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Polyvinylidene fluoride (PVDF) electrospun nanofibrous membranes (ENMs), consolidated with a heatpress process and of various thicknesses were fabricated and tested in a DCMD cell at five different operating conditions. Membranes as thin as 27 μm were sufficiently robust for evaluation and gave a transmembrane flux as high as 60 L/h m 2 . An analytical model was created to estimate the optimal membrane thickness for DCMD operations. It is found that the value of optimal thickness increases with reduced heat transfer coefficients; decreased feed inlet temperature; increased membrane permeability; and increased salinity. Even for 10% NaCl the predicted optimum was estimated to be 13 μm which was too thin for experimental confirmation. Based upon this analysis but with due allowance for the variation of heat transfer coefficients with temperature dependent physical properties a single Matlab model was created to fit the five sets of data. With the introduction of a structural derivation factor, which reflected the experimentally determined variation of porosity and pore size with thickness, the model was found to fit the data very well. [ABSTRACT FROM AUTHOR]

Abstract: This study aims at developing an effective method to modify hydrophobic polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD) which is tolerant to salty feed solution containing mineral oil. A novel approach was explored to obtain a highly hydrophilic membrane surface via plasma induced grafting of polyethylene glycol (PEG) and subsequent TiO2 particles deposition onto the membrane surface. The mechanism of oil affinity onto a membrane surface was quantitatively investigated by the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. A series of characterizations were conducted for the virgin and modified membranes. Membrane distillation tests for synthetic seawater containing 0.01wt% of mineral oil were also performed to investigate the influences of membrane hydrophobicity, membrane surface structure, pH value of the feed solution and hydrodynamic conditions on oil fouling behavior. The SEM and FTIR measurements confirmed the presence of PEG and TiO2 particles on the PVDF membrane substrate. The hydrophilically modified PVDF-PT membrane exhibits a positive interfacial free energy with oil in the feed, indicating the TiO2 deposition effectively improve the anti-fouling property of PVDF-P membrane and PVDF membrane. It was also found that the PVDF-PT membrane presents a stable water flux over 24h of operation without fouling and wetting due to combined effects of enhanced surface hydrophilicity and reduced membrane pore sizes. In addition, the pH value of the feed solution plays an important role in oil fouling. More severe fouling can be observed at the pH which made the membrane surface and oil droplets carry opposite charges. Moreover, a higher flow velocity enhances the shear force exerted on the oil droplets, thus leading to less oil present on the membrane surface and less oil fouling of the membrane. [Copyright &y& Elsevier]

Abstract: Recently membrane distillation (MD) has received intensive interests for a range of applications such as desalinations of seawater and brine. One of the major obstacles for MD application is the lack of an optimized MD membrane that can produce a high and stable flux in long-term operation. Two types of superhydrophobic PVDF nanofiber membranes, integrally-modified and surface-modified PVDF membranes, have been successfully fabricated by electro-spinning followed by surface modification, which includes dopamine surface activation, silver nanoparticle deposition and hydrophobic treatment. The modification is convenient because of mild reactions and wide applicability. These novel composite nanofiber membranes have been characterized by a series of measurements and benchmarked against commercial PVDF flat sheet membrane for MD application. The characterizations reveal that the modifications have altered the membrane surface morphology and topology and made the membrane superhydrophobic due to their hierarchical structures. Compared with unmodified membrane, the integrally-modified membrane (I-PVDF) can achieve a high and stable MD water flux of 31.6Lm−2 h−1 using a 3.5wt% NaCl as the feed solution while the feed and permeate temperatures were fixed at 333K and 293K, respectively. To the best of our knowledge, this result is superior to all other PVDF flat-sheet membranes tested under the same or similar conditions, which is believed to be attributed to the open-surface pore structure and thin thickness of the PVDF nanofiber membrane with the aid of electro-spinning. The superhydrophobic nature of the membrane surface brought by the integral modification on all nanofibers renders the membrane anti-wetting property while remaining high water flux. [Copyright &y& Elsevier]

