Vacuum Membrane Distillation Performance Research Articles

Construction of robust silica-titania polydopamine composite membrane with light-absorption property for photothermal vacuum membrane distillation

A robust silica-titania polydopamine composite membrane was fabricated adding a photothermal polydopamine coating to the surface of polypropylene membrane for photothermal vacuum membrane distillation. Detailly, the surface of an inert PP membrane was modified with atomically controllable titanium and silicon to enhance the hydroxyl groups on the membrane surface, then, a photothermal effect layer of polydopamine was generated on the composite membrane surface through the self-polymerization reaction of dopamine. The polydopamine coating has significant light absorption and heating properties, and it can act as a local heater when exposed to light sources. This local heating effect significantly raises the temperature at the membrane interface, creating a thermal gradient that drives the evaporation of water molecules. Specifically, we quantitatively analyzed the three reasons (surface energy, free radicals, and photothermal effect) for the increase in evaporation rate of the pristion PP membrane caused by the membrane composite membrane. Among these reasons, the photothermal effect caused by black polydopamine is the most important factor for improving evaporation efficiency. The dark-illumination alternating experiment revealed that the PP-TiO2-SiO2-PDA composite membrane delivered a noteworthy enhancement in vacuum membrane distillation performance (approximately 40%), while the improvement of the PP membrane was only 1.7%.

Experimental Investigation of Vacuum Membrane Distillation (VMD) Performance Based on Operational Parameters for Clean Water Production

Freshwater shortage is an ongoing concern across the world, due to increasing populations and climate change. Vacuum membrane distillation (VMD) is a viable approach for producing fresh water to meet the needs of society. In the current study, an experimental investigation has been conducted on a laboratory-scale single-stage module to explore the impact of operational parameters such as feed temperature, vacuum pressure, and feed salinity on the performance of vacuum membrane distillation (VMD), including permeate flux, gained output ratio, and specific thermal energy consumption. Results show that increasing the feed temperature and feed flow rate, and reducing the salinity, increases the permeate flux. As the feed temperature rises from 60 to 70°C, the permeate flux increases from 1.90 to 4.36 kg/m²h at a permeate pressure of 12 kPa and salinity of 30 g/L. Similarly, increasing the vacuum pressure from 12 to 18 kPa reduces the permeate flux. As a result, the specific thermal energy consumption increases from 728 to 803 kWh/m³. From experimental findings, it was observed that the rejected brine from VMD retains sufficient energy that could be utilized in another desalination system.

Numerical Modelling and Performance Evaluation of Vacuum Membrane Distillation for Energy-Efficient Seawater Desalination: Towards Energy-Efficient Solutions

Vacuum membrane distillation (VMD) is a compelling technique for desalinating water because it exhibits superior pure water permeability at lower operating temperatures compared to other membrane distillation technologies. This leads to reduced energy consumption, lower heat loss via conduction across the membrane surface, and minimal heat transfer through conduction due to the low pressure on the permeate side. Detailed modelling of heat and mass transfer in VMD is essential for optimizing the process as it provides valuable insights that contribute to the advancement and successful implementation of seawater desalination using VMD technology. The aim of this study is to establish a comprehensive numerical model that describes the water vapor transfer across a hydrophobic micro-porous membrane in single-stage and multi-stage VMD processes for seawater desalination. The numerical predictions were compared to experimental data in addition to numerical computations based on an existing literature database, and good agreement has been found. The investigation also conducted a sensitivity analysis of process variables and membrane specifications on the VMD performance, as well as an assessment of the impact of temperature and concentration polarization. The obtained results showed that the permeation flux reached 18.42 kg/m2·h at 35 g/L feed concentration, 65 °C feed temperature, 50 L/h feed flow rate, and 3 kPa vacuum pressure. Moreover, the findings revealed that the feed temperature was the most significant factor, while the feed flow rate was the least important in determining the permeation flux. Additionally, the findings suggested that the effectiveness of the VMD process heavily relies on the composition and permeability of the support materials. Finally, the results confirmed that temperature polarization had a more significant effect on the reduction of the permeate flux than the concentration polarization.

