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Hydrophobic PVDF hollow fiber membranes with narrow pore size distribution and ultra-thin skin for the fresh water production through membrane distillation
Abstract
Polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membrane was fabricated through the dry-jet wet phase inversion process. It is found that the PVDF hollow fiber has an ultra-thin skin layer and a porous support layer from the morphology study. The fully porous membrane structure has the advantage of decreasing the vapor transport resistance and enhancing the permeation flux. The fabricated PVDF membrane has a mean pore size of 0.16μm in diameter and a narrow pore size distribution. The rough external surface produces an advancing contact angle of 112±3∘ with water. During direct contact membrane distillation (MD) of 3.5wt% salt solution, PVDF hollow fibers produced a water permeation flux of 41.5kgm-2h-1 (based on the external diameter of hollow fiber) and a NaCl rejection of 99.99% with a hot salt solution at 79.3°C and cold distillate water at 17.5°C. This performance is comparable to or superior to most of commercially available PVDF hollow fiber membranes, indicating that the newly developed PVDF may be suitable for MD applications.
... The HFMs offer several benefits over conventional flat-sheet membranes, including higher packing density [28][29][30], excellent selfmechanical support to be durable against the backwashing process [28][29][30], and greater membrane surface area per unit volume of membrane modules (which leads to higher productivity) [28][29][30]. The surface-area-to-volume ratio is in the range of 350 to 500 m 2 /m 3 for plate and frame, 650À800 m 2 /m 3 for spiral wound modules, and as high as 7000 to 13,000 m 2 /m 3 for hollow fiber modules [31]. ...
... The HFMs offer several benefits over conventional flat-sheet membranes, including higher packing density [28][29][30], excellent selfmechanical support to be durable against the backwashing process [28][29][30], and greater membrane surface area per unit volume of membrane modules (which leads to higher productivity) [28][29][30]. The surface-area-to-volume ratio is in the range of 350 to 500 m 2 /m 3 for plate and frame, 650À800 m 2 /m 3 for spiral wound modules, and as high as 7000 to 13,000 m 2 /m 3 for hollow fiber modules [31]. ...
... The HFMs offer several benefits over conventional flat-sheet membranes, including higher packing density [28][29][30], excellent selfmechanical support to be durable against the backwashing process [28][29][30], and greater membrane surface area per unit volume of membrane modules (which leads to higher productivity) [28][29][30]. The surface-area-to-volume ratio is in the range of 350 to 500 m 2 /m 3 for plate and frame, 650À800 m 2 /m 3 for spiral wound modules, and as high as 7000 to 13,000 m 2 /m 3 for hollow fiber modules [31]. ...
Reinforcing the HFMs provides appropriate mechanical and shear strengths and improves break at elongation. This comprehensive review paper first discusses the fundamental concept of HFMs fabrications, advantages and disadvantages of HFMs, fabrication methods and fabrication materials. Then, reinforcement methods are categorized into porous matrix membrane reinforced method, the fibers (use of threads or filament) reinforced method, and the use of tubular braids, and each one is discussed based on the concept of thermochemical compatibility of the supporting material and the base polymer. Such a compatibility leads to homogeneous- and heterogeneous-reinforcement method with different interfacial bonding state. A comparative study on the mentioned reinforcement methods is also presented based on the tabulated and charted tensile strength and elongation at break data. At the end, a brief look is taken into commercialized products and worldwide projects installed or under installation in the field of reinforced HFMs.
... Polymer polyvinylidene fluoride (PVDF) was evaluated as a suitable candidate for a lot of applications, we can mention for example biomedical energy applications, which are based improvement of piezoelectric properties of PVDF (see review [3]) The other important application is water purification because of chemical properties of PVDF (see for example [4,5]). PVDF is a durable thermoplastic fluoropolymer, which can withstand high mechanical loads and high temperature differences. ...
... PVDF is a durable thermoplastic fluoropolymer, which can withstand high mechanical loads and high temperature differences. However, a serious problem in the use of PVDF for this purpose is its high hydrophobicity [4][5][6][7][8][9]. Researchers solve this problem using various methods and techniques for plasma treatment of polymer surfaces [11][12][13][14][15][16][17][18] to improve wettability and adhesion properties for subsequent chemical modification. ...
Present paper shows special benefits of low energy plasma treatment on PVDF electrospun nanofiber membrane, where the plasma power 120W has been applied between two flat electrodes creating homogeneous electric field. This experimental arrangement was crucial for two specific effects: (1) Surface activation took place by the scission of C-H bonds without defluorination (preserving the stronger C-F bonds) and consequently preserving the chains ordering and (2) Homogeneous electric field by low plasma power led to better chains ordering, higher degree of crystallinity and increase in the proportion of electroactive beta phase to almost the entire sample volume. This is a major difference from previous publications using higher plasma energies (240-500W) that report successful surface activation, but accompanied by defluorination, chain breaks and a decrease in the electroactive beta phase fraction.
... (Fig. 8). The polymeric RO membrane [24] is a dense, non-porous membrane made up of a polymer network in which solutes are dissolved. The first-generation reverse osmosis membrane is made of cellulose acetate (CA). ...
BIOCHEMICAL INVESTIGATION OF MUSHROOM
... The NMP can make gelation rate very slow near the fiber lumen, and thus an alveolar structure near the membrane inner skin could be developed. Such a structure would contain a large volume of air which can reduce the mass transfer resistance, thus enhancing membrane permeability (Wang et al., 2008;Hou et al., 2014). ...
... In the fabrication of MD membranes, the materials employed can be broadly categorized into two groups: polymeric and inorganic materials. The frequently utilized polymeric materials for MD membrane fabrication are polytetrafluoroethylene (PTFE), polypropylene (PP), and polyvinylidene fluoride (PVDF), respectively [39][40][41][42][43]. Among the materials introduced above, PVDF has garnered the most attention in research studies due to its exceptional mechanical strength, impressive chemical resistance, favorable thermal stability, and excellent aging resistance. ...
Membranes have been widely utilized for different applications in various fields such as desalination and separation processes. The membrane-based desalination technologies, accounting for over 65% of desalination plants, share the common objective of extracting freshwater from various feed sources including seawater, high total dissolved solids brine extracted from deep saline aquifers, brackish water, and industrial effluent. However, the transport mechanisms within the membranes of these aforementioned techniques are significantly different in nature. These differences arise from the distinct specifications of membranes used in each technique, as well as the driving forces applied in their respective processes. Therefore, this chapter focuses on analyzing the transport mechanisms involved in the membranes of three common membrane-based desalination processes: reverse osmosis (RO), membrane distillation (MD), and electrodialysis (ED). In addition to the transport mechanisms occurring within the membranes, this chapter examines the driving forces, the required specifications of the membranes, and the performance parameters and energy consumption associated with the processes mentioned above.
... The accuracy and validity of the acquired measurement of the internal pore structure can be verified through the previous anatomical study by Lee et al. [47]. Many researchers have analyzed enlarged facial skin pores [48,49] and measured the pore surface diameter [50] and size distribution [51]. However, the methods used in each study for assessing the facial pores varied largely and there has been no volumetric measurement and analysis of the total internal volumetric structural calculation of skin pores using OCT. ...
Sebum is an important component of the skin that has attracted attention in many fields, including dermatology and cosmetics. Pore expansion due to sebum on the skin can lead to various problems. Therefore, it is necessary to analyze the morphological characteristics of sebum. In this study, we used optical coherence tomography (OCT) to evaluate facial sebum areas. We obtained the OCT maximum amplitude projection (MAP) image and a cross-sectional image of skin pores in the facial area. Subsequently, we detected the sebum in skin pores using the detection algorithm of the ImageJ software to quantitatively determine the size of randomly selected pores in the proposed MAP images. Additionally, the pore size was analyzed by acquiring images before and after facial sebum extraction. According to our research, facial sebum can be morphologically described using the OCT system. Since OCT imaging enables specific analysis of skin parameters, including pores and sebum, skin analysis employing OCT could be an effective method for further research.
... The accuracy and validity of the acquired measurement of the internal pore structure can be verified through the previous anatomical study by Lee et al. [47]. Many researchers have analyzed enlarged facial skin pores [48,49] and measured the pore surface diameter [50] and size distribution [51]. However, the methods used in each study for assessing the facial pores varied largely and there has been no volumetric measurement and analysis of the total internal volumetric structural calculation of skin pores using OCT. ...
Pores are the microscopic openings in the skin that emit oils and sweat. Pores can appear larger due to acne, sun damage, or increased sebum production, a waxy and oily substance that causes oily skin. Investigating and extracting sebum from facial pores is essential for treating skin issues as the enlargement of the pores causes higher susceptibility of the skin to microbe aggressions and inflammatory reactions. In this study, we assessed the volumetric size of pores before and after the sebum extraction using spectral domain optical coherence tomography (SD-OCT). To properly estimate the volume of the sebum before and after extraction, multiple cross-sectional OCT images were selected. The area of a single pixel was calculated from the OCT images using the scanning range. Furthermore, an algorithm was developed to use the pixel area to calculate the full volumetric size of the skin pore. This research illustrates the use of a high-resolution microscopic analysis using SD-OCT in dermatological research and can operate as a guideline for future research investigations in evaluating non-destructively wounded tissue analysis, underlying skin biochemistry, and facial statistical approaches in skin parameters for moisturizer treatment.
... Hence, MD can theoretically reject 100% nonvolatile components in the water (Dong et al., 2021;Zhang et al., 2010). MD is also capable of utilizing low grade heat as driving force during treatment (Alklaibi and Lior, 2005;Deshmukh et al., 2018;El-Bourawi et al., 2006;Gao et al., 2020;Gao et al., 2017;Gryta, 2005;Kai et al., 2008;Khayet et al., 2004;Li et al., 2013a;TOMASZEWSKA, 2003), and is not as sensitive to feed concentration as reverse osmosis Zhang et al., 2010). ...
