Catalyst Selection for Deep Trace CO Removal in Air Separation Units

Air separation units (ASUs) are core equipment for industrial gas production. The carbon monoxide (CO) concentration in ambient air feed typically ranges from 0.5–5 ppm, and can spike above 10 ppm near chemical parks or traffic hubs. If trace CO is not effectively removed, it will continuously concentrate during cryogenic rectification – the top temperature of the nitrogen column is approximately -196°C, and the bottom of the oxygen column is about -183°C. The solubility of CO in liquid oxygen is limited; when the concentration reaches a critical level, solid CO may precipitate and block heat exchanger passages, causing a sharp drop in heat transfer efficiency, increased resistance, and in severe cases, plate‑fin heat exchanger freeze‑up failures and complete plant shutdowns. Moreover, products such as electronic‑grade nitrogen and high‑purity oxygen have extremely stringent limits on CO – in high‑purity nitrogen production, the CO content in the product gas is typically required to be below 1 ppm, and for some ultra‑high‑purity applications, below 0.1 ppm. Catalytic oxidation is the most mature and cost‑effective CO removal technology for air separation prepurifiers (PPUs). Hopcalite (copper‑manganese mixed oxide) catalysts efficiently convert CO to CO₂ at ambient temperature (0–40°C), with a single‑pass conversion typically exceeding 99%. In well‑designed prepurification systems, CO can be reduced from about 1000 ppb to approximately 1.0 ppb, achieving a net removal efficiency of 99.9%. Proper catalyst selection – covering active component system, upstream dehydration, pressure‑drop control, byproduct management, and lifetime matching – is the key to achieving deep CO removal and long‑term safe operation in ASUs.

1. Sources and Hazards of Carbon Monoxide in Air Separation Units

The ambient air drawn into an ASU typically contains 0.5–5 ppm CO, but concentrations can rise above 10 ppm near chemical parks or traffic hubs. Additional sources include high‑temperature cracking of compressor lubricating oil, entrainment of combustion exhaust, and surrounding industrial or vehicular emissions, all of which can elevate the inlet CO level beyond normal ranges. If trace CO is not effectively removed, it poses three major threats to the ASU system.

Cryogenic System Operational Risks

This is the primary concern. The core process of an ASU is cryogenic rectification, operating at extremely low temperatures – the top of the nitrogen column is about -196°C, and the bottom of the oxygen column is about -183°C. At these temperatures, the solubility of CO in liquid oxygen is limited, allowing gradual enrichment. When the CO concentration in liquid oxygen builds up to a critical point, solid CO may precipitate and block heat exchanger passages, leading to a sharp decline in heat transfer efficiency, increased resistance, and in severe cases, plate‑fin heat exchanger freeze‑up failures that force a complete plant shutdown. In addition, adsorption and accumulation of CO on cryogenic heat exchanger surfaces can impair thermal performance.

Failure to Meet High‑Purity Gas Specifications

This is the second major threat. Electronic‑grade nitrogen, high‑purity oxygen, and specialty shielding gases have very strict CO limits. For example, in semiconductor manufacturing, even ppb‑level CO in the protective atmosphere can cause wafer oxidation process instability, degrade thin‑film deposition quality, and result in large‑scale product rejects. In high‑purity nitrogen production, the CO content in the product gas is usually required to be below 1 ppm, and for some ultra‑high‑purity applications, below 0.1 ppm.

Contamination of Downstream Process Equipment

This is the third threat. Adsorbents, membrane separation materials, and precision analytical instruments used in high‑purity applications are sensitive to CO. Insufficient front‑end purification may allow CO to break through subsequent polishing units, shortening equipment life and increasing maintenance costs.

2. Technical Challenges for CO Removal in the PPU

The prepurifier (PPU) is the last line of defense before ambient air enters the cryogenic system. CO removal faces multiple technical challenges:

Low Concentration, High‑Precision Requirement

CO concentration in air is typically 1–10 ppm, but must be reduced to the ppb level to meet process specifications.

Competitive Reaction Interference

The PPU must simultaneously remove H₂, CH₄, and other combustibles, requiring the catalyst to have high selectivity.

Complex Operating Conditions

The PPU operates over a wide temperature range (-40°C to 50°C), and humidity fluctuates during the adsorption cycle, posing continuous challenges to the catalyst's low‑temperature activity and stability.

Long‑Cycle Operation Requirements

Catalyst life must match the molecular sieve regeneration cycle, typically 5–10 years.

