Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

Hydrogen gas mixture separation by CVD silica membrane

In this work we investigate the performance of high flux chemical vapour deposition (CVD) silica membranes for the separation of gas mixtures containing H₂ and CO₂ at various temperatures. The membranes were prepared by a counter diffusion CVD method where tetraethyl orthosilicate (TEOS) and O₂ were used as reactants. Single gas permeation resulted in activated transport for the smaller kinetic diameter gases (H₂ and He) whilst the larger kinetic diameter gases (CO₂ and N₂) showed negative activation energy. The single gas permeation of H₂ increased from 5.1 × 10⁻⁷ to 7.0 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ in the temperature range 100–400 °C, and H₂/CO₂ and H₂/N₂ selectivities reached 36 and 57 at 400 °C, respectively. The H₂ purity in the permeate stream also increased with temperature for H₂:CO₂ binary gas mixture, thus being beneficial for H₂ diffusion. H₂ competitively permeated through the membrane at a several range of gas mixtures, and a saturation level was achieved at H₂:CO₂ 60:40 feed concentration, where the diffusion of CO₂ molecules became negligible delivering ∼99% H₂ purity in the permeate stream. These results substantiate that the counter diffusion CVD method produced thin silica film membranes with a very precise pore size control, in particular suggesting a narrow pore distribution with average pore radius of about 3.1 Å.     

PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications

A new application for used reverse osmosis (RO) membranes as gas separation membranes was studied. In this regard, firstly, three pretreatment procedures were used to remove the foulants from the surface of used membrane and then they were coated with polydimethylsiloxane (PDMS). The results indicated that PDMS-coated used RO membranes were capable of separating O₂/N₂ and CO₂/N₂. The maximum O₂/N₂ and CO₂/N₂ selectivities of coated membranes were 5.9 and 32.5, respectively. The O₂/N₂ and CO₂/N₂ selectivities of PDMS membrane were reported in the range of 2.1–2.2 and 11–12, respectively. Finally, an economic assessment was carried out to compare prepared PDMS coated RO membranes with commercial PPO membrane. This showed that coated membranes are less expensive than PPO membrane for CO₂/N₂ gas separation. The outcome of the research was a simple method for converting used RO membranes to cost effective gas separation membranes.     

Performance and pore characterization of nanoporous carbon membranes for gas separation

The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated.     

Using PDMS coated TFC-RO membranes for CO2/N2 gas separation: Experimental study,modeling and optimization

Thin film composite (TFC) reverse osmosis (RO) membranes are semipermeable membranes that are utilized in water purification or water desalination systems. Discarding these membranes after end-of-life leads to environmental problems. Reusing old TFC-RO membranes is one way to solve this problem. For this reason, in this study, used TFC-RO membranes were coated with polydimethylsiloxane (PDMS) for CO₂/N₂ gas separation application. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was utilized to confirm the crosslinking of coated PDMS. The morphology of PDMS/TFC-RO membranes was characterized using scanning electron microscopy (SEM). The parameters that can affect performance of prepared membranes (N₂ permeance and CO₂/N₂ selectivity) are concentration of PDMS solution, coating time, solvent evaporation time and curing temperature and time. Given that the used membranes don't have uniform surfaces, the first step of this study was to investigate the effect of the above mentioned factors on virgin membranes using fractional factorial design (FFD) of experiments. The results obtained showed that PDMS concentration is the most significant factor that has a negative effect on N₂ permeance and positive effect on CO₂/N₂ selectivity. The reported CO₂/N₂ selectivity of PDMS membranes was 11–12, but this selectivity for prepared PDMS/TFC-RO membranes was in the range of 6.7–22.5. After determining optimum conditions, the gas separation performance of PDMS coated used TFC-RO membrane under these conditions was finally determined. The results showed that the used membranes had a better performance than virgin membranes.     

