Vacuum Membrane Distillation Module Research Articles

Modeling of Integrated Hollow-Fiber Solar-Powered VMD Modules for Desalination for a Better Understanding and Management of Heat Flows.

The direct integration of membrane distillation and solar energy collection in a single module is a promising technology for autonomous seawater desalination in remote regions; however, the modeling and design of such modules are challenging because of the coupling of the radial and longitudinal heat and mass transfers. In a previous study, we provided as a first modeling approach a hollow fiber solar collector vacuum membrane distillation (VMD) module, considering a constant temperature at the shell side and a pure water feed. Here, a full model is developed to describe the coupled effects of the solar collector and a hollow fiber VMD module operating in an outside/in mode with saline water. The model considers all the main phenomena (membrane distillation, temperature and concentration polarization, absorption of solar radiation and energy balances over the solar collector, radial and longitudinal heat and mass transfer, seawater properties, and more than 30 variables). Applied to simulate the behavior of a semi-industrial-scale module, it allows the influence of solar radiation on the performance/limits of the integrated module to be discussed based on the radial and longitudinal profiles and heat flows. The model can be used to identify key points in the module design to better utilize solar radiation and manage heat flows.

Performance comparison of vertical and horizontal vacuum membrane distillation module coupled with mechanical vapor recompression

This paper focuses on the comparative study of vertical and horizontal vacuum membrane distillation (VMD) modules coupled with mechanical vapor recompression (MVR) system to obtain the optimal system layout structure and further improve operation stability and energy efficiency in the wastewater treatment. Firstly, freshwater accumulation characteristic in the vertical VMD module was explored under different working conditions. Then, comparative experiments of vertical and horizontal VMD-MVR systems were conducted. Finally, comprehensive performance of horizontal VMD-MVR system were evaluated. The obtained results showed that freshwater production in the distilling tank and freshwater accumulation in the VMD module were affected by operation time, feed temperature, feed flow rate, feed concentration and vacuum side pressure. Furthermore, the horizontal VMD-MVR system was proved to be a better choice due to lower freshwater accumulation in the VMD module and energy consumption in comparison to the vertical VMD-MVR system. Finally, long term operation experiment of 240 h indicated evaporation rate dropped to 55 kg/h as operation time reached to 180 h owing to membrane fouling. After membrane cleaning, evaporation rate recovered from 55 to 62 kg/h with a higher recovery efficiency of 95.1 %. Compared to conventional wastewater treatment systems, the horizontal VMD-MVR system exhibited excellent comprehensive performance.

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.

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.

Techno-Economic Analysis of Vacuum Membrane Distillation for Seawater Desalination

Seawater desalination is an affordable and viable solution to the growing freshwater scarcity problem in water scarce regions. The current study focuses on cost analysis of Vacuum Membrane Distillation (VMD) setup for removing salts from water. The membrane used in the flat sheet VMD module was Polytetrafluoroethylene (PTFE) with 250 mm × 200 mm dimensions and 165 µm thickness. The experiments were carried out with variations in parameters such as velocity, pressure, concentration, and temperature. For the cost analysis, the operational, maintenance, instrumentation, and capital cost of the lab model was considered and then upscaled. A range of experiments was performed for NaCl and KCl under variations of operating parameters. It was noted that, for the NaCl solution, the increase in temperature from 50 °C to 70 °C doubled the permeate flux. However, for the conditions tested, the concentration shift from 0.25 M to 0.75 M decreased the permeate flux by 1.4% because the increase in ion concentrations along the membrane lowers the vapor pressure, restricting the permeate flux. The results trend for the KCl solution was similar to the NaCl; at temperature T1, it was noted that increased concentration from 0.25 M to 0.75 M significantly reduces the permeate flow. The reduction in permeate flow was nonlinear for a given pressure 30 kPa and velocity 5.22 m/s, but linear for all other variables. It was also observed that with an increase in temperature from 60 °C to 70 °C, the permeate flux for concentration 0.25 M was 49% for all the combinations of pressure and velocity. In addition, permeate flow increased 53% from temperature 50 °C to 60 °C and 49% from temperature 60 °C to 70 °C for both the solutions at a concentration of 0.25 M. This shows that the temperature also had a profound impact on the permeate flux. The economic analysis and market survey shows that the cost of clean water at the lab level was high which can be significantly reduced using a large-scale setup providing 1,000,000 L/H of distilled water.

Performance Assessment of Coupled Concentrated Photovoltaic-Thermal and Vacuum Membrane Distillation (CPVT-VMD) System for Water Desalination

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.

