Defect-free Selective Layer Research Articles

Nanofiltration membrane via EGCG-PEI co-deposition followed by cross-linking on microporous PTFE substrates for desalination

It is imperative to fabricate nanofiltration membrane with promising separation performance and stability for desalination and wastewater treatment. In this work, inspired by polyphenol chemistry, composite nanofiltration membrane is facilely manufactured by a combined strategy of epigallocatechin-3-gallate (EGCG)-polyetheylenimine (PEI) co-deposition and trimesoyl chloride (TMC) cross-linking on porous polytetrafluoroethylene (PTFE) substrate. Separation performance of membrane was optimized by controlling EGCG-PEI coating time, EGCG-PEI mass ratio and TMC concentration. Chemical structures and surface properties of the optimal membrane were characterized systematically. It was found that a dense and defect-free selective layer was formed with negatively charged surface and molecular weight cut off of 380 Da (stokes radius of 0.46 nm). Experimental results showed that the optimal membrane yielded a pure water permeability of 9.15 L/(m2·h·bar), high rejections of salts in a sequence of Na2SO4 (95.5%) > MgSO4 (86.3%) > MgCl2 (79.1%) > NaCl (61.4%) and excellent rejections of dyes (99.9% for neutral red, methyl orange and crystal violet) at 0.2 MPa. Moreover, the membrane exhibited favorable structural stability and long-term operation stability due to the strong EGCG-PEI adhesion property and robust interfacial interactions.

Influence of non-solvent chemistry on polybenzimidazole hollow fiber membrane preparation

Polybenzimidazole membrane materials have attractive H2/CO2 separation characteristics and high thermo-chemical stability for elevated temperature synthesis (syn) gas separations. The development of PBI membranes with a thin defect-free selective layer and porous support morphology is vital to achieving industrially attractive separation performance. This work is focused on developing a fundamental understanding of the liquid-liquid demixing-based phase inversion process for asymmetric PBI hollow fiber membrane (HFM) formation. The development of industrially attractive HFMs is a challenging process due to the complex interplay between phase equilibria, phase inversion kinetics, and interfacial mass transfer that exist during the liquid-liquid demixing process. Numerous parameters including the dope, bore, and outer coagulant chemistries and compositions significantly influence the HFM morphologies produced. Here, a systematic study is conducted to investigate the phase inversion process parameters including the roles of the non-solvent solubility and diffusivity parameters with respect to the solvent and PBI on the phase inversion process. Furthermore, the influence of dope, bore and coagulant chemistries and compositions on PBI HFM morphology are investigated. The fabricated PBI HFMs are evaluated for their ideal H2 and CO2 permeance and H2/CO2 selectivity at 250 °C to benchmark their separation performance.

Fabrication of defect-free Matrimid® asymmetric membranes and the elevated temperature application for N2/SF6 separation

N2/SF6 mixtures have been widely used in gas-insulated equipment. Highly efficient separation approaches are urgently required to recycle SF6 from the mixtures during equipment maintenance. Gas separation with glassy polymeric membranes is regarded as a promising approach due to the great size difference between SF6 and N2 molecules. However, the common membranes are quite low permeable for N2/SF6 mixtures. In this research, the dry-wet phase inversion was customized with sectionalized air gap, i.e., dried air at upper section and humidified air at lower section, to manufacture Matrimid® asymmetric membranes in bulk with an ultrathin and defect-free selective layer. After optimization, the defect-free selective layer could be thinned to 80 nm. The pure gas test at 25 °C revealed that JN2 was increased to 2.58 GPU, and αN2/SF6 was approximately 41, very close to the value in the literature [J. Membr. Sci., 2014, 452, 311]. Without the restriction from the usual PDMS coating layer for defect-blocking, the elevated temperature operation was able to be employed to further enhance gas permeation. At 128 °C, the selectivity αN2/SF6 was maximized to 115, and the relevant JN2 is 9.0 GPU. At 200 °C, the temperature close to the allowable upper limit, JN2 was increased to 15.9 GPU, and the relevant αN2/SF6 was 97. Furthermore, the mixed-gas test demonstrated the favorable long-term stability of the customized membranes, even under the extreme running condition with pressure up to 2.0 MPaG and temperature up to 200 °C. With high efficiency and throughput under elevated temperature operation mode, the Matrimid® asymmetric membrane with ultrathin and defect-free selective layer is a promising approach for the separation and recycle of SF6 from the N2/SF6 mixtures.