Modeling-based investigation on the contribution of radial heating into integrated VMD hollow-fiber modules at different scales for autonomous desalination

Integrating solar energy with vacuum membrane distillation (VMD) is a promising means to provide fresh water in remote areas. Our previous work on integrated solar VMD modules focused on optimizing flat-sheet integrated modules; this study examines the feasibility of up-scaling VMD modules containing outside/in hollow fibers with an external module surface collecting solar heat (HF-SC-VMD). Such modules have been experimented with at the lab scale; however, additional information is required concerning the parameters governing the productivity in such modules at the industrial scale, notably concerning the impacts and limits induced by radial heating. We contribute to this goal by estimating the temperature profiles and the maximal permeate production achievable for an HF-SC-VMD module at a semi-industrial scale in the case of the more favorable heating conditions provided via solar radiation, where the maximum temperature acceptable for the constituent materials of the module is assumed to be the temperature reached by the feed close to the shell wall (Twall). A two-dimensional shell-side model is developed to describe the coupled radial and longitudinal heat and mass transfers accounting for temperature polarization while considering module design parameters and operating conditions for future optimization. The model properly describes the experimental VMD performances (without radial heating) with and without temperature polarization. The influence of Twall on the permeate flux and temperature profiles is examined to discuss the effect of radial heating. The specified semi-industrial-scale HF-SC-VMD module (2.1 m2) could produce 37.8 L/h of drinkable water (covering the daily needs of ~150 people) if the full module design (including the solar collector) allows Twall to reach 70 °C, confirming the interest of continuing research toward optimizing a full HF-SC-VMD module.

Superhydrophobic electrospun FPI/PTFE nanofiber membranes for robust vacuum membrane distillation

In this study, we firstly proposed a fluorinated polyimide (FPI) /polytetrafluoroethylene (PTFE) nanofiber membrane for vacuum membrane distillation (VMD) by electrospinning FPI and double eletcrospraying PTFE particles. The resultant FPI /PTFE nanofiber membrane possessed desirable superhydrophobicity (water contact angle (WCA) of 155.3°), anti-wettability and durability due to the rough micro/nano structure of the membrane surface. The surface morphology and chemical composition of obtained FPI nanofiber membranes were characterized by SEM, FTIR, XRD and XPS, and the effects of PTFE content on the VMD performance were investigated. Results showed that the liquid entry pressure (LEP) of FPI/PTFE nanofiber membranes increased significantly after PTFE particles were introduced (LEP of 158 kPa). In the VMD process, the FPI/PTFE nanofiber membrane showed stable MD flux (33.49 L·m2·h−1) and excellent salt rejection (99.96%). Furthermore, the FPI/PTFE nanofiber membrane maintained its favorable performance, resulting in stable MD flux (30.21 L·m2·h−1) and salt rejection (99.96%) after the 120 h continuous long-term operation. It shows a promising future for VMD of FPI/PTFE nanofiber membranes.

Application of vacuum membrane distillation process for lithium recovery in spent lithium ion batteries (LIBs) recycling process

The feasibility of using vacuum membrane distillation (VMD) for lithium recovery by concentrating the discharged leachate from spent lithium ion batteries (LIBs) recycling process was evaluated. The performance of VMD was compared with that of direct contact membrane distillation (DCMD) in terms of water flux, concentration rate, membrane wetting, and economic feasibility. VMD achieved a higher volume concentration factor (VCF) of 45 compared to the VCF of 25 achieved by DCMD in the LiCl feed solution. In the Li2SO4 feed solution, VMD and DCMD were concentrated to VCF15 and VCF 17, respectively, before wetting occurred. The stability of the MD process was verified using feed solutions containing nickel and manganese, which are cathode materials that can cause scaling even at low concentrations. Low concentrations of nickel and manganese did not significantly affect the maximum VCF or wetting; however, high concentrations of nickel and manganese affected the stability of the MD process. VMD exhibited a higher flux and 32 % lower thermal energy consumption than DCMD. Furthermore, the expected cost of Li2CO3 production with VMD for leachate concentration was $8.31–10.65/kg. The VMD concentration process is a feasible option for recovering lithium from the discharged leachate from spent LIBs recycling process.

