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Defined as an assembly of polymer chains which are tethered by one end to a surface or an interface, polymer brushes have found many applications. In particular, grafting of hydrophilic polymer brushes to the membrane surface was used to mitigate fouling of filtration membranes. As well, hydrophobic polymer brushes were grafted on top of a solid substrate to be used for organic selective pervaporation. In this work, grafting of hydrophobic brushes to the surface of membranes for membrane distillation (MD) is proposed to prevent MD membranes from wetting. This is not an experimental work but a computer simulation of designing the polymer brush or patterned polymer brush to optimize their effect on wetting prevention. It was found that such an effect increases with an increase in the distance between brushes, the contact angle and the radius of the cylindrical polymer brush. Considering the decrease in the effective area of the membrane used for vapor permeation by grafting of polymer brush, the optimal distance between the brushes was found to be twice as large as the brush diameter for millimeter-sized brush and three times as large as the brush diameter of micro- or nanometer sized brushes. [ABSTRACT FROM AUTHOR]

• Flat sheet polyvinylidene fluoride - polytetrafluoroethylene (PVDF - PTFE) nanocomposite membranes were fabricated. • The flux of the nanocomposite membrane was 464% higher than that of the neat PVDF membrane. • Water contact angle increased from 67.8° to 87.4° by incorporating of PTFE. • Salt rejection ratio remained unchanged when the feed temperature was increased. Supported flat sheet polyvinylidene fluoride - polytetrafluoroethylene (PVDF-PTFE) nanocomposite membranes were fabricated for this study using the phase inversion process. Several characterization methods, such as scanning electron microscopy, atomic force microscopy, attenuated total reflectance - Fourier transform infrared spectroscopy, water contact angle, and liquid entry pressure of water (LEP w) measurements, were carried out on the prepared membranes, and vacuum membrane distillation (VMD) experiments were then applied to test membrane permeabilities. The cooperation of PTFE was demonstrated to have great potential in enhancing the permeate flux of PVDF membranes while maintaining an acceptable LEP w value. At a feed temperature of 30 °C and vacuum pressure 97.54 kPa, the VMD pure water permeates flux of the prepared nanocomposite membrane embedded with 40 wt% PTFE was 5.61 kg/m2·h, while that of pure PVDF membrane was 0.71 kg/m2 h. The salt rejection results of the prepared nanocomposite membranes were all above 99.9%. These results indicate the applicability of the PVDF-PTFE nanocomposite membranes supported on non-woven polyester fiber for desalination. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Membranes with multi-walled carbon nanotubes (MWCNTs) and two additives MWCNTs/SiO 2 were fabricated. • With the MWCNTs addition, the average pore size decreases, but the finger-like pores are extended. • Hybrid nanocomposite membranes with 1% SiO 2 , integrated additive shows synergetic effects on vapor flux through membrane. • High SiO 2 content could increase liquid entry pressure of water of membrane compared with membrane blended MWCNTs only. • MWCNTs exhibits different effects on tensile properties of membrane when loading ratio of SiO 2 is different. Abstract Polyvinylidene fluoride (PVDF) was blended with multi-walled carbon nanotubes (MWCNTs) and MWCNTs/SiO 2 nano-additives to fabricate nanocomposite vacuum membrane distillation (VMD) membranes for desalination. The effects of MWCNTs and MWCNTs/SiO 2 integrated additives on the morphology and performance of the PVDF membrane were studied. When only MWCNTs were used, 2 wt% loading of MWCNTs to PVDF had the maximum effect on the improvement of VMD flux, due primarily to the enlargement and elongation of the finger-like pores. Then, one of three different SiO 2 nanoparticles (NPs) (hydrophobic, superhydrophobic and hydrophilic) was further added to the membrane that already contained 2 wt% of MWCNTs. When the amount of SiO 2 NPs was relatively low, VMD flux increased as a result of the synergistic effects between the two filler components, i.e. one is the increase of overall porosity by MWCNTs and the other is the growth of the macrovoids by SiO 2 NPs. But among those two, the effect of MWCNTs was more dominant. Compared with the pristine PVDF membrane and the nanocomposite PVDF membrane with a single filler of MWCNTs, respectively, the flux increase by the addition of SiO 2 was 292% and 55%. On the other hand, when a relatively large amount of SiO 2 NPs was added, the effect of SiO 2 NPs started to dominate. Especially, the asymmetric structure of the membrane disappeared, leading to low mechanical strength. All fabricated membranes exhibited great desalination potential with salt rejection as high as >99.97%. [ABSTRACT FROM AUTHOR]

