How Activated Carbon is Used for Carbon Dioxide Adsorption in Pressure Swing Adsorption (PSA) Systems

1. Core Principle: The "Physical Adsorption + PSA Cycle" Mechanism for Activated Carbon Adsorption of CO₂

Activated carbon's adsorption of CO₂ is essentially physical adsorption (dominated by van der Waals forces), requiring no chemical reaction and being reversible—a key factor in its compatibility with the "recyclable" nature of PSA systems. The specific mechanism can be broken down into two points:

1. The inherent advantages of activated carbon in CO₂ adsorption

Activated carbon's adsorption capacity stems from its unique microstructure and surface properties. Its selective adsorption of CO₂ relies primarily on the following two factors:
High surface area and precise pore structure: Activated carbon possesses well-developed micropores (pore diameter <2nm) and mesopores (2-50nm), resulting in a surface area of ​​500-3000 m²/g. CO₂ molecules, with a diameter of approximately 0.33nm, are efficiently captured by micropores (the "molecular sieve effect"). Other gas molecules, such as nitrogen (N₂, diameter 0.36nm) and oxygen (O₂, diameter 0.35nm), are less well-adsorbed in micropores, enabling the selective separation of CO₂.
Surface chemistry plays a supporting role: Activated carbon surfaces typically contain a small number of oxygen-containing groups (such as hydroxyl, carboxyl, and carbonyl). These polar groups form weak electrostatic attractions (hydrogen bonds or dipole-dipole interactions) with the more polar CO₂ molecules, further enhancing their adsorption affinity for CO₂. Non-polar gases such as N₂ and O₂ are less affected by this, further improving separation selectivity.

II. Key Activated Carbon Selection Requirements: Core Indicators for PSA Process Compatibility

Not all activated carbons are suitable for PSA decarbonization. Activated carbon selection must be tailored to the feed gas composition (e.g., CO₂ concentration, impurity content), process parameters (pressure, temperature), and separation objectives (product gas purity, CO₂ recovery). Key selection criteria include:
1. Pore Structure: The Micropore Ratio is Key
Micropore Volume and Specific Surface Area: Activated carbon must have micropore volume accounting for at least 60% of the total pore volume, and a specific surface area >1000 m²/g to ensure a high CO₂ adsorption capacity (typically 0.5-1.5 mmol/g at 25°C and 1 atm).
Pore Size Distribution: The pore size should be concentrated between 0.3-0.5 nm (matching the diameter of CO₂ molecules), avoiding excessive macropores (>5 nm). While macropores facilitate gas diffusion, they rarely adsorb CO₂, reducing the effective adsorption efficiency per unit mass of activated carbon. 2. Adsorption Performance: Selectivity and Kinetic Balance
CO₂/N₂ Selectivity Coefficient: At PSA operating pressure, the activated carbon's CO₂ adsorption capacity must be significantly higher than N₂ (typically 5-10 times that of CO₂), with a selectivity coefficient (CO₂ adsorption capacity / N₂ adsorption capacity) >8 to ensure product gas purity.

Fast Adsorption/Desorption Kinetics: The activated carbon must have a smooth pore structure (mesopores acting as "diffusion channels" to assist micropore adsorption). This ensures that CO₂ reaches adsorption equilibrium quickly (<10 seconds) at high pressures and desorbs quickly at low pressures. This adapts to the short cycle times of the PSA unit (typically only 1-5 minutes per cycle) and increases the unit's throughput.
3. Mechanical and Chemical Stability
High Strength and Low Dust: Due to the high gas flow rates and large pressure fluctuations in PSA units, the activated carbon must possess high compressive strength (>95%) and abrasion resistance to prevent breakage and dust generation that could clog pipelines or contaminate other equipment. Impurity and Temperature Resistance: If the feed gas contains impurities such as water vapor and sulfides (such as H₂S), activated carbon with a hydrophobic surface or modified (such as impregnated with metal oxides) should be selected to prevent impurities from occupying adsorption sites and causing "poisoning." Furthermore, the activated carbon must be resistant to the temperature fluctuations of the PSA process (typically 0-80°C. High temperatures reduce CO₂ adsorption capacity, necessitating process temperature control).

III. Typical Application Scenarios and Process Optimization

CO₂ separation technology using activated carbon PSA units has been widely used in the energy, chemical, and environmental protection sectors. The process design requires tailored adjustments for different scenarios:

1. Typical Application Scenarios
Flue Gas Decarbonization (CCUS): For flue gas from power plants and steel mills (CO₂ concentration of 10%-20%), activated carbon PSA units are used to increase the CO₂ concentration to above 90%. The CO₂ concentration is then subsequently purified for geological storage or chemical feedstock (such as methanol synthesis). Biogas Purification (Biogas): Biogas primarily consists of CH₄ (50%-70%) and CO₂ (30%-50%). By adsorbing CO₂ with activated carbon PSA, the CH₄ purity can be increased to over 95% and then incorporated into the natural gas grid as biogas.

Air Separation Nitrogen/Oxygen Production: Air contains N₂ (78%), O₂ (21%), and CO₂ (0.04%). Activated carbon PSA can selectively adsorb CO₂ and a small amount of O₂, producing high-purity N₂ (>99.99%). Alternatively, it can be combined with molecular sieves to adsorb N₂ for oxygen production.

2. Process Optimization
Multi-tower Series/Parallel: A 3-4-tower design is used to extend the adsorption time and optimize the pressure equalization step, improving CO₂ recovery (up to over 90%) and product gas purity. Composite adsorbent loading: Activated carbon is layered with other adsorbents (such as 13X molecular sieves and MOFs). The activated carbon preferentially adsorbs CO₂ and water vapor, while the molecular sieve further adsorbs residual impurities, improving separation efficiency.
Temperature-switching assisted desorption (TSA-PSA coupling): Low-temperature (such as cold water) or high-temperature (such as hot air) is introduced simultaneously with low-pressure desorption to accelerate CO₂ desorption, reduce flushing gas usage, and lower energy consumption.