Field demonstration of intensified membrane distillation for treating oilfield produced waters from unconventional wells

Increasing disposal costs and environmental concerns have encouraged desalination projects in the Permian basin to cut down produced water (PW) volumes and improve the handling. This study aims to demonstrate performance of membrane-distillation (MD) for desalinating high-salinity PW. Prior to MD, electrocoagulation (EC) and microfiltration (MF) are used as primary and secondary pretreatments, respectively. These pretreatment steps are necessary to remove constituents such as suspended solids, oil, and grease. The integrated EC-MF process was followed by vacuum MD (VMD) to recover water and concentrate the brine stream. To improve latent heat recovery, vapor compression is integrated into the vacuum membrane distillation (VMD). Our EC-MF-VMD PW treatment process can treat 1000 L of PW per day, with water recoveries in the range of 30–70 %. Suspended solids and turbidity removal of 90 % and total organic carbon (TOC) removal of 60 % were achieved with EC-MF pretreatments, which effectively mitigated PW fouling potential. A custom-designed plate-and-frame VMD module was established which enabled unparalleled service flexibility to change membrane replacement and active area. Results from our systematic evaluation revealed that the flux of the commercial membranes in our VMD module varied from 5 to 66 Lm−2 h−1. The intensified VMD performance achieved by vapor compression led to remarkable improvement in energy efficiency in terms of gained output ratio (GOR = 2.7) and energy consumption per unit of water production (248 kWh m−3). This study demonstrates the capability of the MD process for PW treatment, revealing the reasonable potential for PW reuse and recycling. Although this process might be of interest for unique applications that justifies the cost, further improvements in energy consumption are required to make this technology feasible for commercialized desalination of oilfield produced waters.

Synthesis of novel PVDF-co-PTFE membranes for ethanol–water separation using vacuum membrane distillation

Vacuum membrane distillation (VMD) is an intriguing technique that has the potential to separate ethanol from an ethanol–water mixture. The goal of this work was to synthesize hydrophobic polyvinylidene fluoride–polytetrafluoroethylene (PVDF–PTFE) hybrid membranes that could be used in VMD. For membrane synthesis via phase inversion, the outcomes of different solvents, namely N-methyl-2-pyrrolidone (NMP), N,N-dimethyl acetamide (DMAc), and dimethylformamide (DMF), as well as PTFE loading in PVDF dope solution were also evaluated. The desired membrane hydrophobicity was ensured by increasing contact angle from 69° to 135°. The obtained average maximum pore size fell between 0.19 μm and 0.22 μm. Plenty of work has been done on how the performance of the synthesized membrane depends on the VMD operating parameters (ethanol concentration, flow rate, feed temperature, and permeate pressure). The highest separation factor of 37.68 was offered by the membrane synthesized using DMF as a solvent, while the membrane synthesized with the help of DMAc solvent had the most increased ethanol flux of 10.55 kg m−2 h−1. A thorough report has been made on understanding membrane morphologies and VMD performance.

Polypropylene hollow fiber membrane prepared with green binary diluents via TIPS: Effects of spinning process parameters and VMD desalination performance

Environmental-friendly castor oil (CO) and soybean oil (SO) were used as mixed diluents to spin polypropylene (PP) hollow fiber microporous membrane (HFMM) via thermally induced phase separation. With the addition of castor oil, the cloud point temperature gradually increases, and the region of liquid-liquid phase separation becomes larger. The membrane pore structure changes from a spherulitic form to a continuous sponge-like pore structure. Particularly, the effects of spinning process parameters (spinning speed, spinneret temperature and bore liquid temperature) on HFMM properties (e.g., mechanical properties, water contact angle, pore size and pore size distribution) and desalination performance of vacuum membrane distillation (VMD) were further studied. A higher spinning speed and smaller temperature difference between dope and spinneret/bore fluid increase pore size and reduce membrane porosity. The PP HFMM produced under the spinneret temperature of 160 °C, the bore fluid temperature of 113 °C, and spinning speed of 16.1 m/min reached relatively high VMD desalination performance both feeding 10 g/L NaCl solution (water flux 18.4 kg/m2·h and salt rejection 99.99 %) and petrochemical saline wastewater (water flux 14.0 kg/m2·h and salt rejection 99.5 %). This paper has important guiding significance for the mass production of PP HFMMs.

