Vacuum Membrane Distillation Experiments Research Articles

Novel bentonite modified PVDF membrane for desalination of synthetic-produced water by vacuum membrane distillation

ABSTRACT Membrane distillation (MD) has emerged as a highly promising method for the treatment of intricate and heavily contaminated water sources, such as those containing oily wastewater. The present study endeavors to advance the field by designing and evaluating a novel nanocomposite membrane comprised polyvinylidene fluoride (PVDF) and Bentonite (BNT) for application in vacuum membrane distillation (VMD) processes targeted at treating oily wastewater. A series of five nanocomposite flat sheet membranes were meticulously fabricated, incorporating varying quantities of bentonite (ranging from 0 to 0.8 g). Systematic characterizations were meticulously undertaken to comprehensively elucidate the influence of BNT incorporation on the structural and functional attributes of the developed membranes. These encompassed an array of analytical techniques, including Fourier Transform Infrared Spectroscopy (FTIR), atomic Force Microscopy (AFM), X-Ray diffraction (XRD), field emission scanning electron microscopy (FESEM), contact angle measurements (CA), as well as assessments of membrane thickness, porosity, pore size, and pore size distribution. The findings distinctly showcased the successful integration of BNT within the PVDF matrix, which correspondingly led to discernable augmentations in membrane surface roughness, hydrophobicity, and porosity. Employing consistent operational conditions involving a feed temperature of 50°C, a vacuum pressure of 30 kPa, and a NaCl solution with a concentration of 14.75 g/L, VMD experiments were systematically conducted. The empirical outcomes revealed that the incorporation of BNT notably conferred substantial enhancements upon the nanocomposite membrane’s performance. The pinnacle achievement was evidenced in a flux rate of 34 L/m2.h and a remarkable NaCl rejection rate of 99.92%, a result achieved by the nanocomposite membrane incorporating the highest content of BNT additives.

Tailored PVDF membrane with coordinated interfacial nano/micro-structure for enhanced membrane distillation

Membrane distillation (MD) has attracted growing attention as a sustainable green membrane process technology for seawater and high saline water desalination, yet seriously challenged by the potential membrane wetting and fouling. Herein, a controllable approach to fabricate tailor-made MD membranes was proposed via combining in-situ growth of SiO2 nanoparticles with membrane surface activation and low surface energy substances. SiO2 nanoparticles distributed uniformly on the membrane surface and combined with the intrinsic microstructure of the membrane, a hybrid macro/micro-structure was constructed, further endowing the membrane with superhydrophobicity. The modified membranes exhibited excellent wetting-resistance properties and over 99.9 % salt rejection during vacuum membrane distillation (VMD) process. The surface wetting and anti-nucleation theory special for MD membranes was uncovered. This theory and VMD experiments synchronously indicated that the hybrid macro/micro structure with optimized nanoparticle size rendered the resulting fabricated membranes enhanced permeate flux and anti-fouling properties. The optimal L-F-3 membrane achieved high flux (16.08 kg/(m2·h)) under feed temperature of 60 °C with a water contact angle of 150° and LEP (liquid entry pressure) of 2.9 bar. Finally, the modified membranes possessed the enlarged effective evaporative surface and intensified the nucleation barrier as well as the interfacial nanoscale turbulent flow without potential scaling.

Evaluation of spacer-induced hydrodynamic mixing using particle image velocimetry: Impact on membrane distillation performance

