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This paper deals with a numerical study of heat and mass transfer in a vacuum membrane distillation unit used for desalination. The governing equations expressing the conservation of mass, momentum, energy, and species for the saline solution with coupled boundary conditions, as well as the mass and energy balances on the membrane sides, were solved numerically. The slip boundary condition due to the membrane's hydrophobicity was also taken into consideration. The numerical model was validated with available experimental data and found to be in good agreement. The effects of operational parameters on pure water production and temperature polarization were presented and discussed. A comparison between three flat-sheet membranes commonly used in vacuum membrane distillation was conducted under the same operating conditions. In addition, a comparison between the Nusselt and Sherwood numbers obtained in this study and correlations commonly used in the literature was conducted, which revealed remarkable deviations between models along the channel. The impact of the heat and mass transfer coefficients on the transmembrane permeate mass flux variation is found to be important. [ABSTRACT FROM AUTHOR]

In this study, we developed a comprehensive two-dimensional computational fluid dynamics (CFD) model using COMSOL™ Multiphysics to describe and simulate heat transfer, mass transfer and fluid flow in the flat sheet vacuum membrane distillation (VMD) under laminar flow conditions. A combination of Knudsen and Poiseuille flow was applied to study mass transfer across the membrane. The effect of variation of Reynolds number, inlet feed temperature and degree of vacuum on different parameters (mass flux, temperature polarization coefficient- TPC, concentration polarisation, heat transfer coefficient) was studied. There was a positive impact of the Reynolds number (50–200) on mass flux (13.15%), heat transfer coefficient (2.64%) and TPC (1.42%), while CPC decreased by 56.63%. The increment in the heat transfer coefficient was due to fluid mixing on the feed side, while the increment in the TPC was due to a higher temperature gradient across the membrane surfaces. The increment in the feed temperature (323–343 K) resulted in an increase in mass flux by 132.9%, while TPC decreased from 0.98 to 0.90. The degree of vacuum (640–750 mm Hg) increased mass flux and heat transfer coefficient by 72.52 and 425.83%, respectively, while the TPC decreased by 8.81%. The feed temperature was the most sensitive parameter with respect to mass flux. The developed CFD model was validated with in-house experimental results with reasonable accuracy. HIGHLIGHTS Comprehensive 2D CFD model developed using COMSOL to simulate heat transfer, mass transfer and fluid flow in VMD.; CFD model used to determine the temperature and concentration polarization at the membrane surface.; The effect of Reynolds number, feed temperature, and vacuum degree on VMD performance parameters were studied.; Combination of Knudsen and Poiseuille flow was applied to study mass transfer across the membrane.;

Numerical simulations were carried out to assess the technical and economic feasibility of a solar water desalination system that has a novel hybrid Concentrating Photovoltaic Thermal (CPVT) collector coupled with a Vacuum Membrane Distillation (VMD) process. A special characteristic of this CPVT is its triangular receiver with PV cells facing the reflecting surface. This type of receiver has the advantage of generating more electricity with less PV surface area and great potential to be used to hybridize conventional parabolic thermal collectors. TRNSYS was employed to analyze the annual performance of the CPVT-VMD system evaluating parameters such as solar fraction, specific permeate production and specific energy production for different coastal cities. In the dynamic simulations, local annual weather data and specific information about the characteristics and operating conditions of a real CPVT collector and a VMD module were considered. From the parametric analysis the optimal surface area of collectors and the input temperature of the VDM module were determined. A maximum specific permeate of 218.410 m 3/m2VMD for Acapulco, MX, and a minimum of 170.365 m 3/m2VMD for Singapore, SG, were achieved for the proposed CPVT-VMD system of four solar collectors with an operating set temperature of 55 °C. An economic profit was found after 7 years for Acapulco city, which showed great potential to use solar energy from hybrid CPVT collectors for a VMD process to provide freshwater in coastal cities. [ABSTRACT FROM AUTHOR]

Vacuum membrane distillation (VMD) has attracted increasing interest for various applications besides seawater desalination. Experimental testing of membrane technologies such as VMD on a pilot or large scale can be laborious and costly. Machine learning techniques can be a valuable tool for predicting membrane performance on such scales. In this work, a novel hybrid model was developed based on incorporating a spotted hyena optimizer (SHO) with support vector machine (SVR) to predict the flux pressure in VMD. The SVR–SHO hybrid model was validated with experimental data and benchmarked against other machine learning tools such as artificial neural networks (ANNs), classical SVR, and multiple linear regression (MLR). The results show that the SVR–SHO predicted flux pressure with high accuracy with a correlation coefficient (R) of 0.94. However, other models showed a lower prediction accuracy than SVR–SHO with R-values ranging from 0.801 to 0.902. Global sensitivity analysis was applied to interpret the obtained result, revealing that feed temperature was the most influential operating parameter on flux, with a relative importance score of 52.71 compared to 17.69, 17.16, and 14.44 for feed flowrate, vacuum pressure intensity, and feed concentration, respectively.