Vacuum membrane distillation for seawater concentrate treatment coupled with microbubble aeration cleaning to alleviate membrane fouling

In this study, a laboratory-scale vacuum membrane distillation (VMD) system was applied to treat seawater concentrate using a hydrophobic polytetrafluoroethylene (PTFE) membrane with an effective area of 120 cm2. At the beginning, the VMD performance was investigated under different temperature and vacuum pressure. Results indicated that running temperature and vacuum pressure were optimal at 65 °C, 0.09 MPa, respectively in order to obtain an appropriate permeate water flux. Furthermore, four types of synthetic seawater concentrate with different ionic compositions, namely, SW-1, SW-2, SW-3, SW-4, was used as feed solutions for studying the VMD performance at 65 °C, 0.09 MPa. Results confirmed that the existence or absence of scaling ions would observably affect the VMD performances. For instance, concentration factor in the SW-4 test without scaling ions can reach 3.99 (i.e., 20.09 wt%), compared to the SW-1 test. Besides, we briefly elucidate the whole fouling behavior in VMD, the reason is that scaling ions could easily generate sediment substances on the surface or in the pores of membranes. In view of appearance of membrane fouling, finally, unlike conventional membrane cleaning methods, microbubble aeration (MBA) cleaning as a green approach, without chemicals consumed during cleaning, was proposed to alleviate membrane fouling, meanwhile, the mechanism of performance enhancement by MBA cleaning was also illustrated, with the introduction of microbubbles (MBs), a gas-liquid two-phase flow could intensify the surface shear and adhesion effects near the membrane surface, and also generate an electrostatic repulsion effect between membrane surface and MBs. Together, these results show that MBA cleaning is an effective method to mitigate membrane fouling in VMD.

Tuning poly(vinyl chloride) membrane morphology to suit vacuum membrane distillation: Focusing on membrane preparation process based on phase separation

Nowadays, water scarcity is a paramount concern all over the world. Water desalination has been proposed as an applicable solution to face this problem. In this work, water desalination based on distillation technique using Poly(vinyl chloride) (PVC) membrane was considered. PVC membranes were fabricated via nonsolvent-induced phase separation (NIPS). Parameters involved in membrane production process were changed to tune the PVC membrane morphology for vacuum membrane distillation (VMD). These parameters include (1) polymer concentration in casting dope, (2) exposure of cast solution to the water vapor before immersing in nonsolvent bath, (3) addition of a hydrophilic polymer such as polyvinyl pyrrolidone (PVP) with different concentrations to the casting dope and (4) casting thickness. It was shown that the investigated parameters significantly affect the morphology and VMD performance of the PVC membranes. Additionally, enough evidence was provided to demonstrate that PVC concentration in the casting dope is the parameter using which PVC membrane morphology can be easily and effectively tuned to suit the VMD. Utilizing appropriate PVC concentrations, membranes with a suitable combination of both macrovoids and cellular pores can be produced. Note that although macrovoids are beneficial to enhance membrane flux, they adversely influence the membrane liquid entry pressure of water (LEP w ). An optimum LEP w is essential to resist against membrane wetting and achieve a high flux and long-term performance during the VMD. Therefore, the length and density of macrovoids within the membrane structure must be optimized to obtain a membrane, which is suitable for the VMD. It can be best achieved by changing the PVC concentration in the casting dope. The influence of VMD operational parameters on the performance of PVC membranes produced using a casting solution with appropriate PVC concentration was further investigated. It was shown that in spite of the feed salt concentration, the temperature and flow rate of feed solution positively affect membrane flux while the salt rejection remains unchanged. • Tuning the PVC membrane morphology to suit VMD is comprehensively explored. • The appropriate PVC membrane morphology for VMD is disclosed. • For VMD, membrane morphology can be best tuned by changing solution concentration.

Development of high flux PVDF/modified TNTs membrane with improved properties for desalination by vacuum membrane distillation

In this work, titanium dioxide nanotubes (TNTs) were first synthesized via hydrothermal method at 110 °C. To reduce hydrophilicity, the synthesized TNTs were annealed at 350 °C and then aged in air at ambient temperature. Subsequently, polyvinylidene fluoride (PVDF) mixed matrix membranes (MMMs) containing the modified TNTs were prepared by phase inversion method. Performance of the prepared PVDF/TNTs MMMs was evaluated in vacuum membrane distillation (VMD) process. Effects of polymer concentration and content of the modified TNTs on the fabricated membrane performance in the VMD process were investigated. Hydrophilicity reduction of the synthesized TNTs after annealing and aging was confirmed by FTIR analysis. It was found that the modified TNTs with partial hydrophilicity could improve the VMD performance of the fabricated MMMs. The results showed that the modified TNTs improve the membrane porosity, finger-like cavities, thermal and mechanical properties without having a significant negative effect on the hydrophobicity of the PVDF membrane. Compared to the neat PVDF membrane, water flux and salt rejection of the MMM containing 2 wt% of the modified TNTs were improved by 38.7% and 47.0%, respectively. Response surface methodology (RSM) was used to study the effects of operating parameters of VMD process such as feed flow rate, feed temperature and feed concentration on the membrane performance. The obtained optimum conditions are: flow rate of 36.99 LPH, temperature of 77.79 °C and concentration of 10.18 g/l. Under optimum conditions, water flux and salt rejection were maximized as 92.55 L/m2.h and 99.9%, respectively. The prepared PVDF/TNTs MMMs with improved properties can be used in MD applications successfully.

