Air Gap Membrane Distillation Research Articles

Hyper-branched dendritic structure modified PVDF electrospun membranes for air gap membrane distillation

Dendrimers are a new class of polymeric materials owning unique properties such as hydrophobicity. In this study, hydrophobic hyper-branched dendritic (HB-Den) structures were synthesized via a polycondensation reaction between hydroxyl groups of boehmite and carboxylic groups of nitrilotriacetic acid (NTA) and was used to improve the hydrophobicity of electrospun nanofibrous membranes (ENMs). Effects of the different contents of HB-Den structures on membrane morphology, elemental properties, surface hydrophobicity, desalination performance and antifouling properties were assessed by field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), water contact angle measurements and air gap membrane distillation (AGMD) experiments. Digital microscopy images showed monodisperse dendrimers with a tree-like structure. The water contact angle and liquid entry pressure (LEP) increased from around 129.3° and 101 ± 3.1 kPa for a neat PVDF membrane to 138.3° and 121 ± 2.2 kPa upon loading with 0.075 wt% HB-Den structures. FTIR analysis of the HB-Den containing ENMs confirmed the presence of carboxylic groups of NTA on the membrane surface. After desalination experiments, the 0.075 wt% HB-Den ENM showed a stable flux of 10.7 kg/m2 h and 99.9% NaCl rejection over 15 h filtration of a 3.5 g/l NaCl solution. The anti-fouling properties of the ENMs were also enhanced by incorporation of the dendritic structures. The 0.075 wt% HB-Den ENM showed flux recovery of about 94% after 20 h desalination experiment using real seawater as a feed solution.

Permeate flux enhancement with a helical wired concentric tube in air gap membrane distillation systems

Temperature polarization plays an important role in determining the permeate flux of the membrane distillation in an air gap membrane distillation (AGMD) module. This study aimed to increase the pure water productivity of saline water desalination by applying helical wire on the circumference of a concentric-tube AGMD module to reduce the temperature polarization occurring on the smooth annulus tube of an AGMD module with a normal concentric-tube. Modeling equations of heat and mass transfer in the new design of the AGMD device with helical wire winding on the annulus of a concentric tube have been investigated theoretically and experimentally. The mathematical model proposed in this study was used to correlate a simplified equation for estimating the heat transfer coefficient and to predict the permeate flux accordingly. The effects of various operation parameters, including the feed saline water temperature, feed volumetric flow rate, air gap thickness, and helical wire pitch, on the pure water productivity were also delineated. The permeate flux enhancement for helical wire in the AGMD module could provide the maximum relative increment of up to 31.1. The temperature polarization coefficient found in this study is ranging from 0.5527 to 0.5451 for different hot feed temperature and flow rate, respectively. An optimal helical wire module was assessed when considering both permeate flux enhancement and energy utilization effectiveness. © 2020 Desalination Publications. All rights reserved.

FABRICATION OF COMPOSITE MEMBRANES FOR WATER DESALINATION BY ELECTRON-BEAM DEPOSITION OF A POLYTETRAFLUOROETHYLENE-LIKE COATING ON THE SURFACE OF TRACK-ETCHED MEMBRANE

The present paper describes the method for the formation of polymer coatings on the poly(ethylene terephthalate) track-etched membrane surface by electron-beam deposition of polytetrafluoroethylene-like in a vacuum. It is found that the usage of this method leads to substantial hydrophobization of the membrane surface layer. The water contact angle of the deposited coatings changes from 130° to 155°, depending on the coating thickness. This change is due to the development of roughness of the deposited layer having hydrophobic properties. It is shown that the membranes of the developed sample provide a high selectivity of separation and water production rate during desalination of an aqueous salt solution of by air gap membrane distillation process and can be used for desalination of seawater.

