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Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation
Abstract
The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m2 h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 °C and the cold distillate water at 20.0 °C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.
... Membrane distillation (MD) has been studied over the years for water desalination using various hydrophobic polymers such as polypropylene (PP) [1][2][3][4][5], polyvinylidene fluoride (PVDF) [6][7][8][9][10], and polytetrafluoroethylene (PTFE) [11][12][13][14][15]. Amongst hydrophobic polymers, PVDF has received tremendous attention from researchers due to its excellent chemical, mechanical and thermal properties [16,17]. Additionally, PVDF membranes possess high porosity and are scalable [18][19][20]. ...
... The fabricated PVDF membranes obtained from the above process are chemically modified by a single-step dip-coating approach. The PVDF membranes were dip-coated in fluorine group (F21) containing silane (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyltrichlorosilane) solution (prepared in hexane) for multiple hours. Finally, the PVDF membranes were removed from the silane solution and dried in air for 2 h. ...
... The fabricated PVDF membranes obtained from the above process are chemically modified by a single-step dip-coating approach. The PVDF membranes were dip-coated in fluorine group (F21) containing silane (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyltrichlorosilane) solution (prepared in hexane) for multiple hours. Finally, the PVDF membranes were removed from the silane solution and dried in air for 2 h. ...
Here, a facile and novel route to fabricate superhydrophobic poly (vinlylidene fluoride) (PVDF) polymeric membranes for the desalination of highly saline water is described. The superhydrophobic PVDF membrane (with a high-water contact angle >150°) was fabricated by phase inversion process on a smooth glass surface followed by surface chemical modification with long fluoroalkylsilane chains (F21) via a single dip-coating approach. The superhydrophobicity of the membrane surface after F21 modification was found to be positively correlated with the concentration of the fluorine groups and the dip-coating duration. The performance of these membranes for the treatment of highly saline water was also evaluated using an air gap membrane distillation (AGMD) system. The chemically modified PVDF membrane exhibited excellent and stable salt rejection of over 99.99% (~ 100%) and high mean water permeate flux (19.09 L/m².h) when using synthetic 7 wt.% NaCl feed solution. The modified PVDF membrane showed significant improvement in the mean water flux (>60%) compared to the pristine PVDF membrane due to variations in the membrane’s chemical and bulk structure in the dip coating process. The results suggested that the nature and amount of the endowed fluoroalkylsilane chains on the membrane surface are key design parameters in membrane fabrication for stable and excellent-performing membranes for desalinating highly saline water.
... Figure 5 shows the cross-sectional view of fabricated modified PVDF membranes. As can be clearly seen, the membrane morphology had two layers of finger-like structures, which is a typical structure for PVDF by incorporating PEG additive [5,20]. The LSMM that has been mixed during the PVDF/PEG/LSMM dope solution preparation [21] is successfully dispersed, shown by Figure 5(2b), which exhibits the presence of white particles on the pore surface. ...
... Morphological Structure Figure 5 shows the cross-sectional view of fabricated modified PVDF membranes. As can be clearly seen, the membrane morphology had two layers of finger-like structures, which is a typical structure for PVDF by incorporating PEG additive [5,20]. The LSMM that has been mixed during the PVDF/PEG/LSMM dope solution preparation [21] is successfully dispersed, shown by Figure 5(2b), which exhibits the presence of white particles on the pore surface. ...
... The test resulted in 161 °C of melting point for PVDF/PEG/3LSMM membrane. It was such an improvement of a previous study by Hou et al., which exhibited the melting point of 150 °C for PVDF/PEG membrane [5]. The use Therefore, ideal porosity is considered to meet MD specifications, which is 85% at the maximum. ...
Hollow fiber membranes of polyvinylidene fluoride (PVDF) were prepared by incorporating varying concentrations of hydrophilic surface-modifying macromolecules (LSMM) and a constant amount of polyethylene glycol (PEG) additives. The membranes were fabricated by the dry-wet spinning technique. The prepared hollow fiber membranes were dip-coated by hydrophobic surface-modifying macromolecules (BSMM) as the final step fabrication. The additives combination is aimed to produce hollow fiber membranes with high flux permeation and high salt rejection in the matter of seawater desalination application. This study prepares hollow fiber membranes from the formulation of 18 wt. % of PVDF mixed with 5 wt. % of PEG and 3, 4, and 5 wt. % of LSMM. The membranes are then dip-coated with 1 wt. % of BSMM. The effect of LSMM loading on hydrophobicity, morphology, average pore size, surface porosity, and membrane performance is investigated. Coating modification on LSMM membranes showed an increase in contact angle up to 57% of pure, unmodified PVDF/PEG membranes, which made the fabricated membranes at least passable when hydrophobicity was considered as one main characteristic. Furthermore, The PVDF/PEG/4LSMM-BSMM membrane exhibits 161 °C of melting point as characterized by the DSC. This value indicates an improvement of thermal behavior shows so as the fabricated membranes are desirable for membrane distillation operation conditions range. Based on the results, it can be concluded that PVDF/PEG membranes with the use of LSMM and BSMM combination could enhance the permeate flux up to 81.32 kg·m−2·h−1 at the maximum, with stable salt rejection around 99.9%, and these are found to be potential for seawater desalination application.
... Since the membranes for MD are highly hydrophobic, fouling and scaling of the MD membranes are not as serious as those for hydrophilic membranes. Therefore, pretreatment of the feeds with high concentration is not as strict as RO and also the pretreatment costs are low [2,8]. ...
... PVDF is soluble at room temperature and in many solvents such as DMF, DMAC, and TEP. Phase inversion is a method of PVDF membrane fabrication [8,76,100,137]. Tables 4.6 and 4.7 show the list of commercial flat sheet and capillary membranes that have been frequently used in the MD processes, including the name of manufacturers and their important characteristics. ...
... 8 Permeate flux of different types of fabricated and modified flat sheet membranes for MD applications[72,102,[140][141][142][143][144] ...
Membrane distillation (MD) is a nonisothermal membrane operation that has
received much attention in recent years, especially for salty water desalination. In the MD process, only vapor molecules are allowed to pass through the
hydrophobic membrane pores. The pressure difference resulting from the tempera-
ture difference is applied between the surfaces of the hydrophobic membrane, so the
separation takes place.
... Lithium chloride addition makes the finger-like macrovoids beneath the outer and inner layers of the fiber become smaller and longer and provided more sponge like structure. LiCl is thought to benefit the solid-liquid demixing process and suppress macrovoid formation, which may be related to the increase in the spinning dope viscosity (5800 cp) caused by the interaction of LiCl with solvent, as well as, with electron donor group of PVDF to delay the dope precipitation [25][26][27][28]. It should be noted that the finger/sponge like structure resulted from the addition of LiCl comprise a trade-off choice between permeability and wetting resistance [29]. ...
... The pure water flux experiments were conducted for PV and PVL at 70 o C under VMD mode and the flux values were 37.6 and 48 L/m 2 h for PV and PVL, respectively as presented in Table 4. This may be attributed to the higher porosity obtained after the addition of LiCl, which agrees with results mentioned in the previous sections, where the addition of pore former additive in the polymer dopes generally results in the enhancement of liquid-liquid demixing which enhances the membrane porosity and, consequently, the VMD flux [4,26,28,29,35]. It is worth mentioning that these values are higher than those previously published under comparable VMD conditions for PVDF HF membranes [4,30,31,36,37], but lower than those published by Drioli et al. who used PVP and maleic acid as additives [21]. ...
... 15 PVDF membranes are a promising candidate for DCMD because of the low cost, good chemical stability, and available large-scale fabrication. 16,17 A lot of studies demonstrated that adding filler materials into the polymer matrix could modify the membrane characteristics, such as pore size, porosity, and surface hydrophobicity, so that to improve the DCMD performance. [16][17][18][19][20][21][22][23] Carbon nanotubes (CNTs) have been extensively studied as a filler material for PVDF membranes because CNTs possess high aspect ratio, high thermal and mechanical stabilities, and light weight. ...
... 16,17 A lot of studies demonstrated that adding filler materials into the polymer matrix could modify the membrane characteristics, such as pore size, porosity, and surface hydrophobicity, so that to improve the DCMD performance. [16][17][18][19][20][21][22][23] Carbon nanotubes (CNTs) have been extensively studied as a filler material for PVDF membranes because CNTs possess high aspect ratio, high thermal and mechanical stabilities, and light weight. 7,19,[24][25][26] It has also been reported that surface functionalization, such as silanization and fluorosilanization, can further improve the dispersity of CNTs. ...
Direct contact membrane distillation (DCMD) is a promising method for water purification and it is essential to fabricate membranes with high porosity, narrow pore distribution and high surface hydrophobicity. In this study, carbon-nanotubes (CNTs) with two surface functionalization, i.e., silanization and fluorosilanization, were compounded with polyvinylidene fluoride (PVDF) to prepare electrospun composite nanofiber membranes for DCMD. The effects of CNT surface functionalization on CNT dispersity and membrane pore size, porosity, hydrophobicity, and DCMD performance were investigated. The results showed that CNTs improved the membrane hydrophobicity and surface functionalization of CNTs furthered this improvement. It was found that membranes with co-functionalized CNTs achieved the highest distillate flux (~ 45 Lm-2h-1) and salt rejection (~ 99.99 %) among the tested membranes. This flux was 14 % and 46 % higher than the fluxes on the membranes with unfunctionalized-CNTs and without CNTs, respectively. Characterizations suggested that the superior performance was attributed to the co-functionalization improved dispersity of the CNTs, high porosity (~ 85.5 %), large mean pore size (~ 0.89 μm) and surface superhydrophobicity (contact angle of ~153 o) of the membrane. This work implied that co-functionalization of CNTs is an avenue of great potential to improve the performance of PVDF membranes in DCMD application.
