Can pure-gas permeability data design a mixed-gas module? A correctability map for dual-mode CO2/N2 transport in Matrimid 5218 hollow fibres

Abstract Pure-gas permeabilities are routinely used to size polymeric gas-separation modules, although permeation in glassy polymers depends on pressure and composition through dual-mode sorption. This work asks whether any choice of pure-gas measurement pressure can make such data reproduce mixed-gas module performance, using CO 2 /N 2 in Matrimid 5218 as a case study. An exact partial-immobilization flux with competitive sorption on both membrane faces was embedded in a mass-balance-verified counter-current hollow-fibre model. The governing mobility parameter F·K was calibrated against two independent literature datasets (0.191; 90% confidence interval 0.123–0.272), whereas a single-laboratory literature value (0.859) failed a held-out test. With pure-gas data taken at 1 bar, the design overestimated CO 2 recovery by 2.5 to 3.5 percentage points (1.7–4.8 across the F·K interval) and CO 2 purity by 0.7–1.2 points, corresponding to modules undersized by 5.6–9.6% of the area actually required. Whether this bias can be removed by choosing another measurement pressure depends jointly on feed CO 2 partial pressure and stage cut. At stage cuts of about 10–13%, no measurement pressure corrects the design below a feed CO 2 partial pressure of about 0.84 bar, and correction within 20 bar requires more than about 1.64 bar; this uncorrectable regime contracts with increasing stage cut and was not observed above roughly 18%. It arises where competitive sorption depresses the local mixed-gas CO 2 permeability below the pure-gas Henry's-law limit. Higher temperature reduces the bias but costs more in selectivity. The results identify where mixed-gas characterization cannot be replaced by pure-gas testing. Mapped onto industrial CO₂ sources at 10 bar feed, natural-gas combined-cycle flue gas falls in the uncorrectable regime, coal flue gas in a regime correctable only by impractically high test pressures, and enriched or cement-kiln feeds in a regime correctable by routine testing. A screening procedure is proposed that tells the designer, before any module is sized, whether vendor pure-gas data are adequate, require a stated area margin, or must be replaced by mixed-gas measurements.