Abstract: The main challenges for membranes used in membrane distillation (MD) are well-designed porous structure and hydrophobic surface property. Hydrophobic nanofiber membranes possess high hydrophobicity, high surface porosity and adjustable pore sizes and membrane thickness, which make them an attractive candidate as MD membrane. The current study aims to fabricate and optimize polyvinylidene fluoride (PVDF) nanofiber membranes for MD application. Scanning electron microscopy (SEM), capillary flow porometer, geniometer, a homemade setup for liquid entry pressure (LEP) measurement and direct contact MD (DCMD) setup were used to characterize the resultant nanofiber PVDF membranes. Polymer dope compositions, spinning parameters such as sprayer moving speed and chamber moisture were optimized to examine their effects on pore size distribution of the membranes. Inorganic additives were also added into the dope to improve the electro-spinability of diluted polymer dopes in order to further decrease membrane pore size. Meanwhile, a heat-press post-treatment was considered as a necessary step to improve fresh nanofiber membrane integrity, enhance water permeation flux and help prevent membrane pores from wetting in DCMD operation. The experiment confirmed that all the electrospun membranes exhibit a rough surface with high hydrophobicity (>135° water contact angle). The post-treated PVDF nanofiber membranes were able to present a steady water permeation flux of about 21kgm−2 h−1 throughout the entire testing period of 15h, using a 3.5wt% NaCl solution as the feed under the feed and permeate inlet temperatures of 323K and 293K, respectively. This result was better than these of commercial PVDF membranes and the nanofiber PVDF-clay membrane reported in literature, suggesting the competency of PVDF nanofiber membranes for DCMD applications. [Copyright &y& Elsevier]

Abstract: Five types of novel hollow fiber module configurations with structured-straight fibers, curly fibers, central-tubing for feeding, spacer-wrapped and spacer-knitted fibers, have been designed and constructed for the direct contact membrane distillation (DCMD) process. Their module performances were evaluated based on permeation flux experiments, fluid dynamics studies, and tracer-response tests for flow distribution as well as process heat transfer analysis. The novel designs showed flux enhancement from 53% to 92% compared to the conventional module, and the spacer-knitted module had the best performance. The fluxes of all the modified configurations, except the structured-straight module, were independent of the feed flow velocity, and the modules with undulating membrane surfaces (curly and spacer-knitted fibers) were able to achieve more than 300% flux improvement in the laminar flow regime. The improved performance was attributed to the improved fiber geometries or arrangements that can provide effective boundary layer surface renewal and more uniform flow distribution, confirmed by the sodium chloride tracer response measurements. The heat transfer analysis underscores the advantage of the module with curly fibers with the least temperature polarization effect (temperature polarization coefficient=0.81–0.65 at T m =303–333K), which is favorable for enhancing permeation flux. [Copyright &y& Elsevier]

Abstract: The performance of membrane distillation depends on both membrane and module characteristics. This paper describes strategies to improve the performance of hollow fiber membrane modules used in direct contact membrane distillation (DCMD). Three different types of hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membrane (unmodified, plasma modified and chemically modified) were used in this study of direct contact membrane distillation (DCMD). Compared to the unmodified PVDF hollow fiber membrane, both modified membranes showed greater hydrophobicity and mechanical strength, smaller maximum pore sizes and narrower pore size distributions, leading to more sustainable fluxes and higher water quality (distillate conductiviy<1μscm−1) over a one month test using synthetic seawater (3.5wt% sodium chloride solutions). Comparing the plasma and chemical modification the latter has marginally better performance and provides potentially more homogeneous modification. MD modules based on shell and tube configuration were tested to identify the effects of shell and lumen side flow rates, fiber length and packing density. The MD flux increased to an asymptotic value when shell-side Re f was larger than 2500, while the permeate/lumen side reached an asymptotic value at much lower Re p (>300). By comparing the performance of small and larger modules, it was found that it is important to utilize a higher shell-side Re in the operation to maintain a better mixing near the membrane surface in a larger module. Single fiber tests in combination with heat transfer analysis, verified that a critical fiber length existed that is the required length to assure sufficient driving force along the fiber to maintain adequate MD performance. In addition, for multi-fiber modules the overall MD coefficient decreased with increasing packing density, possibly due to flow maldistribution. This study shows that more hydrophobic membranes with a small maximum pore size and higher liquid entry pressure are attainable and favorable for MD applications. In order to enhance MD performance various factors need to be considered to optimize fluid dynamics and module configurations, such as fiber length, packing density and the effect of module diameter and flow rates. [Copyright &y& Elsevier]

