2.1 Principles and Technical Characteristics of Industrial Oxygen Production Methods

Industrial oxygen production is the process of separating high-concentration oxygen (O₂) from air. It is an indispensable basic technology in modern industry, medicine, and chemical engineering. Its core lies in utilizing the differences in the physical or chemical properties of the various components in air (mainly nitrogen N₂ 78%, oxygen O₂ 21%, argon Ar 0.93%, and other trace gases) for separation. The following is a detailed analysis of the principles, technical characteristics, and applicable scenarios of several mainstream industrial oxygen production methods:

2.1.1 Cryogenic Separation (Deep Cold Air Separation)

Core Principle: Utilizing the significant difference in boiling points of components in air such as nitrogen (N₂, boiling point -195.8°C) and oxygen (O₂, boiling point -183°C). Air that has undergone rigorous purification (removal of water vapor, CO₂, hydrocarbons, etc.) is compressed to high pressure. Through a cascade heat exchanger, the product's coldness is used to cool it to an extremely low temperature (approximately -170°C to -190°C), liquefying it to form "liquid air". Liquid air enters a rectification column system (usually including a high-pressure column and a low-pressure column). Inside the column, multiple gas-liquid equilibrium (rectification) processes occur between the rising vapor and the downward flowing liquid. Because nitrogen has a lower boiling point, it tends to vaporize and rise; oxygen, with a higher boiling point, tends to liquefy and flow downward. Finally, high-purity nitrogen is obtained at the top of the column, and high-purity liquid oxygen or gaseous oxygen is obtained at the bottom of the column.

Main Advantages:

Extremely high oxygen purity (usually ≥99.5%, up to 99.999%).

Huge single-unit production capacity (oxygen output can reach hundreds of thousands of Nm³/h), significant economies of scale.

Simultaneous production of high-purity nitrogen, liquid argon, and rare gases (neon, krypton, xenon, etc.).

Mature and reliable technology, stable operation.

Main Disadvantages:

High initial investment (involving large compressors, expanders, complex heat exchange networks, rectification columns, cryogenic containers, and insulation systems).

Long start-up time (from room temperature to stable liquid oxygen production usually takes several hours to tens of hours).

Relatively high energy consumption (mainly the energy consumption of air compression and cold compensation).

Complex process flow, high operational and maintenance requirements, requiring a professional team.

Large equipment, large footprint.

Typical Applications: Large-scale, ultra-high-purity oxygen demand scenarios, such as large-scale steel smelting (high-furnace oxygen enrichment, converter steelmaking), large-scale chemical synthesis (coal chemical industry, ethylene cracking), electronic-grade gas production, liquid oxygen/liquid nitrogen storage stations.

2.1.2 Pressure Swing Adsorption (PSA)

Core Principle: Utilizing the difference in adsorption capacity of specific adsorbents (mainly zeolite molecular sieves) for different gas molecules at room temperature (preferential adsorption of nitrogen, water vapor, CO₂). Air is pressurized by a compressor and enters an adsorption tower filled with molecular sieves. During the high-pressure adsorption stage, nitrogen and other strongly adsorbed components are captured by the molecular sieves, while oxygen (and argon), which are relatively less easily adsorbed, penetrate the adsorption bed layer and are output as product gas. When the adsorbent is close to saturation, the system switches to a low-pressure (or vacuum) desorption stage, releasing the adsorbed nitrogen and other impurities, regenerating the adsorbent, and preparing for the next adsorption cycle. A dual-tower or multi-tower system is usually used, and the cyclic operation of adsorption, desorption, and pressure equalization steps is achieved through program-controlled valves to continuously produce oxygen.

Main Advantages:

Relatively simple process flow, compact equipment, high degree of modularization.

Room temperature operation, no refrigeration equipment required, fast start-up (a few minutes to half an hour).

Energy consumption is usually lower than cryogenic methods (especially for medium and small scales).

High degree of automation in operation, relatively simple maintenance.

Adjustable oxygen purity (usually between 90%-95% O₂).

Main Disadvantages:

The upper limit of oxygen purity is limited by the selectivity of the adsorbent (argon is difficult to completely remove) and process design, generally lower than cryogenic methods.

Adsorbents have a service life (several years) and need to be replaced periodically, resulting in operating costs.

Product oxygen pressure fluctuation (buffer tank or post-pressurization required).

For large-scale applications (>5000 Nm³/h), the economy may be inferior to cryogenic methods.

