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In view of the proliferating applications involving membrane-based separation, the inevitable membrane fouling phenomenon is well-studied. However, the influence of foulant shape on membrane fouling remained incompletely understood. Therefore, the goal of this study was to investigate the fouling behaviors by three different shapes of latex particles (namely, peanut, pear and sphere) with similar mean diameters in the range of 3.5–3.8 μm. External and internal fouling were studied using polycarbonate track-etched (PCTE) membranes with nominal pore sizes of 2 and 8 μm, respectively. Flux data were taken, and fouling models, optical coherence tomography (OCT) and field-emission scanning electron microscope were employed. Regarding external membrane fouling, the sphere-shaped particles displayed the worst flux decline, which is tied to the greatest tendency to deposit on the membrane pores, as revealed by the network fouling model. As for internal fouling, the flux decline curves were more similar among the particle types. The fouling model shows the sphere-shaped particles gave the densest internal cake, which caused slightly steeper flux decline in the latter part of the filtration. Furthermore, OCT indicates that the highest fouling fractions at 7.5 μm below the membrane were exhibited for the peanut-shaped particles, which is due to the preferential vertical orientation resulting in deeper penetration. Results here highlight the non-negligible influence of foulant particle shape, which warrants further understanding and accounting for membrane fouling studies. [Display omitted] • Used OCT to reveal influence of foulant particle shape on membrane fouling. • Spheres were more prone to pore-blockage and thus gave worst flux decline. • Non-spherical particles gave less dense internal cakes due to rotational effects. • Peanut-shaped particles penetrated deeper due to preferential vertical orientation. • Particle shape affected flux decline more when external fouling was dominant. [ABSTRACT FROM AUTHOR]

In a gas–liquid membrane contactor, a larger pore size can result in a lower membrane mass transfer resistance. However, the membrane pore size is usually limited by the concern of pore wetting, e.g. a large pore size means a higher wetting tendency. As a breakthrough, this paper reported a porous polyetherimide (PEI) hollow fiber membrane with high surface porosity and large pore size to minimize the membrane mass transfer resistance by using a triple-orifice spinneret in the hollow fiber spinning process, and followed by a novel approach of fluorinated silica (fSiO2) nanoparticles (NPs) incorporation to make the membrane surface highly hydrophobic and chemical resistant to prevent the membrane from wetting caused by the large pore size on the membrane surface. The newly developed composite hollow fiber membranes showed the advancing contact angle value of 123.3°, receding contact angle value of 107.2°, and contact angle hysteresis of only 15.9°, indicating the high water resistant property. The composite membrane also exhibited a higher rigidity property compared with the original PEI substrate. The CO2 absorption flux of the composite membranes was investigated in both physical and chemical absorptions in a gas–liquid membrane contactor system. The membrane contactor showed a stable performance throughout the 60d long-term operation using a 2M sodium taurinate aqueous solution as the liquid absorbent. The highly hydrophobic composite hollow fiber membrane was able to outperform a conventional polymeric hydrophobic membrane in term of superior gas absorption flux and outstanding long-term stability, suggesting that the formation of organic–inorganic composite membranes is an effective way to enhance the feasibility of membrane contactor processes for practical applications. The results demonstrated the important role of membrane fabrication and modification techniques in facilitating the commercialization of membrane contactor technology. [ABSTRACT FROM AUTHOR]

Abstract: A systematic simulation has been carried out in membrane contactors to study CO2 capture by water and diethanolamine (DEA) aqueous solutions from a CO2/N2 mixture under the wetted and the non-wetted operation modes. Two types of microporous hollow fiber membrane modules made of polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fibers were used to conduct CO2 absorption experiments. The corresponding experimental data were used to verify simulated results. Experimental study showed that the membrane wetting could not be completely avoided especially in the case of chemical absorption. Both experimental and theoretical study disclosed that the membrane wetting would result in a significant drop of CO2 flux. The simulation results further revealed that for the physical absorption of CO2 by water, the proportion of membrane phase resistance in the overall mass transfer resistance increased from less than 5 to about 90% when the operation mode was shifted from non-wetted to wetted. As for the chemical absorption, analysis on the mass transfer resistance revealed that the ratio of the membrane resistance increased sharply from 10 to 70% when only 10% membrane length was wetted. With the introduction of an extra resistance caused by the membrane wetting, the mass transfer in the wetted membrane phase finally became the rate-controlling step. It was not as effective to enhance the CO2 flux by increasing the inlet gas velocity or the liquid velocity as in the non-wetted mode of operation. [Copyright &y& Elsevier]