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]

Membrane devices, membrane processes and membrane-based conventional chemical engineering processes have achieved extraordinary levels of process intensification (PI). Generally membrane-based devices require smaller equipment to achieve a given device production rate. Further they can eliminate dispersion-based operation and achieve extraordinary selectivity. Exploiting the compartmentalization of two regions on two sides of the membrane, membrane devices can combine two processes carried out on two sides of the membrane in one membrane device. Selected membrane processes and their applications are briefly reviewed here in terms of the level of PI achieved. The membrane processes selected are generally recently commercialized or being commercialized or have great potential for commercialization: membrane bioreactor; membrane gas–liquid contacting; membrane solvent extraction; forward osmosis; pressure-retarded osmosis; membrane distillation; membrane distillation bioreactor. Examples of potential process intensification by membrane processes are briefly illustrated for processing of lignocellulose to biofuels in a biorefinery and for produced water treatment. [ABSTRACT FROM AUTHOR]

Abstract: This study explores the potential of microstructured hollow fiber designs to enhance process performance in a direct contact membrane distillation (DCMD) system. Hollow fibers with 10 different geometries (wavy- and gear-shaped cross sections) were evaluated. A series of three-dimensional computational fluid dynamic (CFD) simulations were carried out to investigate their capability in terms of depolarizing the buildup of liquid boundary layers, thus improving water productivity. Analyses of heat and mass transfer as well as the flow-field distribution in respective MD modules were obtained. It was found that the enhancement of the heat-transfer coefficients, h f , was up to 4.5-fold for a module with a wavy fiber design 07 and an approximate 5.5-fold h p increase for a gear-shaped fiber design. The average temperature polarization coefficient and mass flux N m of the gear-shaped fiber module showed an improvement of 57% and 66%, respectively, over the original straight fiber design, followed by the wavy designs 07 and 08. The enhanced module performance was attributed to the improved hydrodynamics through the flow channels of various fiber geometries, which was confirmed by the visualization of flow-field and temperature profiles in CFD. Investigations of the fiber-length effect showed that the gear-shaped fiber modules exhibited the highest flux enhancement of 57–65% with the same length, compared to the modules with original straight and wavy fibers. In addition, the gear-shaped fiber module is very sensitive to feed velocity changes. Therefore, employing a smart microstructured design on the membrane surface would bring in a significant improvement under adverse flow conditions. Moreover, the computed water production and hydraulic energy consumption (HEC) among the modules with various fiber geometries were compared. With 1.9-fold surface area increase per unit volume, the gear-shaped fiber configuration had the highest water production but the lowest HEC, followed by wavy designs 07 and 08. [Copyright &y& Elsevier]

Abstract: As an extended exploration of process enhancing strategies, nine modified hollow fiber modules with various turbulence promoters were designed and modeled using a two dimensional computational fluid dynamic (CFD) heat-transfer model to investigate their potential in improving heat transfer and module performance for a shell-side feed direct contact membrane distillation (DCMD) system. With the aids of turbulence promoters, the feed heat-transfer coefficient h f of the modified modules generally showed much slower decreasing trends along the fiber length compared to the original (unmodified) module. A 6-fold h f enhancement could be achieved by a modified module with annular baffles and floating round spacers. Consistently, the temperature polarization coefficient (TPC) and mass flux distribution curves of these modified modules presented increasing trends and gained an optimal improvement of 57% and 74%, respectively. With the local flow fields and temperature profiles visualized in CFD simulations, it was confirmed that an appropriate selection of turbulence promoters could promote intense secondary flows and radial mixing to improve the shell-side hydrodynamics and enhance heat transfer. Moreover, an increase of flow velocity was used and compared as a conventional approach to improve hydrodynamics. It was found that a well-designed module could bring more significant enhancement for a liquid-boundary layer dominant heat-transfer process. Finally, the hydraulic energy consumption (HEC) caused by the insertion of turbulence promoters or the increase of circulating velocity was compared. Configurations with attached quad spacers or floating round spacers achieved a good compromise between enhanced permeation fluxes and modest HECs. Overall, the TPC decreases with increasing MD coefficient (C) values and operating temperatures; while the thermal efficiency increases dramatically with increasing C and operating temperatures in a MD system. [Copyright &y& Elsevier]