Surfactant containing wastewater widely exists in textile industry, which hardly to be treated by membrane technology due to its high in salinity and wetting potential. In this study, PVDF membrane was modified by constructing a PDMS-SiO2-PDMS "sandwich" structure on top of its surface via coating to achieve resistance to surfactant induced wetting. The "sandwich" layer was optimized based on the membrane performance during membrane distillation. Compared to the pristine PVDF membrane with contact angle of 92°, the water contact angle of the membrane with a "sandwich" layer of 0.44 μm increased to 153°. For the feed contained 0.5 wt% NaCl and 0.25 wt% surfactant, there was no membrane wetting occurred during the experiment period using the membrane with a "sandwich" structure, in comparison to the pristine PVDF membrane being wetted from beginning. For a challenge experiment to the developed membrane lasting for 100 h using a surfactant containing feed, there is no wetting sign observed and the stable flux is 20 kg·m-2·h-1.
... The formula for calculating the flux recovery rate (FRR) of PVDF membrane is as follows [Eq. (5)] [28]: ...
Polyvinylidene fluoride (PVDF) membranes were surface modified via a simple coating method for improvement of the hydrophilicity performance. In this work, a hydrophilic modified TA/PVDF membrane was prepared by depositing tannic acid (TA). Then, Ethylenediamine (ED) and succinic acid (SA) were grafted onto the surface of the TA coating to prepare TA/ED/PVDF and TA/ED/SA/PVDF membranes. The surface chemical compositions of the modified membrane were investigated by Fourier transform infrared and X-ray photoelectron spectroscopy, and the surface topography of the modified membrane was studied using scanning electron microscopy and atomic force microscopy. Compared with the pristine membrane, the modified membrane has good hydrophilicity and permeability. The water contact angle and water flux of TA/ED/SA/PVDF membrane are 43.3° and 143.8 L·m⁻²·h⁻¹, respectively. When TA/ED/SA/PVDF membrane was used to separate (Bisphenol A) BPA and (Tetracycline) TC, the rejection ratios reached 86.3% and 91.7%. In addition, the influence of membrane surface charge on the rejection ratio of TC solution was discussed. The results showed that by increasing the pH of the solution from 4 to 10, the retention ratio of TA/ED/SA/PVDF membrane for TC solution increased from 88.8 to 94.4%. In the cycle test, the TA/ED/SA/PVDF membrane has good antifouling performance after three cycles, and its Flux recovery rate reached 89.9%. These results indicated a great application potential of the TA/ED/SA/PVDF modified membrane for water purification, especially antibiotics wastewater treatment.
... (Fig. 8). The polymeric RO membrane [24] is a dense, non-porous membrane made up of a polymer network in which solutes are dissolved. The first-generation reverse osmosis membrane is made of cellulose acetate (CA). ...
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... The increase in permeation flux is favored by large porosity and thin walls, however this compromises the mechanical strength of the resulting membranes. [14][15][16][17][18][19][20][21][22][23]. ...
... Only vapor molecules can pass through the porous membrane and the salt is concentrated at the feed side (Alkhudhiri et al. 2012). Direct contact membrane distillation is the most frequently utilized configuration, where the feed of hot seawater is separated from the flood of cold fresh water due to the barrier of the hydrophobic membrane (Aljumaily et al. 2018a, b, c;Wang et al. 2008). To allow the flow of water vapor and to avoid the passage of liquid, it is imperative to employ a hydrophobic, porous, and thin membrane (Dumée et al. 2013). ...
Membrane distillation (MD), as a hybrid desalination technology with super-hydrophobic characteristics, has been emerging in the recent year. In this contest, the outstanding features of carbon-based nanomaterials have promising potential to contribute to the MD process evolution. This work presented an endeavor to impart the super-hydrophobic features of powder activated carbon (PAC) into poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes for DCMD applications. The FTIR indicates that the CNM was successfully coated onto the PVDF-HFP membrane which leading to higher contact angle values (from 83° to 124°, porosity (45% 86.9%) and water distillate flux at each increment in the PAC loading weigh. The novel coated membranes with 30 mg CNM led to an enhancement of the permeate flux (higher fluxes were obtained at higher CNMs loading) presented fluxes around 77 L/m 2 .h and exhibited a high salt rejection (> 99.9%) in most cases.
... Only vapor molecules can pass through the porous membrane and the salt is concentrated at the feed side (Alkhudhiri et al. 2012). Direct contact membrane distillation is the most frequently utilized configuration, where the feed of hot seawater is separated from the flood of cold fresh water due to the barrier of the hydrophobic membrane (Aljumaily et al. 2018a, b, c;Wang et al. 2008). To allow the flow of water vapor and to avoid the passage of liquid, it is imperative to employ a hydrophobic, porous, and thin membrane (Dumée et al. 2013). ...
Membrane distillation (MD), as a hybrid desalination technology with super-hydrophobic characteristics, has been emerging in the recent year. In this contest, the outstanding features of carbon-based nanomaterials have promising potential to contribute to the MD process evolution. This work presented an endeavor to impart the super-hydrophobic features of powder activated carbon (PAC) into poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes for DCMD applications. The FTIR indicates that the CNM was successfully coated onto the PVDF-HFP membrane which leading to higher contact angle values (from 83° to 124°, porosity (45% 86.9%) and water distillate flux at each increment in the PAC loading weigh. The novel coated membranes with 30 mg CNM led to an enhancement of the permeate flux (higher fluxes were obtained at higher CNMs loading) presented fluxes around 77 L/m 2 .h and exhibited a high salt rejection (> 99.9%) in most cases.
... Specific experimental conditions related to the wet-and the dry-wet spinning process are the spinning and coagulation temperature, fibre extrusion and collection rate and the spinneret dimensions. In the last decades, numerous PVDF hollow fibre membranes have been reported for different applications in the field of membrane contactors [15,25] in general, membrane distillation [26][27][28], or water in oil emulsion separation [29] that rely on their high hydrophobicity. Other fields are wastewater treatment [30], biomedical applications [31,32], and pervaporation [33,34]. ...
The gas and organic vapour permeation of integrally skinned asymmetric poly(vinylidene fluoride) (PVDF) hollow fibre membranes with an effective skin thickness of ca. 0.2 μm was studied. Methanol, dichloromethane and acetone showed fundamentally different behaviour related to their interaction with the polymer phase. Strong plasticization by acetone led to a very pronounced increase in permeance as a function of the vapour activity. In contrast, the permeance of methanol was virtually independent of the vapour activity. Dichloromethane showed intermediate behaviour. The data were supported by vapour and liquid phase sorption measurements and liquid phase dilation measurements on 38.5 μm thick flat sheet reference samples. X-ray diffraction analysis revealed a significant decrease in crystallinity upon swelling by acetone, whereas none of the liquids caused a notable change in the crystal polymorphs. In addition, differential scanning calorimetry revealed no irreversible changes in crystallinity or crystal polymorphs. A considerable discrepancy between liquid phase sorption and vapour phase sorption at high activity, known as Schröder's paradox, was observed for C1–C4 alcohols. The anomalous transport properties were investigated in detail and were explained in terms of the ENSIC model for polymer-vapour interactions. The results were discussed in light of the potential use of asymmetric PVDF membranes in gas/vapour separation, vapour/vapour separation or pervaporation.
... The droplet on sample surface in Figure 9a spun at 300 rpm exhibit contact angle of 103.4 ± 4.2°resulting as more hydrophilic than fiber spun at 2000 rpm (Figure 9b), showing the contact angle of 131.8 ± 2.9°. The enhanced contact angle of PVDF fibers spun at 2000 rpm can be sign of higher porosity on fiber surface [46], or it can be caused by higher surface roughness [47], or due to parallel alignment of the fibers [48]. But in general, samples where fibers are disordered as well as fibers ordered in one direction (spun at higher speed) can be considered as hydrophobic since in both cases, the contact angle of 90°determining the hydrophobicity of the material was exceeded. ...
This study is focused on the characterization and investigation of polyvinylidene fluoride (PVDF) nanofibers from the point of view of macro- and nanometer level. The fibers were produced using electrostatic spinning process in air. Two types of fibers were produced since the collector speed ( 300 and 2000) differed as the only one processing parameter. Differences in fiber’s properties were studied by scanning electron microscopy (SEM) with cross-sections observation utilizing focused ion beam (FIB). The phase composition was determined by Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. The crystallinity was determined by differential scanning calorimetry (DSC), and chemical analysis of fiber’s surfaces and bonding states were studied using X-ray photoelectron spectroscopy (XPS). Other methods, such as atomic force microscopy (AFM) and piezoelectric force microscopy (PFM), were employed to describe morphology and piezoelectric response of single fiber, respectively. Moreover, the wetting behavior (hydrophobicity or hydrophilicity) was also studied. It was found that collector speed significantly affects fibers alignment and wettability (directionally ordered fibers produced at 2000 almost super-hydrophobic in comparison with disordered fibers spun at 300 with hydrophilic behavior) as properties at macrolevel. However, it was confirmed that these differences at the macrolevel are closely connected and originate from nanolevel attributes. The study of single individual fibers revealed some protrusions on the fiber’s surface, and fibers spun at 300 had a core-shell design, while fibers spun at 2000 were hollow.
... • This process is simplest type of MD, has recently gained more attention. • It can explain the characteristics of this type briefly [17][18][19][20]: 1-Total (100%) rejection, 2-Intensive to feed concentration, 3-Mild operating conditions, 4-Stable performance at high contaminant concentrations, 5-Osmosis characteristics play big role in achieving treatment. ...
... The pore size distribution and the average pore size of the membrane was determined using a capillary flow porometer (Porolux 1000, IB-FT GmbH, Germany). The membranes were dried overnight and then immersed in Porefil liquid, and then the measurement was conducted following a similar procedure as described by (Yu Wang et al., 2008). The pore size distribution and the average pore size was calculated using a dedicated computer software coupled to the capillary flow porometer. ...
