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This study presented a comprehensive gas permeation model to predict the skin parameters of a complex system, specifically a hollow fiber membrane with an asymmetric structure consisting of two external and internal skin layers. According to the work of Haefer for n segments of pores connected in series as in hollow fiber membranes, applying the Wakao et al. model, a comprehensive model for the transition regime was derived. The model was examined against the various fabricated PES and PEI hollow fiber membranes. Unlike conventional models, the proposed method predicted the mean pore size and mean pore length independent of surface porosity for each skin layer. The study revealed valuable insights into i) the middle pressure, ii) the difference between the skin parameters of the two membrane skins, and iii) the impact of the feeding direction on gas permeation behavior. It was shown that the dependency of permeation behavior on the feeding direction is linked to the influence of middle pressure on viscous flow, which is closely related to surface porosities of each side. It is evident that even the slightest variations in the fabrication parameters can result in different structures. Furthermore, it is evident that the assumption of neglecting slip flow was reasonable within the experienced Knudsen number range. The model also highlighted that the skin parameters of the two skin layers may not be identical, a distinction not captured by the current models. The traditional approach of setting the middle pressure as the average of the upstream and downstream pressures has been improved by expressing it as a linear function of both pressures. In the feed side, higher pressures led to dominant viscous flow, whilst the permeate side was more influenced by the free molecular regime due to lower pressures. There existed a competition between the Knudsen flow and the viscous flow at a specific Knudsen number for the feeding side pores, whether it was the inner or outer surface. The finding emphasized the significance of porosity values on each side in determining the proximity of middle pressure to feed and permeate side pressures, underscoring the importance of accurate knowledge of surface porosities, mean pore sizes, and pore lengths for both skin layers in understanding processes involving porous dual-skin hollow fiber membranes. [Display omitted] • Novel theoretical model to predict HF membranes gas permeance was proposed. • The current model predicts structural parameters for both skins, unlike earlier models. • The model can predict the dependency of permeance on the gas flow direction. • Permeance depends on permeate side pressure, even if average pressure is constant. • Middle pressure is given by a linear function of the P h and P l , not their average. [ABSTRACT FROM AUTHOR]

Porous hydrophobic polyvinylidene fluoride (PVDF) composite hollow fiber membranes were fabricated via phase inversion method by embedding different amounts of hydrophobic calcium carbonate (CaCO 3 ) nano-particles in the polymer matrix. The effects of nano-particle loadings on the morphology, structure and performance of the spun membranes in gas–liquid contactors were investigated. The incorporation of hydrophobic nano-particles into the polymer network enabled the formation of more abundant and narrower finger-like pores in the composite membranes compared to plain PVDF membrane. Moreover, the addition of nano-particles enhanced the surface roughness, permeation rate, porosity and wettability resistance of the composite membranes. CO 2 absorption performance of the fabricated membranes was evaluated via a gas–liquid membrane contactor system. The CO 2 flux was improved to some extent by increasing the mixing ratio of CaCO 3 . Peak absorption performance of 1.52 × 10 −3 mol m −2 s −1 at 300 ml/min absorbent flow rate was achieved when 20/100 weight ratio of CaCO 3 /PVDF was employed. However, further increase of the ratio resulted in a composite membrane with lower absorption performance than the other composite membranes. Moreover, a long-term stability study of the composite membrane with the best CO 2 absorption flux showed no decline in performance in the initial 210 h of operation, indicating that the membrane possesses high potential for gas–liquid contactor applications. [ABSTRACT FROM AUTHOR]

In this work a novel method to evaluate the partial pore wetting of gas absorption membranes is proposed. The method consists of two approaches, one by energy-dispersive X-ray spectrometry (EDX), and the other by calculating the mass transfer resistances from the gas and liquid phase in the pore, using the parameter obtained by He gas permeation experiments. For this purpose, hollow fibers were spun from polyetherimide (PEI) and polyethersulfone (PES). As well, the lumen of a PEI hollow fiber was coated with silicone rubber. The hollow fibers were then characterized by He gas permeation experiments, critical entry pressure of water (CEPw), contact angle, scanning electron microscopy (SEM) and EDX. The hollow fibers were then subjected to CO 2 gas absorption using aqueous NaCl solution as absorbent. After drying the hollow fiber was subjected to the EDX analysis. The cross-sectional profile of Na and Cl content in the hollow fiber revealed that the pores of the PES hollow fibers were partially wetted, particularly at the pore inlet. Calculation of mass transfer resistances, on the other hand, revealed that the resistance came from gas phase in most hollow fibers, except for the PES hollow fiber where the liquid phase contribution was significant. Silicone rubber coating of the PEI hollow fiber increased the surface hydrophobicity and reduced the pore wetting considerably. Thus, it was concluded that the proposed method can be used as a powerful tool to investigate the pore wetting for many membrane contactor applications. [ABSTRACT FROM AUTHOR]