In this paper, an integrated system based on low-concentrated photovoltaic/thermal (LCPV/T) technology and efficient vacuum membrane distillation (VMD) seawater desalination utilizing the energy of solar is established. Through a theoretical analysis and a series of experiments, this paper explores the temperature change of a single VMD process, and the variation trend of single-day membrane flux with solar irradiation and temperature parameters. In addition, the changes in solar irradiation, temperatures of the integrated system, membrane flux, and thermoelectric properties in different seasons are also analyzed. A mathematical model was established to calculate the relationship between membrane flux and temperature difference. The experimental results show that the membrane flux of VMD is 2.73 L/(m2·h); the simulated seawater can achieve a desalination rate of 99.9%. After economic analysis, the operating incomes of the system under sunny weather conditions in different seasons were all positive. [ABSTRACT FROM AUTHOR]

Membrane material plays an important role in the membrane distillation technology. However, traditional membrane materials such as polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) and PP have existing membrane pollution and membrane infiltration problems, which limit further development of the membrane distillation technology. To solve these problems, the 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) has been added to the PTFE solution to create a novel super-hydrophobic PFDTES-PTFE membrane. A series of characterizations and characterization techniques have accordingly been used to study the structure and performance of the prepared membranes, such as atomic force microscope (AFM), water contact angle (WCA) measurements, liquid entry pressure (LEP) tests, vacuum membrane distillation (VMD) experiments, etc. The AFM analysis indicated a decreased roughness of the PFDTES-PTFE membrane. Moreover, the WCA of the membrane was increased from 131.3° to 137.5°, and the LEP increased by 33.3%, which indicated that the PFDTES-PTFE membrane showed excellent hydrophobicity and anti-wetting performances. What is more, the mean pore size was reduced from 269.6 to 205.0 nm, but there was a minor difference in film thickness. Furthermore, the VMD tests were implemented to evaluate the performance of the original and modified membranes with aspect to wetting. Compared to the pristine membrane, the modified PTFE membrane showed a slight decrease in membrane flux in the VMD process. However, the modified membrane kept a stable membrane flux about 0.932 L/(m2·h) with a salt rejection values of 99.8% after distillation for 3 h. In a word, the present study has proposed a new method to extend the membrane life, in favor of further developments of the membrane technology for islands and reef fields. [ABSTRACT FROM AUTHOR]

Numerical simulations were carried out to assess the technical and economic feasibility of a solar water desalination system that has a novel hybrid Concentrating Photovoltaic Thermal (CPVT) collector coupled with a Vacuum Membrane Distillation (VMD) process. A special characteristic of this CPVT is its triangular receiver with PV cells facing the reflecting surface. This type of receiver has the advantage of generating more electricity with less PV surface area and great potential to be used to hybridize conventional parabolic thermal collectors. TRNSYS was employed to analyze the annual performance of the CPVT-VMD system evaluating parameters such as solar fraction, specific permeate production and specific energy production for different coastal cities. In the dynamic simulations, local annual weather data and specific information about the characteristics and operating conditions of a real CPVT collector and a VMD module were considered. From the parametric analysis the optimal surface area of collectors and the input temperature of the VDM module were determined. A maximum specific permeate of 218.410 m 3/m2VMD for Acapulco, MX, and a minimum of 170.365 m 3/m2VMD for Singapore, SG, were achieved for the proposed CPVT-VMD system of four solar collectors with an operating set temperature of 55 °C. An economic profit was found after 7 years for Acapulco city, which showed great potential to use solar energy from hybrid CPVT collectors for a VMD process to provide freshwater in coastal cities.

In this paper, an integrated system based on low-concentrated photovoltaic/thermal (LCPV/T) technology and efficient vacuum membrane distillation (VMD) seawater desalination utilizing the energy of solar is established. Through a theoretical analysis and a series of experiments, this paper explores the temperature change of a single VMD process, and the variation trend of single-day membrane flux with solar irradiation and temperature parameters. In addition, the changes in solar irradiation, temperatures of the integrated system, membrane flux, and thermoelectric properties in different seasons are also analyzed. A mathematical model was established to calculate the relationship between membrane flux and temperature difference. The experimental results show that the membrane flux of VMD is 2.73 L/(m2·h); the simulated seawater can achieve a desalination rate of 99.9%. After economic analysis, the operating incomes of the system under sunny weather conditions in different seasons were all positive.

Seawater desalination using conventional vacuum membrane distillation is facing a grave problem of high specific heat consumption (SHC). In order to deal with this issue, energy of the saline feed is recovered in multi-stage vacuum membrane distillation (MSVMD). The present study is aimed to analyze the energetics of MSVMD in terms of SHC, gained output ratio (GOR), and specific membrane area (SMA) as a function of process parameters. An attempt was made to determine the optimum number of stages for the maximum permeate productivity. Further, the impact of multi-staging on MSVMD energetics was also analyzed. A drop in cumulative SHC, GOR, and SMA was found with the corresponding rise in feed inlet temperature. Rising saline feed flowrates enhanced the cumulative SHC and GOR, however, it declined the SMA. The economics of MSVMD in terms of levelized cost of water (LCOW) were also investigated. It was observed that the permeate enhancing conditions such as a rising saline feed temperature from 60°C to 100°C depreciates the LCOW by about 57 USD/m³. Similarly, a decline in the LCOW was noticed with a surge in saline feed flowrate and vacuum pressure. Multi-staging initially dropped the LCOW, but it was found that after a certain number of stages, further addition would cause an upsurge in the LCOW. [ABSTRACT FROM AUTHOR]