Wastewater produced from chemical industries contains high levels of inorganic and organic pollutants. The beneficial reuse of this wastewater requires the application of membrane technology to remove these organic compounds. This work aims to optimize the synthesis of polyvinylidene fluoride (PVDF) composite membranes for the removal of chemical oxygen demand (COD) from lye wastewater. 3-aminopropyl triethoxysilane modified titanium dioxide/graphene oxide (APTES-TiO2/GO) was introduced into the PVDF solution to prepare the nanocomposite membranes via immersion precipitation. Box-Behnken Design was used to reduce the number of preparation parameters and optimize the permeate flux and COD reduction. The effects (including interacting and quadratic effects) of PVDF (wt.%), APTES molarity (mM), and APTES-TiO2/GO (wt.%) on permeate flux and COD reduction were investigated. Overall, the APTES molarity and the APTES-TiO2/GO loading had a significant effect on the permeate flux, while PVDF and APTES-TiO2/GO loading presents significant effects on the COD reduction. Under optimized conditions of PVDF (17.22 wt.%), APTES (900 mM) and APTES-TiO2/GO (0.1 wt.%), the permeate flux of 65.545 kg m⁻² h⁻¹ and COD reduction of 99.54% was achieved. Meanwhile, the experimental values for permeate flux and COD reduction were 66.79 kg m⁻² h⁻¹ and 99.56%, respectively. Furthermore, the optimum membrane presents a higher permeate flux and COD reduction when compared with pure membrane due to improved hydrophilicity and porosity. Additionally, the fouled membrane could be effectively regenerated by self-cleaning. This hybrid membrane offers promising application prospects for the treatment of wastewater, which is in-line with the requirements of sustainable development and thus promotes the development of cleaner production technologies.
... • This process is simplest type of MD, has recently gained more attention. • It can explain the characteristics of this type briefly [17][18][19][20]: 1-Total (100%) rejection, 2-Intensive to feed concentration, 3-Mild operating conditions, 4-Stable performance at high contaminant concentrations, 5-Osmosis characteristics play big role in achieving treatment. ...
In this research the membranes distillation were used to remove calcium ions from heavy sewers at dairy plants in Iraq. This method is easy to operate, easy to manage and has many economic benefits. A chemical treatment was carried out on it using alum as a coagulant, different tests achieved samples before and after passing MD, according to APHA, AWWA, WEF. The results showed that obtained in water treatment trust that has been manufactured and edited according to the characteristics of the water waste the interests of the dairy capacity of this method to remove the calcium ion to a certain extent it is when the concentration reaches the inside of this ion to 1428.57 mg per liter, where to start the composition of hydroxides of calcium responsible for pain. The optimal working temperature ranges from 20-22 ° C and the flow is around 0.66 kg.m-2.h-1, here was the result of a layer of plaster that led to clogged membrane and folding and was stopped for washing and cleaning.
... The membrane average pore size and pore size distribution were analyzed via a capillary flow porosimeter (Porolux 1000, IB-FT GmbH, Germany). The membrane was completely wetted with Porefil and finally measured according to the procedure described in the literature [42], and then determined by the CFP computer software [43]. The membrane water contact angle (WCA) via the in-air sessile drop method and oil contact angle (OCA) under water was measured via a goniometer (Shengding, China). ...
This study mainly focused on enhancing oil-membrane electrostatic repulsion towards anti-oil-fouling via electrically conductive hydrophobic membrane in electricity-assisted membrane distillation (MD). Carbon nanotubes (CNTs) were coated on commercial membranes to fabricate membrane cathode. For concentrated hexadecane-in-saline water emulsions, the modified membrane exhibited significantly less flux decline, < 5% in dealing with extreme high-concentration oil emulsion (2000 ppm) at cell potential of 3.0 V. The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation with in-situ DI rinse. The anti-fouling mechanism was systematically discussed regarding the hydrophilicity of the membrane interface, charge repulsion between the oil and membrane cathode as well as slippery property at the liquid-gas-solid triple-phase interface. A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode. Thermodynamics analysis suggested enhanced foulant-membrane electrostatic repulsion leads to a significant energy barrier which favored anti-fouling performance. Interestingly, the sliding dynamics of oil droplet along the interface of membrane cathode was found, tuned by the herein weak cell potential, which could also contribute to the fouling mitigation. Our results insight that the electrically conductive membrane cathode could modulate the foulant-membrane interaction which plays an important role in mitigating fouling/wetting occurred at the triple-phase interface.
... With these superior properties, PVDF has been considered for utilization in various membrane-based separations e.g. micro/nano/ultra-filtration [5,6], membrane distillation [7][8][9], pervaporation [10] and membrane gas absorption [11,12]. ...
This study explores the fundamental science of fabricating poly(vinylidene fluoride) (PVDF) hollow fiber membranes as well as elucidates the correlation among membrane morphology, crystallinity, and mechanical properties as functions of nonsolvent additives and dope rheology in the phase inversion process. A series of nonsolvents (i.e., water, methanol, ethanol, isopropanol) are used either as nonsolvent additives in the dope or as a component in the external coagulant. Depending on the strength of the nonsolvent, the phase inversion of semicrystalline PVDF membranes is dominated by liquid-liquid demixing or solid-liquid demixing accompanying crystallization. As a result, the membrane morphology transforms from an interconnected cellular type to an interconnected globule transition type with lower mechanical strengths when adding water, methanol, ethanol, or isopropanol into the spinning dopes or into the coagulation bath. The crystallinity and size of spherulitic globules in the morphology are controlled by the amounts of nonsolvents present in the systems. The rheological behavior of dope solutions is explored and the relationship between elongation viscosity and mechanical properties has been elaborated. Analytical methods and molecular dynamics simulations are employed to provide insight mechanisms from the views of thermodynamic and kinetic aspects as well as the state of polymer chains involved in the phase inversion process.
... Pore size distribution was evaluated by PMI capillary flow porometer from Porous Materials Inc. (USA). Experiments were performed according to [58] using Fluorinert FC-40 (surface tension=0.017 N/m) as wetting liquid. ...
Water and energy are two intimately interconnected issues of strategic relevance for a sustainable industrial development. Herein, we integrated light-harvesting/self-heating membranes and salinity gradient technology with the aim to implement the innovative concept of light-to-heat-to-power conversion.
Novel photothermal membranes, prepared by immobilizing silver nanoparticles (AgNPs) on the top layer of microporous polyvinylidene fluoride (PVDF) matrix, were tested – for the first time – in a Sweep Gas Membrane Distillation (SGMD) unit applied to the desalination of synthetic seawater solution (0.5M NaCl). As a result of the ability of noble metal nanofillers to act as localized thermoplasmonic nano-heaters at membrane-feed interface for efficient water evaporation, an increase of transmembrane flux under UV radiation by about 2.5-fold with respect to unloaded PVDF membrane was observed.
The SGMD retentate, consisting in hypersaline brine (progressively concentrated up to 4M NaCl and rejected at about 40°C) was fed to a Reverse Electrodialysis unit with the aim to harvest electrochemical energy. The maximum power density, measured for a retentate concentration increasing from 1M to 4M, raised from 0.13 to 0.9 W/m²MP (MP: RED membrane pair). Overall, the proposed integrated membrane system allowed to extract about 10% of the energy not employed for water evaporation.
... The hollow fiber modules of polymeric membranes are thought to offer the advantage of small footprint, high surface porosity and surface area, with high packing density which may ultimately translate to higher output in terms of flux in comparison to their flat sheet counterparts [1][2][3]. They have good flexibility and ease of maintenance as they do not require spacers or multiple sealing steps which reduces the cost of assembly [4,5]. However, although hollow fiber membranes with asymmetric fingerlike and spongelike structures formed from NIPS process are associated with higher pore network in comparison to the symmetric denser spherulitic morphology typical of thermally induced phase separation (TIPS) membranes, they often suffer from deterioration in mechanical properties [6]. ...
Self-supporting polymeric hollow fiber membranes prepared using non-solvent induced phase separation (NIPS) often suffer deterioration in mechanical properties due to asymmetric fingerlike and sponge-like morphology giving a porous and fragile structure. Hollow braided fibers of Polyethylene terephthalate (PET) have been used as substrate to increase the strength of hollow fiber membranes. However, problems arise from poor interfacial bonding between the braid and coating polymer resulting in peeling off or delamination at the interface. In this work, we report a method of braid treatment and selection with two alkali treatment steps (NaOH and KOH) to study their effect on pure water flux, water contact angle, tensile strength as well as braid morphology of three different braid samples (B1, B2 and B3). B2 sample treated in KOH demonstrated the highest water flux of 1388 L/m 2 h. Examination of surface morphology of the braids revealed a washing effect and enlargement of braid interspaces making them more porous and hence increased permeability, with a contact angle of 0°with water. The sample also exhibited zero weight loss as well as a remarkable tolerance for high temperature with negligible reduction in tensile strength of only about 0.9%. Membrane fabricated with B2-K was demonstrated to have better adhesion between polymer-braid interface in comparison to the control. The pre-treatment step provides a good braid selection basis for onward membrane development with KOH showing the most favorable outcome without losing braid quality.
... The antifouling performance of the PVDF membrane was analyzed by flux recovery ratio (FRR), and the FRR was calculated using [Eq. (4)] [32]: ...
Separation of dyes and antibiotics is particularly important in wastewater treatment. In this work, the hydrophilic modified polyvinylidene fluoride (PVDF) membranes were prepared by deposition of halloysite nanotubes (HNTs), tannic acid (TA) and ferric chloride (FeCl3). The surface morphology of the modified membranes was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition, the Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to evaluate the chemical compositions of the membrane. The measurements of water contact angle (CA) and pure water flux (Jw) show that the PVDF membrane has excellent permeability and hydrophilicity. The PVDF modified membrane has good separation performance for dyes, and the rejection ratios of methyl orange, methylene blue and rhodamine B were 98.2%, 94.6% and 96.9%, respectively. In addition, the rejection ratios of modified membrane for tetracyclines and bisphenol A were significantly improved. Most important of all, the TA-Fe(III)/HNTs/PVDF membrane has excellent antifouling performance after cycles tests. These results indicate that the TA-Fe(III)/HNTs/PVDF modified membranes have a great application prospect in dye and antibiotic wastewater treatment.
Membrane distillation (MD) is a hybrid of thermal and membrane processes that utilizes a hydrophobic membrane to separate volatile solutes from feed solution at fairly high temperatures. This process is known to be capable of achieving excellent separation efficiency at low pressure. However, the commercialization of MD has been hampered by low flux performance. To date, different membrane designs have been introduced to ameliorate the flux performance of MD membranes. This article aims to review four key membrane innovations that could enhance the flux performance of MD including the phase inversion-based asymmetric membrane structures, hydrophobic/hydrophilic membrane structures, electrospun nanofibrous membranes (ENMs), and the incorporation of carbon materials. The state-of-the-art of these developments and their recent research trends have been extensively discussed. An update-to-date review of high-flux MD membrane innovations have also been provided. The economic potential of MD membranes is addressed in the final part of this article.