The innovative concept of a zero-waste, energy efficient, and therefore sustainable desalination strategy, Zero Thermal Input Membrane Distillation (ZTIMD), is demonstrated to be economically more effective than existing seawater desalination technologies by simulation based on a single-pass Direct Contact Membrane Distillation process using surface seawater as the feed and bottom seawater as the coolant. Thermal energy required for water distillation in the process was satisfied by extracting the enthalpy of the surface seawater using the bottom seawater as the heat sink. Under one of the favorable conditions, the proposed ZTIMD process could produce pure water with a cost of $0.28/m 3 at a specific energy consumption of 0.45 kW h/m 3 , which is significantly lower than that of the major existing seawater desalination processes, including the currently dominating technology, Reverse Osmosis ($0.45–2.00/m 3 ). Some major advantages promised by the ZTIMD include (1) With no requirement of external thermal energy input, ZTIMD is an inherently energy-saving process, (2) it is economically competitive to existing desalination technologies, and (3) it is waste-free. [ABSTRACT FROM AUTHOR]

The aim of this research was the analysis of heat and mass transfer in vacuum membrane distillation (VMD), specifically for a dead-end feed set-up. The influence of support material on the supported membranes VMD performance was identified. A mathematical model was proposed to evaluate the membrane/feed interface temperature, membrane tortuosity, membrane mass transfer coefficient, and temperature polarization coefficient (TPC). The model was solved by an excel solver based on experimental results of feed temperature, system pressure and the evaporative fluxes. The SEM images showed that the thickness of unsupported membrane was reduced by 42% after the VMD test. Pore shrinkage and tortuosity increase were also expected during this pore collapsing process. On the other hand, the cross-sectional views of the supported membranes did not show significant changes. These results show that the support material can help prevent the membrane pore channel structure from collapsing during the VMD experiment. TPC is close to unity at low feed temperatures. However, it decreases with an increase in evaporation flux as the feed temperature increases, or when a high flux supported membrane is applied. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising separation technique capable of being used in the desalination of marine and brackish water. Poly(vinylidene fluoride) (PVDF) flat sheet nano-composite membranes were surface modified by coating with electro-spun PVDF nano-fibres to increase the surface hydrophobicity. For this purpose, the nano-composite membrane containing 7 wt.% superhydrophobic SiO 2 nano-particles, which showed the highest flux in our previous work, was first subjected to pore size augmentation by increasing the concentration of the pore forming agent (Di-ionized water). Then, the prepared flat sheet membranes were subjected to nanofibres coating by electro-spinning. The uncoated and coated composite fabricated membranes were characterized using contact angle, liquid entry pressure of water, and scanning electron microscopy. The membranes were further tested for 6 h desalination by direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD), with a 3.5 wt.% synthetic NaCl aq as the feed. In DCMD the feed liquid and permeate side temperature were maintained at 27.5 °C and 15 °C, respectively. For VMD, the feed liquid temperature was 27 °C and a vacuum of 94.8 kPa was applied on the permeate side. The maximum permeate flux achieved was 3.2 kg/m 2 .h for VMD and 6.5 kg/m 2 .h for DCMD. The salt rejection obtained was higher than 99.98%. The coated membranes showed a more stable flux than the uncoated membranes indicating that the double layered membranes have great potential in solving the pore wetting problem in MD. [ABSTRACT FROM AUTHOR]

Effects of hydrophilic silica nanoparticles and Non-Woven Fabric (NWF) backing material on the properties and performance of the Polyvinylidene Fluoride (PVDF) membrane have been studied in this work. The nanocomposite membranes were prepared by the phase inversion method and characterized in terms of surface pore size, cross-sectional morphology, thickness, porosity, surface roughness and hydrophobicity. The fabricated membranes were subjected to Vacuum Membrane Distillation (VMD) experiments and their performance was evaluated through measuring the pure water flux and salt rejection. According to the results, effects of the hydrophilic nano-additives and NWF polyester enabled much higher VMD flux than the neat PVDF membrane when an appropriate amount of the nano-filler was added, possibly due to increase in the surface pore size and the reduction of the sponge-like layer thickness. The supported nanocomposite membranes possessed appropriate Liquid Entry Pressure (LEP w ) and mechanical strength, which make the membrane applicable in VMD process. When 7.0 wt.% of the silica nanoparticles was incorporated in a NWF supported membrane, the pure water flux became 12749.6 g/m 2 h at feed temperature of 27.5 °C and permeate side pressure of 1.2 kPa, representing a 2456% increase from the neat PVDF membrane. Almost complete NaCl rejection was also achieved when tested with 35 g/L NaCl aqueous solution. [ABSTRACT FROM AUTHOR]