This study involves the preparation of a double-skinned thin film composite (TFC) and thin film nanocomposite (TFN) hollow fiber (HF) membrane for forward osmosis (FO) applications. The porous substrate consisted of a Polyvinyl chloride (PVC) / Polycarbonate (PC) blend HF membrane. Interfacial polymerization (IP) was then applied to coat a polyamide (PA) layer on the lumen surface and the porous substrate's outer surface. In addition, the impact of the outer PA active layer and the addition of nanoparticles to the outer selective layer on the FO flux and internal concentration polarization (ICP) were studied. By adding the second active layer to the substrate, water flux, reverse salt flux and ICP decreased. Also, the decline of water flux decreased over time due to the fouling agent. To compensate for the decrease in water flux in the double-skinned membrane, graphene oxide (GO) nanoparticles with 0.05% and 0.1%wt were added to the outer active layer. Addition of 0.1%wt graphene oxide nanoparticle to the outer active layer can help to improve water flux about 78% without spoiling the reverse salt flux. Moreover, the performance of double-skinned membranes against osmotic dilution process for oily wastewater treatment was investigated. The findings of this study demonstrated that the novel double-skinned TFN HF membrane exhibited high FO performance with low ICP and fouling. [Display omitted] • Inclusion of a second active layer without reducing the reverse salt flux and fouling. • To improve fouling resistance of membrane, GO was added to second active layer. • By adding GO to outer active layer, reverse salt flux and fouling was reduced. • The Hermia model showed intermediate blocking in neat membrane. • The predominant fouling in double-skin membranes is cake formation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Formation of finger-like macrovoids enhanced by the addition of clay nanoparticles. [•] Wetting resistance, surface hydrophobicity and roughness increased with MMT loading. [•] CO2 flux of fabricated MMMs superior to several commercial and in-house membranes. [•] 350h of constant CO2 absorption flux confirmed high reliability of fabricated membrane. [Copyright &y& Elsevier]

Highlights: [•] Blending SMM resulted in higher hydrophobicity and gas permeability. [•] The modified membrane showed higher wetting resistance at 24cm air gap. [•] The membrane showed less mass transfer resistance by removing inner skin. [•] Increasing air gap and SMM concentration resulted in higher CO2 flux. [•] A long-term stable operation was achieved by developing PVDF membranes. [Copyright &y& Elsevier]

In this study, thin-film composite (TFC) hollow fibers (HFs) were fabricated by an in-situ coating of polyamide layer on the lumen side of PVC/PC blend substrates for desalination by forward osmosis (FO). Particularly, the substrate HFs were spun at two air gap distances of 2 and 6 cm, to investigate the effect of the air-gap distance on the FO performance. The substrate HFs were characterized by pure water flux measurement, FESEM, and gas permeability test. The Interfacial polymerization (IP) method was then utilized for coating polyamide (PA) layer on the lumen surface of the substrate to fabricate TFC HFs. The TFC HFs so fabricated were also characterized by FESEM, before their FO performance was tested. Furthermore, the effect of HF length on the FO flux was investigated. The results showed that the novel TFC HFs exhibited good FO performance. The longer the air gap distance, the better becomes the FO performance in terms of simultaneous water flux and alginate fouling resistance, while the effect of the HF length remains a controversial issue. • Higher surface porosity resulted defect free active layer. • Higher surface porosity enhanced mass transfer coefficient, by reducing CP. • Irreversible fouling is suppressed when HF spun with longer air gap distance. • Water flux decreased with the fiber length, contrary to the model simulation. • In high flux mode increasing number of the fibers can increase productivity. [ABSTRACT FROM AUTHOR]

A surface-modifying macromolecule (SMM) was used to enhance the surface hydrophobicity of poly(ether imide) (PEI) hollow-fiber membranes. The membranes were used for a membrane contactor to absorb CO2 with water. The effects of three hollow-fiber fabrication parameters, that is, the PEI concentration in the casting dope, the SMM concentration in the casting dope, and the air gap, on the liquid entry pressure of water (LEPw) and the absorption rate (AR) of CO2 were investigated with response surface methodology. The model developed for LEPw satisfied the criterion for regression but had a low goodness of fit. The model predicted that LEPw would increase with PEI (weight percentage) but decrease with air gap. Furthermore, it showed a minimum value with a change in SMM (weight percentage). The model developed for the AR of CO2 had meaningful statistical parameters and was accurate; this indicated that interactions existed between the fabrication parameters on the AR of CO2. The performance of one of the fabricated membranes was compared with in-house and commercially made hydrophobic membranes in terms of the AR of CO2 with distilled water as an absorbent on the lumen side and pure CO2 on the shell side. The comparison showed a superior CO2 flux in the surface-modified membrane; for example, at a liquid velocity of 0.4 m/s, the surface-modified membrane exhibited a 416% higher AR than the commercial membrane contactor (Celgard MiniModule 0.75X5) made of polypropylene. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013 [ABSTRACT FROM AUTHOR]