The dairy industry is considered one of the main industries not only in Iraq in particular but throughout the world. Different treatment methods have been used over time and in different countries. Here, membrane distillation has been used to treat wastewater generated by this industry to reduce the burden on the environment. In this paper, direct membrane distillation was used to remove nitrates from wastewater discharged from dairy plants. This method is easy to operate, easy to manage, and has many economic benefits. This article has been using the HACH instrument with all kits needed to achieve the tests. For determination of nitrates in samples by the Calmagite Colorimetric method. HACH Method 8030. 100 mL Marked Dropping Bottle. All other instruments have been achieved according to APHA, AWWA, and WEF. On the other hand, scanning electron microscopy (SEM) has been used to watch the formation of nitrate on membrane surfaces. The results obtained in treating the samples that were synthesized and prepared according to the characteristics of dairy wastewater showed the ability of this method to remove nitrates by up to 80-85%, on the other hand, temperature affects the value of MD and is directly relative to flux. The temperature of the treated water plays an important role in determining the percentage of nitrate removal through the membrane, as well as the latter's enjoying chemical and physical features that give them positive points in its removal of wastewater when the liquid temperature had been raised.
The activated carbon has been extensively studied for various applications especially the wastewater treatment. In this paper, we reported a new two-step method for the green preparation of the novel mesoporous carbon that relied on the synthesis of adsorbents by hydrothermal method and the surface modification by freeze-drying process. The freeze-dried carbon microsphere (FCMS) was then studied by a series of characterization techniques to elucidate their physico chemical properties. Batch experiments were conducted to evaluate the adsorption behaviors for the removal of rhodamine B (RhB) and methylene blue (MB) with contact time, pH, and initial concentration from the wastewater. The results demonstrated that the organic dyes RhB and MB could be rapidly adsorbed on FCMS within 20 min without any assistance and exhibited excellent adsorption capacity (170.461 and 216.606 mg g⁻¹) toward RhB and MB. In addition, the results of adsorption/desorption cycle showed that the retention rate of adsorption capacity for MB and RhB can reach 77.7 % and 85.2 %, respectively. Finally, it can be concluded that FCMS should be an efficient and promising candidate for the removal of organic pollutants from wastewater.
Membrane distillation is a promising technology for recovering resources and water from brackish water and wastewater. However, characteristics of current membranes limit the employment of membrane distillation in these areas. In this paper, activated carbon modified membrane has been studied. Agglomeration of activated carbon (AC) particles in the membrane matrices has negative influence on the performance of AC/PVDF composite membrane. To solve this problem in this study, the AC was functionalized with fluoroalkyl groups via the reaction between carboxylic activated carbons and octafluoropentanol (OFP). The AC-OFP/PVDF composite membrane was prepared by blending PVDF and the functionalized AC-OFP particles via a conventional phase-inversion method. In comparison with the AC particles, the dispersion of AC-OFP particles in membrane matrix was improved greatly. The maximum DCMD flux increased from 29.8 kg/m²h (AC/PVDF membrane) to 40.4 kg/m²h (the AC-OFP/ PVDF membrane) by just adding 0.15 wt% AC-OFP particles.
Membrane distillation (MD) demonstrates excellent desalination application potential for high salt concentration wastewater. However, keeping the membrane flux and rejection high during long-term operation remains one of the critical challenges. In this work, a heat-stretching treatment method was proposed to improve the structure of polyvinylidene fluoride (PVDF) hollow fiber membrane and enhance its stability in the MD process. The membrane's pore size decreased after a heat-stretching treatment in water at 120 °C for 1 h. However, the liquid entry pressure of water (LEPw) value and membrane strength increased significantly, making it less prone to pore deformation during the drying or MD process and thus improving the membrane flux and stability in the MD process. The heat-stretching treatment made the PVDF chain arrangement more compact and oriented, avoiding shrinkage in the length direction. Besides stretching, the shrinkage of hollow fiber can be constricted by the combined effects of pore filling and stretching due to water or glycerol filling in the membrane pores, thus eliminating the capillary stress caused by liquid evaporation during the air-drying. Consequently, the heat-stretching treatment broke the trade-off between high permeate flux and high stability in the MD process.
Landfill leachates (LFLs) pose a severe environmental threat given their high strength and the slow degradation nature of their organic and inorganic pollutants constituents. Hence, they are challenging to treat by conventional wastewater treatment methods. Although membrane distillation (MD) is promising for wastewater treatment, membrane wetting and scaling that lead to membrane fouling are the bottlenecks in applying this technology to LFLs. In this work, a commercial Teflon emulsion was used to modify the PVDF membrane via a simple dip-coating method, followed by depositing the PTFE particles onto the membrane surface via thermal curing crosslinking reaction. Hence, a Teflon/PVDF composite membrane was successfully prepared with resistance to sodium dodecyl sulfate wetting, CaSO4 scaling, and humic acid fouling. Effects of the Teflon emulsion concentration and dipping time on the composite membrane's chemical composition, surface characteristics, porosity, mechanical properties, liquid entry pressure (LEPW), and direct contact membrane distillation (DCMD) performance were systematically investigated. The results showed that Teflon emulsion modification could improve the membrane surface's hydrophobicity, reduce pore size, and increase mechanical strength. The Teflon/PVDF composite membrane with 100% Teflon emulsion and 10 min dipping time exhibited the highest hydrophobicity and 144.3º water contact angle. The M100-10 membrane also exhibited stable performance for treating nanofiltration (NF) brine collected from the LFL. When concentrating the NF brine, the M100-10 membrane achieved only a 7.1% flux decay rate. Hence, the Teflon/PVDF composite membrane demonstrated excellent wetting, scaling, and fouling resistance. This dip-coating method is simple, low cost, easy to scale up, and has potential in the large-scale production of hydrophobic porous membranes.
In this research, a two-step procedure was applied to prepare Pt/Cu/C titanium-supported anode as an oxygen evolution reaction (OER) catalyst. First, the copper nanoparticles with an average particle size of 20 nm were chemically reduced on carbon Vulcan powder (Cu/C). Then, galvanic replacement of Pt by Cu was performed by immersing Cu/C powder in a platinum solution. Finally, the catalyst ink was brushed over the activated titanium substrate using Pt/Cu/C powder and polyvinylidene difluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent. The effect of different factors, such as synthesis temperature and platinum loading on the catalytic properties of nanoparticles, including electrochemical surface area (ECSA), activity, stability, and durability for the OER, were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry, and consecutive CV. The results showed that the platinum loading level of 0.5 mg/cm² and the synthesis temperature of 60 °C provide the best catalytic performance. Also, compared to the commercial electrodes of iridium oxide and ruthenium oxide with the same noble metal loading, higher activity is obtained for Pt/Cu/C electrode than the two mentioned electrodes. In addition, from a lifetime standpoint, the Pt/Cu/C electrode showed higher stability than the commercial ruthenium oxide electrode.
Porous hydrophobic membranes are the main elements in MD (membrane distillation) desalination systems and need to be adequately modeled in terms of transport mechanisms to allow for theoretical assessments toward process design. The objective of this work is twofold. First, illustrate the rational choice of models based on the morphology and structure of a couple of selected membranes for which micrographs and experimental results for distillate mass flux are readily available in the literature. Second, provide a comparative analysis of well-established models for membrane effective thermal conductivity and tortuosity, so as to confirm the adequacy of the proposed model selection path. Theoretical results of the distillate flux were obtained from the numerical simulation of the heat and mass transfer phenomena in the MD process for the DCMD (direct contact membrane distillation) configuration, using a multi-region approach with OpenFOAM@. The comparative analysis is performed for PVDF membranes with different morphologies in the transversal section, for both flat and hollow fiber geometries. For the membrane with sponge-like morphology, the employment of the parallel-resistance model for the effective thermal conductivity and the fractal tortuosity model, designed for porous media characterized by randomly distributed obstructions in spherical format, led to the best agreement with respect to the experimental data for the distillate flux. On the other hand, for the membrane with macro voids in the cross-section (presence of finger-like channels), asymmetrical morphological structure, and narrow pore size distribution, it was found that the employment of the parallel-resistance model for effective thermal conductivity and the Euclidian-based tortuosity model for the random arrangement of highly interconnected pores provided the best agreement between experimental and theoretical data. The results confirm the appropriate model selection path for effective thermal conductivity and tortuosity in MD simulation by taking into consideration solely the membrane morphology.
Currently, information on the interaction of ellipsoidal particles with rough topography is limited. In this work, a mathematical model was developed based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and surface element integral (SEI) to simulate the total interaction energy created between ellipsoidal particles with rough surfaces, which were constructed by the modified two-variable Weierstrass-Mandelbrot (WM) function. The simulated results revealed that an increase in the fractal roughness and relative fractal roughness of particle surface would increase the surface roughness of particles and weaken the total interaction energy between ellipsoidal particles. Compared with spherical shape particles, the ellipsoidal particles provided greater interaction energy because an ellipsoidal shape would generate a greater interaction area than a spherical shape between particles. Amplifying the aspect ratio diminished the interaction energy between particles but enlarging particles would strengthen the interaction energy. In addition, the orientation angle of ellipsoidal particles would affect their interaction.
Fluoropolymers are amongst the most common polymers used for the fabrication of filtration membranes. Despite this, commercial production of these membranes remains dominated by simple casting and solvent phase separation. Herein, we show a rapid, simple approach to produce fluoropolymer membranes, with a porous patterned surface, via immersion precipitation printing (ipP). The patterns can act as a permeate spacer, which are traditionally added to a membrane separately to induce turbulent flow and subsequently decreasing membrane fouling. The direct phase inversion of the permeate spacer during membrane production induces a porous morphology. Further, intimate mechanical connection between the membrane surface and permeate spacer was observed. Pure water permeability studies were performed on membranes and membranes with patterns (spacers) fabricated with a combination of different fluoropolymer materials (polyvinylidene difluoride (PVDF) and synthesised dehydrofluorinated PVDF (dPVDF)), with a polyvinylpyrrolidone (PVP) (pore forming agent). This simple ipP approach shows potential as a viable alternative for the production of fluoropolymer membranes where the complete control over pattern height, fidelity, and shape is required.