The re-entrant structure of the membrane surface has attracted researchers' attention recently, due to its ability to make the membrane surface more wetting resistant-either in water or in liquids of low surface tension. The underlying principle of this approach is that an energy barrier has to be overcome when the transition occurs from the Cassie Baxter state to the Wenzel state of the rough membrane surface, which prevents the passage of liquid through the membrane pore. In this work, the energy barrier was evaluated for the cone-shaped pore based on Young's theory of the capillary force. It was found that the energy barrier is enhanced by increasing the angle of the apex of the cone, the pore length, and the contact angle. The effects of the first two geometrical factors, the angle of apex of cone and pore length, are much stronger than the effect of the contact angle. It is believed that this work will offer a guideline for the rational membrane design to reduce, and even prevent, the pore wetting of membranes, particularly for oil/water separation and membrane distillation. [Display omitted] • The energy barrier arising in movement of water/air interface in a cone-shaped pore was evaluated based on Young's theory. • The effects of the apex angle of the cone, pore length, and contact angle on the energy barrier were investigated. • The guideline of the re-entrant structured pore was given to prevent the transition from Cassie-Baxter to Wenzel state. [ABSTRACT FROM AUTHOR]

Membranes were prepared from three poly(vinylidene fluoride) (PVDF) of different molecular weights by the phase inversion process. The membranes were characterized by scanning electron microscopy, gas permeation tests, contact angle (CA) and liquid entry pressure of water (LEPw) measurements, and further subjected to the test of flux for vacuum membrane distillation (VMD) in a scenario that is applicable for cooling processes. The results showed that the increase in PVDF molecular weight increased the viscosity and thermodynamic instability of the casting solution significantly. Regarding characterization and performance testing, the membrane prepared from the intermediate molecular weight of Kynar® MG 15 polymer showed the highest VMD flux (325 g/m 2 h) at the feed temperature of 27 °C and the lowest LEPw (622 kPa) due to the largest pore size (49.8 nm) observed among all the tested membranes. The highest flux of this particular membrane seems also due to the thinnest finger-like and sponge-like layer. [ABSTRACT FROM AUTHOR]

• Nanocomposite membranes with various nanoparticles (NPs) were fabricated for a vacuum membrane distillation (VMD) process. • Able to achieve higher VMD flux using a continuous flow cell than that with a static cell. • With increasing temperature, the continuous flow cell yielded higher VMD flux than that of the static cell. • Incorporation of NPs of larger size showed better performance than that of smaller size. • Finger-like voids reached the bottom of the nanocomposite membrane when the cast film thickness was the lowest. In this study, the effects of incorporation of nanoparticles (NPs), including 7 nm TiO 2 , 200 nm TiO 2 , and hydrophilic and hydrophobic SiO 2 with a mean diameter in the range of 15–20 nm, and their concentrations on the membrane properties and vacuum membrane distillation (VMD) performance were evaluated. The effect of membrane thickness and support materials were also investigated. The membranes were characterized extensively in terms of water contact angle, liquid entrance pressure of water (LEP w), scanning electron microscopy, surface roughness, and pore size. While the best polyvinylidene fluoride (PVDF) nanocomposite membranes with 200 nm TiO 2 NPs were obtained at 2 wt% NPs concentration, the optimal NPs concentration was 5% when 7 nm TiO 2 was integrated. Using a nanocomposite membrane containing 2 wt% TiO 2 − 200 nm NPs, a VMD flux of 3.2 kg/m2h and a LEP w of 34 psi were obtained with 99.99% salt rejection. Furthermore, it was observed that decreasing the membrane thickness would increase the portion of the finger-like layer in the membrane and reduce the spongy-like layer when hydrophilic NPs were used. Using continuous flow VMD, a flux of 3.1 kg/m2h was obtained with neat PVDF membranes, which was higher than the flux obtained by the static VMD with the same membrane at the same temperature and vacuum pressure. The fluxes of both static and continuous flow cells VMD increased with temperature. Furthermore, it was evident that the continuous flow cell VMD at 2 L per minute yielded higher flux than the static cell VMD at any given temperature, indicating strong effects of turbulence provided in the continuous flow cell VMD. [ABSTRACT FROM AUTHOR]

