A Review Of Air Separation Technology And Its Integration With Energy Conversion Processes

Air separation technology is the core of industrial gas production, and its integrated optimization with energy conversion processes is crucial to improving industrial efficiency and reducing costs. This paper systematically reviews the principles, economics and application scenarios of traditional and emerging air separation technologies (cryogenic distillation, adsorption, membrane separation, ion transport membrane, etc.), focusing on the integration of cryogenic and non-cryogenic technologies in gas turbines, coal gasification combined cycle (IGCC) and syngas production. Through technical comparison and case analysis, the development trend and future research direction of air separation technology in the energy field are revealed.

 

 

 

 

Introduction

Industrial gases (oxygen, nitrogen, argon) play a key role in the energy conversion process, and the selection and integration of their production technology directly affect the economy and efficiency of the facility. This article discusses the following core issues:
Applicable scenarios of different air separation technologies (purity, scale, energy consumption)
Integration strategy of air separation unit (ASU) and downstream processes (such as gasification, combustion, power generation)
Challenges and supplements of emerging technologies (such as ion transport membranes) to traditional cryogenic processes
 

Non-cryogenic air separation technology
 

 

Adsorption method (PSA/VSA)

Principle: Utilize the selective adsorption of nitrogen by zeolite or carbon molecular sieve to achieve oxygen and nitrogen separation through pressure swing cycle.
Process: Compressed air enters the adsorption tower, nitrogen is adsorbed, and oxygen-rich gas (93-95% purity) is discharged as a product; the saturated tower is regenerated by pressure reduction (Figure 1).
Advantages: Fast startup (minutes), modular design suitable for small and medium scale (<150 tons/day).
Limitations: Argon cannot be produced, and the byproduct nitrogen has a high oxygen content and requires additional purification.

 

Chemical absorption method

Case: MOLTOXe molten salt process (Figure 2)
Process: Compressed air reacts with molten salt after pretreatment, oxygen is absorbed and desorbed by heating/depressurization.
Advantages: Low power consumption for air compression, and waste heat from the process can be used.
Challenges: Significant high-temperature corrosion problems, not yet commercialized.
 

Polymer membrane separation

Mechanism: Based on the smaller kinetic diameter of oxygen molecules, selective permeation through membrane materials (Figure 3).
Features: No moving parts, suitable for low-purity scenarios (25-50% oxygen-enriched air), the scale is usually <20 tons/day.
Application: Aquaculture oxygenation, coal mine inerting.

Ion transport membrane (ITM)

Technological breakthrough: High-temperature ceramic membranes achieve separation through oxygen ion conduction (Figure 4), with a purity of more than 99%.
Integration potential: When combined with gas turbines, high-pressure nitrogen byproducts can be directly used for power generation, with a thermal efficiency increase of 15-20%.
Current status: In the pilot stage, material durability needs to be optimized.
 

Cryogenic distillation technology: current status and evolution

 

Process core

Principle: Utilize the difference in boiling points of air components (O₂ -183℃, N₂ -196℃) to achieve separation through a two-stage distillation tower (Figure 5).
Key steps:
Compression and pretreatment: Air is compressed to 6-10 bar, and molecular sieves remove CO₂ and water vapor.
Cryogenic separation: Liquefied air is separated into oxygen (tower bottom) and nitrogen (tower top) in the distillation tower, and argon is recovered through the side line.
Energy recovery: The expander uses pressure drop for refrigeration, and the high-efficiency plate-fin heat exchanger recovers cold.

 

Large-scale and integrated innovation

Single set scale: From 500 tons/day in the to the current 5,000 tons/day (Figure 6), the unit investment cost has been reduced by 40%.
Typical integration cases:
Demkolec IGCC power plant (Netherlands): ASU is fully integrated with gas turbine, air is taken from turbine compressor, nitrogen is injected back into combustion end, NOx emission is reduced by 30%, and net power generation efficiency is increased to 47%.
Rozenburg coal gasification project: independent ASU supplies oxygen, nitrogen is used as inert gas for process safety, and syngas production capacity is maximized.

