Geometric restriction of microporous supports on gas permeance efficiency of thin film composite membranes

High-Performance Porous Supports Based on Hydroxyl-Terminated Polysulfone and CO2/CO-Selective Composite Membranes

The scope of this work was to develop a thin-film composite (TFC) membrane for the separation of CO2/CO mixtures, which are relevant for many processes of gas processing and gasification of carbon-based feedstock. Special attention was given to the development of highly permeable porous polysulfone (PSF) supports (more than 26,000 GPU for CO2) since both the selective and support layers contribute significantly to the overall performance of the TFC membrane. The PSF porous support is widely used in commercial and lab-scale TFC membranes, and its porous structure and other exploitation parameters are set during the non-solvent-induced phase separation (NIPS) process. Since the casting solution properties (e.g., viscosity) and the interactions in a three-component system (polymer, solvent, and non-solvent) play noticeable roles in the NIPS process, polysulfone samples in a wide range of molecular weights (Mw = 76,000-122,000 g·mol-1) with terminal hydroxyl groups were synthesized for the first time. Commercial PSF with predominantly terminal chlorine groups (Ultrason® S 6010) was used as a reference. The PSF samples were characterized by NMR, DSC, and TGA methods, and the Hansen solubility parameters were calculated. It was found that increasing the ratio of terminal -OH over -Cl groups improved the "solubility" of PSF in N-methyl-2-pyrrolidone (NMP) and water. A direct dependence of the gas permeance of porous supports on the coagulation rate of the casting solution was identified for the first time. It was shown that the use of synthesized PSF (Mw = 76,000 g·mol-1, Mw/Mn = 3.0, (-OH):(-Cl) ratio of 4.7:1) enabled a porous support with a CO2 permeance of 26,700 GPU to be obtained, while the support formed from a commercial PSF Ultrason® S 6010 (Mw = 68,000 g·mol-1, Mw/Mn = 1.7, (-OH):(-Cl) ratio of 1:1.9) under the same conditions demonstrated 4300 GPU. The siloxane-based materials were used for the selective layer since the thin films based on rubbery polymers do not undergo the same accelerating physical aging as glassy polymers. Two types of materials were screened for the selective layer: synthesized polymethyltrifluoroethylacrylate siloxane-polydecylmethylsiloxane (50F3) copolymer, and polydimethylsiloxane (PDMS). 50F3 siloxane was studied for gas separation applications for the first time. It was shown that the permeance of composite membranes based on high-performance porous supports from the PSF samples synthesized was 3.5 times higher than that from similar composite membranes based on supports from a commercial Ultrason® S 6010 PSF with a permeance value of 4300 GPU for CO2. It was found that the enhanced gas permeance of composite membranes based on the highly permeable porous PSF supports developed was observed for both 50F3 polysiloxane and commercial PDMS. At the same time, the CO2/CO selectivity of the composite membranes with a 50F3-selective layer (9.1-9.3) is 1.5 times higher than that of composite membranes with a PDMS-selective layer. This makes the F-containing 50F3 polysiloxane a promising polymer for CO2/CO separation.

Prediction of CO2 Permeance across ZIF-L@PDMS/PES Composite Membrane

The current work predicted the permeance of CO₂ across a ZIF-L@PDMS/PES composite membrane using two different models. The membrane was fabricated by dipping a PES hollow fiber membrane in a coating solution made using PDMS that contained ZIF-L. First, flat sheet ZIF-L@PDMS membranes were fabricated to verify the role of ZIF-L on the gas separation performance of the membrane. Based on the data, the presence of ZIF-L in the PDMS matrix allowed enhancement of both permeability and selectivity of CO₂, where the maximum value was obtained at 1 wt% of ZIF-L. The performance of ZIF-L@PDMS layer, as a function of ZIF-L loading, was well-predicted by the Cussler model. Such information was then used to model the CO₂ permeance across ZIF-L@PDMS/PES composite membrane via the correction factor, which was introduced in the resistance in series model. This work discovered that the model must consider the penetration depth and the inorganic loading (in the case of ZIF-L@PDMS/PES). The error between the predicted CO₂ permeance and the experimental results was found to be minimal.

