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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: The outer surface of polyvinylidene-co-hexafluoropropylene (PVDF–HFP) asymmetric microporous hollow fiber membrane prepared by phase inversion process was activated by 10wt.% of NaOH solution, and subsequently modified with the mixture solution of α,ω-triethoxysilane terminated perfluoropolyether or Fluorolink®S 10 (FS10) and tetraethoxysilane (TEOS) in order to enhance the hydrophobicity of the membrane. The chemical and physical changes of the membranes were investigated by attenuated total reflection infrared spectroscopy FTIR-ATR and scanning electron microscope (SEM). The characteristics of the modified membrane were examined in terms of liquid entry pressure of water (LEPw), pore size and pore size distribution, contact angle and mechanical properties. The modified fibers were also used in the membrane contactor system to evaluate their performance for CO2 absorption. FTIR-ATR result showed the enrichment of fluorine and ethoxysilane on the modified PVDF–HFP membranes leading to higher LEPw and contact angle than those of the unmodified membrane by 36% and 33%, respectively. The contact angle increased from 95.5° to 127.8°. Compared to the unmodified PVDF–HFP hollow fiber, the cross-sectional morphology and mechanical properties of the modified membrane changed insignificantly, while the membrane mean pore size was reduced from 32.7nm to 25.2nm. The CO2 physical absorption performance of the modified fibers was better than the unmodified fibers. The membrane mass transfer coefficients of the unmodified and modified PVDF–HFP hollow fiber were 5.69×10−5 and 7.56×10−5 m/s, respectively, indicating that the modified membranes have good potential for application in the gas absorption process. [Copyright &y& Elsevier]

This report demonstrates the fabrication of a composite hollow fiber membrane that efficiently recovers dissolved methane (dCH 4) from anaerobic wastewater treatment effluents via membrane contactor (MC) process. We first fabricated highly porous hollow fiber membranes (∼85% porosity) using polyvinylidene difluoride (PVDF). Then, we coated the whole structure (bulk), or lumen side of PVDF supports with polydimethylsiloxane (PDMS), a hydrophobic polymer possessing high gas permeability. The adjustment of the concentration of PDMS precursor mix could effectively serve for tuning the performance characteristics of MC membranes and helped suppress membrane wetting, a major performance-reducing factor. Despite their denser skin layers, the lumen-modified membranes demonstrated superior dCH 4 recovery flux. Mass transfer analyses have confirmed the importance of preserving bulk porosity while coating the membrane surfaces. The long-term performance tests performed using actual anaerobic effluents suggested that the PDMS coating did not accelerate fouling, such that the lumen-modified MC membranes showed an almost steady flux for 8 days with an anaerobic membrane bioreactor effluent. A high-strength effluent, obtained from an up-flow anaerobic sludge blanket reactor, induced faster fouling. However, we observed only a ∼20% flux decline in a 10-day operation. Thus, we propose that the designed PDMS-PVDF composite membranes are promising for industrial practices. Image 1 • We prepared PDMS-coated PVDF porous hollow fiber membranes for membrane contacting. • Both lumen- and bulk-modified PVDF membranes were successful in methane recovery. • Lumen-modified PDMS-PVDF membrane outperformed commercial PP and PDMS membranes. • Lumen-modified PVDF achieved long-term performance with actual anaerobic effluents. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive analysis on the dominant effects for heat and mass transfer in the direct contact membrane distillation (DCMD) process has been performed with the aid of computational fluid dynamics (CFD) simulations for hollow fiber modules without and with annular baffles attached to the shell wall. Potential enhancement strategies under different circumstances have been investigated. Numerical simulations were carried out to investigate the effect of the MD intrinsic mass-transfer coefficient of the membrane (C) on the performance enhancement for both non-baffled and baffled modules. It was found that the temperature polarization coefficient (TPC) decreases significantly with increasing C value regardless of the existence of baffles, signifying a loss of overall driving force. However, the higher C compensated for this and the mass flux showed an increasing trend. A membrane with a lower C value was found to be less vulnerable to the TP effect. In this case, the introduction of turbulence aids such as baffles did not show substantial effect to improve system performance. In contrast, introducing baffles into the module can greatly enhance the mass flux and the TPC for a membrane with a high C value, where the main heat-transfer resistance is determined by the fluid side boundary layers. The effect of operating temperature on heat and mass transfer in the MD process was also studied with a membrane of a lower C value (2.0×10−7 kgm−2 s−1 Pa−1). Although the TPC generally decreased with increasing operating temperatures, the mass flux N m increased significantly when operating temperature increased. A baffled module showed a more significant improvement than a non-baffle module at a higher temperature. Moreover, it was confirmed that higher operating temperatures are preferable for a substantial improvement in the heat/mass transfer as well as MD thermal efficiency, even with a relatively small transmembrane temperature difference of 10K. [Copyright &y& Elsevier]

