Accurate monitoring of gas mixture transport knetics through polymeric membranes

The accurate assessment of the membrane gas separation properties requires improved metrologies when the feed gases are mixtures rich of components such as biogases, flue gases and mixtures produced by 〖CO〗_2 reforming. We present an original mass spectroscopy- based approach that permits to monitor the permeation kinetics of gas mixtures formed by components with overlapping mass signals. In the instrumental setup, sample-holder and quadrupole mass spectrometer are hosted in an Ultra High Vacuum chamber which is kept under dynamic pumping conditions during the experimental run. This setup permits to measure, as a function of time, the permeation flux of each mixture component monitoring kinetics with transient lasting down to the ~ 1 s order. The flux detection limit is ~ 10-6 cm3(STP) m-2 s-1 for species with unambiguously attributable mass signal and no worse than ~ 10-5 cm3(STP) m-2 s-1 for species with overlapping mass signal. Tests were carried out at room temperature with ~ 50 m thick polymer films exposed to 〖CO〗_2- rich gas mixtures at total pressure below 105 Pa. The analysis of permeation flux kinetics allows the evaluation of the diffusion constant of migrating species with  4 % indetermination. In steady-state conditions the permeation flux through a poly(lactic acid) film is measured with accuracy better than  3 % for components with unambiguously attributable mass signal (〖CO〗_2, O_2) and no worse than ~ 6 % for those with overlapping mass signal (N_2, CO).