... Due to its semi-crystalline structure, PVDF has excellent thermal, chemical and mechanical properties [4][5][6]. Because of these good properties, PVDF is widely used for various applications such as microfiltration [7], ultrafiltration [8], pervaporation [9], membrane distillation [10,11] and gas-liquid membrane contactors [12]. ...
Phase inversion is used to prepare poly(vinylidene fluoride) (PVDF) microfiltration membranes by combining it with the hydrophilic polymer additive cellulose acetate (CA). For the filtration of methylene blue (MB) and acid yellow 17 (AY17), the effects of coagulant bath temperature (CBT) on the separation performance and antifouling properties are thoroughly investigated. SEM analysis shows that the membrane at higher CBT has a larger pore diameter than at lower CBT, resulting in differences in membrane surface hydrophilicity. It is found that the increase in surface hydrophilicity causes the permeability of PVDF/CA membranes to be higher at higher CBT than at lower CBT. Rejection values above 90% indicate that the membranes are more effective for MB separation at both lower and higher CBT. Otherwise, lower CBT provides better AC rejection than higher CBT. The Flux Recovery Ratio, which is higher at higher CBT, remains in the 75–95% range at lower CBT. As a result, lower CBT is better for membrane fouling resistance than higher CBT. In addition, the fouling observed at lower CBT is similar to the fouling observed at higher CBT, but lower CBT has a higher percentage of reversible fouling than higher CBT. The membranes with more reversible fouling are therefore easier to clean using the backwashing process. As a result, PVDF/CA membranes manufactured at lower CBT have better separation performance and antifouling characteristics than those manufactured at higher CBT.
... Hydrophobicity of MD membrane is controlled by embedding inorganic additive into membrane. For PVDF hollow-fiber MD membrane, Hou et al. (2009) observed that the contact angle of the membrane increased to 118.5 • from 85.8 • after embedding polyethylene glycol (MW = 1500, PEG-1500) into membrane. However, the membrane embedded with PEG-1500 was not suitable for MD application because it increased membrane pore size. ...
... The increased membrane porosity allows more moisture to penetrate through the membrane pores. As shown in Fig. 6, the permeate flux through the membrane increased with the increasing temperature and membrane porosity [37]. The permeate flux of the membrane with the addition of PVP from 2 % to 5 % increased from 8.193 to 8.672 kg/m 2 h at 80 • C. The distillate quality was continuously monitored and was consistently below 25 PPM, which is much less than the limit recommended by the WHO (300 PPM). ...
... To overcome these issues, various modifications of PVDF membranes were performed for improving the hydrophilicity, membrane antifouling properties, and filtration performance. Several modifications had been reported, such as blending with polymers [19][20][21], combining with inorganic fillers to form mixed matrix membranes [22][23][24][25], and copolymerization of PVDF [26] [27,28]. The addition of hydrophilic materials into PVDF membranes could improve the membrane performance and antifouling properties. ...
Trade-off issue and membrane fouling remain two major issues in the utilization of membrane technology for the water treatment due to reduced membrane permeability and lifetime. In our study, we employed 3-aminopropyltriethoxysilane modified bentonite (BNTAPS) as an anti-fouling modifier to prepare polyvinylidene fluoride (PVDF)-based membranes via the phase inversion method. The effects of BNTAPS concentration on the physical, mechanical, morphological, and filtration performance of the hybrid membranes have been investigated. It was found that the addition of BNTAPS improved the hydrophilicity of the membrane revealed by the decreased water contact angle. Consequently, the pure water flux of PVDF membrane containing 0.5% BNTAPS (PVDF/BNTAPS0.5%) increased to 35.5 L m⁻² h⁻¹. Moreover, the PVDF/BNTAPS membrane showed a smaller pore diameter and porosity compared to pristine PVDF. The membrane performance evaluation was carried out using cationic and anionic dyes, i.e., methylene blue (MB) and acid yellow (AY17), respectively. Our study revealed that the rejection of each dye was slightly increased for the PVDF/BNTAPS0.5%. However, the flux recovery rate of the PVDF/BNTAPS membrane significantly improved, which directly prolonged the membrane lifetime.
... Some functional materials can also be added into the membrane to increase removal efficiency of pollutants (Chen et al., 2020;Huo et al., 2013;Zhang et al., 2022). The anti-wettability of membrane can be achieved by increasing surface J o u r n a l P r e -p r o o f Journal Pre-proof hydrophobicity via forming a rough surface and/or grafting or plasma modification of the surface materials (Fresnais et al., 2006;Hou et al., 2009;Huang et al., 2005;Kim et al., 2012;Meng et al., 2014;Prince et al., 2014a). The highly hydrophobic layer can reduce the affinity of organics/surfactants to the membrane surface due to its low surface energy, and delay the infiltration of hydrophilic substances (Dong et al., 2014). ...
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.
... Importantly, the MD process is not limited by osmotic pressure (because of its thermally-driven nature) and unlike the RO process, this characteristic of MD substantially augmented the water recovery. Temperature of operation in MD would be between 45 and 85°C and increasing the temperature leading to improve the performance of system ( Koo et al., 2015 ;Zare and Kargari, 2018 ;Shirazi et al., 2014 ;Hou et al., 2009 ). ...
The unprecedented situation of the COVID-19 pandemic heavily polluted water bodies whereas the presence of SARS-CoV-2, even in treated wastewater in every corner of the world is reported. The main aim of the present study is to show the effectiveness and feasibility of some well-known desalination technologies which are reverse osmosis (RO), Electrodialysis (ED), Membrane Distillation (MD), multi effect distillation (MED), and multi stage flashing (MSF) during the COVID-19 pandemic. Systems’ effectiveness against the novel coronavirus based on three parameters of nasopharynx/nasal saline-irrigation, temperature of operation and pretreatment methods are evaluated. First, based on previous clinical studies, it showed that using saline solution (hypertonic saline >0.9% concentration) for gargling/irrigating of nasal/nasopharynx/throat results in reducing and replication of the viral in patients, subsequently the feed water of desalination plants which has concentration higher than 3.5% (35000ppm) is preventive against the SARS-CoV-2 virus. Second, the temperature operation of thermally-driven desalination; MSF and MED (70-120°C) and MD (55-85°C) is high enough to inhibit the contamination of plant structure and viral survival in feed water. The third factor is utilizing various pretreatment process such as chlorination, filtration, thermal/precipitation softening, ultrafiltration (mostly for RO, but also for MD, MED and MSF), which are powerful treatment methods against biologically-contaminated feed water particularly the SARS-CoV-2. Eventually, it can be concluded that large-scale desalination plants during COVID-19 and similar situation are completely reliable for providing safe drinking water.
... 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]. ...
... Membrane distillation can recover heat energy and utilise alternative low-cost heat sources, such as heat lost from other processes and renewable energy sources (solar or geothermal energy), reduce operating costs, and provide environmentally friendly separation processes [6]. In addition, the fouling on the MD membrane surface is not severe compared to hydrophilic membranes due to the hydrophobicity of MD [7]. ...
... Membrane distillation can recover heat energy and utilise alternative low-cost heat sources, such as heat lost from other processes and renewable energy sources (solar or geothermal energy), reduce operating costs, and provide environmentally friendly separation processes [6]. In addition, the fouling on the MD membrane surface is not severe compared to hydrophilic membranes due to the hydrophobicity of MD [7]. ...
Ceramic membranes have received more attention currently from researchers in membrane distillation (MD) applications due to their outstanding properties. However, despite their superior mechanical, thermochemical stability, and resistance to harsh operating conditions, several bottlenecks still limit their applications in MD. Although there are several published articles on ceramic membranes in MD, the uniqueness of this review lies in the fact that it discusses the critical bottlenecks that significantly affect the performance of ceramic membranes in long-term operation and limit their scale-up to commercial MD applications. Furthermore, recent advances, strategies, and techniques to mitigate these limitations have also been discussed. A discussion on high ceramic membrane fabrication costs and mitigation strategies using alternative low-cost ceramic materials to erstwhile conventional ceramic materials has been presented. In addition, the inherent problems of the brittleness and wetting/fouling of ceramic membranes and recent advances in strengthening ceramic membranes and fouling/wetting control via the development of superhydrophobic and omniphobic ceramic surfaces in MD have been addressed. Consequently, technical bottlenecks that still exist despite recent development in ceramic membranes for MD applications have been highlighted and future research direction in developing robust ceramic membranes in MD applications has been elaborated.
... The study was carried out at different feed temperature and Reynolds number. D. Hou et al. [10] have studied the PVDF hollow fiber membrane through the direct contact membrane distillation for desalination purpose. The fabricated fibers with a mixture of LiCl and PEG 1500 as non-solvent by phase inversion process. ...