Sluggish kinetics of the cathodic oxygen reduction reaction (ORR) is one of the major challenges hindering the development of microbial fuel cells (MFCs). In this work, a three-dimensional (3D) cellular scaffolds structure is fabricated by in-situ growth of conductive Ni/Co-catecholate bimetal metal-organic frameworks (MOFs) on multiwalled carbon nanotubes (NiCo-CAT/MWCNTs). Such specific structure successfully achieves favorable electronic channels and fast charge-transfer capacity and displays an ultra-low ohmic internal resistance of 7.27 Ω. Meanwhile, NiCo-CAT/MWCNTs exhibit a superior ORR half-wave potential (−0.442V vs. Hg/HgCl2), onset potential (−0.064V vs. Hg/HgCl2) and high limiting current density (8.25 mA cm⁻²), which exceed that of commercial Pt/C. In the MFC with the NiCo-CAT/MWCNTs cathode, the maximum power density is 130% higher than Pt/C cathode. The outstanding performance of MFC is mainly due to the optimized electronic structure and fully exposure of the Ni, Co synergistic catalytic active sites. In addition, oxygen permeability is greatly improved by electrospun polyvinylidene fluoride (PVDF) air diffusion layer replacing the traditional carbon cloth cathode. This study provides a new perspective for MFCs performance improvement by ORR conductive MOFs catalysts.
Desalination technologies can help mitigating the water scarcity problem facing humankind. Membrane distillation (MD) is a desalination technology attractive for its adequacy to use low-grade waste heat or renewable energy sources. This work addresses the modelling of a Direct Contact Membrane Distillation (DCMD) plant with heat recovery. The desalination unit comprises several DCMD modules, each of a shell and hollow fibre tube bundle type. A multiscale approach is needed to address the problem. It includes modelling of the vapour flux through the micro/nano porous membrane, models for the mass and energy conservation at the scale of a single DCMD module, and thermodynamic modelling at the scale of the desalination plant. Analytical solutions of the mass and heat transfer across the membranes are combined with finite volume discretised equations describing heat and mass conservation for the feed and permeate streams. An iterative scheme is devised to solve the model equations in order to determine the flow and temperature variables in the DCMD module. The computational model predictions show good agreement with experimental data available in the literature. Finally, the methodology presented herein is used to simulate the performance of a DCMD desalination plant with heat recovery.
Air pollution is becoming more and more severe, especially in developing countries given the ever-increasing levels of particulate matter, which usually carries viruses and bacteria, has drawn considerable attention as a major threat to public health. Polytetrafluoroethylene (PTFE) based antibacterial microfiltration hollow fiber membranes are fabricated using the cold-pressing method including paste extrusion, stretching and heating by adding silver-carried zirconium phosphate (AgZrP) into the lubricant and then mixed with PTFE powders. The AgZrP concentration has influences on membrane performance, such as morphology, tensile strength, thermal stability, filtration, and antifouling properties. The PTFE hollow fiber membranes containing 1 wt% AgZrP have a particle filtration efficiency of 97.7324 % for particles (0.3 μm) and 99.9984 % for particles (2.5 μm). The membranes, with AgZrP particles (≥1 wt%), possess high antibacterial activity and are capable of killing all the Escherichia coli (E. coli) and more than 98 % of the Staphylococci aureus (S. aureus) within 24 h of contact. After washing, the fouled filter media retains stable filtration performance and antibacterial property, indicating that AgZrP has a broad-spectrum and long-acting antibacterial activity. The PTFE/AgZrP hollow fiber membranes are supposed to be a candidate for its application in air filtration.
Developing membrane for brine waste reclamation via direct contact membrane distillation (DCMD) requires specially membrane characteristics engineering. Toward practical and scalable hydrophobic micromembrane with desirable properties, attaining energy-efficient and durability during hypersaline treatment remains a great challenge. Herein, polyvinylidene fluoride (PVDF) hollow fiber (HF) membranes were developed for particular use in DCMD for treating hypersaline (100,000 mg/L of NaCl solution) via adopting mixture of small molecular weight (MW) additives, including water, lithium chloride (LiCl), and ethylene glycol (EG). Engineered to suppress the limitation of single small MW additives only possessing a unique enhancement in the tailoring of specific properties for improving the partial DCMD performance, the research emphasis is placed on the synergistic effect between small molecular weight additives, resulting in a much-enhanced performance. The dope solution of PVDF/LiCl/H2O/N-Methyl-2-pyrrolidone (NMP) = 12/2/2/84 (wt%) spun HF membrane, P-L-W2, has been shown to reveal the most desirable membrane characteristics for DCMD, including comparable porosity (ε, 74.2 ± 0.4%), superior liquid entry pressure (LEP, 3.05 bar), and robust mechanical properties. The P-L-W2 membrane also exhibited enhanced water flux, energy efficiency, and anti-wetting ability in comparison with the neat PVDF HF membrane. With advantage of easy implement, the concept of adoption of additives mixture provides a novel platform to customize desirable PVDF HF membranes with enhanced DCMD performance.
Emerging contaminants (ECs) have huge impacts on all living beings, and conventional treatment processes like coagulation, precipitation, and chlorination have limited capability for removal. So, a tertiary and combined treatment process is required. Alternative treatment technologies include adsorption, chemical treatment, and membrane filtration. However, the associated operating cost, ECs rejection, fouling propensity, and by-product formation are some of the drawbacks. Membrane distillation (MD) is one of the promising membrane technologies for emerging contaminants removal. In MD, The vapor pressure difference between the hot feed and cold permeate is a driving force. MD technology has some added advantages like low-pressure requirements, less fouling susceptibility, low-temperature requirements, and only vapor mass transfer, i.e., 100% non-volatile compounds retention. MD employs a low temperature and pressure so fouling is less compact and is easily cleanable. MD technology has been studied for desalination, hypersaline brine treatment, chemical separation and can potentially remove emerging contaminants. The MD technology does not require very high-quality heat; solar heat, waste heat, or cogeneration-based heat utilization is possible. This way, MD can be operated on renewable energy and becomes sustainable and carbon neutral. MD technology has also been integrated with other efficient treatment technologies like Forward Osmosis (FO), Reverse osmosis (RO), and Nanofiltration (NF), providing a leading edge compared to other treatment methods. This chapter elaborates on various available MD technologies, possible materials, configurations, operating parameters, and energy requirements. We have also highlighted future research trends and challenges for MD treatment technology’s sustainable and commercial application.
Direct contact membrane distillation (DCMD) has shown great promises in remediating high-salinity oilfield produced water (PW). In our previous work, a hydrophobic silica sol (Si-R) was successfully synthesized to engineer anti-wetting ability of poly(vinylidene fluoride) (PVDF) membrane. Here our research interest lies in customizing PVDF hollow fiber membrane (HFM) with desirable stability for treating challenging oilfield PW via a DCMD process. The PVDF/Si-R hybrid HFM was fabricated under an optimized spinning condition. It is found that the fabricated PVDF/Si-R HFM is sandwich-structured, consisting a highly porous and thick middle layer and small-size macrovoids of outer layer and inner layer with different pore geometry, and the unique sandwich structure endows the membrane with superior wetting resistance. The membrane characterization showed that the liquid entry pressure (LEP) and water contact angle (WCA) of the fabricated PVDF/Si-R HFM impressively reached up to 5.25 bar and 126°, respectively. As a result, the PVDF/Si-R HFM possessed a desirable performance in treating high-salinity oilfield PW in terms of wetting resistance and salt rejection. Specifically, with a 60 h continuous DCMD operation, the PVDF/Si-R membrane exhibited over 99.9% salt rejection with only 7% reduction in permeate water flux. Clearly, the synthesized hydrophobic Si-R is a favorable additive for enhancing the antiwetting ability of PVDF HFM to treat high-salinity oilfield PW via DCMD process.
Membrane distillation (MD) has attracted significant attention to desalinate hypersaline water owing to its unique advantages. However, commercially available membranes used for MD desalination face fouling and wetting issues in...
Hollow fiber membrane (HFM) technology has received significant attention due to its broad range separation and purification applications in the industry. In the current study, we applied bibliometric analysis to evaluate the global research trends on key applications of HFMs by evaluating the global publication outputs. Results obtained from 5626 published articles (1970–2020) from the Scopus database were further manipulated using VOSviewer software through cartography analysis. The study emphasizes the performance of most influential annual publications covering mainstream journals, leading countries, institutions, leading authors and author’s keywords, as well as future research trends. The study found that 62% of the global HFM publications were contributed by China, USA, Singapore, Japan and Malaysia, followed by 77 other countries. This study will stimulate the researchers by showing the future-minded research directions when they select new research areas, particularly in those related to water treatment, biomedical and gas separation applications of HFM.
Membrane distillation (MD) is a separation technology that is gaining increasing importance for desalination because of its optimal separation performance and its ability to treat highly concentrated saline solutions. Among all membrane morphological structures, hollow fibre (HF) exhibits some peculiar advantages, does not require any support to withstand the operation conditions and can be arranged in modules reaching high packing density and optimal fluid dynamics reducing both temperature and concentration polarization effects on MD desalination performance. In general, hollow fibre membranes are prepared by spinning a dope solution following different techniques. The HF membrane morphology can be tuned by modifying a large number of spinning conditions as well as by improving the membrane structure by preparing single layer, mixed matrix or dual layered hollow fibres. This review analyses the research studies developed so far on the design and preparation of different types of hollow fibres together with a critical evaluation of the effects of the involved preparation conditions on MD desalination performance, and some useful remarks to improve hollow fibre characteristics, desalination performance and thermal efficiency. Among the proposed HF for MD desalination, dual layered HF membranes exhibit high permeate fluxes up to 98.6 kg/m² h with good salt rejection factors.