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]

Membrane distillation (MD) has been touted as a promising technology for niche applications such as desalination of surfactant- and oil-containing feed streams. Hitherto, the deployment of conventional hydrophobic MD membranes for such applications is limited and unsatisfactory. This is because the presence of surfactants and oils in aqueous feed streams reduces the surface-tension of these media significantly and the attachment of these contaminants onto hydrophobic membrane surfaces often leads to membrane fouling and pore wetting, which compromises on the quantity and quality of water recovered. Endowing MD membranes with surfaces of special wettabilities has been proposed as a strategy to combat membrane fouling and pore wetting. This involves the design of local kinetic energy barriers such as multilevel re-entrant surface structures, surfaces with ultralow surface-energies, and interfacial hydration layers to impede transition to the fully-wetted Wenzel state. This review critiques the state-of-the-art fabrication and surface-modification methods as well as practices used in the development of omniphobic and Janus MD membranes with specific emphasis on the advances, challenges, and future improvements for application in challenging surfactant- and oil-containing feed streams. Unlabelled Image • This review focuses on MD membrane development for low-surface-tension feed streams. • The development of omniphobic and Janus membranes are critically reviewed. • The impacts of surface chemistry and surface structures are analyzed. • This review lays the groundwork for future MD membrane development for niche applications. [ABSTRACT FROM AUTHOR]

Highlights • Novel thermoelectric coupled sweeping gas membrane distillation (T-SGMD) hybrid. • Hybrid membrane distillation for decentralized water treatment, dehumidification and space cooling. • Latent heat cooling provides for water treatment simultaneously. • Impact of membrane area, cool air recycle and module orientation on T-SGMD efficiency. • Interplay among power consumption, space cooling capacity and condensate rate. Abstract The current concept for cooling the indoors is far from ideal with respect to the total energy consumed and waste discharged. A novel concept for improving the energy efficiency is proposed via hybridizing the heat pump with a membrane distillation (MD) unit for simultaneous space cooling and water treatment. MD is well-acknowledged for utilizing low-quality waste heat for water treatment, which makes it feasible for coupling with a heat pump to make use of both the hot and cold reservoirs of the pump. Accordingly, the objective of the current effort was to investigate via experiments the efficacy of a thermoelectric heat pump coupled with a sweep-gas MD system (T-SGMD) by measuring the cooling capacity, condensate production and power consumption. The results from this study can be extended to other heat pumps. Three key highlights emanated from this study. Firstly, condensate production per unit energy consumed can be doubled with the T-SGMD system relative to thermoelectric dehumidification alone. Secondly, cool air recycle affected the condensate flux the most without a drastic loss of cooling compared to other tested parameters during the operation of the T-SGMD. Lastly, the T-SGMD system was able to provide an increase in condensate produced per unit energy without a loss in cooling capacity per unit energy input. These advantages of coupling heat pumps with MD, leveraging on the current advancements in MD, is promising for a hybridized system for decentralized water treatment, dehumidification and space cooling. [ABSTRACT FROM AUTHOR]

Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in direct-contact membrane distillation (DCMD) processes. However, these PVDF membranes are vulnerable to membrane fouling and pore wetting in low surface tension feeds, restricting its application for water recovery from challenging industrial wastewaters. Therefore, it is of paramount importance to engineer fouling- and wetting-resistant MD membranes for robust long-term applications. In this study, a superoleophobic composite hollow fiber membrane with sandwich structure has been developed via accelerated oxidant-induced polydopamine (PDA) deposition on both the outer and inner surfaces of a commercial hydrophobic PVDF membrane under slightly acidic conditions (pH = 5). The modified surface prevents organics adhesion ascribing to its underwater superoleophobicity while the unmodified pores remain hydrophobic for vapor transport. The long-term robustness of the PDA-decorated membrane in highly saline feeds containing low surface tension contaminants has been evaluated via bench-scale DCMD experiments. In contrast to the pristine PVDF membrane, the PDA-decorated membrane exhibits excellent fouling- and wetting-resistant properties in different surfactant solutions as well as oil-in-water emulsion. The PDA-decorated membrane has also been used for seawater desalination, during which it maintains a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presents a flux enhancement of up to 70% over the pristine PVDF membrane in 3.5 wt% NaCl solution at 333 K. This study demonstrates the potential of the PDA-decorated membrane for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising desalination technology as it is less sensitive to feed's salinity and can handle high-salinity water. However, the industrial application of MD is restricted by the limited availability of cost-effective hydrophobic membranes. This study developed a simple method to fabricate hydrophobic MD membranes through the coating of a fluoroethylene vinyl ether polymer called LUMIFLON™ LF-200. The fluoropolymer was effectively coated on porous nitrocellulose (NC) and nylon membrane through dip-coating followed by a thermal crosslinking. The coating layer was thin and uniform, covering the surface of the whole membrane matrix, while the high porosity and the microstructure of the membranes were still mostly preserved. After modification, the originally hydrophilic NC and nylon membranes were transformed into hydrophobic, with water contact angles of ∼130°. The 12LF-NC membrane showed a high flux of 10.1 kg m−2 h−1 with salt rejection >99.99% over 120 h in the direct contact MD fed with 10 wt% NaCl solution, which outperformed the commercial PVDF membrane. This coating method has the potential to modify various substrates to produce MD membranes with good performance, which broadens the availability of appropriate membranes for MD application. [Display omitted] • Fluorinated copolymer, LUMIFLON LF-200 is used to prepare hydrophobic MD membranes. • LF-200 was simply dip-coated on hydrophilic substrates and crosslinked under mild heating. • The WCAs of modified nitrocellulose and nylon membranes increased to ∼130° from 0°. • The 12LF-NC membrane had excellent flux >10 kg m−2 h−1 treating 10 wt% salt water. • This method has high efficiency, wide versatility and the potential to scale up. [ABSTRACT FROM AUTHOR]

Increasing demand for oil and gas leads to the generation of substantial amount of produced water, bringing about deleterious impacts on the environment. Direct-contact membrane distillation (DCMD) could be a possible option for dewatering oil-in-water (O/W) emulsions because of many benefits brought by the DCMD process. However, these low surface tension solutions pose some difficult issues such as membrane fouling and pore wetting. The mechanisms involved are not fully understood due to the lack of study of the interaction between the emulsions and the membrane surface in the DCMD domain. To address the challenges, this study aims at developing a fundamental understanding of the relationship between surfactant-stabilized O/W emulsions and polyvinylidene fluoride (PVDF) membrane surface in DCMD operations. Effects of surfactant types (Span 20, Tween 20, and sodium dodecyl sulfate), oil concentration, and oil types (petroleum and vacuum pump oil) were systematically studied to better understand the fouling and wetting mechanisms involved. The results reveal that surfactant concentration and hydrophobicity had an influence on the membrane fouling and wetting behaviors. Surfactants with a lower hydrophilic-lipophilic balance (HLB) value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve anti-fouling and anti-wetting properties to make DCMD an energy-efficient and effective technology for treating produced water. [ABSTRACT FROM AUTHOR]

