Membrane Distillation Applications Research Articles

Enhancing flux rates and energy efficiency in membrane distillation through stearic acid-electrosprayed membranes

This study investigates the effects of stearic acid-electrosprayed coatings on commercial PTFE, PVDF, and PA membrane substrates for membrane distillation (MD). Drawing inspiration from prior research involving carbon nanotube modifications, stearic acid was selected for its ability to increase hydrophobicity and reduce heat transfer resistance. The stearic acid was electrosprayed from an ethanol solution onto the substrates, which were subsequently characterized by contact angle, liquid entry pressure, and thickness measurements. The results showed a notable flux increase of 131 % for PTFE and 17 % for PVDF, but a decrease of 56 % for PA. A heat transfer model suggests that stearic acid improves flux by lowering heat transfer resistance in PTFE and PVDF substrates. This study highlights the potential of stearic acid coatings to enhance MD performance, providing valuable insights into improving flux and energy efficiency in MD applications.

Novel post-heat treatment green biodegradable PLA@SiO2 nanocomposite membrane for water desalination

Membrane distillation (MD) has received significant attention because of its energy efficiency and ability to treat industrial wastewater and salty water. However, the synthetic nature of commonly used membranes raises environmental disposal concerns, requiring the development of alternative eco-membranes. To address this issue, a novel, eco-friendly membrane using polylactic acid (PLA) coated with green silica nanoparticles (SiO2 NPs), modified by post-heat treatment, was developed for effective direct contact membrane distillation (DCMD). Herein, two nanofibrous membranes were fabricated via electrospinning/electrospraying: one, PLA, with a 12.5 wt% PLA solution, and another, PLA@SiO2, with 2 % (w/v) SiO2 NPs coating. The PLA@SiO2 membrane demonstrated enhanced hydrophobicity. Various post-heat treatments were applied to optimize membrane properties, such as structure, pore size, hydrophobicity, liquid entry pressure (LEP), thickness, and thermal stability. In addition, the desalination performance of all membranes was evaluated using a DCMD unit for 6 h. Results showed that the heat-pressed then annealed PLA@SiO2 membrane (denoted M44) exhibited notable improvements, including an LEP of 128.7 kPa, a tunable pore size of 0.21 μm, and a contact angle of 135.9°. Furthermore, the M44 membrane demonstrated a porosity of 80.5 %, a stable permeate flux of 14.3 kg.m−2.h−1, and an exceptional salt rejection of 99.99 % over a 24 h DCMD test. This novel eco-membrane shows promising potential for efficient desalination and represents a significant advancement in sustainable MD applications.

Mathematical modeling and experimental analysis of the double-effect DCMD-heat pump integrated system

High energy consumption represents one of the hindrances in enabling large scale application of membrane distillation. In this work, experimental and simulation studies of a double-effect direct-contact membrane distillation integrated with a single-stage vapor-compression heat pump (DE-DCMD-HP) were carried out for concentrating tap water with the aim to evaluate the energy saving of the integrated system. It was found during our experiments that an auxiliary cooler must be added to remove the excess heat from HP to enable the integrated system to reach the steady-state condition. The temperature-heat flux plots were first utilized to analyze how HP and DE-DCMD affected each other and reveal their coupling mechanism. The simulation results showed that the increase in the first-effect feed inlet temperature, Tfi,1 and the flow rate, V caused the thermodynamic cycle line of refrigerant shift to higher temperature, which led to the increase of the input power of the compressor and the auxiliary cooler, ultimately affecting the water production mass rate, ṁd, the coefficient of performance, COP, the gain output ratio, GOR, and the specific energy consumption, SEC. The experimental and simulation results revealed that increasing Tfi,1 and V increased the permeate flux Nh and GOR and reduced the SEC. The performances for three different DCMD configurations were furthermore evaluated via experiments at constant feed temperature whereby the SEC decreased from 2168 kWh·t−1 for single-effect DCMD to 1085 kWh·t−1 for DE-DCMD to 257 kWh·t−1 for DE-DCMD-HP.

