Fabrication and Performance Evaluation of Pebax 1657 / PVDF Composite Hollow Fiber Membranes for Biogas Upgrading: Effect of Feed Pressure and Temperature on CH₄ Purity and Recovery

The upgrading of raw biogas to biomethane of pipeline or vehicle fuel quality requires removal of carbon dioxide from a typical 55–65% CH4 / 35–45% CO2 biogas mixture. Membrane-based gas separation has emerged as one of the most energy-efficient and operationally simple technologies for biogas upgrading, offering significant advantages over water scrubbing, pressure swing adsorption, and amine absorption in terms of equipment compactness, absence of liquid waste streams, and ease of scale-up. This paper reports the fabrication, characterisation, and systematic performance evaluation of Pebax 1657 / PVDF composite hollow fiber membranes for CO2/CH4 separation. Hollow fibers were prepared by dry-wet spinning of Pebax 1657 / PVDF blended in an 80:20 weight ratio dissolved in NMP/acetone solvent. The effect of blend composition (0 to 30 wt% PVDF) on membrane permeability, CO2/CH4 selectivity, mechanical strength, and morphology was characterised by gas permeation testing, tensile testing, SEM, and contact angle measurement. For the optimum 80:20 blend hollow fiber module, the effects of feed pressure (2 to 12 bar) and operating temperature (15 to 55°C) on CH4 purity and recovery from a simulated biogas feed (60% CH4 / 40% CO2) were systematically investigated. Results show that the Pebax 1657/PVDF 80:20 blend membrane achieves a CO2/CH4 selectivity of 31.6 with CO2 permeability of 449 Barrer — a 39.5% selectivity improvement over neat Pebax 1657 — attributed to the hydrogen-bonding interaction between polyether segments of Pebax and PVDF fluorine groups that promotes CO2-selective sorption while limiting CH4 transport. At 6 bar feed pressure and 25°C, the hollow fiber module achieves CH4 purity of 88.9% with 92.4% recovery. Increasing feeding pressure to 12 bar raises CH4 purity to 96.8% while reducing recovery to 83.8%. A two-stage membrane configuration is proposed to simultaneously achieve >95% CH4 purity and >90% CH4 recovery.