Air Separation Membrane Process Design for Hydrogen Purification & Nitrogen/Oxygen Separation
25 Jul 2025
Air Separation Membrane Process Design for Hydrogen Purification & Nitrogen/Oxygen Separation
Design Objectives
- Feed Gas: Industrial off-gas (e.g., ammonia synthesis purge gas) containing:
- H₂: 65-85%
- N₂: 10-30%
- O₂: 2-8%
- Products:
- High-purity hydrogen (≥99.9%)
- Nitrogen-enriched gas (≥95% N₂)
- Oxygen-enriched gas (≥30% O₂)
- Capacity: 500 Nm³/h
I. Process Core: Three-Stage Membrane Separation System
Utilizes permeation-selective membrane combinations, leveraging differential gas permeation rates (H₂ > CO₂ > He > H₂O > O₂ > Ar > CO > N₂ > CH₄).
graph LR
A[Feed Gas Pretreatment] --> B[Stage-1 H₂ Purification Membrane]
B --> C[Stage-2 N₂/O₂ Enrichment Membrane]
C --> D[Stage-3 O₂ Concentration Membrane *Optional*]
II. Detailed Process Flow
Step 1: Feed Gas Pretreatment System
| Unit | Function | Key Equipment Parameters |
|---|---|---|
| Buffer Vessel | Pressure stabilization (0.8-1.2 MPa) | V=5 m³, design pressure 1.5 MPa |
| Coalescing Filter | Remove droplets/particles (>0.01 μm) | Pressure drop <5 kPa |
| Activated Carbon Adsorber | Remove oil (<0.01 ppm oil content) | Adsorption cycle: 120 h |
| Precision Heater | Temperature control 40±5°C (prevent condensation) | Power: 15 kW |
Step 2: Stage-1 H₂ Purification Membrane Unit
- Membrane Type: Polyimide hollow fiber membrane
-
Operating Parameters:
- Feed pressure: 1.2 MPa
- Permeate pressure: 0.3 MPa
- H₂/N₂ selectivity: ≥80
-
Outputs:
- Permeate gas: H₂ concentration ≥95% (recovery: 85%)
- Retentate gas: N₂/O₂ enriched stream (H₂ <10%)
Step 3: Stage-2 N₂/O₂ Enrichment Membrane Unit
- Membrane Type: Carbon Molecular Sieve (CMS) composite membrane
-
Operating Parameters:
- Feed pressure: 1.0 MPa (from Stage-1 retentate)
- Permeate side: Vacuum (-0.08 MPa, vacuum pump)
-
Separation Path:
graph TB Retentate_Gas -->|Feed| Membrane Membrane -->|Permeate| O2_Enriched[O₂ >30%] Membrane -->|Retentate| N2_Enriched[N₂ >95%]
Step 4: Stage-3 O₂ Concentration Membrane (Optional)
- Technology: O₂/N₂ selective membrane (O₂/N₂ ≈6)
- Input: Stage-2 O₂-enriched stream (30% O₂) → Output O₂: 40-50%
III. Key Equipment Configuration Table
| System Module | Equipment Name | Specifications | Qty |
|---|---|---|---|
| Pretreatment | Stainless steel buffer vessel | 1.5 MPa, 316L material | 1 |
| Electric heated precision filter | 0.01 μm, 40°C constant temp | 2 (1 standby) | |
| Membrane Separation | Stage-1 H₂ membrane module | Surface area 150 m², PRISM® PA | 4 |
| Stage-2 N₂/O₂ membrane module | CMS-7 carbon molecular sieve | 3 | |
| Vacuum System | Liquid ring vacuum pump | Capacity 45 m³/h, ultimate vacuum -95 kPa | 2 |
| Compression System | Oil-free screw compressor | Discharge pressure 1.6 MPa, 55 kW | 1 |
IV. Performance Prediction & Energy Efficiency
| Gas Component | Feed Gas (%) | H₂ Product (%) | N₂-Enriched (%) | O₂-Enriched (%) |
|---|---|---|---|---|
| H₂ | 75 | ≥99.9 | <5 | <1 |
| N₂ | 20 | <0.1 | ≥95 | <65 |
| O₂ | 5 | <0.01 | <5 | ≥30 |
| Specific Energy Consumption: 0.25 kWh/Nm³ H₂ (significantly lower than PSA's 0.4~0.6 kWh/Nm³) |
V. Safety & Control System
-
Interlock Protections:
- Activates N₂ dilution system if O₂ concentration >8%
- Automatic feed cut-off if temperature upstream of membrane >50°C
-
Analyzers:
- Online H₂ purity analyzer (±0.1% accuracy)
- O₂ content alarm (three-stage alerts)
-
DCS Control:
- Real-time adjustment of transmembrane pressure differential
- Automatic switching of active membrane banks based on product purity
VI. Economics
| Item | Cost (10k RMB) |
|---|---|
| Equipment Investment | 380 |
| Installation | 60 |
| Annual Operating Cost | 85 (Electricity: 70%) |
| Payback Period | 2.8 years (at H₂ price: ¥1.8/Nm³) |
Technical Advantages
- Highly Integrated: Single-line H₂/N₂/O₂ tri-separation
- Zero Chemicals: No consumable adsorbents, green process
- Fast Response: Startup/shutdown time <15 min (80% faster than PSA)
Applications: Coal chemical purge gas recovery, electronic-grade gas refining, fuel cell hydrogen supply.
Design Basis: GB/T 3634.1-2018 Hydrogen Quality Standard / ISO 15338 Gas Membrane Separation Testing Specification.
Note: Membrane area and pressure parameters can be adjusted based on specific feed composition and scale. ASPEN process simulation recommended for energy efficiency optimization.