Membrane distillation has emerged as a promising technique for treating high-saline water. However, the process suffers from drawbacks such as wettability and fouling due to insufficient hydrophobicity of the membrane material. A practical approach for preparing hydrophobic membranes should involve improving surface roughness and chemistry manipulation. Herein, we explored chemical grafting of different functional groups viz. 1-(4-vinyl benzyl)azepane, N-(4-vinyl benzyl)decan-1-amine and 3-(triethoxysilyl)-N-(4-vinyl benzyl) propane-1-amine on polyvinylidene fluoride side chain to improve the hydrophobic characteristics and vacuum distillation performance (permeate flux and salt rejection). The successful grafting of hydrophobic functional groups on the polyvinylidene fluoride side chain was confirmed by spectral analysis such as 1H nuclear magnetic resonance, Attenuated total reflectance-Fourier transformation infrared spectroscopy, and X-ray photoelectron spectroscopy. The synthesised membranes were further characterised using morphology techniques (scanning electron microscopy, atomic force microscopy) for hydrophobicity (water contact angle, liquid entry pressure) and thermal and mechanical stability. The desalination performance of membranes was assessed by varying feed concentration (2–8 % NaCl) and feed temperature (30–75 °C). The 3-aminopropyltriethoxysilane grafted vinyl benzyl polyvinylidene fluoride membrane exhibited the highest flux (20.35 L m−2 h−1), salt rejection (99.99 %) at 4 % NaCl feed concentration and 60 °C feed temperature attributed to a well-arranged large number of pores and its hydrophobicity. The 3-aminopropyltriethoxysilane grafted membrane exhibited the highest anti-fouling characteristic. Further, high flux and salt rejection of the synthesised membranes for vacuum membrane distillation operation were compared to membranes reported in the literature, demonstrating its potential for high saline water desalination. [Display omitted] • Flat-sheet side chain chemical grafted PVDF membranes were prepared via NIPS. • Effect of feed concentration, and temperature; and anti-fouling characteristics of the membranes were studied. • M-APTES membrane exhibited the highest flux, salt rejection and anti-fouling characteristics. [ABSTRACT FROM AUTHOR]

This study designed and tested a novel type of solar-energy-integrated vacuum membrane distillation (VMD) system for seawater desalination under actual environmental conditions in Wuhan, China. The system consists of eight parts: a seawater tank, solar collector, solar cooker, inclined VMD evaporator, circulating water vacuum pump, heat exchanger, fresh water tank, and brine tank. Natural seawater was used as feed and a hydrophobic hollow-fiber membrane module was used to improve seawater desalination. The experiment was conducted during a typical summer day. Results showed that when the highest ambient temperature was 33 °C, the maximum value of the average solar intensity was 1,080 W/m2. The system was able to generate 36 kg (per m2 membrane module) distilled fresh water during 1 day (7:00 am until 6:00 pm), the retention rate was between 99.67 and 99.987%, and electrical conductivity was between 0.00276 and 0.0673 mS/cm. The average salt rejection was over 90%. The proposed VMD system shows favorable potential application in desalination of brackish waters or high-salt wastewater treatment, as well.

Abstract Membrane fouling, especially inorganic fouling due to salt crystal formation and deposition on the membrane surface, is still a major technical issue in membrane distillation (MD) applications. In this study, microbubble aeration (MBA) was included in a laboratory-scale vacuum membrane distillation (VMD) rig and its effect on a desalination process was examined. Without MBA, serious membrane scaling occurred during desalination of simulated high-salinity sea water (100 g.L−1 salt concentration), which resulted in a dramatic reduction of permeate flux to essentially zero after 120 min. Scanning electron microscopy showed that a layer of large cuboid salt crystals uniformly covered the membrane surface. However, membrane scaling was mitigated with the introduction of MBA, resulting in the improved VMD desalination performance, which was positively correlated with pump pressure in the microbubble (MB) generator. Results showed that the effective processing time of the VMD desalination processing cycle was respectively prolonged to 150, 180, and more than 300 and 360 min (cf. 120 min without MBA) when the pump pressure was respectively at 0.1, 0.2, 0.3 and 0.4 MPa, leading to the increase of cumulative water production. Further studies found that larger numbers of MBs of smaller size were produced at higher pump pressure, which are more beneficial for increasing water vapor production and alleviating salt precipitation. The difference in zeta potential between the MBs in distilled water (about −30 mV) and that in SW100 solution (about -2 mV) demonstrated that MBA not only effectively mitigated the negative effect of concentration polarization by enhancing the surface shear rate at the membrane surface, but also reduced salt precipitation probably due to the MBs attracting counterions to the gas-water interface. Finally, energy consumption analysis of the modified VMD desalination process revealed that MBA, while itself only adding about 3% to the total energy consumption at varied pump pressures, was able to improve the specific energy consumption, especially at higher pump pressures. Together, these results demonstrate that MBA is an effective way of improving the performance of VMD desalination of water. Graphical abstract Image 1 Highlights • Microbubble aeration (MBA) effectively improved VMD desalination performance. • Membrane scaling was alleviated with the introduction of MBA. • MBA markedly improved the specific energy consumption. [ABSTRACT FROM AUTHOR]

Solar-driven membrane distillation (SDMD) for desalination is a feasible method to solve water and energy resource issues. The design and operation of SDMD is different from continuous and steady state processes, such as common chemical plants, due to the intermittent and unpredictive characteristics of solar radiation. Employing the steady state and dynamic simulation models developed on the platform of Aspen Custom Modeler®, this paper presents a two-stage design approach for the SDMD systems using different types of membrane distillation configurations, including AGMD (air gap MD), DCMD (direct contract MD) and VMD (vacuum MD). The first design stage uses the steady state simulation model and determines equipment sizes for different constant-value solar radiation intensities with the objective of minimizing total annual cost. The second design stage is implemented on the SDMD systems with process control to automatically adjust the operating flow rates using the dynamic simulation model. Operated with the yearly solar radiation intensity of Taiwan, the unit production costs (UPCs) of the optimal SDMD systems using AGMD, DCMD, and VMD are $2.71, 5.38, and 10.41 per m3 of water produced, respectively. When the membrane unit cost is decreased from $90/m2 to $36/m2, the UPC of the optimal solar-driven AGMD system can be reduced from $2.71/m3 to $2.04/m3.