3. Technical Principles of Catalytic Oxidation

CO removal is primarily achieved through low‑temperature catalytic oxidation: 2CO + O₂ → 2CO₂.

Hopcalite catalysts are essentially copper‑manganese mixed oxides, composed of active manganese dioxide (MnO₂) and copper oxide (CuO) in a specific ratio, typically MnO₂:CuO = 1:0.8. The reaction does not require elevated temperatures – Hopcalite catalysts work efficiently at ambient temperature (0–40°C) with no external heating. Their activity derives from the reversible redox cycles between Cu²⁺/Cu⁺ and Mn⁴⁺/Mn³⁺. CO first reacts with lattice oxygen on the catalyst surface to form CO₂, and the reduced active sites are then re‑oxidized by gas‑phase O₂, restoring catalytic activity.

In terms of performance, Hopcalite catalysts typically have a specific surface area of 120–220 m²/g and active component content exceeding 80%. Under dry, clean inlet conditions, a single pass through a Hopcalite bed typically achieves CO conversion above 99%. In well‑designed prepurification systems, CO can be reduced from about 1000 ppb to approximately 1.0 ppb, representing a net removal efficiency of 99.9%. The CO₂ generated by catalytic oxidation is subsequently removed by downstream molecular sieves or adsorbents. The combined "catalytic oxidation + CO₂ adsorption" process has become one of the most reliable and cost‑effective CO removal solutions in the ASU and high‑purity gas industry.

4. Key Dimensions of Catalyst Selection

To address the practical pain points of ASU operators, catalyst selection and process design must focus on the following dimensions:

Upstream Dehydration – The Most Critical Prerequisite

Water is a major poison for CO oxidation catalysts. Water molecules compete with CO for active sites on the catalyst surface, leading to poisoning and a significant drop in removal efficiency. Experimental data show that when the relative humidity of the feed gas increases from 30% to 80%, the CO conversion of a copper‑manganese catalyst can decline from above 95% to below 60% within 200 hours. Therefore, in industrial practice, an efficient dehydration device must be installed upstream of the catalyst bed to reduce the relative humidity of the inlet air to below 60%, ensuring the catalyst operates under dry conditions for long‑term stability. If moisture in the feed gas remains consistently high, water molecules will accumulate on the catalyst surface, progressively covering active centers and causing irreversible structural changes, ultimately leading to permanent deactivation – shortening the service life from an expected 5–8 years to less than 2 years. Ignoring dehydration and loading catalyst directly will result in rapid activity decay and drastically shortened service life.

Active Component System

The ASU requires the catalyst to maintain high conversion at ambient/low temperatures (e.g., >99% CO conversion at ≤10°C). Copper‑manganese mixed oxide (Hopcalite‑type) catalysts are the mainstream choice for PPU applications due to their excellent low‑temperature activity. Compared with precious‑metal catalysts (Pt, Pd, which typically operate at 80–200°C), the non‑precious‑metal type offers significant cost advantages and works at ambient temperature.

Pressure‑Drop Control

ASUs process large gas volumes, and the pressure drop across the catalyst bed directly affects blower energy consumption. Monolithic honeycomb structures can reduce pressure drop by more than 50% compared to packed beds, making them suitable for large‑scale ASUs. The honeycomb structure offers very low flow resistance, which is a clear advantage in high‑airflow systems. For throughputs of tens of thousands of Nm³/h, the pressure‑drop difference directly translates into long‑term operating power costs. For example, in a 100,000 Nm³/h class ASU, reducing bed pressure drop from 2 kPa to 1 kPa can save approximately 200,000–300,000 kWh per year in blower electricity.

Byproduct Management

The catalytic oxidation reaction generates trace CO₂, so the downstream molecular sieve must have sufficient adsorption capacity. In addition, if the feed air contains sulfur compounds, olefins, or other impurities, catalyst poisoning may occur. When the H₂S concentration in the feed gas exceeds 0.1 ppm, it is advisable to select a sulfur‑resistant formulation or install a dedicated desulfurization unit upstream to ensure the catalyst's design life. The catalyst solution should be tailored to the specific gas‑source conditions.

Lifetime and Regeneration Cycle Matching

The catalyst life must match the regeneration cycle of the PPU adsorbents (molecular sieves, activated alumina), typically 5–10 years. During selection, attention should be paid to thermal stability and sintering resistance to ensure activity is not degraded by periodic regeneration temperature swings.

