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Robust surface modified polyetherimide hollow fiber membrane for long-term desalination by membrane distillation
Abstract
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).
... Studies on MD have advanced rapidly with a substantial increase in publications since 2010 ( Figure 1). Recently, research on MD has focused on novel hydrophobic membranes with anti-wetting and anti-fouling properties for various MD applications [42][43][44][45][46][47][48][49][50][51][52], Janus membranes with significantly reduced specific energy consumption for MD [53,54], and hybrid MD systems [55][56][57]. In addition, since Lawson and Lloyd [58] published the extensively cited comprehensive review paper in 1997, numerous review papers covering various aspects of MD have been published. ...
... Studies on MD have advanced rapidly with a substantial increase in publications since 2010 ( Figure 1). Recently, research on MD has focused on novel hydrophobic membranes with antiwetting and anti-fouling properties for various MD applications [42][43][44][45][46][47][48][49][50][51][52], Janus membranes with significantly reduced specific energy consumption for MD [53,54], and hybrid MD systems [55][56][57]. In addition, since Lawson and Lloyd [58] published the extensively cited comprehensive review paper in 1997, numerous review papers covering various aspects of MD have been published. ...
High water flux and elevated rejection of salts and contaminants are two primary goals for membrane distillation (MD). It is imperative to study the factors affecting water flux and solute transport in MD, the fundamental mechanisms, and practical applications to improve system performance. In this review, we analyzed in-depth the effects of membrane characteristics (e.g., membrane pore size and distribution, porosity, tortuosity, membrane thickness, hydrophobicity, and liquid entry pressure), feed solution composition (e.g., salts, non-volatile and volatile organics, surfactants such as non-ionic and ionic types, trace organic compounds, natural organic matter, and viscosity), and operating conditions (e.g., temperature, flow velocity, and membrane degradation during long-term operation). Intrinsic interactions between the feed solution and the membrane due to hydrophobic interaction and/or electro-interaction (electro-repulsion and adsorption on membrane surface) were also discussed. The interplay among the factors was developed to qualitatively predict water flux and salt rejection considering feed solution, membrane properties, and operating conditions. This review provides a structured understanding of the intrinsic mechanisms of the factors affecting mass transport, heat transfer, and salt rejection in MD and the intra-relationship between these factors from a systematic perspective.
... Sobre os dados de energia de superfície, os valores não são condizentes com aqueles relatados por Khayet et al. 26 , que obteve 38,8 mN m -1 para a PEI. Nesse caso foi possível evidenciar que os aditivos e as temperaturas do banho de coagulação influenciaram de certa forma nas energias de superfícies das membranas de PEI produzidas, sendo que resíduos destes aditivos podem ter ficado aderidos às membranas. ...
... Most of the UV photo-grafting research is focused on modifying polymeric membranes with hydrophilic monomers such as acrylic acid, 2-hydroxyethyl methacrylate, 2,4-phenylenediamine, ethylenediamine, cellulose, potato starch, and chitosan. 9,13−17 Modifying membranes with hydrophobic groups includes surface segregation, 18 plasma treatment, 19 covalent modification, 20 electrospinning, 2 and spin coating. 21 To our knowledge, no UV photo-grafting works focused on modifying polymeric membranes with 1-hexene have been reported in the literature. ...
The two main challenges for industrial application of membrane distillation (MD) are mitigation of temperature polarization and reduction of high-energy consumption. Despite the development of advanced materials and the configuration improvements of MD units, membrane surface modification is still one of the alternatives to overcome temperature polarization and improve membrane performance. This work reports a novel and simple method to modify the physical and chemical properties of the polypropylene membrane in order to improve its performance in direct contact membrane distillation (DCMD). The membrane was grafted by polymerization with 1-hexene, UV irradiation, and benzophenone as a photoinitiator. A grafting degree of up to 41% was obtained under UV irradiation for 4 h. The performance of the modified membrane in DCMD was evaluated at different temperatures and salt concentrations in the feed. First, it was found that there was an increase of the vapor permeate flux in the MD process within the range of tested temperatures and salt concentrations. The results were analyzed in terms of the physical properties of the membrane, the transport phenomena, and the thermal efficiency of the process. Theoretical analysis of the results indicated that grafting increased the transfer coefficients of mass and heat of the membrane. Hence, it improved the membrane performance and the thermal efficiency of the DCMD process.
... Ceramic membrane is often employed during a long-term study due to its high durability. In 54 d of DCMD operation with polyetherimide (PEI) membrane that was modified with fluorinated polyurethane, the membrane has a high permeate flux of 23.8 LMH with 2.98 wt.% of NaCl solution with 99.9% salt rejection [115]. In another word, there is no membrane wetting occurring for 54 d of DCMD operation with FPS-PEI membrane. ...
Membrane technologies have been expanded to treat seawater, brine water, wastewater, and polluted water to provide clean water. In membrane distillation (MD) application, membrane with low surface free energy is preferable to prevent membrane wetting. Thus, the pristine membrane was often modified with functional material to enhance the membrane performance during operation. In this paper, the mechanism of MD is briefly described according to different membrane configurations. Additionally, case studies for pilot plant MD were discussed. This is followed by the membrane fabrication techniques, functional material used for membrane modification and application of functional membrane in MD system. Finally, a future outlook and conclusion were made based on the review.
... In this case, the hollow fibers represent the tubes in a shell-and-tube heat exchanger for mass transfer in membrane technology. Among these separations, MD with hollow fiber membrane configuration is attractive for dye wastewater treatment [95] and desalination process [71,101,104,195,[569][570][571][572][573][574]. ...
The aggravation of environmental problems such as water scarcity and air pollution has called upon the need for a sustainable solution globally. Membrane technology, owing to its simplicity, sustainability, and cost-effectiveness, has emerged as one of the favorable technologies for water and air purification. Among all of the membrane configurations, hollow fiber membranes hold promise due to their outstanding packing density and ease of module assembly. Herein, this review systematically outlines the fundamentals of hollow fiber membranes, which comprise the structural analyses and phase inversion mechanism. Furthermore, illustrations of the latest advances in the fabrication of organic, inorganic, and composite hollow fiber membranes are presented. Key findings on the utilization of hollow fiber membranes in microfiltration (MF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), pervaporation, gas and vapor separation, membrane distillation, and membrane contactor are also reported. Moreover, the applications in nuclear waste treatment and biomedical fields such as hemodialysis and drug delivery are emphasized. Subsequently, the emerging R&D areas, precisely on green fabrication and modification techniques as well as sustainable materials for hollow fiber membranes, are highlighted. Last but not least, this review offers invigorating perspectives on the future directions for the design of next-generation hollow fiber membranes for various applications. As such, the comprehensive and critical insights gained in this review are anticipated to provide a new research doorway to stimulate the future development and optimization of hollow fiber membranes.
... Apart from the aforementioned polymers, polyamide [34], polyacrlyo nitrile [35], polydopamine [36], polyethylene [37], polyetherimide [38], polyethylene terephthalate [39], and so on have been investigated for the preparation of the membrane for MD applications. As most of these polymers produce hydrophilic membranes, their surface requires hydrophobic modifications prior to their applications. ...
Membrane distillation (MD) is a developing membrane separation technology for water treatment that involves a vapor transport driven by the vapor pressure gradient across the hydrophobic membrane. MD has gained wide attention in the last decade for various separation applications, including the separation of salts, toxic heavy metals, oil, and organic compounds from aqueous solutions. Compared with other conventional separation technologies such as reverse osmosis, nanofiltration, or thermal distillation, MD is very attractive due to mild operating conditions such as low temperature and atmospheric pressure, and 100% theoretical salt rejection. In this review, membrane distillation’s principles, recent MD configurations with their advantages and limitations, membrane materials, fabrication of membranes, and their surface engineering for enhanced hydrophobicity are reviewed. Moreover, different types of membrane fouling and their control methods are discussed. The various applications of standalone MD and hybrid MD configurations reported in the literature are detailed. Furthermore, studies on the MD-based pilot plants installed around the world are covered. The review also highlights challenges in MD performance and future directions.
... all through. Khayet et al. [143] modified the surface of polyetherimide (PEI) hollow fiber membranes to offer robustness via single casting step by surface segregation for longterm MD operation over a period of 54 days. The addition of FPA in the polymer blend significantly increased the hydrophobicity and in turn the MD performance of the membranes, recording salt rejection above 99.99% throughout. ...