Abstract In this work simulation results for the pore wetting of vacuum membrane distillation (VMD) are reported, considering heat and mass balance at liquid/vapor interface when steady state is achieved. Two cases were studied. In case 1, the system is in isothermal condition. In case 2, heat enters only from the pore inlet and is removed at the liquid/vapor interface as liquid vaporizes. The simulation has shown that the liquid/vapor boundary may remain inside the pore at the steady state condition, which means the partial pore wetting is possible. The significant temperature drop from the pore entrance to the liquid/vapor interface has also been predicted. It was found from case 2 simulation that VMD is possible in a wider range of pore radius, r , than the Laplace equation predicts. As well, the possibility of VMD for the contact angle (θ) below 90° was justified. Highlights • The pore wetting was investigated by developing a well-founded model. • The effects of different parameters were studied. • A better understanding of wetting phenomena was provided. [ABSTRACT FROM AUTHOR]

Nanocomposite membranes were prepared for vacuum membrane distillation (VMD) by casting the dope suspension on top of a nonwoven polyester backing material. The dope consisted of 7.0 wt % hydrophilic SiO2 nanoparticles and a polyvinylidene fluoride (PVDF) blend of high molecular weight (H) and low molecular weight (L). The effects of the blend ratio (H:L) and the penetration time (τ), defined as the period between the completion of membrane casting and the immersion in the coagulation bath, on the membrane properties and performance were studied. It was found that the VMD flux is governed by the pore size and thickness of the top layer (defined as the layer formed above the backing material), both of which are affected by the H:L ratio and the penetration time. Results indicate that the VMD flux increased as the portion of L in the casting dope increased at constant τ, while a maximum flux was observed at τ = 2 min when the penetration time was changed at constant H:L ratio. It was also observed that the liquid entry pressure of water (LEPw) of membranes changed with the PVDF blend ratio and penetration time due to alteration of the maximum pore size. Considering all the data collected, the combination of H:L = 2:8 and penetration time of 3 min was identified as the best condition for the preparation of nanocomposite membranes, achieving one of the highest VMD fluxes, 12.1 kg/m² h at 27.5 °C and 1.2 kPa, an improved LEPw of 27.0 psig, and a near complete NaCl rejection. [ABSTRACT FROM AUTHOR]

[Display omitted] Membrane distillation (MD) is an emerging technology for the desalination of brines. In some cases, liquid penetrates into the pores of the membrane, causing pore wetting. MD's commercialization is hindered largely due to the occurrence of pore wetting phenomena since it results in the reduction of flux and/or permeate quality. Hence, it is of crucial importance for MD to prevent pore wetting from occurring. In this paper, the methods of detecting pore wetting and the membrane parameters related to this phenomenon are reviewed and possible sources of MD pore wetting occurrence are identified. Moreover, the methods to prepare membranes specifically designed for the mitigation of membrane wetting, such as the design of membrane materials, membrane surface modification, preparation of nanocomposite membranes by the addition of nanoparticles, dual-layered membranes, and membranes with a re-entrant structure are discussed. Finally, process-based approach for wetting mitigation, mainly by the pretreatment of the feed solution, is elucidated, and models for wetting phenomena are also outlined. Thus, attempts are made in this review to discuss all aspects of the pore wetting of MD membranes. [ABSTRACT FROM AUTHOR]

Wetting phenomenon inside the pore is a significant obstacle hindering membrane distillation (MD) from being fully industrialized. Herein, a new equation is provided for the users, using the combination of computational fluid dynamics (CFD) and genetic programming (GP) tools for estimation of liquid entry pressure (LEP), a parameter closely related to pore wetting. CFD was applied to model the wetting process inside the pore during the gradual increase in feed pressure at different scenarios in which contact angle, pore radius and membrane thickness were changed. Afterwards, GP as an intelligent method was employed to provide a computer program estimating LEP in the whole ranges in which CFD modeling was carried out. Moreover, validation was done using experimental data and then the influence of effective parameters on LEP was studied. This work provides an explicit formula for estimation of LEP in a closer agreement with the experimental data in comparison to the Young-Laplace equation. In addition, the influence of the membrane thickness was added to the equation, providing a more realistic formula for LEP estimation. Unlabelled Image • A CFD-GP model was presented for estimating LEP, considering the thickness of membrane. • The model provided an explicit formula for estimation of LEP. • The influence of effective parameters on LEP was studied using the developed model. [ABSTRACT FROM AUTHOR]