Porous heat exchange tube with ultra-thin dense skin layer via NIPS for AGMD process

In this paper, a kind of poly(vinylidene fluoride) (PVDF) porous heat exchange tube (HET) was prepared via non-solvent induced phase separation (NIPS) method. This porous HET possessed an ultra-thin dense skin layer of 2 μm. Since the thermal conductivity of water is higher than that of PVDF, filling the pores with water can significantly improve the heat performance of the porous HET. A full plastic double-pipe air gap membrane distillation (AGMD) module was fabricated with the porous HETs and PVDF hollow fiber membranes. In the AGMD process, the wetting treatment can make the membrane distillation flux (J) and the gained output ratio (GOR) increased by 30%. During the long-term stability test, the maximum conductivity of distillate water was only 17.2 μS/cm. The experimental results demonstrated that the porous HET can be applied for the AGMD process to treat high saline brine, and the ultra-thin dense layer can prevent the leakage of saltwater.

Polyvinylidene fluoride phase design by two-dimensional boron nitride enables enhanced performance and stability for seawater desalination

The instability of polyvinylidene fluoride (PVDF) membranes in membrane distillation (MD) for seawater desalination is still a problem, despite the tremendous effort expended to resolve this issue. Here, a simple and feasible approach for improving desalination performance through the incorporation of two-dimensional boron nitride nanosheets (BNNSs) in polyvinylidene fluoride-co-hexafluoropropene (PVDF-co-HFP) electrospun nanofiber membrane (BNs-PH) is proposed as well as demonstrate its origin for fundamental understanding. The BNs-PH membrane exhibits a stable water vapor flux (18 LMH) and superior salt rejection (99.99%), even after operation for 280 h (commercial PVDF: steep decay within 28 h; neat PH: wetting within 4 h). From structural/chemical analyses, the BNNSs play a crucial role in forming favorable phases of the PH polymer crystal structure, inducing a superhydrophobic surface with greater nanoporosity and higher heterogeneity as well as enhanced mechanical properties (increase of UTS: 13.4%; modulus: 1.2%) for long-term operation. Theoretical modeling results of an air-gap MD system are consistent with our experimental results. The approach introduced in this study can be applied to other desalination systems to boost various water treatment applications.

Performance increase of membrane distillation pilot scale modules operating in vacuum-enhanced air-gap configuration

This paper presents the first experimental evaluation at pilot scale of the operation of vacuum-enhanced air-gap membrane distillation (V-AGMD) using two commercial spiral-wound modules at Plataforma Solar de Almería's solar desalination test facilities. The main difference between the modules was the channel length (1.5 and 2.7 m) as a result of having different membrane surface area (7.2 m2 and 25.9 m2 respectively) and different number of envelopes. Suction of air from the gap improved the vapour transfer through the membrane pores and the performance of the modules was significantly increased in relation to common air-gap (AGMD) operational mode, especially in the treatment of high salinity feeds. Increases of up to 234% in permeate flux and decreases of 68% in specific thermal energy consumption were measured. Depending on the channel length of the modules, the effect of vacuum led to extreme permeate productivity (8.7 l h−1 m−2) in the shortest, or to extreme energy efficiency (49 kWhth m−3, equivalent to a GOR of 13.5) in the longest. These are the best experimental performances obtained so far with pilot scale modules in membrane distillation.

Analysis of an air-cooled air gap membrane distillation module

An air-cooled design for an air gap membrane distillation (AGMD) process offers potential for significantly lowering energy requirements and reducing cost for desalination by eliminating or reducing the balance of plant components associated with the coolant flow system. Experiments were conducted on an air-cooled AGMD module to study the effects of air gap, support mesh conductivity and hydrophobicity, and the condensing surface hydrophobicity. A novel modular design was developed in which modules could be arranged in a series configuration to increase the distillate flux. The output from the series configuration was found to yield about three times the distillate water compared to a single pass water-cooled system with the same temperature difference (ΔT). The results also indicate that the mesh conductivity has a favorable effect on the flux value whereas the hydrophobicity of the mesh has no significant effect. The hydrophobicity of the condensing surface is favorable on two accounts: first, it increases the distillate flux at temperatures below 60 °C and secondly, the temperature difference of the saline feed when it enters and leaves the module is lower, which can lead to energy savings and higher yield when used in a series configuration.