Net-type spacers are used to promote hydrodynamic mixing in the feed channel of crossflow membrane filtration. The induced fluid flow mitigates concentration and temperature polarization effects on the membrane surface, leading to the enhancement of heat/mass transfer across the membrane. The objective of this research is to study the effect of spacer geometry on fluid flow and heat/mass transfer in the feed channel. The particle image velocimetry (PIV) technique was used to measure flow velocity in the feed channel with net-type spacers. Flow characteristics at a wide range of Reynolds numbers were described in terms of time-averaged mean velocity and turbulence kinetic energy (TKE). The PIV result revealed distinct profiles of mean velocity and TKE at different channel heights with bi-planar spacers. The spacer with the largest hydrodynamic angle of 120° (Net120) was found to be the most efficient in promoting flow unsteadiness because it led to the largest channel-averaged TKE among the selected spacers. The unsteady flow regime on Net120 was accompanied by the greatest pressure drop over the feed channel. Moreover, the effect of spacer-induced mixing on heat/mass transfer across the membrane was further evaluated via the flux measurement with vacuum membrane distillation experiments. In comparison with other spacers, Net120 resulted in a relatively higher flux because of the improved hydrodynamic mixing. These findings reveal the validity of the experimental approach in identifying the impact of key spacer design parameters on hydrodynamic mixing. Nusselt number was calculated to relate the hydrodynamic flow and thermal flow in the feed channel of membrane distillation, and a modified Nusselt number correlation was proposed to better predict convective heat transfer in the spacer-filled channel in VMD.

Sustainable ammonia recovery from anaerobic digestion effluent through pretreating the feed by biomass ash

Ammonia recovery from anaerobic digestion (AD) effluent by vacuum membrane distillation (VMD) is a sustainable and low-carbon footprint strategy for nitrogen fertilizer production and waste treatment. However, due to the good buffering performance of AD effluent, lots of alkali chemicals are consumed to adjust the pH value, thereby increasing the treatment cost. To reduce the consumption of alkali chemicals, biomass ash (BA) is proposed as the alternative to replace the alkali chemical for pretreating the AD effluent before ammonia recovery in this study. Compared with the case using NaOH for pH adjustment, the ammonia separation factor increases by 40 % for BA addition case. Meanwhile, the 30-h VMD experiment for AD effluent treatment is conducted to testify to the stability. Additionally, the high germination index (GI) value (0.84) illustrates the potential of ammonia-removed AD effluent in agricultural applications. Furthermore, the greenhouse emission of ammonia recovery from AD effluent pretreated with BA is only 2.06 kg-CO2 eq/kg, which decreases by 51 % compared with that of the Haber-Bosch process. Finally, BA addition adopted for AD effluent pretreatment has economic superiority to NaOH addition. Inferably, using BA to pretreat the AD effluent can be acted as a high-efficacy and sustainable candidate strategy for ammonia recovery.

A novel silver / activated - polyvinylidene fluoride - polydimethyl siloxane hydrophilic-hydrophobic Janus membrane for vacuum membrane distillation and its anti-oil-fouling ability

In the membrane distillation process, the anti-fouling ability of the membranes is one of the main factors that affect the use of the membranes. In this paper, the Silver/Polyvinylidene fluoride - Polydimethyl siloxane (Ag/PVDF-PDMS) hydrophilic-hydrophobic Janus composite membrane was prepared by chemical reduction method to construct the underwater oleophobic silver layer on the surface of the hydrophobic Polyvinylidene fluoride - Polydimethyl siloxane (PVDF-PDMS) membrane, the silver layer can enhance oil resistance and increase membrane flux. To improve the adhesion between the silver layer and the substrate, the PVDF-PDMS membrane was activated by palladium, and then also by chemical reduction method, the Silver/Activated - Polyvinylidene fluoride - Polydimethyl siloxane (Ag/a-PVDF-PDMS) Janus composite membrane was produced. The properties of the membranes were systematically studied by scanning electron microscope (SEM), X-ray diffractometry (XRD), mechanical properties, contact angle (CA) and vacuum membrane distillation experiments. Compared with PVDF-PDMS and Ag/PVDF-PDMS membranes, Ag/a-PVDF-PDMS Janus composite membrane is better in mechanical property, underwater oleophobic performance and long-term operation stability, after vacuum membrane distillation treatment of crude oil/NaCl emulsion for 10 h, the flux recovery rate could arrive at 85%. This composite membrane will have huge application potential in seawater desalination and sewage treatment.