Membrane distillation (MD) has proved worthwhile in treatment of hypersaline feeds demonstrating near complete rejection of dissolved solutes without any effect on the process conditions. This makes it potential treatment option for hypersaline oilfield produced water (PW) with salinity level far greater than that of seawater. Polymeric membranes have recently garnered more attention than their ceramic counterparts in oily wastewater treatment owing to ease of synthesis and relative cost advantage. However, lower mechanical durability and the propensity for fouling of these membranes due to presence of low surface energy organics in PW ultimately affects MD performance in its treatment. Studies elucidating the mechanism of fouling between PW feed and membranes in MD is scarcely reported in the literature. Various fouling mitigation approaches have shown promise towards the realization of MD as a viable option for PW treatment. Hybridization of MD, use of (super)hydrophobic MD membranes with feed pre-treatment using other technologies and/or membrane post-cleaning, integrated MD systems and recently omniphobic and Janus modifications of MD membranes have all been reported for treatment of PW showing more promise towards achieving ultrapure-distillate treatment. In this article, the performance of these polymeric MD membranes used in PW treatment in comparison to the other conventional treatment options as well as advances in MD as a cost-effective alternative for beneficial re-use of PW is reviewed, highlighting the areas requiring further study for this line of research. Because MD is still largely energy inefficient, several efforts to realize it as an all-round competitive technology focusing on long-term stability, brine handling capacity and potentials for cost savings with alternative and rather sustainable energy source are also discussed.
... As widely reported, the hydrophobicity and surface roughness are two of the main factors determining the water contact angle value of a surface [36,38]. When water droplets are placed on the rough surfaces, the following equation is normally used to predict the water contact angle [39]: ...
A surface acoustic wave (SAW) formaldehyde gas sensor was fabricated on a 42° 75′ ST-cut quartz substrate, with a composite sensing layer of zeolitic imidazolate framework (ZIF)-8 on polyethyleneimine (PEI)/bacterial cellulose (BC) nanofilms. The addition of snowflake-like ZIF-8 structure on the PEI/BC sensitive film significantly improves the hydrophobicity of the SAW sensor and increases its sensitivity to formaldehyde gas. It also significantly increases surface roughness of the sensitive film. The hydrophobic nature of ZIF-8 prevents water molecules from entering into the internal pores of the BC film, thereby avoiding a significant mass loading caused by humidity when the sensor is used to detect low-concentration formaldehyde gas. The Zn²⁺ sites at the surface of ZIF-8 improves the sensor's response to formaldehyde gas through enhanced physical adsorptions of gas molecules. Experimental results show that the ZIF-8@PEI/BC SAW sensor has a response (e.g., frequency shift) of 40.3 kHz to 10 ppm formaldehyde gas at 25 °C and 30% relative humidity (RH). When the relative humidity is increased from 30 to 93%, the response of the sensor only varies ~ 5%, and the change in response is negligible at medium humidity levels (~ 50 to 60% RH).
... Furthermore, a stable permeate flux together with a high salt rejection during long-term operations are essential aspects that need to be considered for large-scale MD applications [14]. Among other different techniques, non-solvent induced phase separation (NIPS) [15][16][17], vapor-induced phase separation (VIPS) [18], combination of NIPS and VIPS [19], thermally induced phase separation (TIPS) [20], combination of NIPS and TIPS [21], and electrospinning [22][23][24] have been used to fabricate membranes suitable for MD applications. Electrospun nanofibrous membranes (ENMs) exhibit several important characteristics, such as the high ratio of surface area to volume, a very high porosity or "void volume fraction" that can easily reach values over 90%, pore size or "inter-fiber space" that should be smaller than few micrometers, interconnected open structure, mechanical integrity and the possibility of constructing a wide variety of fiber sizes and shapes (e. g., different diameters, mixed matrix, and core-shell nanofibers) [25]. ...
A systematic study is carried out to determine the optimum electrospinning preparation condition to prepare an adequate electrospun nanofibrous membrane (ENM) for direct contact membrane distillation (DCMD). A structural properties investigation of ENM was carried out because of the significant impact of its architectural structure, nanofiber diameter, inter-fiber space and ENM thickness, on DCMD performance. The morphology, hydrophobicity, mechanical properties, crystallinity and DCMD desalination were investigated. A long-term DCMD experiment (100h) was carried out using 30 g/L NaCl aqueous solution, both in the feed and permeate side of the optimum ENM membrane to evaluate its potential to produce drinkable water in case of lack of distilled water, for instance in a remote area, emergency situation, and/or portable system. In this case, drinkable water could be produced after 28 h with a permeate flux of 57.5 kg/m².h and a salt rejection factor greater than 99.9%. Another long-term DCMD experiment (65 h) was conducted using 30 g/L NaCl aquesous solution as feed but at a higher temperature and distilled water as permeate to evaluate the desalination stability, wettability and scaling of the optimum ENM. A permeate flux of 58.5 kg/m².h was obtained with a salt rejection factor greater than 99.9%.
... As is well reported, the hydrophobicity and surface roughness of the lm are the two main factors which affect the water contact angle value of the surface [32,34]. When water droplets are placed on the rough surface of the lm, the following equation is normally used to predict the water contact angle [35]: ...
A surface acoustic wave (SAW) formaldehyde gas sensor was fabricated on a 42°75' ST-cut quartz substrate, with a composite sensing layer of zeolitic imidazolate framework (ZIF)-8 on polyethyleneimine (PEI)/ bacterial cellulose (BC) nanofilms. The addition of snowflake-like ZIF-8 structure on the PEI/BC sensitive film significantly improves the hydrophobicity of the SAW sensor and increases the sensor's sensitivity to formaldehyde gas. It also significantly increases the surface roughness of the sensitive film. Its hydrophobic nature prevents water molecules from entering into the internal pores of the BC film, thereby avoiding significant mass loading caused by the humidity change when the sensor is used to detect low-concentration formaldehyde gas. The Zn ²⁺ sites at the surface of ZIF-8 improves the sensor's response to formaldehyde gas through enhancing the physical adsorptions. Experimental results show that the ZIF-8@PEI/BC SAW sensor has a response (e.g., frequency shift) of 40.3 kHz to 10 ppm formaldehyde gas at 25℃ and 30% RH. When the relative humidity was increased from 30% to 93%, the response (frequency shift) of the sensor drifts only ~5%, and there is negligible drift at a medium humidity level (~56% RH).
... Measurement of hydrophobic wetting resistance for the MD method is a significant criterion dependent on its contact angle as well as LEP. 39 The LEP value of the membrane in MD should be sufficient to avoid wetting. The LEP values for various PTFE HF membranes are shown in Figure 3. Compared to the pristine PTFE membrane, which was 0.2132 ± 0.01 bar, the LEP data for all coated PTFE HF membranes showed an increase from 0.2 to 0.99 bar. ...
Membrane distillation (MD) is an attractive technology for the separation of highly saline water used with a polytetrafluoroethylene (PTFE) hollow fiber (HF) membrane. A hydrophobic coating of low-density polyethylene (LDPE) coats the outer surface of the PTFE membrane to resolve membrane wetting as well as increase membrane permeability flux and salt rejection, a critical problem regarding the MD process. LDPE concentrations in coating solution have been studied and optimized. Consequently, the LDPE layer altered membrane morphology by forming a fine nanostructure on the membrane surface that created a hydrophobic layer, a high roughness of membrane, and a uniform LDPE network. The membrane coated with different concentrations of LDPE exhibited high water contact angles of 135.14 ± 0.24 and 138.08 ± 0.01° for membranes M-3 and M-4, respectively, compared to the pristine membrane. In addition, the liquid entry pressure values of LDPE-incorporated PTFE HF membranes (M-1 to M-5) were higher than that of the uncoated membrane (M-0) with a small decrease in the percentage of porosity. The M-3 and M-4 membranes demonstrated higher flux values of 4.12 and 3.3 L m–2 h–1 at 70 °C, respectively. On the other hand, the water permeation flux of 1.95 L m–2 h–1 for M-5 further decreased when LDPE concentration is increased.
