Dense Selective Layer Research Articles

Original Installation for Researching the Process of Forming Polysulfone Hollow Fiber Membranes

In this work an original installation (manipulator) has been created that allows one to obtain up to 30 samples of hollow fiber membranes in one molding cycle, while simultaneously varying the molding conditions in a wide range (polymer concentration, nature of solvent and precipitant, exposure time in air and in a precipitant environment, post-processing and washing modes samples, diameter of the carrier needle). This installation makes it possible to move to a fundamentally higher level of accumulation of experimental data on the relationship "the composition of the spinning solution - the structure of the hollow fiber membrane - the separating properties of the membrane." It will also make it possible to involve in these studies new laboratory samples of polymers whose synthesis volumes are insufficient for the existing methods of obtaining laboratory samples of hollow fiber membranes. The principle of operation of the manipulator was worked out when obtaining mini-samples of hollow fiber PSF membranes from 24 wt. % PSF solution in NMP with the addition of 19 wt. % PEG-400 blowing agent on a carrier needle with external deposition. Mini-samples were obtained for studies of morphology, mechanical, transport and separation properties in one molding cycle of the manipulator. The properties of mini-samples of hollow fiber PSF membrane were compared with the properties of a membrane made by the method of “dry-wet” molding with internal deposition from a solution of the same composition. It was found that the porous structures of the membranes differ significantly from each other. In a hollow fiber PSF membrane obtained on a manipulator, the porous structure was spongy with separate macrovoids of various shapes. However, in the membrane obtained by the “dry-wet” method, a dense selective layer was formed on the inner side of the backing layer of elongated finger-shaped pores. It is the formation of spongy pores along the entire perimeter of the fiber wall that led to a decrease in the permeability of the hollow fiber PSF membrane obtained on the manipulator. Thus, not only the composition of the solution, but also the molding method makes a significant contribution to the properties of the membrane.

Preparation and modification of thin film composite membrane using a bulky dianhydride monomer

AbstractOne of the most effective methods to modify thin film composite (TFC) membranes is changing the chemistry of top selective layer by different monomers and different monomer concentrations. Herein, we report the preparation of modified TFC membranes using a pyromellitic dianhydride (PMDA) mixed with organic phase (trimesoyl chloride) and meta phenylene diamine (MPD). By manipulating the PMDA amount in organic phase, the structures and chemical compositions of polyamide selective layer could be modified. It was realized that the presence of PMDA could result in a modified membrane with higher surface roughness, less dense selective layer, more surface charge density, and better hydrophilic properties and consequently less fouling. The optimum PMDA concentration was found 0.05 wt%, such that the obtained membrane had 35.6 L m−2 h−1 pure water flux, about 1.6‐fold higher than the reference membrane with similar salt rejection. Fouling intensity for the reference membrane was 38.1%, while for the modified membranes it decreased to 16.7%.

Fabricating thin-film composite membranes for pervaporation desalination via photo-crosslinking

Pervaporation (PV) is a low-energy membrane separation technology widely studied for desalination, water treatment and solvent recovery where heat is applied to vaporize the feed solution and a vacuum is applied at the permeate side to drive the separation of miscible mixtures. The heart of this technology – polyvinyl alcohol (PVA) that is commonly used as the separation layer of polymer membranes is often fabricated over long cross-linking durations, high temperatures or toxic cross-linkers to prevent it from being dissolved in water. Here we overcome these intrinsic material limitations with a photo-crosslinkable polymer, poly(vinyl alcohol)-stilbazol quaternized (PVA-SBQ), commonly deployed in the printing industry, as the dense selective layer of a thin-film composite PV membrane. The separation performances of the resultant PVA-SBQ/nanofiber thin-film composite membranes were optimized as a function of photo-crosslinking conditions where ultra high water fluxes of 122.6 ± 10.8 kg·m−2·h−1 were achieved while desalinating 3.5 wt% NaCl from water at 75 °C and a permeate pressure of 100 Pa. This high PV desalination performance was also maintained during brine (containing 10 and 20 wt% of NaCl) treatment.

