, and the rest is vented. The nitrogen discharged from the top of the nitrogen tower is reheated to room temperature through the main heat exchanger and discharged from the cold box to be supplied to users.

▎Schematic diagram of nitrogen production by negative expansion of cryogenic air separation

The cryogenic air separation reflux expansion process is generally applicable to the occasions where the user has certain requirements for the pressure of the nitrogen product (such as higher than 0.2MPa). The raw air enters the main heat exchanger after dust removal, compression, precooling and purification. After being cooled to the saturation temperature and having a certain moisture content by the refluxed dirty nitrogen, it enters the bottom of the nitrogen tower to participate in distillation. In this way, liquid nitrogen and nitrogen gas products are obtained at the top of the nitrogen tower;

Most of the oxygen-rich air drawn from the top of the condenser evaporator directly enters the cold end of the main heat exchanger, is reheated to a certain temperature, is drawn from the middle, and enters the turbine expander for expansion, providing cooling for the entire pure nitrogen equipment. The expanded oxygen-rich air is mixed with another stream of throttled oxygen-rich air and enters the cold end of the main heat exchanger, exchanges heat with the positive air, and is reheated to room temperature before being discharged from the cold box, of which a portion is used for the regeneration of the molecular sieve adsorber, and the rest is vented. The nitrogen drawn from the top of the ammonia tower is reheated to room temperature through the main heat exchanger and then exits the cold box for supply to users.

▎Schematic diagram of nitrogen production by positive expansion of cryogenic air separation