... GO enhances the permeation factor of PEI polymer due to hydrophilic properties of the filler [22][23][24][25][26] like the presence of rich oxygen groups such as hydroxyl, carboxyl, carbonyl and epoxide groups. These rich oxygen groups exhibit polar properties [27,28]. The functional groups' of GO enhances the interface interaction with PEI polymer [29,30]. ...
In the present investigation, laboratory synthesized graphene oxide (GO) as a nano-filler was used in polyetherimide (PEI) flat-sheet membranes (PM). The PEI flat-sheet membrane was fabricated through a dry-thermal treatment (DTT) method. The effects of fabrication method were investigated on polyetherimide-GO membrane prepared by dry-thermal treatment (PMDTT). The morphological structure was investigated via different characterization; Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), contact angle measurement and Raman spectra. The results indicated that, the hybrid PMDTT membrane displayed reasonably better pervaporation separation performance in comparison to neat PMDTT membranes. The concentrations of water at the permeate side of hybrid and neat PMDTT membrane were 99.3 and 90.9 wt.%, respectively. Hybrid membranes showed a 78.3% enhanced permeation rate. Enhancement of pervaporation property of hybrid PMDTT membrane could be ascribed mainly due to the presence of graphene oxide in the dense top layer. Overall, the blending of graphene oxide in hybrid PMDTT membranes could be a promising approach for enhancing the pervaporation properties of the membranes.
... In fact, several studies investigated the long-term operation of membrane distillation technology [27][28][29][30]. However, those investigations focused on a low fouling feed solution in a direct-contact membrane distillation (DCMD) configuration and analyzed the change in surface morphology of the membrane. ...
Membrane distillation (MD) has shown promise for concentrating a wide variety of brines, but the knowledge is limited on how different brines impact salt scaling, flux decline, and subsequent wetting. Furthermore, past studies have lacked critical details and analysis to enable a physical understanding, including the length of experiments, the inclusion of salt kinetics, impact of antiscalants, and variability between feed-water types. To address this gap, we examined the system performance, water recovery, scale formation, and saturation index of a lab-scale vacuum membrane distillation (VMD) in long-running test runs approaching 200 h. The tests provided a comparison of a variety of relevant feed solutions, including a synthetic seawater reverse osmosis brine with a salinity of 8.0 g/L, tap water, and NaCl, and included an antiscalant. Saturation modeling indicated that calcite and aragonite were the main foulants contributing to permeate flux reduction. The longer operation times than typical studies revealed several insights. First, scaling could reduce permeate flux dramatically, seen here as 49% for the synthetic brine, when reaching a high recovery ratio of 91%. Second, salt crystallization on the membrane surface could have a long-delayed but subsequently significant impact, as the permeate flux experienced a precipitous decline only after 72 h of continuous operation. Several scaling-resistant impacts were observed as well. Although use of an antiscalant did not reduce the decrease in flux, it extended membrane operational time before surface foulants caused membrane wetting. Additionally, numerous calcium, magnesium, and carbonate salts, as well as silica, reached very high saturation indices (>1). Despite this, scaling without wetting was often observed, and scaling was consistently reversible and easily washed. Under heavy scaling conditions, many areas lacked deposits, which enabled continued operation; existing MD performance models lack this effect by assuming uniform layers. This work implies that longer times are needed for MD fouling experiments, and provides further scaling-resistant evidence for MD.
... Up to now, distillation technologies have been widely applied in various engineering fields like separation or concentration of solution, desalination, water treatment, etc. [1][2][3][4][5]. The multistage flash (MF) and multi-effect distillation (MED) working at high vacuum are considered to be traditional thermal distillation technologies [6]. ...
The effectiveness-number of transfer units (ε-NTU) method was used to develop the heat and mass transfer model of air-gap diffusion distillation (AGDD). In this model, the latent heat transfer of vapor in air-gap was regarded as an equivalent convective heat transfer to make a relationship with NTU. Compared with the full numerical model developed in our previous work, the new model was simple and performing with dramatic high computational efficiency. The accuracy of the model was as well as the full numerical model. The influences of structure and operation parameters on the performances of AGDD system were discussed furtherly by employing the model. Results shown that the air-gap height, the inlet temperature and feed flow rate of hot stream could be regarded as the strong parameters effected significantly on the performances of the system in comparison with the air-gap thickness and inlet temperature and salt concentration of cold stream. Therefore, these strong parameters should be considered firstly for improving the heat and mass transfer of AGDD.
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.
Mixed matrix membranes (MMMs) combine the advantages of different materials and hopefully break through the limitation of high mass transfer resistance and low permeability in membrane distillation (MD) process. In this study, hydrophobic zeolitic imidazolate framework-71 (ZIF-71)/polyvinylidene fluoride (PVDF) hollow fiber composite membrane was prepared by the dilute solution coating method. ZIF-71 was incorporated into the PVDF dilute solution and a macroporous hydrophobic ZIF-71/PVDF coating layer was formed on the outer surface of PVDF hollow fiber support membrane through the nonsolvent induced phase separation (NIPS) process. PVDF hollow fiber support membrane was fabricated by the dry-jet wet spinning process. Membrane surface morphologies were observed through Field emission scanning electron microscope (FESEM). The effects of zeolitic imidazolate frameworks (ZIFs) content in PVDF coating layer on membrane surface hydrophobicity and desalination performance were investigated through Dynamic water contact angle (WCA) measurements and the vacuum membrane distillation (VMD) experiment, respectively. The VMD experiment was carried out at feed temperature 50 °C and permeation side vacuum 31.3 kPa using 3.5 wt% NaCl aqueous solution as the feed solution. Compared with the uncoated PVDF hollow fiber membrane, both the surface hydrophobicity and the water permeability of the ZIFs-incorporated PVDF hollow fiber composite membranes are improved. The surface WCA and permeation water flux of ZIFs-PVDF composite membrane increase from 94.5 to 136.5 o and 13.5 to 27.1 kg.m-2.h⁻¹ meanwhile the NaCl rejection could maintain at 99.99 %. No obvious pore wetting phenomenon was observed for ZIFs-PVDF hollow fiber composite membrane during the continuous VMD experiment for 60 h. The dilute solution coating is a promising method to prepare high-performance hollow fiber composite membrane for the desalination in MD process.
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.
Oil sorbent membranes are highly effective materials in combating oil spills. Recent technological advancements in membrane preparations permit higher oil-sorption efficiency and easier oil recoverability. This review article discusses the main structural difference between the most commonly fabricated membranes. Next, it highlights the different membrane production methods and their role in producing effective membranes, while focusing on the recent advances in elevating the properties of the membranes through the addition of different nanocomposites. Consequently, the hydrophobic/hydrophilic properties are highlighted, which are critical in the oil-collection mechanism. Finally, it examines prediction and challenges with regards to oil/water separation and recovery. These concepts are discussed with emphasis on modern production methods and oil-sorption proficiency.
Desalination has been used for thousands of years. Greek sailors boiled water to evaporate fresh water away from the salt and Romans used clay filters to trap salt. The most matured and commercially implemented technologies include multi stage flash (MSF), multi effect distillation (MED), vapour compression distillation (VC), reverse osmosis (RO), ion exchange (IX), and electrodialysis (ED). These technologies, however, have their own drawbacks and limitations. For example, MSF requires high capital cost and consumes more energy per cubic meter of product water than other technologies. RO, on the other hand, is susceptible to membrane fouling, has high maintenance and operation cost, and is unable to effectively desalinate brine water (i.e. TDS > 70,000 mg/L) due to elevated applied pressure requirement. To overcome the shortcomings of the readily available commercial technologies, researchers and engineers around the world have been working on new and unconventional desalination technologies. These technologies include novel membrane based processes such as forward osmosis (FO) and membrane distillation (MD). Additionally, adsorption and freezing are among other unconventional desalination technologies that have shown promise in recent years. This chapter discusses these emerging technologies and highlights their recent advancements and their potentiality for scale up and commercialisation.
Membrane Distillation (MD) is a thermally-driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. The driving force in the MD process is the vapour pressure difference induced by the temperature difference across the hydrophobic membrane. This process has various applications, such as desalination, wastewater treatment and in the food industry.This review addresses membrane characteristics, membrane-related heat and mass transfer concepts, fouling and the effects of operating condition. State of the art research results in these different areas will be presented and discussed.