Electrospun nanofibrous polyether-block-amide membrane containing silica nanoparticles for water desalination by vacuum membrane distillation

In this work, hydrophobic nanofibrous membranes based on polyether-block-amide (PEBAX-2533) were prepared by the electrospinning technique for using in the vacuum membrane distillation (VMD) process for water desalination applications. In the first part of this study, the influence of the electrospinning conditions like polymer concentration, surfactant concentration, applied voltage, spinneret to collector distance, feed injection rate and spinneret diameter on the morphology of the electrospun mats was investigated and optimized to obtain uniform, bead-less and defect-free nanofibrous PEBA membranes. In the second part, the electrospun nanofibrous SiO2/PEBAX-2533 membranes were fabricated by incorporating silica nanoparticles into the polymer solution. The morphology, structure, and surface properties of the prepared nanofibrous SiO2/PEBA membranes were characterized by the SEM, FTIR, OCA, and tensile strength tests. Likewise, the VMD performance of the prepared electrospun SiO2/PEBA membranes was studied for the desalination of NaCl aqueous solutions at various operating conditions. It was observed that the VMD performance of the prepared electrospun nanofibrous SiO2/PEBAX-2533 membrane containing 8 wt% of silica nanoparticles had the best separation performance in terms of the water flux, flux decline and TDS content of the permeate stream. Finally, a comparison between the prepared nanofibrous SiO2/PEBAX-2533 membrane and a commercial PVDF membrane revealed that the VMD separation performance and long-term stability of the electrospun membrane were much better than those of the commercial PVDF membrane.

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.

Nafion-PTFE hollow fiber composite membranes for improvement of anti-fouling and anti-wetting properties in vacuum membrane distillation

Nafion-PTFE hollow fiber composite membranes were fabricated and applied in vacuum membrane distillation (VMD) process to improve the anti-fouling and anti-wetting performance in the presence of oil and surfactant. The composite membrane exhibited an underwater super-oilphobic property according to the contact angle test. The VMD performance when treating saline feed containing 3.5 wt% NaCl and 1000 ppm mineral oil for the composite membrane behaved a stable flux for 40 h. In construct, an immediate and sharply-declined flux was observed for the pristine PTFE, indicating the significant improvement in the anti-fouling property. Meanwhile, the composite membrane also alleviated the wetting phenomena caused by the surfactant comparing with the pristine PTFE membrane, and the Nafion-PTFE composite membrane behaves a stable flux and high salt rejection higher than 99.99% for 60 h treating saline feed containing O/W emulsions. The pressure depression effect of the Nafion layer on the mass transfer was analyzed. It was found that the depression could be reduced with the increasing of the feed temperature and the enhancement of the corresponding effective pore diameter of the Nafion layer, that the pressure depression factor decreased from 0.23 to 0.04 with the feed temperature increasing from 60 °C to 75 °C.

Improving the performance of vacuum membrane distillation using a 3D-printed helical baffle and a superhydrophobic nanocomposite membrane

Membrane distillation represents a promising solution for high salinity water desalination (e.g. shale gas produced waste wastewater), but so far it has achieved limited commercial success relative to other thermally driven desalination technologies. This study aims to advance membrane distillation technology by systematically investigating three innovative features which can potentially help improve overall performance: a 3D-printed helical baffle, a superhydrophobic membrane coating, and an intermittent operation mode. Our experimental and numerical results reveal that the 3D-printed helical baffle can enhance the permeate flux by 6–46% (depending on flow rate) and increase the average membrane shear by ~60%, with negligible additional pumping power. The impact of a super-hydrophobic coated membrane was evaluated for a synthetic wastewater (35 g/L of NaCl and 0.1 mM of sodium dodecyl sulfate) which mimicked low surface tension shale gas wastewater. It was found that the modified membrane’s flux and salt rejection were maintained ~25% longer than a conventional uncoated polypropylene (PP). Lastly it was found intermittent operation consisting of 5 h of operation followed by 1 h of regeneration doubled module durability compared to continuous operation for the equivalent volume of permeate production. Based on these results, it was estimated that the synergy between these three enhancements would provide >6% more permeate flux and >100% durability, as compared to the baseline. Thus, these results demonstrate that by improving heat and mass transport while also enhancing the hydrophobicity of underlying membranes, it may be possible to make MD systems a viable option for industrial wastewater treatment.

