Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation

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.     

Innovative hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride (PVDF) composite membrane for vacuum membrane distillation

Though membrane distillation (MD) has gained more and more attention in the field of desalination, the wetting phenomenon was still a non-negligible problem. In this work, a method combined dip-coating and UV in situ polymerization for preparing hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride composite membranes. This composite membrane consisted of a top thin hydrophobic coating layer and hydrophilic substrate membrane. In terms of anti-wetting properties, contact angle and liquid entry pressure of all composite membranes (except for those based on 0.45 μm) exceeded 160° and 0.3 MPa, respectively. In particular, the desalination performance was tested in vacuum membrane distillation tests by feeding 3.5% (mass) saline solution (NaCl) at 60 ℃. The composite membranes with larger support pore size and lower PFPE content had higher membrane distillation flux. And for stability tests (testing the 0.22 μm membrane coated by 5% (mass) PFPE), the highest MD flux 29.08 kg·m^-2·h^-1 and stable salt rejection (over 99.99%) during the period. Except that, the effects of coating material concentration and pore sizes of substrate membrane were also investigated for surface morphology and topography, porosity, mechanical strength and pore size characteristics. This work provided a simple and effective alternative to prepare excellent hydrophobic composite membranes for MD applications.     

Preparation and characterization of novel poly(vinylidene fluoride) membranes using self-assembled dibenzylidene sorbitol for membrane distillation

We proposed a method for the preparation of novel poly(vinylidene fluoride) (PVDF) membranes with self-assembled 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS). The vapor-induced phase separation (VIPS) method with a dry/wet process was used to produce DMDBS/PVDF composite membranes. The resulting membranes exhibited the coexistence of PVDF cellular pores and crystalline particles. The DMDBS molecules self-assembled into nanofibril structures, and a large number of nanofibrils were found on the surfaces and in the cross-sections of the prepared membranes. The DMDBS nanofibril networks in the PVDF matrix acted as reinforcing materials that enhance the hardness and stiffness of the membranes. Moreover, because of the entangled DMDBS networks, a greater strain was required to induce sample failure; therefore, the ductility of the membranes increased with increasing amounts of DMDBS. In addition, in a membrane distillation process, our composite membranes exhibited a good permeate flux that was comparable to that of commercial PVDF membranes.     

Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.     

亲疏水性CuBTC/PVDF复合膜应用于膜蒸馏抗油实验

采用水热法合成亲水性的CuBTC金属有机骨架(MOFs)颗粒,采用聚乙烯醇(PVA)作为黏合剂,用抽滤的方法将CuBTC颗粒附载在聚偏氟乙烯(PVDF)膜上,之后用戊二醛(GA)对PVA进行交联,制备出表层亲水、底层疏水的CuBTC/PVDF复合膜。通过场发射电子显微镜、比表面积及孔径分析仪、接触角测量仪、孔径分析仪、X射线衍射仪等对CuBTC颗粒和不同CuBTC含量的复合膜的表面特征、结构形态和稳定性进行了表征。结果表明,CuBTC颗粒有着较大的比表面积和孔容,CuBTC颗粒可以牢固地抽滤在PVDF膜表面,热稳定性高且有较好的柔韧性。与抽滤前的PVDF膜相比,随着CuBTC颗粒的增多,膜厚度有所增加,孔径和孔隙率有所减小,但对其膜蒸馏膜通量的影响不大,且在CuBTC含量在0.6 g时表现出较好的性能。在以1 g/L原油和35 g/L氯化钠混合溶液为进料液对原膜和复合膜进行直接接触膜蒸馏抗油污实验,发现原膜很快被油污染堵塞毛孔,而复合膜具有良好的抗油污染能力,可以进行长期的膜蒸馏实验。     

Desalination by membrane distillation adopting a hydrophilic membrane

Direct contact membrane distillation (MD) by means of composite membranes with a PVA/PEG hydrophilic layeron a hydrophobic PVDF substrate has been developed for desalination. The effects of brine temperature, salt concentration, running time and the addition of ethanol on the flux of composite membranes have been investigated. Results showed that the flux of the composite membrane did not deteriorate by adopting an additional hydrophilic membrane although durability was obviously improved. More than 99% of the separation coefficient in one run was achieved with the conductivity of the produced fresh water in the range of 6-10 μs/cm. The flux of the composite membrane retained 91% flux of substrate at 70°C, being 23.7 kg/h·m². When the brine temperature rose to 70°C, the composite membrane showed a declined concentration polarisation, with a smaller C_mb/C_b (3.89) than that of the substrate (5.79). Although the flux decreased with the increase of brine concentration, it retained 64% flux of pure water at brine solution containing 20% NaCl and was kept almost constant until 25% NaCl. In the continuous running experiments, there was no obvious drop of flux, even after adding 25% ethanol to the brine and running overnight. It is expected that adopting a hydrophilic layer can prohibit the wetting problem that faced traditional MD with hydrophobic membranes.     