This study aims to compare the effect of host hydrophilic polymer on novel hydrophobic/ hydrophilic composite membrane characteristics and desalination performance by direct contact membrane distillation (DCMD). Two different polymers are used for the host polymer: polyethersulfone (PES) and polyetherimide (PEI). The membranes were prepared by the phase inversion method by blending surface modifying macromolecules (SMM) into the host hydrophilic polymer (PES and PEI). The membranes were characterized using a wide variety of characterization techniques including the gas permeation test, measurement of the liquid entry pressure of water (LEPw), scanning electronic microscopy (SEM), atomic force microscopy (AFM) and contact angle measurement. Furthermore, the membranes were tested by DCMD for desalination of 0.5 M NaCl solution and the results were compared to commercial polytetrafluoroethylene (PTFE) membranes (FGLP 1425, Millipore). The effects of the type of host polymer on membrane morphology and characteristics were identified, which enabled us to link membrane morphology to membrane performance. The PES membrane yielded superior flux to that of the commercial membrane and the PEI membrane when their performance was compared. This result could be attributed to the fact that the nSMM/PES had a higher pore size/porosity ratio and lower LEPw than the nSMM/PEI membrane. It is worth mentioning that all prepared membranes were tested successfully for the desalination application. In other words, NaCl concentrations in the permeate were below 200 ppm.
In this Study, a novel modified PVDF-HFP/GO/ODS hollow fiber membrane fabricated by dry/wet jet spinning and used in direct contact membrane distillation. The precursor of 12 wt% polymer solution in NMP, as a solvent, prepared and mixed with different amounts of graphene oxide (GO) nanosheets (0, 1, 3 and 5%). Then the composite membranes were silanized using ODS in acidic condition. Various techniques such as SEM, AFM, FTIR, LEP, contact angle (CA) and tensile measurement tests were used to investigate the effects of both ODS and GO additions on the structure, surface chemistry, and performance of the membranes. The modified hollow fiber membrane exhibits a competitive permeation flux of 34.1 ± 1.1 kg m −2 h −1. In addition, the water contact angle and LEP were increased from 115°±1.5 and 112±4.5 kPa for the pristine PVDF-HFP membrane to 162°±1.7 and 181±5.3 kPa for modified PVDF-HFP/GO/ODS membrane, respectively. The incorporation of GO in the membrane matrix not only induces the hierarchy roughness on the membrane surface but also provides reactive hydroxyl sites which help the hydrolyzed silane coupling agent to form a robust uniform water-repellent film. Desalination of Caspian Sea water (CSW) showed that salt precipitation and permeation flux reduction partially occurred after 10 days for modified membranes. At last, the membranes recovered by chemical cleaning in HCl solution (pH=5) and used for another 10 days desalination.
To enhance the mechanical properties and wetting resistance of poly(vinylidene fluoride) (PVDF) membranes for membrane distillation (MD) of seawater, we have added n-butylamine modified graphene oxide (GO-NBA) in PVDF flat-sheet and hollow fiber membranes. For the first time, it is found that the addition of GO-NBA leads to PVDF flat sheet membranes with greater mechanical properties than that of conventional GO because of better compatibility, dispersity and crystalline structure. The addition of the GO-NBA also improves the mechanical properties of PVDF hollow fiber membranes in both tensile and hoop directions. With a small loading of 0.5 wt% GO-NBA in PVDF hollow fiber membranes, the maximum tensile stress, maximum tensile stain and Young's modulus increase by 32%, 63% and 71%, respectively, while the liquid entry pressure (LEP) and burst pressure jump by 67% and 15%, respectively. Under direct contact membrane distillation (DCMD), the 0.5 wt% GO-NBA incorporated membrane shows a flux of 61.9 kg m-2 h-1 and a salt rejection of 99.9% using a model seawater as the feed at 80 °C. This work might provide useful insights to fabricate robust MD membranes for seawater desalination.
Membrane distillation (MD) is an emerging thermal separation technique, using a hydrophobic microporous membrane. Initially, the process used MF membranes with a hydrophobic character, such as PTFE and PVDF membranes. These membranes are not wetted by aqueous feed liquids and their microporous structure allows the transport of water vapor molecules through the membrane due to an applied temperature difference. On average 23% of the publications on MD focus on membrane engineering. Besides the traditional stretching and phase inversion technique, many novel techniques are proposed, including electrospinning, the use of carbon nanotubes, surface modification techniques and the design of novel polymers. This article provides guidelines for the evaluation of these membranes measured using a large variety in process conditions at lab scale. In the second part, an overview of the synthesis methods used for MD was given and the advantages, disadvantages and the performance of the respective membrane types in MD were evaluated.
This study aims at further improvement and development of the novel hydro–phobic/–philic composite membranes which are made specifically for membrane distillation (MD). This was attempted by studying the effect of the casting conditions during the membrane preparation process by the phase inversion method. Two variables were chosen to study, which are the evaporation time before gelation and the gelation path temperature. Some of the membranes were allowed to evaporate at room temperature for 2 or 3 minutes to study the effect of evaporation time. The temperature of the gelation path was varied to 4°C, 20°C or 60°C in order to study the gelation path temperature effect. The prepared membranes were characterized using gas permeation test, measurement of the liquid entry pressure of water (LEPw), X–ray photoelectron spectroscopy (XPS), contact angle measurements and atomic force microscopy (AFM). The effects of the casting conditions on the membrane morphology were identified, which enabled us to link the membrane morphology to the membrane performance. The membranes were then tested for desalination of 0.5 M NaCl solution by direct contact membrane distillation (DCMD) and the results were compared to commercial polytetraflouroethylene (PTFE) membrane. It was found that the membrane which was prepared with no evaporation time produced better flux than those with evaporation time. Regarding the gelation path temperature; the membrane prepared with gelation path temperature of 4°C was better than those prepared with gelation path temperature of 20 or 60°C. It should be emphasized that the DCMD flux of the membranes prepared with no evaporation time or with a gelation path temperature of 4°C was superior to the commercial one. Furthermore, all the prepared membranes were tested successfully for the desalination application. In other words, no NaCl was detected in the permeate.
In this chapter, the fundamentals of membrane distillation are shown together with heat and mass transport equations. The commercial membranes used for membrane distillation are summarized. There are four major configurations that can be used for the membrane distillation process. For all these configurations, the literature says that pore wetting is the major drawback because it causes performance deterioration of the membrane. Hence, an attempt was made to develop a mathematical model to understand pore wetting theoretically in more detail. The model, developed on the basis of the force and mass balance of water in the membrane pore, should be considered as an initial attempt, for which further improvement is needed. It also should be noted that a similar model was developed earlier for pervaporation. Thus, it is the authors' attempt to understand membrane distillation and pervaporation from a single uniform view point.
In a gas–liquid membrane contactor, a larger pore size can result in a lower membrane mass transfer resistance. However, the membrane pore size is usually limited by the concern of pore wetting, e.g. a large pore size means a higher wetting tendency. As a breakthrough, this paper reported a porous polyetherimide (PEI) hollow fiber membrane with high surface porosity and large pore size to minimize the membrane mass transfer resistance by using a triple-orifice spinneret in the hollow fiber spinning process, and followed by a novel approach of fluorinated silica (fSiO2) nanoparticles (NPs) incorporation to make the membrane surface highly hydrophobic and chemical resistant to prevent the membrane from wetting caused by the large pore size on the membrane surface. The newly developed composite hollow fiber membranes showed the advancing contact angle value of 123.3°, receding contact angle value of 107.2°, and contact angle hysteresis of only 15.9°, indicating the high water resistant property. The composite membrane also exhibited a higher rigidity property compared with the original PEI substrate. The CO2 absorption flux of the composite membranes was investigated in both physical and chemical absorptions in a gas–liquid membrane contactor system. The membrane contactor showed a stable performance throughout the 60 d long-term operation using a 2 M sodium taurinate aqueous solution as the liquid absorbent.
