Requirements of PSA nitrogen generator for carbon molecular sieves
In the PSA (Pressure Swing Adsorption) nitrogen generation system, carbon molecular sieves (CMS) are the core functional materials that determine the nitrogen generation efficiency, energy consumption, and long-term stability. The requirements for CMS in the nitrogen generator can be systematically elaborated from four aspects: selectivity, kinetic performance, mechanical reliability, and operational adaptability.
Essential requirement: "Kinetic selectivity" for O₂/ N₂
PSA nitrogen production does not rely on thermodynamic equilibrium adsorption, but rather on the difference in diffusion rates (kinetic separation):
O₂ (kinetic diameter ≈ 0.346 nm)
N₂ (dynamic diameter ≈ 0.364 nm)
The core features of a high-quality CMS:
The pore diameters are concentrated in the range of 0.35–0.38 nm.
The diffusion rate of O₂ is significantly higher than that of N₂.
The separation was completed under the condition of an adsorption time of only 1–10 seconds.
?? This directly determines:
Nitrogen production capacity of a single tower
Recovery rate (N₂ Recovery)
Can the cycle period be shortened?
2. Key performance indicators of PSA nitrogen generator for CMS
2.1 Nitrogen purity and adsorption rate
Indicators Typical requirements for PSA nitrogen generator
Nitrogen purity 95% – 99.999%
Dynamic adsorption capacity of O₂ High
O₂/N₂ selectivity High (kinetic advantage)
Initial adsorption rate Fast (second-level)
High-purity nitrogen (≥ 99.9%) has extremely strict requirements for the consistency of pore size distribution in CMS.
Aperture is slightly larger → N₂enters synchronously → purity decreases
2.2 Particle Size and Bed Flow Mechanics
Common particle size: 1.0–2.0 mm (or 1.4–1.7 mm)
The requirements characteristics of PSA for CMS particle size:
Too small → High pressure drop and severe powdering
Cannot be too large → Longer diffusion path and poorer kinetic separation
?? 1–2 mm represents the engineering balance point between performance and pressure drop.
2.3 Mechanical Strength and Anti-Fragility Performance
PSA operating characteristics:
High-frequency pressurization / depressurization (10⁴– 10⁷ cycles)
Periodic expansion and contraction of the bed layer
Therefore, CMS must possess:
High compressive strength
Low wear rate (Attrition)
Long-term operation without powdering or caving in.
Typical Requirements:
Compressive strength ≥ 80–120 N per piece
Wear rate ≤ 0.3 - 0.5 wt%
2.4 Reversibility and Lifespan of Adsorption-Desorption
A high-quality CMS should have the following features:
Quick and complete desorption of O₂
Extremely low "residual adsorption"
The performance degrades minimally during long-term operation.
Project Expectations:
Continuous operation for ≥ 5–10 years
Nitrogen production and purity have decreased by less than 5–10%.
3. The "implicit requirements" of the PSA air purifier for the intake conditions (reversing the requirements of CMS)
The requirements of the nitrogen generator for CMS are also reflected in the strict limitations on the quality of the incoming air:
3.1 Moisture
CMS is highly prone to absorbing moisture.
Water will:
Blocked micropores
Significantly reduce the rate of O₂ diffusion
Typical Requirements:
Entry temperature at Talu point ≤ –40 °C
High-end system: ≤ –60 °C
3.2 Oil Content and Organic Matter
The compressor oil mist / VOC will:
Irreversible sealing channels
Resulting in permanent inactivation
Typical Requirements:
Oil content < 0.01 ppm
Must be configured:
Efficient oil removal
Activated carbon pre-treatment
3.3 CO₂ and the Residual Components
CO₂ diffuses slowly in the micropores.
Long-term enrichment affects the stability of the cycle.
General requirements:
CO₂ concentration < 300 ppm
Configure the CO₂ unit if necessary
4. Key focuses of CMS technology corresponding to different nitrogen production targets
Nitrogen production target CMS technology focus
95–98% N₂ High adsorption capacity, low cost
99–99.9% N₂ Narrow pore size distribution, strong kinetic selectivity
≥99.99% N₂ Extremely high pore size consistency, low defect rate
Small modular units Quick adsorption kinetics
Large industrial installations Strength, lifespan, anti-friability
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