Poly (ethylene oxide) composite membrane synthesized by UV-initiated free radical photopolymerization for CO2 separation

The composite membrane with high CO2 permeance and high CO2/N2 selectivity was synthesized by UV-initiated free radical photopolymerization. Both supporting layer and selective layer were fabricated through photopolymerization. The supporting layer was synthesized by poly (ethylene glycol) diacrylate as monomer and polyethylene glycol as pore-forming additive, while the selective layer was synthesized by poly (ethylene glycol) methyl ether acrylate as monomer and dipentaerythritol hexaacrylate as cross-linker. The morphology, chemical structures and gas transport properties of the composite membranes were investigated by different techniques. Resulted composite membranes had defect-free selective layers with high CO2/N2 selectivity, ranging from 38.8 to 40.9, which were close to the intrinsic selectivity for the selective layer material. In addition, it is found out that the selective layer thickness and permeance were affected severely by varying the consumption of coating solution. The selective layer thickness decreased with the decrement of coating solution consumption, changing from 4.15 to 0.62µm, which led to a significant increase in CO2 permeance, ranging from 182 to 1889 GPU (25°C, 0.2MPa).

Nanocomposite Membranes via the Codeposition of Polydopamine/Polyethylenimine with Silica Nanoparticles for Enhanced Mechanical Strength and High Water Permeability.

A defect-free and stable selective layer is of critical significance for thin film composite membrane with excellent separation performance and service durability. We report a facial strategy for fabricating thin film nanocomposite (TFN) nanofltration membranes (NFMs) based on the codeposition of polydopamine, polyetheylenimine, and silica nanoparticles. Tripled water flux can be obtained from the TFN NFMs as compared with those NFMs without silica nanoparticles. This is ascribed to the improved wettability of the membrane surfaces and the enlarged pore sizes of the selective layer. The interfacial compatibility of the inorganic fillers and the polymer matrices can be enhanced by the electrostatic interactions of silica nanoparticles with polyethylenimine and the adhesive characteristics of polydopamine, resulting in a defect-free selective layer and then good rejection for both bivalent cations and neutral solutes. The rigid silica nanoparticles also improve the surface mechanical strength of the TFN NFMs effectively and lead to structural stability and compaction resistance during the long-term filtration process.

Dual-layer hollow fibers for gas separation processes produced by quadruple spinning

ABSTRACTA novel quadruple spinneret to produce dual-layer hollow fiber membranes by simultaneous spinning of two polymer solutions, using the dual precipitation bath technique is proposed. Hollow fibers aimed at gas separation processes were prepared in extrusion system specifically designed and built for this purpose. A polyurethane polymer was selected as the selective layer (outer-layer), while polyethersulfone was defined as the support (inner-layer). Activated carbon powder was added into the PU solution for further improvement of the transport properties. The hollow fibers showed good adhesion between the polymer layers and a defect-free selective layer. Representative results include a CO2/N2 selectivity of 43.

Nanofiltration membranes via co-deposition of polydopamine/polyethylenimine followed by cross-linking

Novel composite nanofiltration membranes (NFMs) were simply fabricated via co-deposition of mussel-inspired polydopamine (PDA) and polyetheylenimine (PEI) followed by glutaraldehyde (GA) crosslinking. A uniform, robust and defect-free selective layer was generated on the hydrolyzed polyacrylonitrile (HPAN) ultrafiltration membrane substrate, endowing the composite NFMs with high separation performance for multivalent ions. Zeta potential measurements indicate these NFMs are slightly positively charged, resulting in a salts rejection sequence of MgCl2>CaCl2>MgSO4>Na2SO4>NaCl at pH 5.5. The nanofiltration performance can be tuned by changing the co-deposition time and the mass ratio of dopamine/PEI. A mass ratio of 2/2 with 4h co-deposition is the optimum protocol for the membrane performances including surface hydrophilicity, water flux and salt rejection. Moreover, the composite NFMs show a good structural stability for immersing in ethanol or for long-term nanofiltration process.