Popular polymers such as polysulfone and poly(vinylidene fluoride) (PVDF) when electrospun into a membrane, have a much higher contact angle and hence are more hydrophobic when compared to the virgin polymeric material. In direct liquid penetration it is more beneficial if the membrane is hydrophilic so that the flux is not compromised and has less tendency to foul. Hence, this research is focused on generating a highly hydrophilic electrospun membrane (EM) based on PVDF material by blending this polymer with several different types of surface modifying macromolecules (SMMs) prepared from urethane pre-polymer with poly(ethylene glycol)s (PEGs) of various average molecular weights (400, 600, and 1000 Da) and poly(propylene glycol) of average molecular weights 3500 and 425 Da. One of the SMMs, with PEG 1000 Da, had a significant impact on the hydrophilic nature (0° SCA with water) of the EM as compared to the blend casted membrane. This could possibly be due to the orientation of the SMMs hydrophilic groups adopted during electrospinning on the surface, whereas they are either encapsulated or submerged in other SMMs. The water flux at a given pressure of blended EM was higher than the non-blended electrospun PVDF membrane. This study highlights the potential benefits of this new hydrophilic polymeric material in the membrane field, which can achieve high-flux rates at low pressure.
The effects of the polymer polyvinylidene fluoride (PVDF) concentration on the characteristics and direct contact membrane distillation (DCMD) desalination performance of self-sustained electrospun nano-fibrous membranes (ENMs) have been studied. Different polymer concentrations ranging from 15 to 30 wt% were considered in the solvent mixture N,N-dimethyl acetamide and acetone, while all other electrospinning parameters were maintained the same. Viscosity, electrical conductivity and surface tension of the polymer solutions were measured and the effects of the PVDF concentration on fiber diameter, thickness, water contact angle, inter-fiber space, void volume fraction, liquid entry pressure, mechanical and thermal properties of the ENMs were investigated. The minimum polymer concentration, critical chain entanglement concentration, required for electrospinning beaded fibers and the concentration needed for the formation of bead-free fibers were localized. Two groups of ENMs were identified based on the surface structure of the ENMs, their void volume fraction and inter-fiber space. Bead-free ENMs, prepared with PVDF concentration higher than 22.5 wt%, exhibit higher DCMD permeate flux than the beaded ENMs. Beaded ENMs can be used in desalination by DCMD. Among the prepared ENMs, the optimized membrane exhibiting the highest DCMD performance was prepared with 25 wt% PVDF concentration.
Low wettability is a vital characteristic for the membrane used in membrane contactor. Blending surface modifying macromolecule (SMM) in spinning dope is an interesting method to enhance the hydrophobicity of membrane and in this research, the effect of SMM on the properties and structure of polyetherimide (PEI) hollow fiber membrane in terms of mean pore size, liquid entry pressure of water (LEPw), membrane porosity and contact angle was investigated and compared with the properties of the PEI membrane without SMM.Furthermore the performance of PEI surface modified membrane in contactor applications in terms of CO2 absorption with distilled water was evaluated and compared with the absorption flux of PEI membrane without SMM and also, with the absorption flux of commercial and in-house made hydrophobic membranes which shows superior performance of surface modified PEI membrane e.g. in case of water in lumen side and pure CO2 in shell side of contactor and at Vliquid = 0.5 m s−1, the absorption flux of PEI surface modified membrane is 2.94 × 10−3 mol m−2 s−1 which is 114% higher than PEI without SMM and 73% higher than commercial membrane contactor, Celgard MiniModule® 0.75X5.
A novel surface modified polyetherimide (PEI) hollow fiber membrane was fabricated via dry–wet phased inversion process where the spinning dope contains Surface Modifying Macromolecule (SMM). The surface modified membrane exhibited large pore size, higher effective surface porosity, contact angle and porosity but lower Liquid Entry Pressure of water compared to polyetherimide hollow fiber membrane without SMM.The performance of surface modified membrane in contactor application for gas separation process was evaluated based on CO2 absorption process using water as absorbent and pure CO2 and 20:80 CO2/CH4 gas mixture. The results show that surface modified membranes have superior performance compared to commercial and in-house made hydrophobic membranes such as polypropylene and polytetrafluoroethylene, e.g., in the case of pure CO2 in the shell side and distilled water in the lumen side, the surface modified PEI membrane shows 72% higher absorption flux than commercial membrane contactor, Celgard MiniModule® 0.75X5 which is made of polypropylene membranes, when the liquid velocity is 0.4 m s−1.
A fluorinated surface modifying macromolecule (SMM) was synthesized and blended into the casting solution of polyetherimide used as host polymer. A composite porous hydrophobic/hydrophilic membrane was prepared by the phase inversion technique in a single casting step. The membrane was characterized by different techniques. During membrane formation, SMM migrates to the top membrane surface increasing its hydrophobicity and decreasing its pore size, nodule size and roughness parameters. The thickness of the porous hydrophobic top layer was found to be around 4 μm. The membrane was used for desalination by air gap membrane distillation and direct contact membrane distillation. The experiments were performed for different sodium chloride aqueous solutions and various operating conditions. The water production rate was found to be high for direct contact membrane distillation because of the low resistance to mass transport achieved by the diminution of the water vapour transport path length through the hydrophobic thin top-layer of the membrane.
Microporous hydrophobic poly(vinylidene fluoride) (PVDF) hollow fibers were prepared via dry-jet wet phase-inversion method using N,N-dimethylacetamide (DMAc) as solvent, LiCl and PEG-400 as non-solvent additives in the polymer dopes. The effects of various preparation conditions, including the concentration of polymer and additives, bore liquid temperature, air gap, take-up speed and dope extrusion rate on the morphology and properties of membrane were studied. The prepared membranes were characterized through scanning electron microscopy (SEM) observation, gas permeation measurement, and tensile property test. The permeate flux of membrane in both vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD) for desalination was tested. It is found that the permeation property of membrane is mainly determined by membrane porosity, especially effective porosity. The formation of the sponge structure reduced VMD flux more fiercely than DCMD flux. Under the synergetic effect of PEG-400/LiCl, high permeate flux and relatively high mechanical strength of membrane can be simultaneously achieved. There exist the best ranges of bore liquid temperature and of air gap distance for relatively high permeate flux of membrane.
In this study, the polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) composite is used to fabricate hollow fiber membranes for seawater desalination via direct contact membrane distillation (DCMD) application. The incorporation of PTFE particles in the formulated dope solution can efficiently suppress the formation of macrovoids and enhance the outer surface hydrophobicity. Dual-layer hollow fibers with a desirable macrovoid-free morphology and a relatively thin (13±2μm) outer-layer can be obtained via blending 30wt% of PTFE particles in the outer-layer dope. The resultant dual-layer hollow fiber (DL-30) displays a moderately high contact angle of 114.5° and porosity of 81.5%. Compared to the single-layer hollow fiber with 30wt% (SL-30) PFTE particles, the DL-30 fiber exhibits a flux enhancement of approximately 24% that is contributed to the reduction in inner-layer mass transfer resistance. Dual layer membrane configuration with a lower wall thickness as well as larger outer and inner diameters provides even higher water vapor transport is potentially suitable for desalination. Both single- and dual-layer PVDF/PTFE hollow fiber membranes reveal good long-term stability of up to 100h of continuous testing. By utilizing the state-of-the-art dual-layer spinning technology, hollow fiber membranes with better performance (i.e. enhanced flux) and morphology (i.e. macrovoid-free) can be tailored.
A fractional factorial design and a steepest ascent method were applied for possible fabrication of hollow fibers by the dry/wet spinning technique. Seven spinning factors were taken into account. Different concentrations of the copolymer poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP with 400,000g/mol molecular weight and the additive polyethylene glycol, PEG with 10,000g/mol molecular weight were dissolved in N,N-dimethyl acetamide, DMAC. The developed approach permits localization of the region of experimentation, defect-free spinning conditions, to produce hollow fibers. The obtained hollow fiber membranes were characterized by scanning electron microscopy and atomic force microscopy. Penetration liquid in membrane pores and porosity were also determined. Finally the membranes were tested for desalination by direct contact membrane distillation. An optimal hollow fiber membrane was finally fabricated using the determined optimum spinning conditions: a copolymer concentration of 20% w/w, a PEG concentration of 6% w/w, an air gap length of 25cm, an internal/external coagulation temperature of 37.5°C, an internal coagulant flow rate of 19ml/min, a pressure of 0.3bar and free falling. This membrane exhibits the highest performance index and the greatest global desirability (i.e. high permeate flux and salt rejection factor).