Abstract: In this paper, poly(vinylidene fluoride) (PVDF) hollow fiber membranes were prepared to make membrane contactors for CO2 capture. The hollow fiber membranes were spun with two different dope solutions at different shear rates in order to understand the influences of the rheological characteristics of the dope solution on the membrane structure and the system performance for CO2 adsorption. The membrane''s pore size distribution was characterized using the log normal distribution with the measurement of the rejection characteristics of dextran through the membrane. Lab-scale membrane contractors were finally prepared using resultant PVDF hollow fibers to conduct preliminary study on CO2 absorption. With the increase of the shear rate, the water permeation flux was increased and then maintained nearly the same level. The MWCO and CO2 absorption flux of the hollow fiber membranes spun from 17wt.% PVDF dope at different shear rates were almost the same. But for the hollow fibers spun from 20wt.% PVDF dope, increasing the shear rate in the spinning process resulted in bigger geometric mean diameters, larger MWCO and higher CO2 absorption flux. Compared with the hollow fiber membranes spun with high polymer concentration (20wt.%), the hollow fibers spun from low PVDF concentration (17wt.%) had a wider pore size distribution and bigger pores, which dominated the MWCO of hollow fiber membranes and presented a significant influence on the membrane performance. [Copyright &y& Elsevier]

Abstract: In order to gain a better understanding of CO2 absorption in a hollow fiber membrane contactor, theoretical simulations have been performed to describe CO2 capture by distilled water and aqueous diethanolamine (DEA) solutions. The studies were focused on gas phase behaviors by taking the distributions of gas concentration and gas velocity along the flowing direction into account. The corresponding experiments were also carried out in a Celgard MiniModule to verify the simulated results. In the case of physical absorption, both simulation and experimental results indicate that CO2 flux increases with the liquid velocity, while the inlet gas velocity has no significant effect on CO2 flux. The mass transfer resistance mainly exists on the liquid side. In the case of chemical absorption, the CO2 flux is significantly influenced by the inlet gas velocity while the liquid velocity has a limited effect. The analyses of CO2 concentration profiles in the gas and liquid phases as well as DEA concentration profile in the liquid phase reveal that CO2 concentration in the gas phase serves as the rate-determining factor for the reaction between CO2 and DEA. The significant CO2 loss in the gas phase results in an interesting DEA concentration profile near the liquid–gas interface. There exists an effective module length L eff, which suggests that increasing the module length is not a useful approach to enhance CO2 absorption when the module length is longer than L eff. [Copyright &y& Elsevier]

Abstract: Mixed-conducting ceramic membranes for air separation were studied experimentally and theoretically. Dense tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes were successfully prepared by the plastic extrusion method. Oxygen permeation through the membranes was measured at different oxygen partial pressures in the feed stream (0.1925 × 105 to 1.01325 × 105Pa (0.19–1.0atm)) and different temperatures, between 700°C and 900°C. The effects of air flow rate and sweeping helium flow rate on the oxygen permeation were also investigated. A mathematic model has been developed to simulate the process of air separation in the BSCF membrane permeator for four cases, each under co-current or cross-flow patterns with purge or vacuum operation. The calculated results from the cross-flow pattern with purge operation are in good agreement with experimental data. Moreover, parametric study under co-current flow with vacuum operation reveals that the air flow rate should be sufficiently high to fully utilize the separation capacity of the membrane. Higher vacuum levels and smaller inner diameter of the membrane tubes are essential to achieve higher separation efficiencies. The theoretical solutions also indicate that there is an optimal shell side pressure and optimal tube length, under which the oxygen productivity reaches its maximum with a fixed air flow rate. [Copyright &y& Elsevier]