Surface wettability switching of a zeolitic imidazolate framework‐deposited membrane for selective efficient oil/water emulsion separation

Oil/water emulsion separation especially for surfactant ˗ stabilized emulsions remains a big challenge for environment protection. Membrane separation technology based on special wettable materials is promising for oil/water emulsion separation due to low energy consumption and good separation performance. In this contribution, zeolitic imidazolate framework‐8 (ZIF‐8) membranes were prepared by vacuum deposition of ZIF‐8 particles with different sizes on a regenerated cellulose support. Microparticles gave a better performance as compared to nanoparticles in terms of the permeate flux, while the presence of large interparticle voids adversely affected the rejection efficiency. Such the detriment could be overcome by a sequential deposition of microparticles followed by nanoparticles to form a dense selective layer for oil/water emulsion separation. Hydrophobic modification of pre‐formed ZIF‐8 particles switched the surface wettability of ZIF‐8 deposited membranes from hydrophilic to hydrophobic, offering versatility of the deposited membrane for both oil‐in‐water and water‐in‐oil emulsion filtration.

Impacts of Green Synthesis Process on Asymmetric Hybrid PDMS Membrane for Efficient CO2/N2 Separation.

The effects of green processes in hybrid polydimethylsiloxane (PDMS) membranes on CO2 separation have received little attention to date. The effective CO2 separation of the membranes is believed to be controlled by the reaction and curing process. In this study, hybrid PDMS membranes were fabricated on ceramic substrates using the water-in-emulsion method and evaluated for their gas transport properties. The effects of the tetraethylorthosilicate (TEOS) concentration and curing temperature on the morphology and CO2 separation performance were investigated. The viscosity measurement showed that, at specific reaction times, it is benefit beneficial to fabricate the symmetric hybrid PDMS membranes with a uniform and dense selective layer on the substrate. Moreover, the a high TEOS concentration can decrease the reaction time and obtain create the a fully crosslinked structure, allowing more efficient CO2/N2 separation. The separation performance was furtherly improved with in the membrane prepared at a high curing temperature of 120 °C. The developed membrane shows excellent CO2/N2 separation with a CO2 permeance of 27.7 ± 1.3 GPU and a CO2/N2 selectivity of 10.3 ± 0.3. Moreover, the membrane shows a stable gas separation performance of up to 5 bar of pressure.

Electrospun transition layer that enhances the structure and performance of thin-film nanofibrous composite membranes

Electrospun nanofibers with completely interconnected pore structures are increasingly considered as substrates for fabricating thin-film nanofibrous composite (TFNC) nanofiltration (NF) membranes. The poor adhesion between the large-pore substrate and dense selective layer has been considered as the bottleneck for wide application. This work presents the role of a transition layer on membrane structure as well as performance enhancement. Specifically, both polyethyleneimine (PEI)/polyacrylonitrile (PAN) transition layer and PAN substrate layer are formed by electrospinning. Interfacial polymerization (IP) is subsequently performed with trimesoyl chloride and piperazine. The introduction of the PEI modified transition layer improves the hydrophilicity, provides additional reaction sites for IP, and avoids the polymerization in the pores of nanofibers. The resultant composite NF membranes exhibit a compact and defect-free selective layer. Such an optimal structure leads to an excellent separation performance of 95.6% for orange ΙΙ dye (MW = 350.32 g/mol) rejection and 38.5 L m−2h−1 bar-1 for the pure water permeability. It exhibits stable long-term performance during low pressure (0.2 MPa) nanofiltration, which will afford significant directions for the actual application of the TFNC membranes in water purification and other fields.

(Invited) Development of Durable and Low Cost Membrane Humidifiers for Fuel Cell Systems