Hydrophobic polyvinylidene fluoride–polytetrafluoroethylene (PVDF–PTFEPVDF–PTFE) hollow fiber membranes for desalination via direct contact membrane distillation (DCMD) are successfully fabricated. The incorporation of PTFE particles (1μm) into the polymeric matrix enhances the hydrophobicity of the membranes, yielding a resultant water contact angle of 103°. FESEM analysis confirms the formation of macrovoid-free hollow fiber membranes with uniform particle distribution at 50wt.% PTFE loading. All the fabricated membranes exhibit narrow pore size distributions and have relatively small mean pore diameters between 0.116 and 0.308μm. Hollow fiber membranes spun at higher air gaps result in above 6% improvement in permeate flux because of the reduction in membrane wall thickness and greater surface porosity. The fabricated membranes demonstrate high thermal efficiency (EE) of above 80% when subjected to a hot feed solution of 80°C. PVDF–PTFE membranes spun at an air gap of 4cm and with 50wt.% particle loading demonstrate an optimal separation performance of 40.4kg/m2h permeation flux as well as ∼99.8% salt rejection at 80°C. This performance is comparable or even higher than the commercial hydrophobic membranes for DCMD studies.
For the first time, co-extrusion was applied for the fabrication of dual layer hydrophilic–hydrophobic hollow fibers especially for the direct contact membrane distillation (DCMD) process. The effect of different non-solvents on the morphology of the PVDF membranes was investigated and it was found that weak coagulants such as water/methanol (20/80, w/w) can induce a three-dimensional porous structure on PVDF membranes with high surface and bulk porosities, big pore size, sharp pore size distribution, high surface contact angle and high permeability but rather weak mechanical properties. Hydrophobic and hydrophilic clay particles were incorporated into the outer and inner layer dope solutions, respectively, in order to enhance mechanical properties and modify the surface tension properties in the membrane inner and outer layers. Different membrane characterizations such as pore size distribution, gas permeation test, porosity and contact angle measurements were carried out as well. Ultimately, the fabricated hollow fibers were tested for the DCMD process and flux as high as 55kg/(m2h) at 90°C was achieved in the test. This performance is much higher than most of the previous reports, indicating that the application of dual layer hydrophilic–hydrophobic hollow fibers may be a promising approach for MD.
Polyvinylidene fluoride (PVDF) microporous flat membranes were cast with different kinds of PVDFs and four mixed solvents [trimethyl phosphate (TMP)–N,N-dimethylacetamide (DMAc), triethyl phosphate (TEP)–DMAc, tricresyl phosphate (TCP)–DMAc, and tri-n-butyl phosphate (TBP)–DMAc]. The effects of different commercial PVDFs (Solef® 1015, FR 904, Kynar 761, Kynar 741, Kynar 2801) on membrane morphologies and membrane performances of PVDF/TEP–DMAc/PEG200 system were investigated. The membrane morphologies were examined by scanning electron microscopy (SEM). The membrane performances in terms of pure water flux, rejection, porosity, and mean pore radius were measured. The membrane had the high flux of 143.0 ± 0.9 L m−2 h−1 when the content of TMP in the TMP–DMAc mixed solvent reached 60 wt %, which was 2.89 times that of the membrane cast with DMAc as single solvent and was 3.36 times that of the membrane cast with TMP as single solvent. Using mixed solvent with different solvent solubility parameters, different morphologies of PVDF microporous membranes were obtained. TMP–DMAc mixed solvent and TEP–DMAc mixed solvent indicated the stronger solvent power to PVDF due to the lower solubility parameter difference of 1.45 MPa1/2 and the prepared membranes showed the faster precipitation rate and the higher flux. The less macrovoids of the membrane prepared with TEP (60 wt %)–DMAc (40 wt %) as mixed solvent contributed to the higher elongation ratio of 96.61% ± 0.41%. Therefore, using TEP(60 wt %)–DMAc (40 wt %) as mixed solvent, the casting solution had the better solvent power to PVDF, and the membrane possessed the excellent mechanical property. The microporous membranes prepared from casting solutions with different commercial PVDFs exhibited similar morphology, but the water flux increased with the increment of polymer solution viscosity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique using different copolymer concentrations in the dope solutions ranging from 17 to 24wt.%. All the spinning parameters were maintained constant except the copolymer concentration. The morphological properties of the hollow fiber membranes were studied in terms of scanning electron microscopy (SEM), atomic force microscopy (AFM) and void volume fraction. The effects of PVDF-HFP content in the spinning solutions were also studied by measuring the water entry pressure and direct contact membrane distillation (DCMD) permeate flux of the hollow fiber membranes. An increase in the copolymer concentration of the spinning solution resulted in a decrease in the precipitation rate and a transition of the cross-section structure from a finger-type structure to a sponge-type structure. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by AFM. It was observed that the pore size decreased in both the internal and external surfaces of the hollow fiber membranes with increasing the copolymer concentration and reached a minimum value at the outer surface for PVDF-HFP concentrations greater than 20wt.%. Water entry pressure values were decreased whereas both the void volume fraction and the DCMD permeate flux increased with decreasing the copolymer concentration.
A new type of surface modifying macromolecule (nSMM) was designed and synthesized to incorporate polydimethylsiloxane (PDMS) component in its structure. Gel permeation chromatography (GPC) and elemental analysis were used to characterize the synthesized nSMM. Also nSMM blended polyethersulfone (PES) membranes were prepared with different compositions, evaporation temperature and evaporation period. Prepared membranes were characterized by contact angle measurement, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and pure water permeation (PWP) test. According to the results, nSMM migrated to the surface and effectively increased the surface hydrophobicity of PES membranes when blended. Increase in liquid entry pressure of water (LEPw) with an increase of surface contact angle was also observed.
Novel composite membrane distillation membranes were prepared by blending the hydrophilic polysulfone with hydrophobic surface modifying macromolecules (SMMs). Three different types of SMMs were tested. These SMMs were synthesized and characterized for fluorine content, molecular weights and glass transition temperature. Phase inversion method in a single casting step was used to prepare the composite membranes. The membranes were characterized by means of different techniques such as contact angle measurement, gas permeation test, liquid entry pressure of water and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation (DCMD). Different membrane preparation conditions affecting membrane morphology, structure and DCMD performance were investigated. The parameters studied were the SMMs type, polysulfone concentration, solvent type and non-solvent additive concentration in the casting solution. Attempts linking the membrane morphology to its DCMD performance have been made. It was found that increasing the polymer concentration or the non-solvent additive concentration decreased the permeate flux of the porous composite hydrophobic/hydrophilic membranes since the liquid entry pressure of water increased and the ratio of the membrane pore size time the porosity over the effective pore length (rɛ/Lp) decreased. Furthermore, the stoichiometric ratio of the SMMs, type of SMMs, was found to affect considerably the characteristics and permeate flux of the composite membranes. In general, the composite membranes with higher liquid entry pressure of water exhibited smaller permeate fluxes. Moreover, the obtained results were compared to those of a commercial polytetrafluoroethylene membrane and it was observed that some of the SMMs blended polysulfone membranes achieved better DCMD fluxes than those of the commercial membrane. A permeate flux 43% higher than that of the commercial membrane was achieved with 99.9% NaCl separation factor.
Two different types of hydrophobic surface modifying macromolecules (SMMs) were synthesized and characterized for fluorine content, average molecular weight and glass transition temperature. The synthesized SMMs were blended into polyetherimide (PEI) hydrophilic host polymer to form porous hydrophobic/hydrophilic composite membranes by the phase inversion method. The prepared membranes were characterized by the contact angle measurements, X-ray photoelectron spectroscopy test, gas permeation test, liquid entry pressure of water and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation (DCMD). Different parameters affecting the membrane preparation process were studied and their effects on the membrane morphology as well as on the membrane performance in desalination by DCMD were identified. These parameters include the SMMs type, SMMs concentration, solvent type and solvent evaporation time before gelation. An attempt to link the membrane morphology to its performance in DCMD is presented. This leads to better understanding of the effect of the membrane preparation on its performance. It was found that increasing the solvent evaporation time before gelation decreased the membrane flux since smaller pore sizes were observed. The membranes with higher contact angles and fluorine contents (more hydrophobic) exhibited smaller permeate fluxes. The membranes having sponge-like structures at the hydrophilic layer exhibited higher fluxes than those having finger-like structure at the hydrophilic layer. Moreover, the results were compared to a commercial polytetrafluoroethylene (PTFE) membrane. It was observed that most of the SMMs blended PEI membranes achieved better DCMD fluxes than those of the commercial membrane. A permeate flux 55% higher than that of PTFE was achieved. For both PTFE commercial membrane and all SMMs blended PEI membranes, the NaCl separation factor was found to be higher than 99% except for the PEI membrane prepared without SMMs.
