Cryogenic air separation units can be used in various fields such as chemical industry, new energy, and metallurgy, meeting the needs of industrial production that requires large quantities of high-purity nitrogen. Cryogenic air separation units utilize low-temperature condensation to convert air into a liquid state. Due to the different evaporation temperatures of various gases, high-purity nitrogen, oxygen, and other gases required in large quantities for industrial use can be separated and purified from the air.

Cryogenic Air Separation Process

Raw air is compressed by an air compressor to 0.7–0.85 MPa, then pre-cooled to 5–10°C in a pre-cooling unit, where most of the moisture is separated. The remaining moisture, carbon dioxide, and hydrocarbons are adsorbed and filtered out in a cryogenic purifier. The air is then expanded and refrigerated in an expansion turbine to provide the cooling required for the unit. In the main heat exchanger of the fractionation column, the air exchanges heat with the returning oxygen, nitrogen, and waste nitrogen, cooling it to near liquefaction temperature while reheating the returning gases to ambient temperature. Nitrogen is supercooled in a subcooler before throttling the liquid air and liquid nitrogen. The air undergoes fractional distillation in the distillation column, with product nitrogen obtained at the top of the upper column and product oxygen at the bottom of the upper column.

Power System

This mainly refers to the raw air compressor. The air separation unit produces oxygen, nitrogen, and other products by cryogenically separating air, which is essentially achieved through energy conversion. The energy required for the unit is primarily input by the raw air compressor. Correspondingly, the majority of the total energy consumption for air separation is attributed to the raw air compressor.

Purification System

Composed of the air pre-cooling system (air cooling system) and the molecular sieve purification system (purification system). The compressed raw air has a high temperature, and the air pre-cooling system reduces the air temperature through contact heat exchange while washing away harmful impurities such as acidic substances. The molecular sieve purification system further removes moisture, carbon dioxide, acetylene, propylene, propane, heavy hydrocarbons, nitrous oxide, and other substances harmful to the operation of the air separation unit.

Refrigeration System

Cryogenic air separation units achieve refrigeration through expansion, with the entire unit's refrigeration strictly following the classic refrigeration cycle. However, when referring to the refrigeration system of the air separation unit, it mainly refers to the expansion turbine.

Heat Exchange System

The thermal balance of the cryogenic air separation unit is achieved through the refrigeration system and the heat exchange system. With technological advancements, modern heat exchangers primarily use aluminum plate-fin heat exchangers.

Distillation System:

The core of the cryogenic air separation unit and a critical device for achieving low-temperature separation. It typically employs high- and low-pressure two-stage distillation, mainly consisting of the low-pressure column, medium-pressure column, and condenser-evaporator.

The cryogenic air separation reverse flow expansion process is generally suitable for situations where users have specific pressure requirements for nitrogen products (e.g., above 0.2 MPa). The process is as shown in the diagram. After dust removal, compression, pre-cooling, and purification, the raw air enters the main heat exchanger, where it is cooled by the returning waste nitrogen to saturation temperature with a certain moisture content before entering the bottom of the nitrogen column to participate in distillation. This yields product liquid nitrogen and nitrogen gas at the top of the nitrogen column, while the oxygen-rich liquid air at the bottom of the nitrogen column is throttled and enters the evaporation side of the condenser-evaporator to condense the nitrogen gas at the top of the nitrogen column.

Most of the oxygen-rich air extracted from the top of the condenser-evaporator enters the cold end of the main heat exchanger directly. After being reheated to a certain temperature, it is extracted from the middle and enters the turbo-expander for expansion, providing cooling for the entire pure nitrogen unit. 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, exchanging heat with the forward-flow air. After being reheated to ambient temperature, it is discharged from the cold box, with a portion used for the regeneration of the molecular sieve adsorber and the rest vented. The nitrogen gas drawn from the top of the ammonia column is reheated to ambient temperature in the main heat exchanger and then discharged from the cold box to supply to users.