Abstract: Low molecular weight organic compounds were added to the spinning dope as phase inversion promoters and their effects on the structure of polyetherimide (PEI) hollow fibers as well as their performance as membrane contactor were investigated. Water, methanol, ethanol, glycerol and acetic acid were added individually to the solvent NMP to prepare a dope containing 15wt% PEI, 4wt% additive, 81wt% NMP and hollow fiber membranes were fabricated via wet spinning method. The solution containing water as the additive had the lowest thermodynamic stability and highest viscosity, which yielded hollow fiber with a thin skin layer of high porosity and a sublayer with sponge-like structure. The four other polymer solutions were more stable thermodynamically and less viscous. Fast solvent/coagulant exchange yielded thick skin layers of lower porosity and sublayers of finger-like macrovoids. Among all fabricated follow fibers, adding methanol resulted in the highest absorption flux, which was ascribed to its high porosity and low tortuosity. [Copyright &y& Elsevier]

A novel method, Thermally Modified Non-Solvent Induced Phase Separation (T-NIPS) has been developed to fabricate translucent hollow fiber (THF) photocatalytic membrane for membrane's photodegradation efficiency enhancement. The process involves two-step temperature treatment that attacks crystalline property of polyvinylidene fluoride (PVDF). Graphitic carbon nitride in a modified morphological structure (hollow nanofiber) was used as photocatalyst to investigate the effect of translucency on membrane's photodegradation efficiency. UV-Vis analysis coupled with FTIR highlighted different crystalline phase appearance and membrane's translucency. The membranes showed high translucency, ranging from 60.0% to 93.9%, with THF-PVDF/PVP-GCN (2.0) exhibiting the highest. The filtration experiment showed that membrane with GCN photocatalyst had high PWF (1200 L/m2h) and rejection (90%) OPW compared to neat THF-PVDFs. In suspended mode, GCN demonstrated an impressive photodegradation efficiency of 99.98%. When immobilized in opaque PVDF membrane, the photodegradation has decreased substantially to 49.40%. However, when immobilized in a translucent PVDF membrane, the photodegradation efficiency of HN-GCN significantly improved compared to the opaque PVDF membrane, reaching 95%. Translucent membrane was subjected to five cycle regeneration test and showed 90% recovery even after the fifth cycle. The 10% reduction in the recovery was investigated using FESEM analysis revealed that there is a cake layer formation on the membrane surface. Therefore, this study proved that translucency of membrane has significant effect on immobilized photocatalyst's photocatalytic efficiency. [Display omitted] • Novel T-NIPS method to produce THFM is developed. • HN-GCN successfully immobilized in THFM. • HN-GCN immobilized THFM showed increased degradation efficiency of OPW. [ABSTRACT FROM AUTHOR]

The surface hydrophobicity of membrane is a key factor in contactor applications where the penetration of liquid into membrane pores has negative effect on the contactor performance. Therefore, different methods have been proposed for surface modification. Blending hydrophobic polymers such as surface modifying macromolecules (SMM) has been investigated in our earlier researches. In this study, the effect of polyvinylidene fluoride (PVDF) blending on the surface and bulk morphology of polyetherimide (PEI) hollow fiber membrane and its performance in membrane gas absorption process was studied. It was found that the pore size increased from 112 nm to 210 nm from the neat PEI membrane to the PEI membrane spun from the solution blended with 0.25 wt% PVDF. At the same time, the contact angle increased from 78.3 and 84.3. Furthermore, the absorption flux of PVDF blended membrane enhanced by 13% compared to the neat PEI membrane. [ABSTRACT FROM AUTHOR]

Asymmetric PEI hollow fiber mixed matrix membranes (MMMs) with improved structure and wetting resistance were fabricated via wet phase inversion method. The effects of incorporating hydrophobic MMT into polymer matrix in different loadings on the membrane properties were investigated. The membranes were characterized in terms of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), gas permeation test, hydrophobicity, wetting resistance and mechanical stability. All membranes possessed finger-like structures differing in skin layer thickness which increased with clay loading. The results revealed that the MMMs have significantly smaller pore size, higher porosity, hydrophobicity, LEPw and mechanical stability than plain PEI membrane. The membranes were further characterized by CO 2 absorption test via contactor system using distilled water as absorbent and pure CO 2 as solute gas. The results showed that the CO 2 absorption performance increased with addition of MMT nano-clay. The membrane containing 1 wt% MMT recorded the highest absorption flux of 2.35 × 10 −3 mol m −2 s −1 at the liquid velocity of 3 m s −1 , almost 135% higher than the flux of plain membrane at the same velocity. Comparatively, the flux of MMM was superior to several in-house made and commercial membranes. [ABSTRACT FROM AUTHOR]

Surface modified polysulfone (PSf) hollow fiber membranes (HFMs) using surface modifying macromolecules (SMM) were spun with air-gaps of 0–50 cm. Morphological analysis of the prepared fibers were carried out using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The wetting resistances of the membranes were determined by critical water entry pressure (CEPw) and contact angle (CA) measurements. The HMFs were further subjected to CO 2 capture by membrane contactor (MC) where water was used as absorbent. The test results indicated that both CEPw and CA showed a maximum value at 15 cm air-gap distance. The maximum He permeation was also achieved at 15 cm air-gap. The CO 2 flux of prepared membranes showed a maximum of 4.79 × 10 −3 mol/m 2 s at the absorbent flow rate of 300 ml/min. It was deduced that both He permeation and CO 2 absorption flux were governed by the HFM surface porosity. The long-term stability test revealed the reduction of 25% in CO 2 flux during the first 55 h of operation for the PSf membrane prepared at 15 cm air-gap. This study indicated that the surface modified HFMs prepared using an appropriate air-gap could be a promising option to increase membrane performance in MC systems. [ABSTRACT FROM AUTHOR]