We describe here a facile route for the in situ modification of the surface properties of fibers produced by electrospinning polystyrene containing small quantities of compatible polymer additives, end-functionalized with 1–3 fluoroalkyl groups. Such additives undergo spontaneous surface segregation during the electrospinning process, resulting in fibers with low surface energy, fluorine-rich, superhydrophobic surfaces. Surface properties were analyzed using static contact angle measurements (with water as the contact fluid) and X-ray photoelectron spectroscopy. We report the effect of a number of parameters on the surface properties of the resulting polystyrene fibers including the molecular weight and concentration of functionalized additive, the number of fluoroalkyl groups, the effect of annealing, and spinning solvent. The majority of the fibers were successfully produced using THF as the spinning solvent and fibers with a contact angle of 150° were attainable. However, preliminary investigations using a blend of polystyrene and 4 wt % of such an additive, end-functionalized with 3 C8F17 groups in a mixed solvent of DMF/THF (3:1 v/v), resulted in a mat of fibers with a superhydrophobic surface and a contact angle of 158°.
Highly porous hydrophobic hollow fiber membranes with high porosity and sandwich trilayer structure were specially designed to meet the requirements of direct contact membrane distillation (DCMD). Poly(vinylidene fluoride) (PVDF)/Cloisite clay composite hollow fibers were fabricated based on the dry-jet wet phase inversion mechanism by using water as both the external and internal coagulants. Membrane void fraction of up to 90% can be produced to improve the fibers’ thermal insulation and reduce vapor transport resistance. The fiber inner surface was full of streaky pores with pore size less than 1.0 μm in diameter, while the pores on the fiber outer surface were much smaller, less than 50 nm in diameter. This demonstrates that membrane pores fabricated at a nanoscale can achieve high water vapor permeation flux with 100% salt rejection. For example, the fabricated PVDF/clay composite hollow fiber was tested by desalinating a 3.5 wt % NaCl solution and permeation flux as high as 79.2 kg/(m2·h) (calculated on the fiber outer diameter) was achieved at the inlet temperatures of 81.5/17.5 °C. The performance shows almost no decay during 220-h continuous tests. The addition of clay particles may enhance the tensile modulus and improve long-term stability compared to those fibers without particles.
Hollow fibers were spun from a solution of surface-modifying macromolecule blended polyethersulfone in dimethyl acetamide by using dry-wet spinning method at different air gaps and at room temperature. The air gap was varied from 10 to 90 cm. The ultrafiltration performance of hollow fibers was studied by using aqueous solutions of polyethylene glycols and polyethylene oxides of different molecular weights. Significant difference in surface morphology between the inner and outer surface of the hollow fibers was observed by atomic force microscopy (AFM). Similar results were obtained by contact angle measurement and XPS. Mean pore sizes of the inner surface and outer surface were calculated from AFM images and compared with the pore sizes obtained from mass transport data. Pore size distribution curves were drawn from both data, i.e., from AFM images and mass-transport data, both methods gave similar results. Roughness parameters of the inner and outer surfaces and the sizes of nodular aggregates on both surfaces were measured. An attempt was made to correlate the above parameters with the performance of the membranes. Unexpected values of contact angles of both inner surface and outer surface were obtained. It was observed that the studied membranes could be put into two groups: (i) the membranes fabricated between 10 and 50 cm air gap and (ii) fabricated at higher than 50 cm air gap. A plausible mechanism for the unexpected results was discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 710–721, 2007
Surface-modifying macromolecules (SMMs) are oligomeric fluoropolymers synthesized by polyurethane chemistry and tailored with fluorinated end groups. In the literature, several formulations of SMMs have been developed and blended with base polymers of polyurethanes and polyethersulfone for surface modification. It has been shown that SMMs migrate to the surface and the fluorine end groups orient themselves toward the air–polymer interface, reducing the surface energy of the hydrophilic base polymer to values close to that of polytetrafluoroethylene (Teflon). Because only a small amount of SMMs was needed, the bulk properties of the base polymer remained relatively unchanged. The properties of the SMM polymers were characterized, including molecular weights, elemental analysis, and thermal transitions. The morphology and surface properties of the SMM-modified and unmodified membranes were assessed. The use of SMMs has been tested for use in ultrafiltration, pervaporation, and biomedical applications. SMM-modified membranes offer advantages over unmodified membranes and the use of SMMs will continue to be the focus of future studies. This study reviews the recent development of surface-modifying macromolecules (SMMs) and SMM-blended membranes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2902–2916, 2003
Hollow fibre membrane distillation (MD) modules have a more compact structure than flat sheet membrane modules, providing potentially greater advantage in commercial applications. In this paper, a high-flux asymmetrically structured hollow fibre MD module was tested under various conditions. The results show that increasing velocity and temperature are positive for flux, and salt rejection was more than 99% over the entire experimental range. The hollow fibre module also showed great variation in flux when altering the hot feed flow from the lumen side to the shell side of the fibre, and this phenomenon was analysed based on the characterisation of the asymmetric structure of the hollow fibre. The largest mass transfer resistance was determined to be in the small pore size skin layer on the outer surface of the membrane, and having the hot feed closest to this surface provided the greatest vapour pressure difference across this high resistance mass transfer layer. The results also show that placing the suction pump on the permeate outlet increased the flux by lowering the pressure within the pore and hence increased the rate of vapour mass diffusion. A maximum flux of 19 L m−2 h−1 was obtained at 85 °C when hot feed was entering the shell side, and the mass transfer coefficient was relatively constant across the entire temperature range when operated at high velocities. These outcomes suggest that asymmetric hollow fibre MD modules should be operated with hot brine feed closest to the high resistant skin layer, and that vacuum enhanced MD further increases vapour transport and flux.
We have studied the thermal and mechanical properties, miscibility and morphology, and separation performance of polybenzimidazole (PBI)/polyetherimide (PEI) composite hollow fiber membranes. These composite hollow fibers were wet-spun from dopes containing 25.6 wt% solids in N, N-dimethylacetamide (DMAc) with different PBI/PEI ratios. Water was used as the external coagulant, while either H2O or DMAc/H2O mixture was employed as the bore fluid. Thermal mechanical analyzer (TMA) data indicate that these composite hollow fibers are miscible blend membranes. The molecular interaction between PBI and PEI are so strong that their miscibility appears to be independent of bore fluid chemistry, bore fluid flow rate, methanol and hot ethylene glycol treatments. The Tg values of wet-spun PBI/PEI blend hollow fiber follow the theoretical prediction of the Fox equation. Both SEM photographs and gas permeation data indicate that an increase in PBI percentage in the spinning solutions resulted in a hollow fiber with a tighter morphology, a lesser layer of finger-like voids and a significantly lower gas permeance. The tensile strength of wet-spun PBI/PEI blend hollow fiber membranes seems to be independent of PBI concentration, while their elongation at break decreases with an increase in PBI concentration.