Molecular Dynamics Simulation of Membrane Distillation for Different Salt Solutions in Nanopores.

Nanoporous membranes offer significant advantages in direct contact membrane distillation applications due to their high flux and strong resistance to wetting. This study employs molecular dynamics simulations to explore the performance of membrane distillation in a single nanopore, mainly focusing on wetting behavior, liquid entry pressure, and membrane flux variations across different concentrations and types of salt solutions. The findings indicate that increasing the NaCl concentration enhances the wetting of membrane pores, thereby decreasing the entry pressure of the solution. However, at the same salt concentration, the differences in wetting and liquid entry pressure among various salts, including CaCl2, KCl, NaCl, and LiCl, are minimal. The presence of hydrated ions significantly reduces membrane flux. As the concentration of NaCl solutions increases, the number of hydrated ions rises, thereby lowering the membrane flux of the salt solution. Furthermore, the type of salt has a pronounced effect on the structure of hydrated ions. Solutions with Ca2+ and Li+ exhibit the smallest first-layer radius of hydrated ions. Under the same salt concentration, KCl solutions demonstrate the highest membrane distillation flux, while CaCl2 solutions show the lowest flux.

Insights into gypsum and silica scaling behaviors of dense Janus membranes in membrane distillation: The effect of varied surface properties

Dense Janus membranes (JMs) have emerged as potential candidates to address the tough scaling issue in hypersaline wastewater treatments for membrane distillation (MD). However, behaviors and mechanisms of various scalants on dense JMs have not been systematically investigated. Herein, we designed three dense JMs to simultaneously compare their anti-scaling properties against gypsum and silica in MD and focus on the critical role of varied surface property in scaling control. Results showed that dense JMs with highly positive gypsum-membrane interfacial energy and with strong electrical repulsion against silica species were more effective in resisting gypsum and silica, respectively. Scaled membrane characterization, quartz crystal microbalance monitoring, interfacial energy analysis and AFM force measurement revealed the distinct anti-scaling mechanisms. Highly hydrophilic surfaces without specific interactions with scaling ions provided robust thermodynamic barriers and less kinetically affected local supersaturations, preventing gypsum nucleation and growth. In contrast, positively charged surfaces favorably attracted negatively charged silica species with high reactivity, promoting subsequent polymerization with silicic acid that kinetically dominated silica scaling. Moreover, coexisting oversaturated gypsum and silica influenced the scaling behavior of each other, and surface properties of the preferentially formed scaled layer might also matter in scaling process. This study shall provide useful insights into the anti-scaling mechanisms of dense JMs in MD and highlight the potential of dense surface design as an effective scaling control strategy to pave the way for potential MD applications.

Concentration and crystallization of Microbial Xylitol from oil palm empty fruit bunch (OPEFB) using submerged direct contact membrane distillation (DCMD)

Xylitol possess high solubility in water and increase viscosity at high temperature, making the concentration process become challenging. This study demonstrated the applicability of Membrane Distillation (MD) in concentrating xylitol solutions and evaluates its performance at different conditions and feed compositions. MD was capable of concentrating pure xylitol solution to reach the supersaturated condition. However, when Oil Palm Empty Fruit Bunch (OPEFB) hydrolysate fermentation broth was used, fouling was observed due to the presence of impurities, despite the application of feed agitation. The batch crystallization of xylitol were conducted at 5 °C with a seed addition of 0.5–0.8%-wt. Higher seed concentrations were required to induce crystallization in OPEFB hydrolysate due to lower xylitol concentrations and impurity presence. Furthermore, impurities influenced the quality of the crystals, resulting in crystal purities of 98.98% and 94.56% in the synthetic solution and OPEFB hydrolysate, respectively. These findings indicated notable impact of crystallization temperature, seed addition, and impurity on xylitol crystallization.

