Hydrophobic porous polysulfone membrane contactors with gravity-driven Ca(OH)₂ absorption for low-energy CO₂ removal...

The upgrading of biogas to biomethane requires efficient and energy-conservative removal of carbon dioxide (CO₂) to enhance calorific value and downstream usability. In this study, a gravity-driven hydrophobic polysulfone (PSf) hollow-fiber membrane contactor was systematically evaluated for CO₂ removal from a synthetic CH₄/CO₂ (60/40 v/v) gas mixture under ambient conditions (23 ± 1 °C and 1 atm). Unlike conventional membrane contactor systems that rely on mechanically driven liquid circulation, the present configuration operates without liquid pumping, thereby eliminating liquid-phase energy demand and simplifying system architecture. Comparative experiments were conducted using tap water as a physical absorbent and a low-concentration Ca(OH)₂ solution (0.1 wt%) as a reactive absorbent to assess hydrodynamic, mass transfer, and energy performance under realistic decentralized operating conditions. CO₂ removal efficiencies exceeding 90% were achieved for both absorbents at optimized operating conditions (gas flow rate: 122.5 mL min^-1; liquid flow rate: 800 mL min^-1), with maximum values of 92-93% for tap water and approximately 94% for Ca(OH)₂. The gravity-driven falling-film liquid flow regime promoted effective gas-liquid interfacial contact while minimizing liquid-side mass transfer resistance. Specific energy consumption was calculated by accounting exclusively for gas handling and analytical power requirements, yielding low values of 0.47 kWh kg^-1 CO₂ for tap water and 0.45 kWh kg^-1 CO₂ for Ca(OH)₂. Long-term stability tests conducted over 30 days (8 h day^-1) demonstrated sustained CO₂ removal performance with only gradual efficiency decline, attributed primarily to progressive CaCO₃ precipitation rather than membrane wetting or structural degradation. The novelty of this work lies not in the introduction of new membrane materials or absorbents, but in the system-level demonstration of a pump-free, gravity-driven membrane contactor that integrates low energy consumption, operational simplicity, and stable performance. The results provide practical insights into the design of modular, low-maintenance membrane contactor systems for decentralized and energy-efficient biogas upgrading applications.