Preparation and vacuum membrane distillation performance of a superhydrophobic polypropylene hollow fiber membrane modified via ATRP

Superhydrophobic polypropylene hollow fiber membranes were fabricated using single- and multi-step atom transfer radical polymerization methods. The degree of grafting was improved by intermittently adding equal doses of monomers. Membrane morphology and anti-wetting properties of the membranes produced using the two methods were thoroughly investigated to understand the influence of structure on vacuum membrane distillation (VMD) performance. Initial monomer dosages of 20% and 30% facilitated the production of superhydrophobic modified membranes with water contact angles of 167.9° and 179.0°, respectively. The samples with the highest liquid entry pressure were produced using an initial monomer dosage of 20%, and were selected for the VMD experiments. Testing was conducted for 21 h and indicated that the modification method significantly affected VMD performance. This was attributed to the prominent differences in morphology and pore distribution. The average flux of the membrane produced with 20% monomer using the single-step method was 11.01 kg/m2h, which was 4.53 kg/m2h higher than that of the unmodified membrane; the long-term stability was superior. Although the membrane produced using the multi-step method exhibited an excellent average flux of 12.71 kg/m2h, the performance was highly unstable, and flux suddenly dropped after 6 h. Furthermore, the rejection performance was unsatisfactory. Overall, the two new membranes showed pronounced distinctions in VMD performances, which agrees well with our hypotheses.

Vacuum membrane distillation for desalination: Scaling phenomena of brackish water at elevated temperature

Membrane fouling or scaling is one of the main operational problems plaguing membrane distillation (MD) process during desalination. Specifically, scaling by the inorganic compounds is a serious and also common problem in MD, particularly at high temperature. In lieu of this, the effects of temperature, inorganic compounds and the membrane matrix on the membrane fouling phenomena require further investigation. In this work, a commercial hydrophobic polypropylene (PP) hollow fibre membrane with a smaller pore size (0.1 µm) was performed for its scaling and wettability propensity under the elevated feed temperatures condition. Brackish water that contains inorganic compounds was used as feed. Based on the membrane fouling in the brackish water jar test study, it was found that the deposits on the membrane surface were predominantly magnesium hydroxide [Mg(OH)2], magnesium silicates (MgSiO3), magnesium sulphate (MgSO4), silica (SiO2), and organic matter traces. The critical temperature for the significant deposition of Mg-based foulants on the membrane surface is 333 K. In the VMD experiments with feed temperature of 333 K, the commercial PP hollow fibre with smaller pore size (0.1 µm) and lower contact angle (102°) suffered more severe magnesium-based surface scaling compared to the membrane with larger pore size (0.2 µm) and higher contact angle (128°). These findings are important for the subsequent operational design of the membrane in vacuum membrane distillation (VMD) for the desalination process.

Preparation of hydrophobic zeolitic imidazolate framework-71 (ZIF-71)/PVDF hollow fiber composite membrane for membrane distillation through dilute solution coating

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° 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.

Improved desalination properties of hydrophobic GO-incorporated PVDF electrospun nanofibrous composites for vacuum membrane distillation

Hydrophobic nanofibers have attracted great attention in the membrane distillation (MD) process because of their extremely high specific surface area. In this study, hydrophobic nanofibrous composites for vacuum membrane distillation (VMD) consisting of a hydrophobic polyvinylidene fluoride (PVDF) nanofibrous layer and a hydrophobic polypropylene (PP) nonwoven fabric (NWF) substrate were presented. PVDF nanofibrous layer was directly fabricated on the surface of PP NWF substrate by electrospinning technique. 1H, 1H, 2H, 2H -perfluorooctyltriethoxysilane (FTES) functionalized GO nanosheets were incorporated in the PVDF nanofiber layer during the electrospinning process to enhance the hydrophobicity and permeation water flux for VMD. Membrane surface morphology and composition were characterized through SEM and FTIR. The effects of FTES-GO nanosheets content on the VMD desalination properties were investigated. And the transfer mechanism of water vapors through the FTES-GO incorporated nanofibrous layers was also proposed. Compared with the original PVDF nanofibrous layer, the surface hydrophobicity, liquid entry pressure (LEP) and water permeability of the FTES-GO incorporated nanofibrous layers had all been improved. When FTES-GO content was 4 wt%, the WCA increased from 104.0° for the neat PVDF nanofibrous layer to 140.5° for the modified nanofibrous layer. The permeation water flux reached a maximum value of 36.4 kg m−2 h−1. It was two times the water flux of the original membrane and meanwhile the salt rejection remained above 99.9% (50 °C, 3.5 wt% NaCl aqueous solution and permeation pressure of 31.3 kPa). No obvious wetting phenomenon was observed for FTES-GO incorporated membrane during the continuous VMD experiment for 60 h.

