Air Separation Units (ASUs) are essential systems designed to separate oxygen, nitrogen, and trace rare gases from atmospheric air according to industrial demand. As sectors such as steelmaking, chemicals, new energy, electronics, and healthcare increasingly require high-purity gases and reliable on-site supply, air separation technology has evolved from a single process into a diversified set of technical routes. Based on operating principles, application scenarios, and process characteristics, ASUs can be broadly classified into Pressure Swing Adsorption (PSA), cryogenic distillation, membrane separation, and auxiliary purification units such as absorption processes.
Pressure Swing Adsorption (PSA) Air Separation Units
Pressure Swing Adsorption (PSA) technology separates gases by exploiting the differences in adsorption capacity between various gas molecules and specific adsorbents. Through repeated cycles of pressurized adsorption and depressurized desorption, PSA systems achieve effective gas separation.
PSA-based air separation units feature a relatively simple process flow, flexible start-stop capability, and low energy consumption, making them particularly suitable for small to medium-scale nitrogen and oxygen production. PSA nitrogen systems are widely used in electronics manufacturing, food preservation, metal heat treatment, and powder handling, providing nitrogen with purities ranging from 95% to 99.999%.
Although PSA is not designed for ultra-high-purity or very large-scale gas production like cryogenic air separation, it offers significant advantages in equipment investment, operating cost, and ease of maintenance. As a result, PSA units are extensively adopted in small and medium-sized facilities and on-site gas supply installations.
Cryogenic Distillation Air Separation Units
Cryogenic distillation is one of the most widely used and technically mature methods for air separation, particularly suited for large-scale production of high-purity oxygen and nitrogen. In this process, ambient air is compressed, purified, and cooled to near-liquefaction temperatures. Once partially or fully liquefied, the air enters a distillation column, where the differences in boiling points allow oxygen, nitrogen, and argon to be separated step by step.
Cryogenic air separation units can reliably produce oxygen with purities of ≥99.6% and nitrogen with purities up to ≥99.999%. They also enable the recovery of high-purity argon and certain rare gases, making them ideal for steel plants, coal-chemical projects, integrated refining complexes, large ammonia plants, and centralized gas supply operations.
Although cryogenic systems require higher initial investment and place greater demands on power supply, infrastructure, and operational expertise, they offer clear advantages in long-term continuous operation and large-volume gas production. As a result, cryogenic distillation remains the dominant technology route for medium- and large-scale industrial gas projects.
Membrane Separation Air Separation Units
Membrane separation technology relies on the selective permeability of semipermeable membranes to separate gas molecules. Due to differences in molecular size and diffusion characteristics, the membrane material allows certain components to pass through more quickly, producing either oxygen-enriched or nitrogen-enriched gas streams.
Membrane-based air separation units feature a non-cryogenic process, compact system layout, small footprint, and rapid start-up and shutdown. Their modular nature makes them well-suited for skid-mounted or containerized configurations. These units are widely used in pipeline purging, inerting, nitrogen blanketing, and fire protection within the oil and gas industry, as well as in lithium battery manufacturing, chemical processing, and grain storage applications that require medium- to high-purity nitrogen.
It should be noted that membrane separation cannot typically achieve ultra-high gas purities on its own. The technology is most effective for producing nitrogen in the 95%–99.5% purity range or oxygen-enriched streams, and is often integrated upstream of PSA or cryogenic systems as a pre-concentration or energy-saving stage.
Absorption and Other Auxiliary Separation Processes
Absorption processes use liquid solvents to selectively absorb specific components from a gas mixture, achieving separation or purification. In air separation systems, absorption is generally not used as the primary separation method; instead, it plays a key role in impurity removal and gas refining.
Typical applications include the use of chemical solvents to absorb carbon dioxide-preventing CO₂ from freezing and blocking equipment at low temperatures-and the targeted removal of trace impurities such as SO₂, NOx, and certain organic compounds. These steps help improve the operational stability and service life of downstream air separation units.
Absorption processes are commonly integrated with drying, filtration, catalytic oxidation, and other purification steps to form a complete air pre-treatment and tail-gas purification chain. As such, they are essential auxiliary units that ensure long-term stable operation of cryogenic air separation, PSA, and membrane separation systems.
How to Choose the Right Air Separation Unit
Different types of air separation units offer distinct advantages and have specific application boundaries. When selecting an ASU, companies should evaluate the following key factors:
- Production scale and future expansion plans:The required gas volume and the expected capacity growth over the next 5–10 years determine whether a cryogenic ASU, PSA system, or membrane separation unit is more suitable.
- Product purity and supply stability:Applications requiring high purity and minimal fluctuation typically favor cryogenic systems or combined solutions such as "cryogenic + fine purification." For general inerting or combustion-support applications, PSA and membrane units are often sufficient.
- Total cost and energy efficiency:Selection should not rely solely on equipment purchase price. Electricity cost, annual operating hours, maintenance expenses, and labor requirements must all be considered. For long-duration, continuous operation, cryogenic ASUs often offer lower unit gas costs.
- Site conditions and environmental constraints:Available space, power infrastructure, pipeline layout, gas transmission distance, and local environmental compliance requirements all influence the final process configuration.
In practical engineering projects, no single technology can fully meet all requirements. The combined use of cryogenic air separation, PSA, membrane separation, and absorption-based purification has become a common trend. By integrating these processes appropriately, enterprises can ensure gas quality and supply reliability while reducing total lifecycle costs.
Shenger Gas has long specialized in industrial gas equipment and air separation system engineering. Based on the operational needs of industries such as steel, chemicals, new energy, electronics, grain storage, and offshore applications, the company provides integrated solutions covering process design, equipment selection, complete-system manufacturing, installation, commissioning, and long-term operation support. This enables users to achieve an optimal balance between safety, reliability, and overall economic performance.