Modeling the phase separation in binary lipid membrane under externally imposed oscillatory shear flow

By adding external velocity terms, the two-dimensional time-dependent Ginzburg-Landau (TDGL) equations are modified. Based on this, the phase separation in binary lipid membrane under externally imposed oscillatory shear flow is numerically modeled employing the Cell Dynamical System (CDS) approach. Considering shear flows with different frequencies and amplitudes, several aspects of such a phase evolving process are studied. Firstly, visualized results are shown via snapshot figures of the membrane shape. And then, the simulated scattering patterns at typical moments are presented. Furthermore, in order to more quantitatively discuss this phase-separation process, the time growth laws of the characteristic domain sizes in both directions parallel and perpendicular to the flow are investigated for each case. Finally, the peculiar rheological properties of such binary lipid membrane system have been discussed, mainly the normal stress difference and the viscoelastic complex shear moduli.     

Adsorbent filled membranes for gas separation. Part 1. Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents

The effect of the introduction of specific adsorbents on the gas separation properties of polymeric membranes has been studied. For this purpose both carbon molecular sieves and zeolites are considered. The results show that zeolites such as silicate-1, 13X and KY improve to a large extent the separation properties of poorly selective rubbery polymers towards a mixture of carbon dioxide/methane. Some of the filled rubbery polymers achieve intrinsic separation properties comparable to cellulose acetate, polysulfone or polyethersulfone. However, zeolite 5A leads to a decrease in permeability and an unchanged selectivity. This is due to the impermeable character of these particles, i.e. carbon dioxide molecules cannot diffuse through the porous structure under the conditions applied. Using silicate-1 also results in an improvement of the oxygen/nitrogen separation properties which is mainly due to a kinetic effect. Carbon molecular sieves do not improve the separation performances or only to a very small extent. This is caused by a mainly dead-end (not interconnected) porous structure which is inherent to their manufacturing process.     

ZIF-8 based polysulfone hollow fiber membranes for natural gas purification

Mixed matrix membranes (MMMs) have received worldwide attention for natural gas purification due to their superior performance in terms of permeability and selectivity. The zeolitic imidazole framework-8 (ZIF-8) blended polysulfone (PSf) membranes have been fabricated for natural gas purification. ZIF-8 was selected due to its low cost, remarkable thermal and chemical stabilities, and tunable microporous structure. The neat PSf hollow fiber membrane and mixed matrix hollow fiber membranes incorporated with the various ZIF-8 loadings up to 1.25% were fabricated. The prepared membranes were evaluated using field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and gas separation performance. The low loading of ZIF-8 nanoparticles to the MMM improved thermal stability and glass transition temperature and yielded low surface roughness. MMMs were tested using pure gases with a significant improvement of 36% in CO₂ permeability and 28% in CO₂/CH₄ selectivity compared to the neat membrane. However, the high ZIF-8 loading reduced the separation performances. Moreover, CO₂/CH₄ selectivity decreased at elevated pressure (8 and 10 bar) due to CO₂-induced plasticization. Previously, the incorporation of ZIF-8 particles has primarily been subjected to the fabrication of flat sheet membranes, whereas this work focused on hollow fiber membranes which are rarely investigated. Hence, the promising results obtained at low feed pressure in this study demonstrated the potential of ZIF-8 based hollow fiber membrane for natural gas purification.     

Superior gas separation performance of dual-layer hollow fiber membranes with an ultrathin dense-selective layer

A concept demonstration has been made to simultaneously enhance both O₂ and CO₂ gas permeance and O₂/N₂ and CO₂/CH₄ selectivity via intelligently decoupling the effects of elongational and shear rates on dense-selective layer and optimizing spinning conditions in dual-layer hollow fiber fabrication. The dual-layer polyethersulfone hollow fiber membranes developed in this work exhibit an O₂/N₂ selectivity of 6.96 and an O₂ permeance of 4.79 GPU which corresponds to an ultrathin dense-selective layer of 918 Å at room temperature. These hollow fibers also show an impressive CO₂/CH₄ selectivity of 49.8 in the mixed gas system considering the intrinsic value of only 32 for polyethersulfone dense films. To our best knowledge, this is the first time to achieve such a high CO₂/CH₄ selectivity without incorporating any material modification. The above gas separation performance demonstrates that the optimization of dual-layer spinning conditions with balanced elongational and shear rates is an effective approach to produce superior hollow fiber membranes for oxygen enrichment and natural gas separation.     