Solvent resistant membrane distillation (SR-MD) is a novel technology to effectively separate water from waste streams containing solvents with high boiling points, avoiding the high chemical costs and release of harmful gas in conventional treatments. Ceramic membranes are promising for this process as they are highly stable chemically but they require modification to tune the hydrophilicity to hydrophobicity. However, the widely used silanization for hydrophobic modification often involves expensive chemicals and may produce toxic substances. Here, we present a new hydrophobic modification method with the use of fatty acid chloride (FAC), as a greener and cheaper alternative to silanes. In the grafting reaction, the acyl chloride groups react with the –OH groups on the ceramic membranes to form strong ester bonds. We successfully grafted stearoyl chloride (SC) and palmitoyl chloride (PC) on the ceramic tubular membranes. With long carbon chains grafted, the modified membranes exhibited high hydrophobicity with a water contact angle >141° and a liquid entry pressure >3 bar. When tested in SR-MD with a feed solution containing 50 wt% dimethyl sulfoxide, the PC-modified membrane demonstrated a flux of 3.2 kg m−2 h−1, with rejection >98% and separation factor >110 at 60 °C, and a high flux of 4.5 kg m−2 h−1 with rejection >96% at 70 °C. The FAC-modified membranes allow a cost-effective treatment of challenging wastewater containing organic solvents through SR-MD. [Display omitted] • Hydrophobic ceramic membranes for solvent resistant MD were synthesized through fatty acid chloride grafting. • The acyl chloride groups formed strong ester bonds on ceramic surface and the modified membranes had high hydrophobicity. • Flux of 3.2 kg m−2 h−1, rejection >98% and separation factor >110 can be achieved when treating 50 wt% DMSO in SR-MD. • The hydrophobicity and separation performance are comparable to ceramic membrane grafted with silanes. • This study presents a greener and cheaper surface modification chemistry to synthesize hydrophobic ceramic membranes. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) technology is being extensively studied to address operational challenges such as undesired thermal efficiency and scaling phenomenon in recovering valuable solutes and minimizing brine disposal. This study has explored the working mechanisms of utilizing gas–liquid two-phase flow to enhance heat transfer and mitigate scaling formation in MD concentration process, based on the quantification of heat-transfer coefficients and local scaling-resistance associated with bubble size properties. With the aid of direct observation and statistical analysis on the bubble characteristics in a specially-designed direct contact MD (DCMD) module, it was found that the bubbles with small mean bubble size and narrow size distribution were preferred for creating even flow distribution, intensifying mixing and enhancing surface shear rate. Compared to non-bubbling DCMD, the heat-transfer coefficient and temperature polarization coefficient ( TPC ) reached up to 2.30- and 2.13-fold, respectively, at an optimal gas flowrate of 0.2 L min −1 . With the theoretical expressions for local scaling resistance derived based on the resistance-in-series model, the relative permeation flux ( J w/o|t=t1 /J w/o|t=0 ) in non-bubbling MD was quantified and found to rapidly decline by 65% as the concentration process progressed, consistent with the increasing trend of the ratio of local scaling resistance to the overall resistance ( r fl /r ov ). Fortunately, the introduction of gas bubbles has shown benefits for supersaturation brine concentrating MD process – remarkably decreased the local-scaling resistance due to bubble-intensified shear stress and enhanced hydrodynamics. Also, the total water removal for the brine concentration process was significantly improved by 131% and the discharged brine volume was reduced accordingly at appropriately selected gas flow rates. Nevertheless, at inappropriately high gas flowrates, high energy consumption and potential fiber breakage should be avoided. [ABSTRACT FROM AUTHOR]

Abstract: The concentrated salt solution from reverse osmosis (RO) or other desalination plants is a critical environmental problem that can impact water quality and thereby have economic and social consequences. An integration of a membrane distillation (MD) with a crystallization unit could serve as a potential way to mitigate this problem by having an almost complete water recovery and eliminating the secondary disposal problem. Hence, the continuous membrane distillation crystallization (CMDC) process was studied for a high concentration (close to saturation) feed solution. In addition, the operating conditions were optimized using an orthogonal fractional factorial (OFF) experiment design in order to realize a near zero discharge by varying the flow rates and operating temperatures on both the feed and permeate sides. The results from an L9(34) OFF experiment design indicated that the flow rates on the feed and permeate sides are the principal factors controlling CMDC performance, whereas the temperatures either on the feed or permeate sides are not main factors. The optimal operating parameter values based on four factors and three levels are a flow rate and temperature on the feed side of 0.64Lmin−1 and 338K, respectively, and on the permeate side of 0.35Lmin−1 and 303K, respectively. These operating conditions not only affect the pure water and NaCl solid production, but also the quality of the crystals obtained from the crystallizer. Factors affecting the crystal size distribution were investigated and indicated that the sodium chloride crystals had a narrow size distribution. The feed flow rate and inlet temperature had more influence on the mean crystal size and crystal size distribution, whereas the permeate flow rate and inlet temperature had relatively less effect. Therefore, CMDC operation was found not only to produce a high quality water product but also sodium chloride crystals with a relatively narrow size distribution. These results are beneficial to a better understanding of the CMDC process and improving its performance to realize zero salty water discharge to the environment. [Copyright &y& Elsevier]