The vacuum membrane distillation (VMD) is applied in desalination. The simultaneous heat and mass transfer and the operating condition are studied. The experiments were carried out on a laboratory VMD unit, using a pre-prepared flat sheet membrane. Feed solutions used were fresh and saline water ranged from 3000 to 40,000 ppm. The operating parameters studied was feed temperature (298–368K), absolute pressure at downstream of the membrane (80,000–20,000 Pa), feed velocity (0.001–0.004 ms–1). The permeability of the prepared membrane is measured by two methods; the results showed the feasibility and applicability of these methods. The operating conditions and membrane characteristics are proved crucial factors influencing the permeation through the membrane. The experimental data were used to estimate the individual liquid film transfer coefficients. The operating condition dependence is demonstrated. The results are expressed in an equation for each coefficient; temperature (T), pressure (P), feed velocity (v) and concentration (C). Also, the obtained empirical correlations at the liquid-vapor interface are expressed as a function in T, P, v, and C. Moreover, an empirical correlation for the Sherwood number is formulated. The developed empirical correlations are proved to be capable in predicting the VMD transport coefficients, and it may play a key in the process design and performance. [ABSTRACT FROM AUTHOR]

Water productivity and thermal efficiency in membrane distillation (MD) have been the main research targets for the aims of commercial application in desalination. The comprehensive understanding of the influence of module configuration parameters, operating conditions and their interaction on MD performance is the key for MD commercialization. In this paper, the multi-objective modeling and optimization in the vacuum membrane distillation were performed by response surface methodology and desirability function approach. A series of PVDF hollow fiber modules of different scale were used to provide the essential data and to verify the modeling program. The multi-objectives including water permeate flux ( J ), water productivity per unit volume of module ( P v ), gained output ratio ( GOR ), and a comprehensive index ( D m ) assessing the desired MD performances were predicted and experimentally verified. The influence of operating parameters (temperature, velocity, and concentration of feed) and membrane module parameters (membrane packing density and length–diameter ratio of module) and their binary interactions on the multi-objectives were investigated. It is found that among the investigated factors, feed inlet temperature and its interaction effect with module parameters play dominant roles on MD performance. Under the multi-objective optimum conditions, 4.85 × 10 3 kg/(m 3 h) of P v and 0.91 of GOR were achieved within the investigated range. Water productivity and thermal efficiency can be simultaneously enhanced by optimizing operating and module conditions with the approach developed in this study. [ABSTRACT FROM AUTHOR]

ABSTRACT In this article, using the non-solvent induced phase separation process, a new microporous membrane with the semi-interpenetrating polymer network (semi-IPN) structure was produced. For this membrane, polydimethylsiloxane (PDMS) polymer is crosslinking and poly(vinylidene fluoride) (PVDF) polymer is linear, by changing the mass ratio of PDMS/PVDF, the structure and the performance of the prepared membranes were studied. The membranes were also investigated by attenuated total reflection-Fourier transform infrared (ATR-FTIR), scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, thermogravimetric analysis, and water contact angle, etc. ATR-FTIR spectroscopy confirmed the formation of semi-IPN; compared with the PDMS/PVDF polymer without semi-IPNs structure, the viscosity of the semi-IPNs structured casting solution increased, membrane mechanical property increased but its hydrophobicity decreased. Using the resulting membranes for the vacuum membrane distillation desalt of the NaCl solution (30 g/L), 99.9% salt rejection and reasonable flux were obtained. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45792. [ABSTRACT FROM AUTHOR]

• A novel Janus membrane with multilevel hierarchical structures was prepared. • The novel Janus membrane simultaneously possesses antiwetting and high-efficiency desalination properties during VMD. • The coupled effects of super hydrophilicity brought about by PDA layer and self-healing mechanism provided by Fe 3 O 4 nanoparticles contributed to the superior performances of novel Janus membrane. In this study, we developed novel Janus membranes with multilevel hierarchical structures to prevent membrane pore wetting and enhance the performance for desalination simultaneously. Here, a Janus membrane with multilevel roughness was tailored via a polydopamine (PDA) coating followed by deposition of Fe 3 O 4 nanoparticles and PDA sealing. These novel Janus membranes demonstrated outstanding antiwetting properties and salt rejection performances in comparison to pristine polytetrafluoroethylene (PTFE) membrane. Particularly, the specific flux of Janus membranes was maintained in a relatively stable state (0.6–0.7) after membrane cleaning with pure water, meanwhile, the permeate conductivity was maintained in the range of 0–30 µS/cm with the desalination rate of 99.9% during the vacuum membrane distillation (VMD) tests by using 6.98 wt.% feed solutions, indicating that these Janus membranes (i.e., P/Fe/P-PTFE membrane) can improve the performance for VMD desalination without membrane wetting throughout the test duration. Furthermore, we ascribed these Janus membranes' superior performances to the coupled effects of super hydrophilicity brought about by PDA layer and self-healing mechanism provided by Fe 3 O 4 nanoparticles. This work highlights the great potential of novel Janus membranes with multilevel hierarchical structures for water reclamation from challenging high-salinity wastewater through VMD. [ABSTRACT FROM AUTHOR]