Formation and thermal stability of BMI-based interpenetrating polymers for gas separation membranes

Semi-interpenetrating polymer networks (semi-IPNs) were prepared by sol–gel technique through in situ polymerization of bismaleimide (BMI) in thermoplastic polyetherimide (PEI) as well as in polysulfone (PSF). This synthesis route allows arresting thermoset/thermoplastic phase separation at an early stage by solidifying the semi-IPNs through membrane phase inversion. The phase separation could be observed visually in the casting solution or by optical microscope on the surface of the produced membranes. These semi-IPNs with a density lower than their thermoplastic base polymer allowed easier water penetration during membrane phase inversion. This led to improved membrane morphology that was confirmed by scanning electron microscopy. Membranes fabricated from these semi-IPN materials had thinner skin layers and longer straight fingers perpendicular to membrane surface. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that these semi-IPNs membranes have improved glass transition temperatures but a lower thermal stability. However, at ambient conditions, these membranes with their improved structure and morphology showed superior gas separation characteristics compared to base polymers. For example, the permeance was increased by 12–15 times without a significant decrease in the selectivity of oxygen over nitrogen in air separation experiments.     

Investigation of Poly(ether-b-amide)/Nanosilica Membranes for CO2/CH4Separation

The results of molecular dynamics （MD） simulations on transport process of CO2 and CH4 gases in poly（ether-b- amide） （PEBAX）/nanosilica membranes are discussed. The diffusion coefficients for CH4 and CO2 gases at 6 cases with different amounts of nanosilica loading in the simulation boxes are presented. The results show that diffusion coefficients for CO2 gas in all cases are larger than those for the CH4 one. Moreover 10% nanosilica loading case shows maximum effects on diffusion coefficients and improves them by more than 68% and 157% for CO2 and CH4 gases, respectively. Additionally, the results of 3-D Cartesian trajectories and displacements curves are presented and the jumping attempt of CO2 is significantly more than that of CH4. Due to the rubbery state of PEBAX membranes in ambient temperature, the results confirm that channel lifetimes are very short and then back diffusion is not observed for this polymer.     

CO2/CH4 mixed gas separation using poly(ether-b-amide)-ZnO nanocomposite membranes: Experimental and molecular dynamics study

Poly (ether-b-amide) (PEBA) mixed matrix membranes (MMMs) filled by different amounts of nano ZnO (up to 1 wt %) were synthesized and their gas separation performance was evaluated for CO₂, CH₄ and N₂ pure gas and their binary mixtures. The ZnO-filled PEBA MMMs were characterized using ATR-FTIR, FESEM, AFM, TGA, DMTA, XRD and Mechanical tensile strength analyses. Generally, it was revealed that 0.5 wt % loading of ZnO into the polymer matrix caused a ZnO−PEO interaction; while ZnO–ZnO self-association hindered the interaction for the MMMs with other loadings of ZnO. As a result, PEBA-ZnO 0.5 wt % MMM possessed a higher glass transition temperature (T_g). Therefore, the CO₂ permeability through PEBA-ZnO 0.5 wt % enhanced 27% than simple PEBA membrane. Moreover, all the fabricated MMMs were simulated by molecular simulation. Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) methods were also applied to simulate the structural and gas transport properties of the membranes. The RDF, XRD, T_g, FFV and density analysis were compared with experimental results. Also, a binary mixture of CO₂:CH₄ (10:90) was used to determine CO₂ permeability and CO₂/CH₄ selectivity, which were considerably reduced compared to single gas experiments. Moreover, the solubility of the binary gas mixture, the energy distribution and density distribution of both gases within the simulated cell were calculated by molecular simulation.     

Permeation of binary gas mixture through glassy polymer membranes with concentration-dependent diffusivities

An analytical solution has been obtained for the modified dual-mode mobility model for a single gas proposed by Zhou and Stern and extended to a binary gas mixture to describe the pressure dependence of mean permeability coefficients for CO₂ and CH₄ mixtures in homogeneous cellulose triacetate membranes. The permeabilities calculated from the model fitted the corresponding experimental results quite well. Permeation experiments for equimolar CO₂ and CH₄ mixture in a homogeneous membrane of methyl methacrylate and n-sbutyl acrylate copolymer were performed along with sorption experiments for pure CO₂ and CH₄ to test the applicability of the model. The experimental permeabilities were close to those calculated from the model.     

