Water vapor permeation through glassy polyimide membranes and its impact upon carbon dioxide capture operations

© 2012 Dr. George Qiuzhi ChenGas separation membranes of high CO2 permeability and selectivity have great potential in both pre and post-combustion CO2 capture from flue gases. While in the laboratory most workers use dense polymeric films, asymmetric flat sheet membranes are used in industrial spiral-wound modules to maximize the plant productivity. These asymmetric membranes contain a very thin active layer and often exhibit very different permeation behavior compared to dense films. Water vapor, often a minor component in gas separation membrane systems, can hardly be avoided in industrial gas streams. It exhibits very different permeation behavior compared to other gas species because of its high hydrogen bonding affinity. The presence of water vapor is not always desirable in membrane gas separation systems as it often lowers the overall performance of the membrane due to plasticization and/or competitive sorption. In this thesis, the water vapor permeation behavior in two polyimide films (2,2’-bis(3,4’-dicarboxyphenyl) hexafluoropropane dianhydrid-2,3,5,6-tetramethyl-1,4-phenylenediamine (6FDA-TMPDA) and poly(3,3’-4, 4’-benzophenone tetracarboxylic - dianhydride diaminophenylindane) (Matrimid® 5218) has been studied under mixed gas/water vapour feed streams (CH4, CO2 and H2O) at 2 and 7.5 bar, 35 °C. Water vapor permeability was found to increase with water vapor activity for both polyimides. A decrease in permeabilities of both CH4 and CO2 for both pressures was also observed as water vapor was added for both polyimides. This loss in permeability can be related to competitive sorption effects. Mathematical models have also been developed to fundamentally understand the effect of the presence of water vapor in membrane separation units using polyimide membranes in a water vapor-gas multi-component system. These models predicted a decrease in diffusion coefficients, reflecting a reduction in fractional free volume caused by pore-filling effects. Pore filling by water molecules or clusters affected the diffusional pathway for both penetrating vapor and gas molecules. This was further verified by results from positron annihilation lifetime spectroscopy (PALS) measurements which showed reductions in microvoid numbers and size for both polyimides in the presence of water vapor. Overall, the trend of increasing water vapor permeability for both polyimides could be attributed to a combination of increasing vapor solubility and decreasing vapor diffusivity as water vapor activity increased. In addition, water vapor transport characteristics and membrane performance in the presence of water vapor have been investigated for flat-sheet Matrimid membranes of different thickness at 7.5 bar, 35 °C. The fractional free volume of Matrimid films in a wide range of thicknesses was found to decrease with decreasing film thickness. Solubility of water vapor was found to be independent of thickness, while diffusivity reduced with decreasing film thickness as predicted by the FFV-diffusivity correlation. The mathematical models derived as above also predicted a decrease in diffusion coefficient as vapor activity increased due to vapor-induced pore filling effect. A trend of reduction in gas permeabilities and CO2/CH4 selectivities as vapor activity increases was also observed. It was found that 90% of the membrane performance can still be maintained for Matrimid membranes provided that water vapor activity is kept below 0.2 (equivalent to ~1.13 kPa at 35 °C)