Evaluation of commercial PTFE membranes for desalination of brine water through vacuum membrane distillation

A vacuum membrane distillation (VMD) model was developed and the VMD experiment of sodium chloride (NaCl) was carried out. Three different commercial hydrophobic PTFE flat-sheet membranes were used in our VMD experiments. Membrane characteristics, such as morphology, thickness, porosity, pore size distribution, hydrophobic and liquid entry pressure of water (LEPw) were investigated. The VMD experiments were carried out under different conditions of various feed temperatures, feed flow rates, feed concentrations and permeate side vacuum degrees. To validate the model, it was tested with an experimental case study of literature [Naidu et al., 2014]. The VMD model has been validated with the data obtained from our laboratory experiments and has been proved that the model can be used to predict the permeation flux on the overall process performance.

Effect of the exposure time on the structure and performance of hydrophobic polydimethylsiloxane–poly(vinylidene fluoride) membranes via a non‐solvent‐induced phase separation process in a clean room

ABSTRACTThe objective of this study was to investigate the effects of the exposure time on the properties and permeability of polydimethylsiloxane (PDMS)–poly(vinylidene fluoride) (PVDF) blend hydrophobic microporous membranes, which were fabricated via a non‐solvent‐induced phase separation process at 25 °C and 60% relative humidity in a clean‐room circumstance. For the prepared PDMS–PVDF membranes, the membrane morphologies were observed by scanning electron microscopy. Crystalline structures were observed by X‐ray diffraction. Pore structures were analyzed by membrane porosity and mean pore size. Hydrophobicity was measured by contact angle measurement, and the mechanical properties were characterized by tensile strength testing. Our study results show that with increasing exposure time from 10 to 110 s, all of the membranes showed a similar pore structure: a spongelike substrate layer with a thin realm of fingerlike structures under the top surface. Phase separation between PDMS and PVDF occurred. The membrane porosity and mean pore radius decreased, and the membrane thickness increased. The membrane hydrophobicity decreased, and the mechanical properties first increased and then decreased. In addition, vacuum membrane distillation experiments were conducted. With the increase in the exposure time from 10 to 110 s, the membrane permeate flux decreased from 16.54 to 6.65 kg m−2·h−1, and the salt rejection was higher than 99.9%. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43842.

High permeation flux polypropylene/ethylene vinyl acetate co-blending membranes via thermally induced phase separation for vacuum membrane distillation desalination

Isotactic polypropylene (iPP)/ethylene vinyl acetate (EVA) co-blending hydrophobic microporous membranes for vacuum membrane distillation (VMD) were prepared via thermally induced phase separation (TIPS), and the membranes were supported by a nonwoven polyester fabric with special cellular structures. These special cellular structures consist of two parts: a thin dense cellular layer and a thick loose cellular layer. These structures markedly decreased the resistance of mass transfer and improved flux performance during VMD. The co-blending membranes used for VMD led to superior long-term stability and excellent durability relative to neat PP membranes. This result can be attributed to the addition of EVA that significantly reduced membrane crystallinity. The thickness of the dense cellular layer and voids of the loose cellular layer showed a minimum and a maximum at a PP/EVA co-blending ratio of 6:1, respectively. Aqueous NaCl solutions (0.5mol/L) were used in the VMD experiment at a feed temperature of 70°C and a permeate side absolute pressure of 3kPa. The flux of the iPP/EVA co-blending membranes was 27.6kg/(m2h) within 36h of continuous operation, whereas that of the iPP membranes was only approximately 16kg/(m2h) within 6h of continuous operation under the same condition.