High performance of PES-GNs MMMs for gas separation and selectivity

In this study, graphene nanosheets (GNs) were incorporated into polyethersulfone (PES) by phase inversion approach for preparing PES-GNs mixed matrix membranes (MMMs). To investigate the impact of filler content on membrane surface morphology, thermal stability, chemical composition, porosity and mechanical properties, MMMs were constructed with various GNs loadings (0.01, 0.02, 0.03, and 0.04 wt%). ?The performance of prepared MMMs was tested for separation and selectivity of CO₂, N₂, H₂ and CH₄ gases at various pressures from 1 to 6 bar and temperature varying from 20 to 60 °C. It was observed that, compared to the pristine PES membrane, the prepared MMMs significantly improved the gas separation and selectivity performance with adequate mechanical stability. The permeability of CO₂, N₂, H₂ and CH₄ for the PES + 0.04 wt% GNs increases from 9 to 2246, 11 to 2235, 9 to 7151, and 3 to 4176 Barrer respectively, as compared with pure PES membrane at 1 bar and 20 °C due to improving the membrane absorption and porosity. In addition, by increasing the pressure, the permeability and selectivity of CO₂, N₂, H₂ and CH₄ are increased due to the increased driving force for the transport of gas via membranes. Furthermore, the permeability of CO₂, N₂, H₂ and CH₄ increased by increasing the temperature from 20 to 60 °C due to the plasticization in the membranes and the improvement in polymer chain movement. This result proved that the prepared membranes can be used for gas separation applications.     

Fabrication and gas separation properties of polybenzimidazole (PBI)/nanoporous silicates hybrid membranes

Composites of polybenzimidazole (PBI) with proton-exchanged AMH-3 and swollen AMH-3 were prepared, characterized by electron microscopy and X-ray scattering and tested for hydrogen/carbon dioxide ideal selectivity. Proton-exchanged AMH-3 was prepared under mild conditions by the ion exchange of Sr and Na cations in the original AMH-3 using aqueous solution of dl-histidine. Swollen AMH-3 was prepared by sequential intercalation of dodecylamine following the ion exchange in the presence of dl-histidine. Both silicate materials were introduced into a continuous phase of PBI as a selective phase. Mixed matrix nanocomposite membranes, prepared under certain casting conditions, with only 3 wt% of swollen AMH-3 present substantial increase of hydrogen/carbon dioxide ideal selectivity at 35 °C, i.e., more than by a factor of 2 compared to pure PBI membranes (40 vs. 15). Similar ideal selectivity was observed using higher loadings (e.g., 14%) of proton-exchanged AMH-3 particles suggesting that transport of hydrogen is faster than carbon dioxide in AMH-3-derived silicates. However, the ideal selectivity of mixed matrix membranes approaches that of pure polymer as the operating temperature increases to 100 °C and 200 °C. The composite membranes with AMH-3-derived materials were compared with MCM-22/PBI membranes. Composite membranes incorporating MCM-22 plate-like crystals show no selectivity enhancements possibly due to the presence of larger pores in MCM-22.     

Optimization of module configuration in membrane gas separation

Mathematical models have been developed to optimize three configurations for membrane gas separation modules. The three systems include the single stage, the two stage, and the continuous membrane column (CMC). Analysis of the three systems is carried out for the case of enriching a binary mixture of methane and carbon dioxide, where the reject stream is the desired product. The cost optimization function includes the capital cost for compressors and membranes as well as the energy operating cost. The cost function is solved subject to a set of equality and inequality constraints. The equality constraints include the module balance equations and the permeation fluxes across the membrane. The inequality equations include constraints on mole fractions in permeate and reject streams, operating pressure, membrane area, and the amount of methane recovered in reject stream. Model equations for the three systems are solved using GINO, a program for nonlinear optimization. A quasi-Newton search method is selected and found quite efficient for solution of the equations. Over the range of parameters considered in the analysis, results show that the two stage configuration has a lower production cost than the other two systems. In addition, the operating cost for the CMC and the single stage systems are found to be comparable. Irrespective of this, the optimum amount of methane recovered is the highest for the CMC system. Although the optimum operating costs for the CMC and the single stage systems are higher than the two stage system, comparison should consider other factors including higher methane recoveries generated by the CMC system and the simplicity of design and operation for the single stage system.     