A new model for mass transfer in direct contact membrane distillation

The mass transfer process in direct contact membrane distillation (DCMD) for three kinds of membranes was measured. Water fluxes at different temperatures and the membrane distillation coefficients (MDC) for each membrane were obtained directly from experimental data. The fact that the MDC values of membranes with larger pore size increase with temperature indicates that Poiseuille flow plays an important role in the process of mass transfer through the membrane. Based on this conclusion, a three-parameter model, named the Knudsen diffusion-molecular diffusion-Poiseuille flow transition (KMPT) model, was developed to predict MDC and water flux for membrane distillation. The parameters of the KMPT model for each membrane employed in this study, by which MDC at various temperatures can be determined, were evaluated by a nonlinear regression. The values of MDC and water fluxes for each membrane predicted by KMPT model agree well with that obtained directly from the experiment results. A large contribution of Poiseuille flow to mass transfer was observed and can be attributed to the distribution of large pores in the membranes. The KMPT model also provides a method for estimation of the effect pore size using the ratio of the MDCs; the ratio of the Poiseuille flow to molecular diffusion MDC provides the best estimation.     

A new enhancement technique on air gap membrane distillation

The most serious concern in the application of air gap membrane distillation (AGMD) configuration is the low permeate flux caused by an additional transport resistance owing to the air gap, by the temperature and concentration polarization and by the surface fouling. This paper presents an innovative design of a low-cost and high efficient membrane module with an advanced enhancement technique in an AGMD configuration, which not only yields a much improved permeate flux but also requires no additional facility for the enhancement. The new module design includes a tangent directional and rotational inlet turbulent flow of hot feed and a partial contact between the membrane and the cooling plate in a small air gap. The concrete structure of the module is introduced in detail in the paper. Using this new module the permeate flux can be obtained up to 119 kg/m²h for tap water when the temperature of the hot and cold water is 77 °C and 12 °C respectively, which is about a 2.5-fold improvement over the traditional AGMD technique at the almost the same conditions. Within the range of our experimental study, the optimum partial contact area ratio is about 75–80%. Mechanistically, the tangent and rotational inlet turbulent flow can accelerate the diffusive process of mass and heat, reduce the boundary layer thickness of temperature and concentration and wash the membrane surface so as to improve the temperature and concentration polarization near the membrane surface and to raise the efficiencies of mass and heat transfer. Because of the partial contact between the membrane and the cooling plate with a large area, the main heat transfer and permeate condensation in the gap both are carried out on the contact area, which is very different from either the common AGMD or DCMD (direct contact membrane distillation) so as to reduce the transport resistance in the gap and thus to raise the permeate flux significantly. The new enhancement technique is also applied for the desalination of 15 wt.% salt water, which shows the similar improvement in permeate flux.     

Simulation of membrane distillation modules for desalination by developing user's model on Aspen Plus platform

This paper presents a simulation study of direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) for desalination. Simulation models are built on Aspen Plus^® platform as user defined unit operations for these two types of modules, respectively. Large scale modules for practical industrial applications are simulated and studied for the effects of design and operation variables, as well as the importance of heat and mass transfers of each phase. For each type of modules with heat recovery design, the response surface method (RSM) is applied to develop the performance-variables quadratic model, followed by the multivariable optimization. Optimal designs can realize separation efficiencies, defined as the ratio of water produced to the feed, of 8.2% and 5.8% for DCMD and AGMD, respectively.     

Preparation of PET threads reinforced PVDF hollow fiber membrane

PET threads were incorporated in the support layer of hollow fiber membrane in axial direction as a special reinforcement material for the purpose of improving the mechanical properties of PVDF hollow fiber membranes. It was found that the reinforcement threads had a limited effect on the separation-related properties of the membrane, such as porosity and pore size, but the tensile strength of the reinforced membrane was improved several folds. Also, the criterion of choosing reinforced fiber materials was suggested.     