Highlights: [•] Different amounts of SMM were used to improve the surface hydrophobicity of PVDF HFM. [•] The PVDF HFMs were used in contactor application for CO2 stripping from water. [•] The CO2 stripping flux and efficiency increased by increasing SMM concentration. [•] The gas flow rate has no significant effect on the CO2 desorption flux. [ABSTRACT FROM AUTHOR]

Highlights: [•] Recent progress in hollow fiber membrane preparation and new materials for making membranes. [•] Widely discussed recent progress of hollow fiber membranes characterization and application. [•] AFM images (internal and external surface of hollow fibre membranes and possible theories. [•] Recent progress of using hollow fiber membranes in contactor etc. [Copyright &y& Elsevier]

Abstract: Porous asymmetric polysolfone (PSf) hollow fiber membranes were fabricated using wet spinning process. Glycerol (4wt.%) was used as additive. Aqueous solutions of 1-methyl-2-pyrrolidone (NMP) with different composition (0, 50, 70 and 90wt.% NMP) were used as bore fluid. Distilled water was used to preparing aqueous NMP solutions. Structure and performance of resulting membranes in measuring of surface porosity, pore size, critical water entry pressure (CEPw) and CO2 absorption in hollow fiber membrane contactor were investigated. The cross-section, the inner skin layer and the inner surface of the membranes were examined via scanning electronic microscopy (SEM). Using distilled water as bore fluid provided the membrane structure with a finger-like macrovoids which extend from the inner and outer surfaces of the membranes to the middle of the hollow fiber wall. Other hollow fiber membranes formed with a finger-like structure near the other surface and a sponge-like structure with holes beneath, which resulted in a high CEPw and CO2 absorption rate. In addition, since a mixture of distilled water and NMP was used as the neutral bore fluid, the membranes showed a skinless inner surface. Results of gas permeation tests showed with increasing NMP concentration in the bore fluid the N2 permeance decrease significantly and the wetting resistance decrease slightly. CO2 absorption by distilled water was conducted through the gas–liquid membrane contactors. By employing 90% NMP in the bore fluid, the PSf membrane showed a CO2 flux of approximately 110% higher than the PSf membrane fabricated with distilled water as the internal coagulant at the absorbent velocity of 7.9×10−4 m/s. [Copyright &y& Elsevier]

Abstract: Submerged ultrafiltration process was studied for treatment of refinery wastewater using PVDF hollow fiber membranes. The membranes were prepared via the phase inversion method by dispersing LiCl·H2O and TiO2 in the dope to study the effects of surface properties on membrane performance. The comparison of the performance and morphology was conducted on prepared PVDF composite membranes with various LiCl·H2O and TiO2 contents. The hollow fiber membranes were characterized by field emission scanning electron microscope (FESEM) and energy dispersive x-ray (EDX), average pore size and effective porosity measurements, contact angle measurement, permeability and rejection test. Maximum results were observed for membrane hydrophilicity, membrane porosity and average pore size when the TiO2 concentration was 1.95%. It was also found that interactions between the membrane surface and suspended solid constituents strongly influenced the membrane fouling. The maximum flux and rejection of refinery wastewater were 82.5L/m2 h and 98.8%, respectively, when the PVDF composite membrane with TiO2 content of 1.95% is used at pH=6.9. [Copyright &y& Elsevier]

Abstract: Microscopic observation was made to investigate the surface structure of porous polyvinylidene fluoride hollow fiber membranes. Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and the Guerout–Elford–Ferry equation applied for pure water permeation rate were used to determine the average pore sizes. As well, surface roughness parameters and nodule sizes were determined by AFM. The observed unmodified and modified PVDF hollow fiber membranes were prepared by dry-jet wet spinning method from spinning dope mixtures containing 19wt.% PVDF, 0.98wt.% lithium chloride (LiCl.H2O) and 0, 1.95, and 3.85wt.% titanium dioxide (TiO2). Significant difference of spun fibers surface morphology was observed between inner and outer surface by AFM and FESEM. Higher TiO2 concentration formed rougher surfaces, increased average nodule sizes, and decreased the pore size, resulting in lower permeation flux. Pore size obtained from AFM was intermediate between FESEM and the method based on the Guerout–Elford–Ferry equation. [Copyright &y& Elsevier]