Air-to-Air Heat and Moisture Recovery in a Plate-Frame Exchanger Using Composite and Asymmetric Membranes

The present work studied an air-to-air exchanger comprising a flat plate module with a diagonal channel and a counterflow configuration for the air streams. The objective of this study was to remove moisture and sensible heat from an exhaust air stream by indirect contact with another air stream. The temperature and flow rate of the exhaust air was in the range of 40–80 °C and 1–5 L·min−1, respectively, and the fresh ambient air to exhaust air flow ratio was 1–5. An asymmetric porous membrane (P-MEM), a thin film composite membrane (C-MEM), and a kraft paper were used as the core for the heat exchange module. The most influential parameter was the humid air temperature, with a direct positive effect (50–60%) due to the increase in the kinetic energy of the water molecules. The other effective parameter was the flow rate of the humid gas with a reverse effect on the enthalpy exchanger performance (25–37%). The ratio of “fresh” air to “exhaust” air had the lowest positive effect (8–10%) on the total effectiveness. The sensible effectiveness of different membranes under the studied conditions was relatively the same, showing their similar heat conductivity. However, the kraft paper showed the best performance compared to the synthetic membranes due to having a porous/hydrophile texture. P-MEM with an asymmetric porous texture showed the closest performance to kraft paper. Furthermore, it was found that under limited conditions, such as higher temperatures (70 and 80 °C) and flow rates (5 L·min−1) for the humid air, the performance of P-MEM was a little better than the kraft paper. However, C-MEM with the lowest total effectiveness and overall heat transfer coefficient (150–210 W·m−2·K−1) showed that the hydrophile PEBAX layer could not contribute to moisture recovery due to its high thickness.

Lithium chloride (LiCl)-modified polyethersulfone (PES) substrate surface pore architectures on thin poly(dimethylsiloxane) (PDMS) dense layer formation and the composite membrane's performance in gas separation

The use of pore forming agents has been notable for improving the water flux in a water-based separation membrane but are rarely being studied as a methodology to influence the substrate's surface architectures for composite membrane fabrication in gas separation. In this study, the influence of lithium chloride (LiCl) on the surface pore architectures and hence, the gas permeance, has been studied in both bare and composite forms with poly(dimethylsiloxane) (PDMS). 1-4 wt% of LiCl was mixed with the dope solution of PES/NMP in the ratio 0.19 and was casted via the dry-wet phase inversion method. Bare substrates were noted to possess increasingly larger surface pore sizes but at a diminishing surface pore density with maximum surface porosity at 2 wt% LiCl. The permeances were, however, significantly reduced with the increase in the LiCl content from 105 300 to 4300 GPU for N₂ gas, presumably due to the thicker skin layer. Nevertheless, the porous surface morphology was confirmed and exhibited Knudsen selectivity with a CO₂/N₂ selectivity of about 0.8, signifying minimal gas flow resistance by the substrates. Upon coating with a similar amount of thin PDMS layer, the composite permeances retain the same trend with values from 361.9 GPU for 0 wt% LiCl substrates to 68.8 GPU for 4 wt% LiCl substrates for CO₂ gas at a consistent selectivity of about 14. As the PDMS layer of the same volumes were used and no significant difference in the coating thickness was noted, the mixed influence of pore intrusion and lateral diffusion is hypothesised at the substrate-coating interface owing to the different surface pore architectures of the substrates.

Membranes with Intrinsic Micro-Porosity: Structure, Solubility, and Applications

Microporous polymer membranes have been widely studied because of their excellent separation performance. Among them, polymers of intrinsic micro-porosity (PIMs) have been regarded as a potential next-generation membrane material for their ultra-permeable characteristics and their solution-processing ability. Therefore, many reviews have been reported on gas separation and monomers for the preparation of PIMs. This review aims to provide an overview of the structure-solubility property. Different structures such as non-network and network macromolecular structure made of different monomers have been reviewed. Then their solubility with different structures and different separation applications such as nanofiltration, pervaporation, and gas/vapor separation are summarized. Lastly, we also provide our perspectives on the challenges and future directions of the microporous polymer membrane for the structure-property relationship, anti-physical aging, and more.