Structure-Controlled Polyetherimide Hollow Fibers for Biogas Purification

Polyetherimide (Ultem-1000) hollow-fiber membranes were developed for biogas purification with emphasis on the relationship between spinning conditions, membrane morphology, gas transport properties, and module performance. Hollow fibers were prepared from dope solutions based on dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) at different conditions, followed by post-treatment with 1 and 3 wt.% silicone solution in n-heptane to reduce nonselective defects and improve selectivity toward the intrinsic behavior of dense PEI films. SEM analysis revealed that DMF-based fibers formed a more open, macrovoid-rich structure, whereas NMP-based fibers exhibited a more homogeneous sponge-like morphology with a better-defined selective layer. DMF-based fibers experienced faster demixing, which promoted macrovoid formation, increased pore connectivity of the substructure, lowered mass transfer resistance, and at the same time increased the probability of nonselective pathways and defect-related loss of selectivity. This structural evolution was reflected in gas transport properties: untreated DMF fibers showed high mixed-gas permeance but limited selectivity, while NMP fibers demonstrated lower permeance and selectivity values closer to those of the dense film. Silicone post-treatment significantly improved separation performance, with 3 wt.% coating being markedly more effective than 1 wt.% coating. The best compromise between permeance and selectivity was achieved for the DMF-based fibers treated with 3 wt.% silicone, which exhibited CO₂ and H₂S permeances of 39.4 and 47.12 GPU, respectively, together with selectivity values of 22.4, 26.8 and 20.2 for CO₂/CH₄, H₂S/CH₄ and CO₂/N₂. A membrane module containing 500 fibers was studied during the quasi-real biogas upgrading. With increasing stage-cut, the CH₄ concentration in the retentate increased from ~74 to 96 mol.%, while CO₂ decreased from ~21 to 2 mol.%. The results demonstrate that structure control combined with silicone post-treatment is an effective strategy for producing PEI hollow fibers suitable for simultaneous methane enrichment and removal of acid impurities from biogas.