Developing cost-effective methods to enhance the mass transfer (i.e. water vapor flux) in vacuum membrane distillation (VMD) is of vital significance. In this study, we propose and demonstrate a simple but highly effective method to enhance the water vapor flux in VMD by introducing a gas flow on the vacuum side. The effect of the gas flow rate on the mass transfer performance in VMD was investigated by experimentation and computational fluid dynamics (CFD) simulation. Both experimental and modelling results confirm that gas flow on the vacuum side can effectively enhance mass transfer in VMD by minimising the boundary layer effect. A higher gas flow velocity results in a lesser boundary layer effect and reduced temperature polarization, thereby achieving a much higher water vapor flux. Specifically, when the gas flow rate is 10 L/min, the water vapor flux is eight times higher than that without gas flow (45.5 vs. 5.8 g/m2 h) on the vacuum side in VMD under the same vacuum pressure of 10 kPa. This study provides a cost-effective method to enhance the water vapor flux in VMD. It offers significant insights into understanding the mechanisms of vapor condensation and mass transfer in thermally-driven membrane processes. [Display omitted] • We demonstrate an effective way to enhance the water flux in VMD; • Gas flow on the vacuum side can effectively enhance mass transfer in VMD; • The key finding is confirmed by both experimentation and CFD simulation; • This study provides mechanistic understanding of condensation and mass transfer in MD. [ABSTRACT FROM AUTHOR]

[Display omitted] • Studied intensified MD integrated with MVC. • A comparative energy and exergy analysis is performed using the eNRTL model. • MVC enables superior latent heat recovery at the expense of the compression work. • MVC enhances the energy and exergy efficiency of the MSVMD system. Membrane distillation (MD) is a promising candidate for desalinating hypersaline brine, but its poor energy efficiency has remained a major barrier for widespread application. One possible solution to this issue is to recover the latent heat in the process. In this work, a multi-stage vacuum MD (MSVMD) was integrated with a mechanical vapor compressor (MVC) to enhance the latent heat recovery, and the energetic and exergetic performance of this integrated process was examined. A comprehensive energy and exergy analysis is provided to compare MSVMD and MSVMD-MVC processes for desalination of hypersaline brine. This analysis was conducted by examining the effect of the compression ratio on the energetic and exergetic performance, and the findings are reported in terms of specific thermal energy consumption (STEC), specific electricity consumption (SEC), and exergetic efficiency. The energy analysis shows that thermal energy consumption can be reduced as the compression ratio increases, due to the enhancement of latent heat recovery. The MSVMD-MVC process can be operated in a steady-state condition, without the need for thermal heat input; with STEC and SEC of 0 and 49 kWh/m3 at the feed temperature of 50 °C and MVC compression ratio of 2.14. Moreover, exergy analysis demonstrates the efficacy of the eNRTL model in exergy calculation. Exergy destruction can be greatly reduced by increasing the compression ratio to an optimal value. For high salinity brine (124 g/L), MSVMD-MVC achieved a higher exergetic efficiency of 6.85%, compared to 2.42% in MSVMD. The result suggests that the application of MVC can intensify the energy efficiency and exergetic efficiency of the MSVMD system, although this process cannot outperform the current desalination technologies from the standard primary energy point of view. [ABSTRACT FROM AUTHOR]

This study evaluates the recovery of pure water by vacuum membrane distillation (VMD). The VMD process has been experimentally studied using a polytetrafluroethylene (PTFE) membrane for aqueous sodium chloride solution containing salt concentration of 20,000 ppm and permeate pressure of 7 kPa. The percentage recovery was observed to increase with time of operation and in 183 h, it became 86%. A mathematical model for recovery has also been developed and validated with experimental data. Heat transfer and mass transfer correlations have been developed for the range of feed flow rate of 0.5–2 lpm and feed bulk temperature of 40–60°C. The effect of feed salt concentration and membrane fouling was also studied on permeate flux. Permeate flux was found to decrease by 15 and 10% at 55 and 60°C, respectively, on increasing feed salt concentration from 5 to 40 g/l. The permeate flux was observed to decrease by 8% from 26.1 to 24 kg/m2 h after continuous operation of 180 h at 60°C and 20,000 ppm feed salt concentration due to fouling. However, after washing the membrane with water, the flux regained up to 25.7 kg/m2 h. The fouling was confirmed in the study of membrane morphology using SEM images. [ABSTRACT FROM PUBLISHER]

Zeolite membranes represent a potential alternative to the polymeric ones for water desalination because of their high chemical stability and their peculiar ability to reject ions. In this work, supported zeolite membranes having Mordenite Framework Inverted (MFI) topology were scaled-up and membranes 30 cm long were prepared using the secondary growth method coupled with the cross flow seeding procedure. In particular, the influence of rotation time during the seeding procedure was evaluated. The prepared membranes were characterized by scanning electron microscope (SEM), X-ray diffractometry (XRD), Energy Dispersive X-ray (EDX) analysis and single gas permeation, and then underwent to Vacuum Membrane Distillation (VMD) tests using both deionized water and NaCl solutions at different concentrations. The experimental work allowed to identify the membrane able to provide interesting trans-membrane flux and salt rejection values. Specifically, permeate fluxes of 13.8kg m − 2 h − 1 and 8.2 kg m − 2 h − 1 , together with rejections of 99.8% and 99.6%, were achieved for the 0.2 M and 0.9 M feeds, respectively. No changes in the membrane structure after the exposure to salty solutions were detected by SEM and XRD analyses and its original performance was restored by means of an identified cleaning procedure. [ABSTRACT FROM AUTHOR]

The aim of this work was the development of a numerical methodology to simulate and study the dynamic behavior of vacuum membrane distillation (VMD) system. A time-dependent one-dimensional finite difference numerical model is developed to study the flow as well as thermal regimes and the dynamic operational performance of VMD under stepped flow disturbance function in the hot channel. The study simulates the process parameters including mass flow rates, temperatures, vapor pressures, and concentration distributions for the hot feed channels along the flat-plate membrane length with transmembrane flux. The developed dynamic tool predicts the hydrodynamic behaviors and thermal characteristics of the very thin channels of VMD module. A rectangular pulse disturbance function is applied to rise up the inlet hot mass flow rate from steady state (SS) value of 1 to 1.34 kg/s for 60 s. The model ran for 3 min, which included the 60 s of applied dynamic disturbance; it started at the 10th s and finished at the 70th s. The results show that the probability of the pressure rise and the salt precipitation (scaling problem) will appear on the second half of the membrane length more than on the first half. The hydrodynamic influence of the flow conditions on the permeate flux appears to be more significant at the feed side of the VMD module fillet. [ABSTRACT FROM PUBLISHER]