Analysis and optimization of a new cylindrical air gap membrane distillation system

Abstract Membrane distillation is a rate-governed non-isothermal membrane separation technique that utilizes trans-membrane temperature difference for evaporating water and thereby separating it from brackish feed for reproducing fresh water. A novel design of a cylindrical air gap membrane distillation module is presented. The module is fabricated in a way similar to a shell and tube heat exchanger. A PTFE hydrophobic membrane is used and is formed in a cylindrical shape. Design of experiments (DOE) is used to design the experiments statistically and to identify the significant operating parameters. Experiments were performed according to the Taguchi design approach using an L16 orthogonal array. Optimization of the whole process is performed by response surface methodology. It is shown that the feed temperature and feed flow rate have a positive effect, whereas the salinity has a negative impact on flux. The maximum value of flux achieved with this system is 3.6 kg/m2 hr. A high value of flux of 2.6 kg/m2 hr was achieved under optimum conditions at a temperature of 45 °C and a flow rate of 1.5 lpm with a salinity of 5 g/litre.

A Comparison Study of Brine Desalination using Direct Contact and Air Gap Membrane Distillation

Membrane distillation (MD) is a hopeful desalination technique for brine (salty) water. In this research, Direct Contact Membrane Distillation (DCMD) and Air Gap Membrane Distillation (AGMD) will be used. The sample used is from Shat Al –Arab water (TDS=2430 mg/l). A polyvinylidene fluoride (PVDF) flat sheet membrane was used as a flat sheet form with a plate and frame cell. Several parameters were studied, such as; operation time, feed temperature, permeate temperature, feed flow rate. The results showed that with time, the flux decreases because of the accumulated fouling and scaling on the membrane surface. Feed temperature and feed flow rate had a positive effect on the permeate flux, while permeate temperature had a reverse effect on permeate flux. It is noticeable that the flux in DCMD is greater than AGMD, at the same conditions. The flux in DCMD is 10.95LMH, and that in AGMD is 7.14 LMH. In AGMD, the air gap layer made a high resistance. Here the temperature transport reduces in the permeate side of AGMD due to the air gap resistance. The heat needed for AGMD is lower than DCMD, this leads to low permeate flux because the temperature difference between the two sides is very small, so the driving force (vapor pressure) is low. 

Janus Nanofibrous Membranes for Desalination by Air Gap Membrane Distillation

The hydrophobic/hydrophilic dual-layer membrane can be considered as a membrane with two different surface properties. It is also named the Janus membrane by researchers for the mythical god with t...

Zirconium dioxide membranes decorated by silanes based-modifiers for membrane distillation – Material chemistry approach

Zirconium dioxide membranes (3 nm and 200 nm) were chemically modified by silane-based modifiers with different degrees of fluorination. Comprehensive material characterization has been performed before and after the modification process by goniometric, and microscopic techniques. Wetting properties has been accomplished by applying the Zisman method, Kao diagram, spreading pressure, and critical surface tension. The mechanical, tribological, and separation features of all membranes were investigated as a function of grafting time for 5h, 15h, and 35h.The performance of the membrane was assessed in air-gap membrane distillation processes. Non-fluorinated membranes possessed better transport properties and better mechanical stability in comparison with fluorinated ones which enable the generation of porous, hydrophobic, crack-free material, optimized for MD. Grafting with short fluorocarbon chains (FC6) produced highly hydrophobic materials (CA = 145°) with low roughness (Rq = 6.58 nm) compared to grafting with long fluorocarbon chains (FC12). On the other hand, FC6 showed a highly permeable character and elimination of problems related to substantial flux reduction and pore blocking.