Fabrication of cellulose nanocrystals-incorporated dense Janus membranes for enhanced desalination and oily saline wastewater treatment via membrane distillation

Conventional hydrophobic membranes used in membrane distillation (MD) face significant challenges, such as severe membrane fouling and wetting, when treating surfactant-containing oily wastewater. Current strategies to modify surfaces for anti-fouling and anti-wetting purposes are often complex and time consuming, potentially compromising the flux in MD. This study investigates the characteristics and performance of novel fabricated Janus membranes for the treatment of oily hypersaline wastewater in membrane distillation applications. Janus membranes, which have a dense polyamide layer containing cellulose nanocrystal nanoparticles, were synthesized using the reverse interfacial polymerization (R–IP) method on a polyvinylidene fluoride (PVDF) substrate. Characterization using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), water contact angle (WCA) revealed that the modified Janus membranes exhibited an enhanced hydrophilicity and structural integrity due to the incorporation of cellulose nanocrystal nanoparticles. The fabricated membranes showed a dense surface layer without visible pores or cracks, with micron-scale patterned structures and widely distributed wrinkles, particularly at higher concentrations of cellulose nanocrystal nanoparticles. The desalination performance was evaluated in an air gap membrane distillation setup, where the modified Janus membranes demonstrated higher water vapor fluxes and stable salt rejection, even in the presence of surfactants. The wettability and fouling resistance of the Janus membranes were evaluated, with the PVDF-RIP0.5C membrane showing the highest hydrophilicity and underwater oleophobicity, thereby preventing oil adhesion and membrane fouling. These results underline the potential of Janus membranes with incorporated cellulose nanocrystal nanoparticles for efficient desalination and treatment of oily saline wastewater in the MD process.

Developing high-flux dense Janus membrane with robust wetting and scaling resistance: A systematic comparison with Omniphobic membranes

Membrane wetting, scaling and low flux are challenging obstacles for membrane distillation (MD). However, the lack of advanced membranes capable of achieving both high flux and high wetting/scaling resistance constrains MD potential applications. Herein, a dense Janus membrane (JM) was facilely developed by co-depositing a dense PVA/PDA layer on a highly porous PTFE substrate to overcome this dilemma and systematically compared with omniphobic PTFE substrates with varied pore sizes for the first time. Results showed that, unlike flux-resistance “trade-off” observed for PTFE substrates, dense JM effectively resisted 0.4 mM SDS at a high flux of 56.8 LMH and retarded membrane scaling by 20 mM oversaturated gypsum. Robust wetting resistance was primarily attributed to the sieving effect of the sub-nanometer surface layer, which greatly protected substrate from SDS adsorption and feed intrusion, and was less affected by severe surface concentration polarization and relatively low substrate LEP than PTFE substrates. Strongly polar PVA/PDA layer provided a much more thermodynamically unfavorable continuous interface for gypsum adhesion than PTFE substrates, which played a key role in determining anti-scaling behavior. In contrast, gypsum still tended to rapidly cause membrane scaling in stagnant zones within PTFE substrates when flux was increased. This study shall provide new insights in designing high-performance membranes for robust hypersaline wastewater treatment, and highlighting dense JMs with a macroporous substrate and a highly hydrophilic dense surface layer as ideal alternatives.

In-line granular filtration for mitigating gypsum scaling in membrane distillation

Mineral scaling and scaling-induced wetting are major challenges for the widespread application of membrane distillation (MD) in saline wastewater treatment. For the first time, an in-line modified activated alumina (AA) granular filtration coupled with MD process was proposed to achieve effective control of gypsum scaling. Firstly, the AA modified by NaOH impregnation with tuned pore structure and surface characteristics were supplied as the filter media. The optimized MD performance was obtained using in-line filtration with modified AA (MAA) with 20 g/L NaOH and the ratio of AA and NaOH solution of 1:3 for the duration of 24 h. Compared with severe scaling without granular filter, the optimized performance with a final normalized flux higher than 0.7 and a permeate conductivity less than 5 μS/cm at 70 % water recovery was observed. The MAA with rich pore, scale-like structures and high Na loading showed a superior ability in capturing gypsum scaling due to the enhanced heterogeneous scaling and the retention of scaling. In-line granular filtration-MD was also effective in mitigating gypsum scaling and scaling-induced wetting in saline solutions (e.g., 7–12 g/L NaCl). Our findings provide guidance for surface design of granular filter media and scaling control in membrane desalination technology.