In this work, a mathematical model has been developed for vacuum membrane distillation incorporating molecular diffusion–Knudsen diffusion–Poiseuille flow and validated with the experimental data. The feed-side membrane surface temperature used in mathematical model was estimated by computational fluid dynamics modeling. The effects of different operating parameters such as feed flow rate, feed bulk temperature, inlet concentration, and permeate-side pressure on water flux and salt rejection have been studied using PTFE membrane. It has been shown that with variation in feed flow rate from 0.5 to 2 lpm, the permeate flux increased from 20.1 to 25.6 kg/m2 h. Similarly, with increase in feed bulk temperature from 40 to 60°C, the permeate flux increased from 2.10 to 26.6 kg/m2 h. The permeate flux showed the variation from 17.8 to 19.5 kg/m2 h on decreasing permeate pressure from 7 to 5.5 kPa. No significant effect of feed salt concentration was observed on permeate flux. More than 99% rejection was obtained at the feed salt concentration of 40,000 ppm of NaCl. Few runs were also carried out by taking multi-ion feed (Ca++, Mg++, Na+, K+, Cl−,, Fe++,, F−) equivalent to that in ground water; in which case also more than 99% rejection was obtained. [ABSTRACT FROM AUTHOR]

This work presents a study of the heat and mass transfer performance of desalination in a laboratory-scale rectangular cross-flow flat-sheet membrane module by vacuum membrane distillation (VMD) experiments. Results show that the traditional Nusselt and Sherwood correlations, which are frequently employed in the membrane distillation literature, are not suitably used to estimate the heat and mass transfer coefficients in the VMD system for Reynolds numbers ranging from 150 to 1400. In this study, it was observed that approximately 30% of the experimental data fit well with semi-empirical correlations whose empirical constants are a = 2.76 × 10 − 3 , b = 0.97 and c = 3.7909 . The heat transfer process is limited by the resistances in the feed boundary layer and the membrane. The heat transfer resistance in the membrane increases when that in the feed boundary layer decreases and vice versa. More than 50% of the heat transfer resistances occur in the liquid feed phase at feed flow rates below 1200 mL/min, whereas the remaining occur in the membrane itself. At feed flow rates that exceed 1200 mL/min, the heat transfer resistance in the membrane becomes dominant. The Knudsen-viscous resistance controls the mass transfer through the membrane while the mass transfer resistance in the liquid feed phase is absent. [ABSTRACT FROM AUTHOR]

In this work, an artificial neural network (ANN) model based on the experimental data was developed to study the performance of vacuum membrane distillation (VMD) desalination process under different operating parameters such as the feed inlet temperature, the vacuum pressure, the feed flow rate and the feed salt concentration. The proposed model was found to be capable of predicting accurately the unseen data of the VMD desalination process. The correlation coefficient of the overall agreement between the ANN predictions and experimental data was found to be more than 0.994. The calculation value of the coefficient of variation ( CV ) was 0.02622, and there was coincident overlap between the target and the output data from the 3D generalization diagrams. The optimal operating conditions of the VMD process can be obtained from the performance analysis of the ANN model with a maximum permeate flux and an acceptable CV value based on the experiment. [ABSTRACT FROM AUTHOR]

In this work, desalination tests were carried out by vacuum membrane distillation, with the aim of investigating the purity (in terms of conductivity) of the produced permeate. Consecutive experiments, with and without cleaning in between, were made on lab-prepared modules equipped with commercial capillary polypropylene membranes. In particular, by working at fixed operating conditions (feed flow rate and composition, temperature and vacuum pressure) the effects of a partial and of a total cleaning procedure on the permeate conductivity were analyzed and compared. Depending on the used approach, pure water or high-purity water can be obtained as permeate. [ABSTRACT FROM AUTHOR]

In the last years, intense research efforts were dedicated to the development of zeolite membranes in different separation processes. In this work, zeolite membranes were prepared and characterized by means of single gas and pure water permeation test. Moreover, the membranes performance in vacuum membrane distillation was also studied to determine their potentiality for desalination. Uncalcined defect free MFI (silicalite) membranes on different tubular supports were synthesized by using the secondary growth method coupled with the cross flow seeding procedure. Gas permeation tests showed that, using a support with pore size of 70 nm and a zeolite slurry concentration of 0.032 wt.%, uncalcined defect free MFI membranes were synthesized. To seal the defects formed after the calcination, two different curing treatments, new cross flow filtration with small crystals and dip coating with PDMS, were carried out. Very low defects concentration for the membranes cured with small crystals was evidenced in pure water permeation tests. In fact, permeance values in the nanofiltration range were obtained. Furthermore, these zeolite membranes were used in vacuum membrane distillation tests with promising results in terms of fluxes and salt rejections. [ABSTRACT FROM AUTHOR]