Robust high-permeance PTMSP composite membranes for CO2 membrane gas desorption at elevated temperatures and pressures

This work covers the development of robust and stable in time poly[1-(trimethylsilyl)-1-propyne] (PTMSP) thin film composite (TFC) membranes with high CO2 permeance for its application in high pressure/temperature gas–liquid membrane contactors used for amine-based solvents regeneration. For the first time, a novel technique of two-layers coating was proposed and successfully implemented to apply thin top-layer on microfiltration support by adjusting PTMSP solubility in organic solvents. By using different catalytic systems such as NbCl5 and TaCl5/TIBA, it was possible to synthesize two polymer samples (“PTMSP-Nb” and “PTMSP-Ta”), which are insoluble and soluble in hexane, respectively. The first intermediate layer made of “PTMSP-Nb” was cast on the microfiltration porous support from a toluene solution in order to form a PTMSP-coated porous support. The solution of “PTMSP-Ta” in n-hexane was used for subsequent formation of a thin defect-free selective layer (second layer). By using two commercial polymeric (MFFK-1) and metal-ceramic (MC) supports with high surface porosity of 85% and 60%, respectively, it was possible to fabricate the tailor-made TFC membranes “PTMSP/MFFK” (top-layer – 0.5µm) and “PTMSP/MC” (top-layer – 1.2µm) with an initial CO2 permeance of 50 and 36.3m3(STP)/(m2hbar), respectively, and CO2/N2 selectivity of 3.5. A preliminary conditioning of the PTMSP/MC membrane at 100°C for 100h in air was applied for the accelerated relaxation of PTMSP top layer. The resulting robust PTMSP/MC TFC membranes still provided a high CO2 permeance of 1.6m3(STP)/(m2hbar) (~600GPU) without a noticeable decline of membrane performance within the long-term gas permeation testing (at least 250h) at 100°C. The PTMSP/MC membranes were further successfully utilized in HPT MGD for regeneration of 50wt% methyldiethanolamine (MDEA) at elevated temperature (100°C) and pressures (up to 30bar). A stable TFC membrane performance was demonstrated and no solvent leakage through the membranes was observed.

High performance dual-layer hollow fiber fabricated via novel immiscibility induced phase separation (I2PS) process for dehydration of ethanol

This pioneer study discloses a novel immiscibility induced phase separation (I2PS) process to fabricate high performance dual-layer hollow fiber for dehydration of ethanol via pervaporation. The I2PS process takes the advantages of phase separation phenomena occurring in immiscible blend dopes made of incompatible polymers. The immiscible blend dopes were simultaneously extruded through a triple orifice spinneret and fabricated into dual-layer hollow fibers consisting of an outer protective layer and an inner selective layer. A dense selective layer is formed at the outer-surface of the inner-layer due to the incompatibility between the polymer solutions of both inner and outer-layers as well as the shielding effect of the outer protective layer that provides a sufficient time for the formation of an almost defect-free selective layer during the phase inversion process. It is found that the phase inversion kinetics of the outer-layer dope solutions as well as the nature of outer-layer materials play great roles in determining the morphology of the outer protective layers and subsequently affects the permeance of the resultant hollow fibers. On the other hand, the selectivity of the as-spun fibers was found to be controlled by the outer surface of the inner-layer. The novel dual-layer hollow fiber spun from the combination of cellulose triacetate (CTA) and Ultem® possesses a high water permeance (29.33mol/m2hkPa) or water flux (1.77kg/m2h at 50°C) coupled with reasonable water/ethanol selectivity (824mol/mol) as compared to the pervaporation membranes available in the literatures. Heat treatment may enhance the selectivity of the aforementioned hollow fibers with some sacrifices of permeance. The present study may illuminate an innovative approach for hollow fiber fabrication in the future.