Conquering surfactant-induced partial wetting of commercial membrane in membrane distillation through in-situ water flushing

Surfactant-induced wetting impedes the practical implementation of membrane distillation (MD). Addressing this issue demands the development of an effective membrane cleaning strategy that can eliminate surfactants adhering to the membrane surface and restore the membrane hydrophobicity. However, current cleaning methods, such as direct drying and pressurized air backwashing, encounter challenges in thoroughly removing surfactants trapped within the pores while preserving the structural integrity of the membrane. This work presents a refined approach to conquer surfactant-induced wetting in MD by water flushing. Utilizing ultrasonic time domain reflectometry and optical coherence tomography techniques, we identified a critical cleaning depth and showed that the hydrophobicity of a partially wetted membrane can be fully recovered by water flushing when the wetting depth is below the critical threshold. Theoretical models evidenced that in instances of low water temperature and low flow rate conditions, relatively high critical cleaning depths can be realized, thereby expanding the operational scope for achieving complete hydrophobicity recovery. Our results demonstrated the applicability of water flushing to commercial membrane modules without necessitating any modification, emphasizing its substantial potential for advancing MD applications.

Gravity-driven membrane integrated with membrane distillation for efficient shale gas produced water treatment

Substantial volumes of hazardous shale gas produced water (SGPW) generated in unconventional natural gas exploration. Membrane distillation (MD) is a promising approach for SGPW desalination, while membrane fouling, wetting, and permeate deterioration restrict MD application. The integration of gravity-driven membrane (GDM) with MD process was proposed to improve MD performance, and different pretreatment methods (i.e., oxidation, coagulation, and granular filtration) were systematically investigated. Results showed that pretreatment released GDM fouling and improved permeate quality by enrich certain microbes’ community (e.g., Proteobacteria and Nitrosomonadaceae), greatly ensured the efficient desalination of MD. Pretreatment greatly influences GDM fouling layer morphology, leading to different flux performance. Thick/rough/hydrophilic fouling layer formed after coagulation, and thin/loose fouling layer formed after silica sand filtration improved GDM flux by 2.92 and 1.9 times, respectively. Moreover, the beneficial utilization of adsorption-biodegradation effects significantly enhanced GDM permeate quality. 100 % of ammonia and 53.99 % of UV254 were efficiently removed after zeolite filtration-GDM and granular activated carbon filtration-GDM, respectively. Compared to the surged conductivity (41.29 μS/cm) and severe flux decline (>82 %) under water recovery rate of 75 % observed in single MD for SGPW treatment, GDM economically controlled permeate conductivity (1.39-19.9 μS/cm) and MD fouling (flux decline=8.3 %-27.5 %). Exploring the mechanisms, the GDM-MD process has similarity with Janus MD membrane in SGPW treatment, significantly reduced MD fouling and wetting.

Novel catalytic membrane based on γ-FeOOH@PVDF/peroxymonosulfate for efficient ammonia recovery: Self-cleaning mechanism and emerging organic contaminant degradation performance

The resolution of membrane fouling issues is crucial for the commercial application of membrane distillation (MD) ammonia recovery. The peroxymonosulfate-based advanced oxidation process (PMS-AOP) exhibited conspicuous degradation of organic pollutants, and in-situ coupling with the MD process could probably achieve desirable antifouling performance. In this research, a novel γ-FeOOH@PVDF catalytic membrane was fabricated by gas–liquid self-deposition method and fluorination treatment. The γ-FeOOH nanosheets arrayed on the substrate membrane endowed the catalytic membrane with superhydrophobicity. The MD ammonia recovery of the digestate achieved a high recovery rate (97.3%), and no membrane fouling or wetting phenomena were detected by real-time monitoring by optical coherence tomography (OCT). The batch degradation experiments of thiamphenicol (TAP) validated the γ-FeOOH@PVDF/PMS system with superior degradation performance and stability. The electron paramagnetic resonance (EPR) coordinated free radical quenching experiment determined the active substance (SO4•-, 1O2, O2•- and •OH) in the degradation process. In addition, the potential self-cleaning mechanism of the radical and non-radical pathways of γ-FeOOH@PVDF was elucidated via density functionaltheory (DFT) calculation. The quinone groups in the organic pollutants facilitated the redox of Fe(Ⅲ)/Fe(Ⅱ) and accelerated the activation of PMS. Hence, the catalytic membrane developed in this research provided a novel instructive strategy for membrane fouling control and also exhibited significant potential of coupling catalysis and membrane technology for water treatment.

