Adsorption Unit Design for Hydrogen Recovery in Chlor-Alkali Plants

Optimizing PSA Technology for High-Purity Hydrogen Yields

1. Introduction

Hydrogen generated during brine electrolysis in chlor-alkali plants is a valuable by-product. Modern facilities increasingly recover and purify this hydrogen for HCl synthesis, hydrogenation, fuel use, and merchant sales. Pressure Swing Adsorption (PSA) is the industry-preferred technology due to its reliability and ability to achieve ultra-high hydrogen purity.

2. Typical Hydrogen Recovery Scheme

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3. Process Steps

The journey of hydrogen from generation to high-purity product involves several distinct stages:

1. Raw hydrogen from the electrolyzer
2. Dechlorination / scrubbing
3. Cooling and moisture removal
4. PSA purification
5. High-purity hydrogen product
6. PSA tail gas routed to the fuel system

Typical Feed Conditions: H₂ (96–99%), O₂ (trace–1%), moisture saturated, Cl₂/HCl at ppm level.

4. Why PSA is Preferred

PSA is favored over membrane technologies, especially when very high-purity hydrogen is required. Key advantages include:

• Hydrogen Purity: 99.9–99.999%
• Recovery Rate: 75–90%
• No phase change involved
• Modular skid design for easy installation
• Fast startup capabilities and excellent turndown flexibility
• Proven performance in harsh chlor-alkali environments

5. Working Principle

PSA separates gases by preferential adsorption. The fundamental principle relies on pressure dynamics:

• High Pressure: Impurities adsorb onto the bed media.
• Low Pressure: Impurities desorb and are purged.

Hydrogen, being weakly adsorbed by the bed materials, passes through and exits as the purified product. A typical cycle involves: Adsorption → Equalization → Depressurization → Purge → Repressurization.

6. Adsorbent Bed Design

A typical multilayer bed is carefully structured to handle specific impurities sequentially:

• Activated Alumina: Removes moisture and residual HCl.
• Activated Carbon: Removes chlorine traces and hydrocarbons.
• Zeolite (5A/13X): Removes O₂, N₂, CO, and CO₂.

7. Key Design Parameters

Proper sizing and operational parameters dictate the efficiency of the unit.

Typical Operating Ranges

• Adsorption pressure: 12–25 barg
• Desorption pressure: 0.2–1 barg
• Feed temperature: 25–40 °C
• Superficial velocity: 0.1–0.3 m/s
• Cycle time: 4–10 minutes

Performance Benchmarks

8. Critical Pretreatment Requirements

Pretreatment dictates the lifespan of the PSA unit. Inadequate conditioning will result in rapid bed degradation.

• Dechlorination: Must be reduced to <0.1 ppm Cl₂.
• Moisture Removal: Must be conditioned to ≤ −40 °C dew point.
• Feed Temperature Control: Typically maintained below 50 °C.

Even ppm-level chlorine breakthrough can permanently damage the sensitive adsorbent layers, particularly zeolites.

9. Best Practices & Conclusion

Industry best practice includes utilizing multilayer beds, maintaining stable feed pressure, ensuring proper valve sequencing, and integrating online purity monitoring. Most chlor-alkali PSA units utilize 6–10 beds to ensure reliable, continuous operation.

In summary, PSA remains the most reliable solution for hydrogen recovery in chlor-alkali plants. With rigorous pretreatment and optimized cycle design, units routinely achieve 99.9–99.999% purity and 75–90% recovery, enabling plants to maximize the value of their hydrogen by-product while maintaining long adsorbent life and stable plant performance.