Vacuum Membrane Distillation Process Research Articles

Nanoporous organosilica membrane for water desalination: Theoretical study on the water transport

An unconventional nanoporous organosilica membrane has been tested in a vacuum membrane distillation (MD) process for water desalination. We propose a modified approach to understand the transport mechanism of water molecules through the nanopores of this membrane. The modified approach stems from the fact that the membrane has a hydrophilic surface (contact angle <90°) and so capillary pressure, which draws liquid water into the nanopore, must be considered when establishing the mathematical model. However, increased friction arising from the dramatic increase in shear viscosity of water in nano-confined spaces balances the capillary flow against the evaporative mass transport to avoid pore wetting. Notably, the liquid/vapor interface is no longer formed at the pore entrance as with a conventional hydrophobic membrane, but rather exists deeper in the pore channel as a consequence of capillary pressure. This was backed by experimental observations (no pore wetting) and SEM evidence which showed salt nucleation and growth existed only on the membrane surface, and did not infiltrate the membrane support layers. The impacts of pore size, membrane thickness, substrate thickness, concentration polarization, porosity, and contact angle on water flux and pore intrusion depth were tested using the model. Pore size was the most influential parameter with an >80% increase in permeation flux if the pore size increased from 2 to 3nm at 60°C. However, pore wetting is expected if dp>3.4nm, particularly at low temperatures where the slower evaporation rate promoted greater pore intrusion. Concentration polarization was shown to be negligible which agreed well with experimentally observed water fluxes which remained relatively constant despite feed salinity increasing from 0 to 150gL−1. Lastly, the membrane hydrophilicity was found to impact on water flux and pore intrusion in a complex relationship with pore size. Ultimately, hydrophilic pores less than 3nm in diameter offer a good combination of good water flux and minimal water intrusion suggesting that ordered mesoporous organosilica membranes have potential in MD applications.

Simulation of Vacuum Membrane Distillation Process for Desalination with Aspen Plus

This paper presents a simulation study of vacuum membrane distillation (VMD) for desalination. A simulation model was built on the Aspen Plus platform as a user defined unit operation for the VMD module. A simplified mathematical model was verified, and the analysis of process performance based on simulation was also carried out. Temperature and concentration polarization effects are significant in the conditions of higher feed temperature and/or vacuum pressure. The sign of (PI,f – PI,p), difference of the vapor pressures between the membranes at the interfaces, is an indicator of the vacuum pressure threshold. Increasing the vacuum pressure at lower feed temperature is an effective way to increase the permeate flux and reduce the energy consumption simultaneously.

Treatment of bentazon herbicide solutions by vacuum membrane distillation

Experimental investigations were carried out to treat bentazon solutions by the vacuum membrane distillation process (VMD). The effects of several parameters, including feed temperature, feed flow rate, vacuum pressure and initial bentazon feed concentration on flux quality and quantity were studied. The VMD process was found to be effective in the treatment of the bentazon solutions as the permeate was absolutely pure water. Results showed that increasing temperature and flow rate and decreasing vacuum pressure and bentazon concentration enhance permeate flux. Temperature and vacuum pressure were found to be the most important factors for permeate flux. Under the best conditions (a feed temperature of 60°C and a vacuum pressure of 30mbar), a maximum permeate flux of 92.94kg/m2h was obtained. Also, a mathematical model describing the variation of bentazon concentration with time was developed and validated with the experimental results.

Vacuum membrane distillation simulation of desalination using polypropylene hydrophobic microporous membrane

ABSTRACTNumerical simulation is an effective method to get the optimal operating parameters in the chemical engineering process. In this work, the transport mechanism of vacuum membrane distillation (VMD) process was simulated and predicted by mathematical model, which was established based on the convective heat transfer coefficient, and 0.5M aqueous NaCl solution was concentrated with isotactic polypropylene (iPP) hydrophobic microporous membrane prepared via thermally induced phase separation (TIPS) in the VMD process. The as‐presented mathematical model simulated the effects of different operating parameters on the VMD performances for aqueous NaCl solution, such as feed temperature, feed flow rate, absolute pressure of membrane permeate side, temperature coefficient, membrane thickness, and porosity. A comparison between experimental data and simulated data was also considered to verify the proposed mathematical model. Additionally, the salt rejection of aqueous NaCl solution production water in VMD was higher than 99.9%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41632.