An experimental study on liquid regeneration process of a liquid desiccant air conditioning system (LDACs) based on vacuum membrane distillation

In this paper, a liquid regeneration method by vacuum membrane distillation (VMD) is proposed for the liquid desiccant air conditioning system (LDACs), and the experimental study on this method is carried out. VMD regeneration experiments were carried out with LiCl solution. The effects of temperature, concentration of feed solution, length, number of fiber membranes and vacuum pressure on the membrane flux, mass transfer coefficient, rejection rate and regeneration ability were studied. The results show that the error between experimental and calculation results is reduced from less than 15% to less than 5% by the optimized calculation model. The temperature of feed solution has a great influence on the regeneration performance of VMD, and the regeneration ability of VMD process increases approximately exponentially from about 0.1% to 0.8–1.2% with the increase of regeneration temperature. The VMD regeneration process of LiCl solution is the result of Poiseuille flow and Knudsen diffusion, and Poiseuille flow dominates. Under the same regeneration capacity, the regeneration temperature of 30 wt% LiCl solution is about 7 °C higher than that of 20 wt% LiCl solution, and this temperature difference also increases as the target regeneration amount increases. In order to improve the regeneration effect of high concentration solution, the regeneration temperature can be increased appropriately. The water flux of the membrane decreases with the increase of the length of the membrane. The membrane length of Type1 is 2.1 times longer than Type 2, but regeneration capacity of Type 1 is only 1–1.7 times higher than Type 2. Further, both the water flux and regeneration ability of the solution decrease first and then increase with the increasing number of membranes. Therefore, the reasonable selection of number of fiber membranes can significantly save materials and also improve the regeneration ability.

A numerical study on the effects of operational parameters and membrane characteristics on the performance of vacuum membrane distillation (VMD)

A numerical study on the effects of operational parameters and membrane characteristics on the performance of vacuum membrane distillation (VMD)

Improved performance of vacuum membrane distillation in desalination with zeolite membranes

A supported Mordenite Framework Inverted (MFI) zeolite membrane with a length of 30 cm was synthesized by the secondary growth method using a fast heating profile for the template removal (calcination) step. The presence of defects into the zeolite layer (before and after calcination) was assessed by means of single gas permeation tests. Afterwards, the membrane performance was evaluated in Vacuum Membrane Distillation (VMD) for desalination. Two cleaning procedures were applied between consecutive experiments on salty solutions and their efficiency compared. NaCl rejections of 99.9% were achieved for feeds ranging from 0.2 M to 0.9 M, dropping down to 97% for the 1.2 M feed. The highest trans-membrane flux (20.6 kg m−2 h−1) was measured for 0.2 M feed, by working at 70 °C and 120 L/h.

Numerical study of desalination by vacuum membrane distillation – Transient three-dimensional analysis

The performance of vacuum membrane distillation (VMD) modules can be optimized through careful selection of design parameters. The present study examines how the addition of cylindrical filaments in the feed channel increases momentum mixing and the overall performance of VMD modules under different operating inlet conditions. Three-dimensional transient Computational Fluid Dynamics (CFD) simulations are conducted using Wall-Adapting Local Eddy-Viscosity (WALE) subgrid-scale Large Eddy Simulation (LES) turbulence model. Local concentration, temperature, and flux are coupled at the membrane surface to predict the rate of water vapor diffused through the membrane by Knudsen and viscous diffusion mechanisms. The predicted and measured vapor flux agrees reasonably well; validating the employed model. The small-scale eddies induced by the presence of spacer filaments promote mixing in the module, thus the temperature and concentration polarization is alleviated and the water vapor flux is immensely improved. The insertions of filaments in the feed channel increase the water permeate rate by more than 50% at higher feed flow rates and inlet temperatures. The pressure drop by the spacer reduces the allowable module length by one order of magnitude, but the module length increases two folds at feed temperature 80°C. Even though the power consumption of the module containing the filaments is increased, the addition of filaments is strongly recommended since the required power for the process could be supplied from readily available low-grade heat source.