Abstract: In this study, polysulfone (PSF) hollow fiber membranes with enhanced performance for humic acid removal were prepared from a dope solution containing PSF/DMAc/PVP/TiO2. The main reason for adding titanium oxide during dope solution preparation was to enhance the antifouling properties of membranes prepared. In the spinning process, air gap distance was varied in order to produce different properties of the hollow fiber membranes. Characterizations were conducted to determine membrane properties such as pure water flux, molecular weight cut off (MWCO), humic acid (HA) rejection and resistance to fouling tendency. The results indicated that the pure water flux and MWCO of membranes increased with an increase in air gap distance while HA retention decreased significantly with increasing air gap. Due to this, it is found that the PSF/TiO2 membrane spun at zero air gap was the best amongst the membranes produced and demonstrated >90% HA rejection. Analytical results from FESEM and AFM also provided supporting evidence to the experimental results obtained. Based on the anti-fouling performance investigation, it was found that membranes with the addition of TiO2 were excellent in mitigating fouling particularly in reducing the fouling resistances due to concentration polarization, cake layer formation and absorption. [Copyright &y& Elsevier]

Recently, forward osmosis (FO) has attracted a great deal of attention in desalination and wastewater treatment. Nevertheless, there are several critical challenges such as the need for new advances in designing membranes that must be met to enhance the water flux in FO processes, control the reverse salt flux, concentration polarization and fouling. Therefore, designing a suitable membrane with a high-water flux, low reverse salt flux, low fouling, and controlled concentration polarization seems to be essential. Thin film composite (TFC) membranes are the most widely used membranes in the FO field. Extensive research has been performed to fabricate and design high performance TFC membranes which can be exclusively used in FO processes. This paper aims to review three types of TFC membranes i.e. TFC's with polyamide active layer (TFC-A), thin film nanocomposites (TFC-N) and double-skinned TFC membranes (TFC-D) in flat sheet and hollow fiber configuration. Finally, an attempt is made to generate a general performance curve based on the water flux and reverse salt flux of these three TFC FO types and the future direction of the R and D on the FO membrane are discussed. • A general performance curve for TFC-FO membranes was generated. • Active layer modified TFNs and double-skinned membranes possess very high potential for FO. • More researches should be performed on the less considered conditions of FO. [ABSTRACT FROM AUTHOR]

A facile route for the in situ surface modification and mechanical strength-enhancement of electrospun nanofibers is described. Blends of a host hydrophilic polymer, polysulfone (PSF), and small quantities of a fluorinated polyurethane additive (FPA) up to 6 wt% in the PSF blend were considered. During electrospinning, this additive undergoes spontaneous surface segregation resulting in nanofibers with an improved hydrophobic character attributed mainly to the detected fluorine-rich surface and enhanced mechanical properties. We report the effects of the fluorinated additive content in the blend on the morphological and structural characteristics of the electrospun nanofibrous mats (ENMs) including their inter-fiber space and void volume fraction. ENMs with a superhydrophobic surface could be prepared using at least 3 wt% FPA in the PSF blend. The ENMs were tested in desalination by membrane distillation (MD) and competitive permeate fluxes as high as 53.8 kg/m2h, with stable low permeate electrical conductivities (<5.1 μS/cm), were achieved for the ENM prepared with 4.5 wt% FPA in the PSF blend, when the feed was 30 g/L NaCl aqueous solution and the transmembrane temperature difference was 60 °C, without any inter-fiber space wetting being detected. Image 1 • Surface modification of polysulfone (PSF) nanofiber by surface segregation. • Fluorine enrichment of electrospun PSF nanofiber surface and superhydrophobicity. • Use of small amount of fluorinated polyurethane active additive (FPA). • Suitable characteristics for desalination by direct contact membrane distillation. • Competitive and stable permeate fluxes without interfiber space wetting were achieved. [ABSTRACT FROM AUTHOR]

Surface modified polyetherimide (PEI) hollow fibers were prepared by surface segregation of a fluorinated polyurethane additive (FPA). This undergoes spontaneous segregation principally to their outer surfaces resulting in PEI hollow fibers with an improved hydrophobic property of a contact angle up to 132.7°. This high contact angle is attributed mainly to the protrusion of fluorine of FPA at the hollow fiber surface. The addition of FPA also enhanced the mechanical strength and liquid entry pressure of water (LEP) reaching high values in the range 329–541 kPa. The effects of the PEI concentration in the blend on the morphological and structural characteristics of the hollow fibers were studied maintaining the FPA at 2 wt% in the PEI spinning solution. The hollow fibers were tested for desalination by direct contact membrane distillation (DCMD). A very stable DCMD performance was obtained for the FPA hollow fiber prepared with 14 wt% PEI with permeate fluxes around 23.8 kg/m2 h and low permeate electrical conductivity (4.9–5.8 μS/cm) over a period of almost two months (54 days DCMD operation with 29.8 g/L NaCl aqueous solution, 80 °C and 20 °C feed and permeate inlet temperatures, respectively). Unlabelled Image • Surface modification of polyethermidie (PEI) hollow fiber by FPA surface segregation • Synthesis and application of a fluorinated polyurethane active additive (FPA) • Fluorine enrichment of PEI hollow fiber outer surface and improved hydrophobicity • Enhanced mechanical properties and liquid entry pressure of modified hollow fiber • Competitive and stable permeate fluxes without pore wetting over a period of 54 days [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Graphitic carbon nitride (GCN) photocatalyst was incorporated in nanofibers. • Photocatalytic nanofibers were electrospun on alumina hollow fiber membranes. • Cross-flow microfiltration demonstrated purification of oilfield produced water. • Hydrophilic, highly-porous nanofiber coating exhibited excellent fouling resistance. • Photodegradation ability of GCN offered cleaning performance of robust membranes. Abstract Cost-effective purification technology of oilfield produced water (OPW) is becoming a global challenge for future petroleum exploration and production industry. Energy-efficient operation of membrane separation is potentially promising. However, severe fouling problem of oil droplets demands new robust and fouling-resistant membranes with high permeability and rejection efficiency. Here, we propose a photocatalytic nanofiber-coated inorganic hollow fiber membrane suitable for OPW treatment. The membrane was fabricated by coating polyacrylonitrile (PAN) nanofiber incorporated with graphitic carbon nitride (GCN) photocatalyst on an alumina (Al 2 O 3) hollow fiber membrane. While the highly porous coating made of smooth hydrophilic nanofibers facilitated water permeation, the coating effectively captured oil droplets in its opening, resulting in a better rejection efficiency of oil contaminants. Its sparse mesh morphology prevented oil contaminants to form dense fouling film on the membrane surface and maintained high permeate flux even after 180 min filtration. The best permeate flux of 640 L·m−2·h−1 and oil rejection percentage of 99% were recorded for 180 min crossflow filtration of OPW at 2 bar along with the highest pure water flux of 816 L·m−2·h−1. The photocatalytic activity of GCN enabled the coating to degrade the captured oil contaminants under UV irradiation, demonstrating permeate flux of 577 L·m−2·h−1 and oil rejection of 97% after three cycles of 180 min filtration. The excellent fouling resistance and cleaning performances of the membrane are considerably beneficial for a long-term repeated filtration operation. This work will motivate researchers to develop nanofiber-coated hollow fiber membranes for future membrane separation technology. [ABSTRACT FROM AUTHOR]