We have studied the rheological characteristics, membrane formation and CO2-induced plasticization phenomenon of almost defect-free copoly(4,4′-diphenylene oxide/1,5-naphthalene-2,2′-bis(3,4-dicarboxylphenyl) hexafluoropropane diimide) (6FDA-ODA/NDA) hollow fiber membranes. Experimental results indicate that the dope rheology changes from a Newtonian to non-Newtonian fluid with elasticity when increasing shear rate. The asymmetric 6FDA-ODA/NDA hollow fiber membranes exhibit plasticization if the feed pressure is greater than 75 psi. Chemical cross-linking modifications with p-xylenediamine were conducted at ambient temperature in order to enhance anti-plasticization characteristics of 6FDA-ODA/NDA hollow fiber membranes. Chemical modified membranes show a significant reduction in CO2-induced plasticization at least up 550 psi (37.4 atm or 37.9 × 106 Pa) if the immersion time is great 1.5 min. However, deteriorations in permeance and permselectivity are clearly observed if the immerse time is greater than 5 min. FTIR spectra confirm that the 6FDA-ODA/NDA polyimide can be chemically cross-linked with p-xylenediamine, while wide-angle X-ray diffraction (WAXD) spectra imply the d-space of 6FDA-ODA/NDA membranes slightly decreases with an increase in cross-linking time. SEM pictures show that the outer selective skin as well as substructure becomes denser and thicker after chemical modification. The slight decrease in d-space may account for the permselectivity increase for the cross-linked membranes if the immersion time is short. However, the benefit of d-space reduction is offset by a denser and more compact selective skin and substructure when the immersion is prolonged. [Copyright &y& Elsevier]

The transport of olefin and paraffin namely ethane, ethylene, propane and propylene in aromatic poly(1,5-naphthalene-2,2′-bis(3,4-phthalic) hexafluoropropane) diimide (6FDA-1,5-NDA) dense membranes was investigated. The gas permeability coefficients were measured at pressures from 2.5 to 16 atm for the C2 hydrocarbon gases and pressures up to 8.4 atm for C3 systems at 35 °C. This membrane exhibits permeabilities of 0.15, 0.87, 0.023 and 0.24 Barrer with respect to pure ethane, ethylene, propane and propylene, and shows an ideal selectivity of 5.8 for the separation of ethylene/ethane, 10 for propylene/propane, 7.6 for nitrogen/ethane and 50 for nitrogen/propane. The olefins showed a preferred permeability to paraffins and discussion were drawn to the permeability, diffusivity and solubility coefficients. The activation energies of permeation, diffusion and solution were also reported and the effect of temperature on the permeation properties was discussed for the pure gas permeability data obtained from 30 to 50 °C. The plasticisation effect was also found for propane and propylene, respectively, although it was neither detected in the saturated nor unsaturated C2 hydrocarbons at pressures up to 16 atm. [Copyright &y& Elsevier]

We report that 6FDA-2,6-DAT polyimide can be used to fabricate hollow fiber membranes with excellent performances for CO2/CH4 separation. In order to simplify the hollow fiber fabrication process and verify the feasibility of 6FDA-2,6-DAT hollow fiber membranes for CO2/CH4 separation, a new one-polymer and one-solvent spinning system (6FDA-2,6-DAT/N-methyl-pyrrolidone (NMP)) with much simpler processing conditions has been developed and the separation performance of newly developed 6FDA-2,6-DAT hollow fiber membranes has been further studied under the pure and mixed gas systems.Experimental results reveal that 6FDA-2,6-DAT asymmetric composite hollow fiber membranes have a strong tendency to be plasticized by CO2 and suffer severely physical aging with an initial CO2 permeance of 300 GPU drifting to 76 GPU at the steady state. However, the 6FDA-2,6-DAT asymmetric composite hollow fibers still present impressive ultimate stabilized performance with a CO2/CH4 selectivity of 40 and a CO2 permeance of 59 GPU under mixed gas tests. These results manifest that 6FDA-2,6-DAT polyimide is one of promising membrane material candidates for CO2/CH4 separation application. [Copyright &y& Elsevier]