Abstract: The heat and mass transfer processes in direct contact membrane distillation (MD) under laminar flow conditions have been analyzed by computational fluid dynamics (CFD). A two-dimensional heat transfer model was developed by coupling the latent heat, which is generated during the MD process, into the energy conservation equation. In combination with the Navies–Stokes equations, the thermal boundary layer build-up, membrane wall temperatures, temperature polarization coefficient (TPC), local heat transfer coefficients, local mass fluxes as well as the thermal efficiency, etc. were predicted under counter-current flow conditions. The overall performance predicted by the model, in terms of fluxes and temperatures, was verified by single hollow fiber experiments with feed in the shell and permeate in the lumen. Simulations using the model provide insights into counter-current direct contact MD. Based on the predicted temperature profiles, the local heat fluxes are found to increase and then decrease along the fiber length. The deviation of the membrane wall temperature from the fluid bulk phase on the feed and the permeate sides predicts the temperature polarization (TP) effect. The TP coefficient decreases initially and then increase along the fiber length. It is also found that the local Nusselt numbers (Nu) present the highest values at the entrances of the feed/permeate sides. Under the assumed operating conditions, the feed side heat transfer coefficients h f are typically half the h p in the permeate side, suggesting that the shell-side hydrodynamics play an important role in improving the heat transfer in this MD configuration. The model also shows how the mass transfer rate and the thermal efficiency are affected by the operating conditions. Operating the module at higher feed/permeate circulation velocities enhances transmembrane flux; however, the thermal efficiency decreases due to the greater heat loss at a higher permeate velocity. The current study suggests that the CFD simulations can provide qualitative predictions on the influences of various factors on MD performance, which can guide future work on the hollow fiber module design, module scale-up and process optimization to facilitate MD commercialization. [Copyright &y& Elsevier]

A membrane contactor has been considered as one of the most effective tools for biomethane (CH 4) recovery from anaerobic effluents owing to its several strengths. However, without thoroughly understanding various factors ranging from operating parameters (e.g., liquid or gas flow rate) to membrane characteristics (e.g., porosity, pore size, hydrophobicity, inner diameter, membrane area, and fiber length), a membrane contactor could just break even, or even worse, may be operated at a loss before turning a profit. Much of the groundwork is required to make sure to gain net benefits while avoiding those sorts of pitfalls. Herein, we attempted to screen several factors ranging from operation-to membrane-related parameters to ensure whether they meet essential criteria. To be specific, this study covers the following aspects: i) the optimal gas-liquid flow rate (G / L) ratio to minimize both the gas-phase resistance and the dilution of the final sweep gas, ii) the combined effects of G / L ratio and mass transfer area (A M) on the energy recovery performance, iii) the influences of an inner diameter (d i) and A M on the CH 4 recovery (RE CH4) and the net energy production, iv) the influences of the L , porous structure, A M , and hydrophobicity on the RE CH4 and the energy output from the microturbine, v) the minimum criterion for the d i to minimize the energy required for the liquid pump, and vi) correlation between the pore structure and pore clogging. Through the thorough studies, we drew nine useful and meaningful conclusions including the best G / L ratio (0.0425 at the A M of 1 m2), necessary requisites (porous structure, larger A M , and more hydrophobicity) for higher RE CH4 , the lower limit of a d i (190 μm) to minimize the energy required for the liquid pump, and so on. We expect that this comprehensive study can help researchers and engineers opt for operation- and membrane-related parameters leading to the desirable performance of a membrane contactor for energy recovery from anaerobic effluents. Image 1 • The influences of membrane contactor operation parameters on the net energy production were systematically examined. • The influences of membrane characteristics on the energy production were also delineated. • Both theoretical and empirical studies were comprehensively carried out. • The criteria for the two parameters were provided to maximize the net energy production. [ABSTRACT FROM AUTHOR]