Numerical investigation on mass and heat transfer performance for novel vacuum membrane distillation modules enhanced by semicircular spacers

Semicircular spacers are creatively introduced into the micro vacuum membrane distillation (VMD) modules for the purpose of modifying flowing field, decreasing mass transfer resistance and enhancing water production. A two-dimensional computational fluid dynamics model is developed to conduct the performance evaluation of rectangular micro-VMD modules. Calculated results indicate that novel VMD modules obtain remarkable improvement in terms of transmembrane mass flow rate. Local velocity and vorticity magnitude around the semicircular spacers increase with augment of the dimension of semicircular spacers as revealed in flow performance evaluation. Thermal performance displays that temperature distribution evolves into uniform when the semicircular spacers are introduced and noticeable improvement can be observed in transmembrane heat flux rate, local temperature and temperature polarization coefficient (TPC) performance. Whereas, Nu number varies in a certain range depending on the configuration of semicircular spacers. This paper further discloses the effect of operating parameters on the mass transfer, heat transfer and TPC performance. It is theoretically found that performance of proposed VMD modules is more sensitive to feed inlet temperature rather than feed inlet velocity. In conclusion, semicircular spacers are believed to be effective strategies to enhance performance of micro-VMD modules, with remarkable improvement to flow field, heat and mass transfer.

Study on the synergistic heat transfer of double boundary layers in the jacketed vacuum membrane distillation process

During the process of membrane distillation (MD) heat recovery, due to the adverse effects of the heat exchange boundary layer and membrane surface boundary layer, the latent heat is hindered in its transfer and ultimately leading to the loss of heat recovery and mass transfer capacity of the system. In this study, a novel type of jacketed hollow fiber vacuum membrane distillation module (J-HF-VMD-M) has been successfully developed. It has the dual functions of “feed evaporation” and “vapor condensation”, which is helpful in reducing the volume and complexity of the system. Based on the structural characteristics of the jacketed module, a “Synergistic heat transfer model of double boundary layers” (SHT-DBL Model) is proposed for the first time. Compared with the previous “independent heat transfer” method between boundary layers, the “synergistic heat transfer” can overcome the above challenges more effectively. Compared with the non-heat recovery process in experimental range, the equivalent membrane distillation flux of the jacketed-type heat recovery process increased by 246.1 %, especially the gain output ratio increased by 145.1 %, and the corresponding increment is more significant than that of the traditional heat recovery process.

Reclaiming water from a direct air capture plant using vacuum membrane distillation – A bench-scale study

In the present work, vacuum membrane distillation (VMD) was evaluated for its ability to be integrated with a direct air capture (DAC) process as a sustainable combined water-CO2 recovery approach. Experiments were performed using four different VMD modules over a range of feed temperatures, feed flowrates, feed CO2 loadings, and vacuum pressures (50–60 °C, 7.6–17.6 L/h, 0–0.3 mol CO2/mol amine, and 7–12 kPa, respectively). The results showed that a commercially available LiquiCel module and a custom-made hollow fibre module (developed for this study) were the most durable (in terms of their tolerance towards the feed solvent that consisted of a basic CO2 capture solvent) among the four studied modules. These modules also produced the highest distillate fluxes (of 0.17 and 0.91 kg/m2.h for the LiquiCel and custom-made modules, respectively) under a 60 °C feed temperature, a feed flowrate of 17.6 L/h, and a 7 kPa (abs.) vacuum pressure. Moreover, under the same operating conditions, the LiquiCel and the custom-made modules exhibited salinity and dissolved oxygen (DO) removals of 96.47% and 50.98%, and 99.95% and 71.58%, respectively. The pH values of the distillates under these conditions were 10.87 and 10.37 for the custom-made and the LiquiCel modules, respectively, indicating that some basic species moved across the membranes. Furthermore, the results show that, when a CO2-loaded absorption liquid was used as the feed stream, the VMD flux was lower, but had lower conductivity and neutral pH than when a CO2-free solvent was used as the feed. Taken together, this work indicates that coupling DAC with VMD represents a viable pathway for sustainably reclaiming water from industrial processes and carbon capture systems for power plants.

CFD Model Development and Experimental Measurements for Ammonia–Water Separation Using a Vacuum Membrane Distillation Module

CFD Model Development and Experimental Measurements for Ammonia–Water Separation Using a Vacuum Membrane Distillation Module

Comparative study of small-scale flat-plate direct contact membrane distillation and vacuum membrane distillation modules with integrated direct solar heating

An intensified desalination module directly integrating membrane distillation (MD) and solar flat-plate collector (FPC) into the same small equipment is explicitly modeled and studied in the present article. Based on a previous work applying vacuum MD (VMD) to the integrated module and the corresponding recycling system, direct contact MD (DCMD) is also adopted in similar configurations to comparatively study the dynamic performance, the impact of different parameters, and the potential production of both DCMD-FPC and VMD-FPC modules. Simulation results for a 0.35 m2 module and a 12-hour operation show diametrically opposite performances in terms of water production and electricity consumption of these two desalination devices. Analyses on the influences of parameters indicate that the productions of both systems are limited by the incoming solar energy, and the DCMD-FPC system further suffers from conductive heat loss across the membrane. In order to relieve the performance from the restriction of available thermal energy, two approaches by heat recovery and solar concentration are applied to both DCMD-FPC and VMD-FPC to examine the potential enhancement. The results on both approaches all recommend the choice of VMD-FPC for small-scale applications, based on its more relevant productivity and less space footprint.