Brackish water treatment for reuse using vacuum membrane distillation process

The availability of fresh water is vital for all human activities and in particular for improving living conditions, health and overall well-being. Pressure on scarce fresh water resources can be reduced by treating and reusing brackish water by advanced membrane treatment technologies. In this study, brackish water originating from effluent discharge of a local coal mine, seawater, groundwater and salt water swimming pool is treated by a laboratory-scale vacuum membrane distillation (VMD) system. VMD is an emerging technology that has the potential to become as important as conventional distillation system and aims to remove particles and dissolved impurities by evaporation and condensation techniques that mimic what occurs in nature. This study validates the mathematical modeling of the transport mechanisms used in the VMD process using data collected for different experimental situations. The response of flux rate to various process operating parameters, including pressure, temperature, flow rate and salinity concentration, is also demonstrated. This thermally driven separation process enables to remove 99.9% of total dissolved solids (TDS) from brackish water. The quality of the permeate water from all four water sources studied is of acceptable standards for potable use; however, it requires mineralization efforts before direct consumption.

Three-dimensional numerical simulation of aqueous NaCl solution in vacuum membrane distillation process

A three-dimensional numerical simulation of a vacuum membrane distillation (VMD) process for aqueous NaCl solution was conducted to understand the vapor and liquid distribution in the hollow fiber membrane module. This understanding is critical for membrane module design as well as heat and mass transfer enhancement. Computational fluid dynamics were used to simulate the VMD process using FLUENT software, and the mixture model and standard k–ϵ model were used to describe the turbulent flow. The vapor and liquid distribution along the entire membrane module were simulated. The membrane thickness, feed temperature, pressure, parameters on the boundary layer, and cross-section in the VMD process were also studied. It was shown that the pressure change and liquid volume fraction change were mainly in membrane silk. The vapor and liquid were found to be distributed inside the membrane silk and shell, respectively. Most of the changes in pressure, temperature, and phase transition occur on the boundary layer in the membrane silk.

Effect of operating parameters on temperature and concentration polarization in vacuum membrane distillation process

Membrane distillation is a process combining thermal and membrane technology, which uses the vapor pressure difference as driving force because of the temperature difference. Of the MD configurations, VMD can achieve high flux due to the application of vacuum. This paper analyzes operating parameters (vacuum pressure, feed flow rate, and feed concentration) that affect permeate flux in a modified design VMD system vacuum multi-effect membrane distillation (V-MEMD). The study investigated the most suitable operating parameters that would enhance the performance of V-MEMD system at high salinity (1–4M of NaCl) with a moderate bulk temperature of 60°C. The result of this study showed that vacuum pressure is the most influential factor of the selected, and the effect of temperature polarization is more dominant to permeate flux than concentration polarization. Above this, the high-salinity solution can be treated by V-MEMD system because it does not affect the permeate total dissolved solid and flux. A dominant reduction ratio of vapor pressure was observed even at a low ratio of temperature polarization.

Treatment of Industrial Wastewater Containing High Levels of Ammonia and Salt Using Vacuum Membrane Distillation

The wastewater containing extreme high levels of ammonia and salt was treated by vacuum membrane distillation (VMD). The effects of feed flow rate, temperature and vacuum degree on ammonia removal efficiency (ARE) were investigated systematically. The ARE can be promoted by increasing feed flow rate, feed temperature and vacuum degree in this study. The theoretical mass transfer model was obtained based on series of theory derivation, and the theoretical released ammonia is consistent with the experimental data at conditions ranged in this study, indicating that the developed model is suitable to evaluate the ammonia removal during VMD process.

Vacuum membrane distillation to remove ethyl acetate from aqueous solutions

Water pollution of volatile organic compounds (VOCs) was a serious environment problem. Vacuum membrane distillation (VMD) was an environmental friendly membrane separation technology to treat VOCs wastewater. Aqueous binary mixtures of ethyl acetate were treated by VMD process. Comparisons were made between different membrane modules for investigating the influence of channel on the membrane separation performance. Three kinds of channel presented different useful area, shape, width, and depth. The arc flow channel was more easier to reduce resistance than square flow channel. Operation parameters influencing flux and separation performance were examined: feed-flow rate, feed concentration, vacuum degree, and feed temperature. Vacuum degree and feed temperature had more significant influence on membrane flux and separation factor than other parameters. The effect of membrane type on separation properties was also investigated, the smaller pore membrane was better for increasing the separation factor. Three membrane modules with various channels were used for experiments. VMD is very suitable for low concentration of aqueous solution VOC separation and better experimental separation performance could be obtained.

Vacuum membrane distillation–crystallization process of high ammonium salt solutions

The batch mode of vacuum membrane distillation–crystallization (VMDC) process was carried out in the ammonium salt solutions. Vacuum membrane distillation (VMD) was used to permeate water to a desired concentration and the crystallizer was applied for the precipitation of crystals. The influence of concentration time on the permeate flux was investigated and the transfer resistances were estimated. Membrane fouling resistance increased gradually when the solution was concentrated up to the critical point of supersaturation factor. Beyond this critical point, the NH4Cl system revealed an irreversible membrane fouling, whereas the (NH4)2SO4 system revealed a reversible membrane fouling. By analyzing the transfer resistances, the determining concentration in the VMD process could be obtained, which would provide a way of avoiding membrane fouling problem. The permeated total nitrogen (TN) increased significantly from 4.7 to 59.8 mg/L with the NH4Cl solution. However, low level permeated TN (TN < 3 mg/L) with (NH4)2SO4 solution was maintained during the whole concentration process. The NH4Cl crystals with dendritic structure and (NH4)2SO4 crystals of lamellar form were obtained by the crystallizer with the production of 125.7 and 58.4 kg/m3, respectively.

