Membrane Separation (Membrane Separation)

Membrane separation is a biogas upgrading technology that uses bundles of polymeric hollow-fibre membranes to separate methane from CO2 and other impurities based on differential gas permeability through the membrane material. Each gas molecule diffuses through the membrane at a rate determined by its molecular size, polarity, and interaction with the polymer; CO2, H2S, and water vapour permeate roughly 20-60 times faster than methane through commonly used polyimide or cellulose acetate membranes. Raw biogas at 8-16 bar is fed into the lumen of the hollow fibres, methane is retained on the high-pressure side (retentate), and CO2 plus impurities permeate to the low-pressure side (permeate) for venting or further processing.

Modern commercial membrane plants use multi-stage configurations to improve methane purity and reduce methane slip:

• Single-stage: 90-94% methane purity, 5-10% methane slip — economical but loses too much methane for SATAT compliance.
• Two-stage with permeate recycle: 96-98% methane purity, 1-3% methane slip — most common in India.
• Three-stage: 97-99% methane purity, below 1% methane slip — premium configuration.

Typical operating parameters for Indian membrane CBG units:

• Feed pressure: 8-16 bar.
• Electricity demand: 0.25-0.35 kWh per Nm3 biomethane.
• Membrane life: 5-10 years before replacement.
• Capex: 2-4 crore INR per 1,000 Nm3/hr capacity.
• Footprint: smallest among upgrading technologies; the entire unit fits in a 40-foot container.

Pre-treatment is critical: H2S must be reduced to below 10 ppm and water to below 1 ppm before entering the membrane, otherwise polymer degradation rapidly destroys performance. Heavy aromatic hydrocarbons and siloxanes also damage membranes, so activated carbon polishing is typically installed upstream.

Trade-offs against PWS and PSA centre on scale and slip. Membrane systems are the most compact and modular technology — a 1 TPD CBG unit can ship as a single skid — and have no rotating internals beyond compressors. They consume no water and produce no liquid effluent. However, methane slip in two-stage systems remains 2-3% (versus 1-2% for PWS), which can exceed 1 crore INR per year in lost revenue at SATAT prices for a 10 TPD plant. Indian membrane adoption has grown rapidly since 2020 for small-scale and modular CBG plants where compact deployment and zero water demand outweigh the slip penalty.