The entire world is facing with a severe dilemma of water pollution which is mainly concerned with the chemical and biological contaminants that have endangered the quality of drinking water. The Membrane separation technology is vastly acknowledged as an advanced process for water filtration and purification, having the potential to alleviate global matter of contention of freshwater scarcity. Various nanofillers have been used by many in water purification. This methodology has been accepted as feasible and active to produce a multifunctional nanocomposite membrane i.e. membranes with higher performance along with their synergistic effects of organic polymer matrix and inorganic nanomaterials for water and wastewater treatment. Recently, inorganic nanofillers such as inorganic metal oxides (SiO2, TiO2, ZnO, Ag, etc.), carbon based (CNT, GO) and mixed nanoparticles (SiO2-TiO2, GO-TiO2, GO-SiO2, etc.) have been extensively used to prepare polyvinylidene fluoride (PVDF) nanocomposite membranes with desired properties for water purification. This review aims to highlight the performance of nanofiller incorporated PVDF membranes and the novel strategies explored for fabrication of the PVDF nanocomposite membrane to cater to the current requirements and expectations of water purification performances.
This study proposed a novel strategy to fabricate highly permeable and robust supported membranes for membrane distillation, via a nonsolvent thermally induced phase separation method. Different from traditional supported membranes cast on non-woven fabrics, a well-integrated fabric-embedded PVDF (FE-PVDF) membrane was successfully obtained using polyamide 66 gauze supports. The optimal FE-PVDF membrane PA-120 exhibited an interesting structure with pseudo Janus function, which entailed hydrophobic bulk membrane property but a pseudo hydrophilic bottom surface due to the presence of water-filled “50 μm grade” pores. The PA-120 exhibited a DCMD flux of 68.5 kg.m⁻².h⁻¹ at 70 °C with synthetic saline solution and exceptional tensile strength of 21 MPa (lateral)/21 MPa (longitudinal), which was 1.4-fold and 23-fold higher than that of the unsupported benchmark, respectively. The concentration of 10 wt% monosodium glutamate solution with PA-120 was demonstrated at 70 °C up to 55 wt% when crystallization was observed, The flux showed a minor decrease by 16.3% until 40 wt% and a subsequent decline towards 55 wt%. Repeated concentration cycles were conducted with intermediate membrane rinsing by fresh feed, demonstrating highly recoverable performance. This preliminary study brings new perspectives towards pushing the limits of current lab-made MD membranes.
Converting CO2 to valuable chemicals via catalytic CO2 hydrogenation presents a promising way for CO2 utilization. However, many types of CO2 hydrogenation are generally conducted under harsh conditions (T≧200 °C and P≧20 bar), yet these reactions are limited by thermodynamic equilibrium. Water is a common by-product in CO2 hydrogenation reactions. By employing a water permeable membrane reactor to simultaneously remove water from the reaction, not only favors the higher CO2 conversion by overcoming the limitation of thermodynamic equilibrium, but also protects the catalyst from deactivation caused by water. The development of a suitable membrane reactor for this application has received huge attention worldwide. However, water separation from gases (H2, CO2, CO) under high temperature and pressure is a major challenge in developing a good performance membrane. Herein, this review contains various high temperature water/gases separating membranes, and their applications in water permeable membrane reactors for CO2 utilization. The separation and reaction mechanism in the catalytic membrane reactors are discussed in detail. Furthermore, challenges and prospects in the application of this type of catalytic membrane reactor in CO2 conversion is provided for the future development.
Brackish water poses a health hazard worldwide, but especially to rural residents in China. In this study, a direct contact hollow fiber membrane distillation system was established based on a filter-pretreated hot spring resource in the Jiaodong area of China. The effects of the operating temperature, flow rate, and composition of brackish water on the properties of membrane distillation were investigated. A multistage system model was established using the Aspen plus platform and the economic cost of the system was analyzed. The results showed that the permeate flux could reach 12.599 kg/m²·h. The effect of the MgSO4 content of the brackish water on the flux decreased above 20 g/L. Electron microscopy and elemental analysis showed that carbonate crystallization occurred mainly in the scaling part. The simulation results showed that the inlet temperature of the feed side had major effects on the increase in the flux and gain output ratio. A parallel layout was more effective and the regenerator improved the gain output ratio (GOR), with a 30% maximum increase for the system. The unit water production cost decreased to $18.2/m³ as the subsystem number increased to 10. These findings may help to improve the performance and economic benefits of this special type of distillation system.
Fruit juices have been traditionally concentrated by multi-stage vacuum evaporation, resulting in a loss of fresh juice flavors, color degradation and a “cooked” taste due to the thermal effects. The promising alternative is reverse osmosis concentration. However, it cannot reach concentrations larger than 25–30°Brix with a single-stage RO system due to high osmotic pressure limitation, which is quite below the value of 45–65°Brix for standard products obtained by evaporation. Technological advances related to the development of new membranes and improvements in process engineering have been proved to overcome this limitation. New membrane processes, including membrane distillation and osmotic distillation, and integrated membrane processes are still being identified and developed in concentrated fruit juice processing to improve product quality and reduce energy consumption. Recent advances and developments of the use of membrane processes for concentrating fruit juice are reviewed and discussed in this paper. Major attentions are focused on the application of new membrane processes and integrated membrane systems.
Three membrane distillation processes, direct contact membrane distillation, sweeping gas membrane distillation and vacuum membrane distillation, have been experimentally studied in a shell-and-tube capillary membrane module. Preliminary experiments were conducted using distilled water and sodium chloride aqueous solutions as feed. The effects of the operating parameters - flow rate, temperature and salt concentration - have been investigated. A theoretical analysis that considers the heat and mass transfer through microporous hydrophobic membrane has been developed. A comparative study was made between the three membrane distillation configurations. Membrane distillation can be an alternative for liquid nuclear waste treatment.
This paper provides a state-of-the-art review of the separation process known as membrane distillation, MD. An introduction to the terminology and fundamental concepts associated with MD as well as a historical review of the developments in MD are presented. Membrane properties, transport phenomena, and module design are discussed in detail. A critical evaluation of the MD literature is incorporated throughout this review.
Pure water direct contact membrane distillation (DCMD) experiments were used to measure the permeability parameter associated with the molecular diffusion in membrane distillation (MD). The fluxes given by a recently reported MD model, which is based on the dusty-gas model of gas transport through porous media, showed good agreement with the experimental results over the entire range of feed temperatures studied. The model was also capable of predicting flux as a function of the difference between bulk feed and permeate temperatures for the limiting case in which only molecular diffusion contributes to flow. The DCMD experiments were performed in this work with a new laboratory-scale module that does not require a support for flat-sheet membranes. The resulting DCMD fluxes were two to three times higher than those reported in the literature for either DCMD or reverse osmosis. The MD model was also used to predict the performance of DCMD desalination, and the results were compared to those of reverse osmosis, in terms of both water production rates and NaCl rejection.
Membrane contactors represent an emerging technology in which the membrane is used as a tool for inter phase mass transfer operations: the membrane does not act as a selective barrier, but the separation is based on the phase equilibrium. In principle, all traditional stripping, scrubbing, absorption, evaporation, distillation, crystallization, emulsification, liquid‐liquid extraction, and mass transfer catalysis processes can be carried out according to this configuration. This review, specifically addressed to membrane distillation (MD), osmotic distillation (OD), and membrane crystallization (MCr), illustrates the fundamental concepts related to heat and mass transport phenomena through microporous membranes, appropriate membrane properties, and module design criteria. The most significant applications of these novel membrane operations, concerning pure/fresh water production, wastewater treatment, concentration of agro food solutions, and concentration/crystallization of organic and biological solutions, are also presented and discussed.
In the direct contact membrane distillation (DCMD) process for desalination, the water vapor flux is strongly affected by the hot brine heat transfer coefficient, conductive heat loss, and long-term flux decline due to membrane pore wetting/fouling, etc. The DCMD process has been explored here using porous hydrophobic polypropylene hollow fibers having three different dimensions and two different wall thicknesses. The outside surfaces of the fibers have been coated with a variety of microporous plasmapolymerized silicone-fluoropolymer coating. A large number of rectangular modules having the hot brine in cross flow over the outside of the fibers and cold distillate flowing in the tube side have been investigated for their DCMD performances with hot brine (1% NaCl) over a brine temperature range of 60-90 degreesC. The module MXFR 3 containing fibers with larger internal diameter (i.d.) and wall thickness, and having the best performance, was tested in a continuous DCMD run for 120 h with 85 degreesC brine flowing at a Reynolds number of 70. The remarkably high water vapor flux (41-79 kg/m(2.)h) obtained in such modules and the complete absence of pore wetting over 400 h of experiments without any module cleaning demonstrate the excellent DCMD potential of such hollow fiber membranes and modules.
For the first time, co-extrusion was applied for the fabrication of dual layer hydrophilic–hydrophobic hollow fibers especially for the direct contact membrane distillation (DCMD) process. The effect of different non-solvents on the morphology of the PVDF membranes was investigated and it was found that weak coagulants such as water/methanol (20/80, w/w) can induce a three-dimensional porous structure on PVDF membranes with high surface and bulk porosities, big pore size, sharp pore size distribution, high surface contact angle and high permeability but rather weak mechanical properties. Hydrophobic and hydrophilic clay particles were incorporated into the outer and inner layer dope solutions, respectively, in order to enhance mechanical properties and modify the surface tension properties in the membrane inner and outer layers. Different membrane characterizations such as pore size distribution, gas permeation test, porosity and contact angle measurements were carried out as well. Ultimately, the fabricated hollow fibers were tested for the DCMD process and flux as high as 55kg/(m2h) at 90°C was achieved in the test. This performance is much higher than most of the previous reports, indicating that the application of dual layer hydrophilic–hydrophobic hollow fibers may be a promising approach for MD.
The results of the over 3 years’ time research on the direct contact membrane distillation applied for production of the demineralised water have been presented. The hydrophobic capillary polypropylene membranes (Accurel PP S6/2) were used in these studies. The inlet temperature of the feed and the distillate was 353 and 293K, respectively. The membranes were found to be thermally stable, and good separation characteristics was maintained throughout the whole period of the investigations. The SEM observations of the capillary membrane confirmed that their morphology remained unchanged. It was found that water (permeate from reverse osmosis process) did not cause the wetting of the used membranes despite a long-term membrane module exploitation. The precipitation of CaCO3 on the membrane surface was observed when tap water was used directly as a feed. A partial wetting of the membrane was found in this case. The wettability resulted in the increased electrical conductivity of the distillate obtained from 0.9 to 2.5μS/cm.
