PSA (PSA)

Pressure Swing Adsorption (PSA) is a cyclic gas separation technology that selectively removes CO2, H2S, water vapour, and other impurities from raw biogas by passing the gas through columns packed with solid adsorbents — typically zeolite 13X, carbon molecular sieve, or activated carbon — at elevated pressure, then regenerating the adsorbent at low pressure. The methane molecule passes through largely unimpeded because its kinetic diameter (3.8 Angstrom) is slightly larger than the pore size of the molecular sieve, while CO2 (3.3 Angstrom) and H2S adsorb strongly. This selectivity allows PSA to upgrade raw biogas from 55–65% methane to 96–99% methane in a single pass through 4–6 columns.

The operating cycle has four phases per column. Adsorption at 5–8 bar — feed gas enters the bottom, methane exits the top, CO2 and H2S accumulate in the adsorbent (60–120 seconds). Depressurisation — the column is depressurised in stages, recovering high-pressure methane that displaces the adsorbent and routing it to a parallel column (15–30 seconds). Purge — a small slip-stream of product methane at low pressure desorbs residual CO2 (30–60 seconds). Repressurisation — the column is repressurised with product gas to prepare for the next cycle (15–30 seconds). The vented CO2-rich off-gas contains 1–3% methane that escapes as methane slip, a critical environmental and economic parameter because methane has 28x the GWP of CO2 and represents lost revenue.

For Indian CBG operators, PSA adoption decisions hinge on five trade-offs against water scrubbing (the alternative dominant technology). PSA wins on water consumption (no process water needed, valuable in water-stressed regions like Maharashtra, Karnataka, Rajasthan) and on methane purity (96–99% versus 95–97%). PSA loses on methane slip (1.5–3% versus 0.5–1.5% for water scrubbing) and capital cost per Nm3 (10–20% higher). PSA is also more sensitive to pre-treatment quality — incoming H2S above 200 ppm rapidly poisons the catalyst, so a dedicated H2S removal stage using iron sponge or biological scrubbing is mandatory upstream. Capital cost for a 1,000 Nm3/hr PSA skid is approximately Rs 4–8 crore, with operating cost dominated by power (0.20–0.30 kWh per Nm3) and catalyst replacement every 5–7 years (Rs 25–40 lakh). Operators in the SATAT cohort increasingly select PSA for plants above 4 tonnes per day capacity where water availability is constrained.