Porous polyetherimide (PEI) and polyethersulfone (PES) hollow fiber membranes were spun under the same spinning conditions and the fabricated membranes were characterized using various analytical methods. The membranes were then subjected to gas–liquid contactor experiments. While the PEI casing solution was thermodynamically less stable, the PES casting solution was less viscous which resulted in a more porous membrane structure. The mean pore size and effective surface porosity obtained by the gas permeation test showed superior properties of the PES membrane. Both PEI and PES membranes had sufficiently high LEPw values even though both polymers were known to be relatively hydrophilic, most likely due to their small pore sizes. The absorption flux of the PES membrane was 4.4×10 −3 mol m −2 s −1 at the liquid flow velocity V liquid of 1.8 m s −1 , which was 267% higher than that of the PEI membrane (1.2×10 −3 mol m −2 s −1 ) due to the larger pore size and smaller tortuosity of the PES membrane. [ABSTRACT FROM AUTHOR]

Porous PVDF-hydrophobic montmorillonite (MMT) mixed matrix membranes (MMMs) were fabricated via wet spin-ning method and used in membrane gas absorption process. The effects of hydrophobic MMT nano-clay loadings (1, 3 and 5 wt% of polymer) on the structure and performance were investigated. The fabricated membranes showed both finger-like and sponge-like structure with an increase in the length of finger-like pores in their cross-section, which resulted in higher permeability and lower mass transfer resistance compared to plain PVDF membrane. Also, significant improvements for surface hydrophobicity, critical entry pressure of water and porosity with the addition of filler were observed. The CO2 absorption test was conducted through the gas-liquid membrane contactor and demonstrated a significant improvement in the CO2 flux with MMT loading and the membrane with 5 wt% MMT presented highest performance. For example, at the liquid water velocity of 0.5 ms-1, CO2 flux of the MMM with 5 wt% MMT of 9.73 × 10-4 mol m-2 s-1 was approximately 56% higher than the PVDF membrane without nano-filler. In conclusion, MMMs with improved absorption properties can be a promising candidate for CO2 absorption and separation processes through membrane contactors. [ABSTRACT FROM AUTHOR]

Surface modified polyvinylidene fluoride (PVDF) hollow fiber membranes (HFMs) were spun via dry-wet spinning technique at different air-gap lengths (0-20 cm). The morphology of prepared membranes was evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Membranes were also characterized in terms of gas permeation, overall porosity, collapsing pressure, critical water entry pressure (CEPw) and contact angle. To determine the CO2 absorption flux of HFMs, a gas-liquid membrane contactor system was used. Experimental results of this study revealed that by increasing the air-gap distance from 0 to 20 cm, wetting resistance and contact angle of fabricated membranes increased due to enhancement of membrane surface hydrophobicity. The highest helium (He) permeation was achieved for the spun fiber at the air-gap of 10 cm. From CO2 absorption experiment it was found that the hollow fiber spun at the air-gap of 10 cm had the maximum CO2 absorption flux of 1.41 x 10-3 mol/m² s at the absorbent flow rate of 300 ml/min, which was significantly higher than CO2 absorption flux obtained by other researchers. It was also found that both highest He gas permeance and CO2 absorption flux were controlled by the surface porosity of the hollow fiber due to the maximum values obtained. Thus, the choice of an appropriate air-gap distance for fabrication of surface modified membranes was found to be a promising method to improve CO2 removal in membrane contactor systems. [ABSTRACT FROM AUTHOR]