The heat and mass transfer of vacuum membrane distillation: Effect of active layer morphology with and without support material

Abstract The aim of this research was the analysis of heat and mass transfer in vacuum membrane distillation (VMD), specifically for a dead-end feed set-up. The influence of support material on the supported membranes VMD performance was identified. A mathematical model was proposed to evaluate the membrane/feed interface temperature, membrane tortuosity, membrane mass transfer coefficient, and temperature polarization coefficient (TPC). The model was solved by an excel solver based on experimental results of feed temperature, system pressure and the evaporative fluxes. The SEM images showed that the thickness of unsupported membrane was reduced by 42% after the VMD test. Pore shrinkage and tortuosity increase were also expected during this pore collapsing process. On the other hand, the cross-sectional views of the supported membranes did not show significant changes. These results show that the support material can help prevent the membrane pore channel structure from collapsing during the VMD experiment. TPC is close to unity at low feed temperatures. However, it decreases with an increase in evaporation flux as the feed temperature increases, or when a high flux supported membrane is applied.

Performance Investigation of O-Ring Vacuum Membrane Distillation Module for Water Desalination

A new O-ring flat sheet membrane module design was used to investigate the performance of Vacuum Membrane Distillation (VMD) for water desalination using two commercial polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) flat sheet hydrophobic membranes. The design of the membrane module proved its applicability for achieving a high heat transfer coefficient of the order of 103 (W/m2 K) and a high Reynolds number (Re). VMD experiments were conducted to measure the heat and mass transfer coefficients within the membrane module. The effects of the process parameters, such as the feed temperature, feed flow rate, vacuum degree, and feed concentration, on the permeate flux have been investigated. The feed temperature, feed flow rate, and vacuum degree play an important role in enhancing the performance of the VMD process; therefore, optimizing all of these parameters is the best way to achieve a high permeate flux. The PTFE membrane showed better performance than the PVDF membrane in VMD desalination. The obtained water flux is relatively high compared to that reported in the literature, reaching 43.8 and 52.6 (kg/m2 h) for PVDF and PTFE, respectively. The salt rejection of NaCl was higher than 99% for both membranes.

Preparation and characterization of PDMS-PVDF hydrophobic microporous membrane for membrane distillation

PDMS-PVDF hydrophobic microporous membranes were prepared through NIPS process for the vacuum membrane distillation (VMD). The influences of PDMS/PVDF mass ratio on the morphology, crystalline structure, pore structure, hydrophobicity, mechanical characteristics and permeability of the membranes were investigated. Compared with the finger-like pore PVDF membrane, PDMS-PVDF membrane had two kinds of pores which are finger-like pore and sponge-like pore and had higher porosity, bigger mean pore size, higher hydrophobicity, stronger mechanical properties and better VMD performances. The VMD experiment for a 20g/L NaCl aqueous was carried out at feed temperature 50°C and permeate-side vacuum 80kPa. Compared with PVDF membrane, PDMS-PVDF blend membrane exhibited striking advantages in permeation performances, and when PDMS/PVDF mass ratio was 1:5, the maximum permeate flux was 15.23kg/(m2·h), which was equal or even better than most results of the other studies.

Separation of ethylene glycol solution by vacuum membrane distillation (VMD)

The feasibility of VMD for concentration of ethylene glycol (EG) from a used coolant liquid is presented. VMD experiments were performed with water-EG mixtures using a flat-sheet polypropylene membrane at various temperatures and recirculation rates. Results show that the membrane almost completely rejects EG and the desired concentration was achieved. This is due to the vapor pressure of EG being negligible compared to that of water. Increasing temperature and recirculation rate increased flux with no effect on EG rejection. Increasing of feed flow rate higher than 0.67 had no strong effect on permeate flux.