J.N. Barsema

Preparation and characterization of highly selective dense and hollow fiber asymmetric membranes based on BTDA-TDI/MDI co-polyimide

In this work, the preparation, the characterization, and the permeation properties of dense flat sheet and asymmetric hollow fiber membranes, based on BTDA-TDI/MDI co-polyimide (P84), are reported. Results are shown for pure gases and for the separation of a CO2/N2 (80/20) mixture. Dope viscosity measurements were performed to locate the polymer concentrations where significant chain entanglement occurs. Asymmetric hollow fibers were spun, using the dry/wet phase inversion process. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and the structure of the developed fibers. The permeation rates of He, CO2, O2, and N2 were measured by the variable pressure method at different feed pressures and temperatures. P84 co-polyimide proved to be one of the most selective glassy polymers. The achieved ideal selectivity coefficients are: 285–300 for He/N2, 45–50 for CO2/N2, and 8.3–10 for O2/N2, which are in the range of the highest values reported ever for polymeric membranes. The permeability of CO2 is relatively low (1 barrer, 25 °C), however it is independent of feed pressure indicating that the P84 dense membranes are not plasticized at CO2 feed pressures up to 30 bar. To the contrary, the permeance of CO2 through the asymmetric hollow fiber membranes increases with pressure, indicating that the plasticization behavior of asymmetric membranes differs from the respective dense ones. However, no evidence of plasticization was observed when a CO2/N2 (80/20) mixture was fed to the hollow fiber membranes for pressures up to 30 bar. In all cases, CO2 permeance decreased with pressure while that of N2 remained constant.

Functionalized Carbon Molecular Sieve membranes containing Ag-nanoclusters

In Carbon Molecular Sieve (CMS) membranes, the separation of O2 and N2 is primarily based on the difference in size between the gas molecules. To enhance the separation properties of these CMS membranes it is necessary to functionalize the carbon matrix with materials that show a high affinity to one of the permeating gas species. Adding Ag-nanoclusters increases the selectivity of O2 over N2 by a factor 1.6 compared to a non-functionalized CMS membrane prepared by the same pyrolysis procedure. We have analyzed the structure of Ag-nanocluster (dcluster≈50 nm) containing membranes produced from different Ag sources, AgNO3 and AgAc, and with different Ag content (0, 6, 25, and 40 wt.%). By measuring the pure gas permeabilities of He, CO2, O2, and N2 we have determined the effect of Ag-nanoclusters in the carbon matrix, concluding that in the case of pure gases, the Ag-nanoclusters act primarily as a spacer at pyrolysis end temperatures up to 600 °C, increasing the O2 permeability by a factor of 2.4. However, they enhance the separation of O2 over N2 at higher pyrolysis end temperatures (700 and 800 °C). It was shown that the build up of an Ag layer on the surface of the membrane reduces the permeability, but does not affect the selectivity.

Carbon molecular sieve membranes prepared from porous fiber precursor

Carbon molecular sieve (CMS) membranes are usually prepared from dense polymeric precursors that already show intrinsic gas separation properties. The rationale behind this approach is that the occurrence of any kind of initial porosity will deteriorate the final CMS performance. We will show that it is not necessary to produce a non-porous precursor in order to obtain a selective CMS membrane. We used tight ultra-filtration (UF) fiber membranes as a precursor. These fibers did not have any gas separation properties before the pyrolysis treatment, nor were coatings applied to these fibers before or subsequent to the pyrolysis. After a heat treatment in air followed by a pyrolysis in a nitrogen atmosphere CMS fiber membranes were obtained. The CMS fibers were analyzed using scanning electron microscopy, thermo gravimetrical analysis, and gas permeation. From the permeation rates and permselectivity values measured for He, H2, CO2, Ar, O2, N2, CH4, C2H4, C2H6, C3H6, C3H8 and SF6 the evolution of the mean pore diameter was investigated. It was found that the pore diameter increases with pyrolysis temperature up to 800 °C, but decreases as the temperature is raised to 900 °C. The overall porosity reaches its highest value at 900 °C.

Ag functionalized Carbon Molecular Sieves membranes for separating O 2 and N 2

In the last two decades substantial progress has been made in the preparation of Carbon Molecular Sieve (CMS) membranes for gas separation. Today, researchers actively study precursor materials and pyrolysis routes to fully explore the merits of CMS membranes. Successful separation of permanent gas mixtures in which the gaseous components posses only little to no affinity to adsorb onto the internal CMS surface relies highly on the exact tailoring of microsieving regions. Preparation of CMS structures, which are highly permselective towards one of such mixture components, is especially cumbersome if the molecular sizes differ only slightly (e.g. O 2 /N 2 ). To facilitate the separation of O 2 and N 2 we have chosen to functionalize the carbon matrix. By introducing nano-sized (40 nm) metallic Ag-clusters, the affinity of the membrane matrix for O 2 significantly increases. We have added a silver containing salt, AgNO 3 , to a solution of BMTA-TDI/MDI co-polyimide (P84, Lenzing) in NMP in the absence of light to obtain homogeneous flat film polymeric precursors. These precursors were pyrolysed, reducing Ag + to Ag, at different temperatures (350, 500, 600, 700, and 800 °C) in a N 2 atmosphere and characterized using Scanning Electron Microscopy, Atomic Force Microscopy, and gas permeation. Thermo Gravimetrical Analysis was used to follow the pyrolysis in detail. From this we observed an increase of the ideal separation factor of 1.6. Moreover, we observed an increase of the permeability to a maximum of 240 %.