Nowadays, in dry and rural areas, solar-powered membrane distillation (SPMD) technology is considered a feasiblemeans for the production of pure water from brackish water. Prior to the design and construction of a SPMD pilot plant, there is a need to predict its performance theoretically by means of a computational simulation program. Unlike previous approaches followed by other investigators to develop a mathematical model that can describe the components of a SPMD pilot plant, the developed mathematical model in this study is based on the fact that the SPMD process by nature is unsteady. The performance of a proposed SPMD pilot plant is then obtained by means of a numerical solution of the model with the aid of a simulation computer program. The results reveal that the proposed SPMD pilot plant has some unique features, which differ from a similar MD process operated at steady-state conditions in a laboratory. The analysis of the system has shown that heat recovery via an external heat exchanger is not only possible, but even effective, and an economical way to intensify the SPMD process. The plant productivity can be improved by increasing the heat-exchanger capacity (KA), decreasing the flow rates of both feed and permeate or otherwise by increasing the effective surface area of the membrane. The achieved enhancements in the SPMD pilot plant productivity are directly related to an improved heat recovery rate in the heat exchanger. However, further analysis reported in this paper shows that the increase in KA and membrane area should be optimized for any planned capacity in the design of a SPMD pilot plant.
This work focuses on vacuum membrane distillation (VMD) using hollow-fibre modules for seawater desalination. VMD is an evaporative process using porous and hydrophobic membranes, which physically separate the aqueous liquid feed from the gaseous permeate kept under vacuum. In the case of a salty water, the water passes through the membrane as a vapour without salt and is then condensed outside the module. We had previously shown that for commercially available membranes, operating conditions (velocity, temperature, pressure) and process design could be chosen to minimise energy consumption. VMD operated with these commercial membranes could clearly compete energetically with RO, However, the flux and thus the membrane area to be installed would be higher than for R0. On the basis of computations, the present study shows that for membranes 100 times more permeable than the experimental membranes, and at low temperature, DMV can compete with RO on energy consumption (less than 2 kWhlm3) with a same level of permeate flux (between 5 to 15/h.m3). Moreover, if coupled with solar energy and operated at high temperature, the energy to be provided to the system can be maintained at a low level and the permeate flux could be enhanced to 45–851/h.m2. On this basis, a discontinuous process coupling VMD and solar energy could clearly compete with RO. Process feasibility is clearly linked to the membrane properties and mainly to their permeability.
Concentration of NaCl solutions by direct-contact membrane distillation (DCMD) integrated with salt crystallization has been studied. The salt crystallization was carried out in a batch mode or continuously. The influence of process parameters (flow rates, temperatures, and concentrations) on permeate flux has been investigated. To evaluate the polarization phenomena, a simple model was used. This model showed good agreement with the experimental results. Two types of membrane distillation MD capillary modules, with the membranes arranged in a form of braided capillaries or helically wounded, were tested. The membrane wettability during long-term testing was investigated. A slow decline of the module efficiency was observed since the membranes were partially wetted during the process. The MD process integrated with continuous crystallization yielded an average NaCl production of 100 kg m−2 d−1.
The demineralization of water by membrane distillation (MD) has been investigated. In the first stage of investigations the tap water or boiled tap water was employed as a feed, and the water recovery coefficient exceeding 75% was achieved. The obtained concentrate was supplied to the second stage of MD installation. The quality of distillate was stable and practically independent of the feed concentration. The produced distillate has the electrical conductivity in the range of 1.4–2.5 µS/cm. The precipitation of salt deposit on the membrane surface was observed during the water demineralisation, especially in the first stage of MD. The membrane morphology and the composition of precipitate layer were studied using scanning electron microscopy coupled with energy dispersion spectrometry. The formed deposit caused clogging of the membrane surface (pores), and resulted in a gradual decline of the module efficiency. Moreover, the formation of the deposit on the membrane surface was the major reason of the partial membrane wettability.
Asymmetric polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by a phase inversion method using dimethylacetamide (DMAc) and a mixture of water/LiCl as solvent and a nonsolvent additive, respectively. The prepared membranes were characterized by scanning electron microscopy (SEM) for observing its microstructures and by a gas permeation method for measuring its surface porosity, pore size, and pore size distribution. Wetting pressures of the dry hollow fiber membranes were also measured. Using the prepared PVDF hollow fiber membranes, a membrane module was fabricated for removal of benzene/toluene from water. Effects of various operating parameters such as downstream vacuum levels, feed temperatures, and feed flow rates on performances of the module were investigated experimentally. The benzene/toluene removal was achieved over 99% under an optimal operating condition. Mass transfer of benzene or toluene removal is controlled not only by the liquid phase resistance but also by the membrane and gas phase resistances. Benzene and toluene can be removed from water simultaneously with no adverse coupling effects.
This review looks at the work carried out over the past 15 years on membrane distillation and reports the conditions utilized for research. The process is still used mainly at the laboratory scale, but a few pilot plants have been built across the world, mostly for desalination and the production of potable water. Studies into membrane distillation have been concerned with the effect of mass transfer, heat transfer, and stirring rate, but the most important effect that has to be considered with this process is temperature polarization. A section on temperature polarization and the effect of boundary layers is included in this review.
When a microporous membrane separates two aqueous solutions at different temperatures, selective mass transfer across the membrane can be obtained. The driving force is the vapour pressure difference between the liquids at the two solution-membrane interfaces. A capillary polypropylene membrane has been tested at different temperature differences, solute concentrations, etc. Interesting transmembrane fluxes and selectivities have been consistently observed. The possibility of operating at very high solute concentrations up to saturation has been demonstrated.
The preparation of very hydrophobic poly(vinylidene fluoride) (PVDF) membranes was explored by using two methods. The first one was the modified phase inversion method using a water/N,N-dimethylacetamide (DMAc) mixture instead of pure water as a soft precipitation bath. The second method was a precipitation-bath free method, that is, the PVDF/DMAc casting solution underwent gelation in the open air instead of being immersed into a precipitation bath. The morphology of the surface and cross section of the membranes was investigated by using scanning electron microscopy (SEM). It was found that the membranes exhibited certain micro- and nanoscale hierarchical roughness on the surface, which brought about an enhanced hydrophobicity of the membrane. The contact angle (CA) of the samples obtained by the second method was as high as 150° with water. The conventional phase inversion method preparing PVDF porous membrane using pure water as precipitation bath usually results in an asymmetric membrane with a dense skin layer having a CA close to that of a smooth PVDF surface. The modified approach avoided the formation of a skin-layer and resulted in a porous and highly hydrophobic surface of PVDF. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1358–1363, 2005
Super-hydrophobic fluorinated membranes were tailored by combining traditional dry-wet phase inversion and wet chemical treatment techniques. PVDF and HYFLON AD 60X were selected as raw polymers for their chemical and mechanical resistance as well as hydrophobic and solvophobic properties. Membranes with modulated pore size, narrow distribution and high overall porosity were manufactured without using additional additives or modifiers. High resistance to liquid water entry pressure (LEPw), high mass transfer and low surface free energy of the membrane surfaces were fully achieved. The combination of these two fluorinated polymers improved dramatically the mechanical resistance and the water repellence of the PVDF membranes. Well-controlled structure combined with aimed supra-molecular chemistry makes these porous layers ideal interfaces to be processed in membrane contactor devices.
Tunisia has five major geothermal districts. Recorded hot spring temperatures range from 294 to 340 K with flow rates 0.1–10 2 L/s. In southern parts available groundwater resources are mostly hard and brackish (3 g/L). Recently, Tunisia has resorted to this resource for agriculture and potable water uses. Cooling towers have been built to lower down the temperature and hardness. Cooled brackish water irrigates greenhouses and feeds desalination plants. The cooling operation of groundwater rejects an important quantity of thermal energy in the atmosphere. However, energy requirements of thermal desalination plants are too excessive to be supplied by a geothermal resource. Membrane distillation (MD) is an emerged desalination technology, which can be driven by a thermal energy at low enthalpy (less than 363 K) as geothermal energy, and a fluidised bed crystalliser can ensure reduction of an important portion of hardness without significant loss of temperature. MD is realised by means of a microporous hydrophobic membrane separating a warm solution from the cooling chamber, which contains either liquid or gas. A fluidised bed crystalliser permits production of from it of a granular crystal calcium carbonate from the consumption rates of calcium ions, using aragonite or sand as seeding materials. The purpose of this paper is to realise experimental assembly constituted of fluidised bed crystalliser (FBC) and a cell of air gap membrane distillation (AGMD). It presents preliminary results related to characterisation of fluidised bed, growth of seeded particles size of CaCO 3 and operating parameters of AGMD. In laboratory scale, the technical feasibility has been shown. More investigation is needed to prove the efficiency and availability of coupled AGMD with FBC.
Two separation processes, pervaporation (PV) and vacuum membrane distillation (VMD), were studied using polyvinylidene fluoride (PVDF) flat-sheet membranes for the separation of chloroform–water mixtures. Both PV and VMD membranes were prepared using the phase-inversion method and the same polymer material. VMD membranes with different pore sizes were prepared using pure water as a pore-forming additive in the PVDF/dimethylacetamide casting solution, whereas PV membranes were obtained with higher polymer concentration, without nonsolvent additives and with solvent evaporation before gelation. The mean pore size, porosity, and pore size distributions of the VMD membranes were determined. Water and formamide advancing and receding contact angles of PV membranes were measured. The swelling degree, the solubility parameter of PV membranes, and the interaction of the permeants with the PVDF polymer were calculated. In the VMD process, a more general theoretical model that considers the pore size distribution, the solution–diffusion contribution through nonporous membrane portion, and the gas transport mechanisms through membrane pores was developed based on the kinetic theory of gases. The contribution of each mechanism was analyzed. A comparative study was made between both membrane separation technologies. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1697–1712, 2004
The flat-sheet membranes from poly(vinylidene fluoride) were prepared by the phase inversion process. The effects of the casting solution composition, exposure time prior to coagulation and temperature of the coagulation bath on properties of prepared membranes were investigated. LiCl was used as a modifying agent. The membrane structure was studied by scanning electron microscopy. For all prepared membranes an asymmetric structure, sometimes without a dense skin layer, was observed. The porosity of prepared membranes before the drying process varied from 72–88%. After drying the membranes become hydrophobic. A contact angle of water droplet on the membrane surface was 107°. The nitrogen permeability varied from 12-2,205 m3/m2 d, depending on the preparation conditions. The maximum pore size, LEPW and mechanical properties were also determined. The membrane distillation process of 1–2% aqueous NaCl solution was applied as a final test of membrane performance. The permeate flux up to 233 dm3/m2 d was achieved at the temperature of the feed and permeate of 333 K and 293 K, respectively. A chloride elimination in the permeate higher than 99% was reached.