膜蒸馏和膜吸收技术现状及发展

膜蒸馏和膜吸收是一种以蒸汽压差为推动力的新型高效的膜分离技术,作者介绍了膜蒸馏和膜吸收技术发展现状,机理及优缺点,并对其在有机废水处理中的应用及今后发展方向进行了论述。     

Preparation of novel Polyvinylidene fluoride/boehmite composite membrane made using nonsolvent induced phase separation method for arsenate ion removal from water

Polyvinylidene fluoride/boehmite composite membranes with hydrophilic surfaces were prepared using the nonsolvent induced phase inversion technique (NIPS) method to remove arsenate ions from water. The nanoparticles were added with different boehmite filler content. The contact angle of the bare membrane is 85⁰, and it is reduced to 53⁰, imparting hydrophilicity due to the presence of boehmite nanoparticles. The mechanical characteristics of the membranes have significantly improved. The BET and SEM results have shown that the average pore diameter gets reduced with boehmite addition, and surface area increases. Additionally, the use of nanoparticles enhanced the membranes' thermal stability. The nanofiltration unit is used to filter the arsenic-contaminated water. The transmembrane pressure of ~4 bar is applied to all the membranes, and arsenic rejection; the flux of the membranes was calculated. The membrane has shown the highest rejection of 55% with a flux of 3.5659 L/m².h.     

Fabrication of low thermal conductivity yttrium silicate ceramic flat membrane for membrane distillation

This paper reports on the successful fabrication of γ-Y₂Si₂O₇ membranes with low thermal conductivity, which is an important membrane property for achieving high performance in membrane distillation process. Single-phase γ-Y₂Si₂O₇ powder was first synthesized by calcination of SiO₂ and Y₂O₃ powders, with 3 wt% LiYO₂ as a sintering aid. The membrane was produced by tape-casting of a suspension of this powder. After sintering at 1300 °C for 4 h, a flat membrane was obtained, which had a thickness of 0.5 mm, 49% porosity, 0.9 μm pore diameter, and low thermal conductivity of 0.497 W/m⋅K at 32 °C, and 0.528 W/m⋅K at 100 °C. The obtained membrane presented hydrophobic features (water contact angle was 132°) after surface modification, which resulted in formation of a strongly adhered robust hydrophobic SiNCO nanoparticle layer on its surface. The resultant hydrophobic membrane was tested in water desalination experiments using a sweeping gas membrane distillation (SGMD) device. High water flux of 10.07 L⋅ m⁻²⋅ h⁻¹ was achieved for a 20 wt% NaCl feeding solution and a temperature at the feed of 90 °C. Stable water flux and rejection rates were recorded in long-term experiments (>400 h).     

Energetic Performance and Permeate Flux Investigation of Direct-Contact Membrane Distillation for Seawater Desalination

The performance of the direct-contact membrane distillation (DCMD) process in desalting Mediterranean seawater was investigated. Theoretical and experimental optimization of various operating parameters was conducted. The effects of temperature differences, feed velocity, and membrane characteristics were studied. When using commercial polyvinylidene fluoride membranes, the vapor transfer throughout the membrane pores is dominated by the Knudsen-molecular diffusion model. Maximum permeate flux was obtained when increasing temperature, feed velocity, membrane pore size, and porosity and decreasing membrane tortuosity and thickness. Thermal efficiency, gained output ratio, and specific thermal energy consumption were improved when increasing feed temperature. By application of the DCMD process to Mediterranean seawater, a high water quality was obtained.     

Exploring the spinning and operations of multibore hollow fiber membranes for vacuum membrane distillation

Hollow fiber membranes with a multibore configuration have demonstrated their advantages with high mechanical strength, easy module fabrication, and excellent stability for membrane distillation (MD). In this work, the microstructure of multibore fibers was optimized for vacuum MD (VMD). A microstructure consisting of a tight liquid contact surface and a fully porous cross‐section is proposed and fabricated to maximize the wetting resistance and VMD desalination performance. The new membrane exhibited a high VMD flux of 71.8 L m^?2 h^?1 with a 78°C model seawater feed. Investigations were also carried to examine various effects of VMD operational conditions on desalination performance. The 7‐bore membrane showed higher flux and superior thermal efficiency under the VMD configuration than the direct contact MD configuration. Different from the traditional single‐bore hollow fiber, the VMD flux of multibore membrane at the lumen‐side feed configuration was higher than that of the shell‐side feed due to the additional evaporation surface of multibore geometry. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1078–1090, 2014     