Plasticization-resistant hollow fiber membranes for CO 2/CH 4 separation based on a thermally crosslinkable polyimide

Decarboxylation-induced thermal crosslinking has been demonstrated to be effective for stabilizing membranes against plasticization in dense films. This study extends this promising crosslinking approach from dense films to industrially relevant asymmetric hollow fiber membranes. Crosslinkable asymmetric hollow fiber membranes were spun from a carboxylic acid containing polyimide, 6FDA-DAM:DABA. Dope and spinning conditions were optimized to obtain fibers with a defect-free selective skin layer. It is found that slightly defective fibers suffered severe selectivity loss after thermal crosslinking, suggesting that defect-free property is essential to the performance of the resulting crosslinked hollow fiber membranes. The crosslinked fibers were tested for CO 2/CH 4 separation. The excellent plasticization resistance under high pressure feeds (with highest CO 2 partial pressure of 400 psia) suggests that these robust membranes are promising for aggressive natural gas purification.

Factors affect defect-free Matrimid® hollow fiber gas separation performance in natural gas purification

Asymmetric hollow fibers have increasingly become attractive for high volume gas separation applications including natural gas purification. The formation of asymmetric structure with ultrathin defect-free selective layer and mechanically robust porous supporting layer without additional coatings is one of the major challenges. In this study, a commercially available polyimide material Matrimid® was used as the benchmark polymer material in fabrication of hollow fibers for natural gas purification. Major fabrication process variables including polymer solution formulation, bore fluid composition, dope and bore flow rates, and air-gap length were systematically investigated and optimized for fabrication of hollow fibers to achieve high CO2/CH4 separation performance. SEM images of cross-section indicated asymmetric structure of hollow fiber with a skin layer and sponge-like supporting structure. Hansen's solubility parameters were calculated for different solvent systems and coagulation medium, and the predicted trends of their effect to the formation of skin layer and gas separation performance were consistent with the experimental observation. High CO2/CH4 separation factors (ranging up to 67) are achieved which are among the highest reported for purely polymeric hollow fibers without post-treatments and exceeded the commonly reported bulk values of Matrimid®.

Dynamically formed inner skin hollow fiber polydimethylsiloxane/polysulfone composite membrane for alcohol permselective pervaporation

This work is concerned with the preparation of alcohol permselective inner skin hollow fiber composite membrane using a dynamic cross-flow coating method. The membranes were successfully prepared by dynamically coating polydimethylsiloxane (PDMS) on the inner surface of the polysulfone (PS) hollow fibers under a pressure-driven process. The strategy of alternating PDMS casting solution direction was firstly explored to ensure the formation of a uniform and defect-free selective layer. Subsequently, the effects of coating time, polymer concentration, and crosslink agent concentration on alcohol permselective performance were investigated. The appropriate conditions were: coating time, 60 min; PDMS concentration, 22.8 wt.%; and TEOS concentration, 1.5 wt.%. Under the given conditions, in the case of pervaporation separation of 8 wt.% ethanol–water mixture (50 °C), the membrane had a separation factor of 6.4 and a permeate flux of 265 g/(m 2 h), respectively. The pervaporation separation behavior of various alcohol/water mixtures with the alcohols being t-butanol, 2-propanol was also investigated. SEM, AFM and contact angle analyses confirmed the formation of selective layer on the inner surface of hollow fibers. Therefore, it indicates that the dynamic cross-flow coating technique is a unique approach to the construction of selective layer on porous hollow fiber substrates, which could emerge as a powerful technique for the preparation of a range of separation membranes.