5. Integrated Application of Catalysts in the PPU Process

The PPU typically employs a dual‑stage "adsorption + catalysis" purification train:

1. Upstream adsorber: removes H₂O, CO₂, and some hydrocarbons.
2. Catalytic reaction layer: CO removal catalyst bed, where residual O₂ reacts with CO to form CO₂.
3. Downstream adsorber: traps the generated CO₂ to protect downstream cryogenic equipment.

The catalyst bed is usually located after the pretreatment system (dehydration, de‑oiling, dust removal) and before the cryogenic system. This arrangement is based on two engineering principles: first, it ensures that the gas entering the catalyst bed is thoroughly dried (the catalyst is moisture‑sensitive); second, the CO₂ generated by catalysis must be adsorbed downstream to prevent it from entering the cold box.

Engineering Case Study

A 60,000 Nm³/h class ASU at a steel mill experienced CO concentrations of 8–12 ppm in the inlet air during winter heating periods. The plant installed a copper‑manganese catalyst bed with a designed loading volume of 18 m³, bed diameter 3.2 m, bed height 2.4 m, operating space velocity 4,800 h⁻¹, and bed pressure drop of about 1.8 kPa. At inlet temperatures of 10–15°C, the outlet CO concentration was consistently maintained below <5 ppb, and the catalyst operated continuously for 7 years without replacement, well synchronized with the molecular sieve regeneration cycle. For this plant, a single unplanned shutdown due to cold‑box freeze‑up could cost several million RMB. The upstream catalytic oxidation system, at less than 2% of the total equipment investment, effectively mitigated this major operational risk.

Boundary Condition Advisory

When the CO concentration in the ASU feed air consistently exceeds 15 ppm, or the H₂S content is higher than 0.1 ppm, or the relative humidity is persistently above 70% due to geographic location and cannot be reduced below 60% by conventional cold drying, the standard selection scheme described above may face rapid performance degradation. Under such boundary conditions, it is recommended to conduct a comprehensive evaluation of the gas‑source conditions during the catalyst selection phase, and, if necessary, add upstream deep purification units or choose poison‑resistant catalyst formulations to avoid premature deactivation due to improper selection.

6. Summary of Selection Strategy

Based on the above analysis, the catalyst selection strategy for trace CO removal in ASU systems can be summarized as follows:

1. Define operating parameters: Accurately determine the inlet CO concentration, temperature range, humidity level, and impurity profile. If CO concentration consistently exceeds 15 ppm or sulfur compounds exceed 0.1 ppm, a boundary‑condition response plan should be activated.
2. Prioritize dehydration: Catalyst selection must be coordinated with upstream dehydration process design. "Dehydrate first, catalyze later" is the core principle for CO removal in ASUs. Ensure that the relative humidity entering the catalyst bed is consistently below 60%.
3. Match active component: For ambient‑temperature operation, copper‑manganese mixed oxide (Hopcalite‑type) catalysts are the first choice. If the operating temperature is higher or there are special lifetime requirements, precious‑metal types may be considered.
4. Evaluate pressure drop and economics: For large ASUs, prefer monolithic honeycomb structures to reduce long‑term energy consumption. Request pressure‑drop curves under various space velocities from the catalyst supplier as a basis for blower sizing and energy consumption calculations.
5. Verify lifetime matching: Confirm that the catalyst's design life matches the PPU molecular sieve regeneration cycle to avoid additional shutdown losses due to mismatched replacement intervals. Request performance data from the supplier under similar operating conditions as a reference.

7. Conclusion

Deep removal of trace carbon monoxide in air separation units is a critical factor for ensuring safe plant operation and high‑purity gas product quality. Enrichment of CO under cryogenic conditions can lead to heat exchanger freeze‑up and even safety incidents, while ppb‑level CO residues can compromise the yield of downstream precision manufacturing processes such as semiconductor fabrication. Catalytic oxidation offers the comprehensive advantages of ambient‑temperature operation, high conversion efficiency (single‑pass >99%), and no secondary pollution, making it the technology of choice for PPU prepurification. Proper catalyst selection – covering active component system, upstream dehydration, pressure‑drop control, byproduct management, and lifetime matching – combined with sound process design, is essential to achieve long‑term stable and cost‑effective CO removal in ASUs. For ASU operators, establishing a complete technical chain of "condition diagnosis – catalyst selection – process integration – performance monitoring" is the fundamental guarantee for long‑term safe operation.