Performance of a newly developed titanium oxide nanotubes/polyethersulfone blend membrane for water desalination using vacuum membrane distillation

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/m2h 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.

FEP Hollow Fiber Membrane and Its Membrane Distillation Process

The influence of draw ratio on the structure and properties of poly(tetrafluoroethylene-cohexafluoropropylene)(FEP) hollow fiber membrane which was fabricated by melt-spinning stretching(MS) method was investigated in this article.The primary FEP hollow fiber membrane structure was compact while the porous structure occurs after stretching.Furthermore,the increase of membrane draw ratios induced the obvious increase of membrane porosity and the N2flux while the decrease of the liquid entrance pressure(LEP).Meanwhile,the FEP hollow fiber membrane was applied to the vacuum membrane distillation(VMD) process to compared with M-S PP hollow fiber membrane which was the commercial product.The results show that the hydrophobic properties,LEP and mechanical strength of FEP hollow fiber membrane was better than PP hollow fiber membrane.The strong and stable of hydrophobic properties enabled the FEP membrane unpenetrated,which kept the desalination rate maintain up to 99%.The larger inner diameter(about 0.74 mm) of FEP hollow fiber membrane induced the use of internal pressure VMD type which brought about the higher VMD flux when the feed temperature increased.

지열에너지와 진공 막 증류법을 활용한 해수담수화 연계형 공정의 경제성 분석

Because of the water scarcity caused by the increase of salinity in the underground water, seawater desalination stands out as one of the most promising solution. As there are so much energy costs in operating desalination plants, new hybrid process which is more effective should be researched. A geothermal VMD (vacuum membrane distillation) hybrid process is a competitive alternative for seawater desalination. Because geothermal energy has significant characteristics of high capacity factor to operate the power plant at full capacity for 24 hour per day, it can be a priority heat source of VMD superior to any other renewable energies such as solar and wind power. In this study, we design a geothermal VMD hybrid process, analyze it economically and finally compare the result with a case of conventional VMD process. Geothermal VMD hybrid process generates $23,822,409 of NPV (net present value) more than the conventional VMD process in case of 5% discount rate. The break-even point between these processes is 5.36 year. Sensitivity analysis indicates that steam cost is the most decisive influence variable to the economic outcome.

Numerical modeling and optimization of vacuum membrane distillation module for low-cost water production

A two-dimensional numerical model, involving energy conservation, momentum transport and continuity equations, was developed to provide the profiles of temperature, velocity and pressure of vacuum membrane distillation (VMD) process using hollow fiber module. The theoretical prediction was in good agreement with the experimental results from literature. Four design and operation variables, including feed temperature, hollow fiber length, feed volume flow rate and vacuum pressure, were optimized to minimize water production cost (WPC) using genetic algorithm (GA). A case study shows that the WPC can be decreased by 38.1% in comparison with the non-optimized VMD process. Meanwhile, a general guidance for reducing the WPC was also provided. The feed temperature should be adopted as high as possible, while the hollow fiber length cannot be too long to result in a decrease in water flux and an increase in pressure drop. The flow rate is recommended to take a lower bound value and the moderate degree of vacuum is preferable. This study indicates the importance of VMD system optimization to minimize water production cost.

A Pervaporation Study of Ammonia Solutions Using Molecular Sieve Silica Membranes

An innovative concept is proposed to recover ammonia from industrial wastewater using a molecular sieve silica membrane in pervaporation (PV), benchmarked against vacuum membrane distillation (VMD). Cobalt and iron doped molecular sieve silica-based ceramic membranes were evaluated based on the ammonia concentration factor downstream and long-term performance. A modified low-temperature membrane evaluation system was utilized, featuring the ability to capture and measure ammonia in the permeate. It was found that the silica membrane with confirmed molecular sieving features had higher water selectivity over ammonia. This was due to a size selectivity mechanism that favoured water, but blocked ammonia. However, a cobalt doped silica membrane previously treated with high temperature water solutions demonstrated extraordinary preference towards ammonia by achieving up to a 50,000 mg/L ammonia concentration (a reusable concentration level) measured in the permeate when fed with 800 mg/L of ammonia solution. This exceeded the concentration factor expected by the benchmark VMD process by four-fold, suspected to be due to the competitive adsorption of ammonia over water into the silica structure with pores now large enough to accommodate ammonia. However, this membrane showed a gradual decline in selectivity, suspected to be due to the degradation of the silica material/pore structure after several hours of operation.