Experimental investigation on the mechanical vapor recompression evaporation system coupled with multiple vacuum membrane distillation modules to treat industrial wastewater

• A multiple vacuum membrane distillation modules system (MVMD) was constructed. • A mechanical vapor recompression system coupled with MVMD was constructed. • An experimental study of the MVMD and MVR-MVMD systems was conducted. • Economic assessment of the MVMD and MVR-MVMD systems was made. • The MVR-MVMD system can save 79.24% energy than that of the MVMD system. Excessive population growth has increased the importances of industrial wastewater treatment as well as energy conservation. This paper presented an experimental study of a mechanical vapor recompression evaporation system coupled with multiple vacuum membrane distillation modules (MVR-MVMD) for energy saving in the wastewater treatment process. For this aim, the MVMD system was constructed to investigate the water production performance in the VMD process, and then the MVR was joined to the MVMD system to examine the energy-saving performance. The experiment results showed that: In the experiment of the MVMD system, an excellent water production performance was observed with the sodium chloride solution as evaporation object, and the parameters that significantly affected the water production performance were obtained as follows: feed temperature was the highest effect, and then permeate side pressure, feed flow rate, and feed concentration had the lowest effect through an orthogonal multi-factor experiment. In the experiment of the MVR-MVMD system, the specific heating energy consumption was 81.95 kWh t −1 , which could save 79.24% energy than that of the conventional MVMD system, at conditions of feed concentration of 5%, feed flow rate of 25 m 3 /h and membrane area of 200 m 2 . Finally, a detailed economic assessment had been made concerning the capital investment, operation, and environment costs compared to the MVMD system. From a long-term perspective, the MVR-MVMD system showed excellent economic performance comparable to the MVMD system.

Multiphysics Modeling and Analysis of a Solar Desalination Process Based on Vacuum Membrane Distillation.

A hollow fiber vacuum membrane distillation (VMD) module was modeled using finite element analysis, and the results were used to conduct an exergy efficiency analysis for a solar-thermal desalination scheme. The performance of the VMD module was simulated under various operating conditions and membrane parameters. Membrane porosity, tortuosity, pore diameter, thickness, and fiber length were varied, along with feed temperature and feed configuration. In all cases, polarization phenomena were seen to inhibit the performance of the module. Under VMD operation, polarization of salt concentration was seen to be the main determining factor in the reduction of permeate flux. Within the boundary layer, salt concentration was seen to rapidly increase from the feed mass fraction of 0.035 to the saturation point. The increase in salt concentration led to a decrease in saturation pressure, the driving force for separation. Charging the feed into the shell instead of the lumen side of the membranes resulted in a further decrease in permeate flux. It is shown that adding a baffling scheme to the surface of the fibers can effectively reduce polarization phenomena and improve permeate flux. Increasing the overall recovery ratio was seen to increase the exergy efficiency of the system. Exergy efficiency was seen to have almost no dependency on membrane parameters due to the low recovery ratio in a single pass and the high heating duty required to reach the desired temperature for the feed stream.

The potential of hollow fiber vacuum multi-effect membrane distillation for brine treatment

Vacuum membrane distillation can extract pure water from degraded sources, but it requires relatively high energy inputs as compared to other thermal-driven technologies. As a commercially successful example, Memsys Water Technologies GmbH has addressed this key limitation by developing a flat sheet-based vacuum membrane distillation module where the latent heat recycled internally by using multiple distillation effects. In this paper, we propose an alternative hollow fiber-based design which also recycles the latent heat with multiple effects, but with an even more compact membrane packing format. The proposed design uses 3-dimensional printing to ‘unlock’ this configuration via a hollow aluminium alloy baffle which relies on its low thermal resistance to recover the latent heat effectively. The printed metal baffle (0.8 mm wall thickness) was calculated to have a very high conductive heat transfer coefficient, ~180 kW/m2K (surpassing even the ~20 μm polypropylene foil used in the Memsys module, which has a conductance of ~10 kW/m2K). Our experimental and theoretical results indicate that this design uses a condensation-convection-distillation heat and mass transfer mechanism which enables a three-effect system to reduce the energy consumption by ~60% over a single-effect design (i.e. from 672 kWh/m3 to 263 kWh/m3) for synthetic geothermal brine (~200 g/L salt concentration). Furthermore, the prototype reached a high average permeate flux of ~5.1 LMH and a salt rejection rate of >99.99%, approaching zero liquid discharge. Overall, this work suggests that hollow fiber membranes can indeed be used in a multi-effect mode and represents a promising new pathway for membrane distillation.