Potential of membrane distillation for water recovery and reuse in water stress scenarios: Perspective from a bibliometric analysis

Climate change and related issues such as water scarcity and the urgent need for a transition to sustainable energy sources have motivated the development of novel processes and techniques that can be applied for water treatment, recovery and reuse. Among these emerging processes, membrane distillation in its different configurations has been extensively studied and reported in the literature. A bibliometric analysis was performed to investigate the research documents published in the scientific database Scopus until 2024, related to use of membrane distillation for water recovery and reuse. The main quantitative attributes of the research during this period were identified. The results of this analysis showed a very low scientific production until 2005, followed by a rapid exponential increase since then. The combined contribution of the two most productive countries (China and USA respectively) accounted for more than 41 % of the total number of publications, followed by Australia and Saudi Arabia, which occupied the third and fourth positions in the ranking. Chemical engineering was the most important subject category (57.4 % contribution), closely followed by environmental sciences. The review of the most cited documents and keywords showed that membrane wetting is one of the main drawbacks that can reduce the performance of membrane distillation and the use of innovative superhydrophobic, omniphobic or Janus membranes resulted an adequate solution to avoid this problem. Moreover, the desalination of seawater, brines and other highly saline solutions can be considered as the most studied application of membrane distillation. The consideration of the energetic aspects of membrane distillation and its coupling with other technologies were identified as relevant topics during the bibliometric network analysis. Special attention was given to the potential of membrane distillation for water recovery and reuse in the Chilean context of water stress for the production of drinking water and the treatment of domestic greywater and wastewater from mining and other industrial activities.

Mathematical Modeling of NaCl Scaling Development in Long-Distance Membrane Distillation for Improved Scaling Control.

Membrane distillation is a novel membrane-based separation technology with the potential to produce pure water from high-salinity brine. It couples transport behaviors along the membrane and across the membrane. The brine in the feed is gradually concentrated due to the permeate flux across the membrane, which is a significant factor in initiating the scaling behavior on the membrane surface along the feed flow direction. It is of great interest to investigate and estimate the development of scaling on the membrane surface. This work specifically focuses on a long-distance membrane distillation process with a sodium chloride solution as the feed. A modeling approach has been developed to estimate the sodium chloride scaling development on the membrane surface along the flow direction. A set of experiments was conducted to validate the results. Based on mathematical simplification and analytical fitting, a simplified model was summarized to predict the initiating position of sodium chloride scaling on the membrane, which is meaningful for scaling control in industrial-scale applications of membrane distillation.

Enhanced water permeability and antifouling properties of cross-linked graphene oxide composite membranes with tunable d-spacings

Low flux and membrane fouling are major challenges in membrane distillation (MD) for treating concentrated wastewater. This study compared the performance of GO composite membranes with different interlayer distances, by covalently bonding the terminal amine groups of ethylenediamine (EDA) and 1,12-diaminododecane (DADD) with the carboxyl groups present on the GO sheets. The results indicated that the GO-DADD/PTFE membrane, with longer carbon chain cross-linking, achieved the highest flux and antifouling properties. At 70 °C, the pure water flux reached 68.5 kg/m2·h, and the fluxes for treating NaCl, HA containing NaCl, and BSA containing NaCl were 29.8 %, 37.1 %, and 38.5 % higher than the uncrosslinked GO/PTFE, and 95.7 %, 121.2 %, and 146.9 % higher than the PTFE membrane, respectively. Through mathematical models for mass and heat transfer, the study identified the key to this enhancement as the increased d-spacing within the GO layer due to cross-linking, which weakened the Kelvin effect and enhanced the vapor partial pressure on the hot side. The unique surface structure and electrostatic interactions induced by long-chain cross-linking further boosted the antifouling effect. These modifications not only overcome the typical trade-off between retention rates and flux but also offer a scalable and efficient solution for advanced membrane distillation applications.