Computational fluid dynamics (CFD) simulations were carried out to study the performance of a single hollow fiber in a direct contact membrane distillation (HF-DCMD) module for a desalination process. The feed solution is seawater with a salt (NaCl) concentration of 35 g/L. The Navier-Stokes, mass and energy transport equations are considered, with a coupled boundary condition imposed by the membrane. The system is investigated to examine sensitivity toward the membrane thickness, pore size, and inner hollow fiber diameter based on existing commercial fibers. Two membrane thicknesses (300 μm, 500 μm), two membrane pores sizes (0.2 μm, 0.45 μm), and two inner diameters (1.2 mm, 1.8 mm) are studied in the laminar regime at fixed operating conditions. The presence of temperature polarization causes a significant drop in the water permeation in the DCMD system due to reduced driving force across the membrane. The characterization of polarization inside the hollow fiber will be the focus of this work. It was found that the vapor flux was most sensitive to the membrane thickness (a 30% flux increase) versus the inner diameter (2% flux increase). The pore size is also a very influential parameter (20% flux increase), moving from optimal to less-optimal properties.
... It could be found the contact angles of the three types of membranes had decreased after 20 days long-term operation, and the decrease range was PVDF-SiO2-HDTMS<PVDF-HDTMS<PVDF-PA-8. According to M. Sadoogh et al.[26] studies, the degradation of PVDF membrane performance may be due to the chemical degradation of the interface between PVDF and MEA, which leaded to the dehydrogenation and fluorination of the PVDF surface. Therefore, the reason for the lower drop of the contact angle of PVDF-SiO2-HDTMS was the surface of PVDF-SiO2-HDTMS membrane grafted with a large amount of superhydrophobic SiO2 with high hydrophobicity and chemical stability, which effectively prevented the PVDF substrate from being in the MEA solution. ...
To obtain a long-term stable operation of the hollow fiber membrane for using in membrane contact absorption of carbon dioxide (CO 2 ), hybrid polyvinylidene fluoride-silica-hexadecyltrimethoxysilane (PVDF-SiO 2 -HDTMS) membrane were fabricated via the non-solvent induced phase-inversion method. The surface properties, performance characteristics and long-term stable operation performance of the prepared membranes were compared and analyzed. The results show the outer surface of the prepared membranes exhibited superhydrophobicity because of the formation of rough nano-scale microstructures and the low surface free energy. Due to the addition of inorganic nanoparticles, the mechanical strength of PVDF-SiO 2 -HDTMS membranes were improved. The long-term stable operation experiments were carried out with the inlet gas (CO 2 /N 2 = 19/81, v/v) at a flow rate of 20 mL/min and the absorbent liquid (1 mol/L DEA) at a flow rate of 50 mL/min. And the result showed that the mass transfer flux of PVDF-SiO 2 -HDTMS membrane decreased from the initial value of 2.39×10 ⁻³ mol/m2s to 2.31×10 ⁻³ mol/m ² s, which was a decrease of 3% after 20 days. The main benefit is the addition of inorganic nanoparticles, which have strong chemical resistance and high hydrophobicity, thereby preventing structural damage and pore wetting of the membrane. PVDF-SiO 2 -HDTMS membrane exhibits excellent long-term stable operation performance of CO 2 .
... Also, it has been shown that nanofiber membranes are more durable than commercial hydrophobic membranes in terms of wetting resistance in longterm MD operation [7,8]. In this nanofiber membrane fabrication, many materials such as polyvinylidenefluoride (PVDF), polytetrafluoroethylene (PTFE), polypropylene (PP), polysulfone (PS), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and polyethersulfone (PES) are used [9][10][11][12]. Additives such as silica nanoparticles, graphene oxide (GO), reduced graphene oxide and titanium dioxide (TiO 2 ) are also used to improve the material properties of these materials to obtain better nanofibers [13][14][15]. ...
In this work, fabrication of PVDF nanofiber membrane with reduced graphene oxide using electrospinning technique has been carried out to improve AGMD performance. Membrane morphology and different membrane characteristics such as contact angle, thickness, liquid entry pressure (LEP), young module, scanning electron microscopy (SEM) and surface roughness of membrane were analyzed for the characterization of hydrophobic nanofiber membranes. Firstly, rGO/PVDF membranes with different rGO concentrations were produced using the electrospinning method. All fabricated rGO/PVDF membranes were characterized and used in the AGMD system for the treatment of saline water. Secondly, the selected optimum membrane according to characterization and filtration results was used in the AGMD system for the treatment of synthetic and real geothermal water. Results showed that all rGO membranes exhibit higher permeate water flux and salt rejection, lower permeate boron or salt concentration than pure PVDF membrane. In real geothermal water experiments, rGO/PVDF membrane (0.039 wt.%) were improved permeate water flux from 19.20 to 30 L/m².h, boron rejection from 96.89 % to 98.16 %. Also, permeate boron concentration was decreased using rGO/PVDF membrane (0.039 wt.%) from 0.305 mg/L to 0.226 mg/L. As a result, fabricated rGO/PVDF membranes were achieved to the enhanced performance of the AGMD system.
... Even though PVDF is highly resistant to chemicals but still it is incompatible with strong alkalis, esters, and many ketones [21]. On being exposed to strong alkaline solutions, PVDF properties begin to weaken and decline gradually reported in literature [22,23]. The PVDF membrane interaction with alkaline solution becomes inevitable when it comes to practical applications since NaOH solution is mostly used as an absorbent materials and cleaning product in gas absorption system or membrane cleaning system. ...
Polyvinylidene fluoride (PVDF) is known as a favorite polymer from the family of fluoropolymers due to its excellent piezoelectric properties, thermal stability, and mechanical strength. It has a good processability, and it also possess chemical resistance property to different materials such as different acids, bases, organic solvents, oil, and fat. The present study reveals an overview of the recent growth and development in the application perspective and modification of PVDF membranes, majorly focus on sensors and actuators, spin-valve devices, magnetoelectric materials, energy harvesting applications, tissue engineering, modeling engineering, and other biomedical engineering and devices. From an electronic point of view, ferroelectric polymers are insulating, polar, and possess a non-conjugated backbone; therefore, they are termed as highly insulating materials. The insulating polymers are most attractive for the study of charge transportation and storage. Such polymers also yield the best electrets for practical application because of their insulating properties and high concentration of deep trapping sites. Thus, the objective of this review is to present the physical, chemical, thermal, and mechanical properties of PVDF. This article also provides an intelligent direction in the progress of PVDF in different fields of science and technology.
Graphical abstract
... 1. PEG-1500 added to PVDF hollow fiber membranes produced a membrane with a contact angle of 118.5° [99]. 2. PTFE microparticles and fluorinated silica particles were added to PVDF hollow fiber membranes and led to superhydrophobic membrane surface with contact angles ranging from 130°to 147° [100,101]. ...
Membrane distillation (MD) is a novel desalination technology that has potential to produce distilled quality water from high salinity brine streams. The driving force for MD is the vapor pressure difference across a hydrophobic membrane resulting in transfer of water vapor from hot to cold side. This vapor contacts a cold surface and condenses to produce distillate. This paper reviews recent and/or multi-year research programs that focused on MD pilot or field testing. The various investigations concluded that while MD can produce distilled water quality, the energy efficiency remains the key bottleneck for future deployment of MD. Membrane wetting and fouling also presents key challenges for desalination due to both the high salinity and the presence of organics in the feed water. The authors contacted several MD vendors requesting updates on their latest products and technology developments. MD vendors with innovative module designs, some of which promise a step change in performance, have recently emerged on the market. In addition to water desalination, MD has a wide range of industrial applications such as hydrogen sulfide removal, the treatment of wastewater from the pharmaceutical, metal finishing industries, direct sewer mining, oily wastewater, and water recovery from flue gas. This paper also reviews novel membrane chemistries with emphasis on membranes prepared by phase inversion and electrospinning techniques to which nanomaterials have been added. The primary objectives in adding various nanomaterials (e.g., carbon nanotubes, graphene, silicon dioxide, fluorinated compounds) are to increase hydrophobicity (to reduce wetting) and increase mass transfer rates (to increase flux and lower cost).
... TiO 2 , Ag, zeolite), inorganic salts (e.g. LiCl and LiNO 3 ), water and mix add of LiCl/PEG can provide high porosity and good hydrophobicity of the fibres [71,72]. Polyvinyl pyrrolidone (PVP) is used to adjust the pore size, produce hydrophilic membrane, increase membrane permeability and prevent fouling. ...
In this article, the most advanced applications and the major operating problems of hollow fibre direct contact membrane distillation (HF-DCMD) are discussed. Over the past few years, direct contact membrane distillation (DCMD) became one of the most preferred technology not only for high-saline desalination process, but also for the water treatment process. The fundamentals of the DCMD and governing equations (from heat and mass transfer principles) that describe performance parameters are first explained. To increase the desalination performance of DCMD technology, some of the DCMD models were upgraded with the hollow fibre module. The efficiency of the HF-DCMD is explained further in this review. Major problems in the HF-DCMD technologies especially on membrane fouling and operating conditions are also discussed in this article. However, it is expected that HF-DCMD offers promising prospects in the future and potential outlook for better performance.
... This allows changing the membrane morphology, inducing pores in the membrane matrix, improving pore interconnections and, as a result, increasing the performance of the processes. Additionally, increases the viscosity of the polymer solutions, and changes both the kinetic and the thermodynamic properties of the coagulation process affecting the final membrane structure [125,126]. Different types of additives have been utilized for preparation of membranes for MCs. For example, inorganic salts such as lithium chloride and lithium perchlorate, polymeric additives such as polyvinylpyrrolidone, poly(ethylene glycol), or any other type of nonsolvent additives capable of increasing both the pore size and the porosity of the prepared membranes, have been employed [127,128]. ...
