Integrated Materials-to-Process Design of a Two-Stage PSF/PEI Membrane System for Biogas Upgrading

Biogas upgrading to biomethane is an important route for increasing the energy value of renewable gas streams and enabling their wider use in fuel, heat, and power applications. In the present study, a two-stage membrane process for biogas upgrading was developed and validated using polysulfone (PSF) and polyetherimide (PEI) hollow-fiber membranes. The main original aspects of this work include the formulation of a PEI/DMF/IPA spinning dope composition (27/60/13 wt.%), the mixed-gas testing of PSF and PEI hollow-fiber membranes in a six-component model biogas mixture, and the combined simulation, experimental validation, and techno-economic evaluation of a two-stage membrane process. Mixed-gas permeation experiments with a six-component model biogas mixture were used as the basis for process simulation of four membrane-stage configurations. Only the PSF + PEI cascade simultaneously provided methane recovery above 90%, methane concentration in the product above 95 mol.%, and residual carbon dioxide content below 2 mol.%. Experimental verification confirmed the modeled process concept and demonstrated that the target biomethane quality was achieved at feed flow rates of 0.9–1.1 L min−1, where methane recovery reached 93.5–95.2% and methane purity was 95.5–96.2 mol.%. A preliminary techno-economic analysis for a 100 m3 h−1 unit indicated an upgrading penalty of USD 96.4 per ton of 95.5 mol.% CH4. PSF membranes provided higher carbon dioxide permeance, whereas PEI membranes exhibited higher CO2/CH4 selectivity, which explains the efficiency of their combination in the two-stage scheme. The results show that the proposed PSF + PEI cascade is a promising membrane-based approach for energy-efficient biogas upgrading.