Abstract: Among four membrane distillation (MD) configurations, direct contact MD (DCMD) and vacuum MD (VMD) exhibit attractive characteristics from different perspectives and have great potential in treating reverse osmosis (RO) brine. Aiming at establishing a quick approach to predict the key output parameters associated with MD module performance and process efficiency, Aspen plus was employed to conduct systematic evaluation for both DCMD and VMD. Due to the lack of built-in MD operation models in Aspen Plus, one dimensional transport models were developed and complied as user customized units to simulate the hollow fiber-based DCMD and VMD modules. The corresponding programming was coded in FORTRAN language. The mathematical models for DCMD and VMD were verified by comparing the simulations results with the experimental and literature data, respectively. By incorporating the boundary-layer effect into the newly-established transport models, steady-state simulations of the respective DCMD and VMD flowsheets were carried out. The results showed that the DCMD presented much lower process efficiency than VMD in terms of permeation flux and specific energy consumption per kg distillate generated, even though it is considered as the simplest and most commonly employed configuration. With the same module specifications and operating conditions at equivalent energy cost, the VMD system demonstrated a minimal 2.5-fold higher average vapor flux (e.g., water recovery capacity) when compared to DCMD. The fundamental difference between the two configurations was revealed through MD mass- and heat-transfer analysis. Based on simulated temperature profiles, it was found that the VMD configuration presented a much higher driving force (transmembrane temperature difference) and negligible conductive heat loss to the membrane, which is a promising feature for achieving high thermal efficiency. [Copyright &y& Elsevier]

Abstract: The present paper introduces a comprehensive study of the performance of newly developed titanium oxide nanotubes (TNTs) incorporated into a Polyethersulfone (PES) blend membrane for desalination using vacuum membrane distillation (VMD) process. The study examines the effect of different operating conditions. The results showed a maximum salt rejection of 98% and a permeate flux of 15.2kg/m2 h at 7000ppm feed salt concentration for the TNTs–PES membrane at a temperature of 65°C and a vacuum pressure of 300mbar with feed flow rate of 11mL/s. A comparison between the performance of the developed TNTs-PES membrane, and commercial Polytetrafluoroethylene (PTFE) membrane was performed at different feed salt concentrations. The achieved results showed a significant improvement in the performance of the new membrane compared to the commercial PTFE membrane, where the salt rejection reached 99.3% at feed concentration 3000ppm and 96.7% at 35,000ppm using the new membrane, compared to salt rejection of up to 90.6% at 3000ppm and 62.5% at 35,000ppm using PTFE membrane. The dense TNTs layer formed on the top surface of the TNTs-PES blend membrane is considered a selective layer that prevents salt passage through the membrane. The decline in permeate flux may be overcome by membrane washing every hour. [Copyright &y& Elsevier]

A novel membrane distillation (MD) process enhanced by an asymmetric electric field was proposed. By applying an asymmetric electric field across a membrane with an increased field intensity in the direction of permeation, the permeation of water molecules through the membrane will be facilitated by the electric potential gradient, resulting in a higher flux than that achieved with the conventional process. The effectiveness of using asymmetric electric field to enhance mass transport was confirmed experimentally, and the effects of electric field parameters (including voltage, electrode distance, and electrode geometry) on water permeation were investigated. The novel process was tested for water desalination by MD and the results showed that both permeation flux and salt rejection were increased by the electric field. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2307-2313, 2014 [ABSTRACT FROM AUTHOR]

Abstract: Vacuum membrane distillation (VMD) for desalination of water using hollow fiber membranes with a shell-side feed configuration was investigated. The effects of membrane permeability, water salinity, feed temperature and flow rate on the water permeation rate were evaluated, and the water vapor pressure buildup in the fiber lumen was analyzed. The pressure buildup of water vapor in the fiber lumen was shown to adversely affect the driving force for water vapor permeation through the membrane pores, and a mathematical model was developed to describe the mass transfer in VMD by incorporating the permeate pressure build up. The model predictions were validated with experimental data. The higher the membrane permeability, the more significant the permeate pressure build-up. This is especially important to consider in module design for practical applications. For VMD of saline water, the membrane played a dominant role in the overall mass transfer process, and the significance of the effect of liquid phase resistance on water permeation depended on the membrane permeability. An increase in operating temperature increased the water productivity in VMD, primarily due to increased driving force for permeation, whereas the permeability of the membrane was not significantly affected. A factorial design experiment was carried out to illustrate the effects of main factors involving membrane permeability and operating parameters (feed concentration, temperature and flow rate) on the VMD performance, and some interactions among the effects were shown to occur as well. [Copyright &y& Elsevier]

Abstract: This paper presents the energy evaluation of the cross-flow vacuum membrane distillation (VMD) for three types of lab-fabricated polyvinylidene fluoride (PVDF) membranes and the commercial Westran S PVDF membrane. Membranes with the effective area 23.5cm2 are tested with distilled water and geothermal water as the feed solutions. Results show that the membrane porosity controlled the flux through the fabricated membranes and the commercial membrane. The commercial membrane with porosity of approximately 76.5%, which was the most porous among the tested membranes, gave the highest flux at 9.28kg/m2 h under the optimum conditions of 33.2L/h feed flow rate and 30kPa downstream pressure. The corresponding specific energy consumption was 66.03kW/kgh−1 when distilled water was examined. Heating energy of 87–89kW/kgh−1, which is approximately 95% of the total energy consumption, could be saved when the warm geothermal water is fed directly into the VMD system. The water produced meets the drinking water quality with the TDS varying between 102 and 119ppm, thus the geothermal water desalination using the VMD system to produce the drinking water is satisfactory. An economic analysis for a 20,000m3/d VMD desalination plant finds that the water production costs are $0.50/m3 and $1.22/m3 respectively for the plant operated with and without geothermal energy (GE). Compare to the plant without GE utilisation, the water production costs of the plant operated with GE are less than $0.50/m3 that is at least $0.72/m3 or approximately 59% in cost saving when the water fluxes are larger than 6.6kg/m2 h. The specific membrane cost reduced from $0.058/m3 to $0.035/m3 when the membrane life extended from 3 to 5years. [Copyright &y& Elsevier]