... This allows changing the membrane morphology, inducing pores in the membrane matrix, improving pore interconnections and, as a result, increasing the performance of the processes. Additionally, increases the viscosity of the polymer solutions, and changes both the kinetic and the thermodynamic properties of the coagulation process affecting the final membrane structure [125,126]. Different types of additives have been utilized for preparation of membranes for MCs. For example, inorganic salts such as lithium chloride and lithium perchlorate, polymeric additives such as polyvinylpyrrolidone, poly(ethylene glycol), or any other type of nonsolvent additives capable of increasing both the pore size and the porosity of the prepared membranes, have been employed [127,128]. ...
Water scarcity is one of the major resource crisis worldwide. Only 3% of Earth’s water is fresh and suitable for human consumption. Given the abundance of salty water from seas and oceans, there is a need to purify such water using economical and environmentally friendly processes. Seawater desalination has long been practiced on this issue. The most widely used commercial desalination techniques are reverse osmosis (RO), electrodialysis, and nanofiltration. They are all energy- and capital-intensive. Nanoporous membranes promise to be more efficient than the existing technology and may yield savings in energy consumption during RO operations. Experimental and simulation studies manifest that membranes having a nanosized pore network can operate as molecular sieves, likely letting only the water molecules pass through the cavities while excluding the ions of the solvent. Carbon-based membranes with micropore size distributions have earned a great merit for research on this outlook. This chapter reviews existing models of carbonaceous membrane systems for desalination. These models depict structures of well-ordered and arranged carbon nanotubes or they define porous stackings of graphene sheets and graphene oxides. We discuss some examples of biomimetic models that have been also proposed for water treatment. Desalination process of these systems has been modeled with either molecular dynamics or Monte Carlo methods.
... This allows changing the membrane morphology, inducing pores in the membrane matrix, improving pore interconnections and, as a result, increasing the performance of the processes. Additionally, increases the viscosity of the polymer solutions, and changes both the kinetic and the thermodynamic properties of the coagulation process affecting the final membrane structure [125,126]. Different types of additives have been utilized for preparation of membranes for MCs. For example, inorganic salts such as lithium chloride and lithium perchlorate, polymeric additives such as polyvinylpyrrolidone, poly(ethylene glycol), or any other type of nonsolvent additives capable of increasing both the pore size and the porosity of the prepared membranes, have been employed [127,128]. ...
... For instance, all produced from polymer of PVDF, compared to membranes with flat-sheet (83°, 13.6 L m −2 h -1 , LMH) and hollow fibrous structure (105°, 11.5 LMH), ENMs displayed much higher water contact angle (138°) and flux (21.4, LMH) [17][18][19]. Consequently, the feasibility and properties of ENMs in MD have been widely investigated in recent ten years. ...
Although with great potential, membrane distillation (MD) is still far from satisfaction due to deficiency of ideal membranes with low cost and high performance. In this work, electrospun nanofibrous membranes (ENMs) fabricated using polyethylene terephthalate (PET) derived from recycled Coca Cola plastic bottles were evaluated in direct contact (DC) MD application. Heat-pressing was used to tune physicochemical properties of PET ENMs to get them better qualified for MD. Effect of heat-pressing on membrane structure and properties was emphasized. The preferred PET ENMs with nanofiber diameter of 517 nm exhibited high porosity of 77 %, high hydrophobicity with contact angle of ≥130°, and high liquid entry pressure of water (LEPw) of 49.8 kPa. The optimized PET ENMs displayed permeation of about 11−23 L m⁻² h⁻¹ (LMH) with varied temperature differences and salt rejection of 99.9 %. Stable permeation was noticed in the long-run membrane operation without significant permeation loss. Finally, 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS) was used to modify the surface of membrane to enhance their antiwetting capability against the oil. Surface modification rendered excellent endurance to the membrane even with addition of 0.3 M Sodium Dodecyl Sulfate (SDS, a model surfactant) while the pristine membrane failed to work once the SDS was added. This work paves the way for developing a low-cost and acceptable industrial-grade membrane with great potential in DCMD applications.
... By introducing small molecule pore forming agents including polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), ethylene glycol, alcohols, and water during phase inversion, flux is enhanced and pore size can be maintained [132]. Hou et al. [133] added PEG-1500 as pore forming agent during the fabrication of PVDF hollow fiber using NIPS technique. They found out that hydrophobicity increased from 85.82 to 118.5 and roughness increases. ...
Membrane distillation (MD), a second-generation separation technology has been identified as vital industrial process due to outstanding properties for seawater desalination. In this article, an overview of recent progress in the membrane improvements to enhance MD and desalination performance. MD membranes with better performance perspective can be enhanced by blending and surface modification strategies. In the literatures, the emergence of the nanotechnological techniques has shown to be of greater impact on MD efficiency. Further efforts are needed towards the functionalization of nanomaterials and the development of innovative polymers. In order to achieve sustainability in MD, there is also need of life cycle assessment in MD plants so as to quantify their environmental impact. It is expected that this article will provide a perception on potential outlook for future MD improvement and performance.
... For example, Tomaszewska used DMF and DMAc to cast flat sheet membranes with a permeation flux of about 10 kg/m 2 ·h [17]. Hou et al. employed DMAc to fabricate single-layer hollow fiber membranes with a permeation flux of about 20 kg/m 2 ·h [18]. Wang et al. utilized NMP to produce dual-layer hollow fiber membranes with a permeation flux over 40 kg/m 2 ·h [7]. ...
... During the fabrication of microporous PVDF hollow fiber membranes, two non-solvent additives PEG 400/1500 and LiCl were added to the feed N,Ndimethylacetamide (DMAc) solution containing PVDF powder [43][44][45][46]. The experiment factors such as dope extrusion rate, take-up speed, air-gap concentration of polymer and additives, and bore liquid temperature were investigated in the membrane fabrication through a dry-jet wet phase inversion process, and the obtained membranes were suitable for DCMD than VMD. ...
The East Qinling Molybdenum Belt (EQMB), which is located on the southern margin of the North China Craton (NCC), is the largest Mo province in the world. This belt hosts a significant number of Mesozoic magmatic-hydrothermal Mo deposits and a small portion of pre-Mesozoic Mo deposits. Understanding the mineralization timing and mechanism of the unique pre-Mesozoic Mo deposits is essential to comprehend the evolution of the EQMB, the pre-Mesozoic Mo enrichment, and the Mesozoic Mo miner-alization event. The recently discovered Zhaiwa deposit is a porphyry Mo deposit located in the Xiong'er Ter-rane of the EQMB. In this study, five molybdenite samples from the Mo-bearing quartz veins were analyzed for Re-Os isotopes composition. These samples yield an isochron age of 1794 ± 45 Ma, which represents the age of mineralization. The mineralization is mostly hosted within the biotite-amphibole plagiogneiss and granite porphyry. LA-ICP-MS U-Pb data of zircons constrain the crystallization age of the granite porphyry to be at 1791 ± 16 Ma. The close spatial and temporal association suggests that the granite porphyry is the causative rocks of the Mo mineralization. The δ 34 S values of pyrite vary from 5.3 to 6.8‰, suggesting that the S was mainly derived from magmatic source. The intrusion of magmas and associated Mo mineralization are contemporane-ous to the regional Xiong'er volcanism that occurred during the late Paleoproterozoic. The Xiong'er volcanism was triggered by partial melting of lithospheric mantle in an extensional setting. The results of our study provide robust evidence for a late Paleoproterozoic Mo metallogenic event along the southern margin of the NCC. Future exploration should also consider the potential of late Paleoproterozoic porphyry Mo mineralization existing in the EQMB, which is closely associated with the Xiong'er volcanism.
There is an urgent need for the development of new water resources in order to solve the problem of the world’s growing demand for clean water. Membrane distillation (MD) is a promising alternative to conventional seawater desalination. Although MD itself is often defined as sustainable desalination technology, there are many aspects within the membrane manufacture and process operation that make it far from being green. For instance, non-biodegradable polymers, toxic solvents and fluoroalkyl silanes are typical chemicals that unfortunately are used in membrane fabrication protocols. Additionally, the huge amount of wastewater generated from membrane fabrication processes makes solvent-free methods more attractive and desirable for extensive investigation. Apart from this, the low energy efficiency of the MD process can be effectively overcome by integrating MD systems with low-grade waste heat. This review critically addresses and discusses the recent advances in methods and strategies to improve the sustainability of MD technology, which is not a common scope of study among the research community. Here, our attention has been devoted to the main aspects of MD membrane fabrication, such as polymers, solvents (and their costs), nonsolvents, additives, solvent-free fabrication procedures, fluoro-free post-modification, and MD operation (energy consumption). This review intends to introduce inspiration for membrane scientists for the development of the next-generation MD process, by promoting the sustainable transformation of today’s approaches into a greener way. In this latter scenario, we provide some timely considerations that could be followed by the researchers in the field.