 

Process optimization

Low pressure (LP) vs high pressure (EP) cycle:
LP cycle: air pressure 65-100 psia, suitable for nitrogen venting scenarios.
EP cycle: pressure > 100 psia, nitrogen can be directly used for process compression, reducing additional power consumption.
Pumping liquid cycle: liquid products are pumped to high pressure to avoid gas compression energy consumption, suitable for scenarios requiring high pressure oxygen (such as coal chemical industry).

Technology comparison and selection guide

Technology Maturity Economic scale (tons/day) Purity (vol.%) Start-up time By-product capacity
Cryogenic distillation Mature >20 ≥99 hours Nitrogen and argon efficient recovery
PSA adsorption Semi-mature <150 93-95 minutes Nitrogen needs to be purified
Membrane separation Semi-mature <20 ≤40 Immediate No
ITM Under development Undetermined ≥99 hours Nitrogen needs to be treated

Selection logic:
Large-scale high-purity demand (such as steel, chemical industry): Prioritize cryogenic distillation, taking into account the value of by-products.
Small and medium-scale flexible scenarios (medical, remote areas): PSA or membrane separation, focusing on rapid deployment and low maintenance.
Future high value-added scenarios: ITM combined with renewable energy, suitable for distributed oxygen production and carbon capture.

 

Integrated technology: the key to improving energy efficiency

 

Thermal integration with gas turbines

Air extraction: extract part of the air from the gas turbine compressor to the ASU to reduce the power consumption of the independent air compressor (Figure 7).
Nitrogen reinjection: high-pressure nitrogen is injected into the combustion chamber to reduce the flame temperature (NOx↓50%), and at the same time act as a diluent to improve fuel utilization (power generation↑10-15%).

 

and coal gasification combined cycle (IGCC)

Case: The Tampa Electric project uses high-pressure ASU, the air pressure matches the gas turbine, and nitrogen is used for synthesis gas cooling, and the overall thermal efficiency of the system is increased to 52%.
Advantages: shared compression equipment, waste heat recovery network, and capital costs are reduced by 15-20%

 

Chemical process integration

Synthesis gas production: ASU oxygen is used for partial oxidation reaction, and the by-product nitrogen is used as a raw material for synthetic ammonia, realizing the "gas-chemical-fertilizer" co-production.
Carbon capture: High-concentration CO₂ produced by oxygen-enriched combustion can be directly sealed to help industrial decarbonization.
Emerging trends and challenges

 

Technological innovation direction

Material breakthrough:
High-performance adsorbents (such as MOFs) improve PSA selectivity, with a purity of more than 97%.
Composite ceramic membranes solve the problem of high-temperature sealing of ITMs, with a target lifespan extended to 50,000 hours.
Digital transformation: AI algorithms optimize ASU operating parameters, and predictive maintenance reduces downtime by 30%.
 

Sustainable development needs

Low-carbon process: Use wind power/photovoltaic-driven electrosorption (E-PSA) to achieve a "green electricity-green oxygen" closed loop.
Waste heat utilization: Use chemical waste heat for TSA regeneration, reducing overall energy consumption by 10-12%.

 

Challenges

Scale bottleneck of non-cryogenic technology: PSA and membrane separation still cannot replace cryogenic distillation in high-purity and high-flow scenarios.
Engineering barriers of ITM: large-scale manufacturing process is immature, and the cost is more than 40% higher than that of low-temperature process.

 

Conclusion

The development of air separation technology presents a pattern of "low temperature dominates large-scale, non-low temperature fills the niche scenarios", and deep integration with energy conversion process is the core of future competition. Cryogenic distillation continues to optimize efficiency through large-scale and heat integration, while non-cryogenic technology shows potential in flexibility and low carbonization. With the advancement of materials science and digital technology, new models driven by hybrid processes (such as cryogenic + ITM) and renewable energy will reshape the industry landscape and provide key support for carbon neutrality goals.