Abstract: An experimental based ANN model is constructed to describe the performance of vacuum membrane distillation process for desalination in different operating conditions. The vacuum pressure, the feed inlet temperature, the concentration of the feed salt aqueous solution and the feed flow rate are the input variables of this process, whereas the response is the permeate flux. The neural network approach was found to be capable for modeling this membrane distillation configuration. The application of Genetic Algorithm (GA) to optimize the ANN model parameters was also investigated. [Copyright &y& Elsevier]

Abstract: Membrane distillation (MD) has shown potential as a means of desalination and water purification. As a thermally driven membrane technology which runs at relatively low pressure, which can withstand high salinity feed streams, and which is potentially more resistant to fouling, MD could be used for desalination where reverse osmosis is not a good option. The use of thermal energy, rather than electrical energy, and the fact that MD membranes can withstand dryout make this technology attractive for renewable power applications as well. However, most research on MD has focused on maximizing membrane flux as opposed to minimizing energy consumption and cost, and current MD systems suffer from poor energy efficiency compared to other desalination systems. In solar driven systems, the reported thermal performance has not been much better than a simple solar still. This paper examines the energy efficiency of single-stage MD-based desalination cycles in each of the MD configurations commonly used for desalination (direct contact, air gap, and vacuum) and compares the gained output ratio, or GOR, of each configuration across the range of membrane module geometries, and operating conditions. Limitations of each configuration are identified. Direct contact MD and air gap MD, in particular, have potential for high GOR. [Copyright &y& Elsevier]

Abstract: Microporous hydrophobic poly(vinylidene fluoride) (PVDF) hollow fibres were prepared by the dry–wet spinning technique. N,N-Dimethylformamide (DMF) was used as solvent, while water and poly(vinyl pyrrolidone) (PVP) were used as pore forming additives. Mixtures of DMF or ethanol in water were employed as bore fluids. The influence of different parameters on the fibres characteristics was explored. Particular attention was focused on the PVP concentration and on the composition of the bore fluid. The obtained fibres exhibit good structure, excellent mechanical properties, high porosity (up to 80%) and an average pore size ranging from 0.12 to 0.27μm. Fibres were tested in vacuum membrane distillation (VMD) configuration, using distilled water as feed. The effect of membrane properties on the water vapor fluxes was investigated. For the prepared membranes, the measured fluxes ranged between 3.5 and 18kg/m2 h at 50°C and 20mbar vacuum pressure. Some selected fibres were assessed for long term stability. Constant performances were observed up to 2 months. Furthermore, preliminary VMD tests on salty water were also performed, in order to verify fibres suitability for seawater desalination. [ABSTRACT FROM AUTHOR]

Abstract: This research focuses on desalination via vacuum membrane distillation (VMD). In order to enhance the performance of VMD in desalination and to get more flux, effects of operating parameters on the yield of distillate water were studied. Four parameters at three levels were selected: temperature (35, 45, and 55°C), vacuum pressure (30, 80, and 130mbar), flow rate (15, 30, and 60mL/s) and concentration (50, 100, and 150g/L). Taguchi method was used to plan a minimum number of experiments. The optimal levels thus determined for the four factors were: temperature 55°C, vacuum pressure 30mbar, flow rate 30mL/s and concentration 50g/L. The results show that increasing temperature and decreasing vacuum pressure improve permeate flux. However, the permeate flux increases with increasing flow rate initially and then reaches to a maximum value at 30mL/s and then decreases with increasing the flow rate. [Copyright &y& Elsevier]

Abstract: This research focuses on vacuum membrane distillation (VMD) using high-concentration NaCl aqueous solutions as feed. A new membrane module was investigated to improve water desalination and experiments were carried out using a commercial polypropylene (PP) membrane with a pore size of 0.2μm. In order to enhance performance of VMD in desalination and to get more flux, effects of operating parameters on the performance were studied. Water fluxes were measured at different feed temperatures, feed concentrations, vacuum pressures and flow rates. The new configuration provides better mixing and this increases heat and mass transfer coefficients, and as a result, reduces temperature and concentration polarization effects. [Copyright &y& Elsevier]

Abstract: In this paper, a pilot vacuum membrane distillation device, using polypropylene hollow fiber, was designed and installed on a seagoing vessel to investigate the operating conditions. During operation, the incoming seawater was heated by the waste-heat generated from the vessel engine. It was found that this device could reach a desalting degree of 99.99% and a membrane flux of 5.4 kg/m2h at 55°C and −0.093 Mpa. The results obtained from this pilot test are in accord with the previous lab experiment results. [Copyright &y& Elsevier]

In order to enhance the performance of the vacuum membrane distillation process in desalination, i.e. to get more flux, it is necessary to study the effect of operating parameters on the yield of distillate water. Simple techniques including the normalized dimensionless sensitivity factor and temperature polarization coefficient as well as the solution of the transport models were used for this purpose. The sensitivity of the mass flux to the process operating parameters including downstream pressure, feed temperature, feed flow rate, and membrane permeability were investigated. The mass flux of distillate water was highly sensitive to the feed temperature especially at high values of vacuum pressure. The mass flux was more sensitive to the vacuum pressure at low feed temperature levels than at the high ones. Since lowering the temperature polarization coefficient is essential to enhance the process performance considerable efforts should be directed toward maximizing the heat transfer coefficient through better module design. The predictions of the normalized dimensionless sensitivity analysis were in agreement with the results obtained from solving the transport model of the process. [Copyright &y& Elsevier]