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead. [Display omitted] • Concepts and mechanisms of bio-inspired super liquid-repellent surfaces were elaborated. • Methods to develop super liquid-repellent MD surfaces were summarized and discussed. • Their MD performance and mechanisms were summarized and analysed. • The perspectives of these super liquid-repellent membranes were illustrated. [ABSTRACT FROM AUTHOR]

Hydrophobic membranes suffer from wetting and scaling when treating hypersaline brine using membrane distillation (MD). Thus omniphobic membranes with re-entrant surfaces have been developed by diverse modifications to enhance anti-wetting/scaling properties. However, those post-modifications usually sacrifice membrane permeability in spite of improved stability. In this work, omniphobic membranes were fabricated by decorating polyvinylidene fluoride (PVDF) nanofibrous membranes with different silica nanoparticles (SiNPs) followed by fluorination. Compared to unmodified nanofibrous and commercial PVDF membranes, both smaller SiNPs coated membrane #P-Min and larger SiNPs coated membrane #P-Max can perfectly sustain liquids with low surface tensions on their surfaces without wetting. In contrast to #P-Max with an oil contact angle of 136 ± 1°, #P-Min exhibited better oleophobicity with an oil contact angle of 152 ± 1°. In addition, the rigidity of their omniphobic properties was confirmed by testing in harsh conditions. Moreover, #P-Min successfully overcame the trade-off relation between membrane permeability and rejection with an enhanced MD flux due to more effective water evaporation area on membrane surface and a higher membrane porosity. While #P-Max was wetted by a hypersaline feed solution composed of 25 wt% NaCl, #P-Min could maintain a stable flux of 15 L m−2h−1 in a continuous 7-h MD operation, which proved its excellent performance for brine treatment. Furthermore, #P-Min exhibited less scaling tendency when feed was a saturated gypsum solution. Compared to #P-Max , the better anti-scaling properties of #P-Min should be due to its higher surface energy barrier, better slippery property and less solid-liquid contact area, which effectively impede crystal nucleation and attachment. Image 1 • Effects of nano-scaled roughness on membrane omniphobicity were studied. • As-prepared omniphobic membranes were stable under harsh conditions. • Omniphobic membrane coated with smaller NPs exhibited better performance. • It should be due to high surface energy barrier, slippery property and less solid-liquid contact area. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) has attracted increasing attention for seawater and brackish water desalination, wastewater treatment, and concentration of juices and pharmaceutical solutions due to its feasibility of using low-grade energy and achieving high solute rejection. However, the successful implementation of MD technology is still impeded by membrane wetting resulted from membrane scaling by inorganic crystals and membrane fouling by organic matters in the feed solutions, especially when dealing with challenging wastewaters. Herein, this study describes a novel strategy to develop a robust and superhydrophobic membrane via electrospinning followed by electrospray to enhance membrane anti-wetting properties caused by scaling and fouling. The robust superhydrophobic membrane PDMS-3 comprised a superhydrophobic surface layer fabricated by electrospraying a PVDF/PDMS/silica fume blended solution and an electrospun nanofibrous PVDF support. The newly developed membrane PDMS-3 not only exhibited an outstanding superhydrophobicity with a water contact angle of 170⁰ ± 1⁰, but also showed a unique anti-abrasive feature, which still possessed a slippery superhydrophobic property after 40 abrasion cycles. Moreover, the DCMD tests revealed that this rigid superhydrophobic surface rendered PDMS-3 with the best anti-scaling and anti-fouling properties compared to the nanofibrous and commercial PVDF membranes. It could achieve a superior MD flux of 28 kg m−2 h−1and a stable salt rejection above 99.99% during a continuous 160-h DCMD operation when the feed and permeation solutions were 3.5 wt% NaCl solution at 333K and distillate water at 293 K, respectively. These results underline the importance of anti-abrasive superhydrophobic membrane development to deal with wastewaters containing diverse inorganic and organic matters via MD. • The robust and superhydrophobic composite membranes were fabricated by electrospinning and electrospray. • The abrasive resistance of as-developed superhydrophobic membranes were examined. • PDMS-3 with the highest silica content exhibits the best anti-abrasive property. • Compared to nanofibrous PVDF membrane, PDMS-3 exhibits better anti-scaling and anti-fouling properties. • The surface property of PDMS-3 is significant in determining its long-term stability in MD. [ABSTRACT FROM AUTHOR]