膜蒸馏脱盐中膜污染与膜润湿的研究进展

膜蒸馏作为一种脱盐的新兴技术受到广泛关注。然而,因为实际水质组分复杂,可能含有如表面活性剂、油类物质、易结垢盐和有机溶剂等污染物,导致一般疏水膜在长时间运行情况下极易发生膜污染或者膜润湿,最终造成膜通量或截留性能降低。本文首先简述了不同种类的膜污染和膜润湿的特点及形成原因,并分析了膜污染和膜润湿之间的区别和联系。对膜蒸馏过程中膜污染和膜润湿的监测和预测手段进行了简要介绍,最后针对膜蒸馏脱盐过程,重点介绍了近几年国内外预防膜污染和膜润湿的研究进展。研究者一般从污染物与疏水膜的相互作用力着手对疏水膜进行表面改性制备全疏膜和Janus复合膜,避免污染物在膜面的吸附以及抑制表面或孔道润湿。越来越多的研究人员采用致密亲水膜的渗透汽化脱盐来从根本上避免疏水膜带来的润湿。除此之外,对进料液进行预处理也能显著延迟膜的污染和润湿,如混凝/沉淀、膜过滤、煮沸、pH调控等,还可通过改变进料方式、辅助外加磁场等措施控制膜表面局部区域的流体力学状态,减少污染物的附着。适当的膜后处理措施也能恢复膜性能。最后,文章指出了解决膜蒸馏中膜污染和膜润湿的研究方向。     

In‐situ cross‐linked PVDF membranes with enhanced mechanical durability for vacuum membrane distillation

A novel and effective one‐step method has been demonstrated to fabricate cross‐linked polyvinylidene fluoride (PVDF) membranes with better mechanical properties and flux for seawater desalination via vacuum membrane distillation (VMD). This method involves the addition of two functional nonsolvent additives; namely, water and ethylenediamine (EDA), into the polymer casting solution. The former acts as a pore forming agent, while the latter performs as a cross‐linking inducer. The incorporation of water tends to increase membrane flux via increasing porosity and pore size but sacrifices membrane mechanical properties. Conversely, the presence of EDA enhances membrane mechanical properties through in‐situ cross‐linking reaction. Therefore, by synergistically combining the effects of both functional additives, the resultant PVDF membranes have shown good MD performance and mechanical properties simultaneously. The parameters that affect the cross‐link reaction and membrane mechanical properties such as reaction duration and EDA concentration have been systematically studied. The membranes cast from an optimal reaction condition comprising 0.8 wt % EDA and 3‐hour reaction not only shows a 40% enhancement in membrane Young's Modulus compared to the one without EDA but also achieves a good VMD flux of 43.6 L/m²‐h at 60°C. This study may open up a totally new approach to design next‐generation high performance MD membranes. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4013–4022, 2016     

膜蒸馏技术最新研究现状及进展

综述了新型分离技术——膜蒸馏技术的最新研究进展,对膜蒸馏的历史、分类,膜蒸馏用膜的制备、膜蒸馏过程中的传递现象、膜组件的优化设计及应用研究等进行了深入的评述,提出了今后亟待研究和解决的问题。     

Effect of nano-ZnO with different particle size on the performance of PVDF composite membrane

To evaluate the effect of nanoparticle size on the structure and performance of the membranes, polyvinylidene fluoride composite membranes modified by nano-ZnO with different particle size were successfully fabricated by the non-solvent induced phase inversion method. A series of tests, including contact angle measurement, porosity and pore size measurement, scanning electron microscope, atomic force microscopy, differential scanning calorimeter and filtration experiments were performed to characterise the composite membranes. The results showed that the size of nano-ZnO particle has an influence on the structure and performance of the composite membranes. The membrane with 90?nm nano-ZnO particle has abundant porosity, high hydrophilicity and low surface roughness. These features are responsible for its excellent permeability and antifouling property.     

Hydrophobic polyethersulfone porous membranes for membrane distillation

Membrane distillation (MD) is a thermal, vapor-driven transportation process through micro porous hydrophobic membranes that is increasingly being applied to seawater and brine desalination processes. Two types of hydrophobic microporous polyethersulfone flat sheet membranes, namely, annealed polyethersulfone and a polyethersulfone/tetraethoxysilane (PES/TEOS) blend were prepared by a phase inversion process. The membranes were characterized and their performances were investigated using the vacuum membrane distillation of an aqueous NaCl solution. The performances of the prepared membranes were also compared with two commercially available hydrophobic membranes, polytetrafluorethylene and polyvinylidene fluoride. The influence of operational parameters such as feed temperature (25–65 °C), permeate vacuum pressure (200–800 mbar), feed flow rate (8–22 mL/s) and feed salt concentration (3000 to 35000 mg/L) on the MD permeation flux were investigated for the four membranes. The hydrophobic PES/TEOS membrane had the highest salt rejection (99.7%) and permeate flux (86 kg/m²·h) at 65 °C, with a feed of 7000 ppm and a pressure of 200 mbar.