Electrospun bio-polymeric nanofibrous membrane for membrane distillation desalination application

Polylactic acid (PLA) has been explored as a sustainable material for membrane distillation (MD) membranes using electrospinning for the first time. Adjusting the electrospinning parameters of PLA nanofiber, such as concentration, voltage, collector distance, humidity, and flow rate, resulted in uniformly distributed and smaller nanofibrous structures. Heat treatments were also performed to enhance the tackiness and fusibility of the fibers within the nanofibrous structure, optimizing the pore size, hydrophobicity, and surface roughness of the PLA membranes. The hierarchical structure of the PLA membranes improved the liquid entry pressure (LEP) and mechanical strength by 320 % and 230 %, respectively. The membrane characteristics were assessed in terms of porosity, pore size, surface roughness, and contact angle, which determined the impact of the wettability of the membrane under different conditions. The results showed that under certain constraints, PLA membranes could be effectively used in the air gap membrane distillation (AGMD) process, achieving a permeated flux of ~2 kg/m2h and a salt rejection of 99 %. However, PLA-based nanofibrous membranes are unsuitable for MD applications at feed temperatures above 60 °C and a flow rate of 60 LHP. This study provides a strong basis for developing and manufacturing eco-friendly bio-based nanofibers materials for desalinating saltwater.

Preparation of PVDF-nSiO2/PVSQ high-flux composite membrane by chemical grafting and separation of composite brine by membrane distillation

The large-scale application of membrane distillation (MD), an important method for desalting seawater, is limited due to membrane fouling. In this study, ethyl orthosilicate (TEOS) was polymerized to obtain different nSiO2 particles. Vinyl triethoxysilane (VTES) was hydrolysed and polycondensation was used to produce spherical polyethylene-sesquioxane (PVSQ) microparticles, which were modified to construct micro/nano particles. Then grafted on the surface of polyvinylidene fluoride (PVDF) composite membrane. The low-surface-energy hydrophobic groups present on the surfaces of the vinyl and ethoxy particles greatly enhanced the membrane hydrophobicity. The particle size of nSiO2 can change the properties of the membrane. The separation performance of the membrane was tested by direct contact membrane distillation (DCMD). In the separation of a constant concentration of feed solution (3.5 wt% NaCl), the membrane flux was relatively stable at 23.8 kg·m−2·h−1, and the salt interception rate reached 99.99 %. In the subsequent separation of a salt/humic acid solution, the flux remained stable and the good anti-fouling performance indicating that this system outperformed the unmodified membrane. Finally, the effect of different cations on the water diffusivity was studied using a molecular simulation method. Both Ca2+ and Mg2+ reduced the diffusion coefficient of water. However, when these two ions were present simultaneously, Ca2+ inhibited the binding of Mg2+ to water, and the diffusion coefficient of water increased. This study provides a novel approach for preparing hydrophobic membranes for the separation of complex brines.

Eco-Friendly Superhydrophobic Modification of Low-Cost Multi-Layer Composite Mullite Base Tubular Ceramic Membrane for Water Desalination