In order to operate effectively, currently commercialized fuel cell proton exchange membranes require sufficient hydration to ensure good proton transport. Hydration also ensures that membrane in the fuel cell stack will have a longer operational lifetime. In order to ensure adequate hydration in the stack, most liquid-cooled PEM fuel cell systems use external humidification. Without external humidification, the fuel cell must be operated at a lower operating temperature to ensure that the stack will not dry out. Operating the stack at lower temperature requires that the coolant loop in the system must reject a greater amount of heat, necessitating a large radiator, which increase system volume. Furthermore, operating at lower fuel cell stack temperatures leads to a significant trade-off in terms of stack power output and acceptable environmental operating ranges.Membrane humidifiers are an efficient form of external humidification that transport humidity through a membrane from the cathode exhaust to the incoming cathode supply air stream. In these humidifiers, membranes with a dense selective layer are used to transport water vapour without allowing oxygen and nitrogen to diffuse through the membrane. Since the membrane humidifier is coupled to the stack cathode exhaust and supply, in order to maintain its function, the membrane humidifier must not leak air or else reactant oxygen will bypass the stack through the humidifier. The humidifier must maintain a high level of water vapour transport and low air leakage over the lifetime of the device.In this presentation we present our work developing low-cost selective membranes for fuel cell humidification. These membranes are functional over a wide temperature range (-30 to +110°C) and have high water vapour permeance. They are resistant to contamination, relative humidity cycling, freeze-thaw cycling, oxidation, hydrolysis, and acidic conditions that are present in the cathode exhaust.This presentation will also discuss the incorporation of these flat-sheet membranes in to heat and mass exchangers (membrane humidifier cartridges). These cartridges are compact with a high membrane-to-volume ratio, high water vapour transport, and have low pressure loss. The membrane humidifiers are robust to a wide range of operating conditions. Furthermore, the design of these cartridges allows the humidifier to be manufactured at a low cost in high manufacturing volumes.Finally, this presentation will discuss membrane-based heat and mass exchanger performance at different operating conditions. A performance model will be presented which summarizes humidifier performance as a function of membrane properties, exchanger design and architecture, and fuel cell operating conditions.

Electrostatic assembly construction of polysaccharide functionalized hybrid membrane for enhanced antimony removal

There is a growing demand for heavy metal removal by membrane technology in real applications. However, few studies were reported concerning antimony (Sb) removal by membrane technology. Herein, a novel thin film nanocomposite (TFN) membrane comprising an alginate (SA) selective layer and a polyether sulfone (PSF) support membrane incorporating chitosan functionalized iron nanocomposite has been firstly developed for Sb removal via electrostatic self-assembly. The support matrix membrane contained iron nanocomposite (denoted as CIM) retained high water flux and porosity, and it reached a maximum removal capacity of 16.5 and 13.6 mg/g for Sb(III) and Sb(V) with nanofiller loading rate of 20% during static experiments, respectively. The coated SA top layer endowed the hybrid membrane (denoted as SA-CIM) to have a lower membrane flux, and have stronger retention abilities for Sb species than that by CIM during dynamic filtration experiments. The SA-CIM membranes also possess tolerable reversibility towards Sb removal. Benefiting from the negatively-charged dense selective layer and high adsorption capacity of the iron nanocomposites, the SA-CIM membranes demonstrated an enhanced removal capacity for Sb species via steric hindrance effect, electrostatic repulsion and adsorption. Our study offers a simple method to remove Sb by a novel polysaccharide functionalized hybrid membrane.

Super selective dual nature GO bridging PSF-GO-Pebax thin film nanocomposite membrane for IPA dehydration

ABSTRACT In this work, Graphene Oxide (GO) is embedded in both selective hydrophilic layer and porous hydrophobic substrate creating a mutual bridge between the two surfaces. Pristine 1–3 µm microporous PSF prepared via dry/wet phase inversion techniques with contact angle of 74.12° has been further study with GO embedded Pebax dense selective layer. This dual nature thin film nano composite TFNC membranes managed to reduce the water contact angle down to 37.18°. As for the IPA dehydration study, the total flux up to 1.19 kgm−2h−1 and 0 wt% IPA detected in permeate was achieved with 20 wt% water feed at 30°C.

Polydopamine nanoparticles modified nanofiber supported thin film composite membrane with enhanced adhesion strength for forward osmosis

To minimize the internal polarization concentration (ICP) in FO process and improve the structural stability of thin film composite (TFC) forward osmosis (FO) membrane, a reactable hydrophilic nanofiber substrate was prepared by vacuum filtrating an interlayer composed of polydopamine nanoparticles (PDA NPs) onto an electrospun polyacrylonitrile nanofiber substrate. Then a TFC FO membrane with dense selective layer, low structure parameter, high water permeability and low salt permeability was prepared by the interfacial polymerization thereon. According to the T-peel test, the resulting TFC FO membrane showed greatly improved adhesion strength between selective layer and substrate due to the chemical bonding and entanglement between polyamides and the PDA NPs, and the strong adhesion between PDA NPs and the nanofibers. The ICP in FO process for the resulting membranes were minimized due to the application of the nanofiber substrates with low structure parameter (in the range of 290 ± 36 μm–354 ± 20 μm). These FO membranes performed well in the heavy metal ions contained water treatment, showed 1–2 times higher water flux (29.2 ± 0.3 L/m2h) compared with commercial TFC membrane (HTI-TFC) and high heavy metal ions rejection (99.5% ± 0.4%, 99.1% ± 0.5%, 98.1% ± 0.7% for Cr3+, Cu2+ and Ni2+, respectively) when using 2.0 M NaCl as draw solution. The findings provide the pathway for the design of nanofiber based FO membrane with improved structure stability and high performance by interfacial polymerization.