Porous polyethersulfone hollow fiber membranes were fabricated via dry-wet phase inversion method with the polymer concentration in the spinning dope either 13wt% or 15 wt%. The fabricated hollow fiber membranes were characterized by different test methods and the performance of membranes in contactor applications was tested by CO2 absorption. The mean pore size, effective surface porosity and membrane porosity decreased while the membrane density and Liquid Entry Pressure (LEPw) increased as polymer concentration increased. The CO2 absorption flux of the fabricated membranes was measured in two cases; i.e. when the absorbent, distilled water, was in the lumen side or in the shell side. The CO2 flux for the membrane, fabricated from 13wt% PES solution, was compared with some commercial and in-house made membranes. The former membrane had 111% higher flux than a commercial PTFE membrane. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effects of tapioca starch and CGTase concentrations on the yield of CDs were studied. [•] Production of CDs in an EMR. [•] The weak adsorption (ra1 ) was the major fouling mechanism. [ABSTRACT FROM AUTHOR]

Highlights: [•] Different amount of SMM were used to improve the surface hydrophobicity of PVDF HFM. [•] Average pore size and contact angle increased by increasing SMM concentration. [•] The fabricated PVDF HFMs were used in contactor application for CO2 absorption. [•] The CO2 flux of the hollow fiber increased by increasing SMM concentration. [•] The membrane mass transfer resistance decreased by enhancement of SMM concentration. [Copyright &y& Elsevier]

Highlights: [•] AFM analysis on the PVDF hollow fiber membrane’s surface for CO2 stripping. [•] Increasing LiCl concentration has decreasing effect on membrane surface parameters. [•] Addition of LiCl has improved the overall membrane mass transfer coefficient. [Copyright &y& Elsevier]

Abstract: Carbon dioxide (CO2) stripping from water was conducted through the porous asymmetric polysulfone (PSf) hollow fiber membrane contactor. The effect of the liquid and gas flow rates on the stripping performance, the liquid phase CO2 concentration and the CO2 stripping efficiency of the membrane module and the effect of liquid phase temperature on CO2 stripping flux were studied. The experimental results showed that the stripping gas velocity had a minor effect on the CO2 desorption flux while the increase in the liquid velocity could enhance CO2 desorption flux in the gas stripping membrane contactor. By increasing liquid flow rate to 200ml/min, the maximum CO2 stripping efficiency of almost 66% was achieved. Enhancement of liquid flow rate from 50 to 200ml/min increased the CO2 flux around 482%. It was found that the CO2 stripping flux was significantly affected by the liquid phase temperature. By increasing liquid temperature from 80 to 90°C, the CO2 stripping flux increased from 1.3×10−4 to 4.9×10−4 molm−2 s−1 at liquid velocity of 200mlmin−1. Hence, the higher stripping efficiency can be achieved by applying the higher liquid flow rate in the membrane contactor module. As well, the liquid phase temperature is a key parameter that needs to be controlled. [Copyright &y& Elsevier]

Abstract: Porous asymmetric polyvinylidene fluoride (PVDF) hollow fiber membrane were fabricated via a phase-inversion method using surface modifying macromolecules (SMM) (1wt%) as the additive in the spinning dope. Distilled water and tap water were used as internal and external coagulation bath, respectively. The membranes were characterized in terms of gas permeation, wetting resistance, overall porosity, contact angle and collapsing pressure. CO2 stripping from diethanolamine (DEA) solutions was conducted through the gas–liquid membrane contactor. The effect of some operating conditions such as gas and liquid velocities, DEA concentration and rich solution temperature on the CO2 stripping flux and efficiency were investigated. By increasing liquid flow rate to 200ml.min−1, the maximum CO2 stripping efficiency of almost 82% was achieved. In addition, an increase in the liquid flow rate resulted in a significant increase of CO2 stripping flux. By increasing the liquid flow rate from 50 to 200mlmin−1, the CO2 flux increased by 900%. By increasing the gas velocity the CO2 desorption flux increased but this changing was negligible. The effect of rich solution temperature was investigated and the results showed that the CO2 desorption flux increased with increasing the solution temperature from 80 to 90°C. The results of DEA concentration enhancement on the CO2 desorption flux showed that the CO2 stripping flux increased drastically with enrichment of the DEA concentration from 1.0 to 3.0 (moll−1). Therefore, the higher stripping efficiency can be achieved by applying the higher liquid flow rate, temperature and DEA concentration in the absorbent liquid in the membrane contactor module. [Copyright &y& Elsevier]