Exergy analysis of a vacuum membrane distillation system integrated with mechanical vapor recompression for sulfuric acid waste treatment

A vacuum membrane distillation (VMD) system integrated with mechanical vapor recompression (MVR) concentrating sulfuric acid solution was proposed. Mathematical models were established based on the first and second laws of thermodynamics. The effects of operating parameters on exergy efficiency of the system were investigated by Matlab simulation. Besides, further experiment using pure water as feed was implemented to validate the feasibility of the integration of VMD in MVR process, and then the energy and exergy analysis were performed comprehensively. The obtained results showed that the total input power increased but exergy efficiency decreased at higher feed concentration and heat transfer temperature difference of heat exchanger, lower feed temperature and velocity. The energy efficiency was improved through recovering the latent heat of vaporization in the VMD process. But the current coupled system was still inefficient and far from perfect with lower exergy efficiency of 3.72% and higher exergy destruction of 3.563 kW, the compressor was identified as the most exergy destruction of 36.99% followed by VMD module, heat exchanger and circulating pump with 18.16%, 11.98% and 11.34%, respectively. Therefore, the incorporation of enhancingheat-insulatingproperty and using more efficient compressor, heat exchanger and VMD module is expected to improve the exergy performance.

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.

Performance investigation of MEMSYS vacuum membrane distillation system in single effect and multi-effect mode

With increase in fresh water demand and lack of fresh water resources, the current water scarcity can only be solved with seawater desalination. However, due to high dependence of current desalination technologies on fossil fuels, especially in GCC countries where the share of thermal desalination systems is dominating, the environmental sustainability is at risk. Despite high operational and maintenance cost, electricity operated membrane based reverse osmosis (RO) system provides simple configuration with less capital cost. Therefore, for future sustainable desalination, more innovative and energy efficient methods have to be sought out which will not only have the low operational cost of the thermal desalination systems but they can also have simple design and fabrication cost of membrane based systems. Vacuum membrane distillation (VMD) is a thermal distillation technique that works on the vapor pressure across the hydrophobic membrane. With the introduction of heat recovery scheme within the VMD modules in form of the multi-effect VMD operation, a detailed performance analysis of the VMD system is presented in this study under different operating conditions. The performance of system is investigated on components level with comparison between single effect and multi-effect operation.

Direct integration of a vacuum membrane distillation module within a solar collector for small-scale units adapted to seawater desalination in remote places: Design, modeling & evaluation of a flat-plate equipment

Aiming to design a small-scale compact seawater desalination unit for remote coastal areas or islands, an integrated module coupling vacuum membrane distillation (VMD) and a solar flat-plate collector (FPC) was modeled and studied in the present work, with an extensive description of simultaneous mass and heat transfer for the hybrid solar-VMD module. The VMD model was built both at the scale of the membrane (membrane pore model) and at the scale of the module (feed bulk longitudinal model), and was then coupled with a detailed solar energy model. Effects of various solar and VMD parameters on daily water production, energy consumption and gained output ratio (GOR) were analyzed through several series of simulations. Results showed that without condensation heat recovery, lower operating temperatures were more favorable where a daily water production of 8 kg m−2 of the collector or the membrane area for domestic drinking could be obtained over a 12-h operation. GOR was found to be above 0.7, which is quite comparable to simple-effect single-stage membrane distillation systems driven by indirect supplied heat. Further evaluations also revealed that the introduction of condensation heat recovery facility could markedly improve the daily water production up to 40 kg m−2 for the same operation.

An Experimental Study on the Characteristic of Thermal Performance according to Feed Water Conditions to of Vacuum Membrane Distillation Module using PVDF Hollow Fiber

An Experimental Study on the Characteristic of Thermal Performance according to Feed Water Conditions to of Vacuum Membrane Distillation Module using PVDF Hollow Fiber

A review on membrane applications and transport mechanisms in vacuum membrane distillation

AbstractThe main aim of this article is to provide a state-of-the-art review of the experimental studies on vacuum membrane distillation (VMD) process. An introduction to the history of VMD is carried out along with the other membrane distillation configurations. Recent developments in process, characterization of membrane, module design, transport phenomena, and effect of operating parameters on permeate flux are discussed for VMD in detail. Several heat and mass transfer correlations obtained by various researchers for different VMD modules have been discussed. The impact of membrane fouling with its control in VMD is discussed in detail. In this paper, temperature polarization coefficient and concentration polarization coefficient are elaborated in detail. Integration of VMD with other membrane separation processes/industrial processes have been explained to improve the performance of the system and make it more energy efficient. A critical evaluation of the VMD literature is incorporated throughout this review.