Selection of crosslinkers and control of microstructure of vapor-phase crosslinked composite membranes for organic solvent nanofiltration

A fast and green chemical modification method has been developed by using amine vapor to fabricate hybrid composite membranes consisting of crosslinked polyimides and nano-size UiO-66-NH2 metal-organic frameworks (MOFs) with higher selectivity for organic solvent nanofiltration (OSN). Three amine vapor-phase crosslinking (VPC) reagents; namely, N,N′-Bis(3-aminopropyl)-1,3-propanediamine (APPD), 1,4-Bis(3-aminopropyl) piperazine (BAPP) and polyethylenimine (PEI) were employed. It was found that APPD and PEI vapor are more effective to generate smaller pores in the dense-selective layer than the BAPP vapor. In addition, the PEI modified membranes are more suitable for polar and aprotic solvents nanofiltration applications, while the APPD and BAPP modified membranes are for applications involving wider solvents. The selected hybrid membranes cast on polyester (PET) non-woven fabrics have rejections of tetracycline (MW = 444 gmol-1) more than 90% in four organic solvents under continuous cross-flow filtration tests for over 120 h. All fluxes and selectivities remain stable without showing significant fluctuations, evidencing great potentials of designed membranes for industrial applications. Besides, the VPC modification process is environmental-friendly because it uses a tiny amount of amine vapors without extra chemical waste production. Therefore, this study offers a promising sustainable, greener and scalable strategy to produce high performance OSN membranes.

Effect of the Formation of Ultrathin Selective Layers on the Structure and Performance of Thin-Film Composite Chitosan/PAN Membranes for Pervaporation Dehydration.

The aim of the study is to improve the performance of thin-film composite (TFC) membranes with a thin selective layer based on chitosan (CS) via different approaches by: (1) varying the concentration of the CS solution; (2) changing the porosity of substrates from polyacrylonitrile (PAN); (3) deposition of the additional ultrathin layers on the surface of the selective CS layer using interfacial polymerization and layer-by-layer assembly. The developed membranes were characterized by different methods of analyses (SEM and AFM, IR spectroscopy, measuring of water contact angles and porosity). The transport characteristics of the developed TFC membranes were studied in pervaporation separation of isopropanol/water mixtures. It was found that the application of the most porous PAN-4 substrate with combination of formation of an additional polyamide selective layer by interfacial polymerization on the surface of a dense selective CS layer with the subsequent layer-by-layer deposition of five bilayers of poly (sodium 4-styrenesulfonate)/CS polyelectrolyte pair led to the significant improvement of permeance and high selectivity for the entire concentration feed range. Thus, for TFC membrane on the PAN-4 substrate the optimal transport characteristics in pervaporation dehydration of isopropanol (12–90 wt.% water) were achieved: 0.22–1.30 kg/(m2h), 99.9 wt.% water in the permeate.

Fouling of composite water vapor transport membranes by aerosol nanoparticles

Composite membranes combining a structural microporous substrate with a dense selective layer are used in various water vapor transport applications. The impact of exposure to aerosols on the water vapor permeance is investigated to develop an understanding of the potential for air pollution to degrade the membrane performance. Samples of commercial membranes were loaded with hygroscopic NaCl and non-hygroscopic graphite nanoparticles. Permeance measurements showed that the deposition of both particle types under dry loading conditions (RH<20%) had minimal influence on the membrane. However, when membranes loaded with hygroscopic particles in dry conditions were subsequently exposed to an elevated humidity (RH>75%) and the associated surface condensation, the membrane permeance reduced by up to 16%. This permeance reduction is shown to be caused by pore narrowing, which results in increased resistance of the microporous substrate. SEM analysis of the fouled membrane surfaces confirmed a reduction in the average pore diameter of degraded samples, proportional to the fouling degree. The reversibility of the fouled membrane permeance, achieved through membrane cleaning, implies that re-crystallization of salt ions, entrained onto the edges of the substrate pores in an aqueous form, is a potential explanation for the permeance changes observed.