The stoichiometric ratio for the synthesis components of hydrophobic new surface modifying macromolecules (nSMM) was altered systematically to produce three different types of nSMMs, which are called hereafter nSMM1, nSMM2, and nSMM3. The newly synthesized SMMs were characterized for fluorine content, average molecular weight, and glass transition temperature. The results showed that fluorine content decreased with increasing the ratio of α,ω-aminopropyl poly(dimethyl siloxane) to 4,4′-methylene bis(phenyl isocyanate). The synthesized nSMMs were blended into hydrophilic polyetherimide (PEI) host polymer to form porous hydrophobic/hydrophilic composite membranes by the phase inversion method. The prepared membranes were characterized by the contact angle measurement, X-ray photoelectron spectroscopy, gas permeation test, measurement of liquid entry pressure of water, and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation and the results were compared with those of commercial polytetraflouroethylene membrane. The effects of the nSMM type on the membrane morphology were identified, which enabled us to link the membrane morphology to the membrane performance. It was found that the nSMM2/PEI membrane yielded the best performance among the tested membranes. In particular, it should be emphasized that the above membrane was superior to the commercial one. © 2009 American Institute of Chemical Engineers AIChE J, 2009
A systematic study of the air gap effects on both the internal and the external morphology, permeability and separation performance of polyvinylidene fluoride (PVDF) hollow fiber membranes has been carried out. The hollow fibers were prepared using the dry-jet wet spinning process using a dope solution containing PVDF/ethylene glycol/N, N-dimethylacetamide with a weight ratio of 23/4/73. Ethanol aqueous solution, 50% by volume, was used as internal and external coagulants. The inner and the outer surfaces of the prepared hollow fibers were analyzed by atomic force microscopy (AFM), while their cross-sectional structure was studied by scanning electron microscopy (SEM). Ultrafiltration experiments were conducted using non-ionic solutes of different molecular weights. The results show that both the pore sizes and nodule sizes have a log-normal distribution. The pore size, nodule size and roughness parameters of the inner and outer surfaces of the hollow fibers were affected by the air gap distance. Alignment of nodules to the spinning direction was observed. Experimental results indicate that an increase in air gap distance, from 1 to , results in a hollow fiber with a lower permeation flux and a higher solute separation performance due to the decrease of the pore size. AFM analysis reveals that the air gap introduces an elongational stress because of gravity on the internal or external surfaces of the PVDF hollow fibers. At low air gap distance, the inner surface controls the ultrafiltration performance of the PVDF hollow fiber membranes because of its lower pore size, while at high air gap lengths the inner pore size becomes larger than the outer pore size. The turning point was observed at an air gap distance of .
Polyamide 6 (PA6) nanofibers were prepared from formic acid solutions by using electrospinning technique. The fibers were smooth, defects free and with diameters smaller than 200 nm. Small amounts of a perfluorinated acridine were added as dopant to the feed solution to modify the wettability of the fibers. The effect of doping on the contact angle values is well apparent. The contact angle values go from 50° of pure PA6 to 120° when 6% of acridine is added. A comparison between fibers and films of pure and doped polyamide 6 was carried out in order to determine the effect of morphology on wettability. Thermal annealing near the Tg of the polymer promoted the segregation of the molecules to the surface, reaching contact angles of 131° with smaller amounts (4%) of acridine. The surface segregation was also promoted by time aging.
We have demonstrated the effect of wet and dry-jet wet spinning on the shear-induced orientation during hollow fiber membrane formation by characterizing the permeability, separation performance and thermomechanical properties of hollow fiber ultrafiltration membranes. Both wet-spun and dry-jet wet spun fibers were prepared by using the phase inversion process. An air gap of 1 cm was chosen for the dry-jet wet spinning process in order to minimize gravity effect. To generalize our conclusion, various hollow fiber UF membranes with different structures were prepared using six spinning dopes with different kinds of polymers, solvents and additives under different shear rates. Experimental results show that pure water flux, coefficient of thermal expansion (CTE) and elongation of the wet spun fibers are lower than that of the dry-jet spun fibers but separation performance, storage modulus, loss modulus and tensile strength of the wet spun fibers are higher. The results indicate that the wet spun fiber has smaller pore size and/or a denser skin than the dry-jet wet spun fiber. These results also confirm our hypothesis that the molecular orientation induced at the outer skin of the nascent fiber by shear stress within the spinneret can be frozen into the wet-spun fiber but relax in a small air gap region for the dry-jet wet-spun fiber. Experimental results strongly indicate that an air gap of 1 cm does make significant impact to the membrane performance. This conclusion arises from the fact that wet spun fibers has the greater shear-induced molecular orientation (from spinneret) than the dry-jet wet spun fiber and there is strong molecular relaxation in air gap. A hollow fiber UF membrane with high flux of 1220 L/h m2 bar can be prepared by the approach proposed in this paper.
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.
This paper presents an assessment of membrane distillation (MD) based on the available state of the art and on ourpreliminary analysis. The process has many desirable properties such as low energy consumption, ability to use low temperature heat, compactness, and perceivably more immunity to fouling than other membrane processes. Within the tested range, the operating parameters of conventional MD configurations have the following effects:(1) the permeate fluxes can significantly be improved by increasing the hot feed temperature (increasing the temperature from 50 to 70°C increases the flux by more than three-fold), and by reducing the vapor/air gap (reducing the vapor air gap thickness from 5 to 1 mm increase the flux 2.3-fold); (2) the mass flow rate of the feed solution has a smaller effect: increasing it three-fold increases the flux by about 1.3-fold; (3) the concentration of the solute has slight effect: increasing the concentration by more than five-fold decreases the flux by just 1.15-fold; (4) the cold side conditions have a lower effect (about half) on the flux than the hot side; (5) the coolant mass flow rate has a negligible effect; (6) the coolant temperature has a lower effect than the mass flow rate of the hot solution. Fouling effects, membranes used, energy consumption, system applications and configurations, and very approximate cost estimates are presented. The permeate fluxes obtained by the different researchers seem to disagree by an order of magnitude, and better experimental work is needed.
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.
The morphology of the inner and the outer surfaces of polyetherimide (PEI) hollow fibers were studied by atomic force microscopy (AFM). The effect of the spinning conditions, namely, the bore liquid flow rate on the surface characteristics was investigated. Bore liquid flow rate was varied between 0.1 and 0.4 ml/min. The pore sizes at the outer and inner surfaces of the hollow fibers were affected by the bore liquid flow rates. Bore liquid flow rates also influenced the ultrafiltration performances of the hollow fiber membranes. The alignment of nodule aggregates to the direction of bore liquid flow was observed.
Polyethersulfone hollow fiber membranes were prepared by the dry/wet spinning technique under different gas gaps, namely, air, oxygen, nitrogen, carbon dioxide and argon. All spinning parameters were maintained the same. The effects of the gas type on the morphological properties of the hollow fibers were studied in terms of atomic force microscopy and solute transport using ultrafiltration of the non-ionic solutes; i.e. polyethylene glycol and polyethylene oxide of different molecular weights. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by atomic force microscopy. Pure water permeation, performance ratio, mean pore size, pore size distribution, pore density, porosity and molecular weight cut off were obtained from solute transport analysis. The studied polyethersulfone hollow fiber membranes could be divided into two groups. A group of hollow fibers prepared under gases exhibiting higher molecular mass and lower thermal conduc
A new strategy to enhance the desalination performance of polyvinylidene fluoride (PVDF) hollow fiber membrane for membrane distillation (MD) via architecture of morphological characteristics is explored in this study. It is proposed that a dual-layer hollow fiber consisting of a fully finger-like macrovoid inner-layer and a sponge-like outer-layer may effectively enhance the permeation flux while maintaining the wetting resistance. Dual-layer fibers with the proposed morphology have been fabricated by the dry-jet wet spinning process via careful choice of dopes composition and coagulation conditions. In addition to high energy efficiency (EE) of 94%, a superior flux of 98.6 L m(-2) h(-1) is obtained during the direct contact membrane distillation (DCMD) desalination experiments. Moreover, the liquid entry pressure (LEP) and long-term DCMD performance test show high wetting resistance and long-term stability. Mathematical modeling has been conducted to investigate the membrane mass transfer properties in terms of temperature profile and apparent diffusivity of the membranes. It is concluded that the enhancement in permeation flux arises from the coupling effect of two mechanisms; namely, a higher driving force and a lower mass transfer resistance, while the later is the major contribution. This work provides an insight on MD fundamentals and strategy to tailor making ideal membranes for DCMD application in desalination industry.
Poly(methyl methacrylate) (PMMA) was electrospun in the presence of a low molecular weight, hyperbranched poly(ethylene imine) additive partially functionalized with perfluorinated and aliphatic end-groups (M(n) approximately 1600 g/mol). The additive exhibited surface segregation with an insignificant influence on the rheological behavior of PMMA solutions. A morphological transition from beaded electrospun fibers to uniform fibers was observed upon introduction of additive at low PMMA concentrations. XPS revealed a surface enrichment of fluorine and nitrogen, which are both present in the hyperbranched additive. Surface fluorine content depended primarily on the amount of additive in solution, and a dependency on the PMMA/additive weight ratio was not observed.