Polytetrafluoroethylene (PTFE) nano-porous membrane has been widely used in various fields due to its excellent thermal stability and chemical stability. However, PTFE nanofiber membranes with simultaneous hydrophobic and oleophobic properties are essential to promote the application of PTFE. Here, based on inorganic/organic nanohybridization, we report a strategy for constructing nanostructures on fibers by adding nanoparticles, such as MOF and ZrO2. The results of FTIR and XRD confirmed MOF was synthesized successfully. At the same time, the SEM results showed UiO-66-(COOH)2 is spherical with an average diameter of 152 nm, and there is no agglomeration, which is suitable for electrospinning. Further, MOF and ZrO2 were payload into PTFE nanofibers. The results of SEM and AFM confirmed nanostructures will be more uniform and pronounced with the increase of UiO-66-(COOH)2 content, and nanostructures are most obvious when the content of UiO-66-(COOH)2 is 15%. The introduced nanostructures can increase the oil contact angle of the PTFE nano-porous membrane to 110° without introducing other groups, and further improve the water contact angle from 133° to 145°. Meanwhile, the introduction of a certain amount of hydrophilia groups can increase the oil contact angle to more than 120°. The simple strategy is of great significance to expand the application of PTFE fiber membrane in dealing with waste water treatment fields.
Poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) is a highly versatile polymer used for water remediation due to its chemical robustness and processability. By incorporating metal‐organic frameworks (MOFs) into PVDF‐HFP membranes, the material can gain metal‐adsorption properties. It is well known that the effectiveness of these composites removing heavy metals depends on the MOF's chemical encoding and the extent of encapsulation within the polymer. In this study, it is examined how the micro to nanoscale structure of PVDF‐HFP@MOF membranes influences their adsorption performance for Cr VI . To this end, the micro‐ and nanostructure of PVDF‐HFP@MOF membranes are thoroughly studied by a set of complementary techniques. In particular, small‐angle X‐ray and neutron scattering allow to precisely describe the nanostructure of the polymer‐MOF complex systems, while scanning microscopy and mercury porosimetry give a clear insight into the macro and mesoporosity of the system. By correlating nanoscale structural features with the adsorption capacity of the MOF nanoparticles, different degrees of full encapsulation‐based on the PVDF‐HFP processing and structuration from the macro to nanometer scale are observed. Additionally, the in situ functionalization of MOF nanoparticles with cysteine is investigated to enhance their adsorption toward Hg II . This functionalization enhanced the adsorption capacity of the MOFs from 8 to 30 mg·g ⁻¹ .
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.
Direct contact membrane distillation (DCMD) is an emerging technology gaining attraction in seawater desalination and concentration of aqueous solutions. In this study, a facile surface modification strategy using hydrophobic ZIF-8 and a fluoroalkylsilane (FAS) was employed to improve the performance of commercial PVDF and PVDF/PTFE blend membranes. The PVDF/PTFE membrane modified using 10 wt./vol % hydrophobic ZIF-8 and 1 H, 1 H, 2 H, 2 H-perfluorooctyltriethoxysilane (PFOTES) exhibited the highest water contact angle (121 ± 4.7°), improved surface roughness (51.6 ± 1.4 nm), most significant vapor flux (17.6 ± 2 L/m²h), and salt rejection of 99.98% with 10,000 ppm NaCl synthetic feed solution at the optimum operating conditions. The surface-engineered membranes exhibited excellent anti-fouling (360 minutes) and anti-wetting performance (240 minutes) when challenged with saline feed solution containing 50 ppm sodium alginate solution and 0.1 mM sodium dodecyl sulfate, respectively. The surface engineered membranes produced permeate of uniform quality for>10 h and could recover~31% water during actual seawater desalination (10 h). Moreover, these membranes could concentrate propylene glycol by a factor of 1.24 within 6 h and retain>80% of it. This facile surface modification strategy holds an excellent potential to be further explored for commercial applications.
In this paper, triangular hollow porous polyacrylonitrile (PAN) fibers were prepared by coaxial wet spinning inspired by silver ant hair. These PAN fibers with ideal triangular hollow cross section structure were obtained by adding inorganic salts into the cortex and core spinning solution combined with proper drawing ratio. Its strength increases to 4.3 times compared with that of the nascent fiber without inorganic salt. Subsequently, the light reflection as well as thermal insulation properties of triangular and circular hollow porous fibrous mats were compared and analyzed. The results show that the triangular hollow porous fibers have higher light reflectance and heat insulation performance than the circular fibers, which endows the fabric a shimmering gloss. This research further expands the application of hollow porous fiber in both thermal insulation field and optical field, which is conducive to the design of “heat stealth” materials and flash materials.
Population growth, industrialization and water mismanagement have played a key role depleting water resources. Membrane Distillation (MD) has emerged as an attractive technology for seawater desalination and water recovery from wastewater. MD exploits the vapor pressure difference across the membranes to selectively transport water vapor from the feed leaving behind all dissolved organic/inorganic contaminants. This chapter provides a comprehensive overview of the MD process, the various MD configurations, the limitations of the process, and the drawbacks of the membranes prepared by conventional techniques. To overcome the limitations of conventional membranes, electrospinning technique is chosen to fabricate membranes with higher pore size, greater porosity, low tortuosity, and low liquid-entry pressure. The various solution, process, and environmental parameters affecting the characteristic of the nanofibers are explained in detail. The membrane properties, posttreatment strategies, and limitations of pristine electrospun nanofibrous membranes (ENMs) have also been discussed. The following section deals with nanoparticle (metal/metal oxide, clays, carbon materials, MOFs, etc.) incorporated ENMs with enhanced permeation rates and outstanding rejection of contaminants. The final section deals with the challenges limiting the adoption of ENMs and the future direction of research employing the ENMs for MD.
Polytetrafluoroethylene (PTFE) nanofiber membranes with novel crosshatched structures are developed and applied to both water desalination by direct contact membrane distillation (MD) and CO2 separation by membrane gas absorption. Crosshatched structures are produced from a PTFE‐poly(ethylene oxide)(PEO) emulsion by depositing alternating layers of aligned fibers oriented in perpendicular directions. This is followed by sintering to remove the PEO and to stabilize the structure. The crosshatched structure allows for rapid gas and vapor transport due to the low tortuosity and high porosity, leading to fast and effective separation. PTFE nanofiber membranes with these novel structures are ideal for membrane CO2 stripping as this polymer is inherently strong and very hydrophobic. The mass transfer in both MD and CO2 stripping is greatly improved in the crosshatched nanofibers (CNF) as well as in composite membranes with microparticles (CNF‐MP), as compared with conventional random nanofibers. The membranes exhibit a MD flux up to 98.5 ± 1.2 kg m−2h−1, significantly greater than a standard PTFE membrane with asymmetric morphology, when tested with a 3.5 wt% sodium chloride feed solution at 80 °C in direct contact with water at 20 °C. Crosshatched structures of polytetrafluoroethylene (PTFE) are prepared by depositing alternating layers of aligned fibers oriented in orthogonal directions to form a nanofiber membrane. The crosshatched structure allows for rapid gas and vapor transport due to the low tortuosity and high porosity, significantly greater than a standard PTFE membrane with asymmetric morphology, leading to fast and effective separation.
Methanol permeation is the main issue of Nafion membranes when they are used as a
polymer electrolyte membrane (PEM) in direct methanol fuel cells (DMFCs). In the
current study, novel nanocomposite polymer membranes are prepared by the
integration of surface-modified sepiolite (MS) in polyvinylidene fluoride grafted
polystyrene (PVDF-g-PS) copolymer as PEM in DMFCs. Sepiolite surface is chemically
modified using vinyltriethoxysilane and analyzed by fourier-transform infrared (FTIR)
spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Nanocomposite PVDF-g-PS/MS membranes are prepared by phase inversion
technique and subsequently treated with chlorosulfonic acid to induce sulfonic acid
(SO3H) active sites at the membrane surface. The prepared nanocomposite
membranes (S-PPMS) are analyzed for their physicochemical characteristics in terms
of water uptake percentage, cation exchange capacity (CEC), proton conductivity (σ),
and methanol permeability. MS dispersion in the copolymer matrix is proved through
morphological SEM examination. The S-PPMS membranes exhibit increased proton
conductivity due to the presence of well-dispersed MS and surface functional –SO3H
groups. A peak power density of 210 mWcm−2 is recorded for S-PPMS10 at 110 °C
which is higher than the output obtained from Nafion-117. These promising results
indicate the potential utilization of prepared nanocomposite PEMs for DMFC
application
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.
An alkaline-modified natural sponge was applied as support of gold and silver nanocatalysts and a deformable reactor to hold the reaction solution, quickly adsorb several kinds of environmental pollutants (4-nitrophenol, hexavalent chromium, and methyl orange) and efficiently catalyse their conversion. As the reaction solution is sustained in the connecting micro-pores constructed by the stretched and knotted sponge fibers, the catalytic reaction is free from external mass transfer and suffers slight internal mass transfer limitation. This batch-wise reaction system employing an operation procedure of injection-extrusion achieved a nearly complete conversion of the reactants within 1 min or much less. This compressible support can also be packed in a continuous flow reaction system and can be repeated use for 37 times without any loss of catalytic activity. This study provides a generalizable and straightforward strategy for utilizing deformable soft supports that may inspire some unique industrial applications.
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.
