Pressure Swing Adsorption (PSA) (PSA)

Pressure Swing Adsorption (PSA) is a gas separation technology that exploits differences in how strongly gases adsorb onto solid porous materials under varying pressure. Raw biogas is fed at 6-10 bar into a column packed with adsorbent — typically carbon molecular sieve (CMS), zeolite 13X, or activated carbon — that selectively binds CO2, H2S, and water vapour while allowing methane to pass through. When the adsorbent saturates, the column is switched to a regeneration cycle: pressure is dropped to near-vacuum, the adsorbed gases desorb and are vented, and the column returns to service. PSA delivers biomethane at 96-98% methane purity and is the second-most-deployed biogas upgrading technology in India after water scrubbing.

A commercial PSA plant uses 4 to 8 adsorbent columns operating in parallel through a sequenced cycle: adsorption, pressure equalisation, blowdown, evacuation/purge, and repressurisation. The cycle time is typically 2-5 minutes per column, with electrically actuated valves orchestrated by a PLC. Vacuum PSA (VPSA) adds a vacuum pump on the desorption side to deepen the regeneration pressure swing, improving methane recovery from 96% to 98-99% and reducing methane slip from 3-4% down to 0.5-1%.

Typical PSA performance characteristics for Indian CBG installations:

• Methane purity: 96-98% (VPSA up to 99%).
• Methane slip: 1-3% (VPSA below 1%).
• Electricity demand: 0.30-0.45 kWh per Nm3 biomethane.
• Adsorbent life: 5-10 years.
• Capex: 2-4 crore INR per 1,000 Nm3/hr capacity (about 30-50% higher than equivalent PWS).

The trade-offs against water scrubbing are favourable in several dimensions: PSA eliminates water consumption entirely (critical in water-stressed regions like Rajasthan, Tamil Nadu, Maharashtra interior), has a smaller footprint (40-60% less floor area), and operates at higher methane purity. The drawbacks are higher capex, higher electricity demand, mandatory H2S pre-removal (typically iron sponge or activated carbon beds upstream) to protect adsorbent life, and more complex maintenance involving valve overhauls every 18-24 months. Indian CBG operators increasingly select PSA for plants under 5 TPD or in water-constrained locations, while PWS remains dominant for 10-30 TPD plants where its operational simplicity and lower capex outweigh its water demand.