Water reclamation from shale gas drilling flow-back fluid using a novel forward osmosis–vacuum membrane distillation hybrid system

This study examined the performance of a novel hybrid system of forward osmosis (FO) combined with vacuum membrane distillation (VMD) for reclaiming water from shale gas drilling flow-back fluid (SGDF). In the hybrid FO-VMD system, water permeated through the FO membrane into a draw solution reservoir, and the VMD process was used for draw solute recovery and clean water production. Using a SGDF sample obtained from a drilling site in China, the hybrid system could achieve almost 90% water recovery. Quality of the reclaimed water was comparable to that of bottled water. In the hybrid FO-VMD system, FO functions as a pre-treatment step to remove most contaminants and constituents that may foul or scale the membrane distillation (MD) membrane, whereas MD produces high quality water. It is envisioned that the FO-VMD system can recover high quality water not only from SGDF but also other wastewaters with high salinity and complex compositions.

Numerical modeling of the vacuum membrane distillation process

In this study, one-dimensional vacuum membrane distillation (VMD) model is suggested for predicting the performance of seawater desalination to evaluate the performance of hollow fiber type VMD module. The energy and momentum balance equations, as well as the heat and mass transfer equations, are simultaneously solved to determine the concentration of NaCl, temperature and velocity distribution of feed side along the module length, productivity of distilled water and specific energy consumption. The productivity increases with an increase in the inlet feed temperature and velocity, the number of fibers and the total module length. However, it decreases with an increase in the mass fraction of the feed. Specific heat energy consumption decreases with an increase in the inlet feed temperature and velocity and total module length, but it increases with an increase in mass fraction of the feed and number of fibers. The specific electrical energy consumption increases with an increase in mass fraction, velocity and total module length and a decrease in inlet feed temperature and number of fibers, but the specific electrical energy consumption is too small and negligible in comparison with the specific heat energy consumption (less than 2%) on various operating conditions and module dimensions.

Numerical simulation studies on heat and mass transfer using vacuum membrane distillation

A general 2D mathematical model was developed to simulate the purification of water from volatile organic compounds (VOCs) via vacuum membrane distillation (VMD) process in hollow fiber membrane contactors. The model was developed for hydrophobic membrane material conditions, taking into consideration axial and radial diffusion in the tube, membrane and compartments of the contactor and was simplified to the two-dimensional structure with a single porous membrane wall. The simulation has studied the mass and heat transfer of VMD system in the porous media, in which aqueous volatile organic solution was considered as an incompressible and steady fluid. Effect of the downstream pressure on the removal of 1, 1, 1-trichloroethane (TCA) was studied to validation of simulation results with experimental data that it was obtained from literature. The temperature, Reynolds number, and total mass flux (convective and diffusive) distribution of TCA are determined in the membrane module. POLYM. ENG. SCI., 54:2553–2559, 2014. © 2013 Society of Plastics Engineers

Effect of pretreatment on membrane fouling and VMD performance in the treatment of RO-concentrated wastewater

The polyvinylidene fluoride hollow fiber hydrophobic membrane was applied in vacuum membrane distillation (VMD) process for the treatment of reverse osmosis-concentrated wastewater from a steel plant. The effects of pretreatment methods on the CODcr removal, the performance and membrane fouling of VMD process were studied. The morphology and composition of the fouling layer were studied by scanning electron microscope and energy dispersive spectroscopy, respectively. The reason for the flux declination in VMD process was discussed. The results showed that the CODcr removal reached 40%, and the flux of VMD process was increased after the pretreatment of coagulation and ultrafiltration, when the concentration factor reached eight, the flux of VMD process using properly pretreated solution was 30% higher than that using un-pretreated solution as the feed. The membrane surface morphology and elemental analysis testified that the fouling of membrane surface was obviously alleviated by pretreatment.

A ballistic transport model for vacuum membrane distillation

Vacuum Membrane Distillation (VMD) as other membrane distillation (MD) techniques is receiving much attention for its ability to produce drinkable water at low cost. Aiming to analyze the VMD process and particularly mass transfer through porous membranes, a ballistic flow model is developed to predict fluxes out from high aspect ratio cylindrical pores. The model is based on the estimation of direct and indirect transmission probabilities by direct integration of species flux. A power law that uses the pore aspect ratio is developed to enable a quick estimation of the flux fraction exiting the pore. The flux entering the pore is calculated using Hertz–Knudsen–Schrage equation and depends on the values of the condensation and evaporation coefficients of the transferred species. Simulation results are compared to classical Knudsen diffusion predictions as well as to experimental data.