Abstract: Porous asymmetric polyetherimide (PEI) hollow fiber membranes were fabricated via a phase-inversion method using ethanol, glycerol and acetone as the additives in the spinning dope. Also, hollow fiber PEI membrane without additives was fabricated. An aqueous solution of 1-methyl-2-pyrrolidone (80wt.%) was used as bore fluid to prevent forming of an inner dense skin layer. The precipitation rate of the polymer dopes with the different additives was studied using cloud point measurement. The effect of the additives on the resulting membrane structure, surface porosity, pore size, critical water entry pressure, collapsing pressure and physical and chemical CO2 absorption performance by distilled water and NaOH (1M) solution in a gas–liquid membrane contactor system were investigated and compared. Cloud point diagrams indicated that the precipitation rate of the polymer dopes increased following the trend of ethanol>acetone>glycerol. Results of gas permeation tests showed that ethanol and glycerol as additives provided the membranes with the largest and smallest pore size, respectively. Moreover, all the additives resulted in an increase in the effective surface porosity. The cross-section of the membranes was examined via a scanning electron microscopy. Ethanol in the spinning dope provided the membrane structure with a sublayer with finger-like macrovoids, originating from the inner and outer surfaces of the hollow fiber and extending to the middle section of the hollow fiber wall, which resulted in a larger pore size and higher CO2 absorption rate than the other PEI hollow fiber membranes. [Copyright &y& Elsevier]

Abstract: The effect of bore fluid type on the structure and performance of polyetherimide hollow fiber membranes in contactor application was investigated. Water was used as phase inversion promoter in spinning dope and water and pure NMP were used as bore fluid. SEM micrographs show that the major parts of both membranes consist of spongelike structure which is related to the high viscosity of spinning dope that reduces the diffusion of coagulant (water) into membrane sublayer and decreases the rate of phase inversion. In the case of water as bore fluid (membrane #M1) there is skin layer on the inner surface of membrane and some drop-shaped voids in the structure of membrane but in the case of pure NMP as bore fluid (membrane #M2), the inner surface of membrane is skinless with big pores and there are fingerlike macrovoids, originating from the inner surface and extending to the vicinity of outer surface which is related to the penetration of bore fluid and dissolving the polymer. Furthermore, membrane #M2 has higher mean pore size and effective surface porosity. The absorption flux of both fabricated membranes was investigated in the case of liquid in lumen side (case #1) and liquid in shell side (case #2) where in case #1, membrane #M1 has higher absorption flux but in case #2, membrane #M2 has higher absorption flux. The different trend in absorption flux confirms that the surface of membrane in contact with the gas phase in a membrane contactor should be skinless with big pores to facilitates the diffusion of solute gas through membrane but the pore size on the surface of membrane in contact with the liquid phase should be adjusted to obtain high absorption flux and low wettability. [Copyright &y& Elsevier]

Abstract: Low wettability is a vital characteristic for the membrane used in membrane contactor. Blending surface modifying macromolecule (SMM) in spinning dope is an interesting method to enhance the hydrophobicity of membrane and in this research, the effect of SMM on the properties and structure of polyetherimide (PEI) hollow fiber membrane in terms of mean pore size, liquid entry pressure of water (LEPw), membrane porosity and contact angle was investigated and compared with the properties of the PEI membrane without SMM. Furthermore the performance of PEI surface modified membrane in contactor applications in terms of CO2 absorption with distilled water was evaluated and compared with the absorption flux of PEI membrane without SMM and also, with the absorption flux of commercial and in-house made hydrophobic membranes which shows superior performance of surface modified PEI membrane e.g. in case of water in lumen side and pure CO2 in shell side of contactor and at V liquid =0.5ms−1, the absorption flux of PEI surface modified membrane is 2.94×10−3 molm−2 s−1 which is 114% higher than PEI without SMM and 73% higher than commercial membrane contactor, Celgard MiniModule® 0.75X5. [Copyright &y& Elsevier]

Abstract: In this paper, polyetherimide (PEI) hollow fiber membranes were used in gas–liquid contacting process. Porous polyetherimide hollow fiber membranes were fabricated via wet phase inversion method. The polymer concentration in dope solution varied from 10 to 15wt%. Water was used as internal and external coagulant. Gas permeation test using helium as test gas, liquid entry pressure of water (LEPw) test and scanning electron microscopy (SEM) were used for membrane characterization. The mean pore size, effective surface porosity and void fraction of membranes decreased as the polymer concentration increased, while LEPw and membrane density increased. CO2 absorption rate of fabricated membranes were measured in a gas–liquid hollow fiber membrane contactor system using distilled water as absorbent and the results showed that CO2 absorption rate of membranes increases as polymer concentration decreases. In addition, CO2 absorption rate of hollow fiber membranes in which polyetherimide concentration was between 10 and 12wt%, was higher than commercial PVDF hollow fiber membranes. [Copyright &y& Elsevier]

Abstract: Removal of endocrine disrupting compound (viz., bisphenol A, BPA) by laboratory-fabricated hollow fiber polyethersulfone (PES) nanofiltration (NF) membranes have been investigated. The tailor made charged PES hollow fiber NF membranes have been developed by blending negatively charged surface modifying macromolecule (cSMM). It is resulted more than 90% removal of BPA, mainly due to negatively charged properties of the membrane and the negatively charged solute under the influence of higher pH feed water matrix. The PES hollow fiber modified by blending cSMM is associated with sulfonic groups, confirmed by energy dispersive X-ray and Fourier transform infrared analysis. The blending of cSMM has successfully modified PES hollow fiber which is resulted twice better removal of BPA compared to the unmodified PES. [Copyright &y& Elsevier]