In this study, structured photocatalytic systems were successfully developed by a facile method based on Alginate molds and a wet-spinning/cross-linking technique, yielding commercial photocatalyst (Degussa P25) in the form of all-ceramic hollow fibers (HFs). Taking advantage of alginate’s exceptional sorption properties, copper augmented HFs were also developed. The structured photocatalysts were thoroughly characterised by a variety of techniques, including nitrogen adsorption, SEM/EDS, XRD, XPS and Raman. Synthesis and heat treatment parameters were found to affect the fibers’ properties, allowing their optimization. Treatment at 600 °C under Ar was found to produce the best performing photocatalysts in terms mechanical stability, resistance to attrition and photocatalytic performance. Ca-Alginate precursors led to structures with increased mechanical stability, while Cu-Alginate decorated the surface of the photocatalyst with highly dispersed copper nanoparticles, in the state of metallic and CuO. The developed materials were photo-catalytically active, while the copper decorated ceramic HFs exhibited the highest MO adsorption and photocatalytic degradation performance, reaching a MO removal of 73.4%. The synergestic effect of adsorption on the MO degradation performance was also noticed. Moreover, the copper addition facilitated the photocatalytic process by improving the electron-hole separation and inhibiting the recombination effects. The presence of carbon residue was also beneficial, enhancing the MO sorption on the photocatalysts. It is noteworthy that the structured photocatalysts retained their efficiency for at least four photocatalytic cycles. The prepared ceramic HFs exhibited enhanced mechanical properties and excellent resistance to attrition after subsequent cycles, rendering them excellent candidates for application in industrial wastewater processes.
In-situ construction of superhydrophobic polyvinylidene fluoride (PVDF) membrane was prepared through nonsolvent induced phase inversion process for direct contact membrane distillation (DCMD). Saturated sodium chloride aqueous solution (NaCl-H2O) was added to the casting solution as a green phase inversion inducing agent. The variation of polymer crystal and the formation mechanism of micro-nano structure on membrane surface were studied. The results showed that NaCl crystals could act as nucleus for the pre-gelation and crystallization of PVDF chains. The “harsh” nonsolvent, water, accelerated the gelation process. When the optimum amount of NaCl-H2O was added, superhydrophobic membrane with lotus leaf like micro-nano spherical structure and interconnected channels was obtained. The pure water contact angle reached 155.3° with DCMD flux of 54.5 L/m²∙h, which was about 1.6 times for the membrane without NaCl-H2O addition. The stability was also improved owing to the superhydrophobic structure.
A systematic study was carried out on the initial casting solutions of asymmetric PVDF ultrafiltration membranes to observe the aggregate structure of polymer building units in solution and the properties of the membranes in terms of microstructure morphology, crystallinity, average pore size, pore‐size distribution, total porosity, hydrophobicity, wetting energy, molecular weight cut‐off, and permeability. Hydrophobic membrane of higher porosity exhibiting higher flux was obtained from the solution of bigger polymer structural units while the membrane obtained from solution of smaller polymer structural units exhibited relatively rougher and hydrophilic surface and smaller uniform pores with lesser flux.
Membrane distillation (MD) is a thermally driven membrane separation process in which only vapor molecules can be transferred through hydrophobic membrane. Significant efforts have been made with MD membrane especially for seawater desalination, but its application is expanded quickly for wastewater treatment and reuse. Membrane performance in MD process varies strongly depending upon the intrinsic properties of membrane materials. This paper provides critical reviews on MD membrane focusing on fabrication methods and membrane materials as well as its applications for wastewater treatment and reuse. To achieve high effluent quality for wastewater reuse purposes, the MD process is integrated with membrane bioreactor (MBR) although both organic and inorganic fouling are the main issues to be resolved in hybrid MD system.
Hydrophobic polyvinylidene fluoride (PVDF) hollow fibre composite membranes were prepared by the dilute solution coating process to build a special surface structure that was similar to the dual micro‐nano structure on the lotus leaf. Poly(vinylidene fluoride‐co‐hexafluoropropene) was chosen as the hydrophobic polymer candidate in dilute solution. Membrane morphology and surface hydrophobicity were evaluated by scanning electron microscopy and dynamic water contact angle measurement. The prepared PVDF hollow fibre membranes were employed to separate dyes (Congo Red and Methylene Blue) from water by vacuum membrane distillation. The effects of operational conditions (feed temperature, vacuum pressure and feed flow rate) on the vacuum membrane distillation performance of different PVDF membranes were investigated. The results indicated that the water contact angle values of PVDF composite membrane surfaces improved from 93.6° to 130.8°, which was mainly attributed to the formation of micro‐nano rods. This structure was similar to the dual micro‐nano structure on the lotus leaf. Under test feed temperature, vacuum pressure and feed flow rate conditions, the dye rejection rate of Congo Red and Methylene Blue by the hydrophobic PVDF hollow fibre membrane remained above 99.5% and 99%, which was higher than that of the pristine PVDF membrane (99% and 98%, respectively). In addition, the hydrophobic PVDF hollow fibre composite membrane showed higher permeation flux under different conditions compared with the pristine PVDF membrane, which was attributed to membrane surface hydrophobicity and the electrostatic interactions between dyes and the PVDF membrane surface.
Surface modified polyetherimide (PEI) hollow fibers were prepared by surface segregation of a fluorinated polyurethane additive (FPA). This undergoes spontaneous segregation principally to their outer surfaces resulting in PEI hollow fibers with an improved hydrophobic property of a contact angle up to 132.7°. This high contact angle is attributed mainly to the protrusion of fluorine of FPA at the hollow fiber surface. The addition of FPA also enhanced the mechanical strength and liquid entry pressure of water (LEP) reaching high values in the range 329–541 kPa. The effects of the PEI concentration in the blend on the morphological and structural characteristics of the hollow fibers were studied maintaining the FPA at 2 wt% in the PEI spinning solution. The hollow fibers were tested for desalination by direct contact membrane distillation (DCMD). A very stable DCMD performance was obtained for the FPA hollow fiber prepared with 14 wt% PEI with permeate fluxes around 23.8 kg/m² h and low permeate electrical conductivity (4.9–5.8 μS/cm) over a period of almost two months (54 days DCMD operation with 29.8 g/L NaCl aqueous solution, 80 °C and 20 °C feed and permeate inlet temperatures, respectively).
Direct contact membrane distillation process has been studied using
microporous polytetrafluoroethylene and polyvinylidene fluoride
membranes. The membranes were characterized in terms of their
non-wettability, pore size distribution and porosity. The mean pore
sizes and pore size distributions were obtained by means of wet/dry flow
method. The mean pore size and the effective porosity of the membranes
were also determined from the gas permeation test. A theoretical model
that considers the pore size distribution together with the gas
transport mechanisms through the membrane pores was developed for this
process. The contribution of each mass transport mechanism was analyzed.
It was found that both membranes have pore size distributions in the
Knudsen region and in the transition between Knudsen and ordinary
diffusion region. The transition region was the major contribution to
mass transport. The predicted water vapor permeability of the membranes
were compared with the experimental ones. The effect of considering pore
size distribution instead of mean pore size to predict the water vapor
permeability of the membranes was investigated.
Investigation of membrane distillation (MD) with a laminar flow of the streams in a module has been performed. The equations describing the heat transfer in MD capillary modules were presented and verified experimentally. The equations were derived for the calculation of the feed and distillate temperature at a layer adjacent to the membrane. The heat transfer correlations were implemented in a physical model of the MD process and the applicability of the model was validated. (C) 1998 Elsevier Science B.V.
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.
A porous membrane based on poly(vinylidene fluoride) (PVdF) is prepared by the addition of a non-solvent before the phase-inversion step and it is stretched uni-axially to achieve high mechanical strength without sacrificing high ionic conductivity. The tensile strength of the PVdF membrane is 23MPa without stretching, which is around twice that of a PVdF membrane made without addition of non-solvent before phase inversion. The former membrane can be stretched by up to 350% at room temperature and there by its tensile strength is increased to 126MPa. In addition, the ionic conductivity is greatly enhanced due to an increase in porosity. The discharge capacity of the cell based on the stretched PVdF membrane is around 90% of the initial discharge capacity after 200 cycles.
Direct contact membrane distillation (DCMD) experiments using distilled water are reported. Influence on the process of feed and permeate flow rates through the cell has been investigated in a wide flow range, from 2 to 8l/min. Two main effects have been studied, its effect on the heat transfer coefficient and on the effective membrane thickness. An empiric dependence of the membrane thickness with linear velocity through the cell has been included in the equation for mass flux through the membrane obtained from the “Dusty-Gas” model with satisfactory results.
The effects of some membrane preparation conditions on the characteristics and performance of poly(vinylidene fluoride)/polyethersulfone blend membranes were studied. Poly(vinylidene fluoride) (PVDF) and polyethersulfone (PES) were employed as base polymers. Four different solvents including dimethyl sulphoxide (DMSO), dimethyl formamide (DMF), 1-methyl-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc) were used as the solvents. Polyvinylpyrrolidone (PVP) was used as pore-forming additive. The preparation variables in this work were sort of solvents, content proportion of PVDF/PES, and concentration of PVP. The flat-sheet membranes prepared by phase inversion were characterized by using scanning electron microscopy (SEM). Membrane performance was evaluated by shrinkage ratio, pure water flux and retention to bovine serum albumin (BSA). The shrinkage ratio of PVDF/PES blend membrane was much reduced when DMAc was used as a solvent. The pure water flux of blend membranes reaches the maximum and the retention to BSA reaches the minimum when the casting solution contains 1.5wt% PES and 5wt% PVP.