This study aimed to investigate and develop a cost-effective and superhydrophobic ceramic membrane for direct contact membrane distillation (DCMD) applications. Two types of mullite-based composite membranes were prepared via extrusion and sintering techniques. To create a small and narrow pore diameter distribution on the membrane surface, the dip-coating technique with 1 µm alumina was employed. The hexadecyltrimethoxysilane eco-friendly grafting agent was adopted to modify low-cost multilayer mullite-based composite membranes, transforming them from hydrophilic to superhydrophobic. The prepared membranes were characterized via field emission scanning electron microscopy (FESEM), energy-dispersive spectrometry (EDS), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), liquid entire pressure (LEP), contact angle, atomic force microscopy (AFM), porosity, and membrane permeability. The results of the prepared membranes validate the appropriateness of the material for membrane distillation applications. The optimized membrane, with a contact angle of 160° and LEP = 1.5 bar, was tested under DCMD using a 3.5 wt.% sodium chloride (NaCl) synthetic solution and Persian Gulf seawater as a feed. Based on the acquired results, an average permeate flux of 3.15 kg/(m2·h) and salt rejection (R%) of 99.62% were found for the 3.5 wt.% NaCl solution. Moreover, seawater desalination showed an average permeate flux of 2.37 kg/(m2·h) and salt rejection of 99.81% for a 20-h test without any pore wetting. Membrane distillation with a hydrophobic membrane decreased the turbidity of seawater by 93.13%.

Fabrication of PVDF membranes via VIPS-NIPS technique for water desalination: Effect of preparation condition

The operating cost of the desalination process can be decreased by using the membrane distillation (MD) process due to very low operating pressure and mild operating temperature. Attempts to find the proper membrane for application in MD attract the attention of many researchers. In this paper, PVDF powder was employed for assembling membranes via the VIPS method for operating in air-gap membrane distillation (AGMD). The influence of the ethanol concentration in the coagulation bath and VIPS time was studied. PVDF membrane characteristics were analyzed via pore size distribution, volumetric porosity, thickness, scanning electron microscope (SEM), measuring water contact angle, water flux, and salt rejection. A nodular structure was found for the membranes resulting from delayed demixing and solid-liquid phase separation. Moreover, the results indicated a larger membrane pore size was formed and the hydrophobicity was improved. The hydrophobicity and pore size are two important factors for membrane distillation that can affect and enhance membrane permeability. Moreover, studying the influence of nanoparticles showed that SiO2 nanoparticles are more effective than TiO2. The highest water contact angle was obtained when SiO2 nanoparticles were sprayed on the surface resulting in higher permeability and salt rejection in AGMD. The AGMD experiments' results showed that the membrane permeability increased from 0.3 to 2.41 L/m2.h, and salt rejection over 99.5 % was obtained.

Facile preparation of superhydrophobic PVDF/MWCNTs distillation membranes: Synthesis, characteristics and separation performance

In recent years, membrane distillation (MD) has emerged as an effective solution to mitigate the high energy consumption associated with conventional fermentable sugar concentration processes. However, the inherent low mechanical strength of traditional organic membrane surfaces can result in issues related to membrane wetting during long-term MD experiments. In this work, superhydrophobic PVDF/MWCNTs (multi-walled carbon nanotubes) distillation membranes with high mechanical strength were prepared by a combination of templating and co-blending techniques. The process involved peeling the semi-gelation membrane from a sandpaper template, subsequently forming a high aspect ratio microstructure on the membrane surface via tearing and pulling. Additionally, the incorporation of MWCNTs further bolstered the mechanical strength of the membrane. At a peeling time of 90 s and a MWCNTs content of 0.08 wt%, the distillation membrane exhibited an impressive water contact angle of 153.2°, indicating it possess exceptional hydrophobicity while maintaining high mechanical strength. The separation performance of the prepared distillation membranes has been inspected by conducting both experimental analyses and computational fluid dynamics (CFD) modeling of the entire MD process. An appropriate elevation in both feed flow rate and temperature could effectively mitigate the problem of temperature and concentration polarization on the membrane surface. PVDF/MWCNTs distillation membrane demonstrated a modest reduction in permeate flux of about 11.69 % and a stable rejection rate exceeding 99.95 % throughout a 100-hour operational period, and it required only 15 h to concentration of aqueous glucose solution to 100 g/L, significantly enhancing the efficiency of the process. This study also employed a combination of experiments and simulations to optimizes the process conditions guiding the vacuum membrane distillation (VMD) process and could provide guidance for the future application of membrane distillation in the concentration of sugar solutions.