Typical Applications: Medium and small-scale industrial oxygen use, such as glass furnaces, non-ferrous metal smelting, wastewater treatment, chemical oxidation processes, oxygen-enriched combustion, aquaculture, and medical institutions (central oxygen supply), etc.

2.1.3 Vacuum Pressure Swing Adsorption (VPSA)

Core Principle: An improved version of PSA technology. The core difference is that in the desorption stage, the pressure is not only reduced to normal pressure, but a vacuum pump is also used to draw the adsorption tower to a negative pressure (usually 0.3-0.6 barabs), more thoroughly desorbing the adsorbed nitrogen and other impurities, significantly improving the regeneration efficiency and utilization rate of the adsorbent. This allows for higher oxygen recovery rates and greater single-unit production capacity at lower adsorption pressures (usually 1.2-1.6 barabs).

Main Advantages (Compared to PSA):

Low adsorption pressure, significantly reduced air compressor power consumption.

More thorough desorption, higher adsorbent utilization efficiency, lower unit energy consumption.

Larger single-unit production capacity (up to tens of thousands of Nm³/h), more suitable for medium to large-scale demand.

Higher oxygen recovery rate.

Main Disadvantages:

The addition of a vacuum pump system slightly increases equipment investment and maintenance costs compared to PSA.

More complex system control logic.

Typical Applications: Medium to large-scale (thousands to tens of thousands of Nm³/h) industrial oxygen applications with purity requirements of 90-95%, such as large glass factories, oxygen-enriched combustion in cement plants, wastewater treatment aeration, pulp bleaching, etc. It is a major competitor to cryogenic methods in the medium-scale market.

2.1.4 Membrane Separation

Core Principle: Utilizing the difference in solubility and diffusion rate of oxygen and nitrogen in specific polymeric membranes (such as polysulfone, polyimide, silicone rubber composite membranes, etc.). Oxygen (O₂) typically has a higher permeation rate. When compressed air flows across the surface of the hollow fiber membrane or flat sheet membrane module, oxygen (O₂), water vapor (H₂O), and carbon dioxide (CO₂) with higher permeation rates preferentially permeate through the membrane wall into the permeate side (low-pressure side) as product gas, while nitrogen (N₂) and argon (Ar) with slower permeation rates remain on the non-permeate side (high-pressure side) as tail gas.

Main Advantages:

The equipment is extremely simple and compact, with no moving parts (except for the air compressor), and operates silently.

Start-up and shutdown are instantaneous. Operation and maintenance are extremely simple and reliable.

Low operating costs (mainly the energy consumption of compressed air).

Main Disadvantages:

Oxygen purity is relatively low (typically between 25%-45% O₂).

Oxygen recovery rate is relatively low (limited by purity requirements).

Membrane materials are susceptible to contamination by oil, solvents, etc., and require high air quality (strict pre-filtration is required).

Increasing purity requires multi-stage series connection or use with PSA, increasing complexity and cost.

Typical applications: Small-scale, low-purity oxygen-enriched applications, such as small burner combustion assistance, healthcare (home or portable oxygen generators), fermentation oxygenation, aquaculture oxygenation, mine rescue equipment, etc.

2.2、Technology Selection and Summary

2.3、Selection Suggestions

1. For ultra-high purity (>99.5%) and large-scale/co-production of gases, cryogenic distillation is the only option.
2. For medium scale (thousands to tens of thousands Nm³/h) and purity requirements of 90-95%, the VPSA method is usually the most competitive in terms of energy consumption and cost.
3. For small and medium scale (hundreds to thousands Nm³/h) and purity requirements of 90-95%, the PSA method is the mainstream mature technology.
4. For small scale, on-site oxygen production, low purity requirements (<45%), and emphasis on simplicity and portability, membrane separation has a clear advantage.
5. If both ultra-high purity oxygen and hydrogen are required and high energy consumption is acceptable, water electrolysis is a solution for specific scenarios.

The final selection should comprehensively consider factors such as oxygen demand, purity requirements, operating pressure, initial investment budget, operating costs (electricity), site conditions, whether co-production of gases is required, and gas supply stability requirements, and conduct a techno-economic assessment. With the advancement of materials science, process control, and energy efficiency technologies, PSA/VPSA continues to expand its share in the medium-purity market, membrane separation technology maintains a firm position in specific low-purity fields, and cryogenic methods remain irreplaceable in large-scale high-purity fields.

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