Asymmetric mixed-matrix membranes incorporated with nitrogen-doped graphene nanosheets for highly selective gas separation

Conventional mixed-matrix membranes (MMMs) possess a dense structure with a filler material uniformly dispersed within the polymer matrix to engineer the transport properties and achieve enhanced gas separation performances with respect to pure polymeric membranes. However, a dense membrane structure increases the transport resistance and undesirably requires a high filler loading to see a substantial enhancement in separation performances. To address this problem, we fabricated asymmetric MMMs that have a thin-selective layer of around 0.5 μm and a low loading (0.03–0.10 wt%) of nitrogen-doped graphene (N-G) nanosheets. The presence of the nitrogen- and oxygen-containing functional groups on the N-G nanosheets ensured good compatibility between filler and polymer matrix, which resulted in strong polymer/filler interfacial adhesions. The N-G nanosheets were also found to be capable of migrating to the top of the membranes during phase inversion. Hence, despite the low filler loadings used in this work, the capacity of the dense selective layers was greatly enhanced. Based on our experimental results, 0.07 wt% and 0.10 wt% loading of N-G nanosheets can improve both O2/N2 (126.9%) and CO2/N2 (45.8%) selectivities with respect to pure polymeric membranes, resulting in O2/N2 separation performance surpassing the Robeson upper bound.

Application prospects of dense gas separation hollow fibers based on poly(4-methyl-1-pentene)

The paper presents the new data of gas transfer characteristics of semicrystalline poly(4-methyl-1-pentene) (PMP) as for films as for hollow fibers (HFs) with dense selective layer. The experimental data obtained earlier made it possible to calculate the gas transport characteristics of the amorphous and crystalline phases of PMP for some gases at temperature from 10 to 80 °C (below and above Tg). On the basis of these data and using the correlation approach, an assessment of the expected gas transport characteristics of HFs based on PMP was carried out in this work for helium and lower hydrocarbons, as well as for water vapors. It is shown that crystalline phase is very permeable and higher crystallinity degree leads to the increasing of PMP selectivity. Laboratory-scale membrane module with commercial hollow fibers of PMP was prepared and successfully tested by the differential method of gas permeability. Application of crazing method which should increase the crystallinity degree of PMP seems promising for the production of perspective HFs with improved transport and separation performance compared with the existing ones.

The Influence of Polymer Concentration and Formation Technique on Gas Transport and Gas Sorption Properties of Copolyetherimide-Based Composite Membranes Containing MIL-101 Filler

Composite mixed-matrix membranes for gas separation containing copolyetherimide Siltem® as the polymer matrix and metal-organic framework MIL-101 (10 wt %) as the active filler, were obtained using dry and wet-dry formation techniques. It has been found that the polymer concentration in the initial solution does not significantly affect the CO2 and CH4 permeability of the film membranes obtained by dry molding. The addition of MIL-101 increases the CO2/CH4 selectivity of the dry-formed membranes approximately by 2 times compared to the selectivity of the filler-free polymer membranes. The materials synthesized by the wet-dry formation possess increased permeability and inverted CO2/CH4 selectivity, which indicates a change in the gas transport mechanism. With the increase of polymer concentration, the selectivity of the membranes obtained by the wet-dry technique, increases significantly due to the formation of the dense selective layer.