Development of Integrally Skinned Asymmetric Polyaniline Hollow Fibers for Membrane Applications

Fabricating integrally-skinned asymmetric hollow fiber membranes from concentrated polyaniline solutions is technically challenging due to the complex issues associated with rapid gelation of the polymer solution and controlling the phase inversion process to create porous structures. Different processing parameters, such as solution composition, coagulants, and spinning conditions were shown to dramatically affect the morphology of immersion-precipitated polyaniline hollow fiber membranes. This study revealed that it is possible to minimize macrovoid formation while controlling the thickness of the selective layer and the porosity of the support layer. Asymmetric polyaniline hollow fibers with morphologies that range from a highly permeable microporous interface to a dense selective layer have been successfully produced. The membrane properties can be tailored by doping the polyaniline hollow fiber with the desired dopant acid during the fiber-spinning process. Asymmetric integrally-skinned polyaniline hollow fiber membranes with an ultra-thin (<1 μm), defect-free selective layer have been produced on a continuous basis (150 m/h). Furthermore, the thickness of the dense layer can be systematically increased up to 30 μm for combined heat-mass transfer applications such as the dehumidification of air.

Aqueous quenched asymmetric polysulfone hollow fibers prepared by dry/wet phase separation

A dry/wet phase separation process has been applied to a dry/wet spinning process to prepare asymmetric polysulfone hollow fibers for gas separations. A spinning solution consisting of tetrahydrofuran as a volatile solvent, dimethylacetamide as a less volatile solvent, and ethanol as a nonsolvent was used to induce phase separation in nascent hollow fiber membranes prior to their coagulation in water. Statistical experimental designs were used to optimize the permeation properties of these fibers which showed essentially defect-free selective skin layers with thicknesses of t̃ 1,200 Å (α O2/N2=5.8, ( P/L) O2=8.8 GPU, o.d.=580 μm). Scanning electron photomicrographs revealed many noncircular distortions in the hollow fiber lumens which appeared more prevalent at low draw ratios and when higher “water-activity” bore fluids were used. The influence of water activity in the bore fluid was investigated in detail with both salt/water and solvent/water mixtures.

Aqueous quenched asymmetric polysulfone membranes prepared by dry/wet phase separation

A process for producing ultrathin and defect-free selecting layers on asymmetric membranes using dry/wet phase separation has been reported earlier for organic coagulation media. The present study seeks to generalize this process by demonstrating the ability to form equally thin defect-free polysulfone membrane using an aqueous coagulation medium. In addition, this study presents the fundamental issues involved in altering flat sheet casting solutions to produce solutions, potentially useful for preparing hollow fibers within, defect free selective layers. Transport properties of integrally-skinned asymmetric polysulfone membranes coagulated in water are reported for the separation of various gases of 24°C and upstream driving pressures of 50 to 100 psig. These asymmetric membranes have essentially the intrinsic permselectivities of dense polysulfone films formed by simple solvent casting techniques. The novel aspect of both the organic and the aqueous quenched structures in their ultrathin selective layers, which can be made as thin as 200 to 800 Å under optimum formation conditions without introducing defects that reduce permselectivity. Unlike other thin-skinned structures reported recently, the present membranes do not require any post-formation solvent treatments or silicone coatings to achieve essentially the intrinsic separation factors of the material. High resolution scanning electron photomicrographs of the membrane structures are presented to clarify the morphologies responsible for their desirable properties.

Gas permeation through composite membranes

Composite gas separation membranes generally consist of a selective, ultrathin top layer backed by a nonselective porous support. The top layer performs the separation and the support provides mechanical strength. A composite membrane will possess a selectivity close to the intrinsic selectivity of the top layer only if most of the permeation resistance lies within this top layer. To make high flux composite membranes, it is necessary to minimize the thickness of the selective layer. This, in turn, means the porous support must be very permeable. A simple model shows that the minimum thickness of a defect-free selective layer in a composite membrane having the intrinsic selectivity of the permselective layer is limited by the resistance of the porous support membrane. The model is supported by experimental gas and vapor permeation data for polysulfone-silicone rubber composites.