Preparation and characterization of silica—polyimide composite membranes coated on porous tubes for CO2 separation

Silica-polyimide microcomposite membranes were prepared on γ-alumina-coated α-alumina support tubes, and their gas permeation properties were evaluated with He, N₂ and CO₂. Smoothing of the substrate surface and hybridization of silica and polyamic acid were both effective to form defect-free thin composite membranes. The CO₂ permeance of a membrane with a silica content of 68 wt% was one order of magnitude higher than that of a polyimide membrane having the same thickness. The permselectivity of CO₂ to N₂ was 30 at 30°C and 13 at 100°C. Contributions of the silica and polyimide phases to permeance of the composite membrane were analyzed with a two-phase permeation model. The effective thickness of the rate-controlling polyimide phase was less than one-tenth of the total thickness of the silica-polyimide membrane.     

New amorphous perfluoro polymers: perfluorodioxolane polymers for use as plastic optical fibers and gas separation membranes

To address the need for perfluoro polymers with higher T_g, we have prepared and characterized various perfluorodioxolane monomers via direct fluorination of the hydrocarbon precursors. These monomers were readily polymerized in bulk or in solution initiated by perfluorodibenzoyl peroxide. The polymers obtained have relatively high T_g(~160°C) and exhibited low material dispersion. These polymers are completely amorphous and soluble in fluorinated solvents. The polymers are also chemically and thermally stable (T_g > 300°C). Thus, these perfluorodioxolane polymers may be used as plastic optical fiber material where high T_g is required, such as in automobile and aircraft application. These perfluorodioxolane polymers were also investigated for use as gas separation membrane. Among these polymers, the copolymer of perfluoro (2‐methylene‐1,3‐dioxolane) and perfluoro (2‐methylene‐4,5‐dimethyl dioxolane) showed superior gas separation performance compared with the commercial perfluoro polymers for a number of gas pair, including CO₂/CH₄, H_e/CH₄, H₂/CH₄, and N₂/CH₄. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication of Matrimid/polyethersulfone dual-layer hollow fiber membranes for gas separation

We have developed almost defect-free Matrimid/polyethersulfone (PES) dual-layer hollow fibers with an ultra-thin outer layer of about 10 × 10⁻⁶ m (10 μm), studied the effects of spinneret and coagulant temperatures and dope flow rates on membrane morphology and separation performance, and highlighted the process similarities and differences between single-layer and dual-layer hollow fiber fabrications. The compositions of the outer and inner layer dopes were 26.2/58.8/15.0 (in wt.%) Matrimid/NMP/methanol and 36/51.2/12.8 (in wt.%) PES/NMP/ethanol, respectively. It is found that 25 °C for both spinneret and coagulant is a better condition, and the fibers thus spun exhibit an O₂/N₂ selectivity of 6.26 which is within the 87% of the intrinsic value and a calculated apparent dense-layer thickness of about 2886 × 10⁻¹⁰ m (2886 Å). These dual-layer membranes also have impressive CO₂/CH₄ selectivity of around 40 in mixed gas tests. The scanning electron microscopy (SEM) studies show that low coagulant temperatures produce dual-layer hollow fibers with an overall thicker thickness and tighter interfacial structure which may result in a higher substructure resistance and decrease the permeance and selectivity simultaneously. The elemental analysis of the interface skins confirms that a faster inter-layer diffusion occurs when the fibers are spun at higher spinneret temperatures. Experimental results also reveal that the separation performance of dual-layer hollow fiber membranes is extremely sensitive to the outer layer dope flow rate, and the inner layer dope flow rate also has some influence. SEM pictures indicate that the macrovoid formation in dual-layer asymmetric hollow fiber membranes is quite similar to that in single-layer ones. It appears that macrovoids observed in this study likely start from local stress imbalance and weak points.