Influence of integrating graphene oxide quantum dots on the fine structure characterization and alcohol dehydration performance of pervaporation composite membrane

Graphene oxide quantum dots (GOQDs), a carbon-based nanomaterial resembling the structure of graphene oxide but with a smaller size of <100 nm, was incorporated in a poly(vinyl alcohol) (PVA) dense selective layer supported on a polysulfone substrate to prepare a composite hybrid pervaporation (PV) membrane for alcohol dehydration. The addition of GOQDs into the PVA polymer matrix provided a tortuous path for the permeating molecules but augments the separation efficiency of the membrane. The presence of hydroxyl and carboxyl groups on the confined nanosize of GOQDs sheets attract the polar groups of the permeants within the polymer matrix. The physicochemical properties of the membrane were characterized. An interesting observation was found when the membranes with GOQD (PVAx-GOQD300) were tested for aqueous propanol isomers mixture, isopropanol (i-PrOH) and n-propanol (n-PrOH). The PVAx-GOQD300 membrane showed lower separation performance for linear alcohol compared with the sterically hindered alcohol. Positron annihilation lifetime spectroscopy (PALS) results revealed that membranes integrated with GOQD behaved differently in both propanol isomers. The PVAx-GOQD300 membrane showed larger free volume size in n-PrOH/water than i-PrOH/water mixtures while there was not much difference in the absence of GOQDs. PALS results justified the low separation of the membrane with linear alcohol/water mixtures. Furthermore, the long operating time, high operating temperature, and high water content in feed shows the potential stability of the membrane for practical application.

Combination of potential nanomaterials for intermediate temperature oxygen membranes on the base of δ-Bi2O3/Ag

The new architecture of IT thin oxygen membranes on a flexible support made of stainless steel grid is proposed. This membrane is designed to realize clean coal energy with combustion of pulverized low ranked solid fuels in oxygen enriched atmosphere using purified flue gases as a sweep gas. Multilayer membrane with thickness about 40 μm is consist of metal grid with deposited protected layer and filled by porous agglomerates of e-conducting perovskite, dense selective layer on the base of multilevel nanocermet δ-Bi2O3/Ag, protected porous layer and supported Pd/Pt-catalyst layer. Mechanochemical ceramic method is the main approach to designing of nanomaterials with interpenetrating and core-shell microstructures. Compatible combination of materials and operations is determined for creating oxygen membranes with operating temperature 500-550 °C. The best combination of materials successfully passed the degradation test at 500 °C for 500 hours.

Nanofiltration membrane cross‐linked by m‐phenylenediamine for dye removal from textile wastewater

In this work, m‐phenylenediamine (MPD) is used to prepare cross‐linked polyetherimide (PEI)‐based nanofiltration (NF) membrane for treatment of dye containing wastewater. The effects of dope solution composition, cross‐linking time, and dye concentration on membrane performance are investigated. Results indicate that the rejection of dye is increased with the increase of acetone concentration in the dope solution, cross‐linking time, and dye concentration. Meanwhile, membrane flux showed the opposite trend. With the aid of SEM and FTIR analysis, the cross‐linking between MPD and PEI is confirmed. The cross‐linked membrane has thicker and dense selective layer compared to the unmodified membrane. The cross‐linked NF membrane (PEI: 15 wt%; acetone: 20 wt%; cross‐linking time: 10 minutes) showed good performance in filtration of synthetic dye wastewater (Reactive Red 120, 1500 ppm) with 98% dye rejection and 0.013 L m−2 hour−1 of flux at relatively low operating pressure (60 psi).

Modeling and simulation of CO2 capture in aqueous ammonia with hollow fiber composite membrane contactors using a selective dense layer

Aqueous ammonia is a promising chemical absorbent for CO2 capture but its high volatility leads to important solvent leakage necessitating expensive solvent recovery strategies. This study investigates the potential of using hollow fiber membrane contactors with composite membranes instead of packed columns to reduce solvent leakage. In this study, we used a composite membrane with a thin, dense selective layer (non-porous) coated on a microporous support to favor CO2 transfer over NH3. We developed one-dimensional adiabatic multi-component transfer models to simulate the capture process using both hollow fiber membrane contactors and packed columns. These models were validated with laboratory-scale and pilot-scale data. Simulations under industrial relevant operation conditions were conducted to investigate process performance as a function of membrane characteristics, i.e. membrane dense layer thickness, selectivity and the micro-porous support mass-transfer coefficient. For contactors using homemade selective membranes, the CO2 specific absorption capacity was of 2.7 mol/m3/s, which is roughly twenty times higher than values for our simulations in packed columns. The corresponding NH3 slip reduction was of 4.3%. A parametric study revealed that thick dense membrane layers led to greater reductions of ammonia slip but that this corresponded to lower specific CO2 absorption capacity, highlighting an important trade-off between two performance parameters.