Mixed matrix membranes using carbon molecular sieves: II. Modeling permeation behavior

Mixed matrix membranes have been formed from flat-sheet solution casting of carbon molecular sieves (CMS) dispersed within two different polymer matrices. In a preceding paper (Part I), the performance of these mixed matrix membrane films has been demonstrated for gas separations. It was shown that the CMS particles produced a mixed matrix or heterogeneous membrane having significantly enhanced effective permselectivities (CO2/CH4 and O2/N2) and fast-gas permeabilities (CO2 and O2) over the intrinsic properties of the pure polymer matrix phase alone. For the CO2/CH4 separation, enhancements by as much as 45% in CO2/CH4 permselectivity and 200% in CO2 permeability over the corresponding intrinsic permeation properties of the pure polymer matrix phases were observed. These performance enhancements increased incrementally as the loading of CMS particles (up to 35% by weight) being dispersed within the two polymer matrices (Matrimid® 5218 and Ultem® 1000) increased. In this paper, these experimental findings are compared with two theoretical mixed matrix models, the Maxwell and the Bruggeman Models. In general, the Bruggeman Model was observed to consistently predict both higher permeabilities and permselectivities than the Maxwell Model. For the Ultem®-CMS mixed matrix films, the Bruggeman Model reasonably predicted the experimental permeability data, while the Maxwell Model underpredicted the gas permeabilities. Both models, however, gave permselectivity predictions that compared reasonably well with experimental values for the Ultem®-CMS mixed matrix films. For the Matrimid®-CMS mixed matrix films, both models significantly overpredicted the observed gas permeabilities and permselectivities. A matrix rigidification phenomenon in Matrimid® was postulated to be occurring in the region or zone near the CMS surface. An ad hoc adaptation to the Maxwell Model was employed to account for this phenomenon. Predictions from this modified approach gave better agreement with experimental values for the Matrimid®-CMS mixed matrix films.