To investigate the effect of poly(ethylene glycol) (PEG) 200 on membrane performance, asymmetric polyetherimide (PEI) membranes with a small pore size were prepared by dry/wet-phase inversion from the casting solution containing N-methyl-2-pyrrolidone as a solvent and poly(ethylene glycol) 200 as an additive. Our experiment revealed that the addition of PEG 200 has an influence on the casting solution properties, permeation properties, and resulting membrane structures. Moreover, a drying process also affects the formation of a dense skin layer. Increasing the amount of PEG 200 drastically improved the solute rejection rate. The drying process improved the rejection rate. We also observed the effect of the mixed solvent (water/ethanol) on permeation through the membranes with various pore sizes. In the case of the membrane with a dense skin layer, the solvent permeation showed relationships with solution viscosity, surface tension, and membrane-solvent interaction.
The preparation and gas permeation performance (H2, CO2, O2, N2, CH4) of silicone rubber/poly(4-vinylpyridine)/polyetherimide (SR/P4VP/PEI) multi-layer composite hollow fiber membranes are described. Dilute P4VP solutions form a defective coating layer on the top of PEI hollow fiber surface but reduce the surface porosity significantly, which makes the SR plugging feasible to regain the gas permselectivity of P4VP/PEI composite hollow fiber. Polyethylene glycol (PEG) additive in the PEI spinning dopes suppresses the growth of macro-voids and produces a membrane morphology having a more porous skin surface and more compact substructure, which provides a lower skin resistance and a higher substructure resistance for gas permeation. Substantial substructure resistance can deteriorate the membrane performance. Since the unfavorable influence of substructure resistance on the gas permeance is more pronounced for the fast permeating gas and in the order of H2>CO2>O2>N2>CH4, the gas selectivities of H2/N2, CO2/CH4, and O2/N2 decrease accordingly. The PEI hollow fibers prepared from a PEI/PEG/NMP (23/0/77) spinning dope, after coated with 0.2wt.% P4VP and 3wt.% SR coating solutions, have gas permeances of H2=41, CO2=7.4, and O2=2.0 GPU (1 GPU=1×10−6cm3(STP)/cm2-s-cmHg) with selectivities of H2/N2=117, CO2/CH4=62, and O2/N2=5.8.
For membranes synthesized from crystalline polymers by phase inversion method, crystallization can sometimes dominate the precipitation process to form a membrane characterized by the so-called particulate structure. Such is the case for PVDF membranes prepared by immersion-precipitation from water/ triethylphosphate solutions. The structure of the membrane formed from this system was studied; in particular, the nano-scale fine structure of the crystallites, which form the matrix of the membrane. The phase diagram of the water/TEP/PVDF was determined using the cloud point method. Membranes were observed using LVSEM at low voltage (e.g., 1 KV) and high magnifications (e.g., 100 KX) to reveal the fine structure of the membranes.
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.
A comparative study of an integrated hybrid membrane –based system with an earlier locally designed RO unit, such system comprises of Nanofiltration (NF), Reverse osmosis (RO) and Membrane Distillation (MD) subsystems. The comparison is essentially based on using the NF technique in pretreatment section, while the MD was contributed to concentrate the two brine streams from both NF and RO. The proposed system was economically evaluated and compared with the RO unit.
Porous asymmetric hydrophobic membranes were prepared from poly(vinylidenefluoride-co-hexafluoropropylene) and polyvinylidenefluoride (PVDF) homopolymer by the phase inversion process induced by a nonsolvent. The effect of pore-forming hydrophilic additives on the membrane morphology and transport properties was investigated. Mean pores size and effective porosity were calculated by the gas permeation method. It was found that membranes made of PVDF copolymer offer a higher resistance to mass transport compared to membranes prepared from PVDF homopolymer under the same experimental conditions. This result is due to lower membrane porosity and pore size and to the presence of a double skin layer (on the upper and bottom surface) for these membranes, reducing the permeate flux. However, membrane morphology and transport properties can be modified by additives in the casting solution. In particular, it was found that polyvinylpyrrolidone improves membrane permeability; LiCl can be used in order to reduce macrovoid formation and increase the mechanical stability of the membranes.
This research focuses on vacuum membrane distillation (VMD) using high-concentration NaCl aqueous solutions as feed. A new membrane module was investigated to improve water desalination and experiments were carried out using a commercial polypropylene (PP) membrane with a pore size of 0.2μm. In order to enhance performance of VMD in desalination and to get more flux, effects of operating parameters on the performance were studied. Water fluxes were measured at different feed temperatures, feed concentrations, vacuum pressures and flow rates. The new configuration provides better mixing and this increases heat and mass transfer coefficients, and as a result, reduces temperature and concentration polarization effects.
The concentration of NaCl solution containing natural organic matter by membrane distillation (MD) has been performed. The salt solution produced during animal intestines processing was used as a feed. The presence of organic compounds in the feed caused the fouling of MD membranes. The experiments were performed with polypropylene capillary membranes. A rapid flux decline caused by the deposition of organic matter on the membrane surface has been observed. The morphology and composition of the fouling layer was studied using scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS) and Fourier transform infrared with diffuse reflectance spectroscopy (FTIR-DRS). Protein and sodium chloride constituted the major components of the gel layer. Rinsing of the MD module with a 2wt.% citric acid solution removed a part of the fouling layer. Boiling of spent NaCl solution followed by filtration resulted in the separation of the organic matter in the form of a deposit. This enabled a significant reduction in the occurrence of fouling phenomenon.
We report here direct contact membrane distillation results from modules having 0.28 m2 of membrane surface area employing porous hydrophobic polypropylene hollow fibers of internal diameter (330 μm) and wall thickness (150 μm) with a porous fluorosilicone coating on the outside surface. The brine salt concentration and temperature and the distillate temperature and velocity were varied. Water vapor fluxes approach values obtained earlier in much smaller modules. As the brine temperature was increased from 40 to 92 °C, water vapor flux increased almost exponentially. Increasing the distillate temperature to 60 from 32 °C yielded reasonable fluxes. Salt concentration increases to 10% led to a small flux reduction. An extended 5-day run did not show any pore wetting. A model using the mass transfer coefficient km as an adjustable parameter predicted the brine temperature drop, distillate temperature rise, and water vapor flux well for the large module and the smaller module of 119-cm2 surface area.
In this paper the experimental results of counter current flow air gap membrane distillation experiments are presented and compared with predictive model calculations. Measurements were carried out with a cylindrical test module containing a single hollow fibre membrane in the centre and a well-defined air gap situated around the fibre. The experimental results show that the previous developed predictive model, with membrane parameters determined from gas permeation experiments, describes correctly the dependence of water vapour flux on temperature level, temperature difference, air gap total pressure, hot water flow and membrane type. At atmospheric air gap pressure, the measured fluxes per saturated water vapour pressure difference between the bulk flows (0.08–0.10 kg/m2h mbar) are comparable with those presented in literature. A reduction of the total air gap pressure to the saturated water vapour pressure of the hot water feed flow temperature of 65 °C, raises the flux by a factor of three. Next to the water vapour flux, the energy efficiency of the process is very important. The measured energy efficiencies (typically 85–90% for a 3 mm air gap and a hot water feed temperature of 65 °C) are slightly below the theoretical values (95–98%), which could be explained by a small heat loss to the surroundings. For air gaps of 1.5 mm or smaller, the energy efficiency is reduced to less than 70%, due to thermal conduction across product water bridges between the membrane fibre and the condenser surface. An optimal air gap is about 3 mm wide and has a total pressure that is equal to or slightly below the saturated water vapour pressure of the hot water entering the hollow fibre membrane.
An improved gas permeation method for characterising microporous asymmetric hollow fibre membranes has been adapted in this study where the pore size distribution of the membranes was taken into consideration. Both standard normal and log-normal distribution functions were employed in the relevant equations. Parameters concerning membrane structures such as pore size, pore size distribution and effective porosity were determined through regression of analysis of gas permeation data. These parameters were then used to predict the permeation coefficients over a range of operating pressures. The predicted permeation coefficients were found to be in good agreement with the actual experimental data. The membrane structure parameters were also used in predicting the theoretical membrane’s coefficients for comparison with H2S gas absorption experimental data. The results indicate that log-normal distribution function analysis is better off in producing a realistic predictions of membrane’s coefficients, which are commonly used to indicate the efficiencies of membrane-based absorption for soluble gas removals.
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.
Preparation of poly(vinylidene fluoride-co-hexafluoro propylene) (F2.6) flat-sheet asymmetric porous membrane has been studied for the first time. Factors affecting F2.6 membrane pore structure and permeate performance, such as macromolecule pore formers (polyethylene glycol-400, 1000, 1540, 2000 and 6000), the small molecule former (glycerol), swelling agent (trimethyl phosphate) in casting solution, precipitating bath component and temperature, exposure time and ambient humidity, were investigated in detail. Average pore radius and porosity were used to characterize F2.6 membrane structure, and respectively, determined by ultrafiltration and gravimetric method for the wet membrane. Morphology of the resultant membranes was observed by scanning electronic microscopy (SEM). Final test on permeate performance of F2.6 porous membrane was carried out by a direct contact membrane distillation (DCMD) setup. The experimental F2.6 membrane exhibits a higher distilled flux than PVDF membrane under the same operational situations. The determination of contact angle to distilled water also reveals higher hydrophobic nature than that of PVDF 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.