Gas-liquid membrane contactor (GLMC) has been regarded as a promising alternative to conventional contacting processes for CO 2 absorption. In this work, a composite hollow fiber (HF) membrane with an aminosilane-modified zeolitic imidazolate framework-8 (mZIF-8) based dense skin layer was designed and synthesized for high-efficiency biogas upgrading in the GLMC process, by dispersing mZIF-8 nanocrystals into poly(dimethylsiloxane) matrix and then depositing on a porous polyvinylidene fluoride (PVDF) substrate. (3-aminopropyl)triethoxysilane was introduced to modify the ZIF-8 nanocrystals, thereby enabling the chemical bonding with PDMS chains for avoiding interfacial voids and further enhancing hydrophobicity. Compared with the control membrane, the newly developed mZIF-8 based composite membrane with a dense skin exhibited competitive hydrophobicity with a contact angle of 130°, ensuring its anti-wetting ability. It exhibited an enhanced biogas upgrading performance with the absorption fluxes of 2.3 and 3.8 × 10−3 mol m−2·s−1 using water and 1 M monoethanolamine (MEA) as absorbents, respectively (liquid velocity = 0.25 m s−1). In particular, a comparable selectivity of CO 2 /CH 4 with the value of ∼20 was achieved by using MEA as absorbent in the GLMC process. A robust long-term stability of the mZIF-8 based composite HF membrane was also achieved in a 15-day operation. This work offers a new perspective for promoting CO 2 mass transfer with mZIF-8 based composite HF membranes, thereby improving the biogas upgrading performance in GLMC applications. • A composite hollow fiber membrane with a ZIF-8 based dense skin layer was designed and synthesized. • The ZIF-8 nanocrystals were modified to cross-link with PDMS chains for further hydrophobicity enhancement. • The newly developed membrane exhibited competitive hydrophobicity, ensuring its anti-wetting ability. • An enhanced biogas upgrading performance was achieved in gas-liquid membrane contact applications. [ABSTRACT FROM AUTHOR]

Several factors affect the performance of membrane-contacting systems for recovering dissolved methane (CH 4) from anaerobic effluents, from membrane- to operations-related parameters. However, the conditions necessary to achieve high-efficiency CH 4 recovery remain unclear. To improve our understanding of these dynamics, we systematically investigated the effects of membranes' physicochemical characteristics and operating parameters in a membrane-contacting system. We not only explored theoretical approaches to weigh the influence of diverse parameters but also evaluated the performance of four types of membranes with distinct morphological and chemical characteristics at various liquid velocities. According to our theoretical calculations, enlarging the mass transfer area (A M) decreases CH 4 flux while a higher liquid flow rate (L) results in a reduction in CH 4 recovery. Meanwhile, increasing the membrane's inner diameter (d i) has a negative influence on both CH 4 flux and recovery. However, a larger A M and a higher L are required to achieve higher recovery and flux, respectively. Our experimental results demonstrate that the capillary pressure imposed by the module inlet, pore wetting, and membrane resistance can also significantly affect the overall mass transfer resistance, thereby having a profound effect on CH 4 flux. We clarify the underlying mechanisms to explain how each parameter influences CH 4 flux and recovery, which leads to a system optimization to achieve the greatest CH 4 recovery efficiency. Image 1 • The influence of membrane characteristics on methane (CH 4) recovery in a membrane contactor was investigated. • The influence of operating parameters on CH 4 recovery was also evaluated. • Both theoretical and experimental analyses were conducted for a comprehensive understanding. • The requirements to obtain the optimum CH 4 recovery efficiency were provided. [ABSTRACT FROM AUTHOR]

An inorganic-organic fluorinated titania-silica (fTiO 2 -SiO 2 )/polyvinylidene fluoride (PVDF) composite membrane was fabricated, via facile in-situ vapor-induced hydrolyzation method followed by hydrophobic modification. This low mass-transfer-resistance membrane, composing of a mesoporous layer deposited onto macroporous substrate, was designed for biogas upgrading in gas-liquid membrane contactor (GLMC) application. The surface hydroxylation was introduced to facilitate the bridging of TiO 2 -SiO 2 nanoparticles and PVDF substrate, which resulted in a more coherent deposition of the fTiO 2 -SiO 2 layer onto the substrate. The surface microstructure was fine-tuned by controlling the amount of doped Si precursor, forming an integrated mesoporous fTiO 2 -SiO 2 layer. The resultant fTiO 2 -SiO 2 /PVDF composite hollow fiber membrane exhibited a tighter pore size of ~25 nm and a desired water contact angle of ~124°, which effectively prevented membrane wetting. The CO 2 absorption fluxes of 8.0 and 5.6 × 10 −3 mol m −2 s −1 were achieved due to the lower mass transfer resistance, by using 1 M of monoethanolamine (MEA) and sodium taurinate as absorbents with a liquid velocity of 0.25 m s −1 , respectively. The long-term stability test showed a good integrity between the fTiO 2 -SiO 2 layer and the PVDF substrate after 31-days of GLMC operation. The main benefit is the robust fluorinated inorganic layer which exhibited strong chemical resistance and high hydrophobicity, thus preventing membrane damage and pore wetting. Overall, this work provides an insight into the preparation of high–performance inorganic/organic composite hollow fiber membranes for carbon dioxide (CO 2 ) removal in GLMC application. [ABSTRACT FROM AUTHOR]