Membrane fouling persists to be among key obstacles in liquid membrane-based separation processes, and thereby the underlying understanding has actively been pursued. The two gaps attempted to be addressed here are namely, (i) internal fouling, which has limited studies due to limitations of instruments available; and (ii) fouling by mixtures of oppositely charged particles, as an extension to the studies on effect of surface charge of pure foulants on fouling. With the sub-micron pure constituents (namely, negatively charged carboxylated latex particles and positively charged aminated latex particles) as benchmarks, mixtures of three different compositions of the two particle types were investigated. The membrane was polycarbonate track-etched (PCTE) membrane. Surprisingly, the mixtures gave worse flux decline than that by purely positively charged aminated latex, which contradicts electrostatic considerations. The fouling model indicates that the mixtures exhibited higher pore blockage parameters (α) than either of the pure constituents, which agree with the flux decline trends. On the other hand, the mixtures gave internal cake resistance parameters (R c /R m) in between that of the pure constituents, which disagree with the fouling trends. Furthermore, optical coherence tomography (OCT) analysis indicates that the mixtures had taller buildups on the membrane as well as heterogenous internal cakes. Therefore, the steepest flux declines for the mixtures seem to be better correlated with the extensive external fouling as manifested in the taller buildups on the membrane pores, rather than internal fouling since the internal cakes were more porous. The contradictory extensive external and mitigated internal fouling phenomena are unexpected based on electrostatic considerations, and thereby highlight that fouling resulting from mixtures may not be readily extrapolated from understanding based on pure particle feeds. Image 1 • Steeper flux decline by mixtures of oppositely charged constituents than pure ones. • Mixtures gave greater pore blockage than constituents, agreeing with flux trends. • Mixtures gave intermediate internal-cake resistance, deviating from flux trends. • OCT trends affirm taller external cake and porous internal cake by mixtures. • Contradictory extensive external fouling and mitigated internal fouling by mixtures. [ABSTRACT FROM AUTHOR]

Tailoring the hydrophobic properties of membranes is an important requirement in gas-liquid membrane contactor processes because the membrane can be wetted by liquid. This paper describes the hydrophobic modification of the chemical surface of a polyvinylidene fluoride hollow-fiber membrane using a commercial perfluoropolyether, Fluorolink S10. The contact angle of the modified membrane was influenced by the modification parameters, and hence parameter optimization was required to obtain a highly hydrophobic membrane. The sodium hydroxide concentration (pH 8, 10 and 12), dehydrofluorination time (30, 60 and 90 min), and chemical solution grafting time (30, 60 and 90 min) were identified as the control parameters of this modification and were optimized using the Taguchi approach. The optimized membrane was then applied to recover methane from anaerobic membrane bioreactor effluent and benchmarked with a commercial polypropylene membrane manufactured for degasification of water. The contact angle of the polyvinylidene fluoride membrane was improved by 32% after the surface modification (under the optimal modification condition indicated by the Taguchi method). The membrane mass transfer coefficient was measured as 1.53 × 10 −4 m/s. Our membrane showed better methane recovery performance than the commercial polypropylene membrane and a very stable flux without pore wetting during 10 days of operation with synthetic effluent (made by saturating a 60:40 methane/carbon dioxide mixture into tap water). In addition, the membrane prepared in this study could be operated without significant fouling (7% flux drop during 8 days of operation) with real anaerobic membrane bioreactor effluent. [ABSTRACT FROM AUTHOR]

To separate oil/water mixtures and emlusions, superwetting membranes have been developed via synergy between surface chemistry and topography. However, challenges remain as most of them exhibit unsatisfactory performance, limited strength and durability. Herein, we report a novel membrane with switchable super-wettability for oil and water. This novel composite membrane was developed by electrospinning a hierarchically porous polyvinylidene fluoride (PVDF)-silica composite nano/micro-beaded top layer and a PVDF nanofibrous intermediate layer on a non-woven support. The surface of the composite membrane was modified to exhibit high surface free energy. The membrane is in-air superamphiphilic, underwater superoleophobic and under-oil superhydrophobic. It can treat various types of oil/water mixtures, from simple layered solutions to emulsions (including surfactant-free or surfactant-stabilized oil-in-water and water-in-oil emulsions) without external driving force or under an ultralow pressure (0.1 bar) in a cross-flow filtration process. It shows a flux up to 2000 L/m 2 h and high separation efficiency (> 99.99% in terms of water and oil purities in the permeation) in the cross-flow filtration process. The membrane also exhibits an excellent robustness under harsh conditions, including strong acidic or alkaline solutions, hot water and petroleum. In addition, it presents remarkable antifouling and easy-cleaning properties, which were demonstrated in a 50-h continuous operation. [ABSTRACT FROM AUTHOR]