A new type of composite hydrophobic/hydrophilic porous membrane is proposed for application in membrane distillation. The membranes were prepared by the phase inversion technique in a single casting step from polymer solutions containing surface-modifying macromolecules. Polyetherimide was used as a hydrophilic base polymer. The effects of polyetherimide concentration in the casting solution on the permeate flux and on the membrane characteristics were investigated. The liquid entry pressure of water, mean pore size, effective porosity and mean roughness of the membranes were determined. Direct contact membrane distillation experiments were performed for pure water and aqueous solutions of sodium chloride as feed. The effect of the mean temperature, stirring rate and salt concentration were studied. Similar experiments were carried out for two commercial polytetrafluoroethylene membranes of different pore sizes, often used for membrane distillation purposes. The water vapor permeability of the proposed membranes was determined and compared with that of the commercial membranes. The new membranes seem to be promising in the field of membrane distillation as they combine a low resistance to mass flux achieved by the diminution of the water vapor transport path length through the hydrophobic thin top-layer of the membrane and a low conductive heat loss through the membrane obtained by using thicker hydrophilic sub-layer of the membrane.
Membrane distillation (MD) is an emerging technology for separations that are traditionally accomplished by conventional separation processes such as distillation or reverse osmosis. Since its appearance in the late of the 1960s and its development in the early of 1980s with the growth of membrane engineering, MD claims to be a cost effective separation process that can utilize low-grade waste and/or alternative energy sources such as solar and geothermal energy. As an attractive separation process, MD has been the subject of worldwide academic studies by many experimentalist and theoreticians. Unfortunately from the commercial stand point, MD has gained only little acceptance and yet to be implemented in industry. The major barriers include MD membrane and module design, membrane pore wetting, low permeate flow rate and flux decay as well as uncertain energetic and economic costs. This study is an attempt to establish a framework for better understanding the MD process and to consider all possible solutions developed so far to overcome its barriers. Unlike the usual trend pursued in review papers, MD studies have been cited in the present manuscript and classified in tables according to their most important contribution in MD development. These tables cover most important aspects of the MD process and are presented in a simple manner for a glance understanding the effects of different factors and operating variables on the productivity of each MD configuration. Among the different MD papers, those involving theoretical models are pointed out. The areas within the MD field that are either usually or rarely studied are highlighted. Some useful technical discussions based on acquired knowledge from experience and information gathered from MD literature are included. In some way, this paper will help new researchers in the field of MD to quickly be updated avoiding repetition of already known studies. In fact, although the effects of some operating parameters are generally agreed upon, still new researches appear with almost the same results.
Preparation of polyvinylidene fluoride (PVDF) asymmetric hollow fiber membranes was studied by introducing small molecular additives, which include nonsolvents (water, ethanol and 1-propanol) and inorganic salt (LiCl). Dimethylacetamide (DMAC) was used as a solvent. Water was used as an external coagulant, while water, ethanol or a mixture of water and ethanol was used as an internal coagulant. The prepared PVDF hollow fiber membranes were characterized in terms of water flux, and molecular weight cut-off for the wet membranes. Average pore size and effective surface porosity were determined using the gas permeation method for the dried membranes. The cross-sectional structure of the hollow fibers was examined by scanning electron microscopy. The collapsing pressure and wetting pressure of the dried membranes were also tested. The effect of polymer concentration, non-solvents, the mixture of non-solvent and LiCl, internal coagulant and post-treatment was studied in details. The PVDF hollow fibers prepared from the small molecular additives exhibit good mechanical strength and excellent hydophobicity. The PVDF hollow fiber spun from the non-solvent alone exhibits a quite low permeability, while good PVDF porous hollow fiber membranes were prepared using a mixture of the water/LiCl, or 1-propanol/LiCl as the additive.
The concept of mass transfer regions within the membranes was introduced to study the mass transport in membrane distillation processes. Mass transfer model for direct contact membrane distillation (DCMD) was derived to examine the influence of pore size distribution and air fluxes on water vapor fluxes across the membranes. The pore size distributions of the membranes were determined by field emission scanning electron microscopy (FESEM) and the image analysis program. DCMD experiments with pure water were carried out under laminar and turbulent flow conditions so as to compare the experimental results with the predictions.The calculation results showed that Knudsen and transition regions were found in the membranes studied, while the transition region was the major contribution to mass transport. The model including the effect of pore size distribution and air fluxes predicted water fluxes with the average discrepancy 5% of the experimental results. The mass transfer analysis indicated that the influence of pore size distribution and air fluxes on water fluxes was insignificant. Therefore, the mass transfer model with the assumptions of air trapped in membrane pores and single pore size is adequate to describe mass transport in DCMD. The concept of mass transfer regions was also applied to analyze the effect of pore size distribution on flux in vacuum membrane distillation and gas permeation.
Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm3/m2 s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO3 deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.
Direct contact membrane distillation (DCMD) process was chosen to produce a highly concentrated apple juice using hollow fiber modules. A high 64°Brix concentration was achieved. The trans-membrane driving force decreased with increasing extra-fiber temperature but increased with higher feed and distillate flow rates in the intra- and extra-fiber volumes, respectively. Flux inversion was sensitive to small differences in temperature between the intra- and extra-fiber volumes and could be prevented by increasing the intra-fiber feed temperature by 2–4°C. Flux rates were dependent upon the temperature polarisation coefficient (TPC) and the effect of the concentration polarisation coefficient (CPC) was negligible. Trans-membrane flux was also significantly increased by thermal-osmotic distillation (TOD) using a high concentration of CaCl2 as the permeate solution.A new model describing the fluid dynamics and membrane behaviour within the DCMD system is presented. In particular, the influence of various properties of membrane morphology, such as the distribution of pores of different diameters and elastic and other mechanical properties upon flux were taken into account in this model.
The improvement of an innovative dual membrane contactor process for treatment of combined hygiene and metabolic wastewater was investigated. Flux and solute rejection in the combined direct osmosis/osmotic distillation (DO/OD) process were enhanced by incorporating membrane distillation (MD) concepts into the process. Two new configurations were investigated: DO/MD, in which the driving force was temperature gradient only, and DO/membrane osmotic distillation (DO/MOD) in which the driving forces were temperature gradient and concentration gradient. Development of a temperature gradient across the membranes substantially enhances the flux of the dual membrane process. It was demonstrated that water flux could be increased by up to 25 times with only a 3–5 °C temperature difference across the membranes. Solutes in the feed wastewater, including urea, were completely rejected. It was demonstrated that complex wastewaters that cannot be treated by one process only could be well treated using a dual membrane process.
The effects of dope flow rate and flow angle within a spinneret during spinning hollow fiber membranes on the morphology, water permeability and separation performance of poly(ethersulfone) ultrafiltration hollow fiber membranes were investigated. For this purpose, two spinnerets with different flow angles were designed and used. The dope solution, containing polyethersulphone (PES)/N-methyl-2-pyrrolidone (NMP)/diethylene glycol (DG) with a weight ratio of 23/41/36, which was very close to its cloud point (binodal line), was used in order to speed up the coagulation of nascent fibers so that the relaxation effect on molecular orientation was reduced. The wet-spinning process was purposely chosen to fabricate the hollow fibers without extra drawing. Therefore, the effects of gravity and elongation stress on fiber formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibers. Experimental results suggest that higher dope flow rates (shear rates) in the spinneret produce UF hollow fiber membranes with smaller pore sizes and denser skin layers due to the enhanced molecular orientation. Hence, the pore size and the water permeability decrease, but the solute separation increases. Hollow fibers spun from a conical spinneret have smaller mean pore sizes with larger geometric standard deviations, thus exhibiting lower water flux and greater solute separation than hollow fibers spun from a traditional straight spinneret. In addition, SEM studies indicate macrovoids response differently for the 90° straight and 60° conical spinnerets when increasing the dope flow rate. Macrovoids can be significantly suppressed and almost disappear in the 90° spinneret at high dope flow rates. This phenomenon cannot be observed for the 60° conic spinneret.
This work aims to provide detailed understanding of heat transport in direct contact membrane distillation (DCMD). The influence of mass transfer on heat transfer rates and on the heat transfer coefficient was identified, and the relative significance of each heat transfer mechanism was evaluated. The role of spacers in heat transfer improvement was analyzed. Alternative methods to evaluate the membrane thermal conductivity were also proposed.The heat transfer analysis of the experimental results showed that the effects of mass transfer on the heat transfer rates and on the film heat transfer coefficients were negligible. The heat transfer due to the vapor flow (qv) in the membrane was equal to or greater than the heat conduction (qc) for the membranes studied and increased with the feed temperature. When the feed temperature was lower than 323 K, the heat loss due to heat conduction across the membrane was the major contribution of the total heat transfer in the membrane. In addition, the temperature distributions in the membranes were closely linear. The membrane distillation (MD) coefficients for each membrane were constant over the flow rates and temperatures studied. The flow pattern in the spacer-filled channel was probably transition flow rather than turbulent flow. The alternative models for calculating the membrane thermal conductivity showed better agreement than the commonly used model.
Membrane distillation (review) Novel membrane and device for direct contact membrane distillation-based desalination process Characterization of three hydrophobic porous membranes used in membrane distillation
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