Three kinds of amphiphilic polymers, including the tri-block copolymer of (polyethylene oxide)–(polypropylene oxide)–(polyethylene oxide) (I, EPTBP), the comb-like copolymer of polysiloxane with polyethylene oxide and polypropylene oxide side chains (II, ACPS) and the hyperbranched star copolymer of polyester-g-methoxyl polyethylene glycol (III, HPE-g-MPEG), were blended with PVDF to fabricate porous membranes via a phase inversion process, respectively, and the effects of the different structures of the amphiphilic polymers on the properties of the blend membranes were compared. The membranes were characterized by scanning electron microscopy (SEM), elemental analysis, X-ray photoelectron spectroscopy (XPS) analysis, mercury porosimetry, water contact angle measurements, etc. The anti-fouling properties of the prepared membranes were evaluated by static and dynamic bovine serum albumin (BSA) adsorptions. Specially, the stabilities of these amphiphilic polymers in the final membranes were estimated by continuous leaching tests. At the same time, the properties of the membranes using the amphiphilic polymers as modifiers were compared with those of the membrane using poly(ethylene glycol) (PEG) as modifier.
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.
This contribution describes a methodology to convert commercially available, microporous membranes into ion-exchange membranes using primary anchoring polymer (mono)layers and graft polymerization from the surfaces of the membranes. Atom transfer radical polymerization (ATRP) was used to modify the membranes with pyridinium exchange groups. Polymerization time was used as the independent variable to manipulate the amount of grafted poly(2-vinylpyridine) on the membrane surface. Results indicate that by changing polymerization time, it is possible to tune the ion-exchange capacity and the average pore size in rational ways. Equally important, membranes with initially broad pore-size distributions had narrower pore-size distributions following polymerization. A polymerization time of 24 h reduced the pore-diameter polydispersity (PDP) from 2.05 to 1.44. A polymerization time of 8 h resulted in a static ion-exchange capacity of 7.32 × 10−2 mmol/g (7.32 × 10−2 meq/g) of dry membrane.
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.
This study investigates the effect of PEG additive as a pore-former on the structure formation of membranes and their permeation properties connected with the changes of thermodynamic and kinetic properties in phase inversion process. The membranes were prepared by using polysulfone (PSf)/N-methyl-2-pyrrolidone (NMP)/poly(ethylene glycol) (PEG) casting solution and water coagulant. The resulting membranes prepared by changing the molecular weight of PEG additive and the ratio of PEG to NMP were characterized by scanning electron microscope observations, measurements of water flux and PEG rejection. The thermodynamic and kinetic properties of membrane-forming system were studied through coagulation value, light transmittance and viscosity. The correlations between the final membrane structure/permeation properties and thermodynamic/kinetic properties of membrane forming system are discussed extensively.
Asymmetric blend hollow fiber membranes had been made from a new casting dope containing poly(vinylidene fluoride) (PVDF)/Thermoplastic polyurethane (TPU)/Polyvinylpyrrolidone (PVP) N, N-dimethylacetamide (DMAc). The effect of hydrophilic additive, polyvinylpyrrolidone, on the morphology and crystal structure of PVDF/TPU blends membranes by phase inversion process was studied. The separation property, microstructure and crystalline phase of membranes were characterized by bovine serum albumin (BSA) retention experiments, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and differential scanning calorimetric (DSC), respectively. The results showed that adding low concentration (less than 3 wt%) of PVP can reduce the hydrophobicity of PVDF and increase its hydrophilicity. With further increment of PVP, the solution demixing is delayed and the kinetic hindrance due to the increase of viscosity. By SEM it was observed that with the increase of PVP the pyriform voids were replaced by the ‘finger-like’ voids, which became longer and widespread. In further increment of PVP (5 wt%), PVP contributes to the suppression rather than the enlargement of macrovoid structure in the membranes. For formed membranes by additive-free casting dope, FTIR–ATR indicated that PVDF crystallized including α and β phase. TPU and low concentration of PVP both can reduce the crystallinity of PVDF membranes. When PVDF/TPU was precipitated from increasing amount of PVP in dopes (e.g. ≥3 wt%), some α phase disappeared and the water flux decreased little by little, the enthalpy of fusion and crystallinity of membranes also declined. When the addition of PVP increased as high as 10 wt%, the heat of fusion and crystallinity of membranes increased slightly compared with membranes adding 3 wt% PVP. In addition the water flux dropped significantly rather than improved.
A theoretical model is presented that describes sweeping gas membrane distillation processes through porous hydrophobic membranes. The approach considers the case in which the liquid feed and the sweeping gas counterflow in a plate and frame membrane module. The model developed emphasises the importance of the heat fluxes in the directions parallel and perpendicular to the membrane surface and permits to obtain the temperature profiles inside the fluid phases. In order to check the model, two membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids has been studied. The theoretical predictions of the model have been applied to the obtained results and the accordance may be considered good.
New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.
Flat-sheet microporous membranes from F2.4 for membrane distillation (MD) were prepared by phase inversion process. Dimethylacetamide (DMAC) and LiClO4·3H2O/trimethyl phosphate (TMP) were, respectively, used as solvent and pore-forming additives. The effects of casting solution composition, exposure time prior to coagulation and temperature of precipitation bath on F2.4 membrane structure were investigated. The morphology of resultant porous membrane was observed by scanning electron microcopy. Some natures of F2.4 porous membrane after drying in air, such as mechanical properties and hydrophobicity, were exhibited and compared with poly(vinylidene fluoride) (PVDF) membrane prepared by the same ways. Stress-at-break and strength stress of F2.4 microporous membrane are higher than that of PVDF membrane, and elongation percentage of F2.4 membrane at break is about eight-fold as great as that of PVDF membrane. Contact angle of F2.4 microporous membrane to water (86.6±0.51°) was also larger than that of PVDF membrane (80.0±0.78°). MD experiment was carried out using a direct contact membrane distillation (DCMD) configuration as final test to permeate performance of resultant microporous membrane. The permeate flux of F2.4 experimental membrane was higher than that of PVDF membrane under the same operational conditions.
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.
Flat membranes with controlled morphology, pore dimensions, mechanical properties and crystal structure were prepared by wet and dry wet phase inversion from polyvinylidene fluoride (PVDF). The effects of several parameters such as precipitation temperature, composition of the polymer solution (concentration, type of solvent), exposure time before immersion in the coagulation bath, type of coagulant on the sequence and the extent of the two phase separation processes, i.e. liquid–liquid and liquid–solid demixing (crystallization), were studied.Using solvent/nonsolvent pairs with different mutual affinity (DMA/water, DMA/C1–C8 alcohols), different morphologies were obtained. High casting solution temperature plays important role to increase the rate of the liquid–liquid demixing on the crystallization, i.e. the type of crystallites formed (α-type) also by using a soft coagulation bath. Exposure time before immersion favours the first type of phase separation and therefore once again crystallites of α type were observed. At room temperature, using C1–C8 alcohols as nonsolvents, the presence of crystallites of α type can be related to molar volume of the coagulant.
Polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared using the solvent spinning method. N,N-dimethylacetamide was the solvent and ethylene glycol was employed as non-solvent additive. The effect of the concentration of ethylene glycol in the PVDF spinning solution as well as the effect of ethanol either in the internal or the external coagulant on the morphology of the hollow fibers was investigated. The prepared membranes were characterized in terms of the liquid entry pressure of water measurements, the gas permeation tests, the scanning electron microscopy, the atomic force microscopy, and the solute transport experiments. Ultrafiltration experiments were conducted using polyethylene glycol and polyethylene oxides of different molecular weights cut-off as solutes. A comparative analysis was made between the membrane characteristic parameters obtained from the different characterization techniques.
In this study, the influence of the morphological parameters of microporous poly(vinylidene fluoride) (PVDF) membranes on the heterogeneous nucleation rate of hen egg white lysozyme (HEWL) crystals has been investigated. Experiments have been carried out on membranes prepared by non-solvent-induced phase inversion method, using PVDF-co-hexafluoropropylene (Kynarflex 2800) and PVDF homopolymer (Kinar 460), and adding LiCl and poly(vinylpyrrolidone) (PVP) in order to modulate the pore structure. From a theoretical point of view, the free Gibbs energy balance for the formation of a critical nucleus has been adapted to nonporous surfaces, thus obtaining a mathematical correlation between the energy nucleation barrier, the membrane porosity, and the contact angle between protein solution and polymeric substrate. The energetic barrier to heterogeneous nucleation was found to increase at higher contact angles-according to the prediction of classical theory-and to decrease at higher porosity. For instance, the predicted deltaG(het)/deltaG(hom) ratio for PVDF-Kynarflex (PVP 2.5%) membrane with porosity of 0.11 was 0.30, 35% lower with respect to the value calculated by the Volmer equation for a dense polymeric matrix having the same contact angle (87.4 +/- 5.8 degrees). In addition, the effect of the membrane pore size, porosity, and thickness on the removal rate of solvent have been discussed. For example, the transmembrane flux through PVDF-Kynar (LiCl 5.0%) membrane was 12% inferior than the one measured under the same experimental conditions through PVDF-Kynarflex (LiCl 7.5%) membrane, the latter having similar pore size and thickness but higher porosity (0.44 vs 0.32). The possibility to achieve rapidly a high level of supersaturation is expected to increase the nucleation rate. In general, measurements performed during crystallization tests carried out at pH 4.5 in NaAc 0.05 M buffer with different precipitant (NaCl) concentrations agree with the predicted trends.
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