Pressure-retarded osmosis (PRO) has attracted worldwide attention for its potential applications in renewable osmotic energy harvesting, low energy seawater desalination, and industrial waste brine disposal. However, membrane fouling is one of the major issues limiting PRO performance with the use of a high-salinity draw solution and a low-value impaired feedwater. This study systematically investigated membrane fouling in the PRO process using a real wastewater retentate (a waste byproduct from wastewater reclamation plants) as the PRO feed. Organic fouling (e.g., by humic substances) and inorganic fouling (e.g., by calcium phosphate) were identified to be the major types of fouling, where the latter dominated the overall water-flux decline. The internal PRO membrane fouling was exacerbated by internal concentration polarization (ICP) and intermolecular interactions between the organic macromolecules and the scaling precursor ions (e.g., Ca 2+ ). A limiting flux that was independent of the initial flux (i.e., applied pressure) was observed during the PRO fouling and was explained by a novel PRO limiting flux model. A membrane with a smaller structural parameter could achieve a higher limiting flux in PRO. A simple and effective pressure-assisted osmotic backwashing protocol was developed to clean the internally fouled membrane that could restore over 92% of the water flux within a short cleaning period (15 mins). The PRO performance could also be significantly improved by decreasing the feedwater pH that mitigated alkaline scaling. Finally, performing the PRO membrane fouling test in the active-layer-facing-feed-solution (AL-FS) orientation revealed that the integral cellulose triacetate (CTA) membrane exhibited strong mechanical stability and anti-fouling tendency whereas the thin-film composite (TFC) membrane was not mechanically stable in this orientation. This study for the first time identified that an integral membrane instead of a TFC membrane is an excellent candidate for PRO operation in the AL-FS mode for fouling control. [ABSTRACT FROM AUTHOR]

The performance of pressure retarded osmosis (PRO) processes is significantly limited by membrane fouling. This study systematically investigated PRO scaling by silica, which is one of the most common salts causing membrane scaling. In particular, the effect of silica concentration, the initial water flux, the chemistry of feed solution (FS) /draw solution (DS), and the type of membrane were studied in the PRO mode of operation. It has been observed that a low silica concentration (e.g., 10 mg/L SiO 2 ) could even cause membrane scaling during a PRO process. The rate and extent of the scaling was mainly governed by the internal concentration of silica in the membrane support layer and the solution chemistry of the FS/DS. The results suggested that the scaling could be mitigated by decreasing the internal concentration through the reduction of bulk silica concentration, the optimization of operating water flux, and the decrease in membrane structural parameter. The structural parameter seems to play a dominant role in PRO scaling by silica than the membrane materials and other physico-chemical properties. In addition, silica scaling can be controlled by lowering the pH of the FS and/or DS. For the first time, it has been demonstrated that utilizing an acidic DS was highly effective in controlling PRO scaling, which can be attributed to the low local pH in the membrane support layer as a result of the internal concentration polarization (ICP) of the reversely diffused H + . This method opens a new dimension for PRO scaling/fouling control. [ABSTRACT FROM AUTHOR]

Inspired by mussels’ byssus with remarkable adhesive power that is neither degraded nor deformed in the marine environment, dopamine coating has emerged as an option for membrane surface modification. With its many advantages, direct-contact membrane distillation (DCMD) appears to hold potential for the recovery of high quality water from produced water. However, membrane fouling and pore wetting are challenging issues of long-term DCMD operations due to the presence of oils and surfactants in these waters. Hence, it is paramount to develop robust membranes with anti-fouling and anti-wetting properties for effective produced water treatment. In this study, we fabricated a composite hollow fiber membrane by single-step co-deposition of polydopamine (PDA)/polyethylenimine (PEI) onto the outer surface of a commercial hydrophobic polyvinylidene fluoride (PVDF) substrate. The successful co-deposition was verified using different characterization techniques. The composite membrane exhibited Janus wettability (Janus membranes have opposing properties at an interface) with its modified outer surface being in-air hydrophilic/underwater superoleophobic for preventing organics adhesion while insuring that unmodified pores beneath the surface remained hydrophobic for vapor transport. The anti-fouling and anti-wetting properties of the modified membrane were investigated via DCMD experiments by feeding a series of low surface tension solutions. In comparison to the pristine PVDF membrane, the modified membrane exhibited promising wetting resistant property against different surfactant types and excellent fouling resistant property against nonionic and cationic surfactants. Moreover, the modified membrane presented a promising long-term performance when feeding a cationic surfactant-stabilized petroleum-in-water emulsion mimicking produced water, during which a stable flux and excellent permeate quality were maintained throughout 7 days of operation. The efficacious combined effects of a hydration layer and protonated amine-functional groups on the PDA/PEI grafted layer could prevent membrane fouling and pore wetting. The results suggest that the mussel-inspired composite PVDF membrane could potentially be used for long-term water recovery from produced water via DCMD. [ABSTRACT FROM AUTHOR]