PSA/VPSA (PSA)

PSA stands for Pressure Swing Adsorption and VPSA for Vacuum Pressure Swing Adsorption — two related industrial gas separation technologies that exploit the selective adsorption of gas molecules onto solid adsorbent materials (zeolites, activated carbon, carbon molecular sieves) under pressure, followed by desorption at lower pressure. They are central to biogas upgrading because they can separate CO2 from methane efficiently and at scale, producing biomethane streams with 96–99% methane content suitable for vehicle fuel use and grid injection.

The operating principle is cyclic. In a PSA system, raw biogas at 5–8 bar passes through a column packed with carbon molecular sieve or zeolite that preferentially adsorbs CO2, H2S, and water — methane passes through and is collected as product. When the adsorbent is saturated, the column is depressurised (the 'pressure swing'), releasing the adsorbed gases as a CO2-rich off-gas stream. A second column takes over feed duty during regeneration, and a typical commercial unit cycles through 4–6 columns to maintain continuous production. VPSA operates at lower pressure (1–2 bar feed, vacuum regeneration at -0.6 to -0.9 bar), reducing compression energy at the expense of larger column footprint. The cycle time is typically 60–180 seconds per column.

For Indian CBG plants, PSA/VPSA captures roughly 25–30% of the upgrading market (against ~60% for water scrubbing), particularly at medium-to-large plant scale of 2,000–10,000 Nm3/hr raw biogas. The technology's strengths are high methane purity (>97% routinely, >99% with optimised cycles), low water and chemical consumption (no consumables beyond catalyst replacement every 5–7 years), and compact footprint compared with water scrubbers. The weaknesses are methane slip of 1.5–3% (versus 0.5–1.5% for water scrubbing) and significant power consumption of 0.20–0.30 kWh per Nm3 raw biogas processed. Indian OEMs and EPC contractors offering PSA include Pentair, Ecospray, Greenlane, and several Indian players — capital cost is approximately Rs 4–8 crore for a 1,000 Nm3/hr upgrading skid. The technology choice between PSA and water scrubbing typically hinges on water availability at site (water scrubbing needs 0.5–2 m3 of clean water per Nm3 biogas) and the methane slip penalty under the operator's offtake contract.