Gas-liquid membrane contactor (GLMC) is a promising method to attain high efficiency for CO 2 capture from flue gas, biogas and natural gas. However, membranes used in GLMC are prone to pore wetting due to insufficient hydrophobicity and low chemical resistance, resulting in significant increase in mass transfer resistance. To mitigate this issue, inorganic-organic fluorinated titania/polyvinylidene fluoride (fTiO 2 /PVDF) composite hollow fiber (HF) membranes was prepared via facile in-situ vapor induced hydrolyzation method, followed by hydrophobic modification. The proposed composite membranes were expected to couple the superb chemical stability of inorganic and high permeability/low cost of organic materials. The continuous fTiO 2 layer deposited on top of PVDF substrate was found to possess a tighter microstructure and better hydrophobicity, which effectively prevented the membrane from wetting and lead to a high CO 2 absorption flux (12.7 × 10 −3  mol m −2  s −1 ). In a stability test with 21-day operation of GLMC using 1M monoethanolamine (MEA) as the absorbent, the fTiO 2 /PVDF membrane remained to be intact with a CO 2 absorption flux decline of ∼16%, while the pristine PVDF membrane suffered from a flux decline of ∼80% due to membrane damage. Overall, this work provides an insight into the preparation of high-quality inorganic/organic composite HF membranes for CO 2 capture in GLMC application. [ABSTRACT FROM AUTHOR]

A significant amount of methane (CH 4 ) produced from anaerobic digestions of wastewater is dissolved in liquid effluent and discharged. The recovery of dissolved CH 4 is therefore essential in ensuring an enhanced energy production of the anaerobic processes, and minimizing environmental impacts of the greenhouse gas. In this work, a membrane contactor is employed as a mass transfer equipment for the CH 4 recovery. A mathematical model considering simultaneous desorption of CH 4 and carbon dioxide (CO 2 ) is developed using a resistance-in-series model to calculate the overall mass transfer coefficients. The simulations were validated with experimental results obtained using an in-house fabricated hollow fiber membrane as well as a real effluent from Anaerobic Membrane Bioreactor (AnMBR) and synthetic effluent made of water saturated with biogas. Results showed that the CO 2 fluxes were higher than those of CH 4 fluxes due to its higher concentration in liquid phase. A decrease of liquid phase mass transfer resistance by an increase in liquid velocity significantly enhanced both CH 4 and CO 2 fluxes. While, an increase in gas velocity slightly affected the CH 4 flux but enhanced the CO 2 flux considerably. It was also found that the CO 2 desorption increased the CH 4 recovery rate. The desorbed CO 2 helped to increase the mass transfer driving force by reducing the partial pressure of CH 4 in the gas side, and enhancing the gas phase mass transfer coefficient to facilitate CH 4 desorption. The increase of liquid velocity increased mole fraction of CH 4 in the gas outlet but decreased the rate of CH 4 recovery. On the other hand, applying vacuum conditions to decrease gas pressure enhanced the rate of CH 4 recovery but lower the CH 4 mole fraction in the product gas. [ABSTRACT FROM AUTHOR]

Abstract: The performance of a forward osmosis (FO) system may be characterised by the assessment parameters: FO–RO flux ratio (J w/J w(RO)), apparent FO water permeability (J w/(π ds − π ml)), and the newly developed flux efficiency factor (J w,ob/J w,re). The former two parameters offer information on extent of internal concentration polarisation and driving force utilisation, respectively. The J w,ob/J w,re factor has practical relevance, and reveals the inevitable trade-off between flux and recovery (φ) for a FO system. The derived J w,ob/J w,re factors corresponded well to experimental observations. High water permeability, low salt-to-water permeability ratio, and large mass transfer coefficient improve the performance of a FO system, but these may also be influenced by operational and fouling effects, such as draw solute transmission, fouling resistance and cake-enhanced concentration polarisation. It was shown that membrane properties also play a significant role in fouling behaviour. Fouling amelioration factors include aeration and osmotic backwash. A thin-film composite membrane showed potential for FO application with favourable intrinsic transport parameters. It was demonstrated that a FO system could achieve stable water production with both relatively high flux efficiency (J w,ob/J w,re =0.8) and high recovery (φ =95.8%), which attested to the technology potential. [Copyright &y& Elsevier]