Low-temperature air separation equipment generally consists of four main sections: air compression and expansion refrigeration; removal of moisture and impurities from air; heat exchange, cooling, and liquefaction of air; air rectification and separation; and cold recovery and compression of low-temperature products. Different combinations of methods and equipment in each section result in different processes.
(1) Classification by Refrigeration Method
By Working Pressure: High-pressure, medium-pressure, and low-pressure processes.
High-pressure process: Operating pressure reaches 10.0–20.0 MPa. Refrigeration capacity relies entirely on the throttling effect without requiring an expansion machine. It features simple operation and is only suitable for small oxygen generators or liquid nitrogen generators.
Medium-pressure process: Operating pressure ranges from 1.0 to 5.0 MPa. For small air separation units, due to high specific cold loss, a larger specific refrigeration capacity is needed to balance it, thus requiring higher working pressure. The refrigeration capacity mainly depends on the expansion machine, though the throttling effect also contributes significantly.
Low-pressure process: Working pressure is close to the lower column pressure. It is currently the most widely applied process, featuring low specific energy consumption.
By Expansion Machine Type: Piston type, turbo type, and booster-assisted turbo type.
Piston expansion machine: Small expansion volume and low efficiency, used only in some old small-scale units.
Turbo expansion machine: High efficiency and most widely applied. For low-pressure air separation units, since the expanded air enters the upper column for rectification, it is desirable to minimize the expansion volume while meeting refrigeration requirements to improve rectification separation efficiency.
Booster-assisted turbo expansion machine: Utilizes the output work of the expansion machine to drive a booster compressor that compresses the expansion air from the air compressor to a higher pressure before supplying it to the expansion machine, thereby increasing the unit refrigeration capacity and reducing the expansion volume. This is increasingly widely applied in new low-pressure air separation processes.
By Expansion Gas: Air expansion process and nitrogen expansion process.
Expanded air entering the upper column affects rectification.
Nitrogen expansion reduces nitrogen condensation in the main condenser, i.e., decreases the reflux liquid entering the upper column, also affecting upper column rectification.
Both have their respective advantages and disadvantages.
(2) Classification by Purification Method
Freezing Method for Removing Moisture and CO₂: During air cooling, moisture and CO₂ precipitate and freeze inside the heat exchanger channels. After a certain period, the channels are switched, and the frozen impurities are removed by the return waste nitrogen gas. Depending on the heat exchanger type, this is further divided into regenerators and plate-fin switchable heat exchangers. This method involves frequent switching operations, complex startup procedures, high technical requirements, and an operating cycle of approximately one year.
Molecular Sieve Adsorption Purification Process: Before entering the main heat exchanger, air is already purified by adsorbers. The long switching cycle of adsorbers greatly simplifies operation. The pure nitrogen product volume is no longer limited by the return gas flow requirement. The operating cycle can reach two years or more and is currently receiving increasingly wide application.
(3) Classification by Separation Method
Low-temperature air separation relies on the rectification process inside the rectification column.
By Product Type: Processes producing single high-purity product, dual high-purity products, simultaneously extracting argon products, or fully extracting rare gases, etc.
By Rectification Equipment: Sieve tray columns and structured packing columns, etc.
(4) Classification by Product Compression Method
This can be divided into external compression and internal compression of the separation unit.
External compression: A separate product gas compressor is installed, which has no direct impact on the operation of the unit.
Internal compression: Liquid products are compressed by pumps, then reheated, vaporized, and delivered outside the unit. Relatively speaking, internal compression is safer; however, the proper functioning of the liquid pump directly affects unit operation.