Electrothermally Driven Membrane Distillation for Low-Energy Consumption and Wetting Mitigation.

Membrane distillation (MD) is a promising alternative approach for desalination, especially for high-salinity brines. Its application has been limited by its high operational cost because of the energy consumption required for hydraulic circulation and heating the entire circulating feed. Localized heating of the feed by Joule heating diminishes energy consumption, but the potential charging on the electrothermal material surface causes water splitting and membrane degradation in high-salinity environments. Herein, a novel reverse Joule-heating air gap MD method was designed in which an electrothermal material was placed at the air gap, isolating itself from saline water. Even though the Joule-heating layer was at the air gap side, 90.56% of heat flowed into the saline water for heating the feed. The opposite temperature gradient in the membrane matrix as opposed to conventional MD-mitigated membrane wetting was caused by capillary condensation. This novel electrothermal-driven MD configuration is worthy to be introduced into applications.

Techno-Economic Assessment of Air and Water Gap Membrane Distillation for Seawater Desalination under Different Heat Source Scenarios

Membrane distillation (MD) has a great deal of potential and this is currently being explored by the scientific community. However, this technology has not yet been implemented by industry, and an estimation of final product costs is key to its commercial success. In this study a techno-economic assessment of air gap MD (AGMD) and water gap MD (WGMD) for seawater desalination under different capacities and heat source scenarios was developed. The simplified cost of water (SCOW) method, which estimates investment costs, fixed and variable costs, as well as amortization factors and price influence over time was applied. In addition, experimental data from a laboratory-scale MD desalination plant were also used. The results showed water costs in the range of 1.56 to 7.53 €/m3 for WGMD and 2.38 to 9.60 €/m3 for AGMD. Specifically, the most feasible scenario was obtained for WGMD with a capacity of 1000 m3 daily using waste and solar heat. Finally, the costs obtained for MD were similar to those of conventional desalination technologies at the same scale factor. Therefore, although large-scale pilot studies and optimization of manufacturing processes are needed, MD shows very promising results that should be considered further.

Performance assessment of chemical mechanical planarization wastewater treatment in nano-electronics industries using membrane distillation

Wastewater from chemical mechanical planarization (CMP) processes in nano-electronics industries must be treated properly in order to fulfil local and international environmental regulations. This study is focused on a performance assessment of membrane distillation (MD) technology for CMP wastewater treatment. A new prototype of air gap membrane distillation (AGMD) module was utilized, with feed water consisting of CMP wastewater collected from imec, Belgium. The module was tested at different operating conditions (temperatures, flow rates and filtration time) and responses in terms of separation efficiency, permeate water quality, transmembrane flux, specific heat demand and exergy efficiency were determined. High quality permeate was produced in all trials, i.e. conductivity ~2.11 µS/cm, pH ~5.4, TOC ~1.13 ppm, IC ~0.24 ppm, TDS ~1.18 ppm and COD ~ 1.9 ppm; for most of the contaminants the separation efficiency was >99%. These findings clearly show that the resulting MD permeate does not exceed environmental regulations for release to recipient, and the permeate can even be considered for reuse. Moreover, the determined specific heat demand at different operating conditions was varying between 1390 and 2170 kWh/m3 whereas; the achievable exergy efficiency was ~19%.

Mass transfer during membrane distillation treatment of wastewater from hot-dip galvanization

Hot-dip galvanized products are widely applied in many fields in China. The treatment of the wastewater containing acid and many metal ions from the hot-dip galvanizing industry is an important environmental issue. In this paper, we experimentally investigated the membrane distillation technique to treat the wastewater from the hot-dip galvanizing industry, and determined the factors that affect the membrane distillation performance. The transport behavior of multi-components (i.e. HCl and H2O) through the porous membranes was studied in detail. The results indicate that it is promising to use the air gap membrane distillation configuration to treat the wastewater and recover hydrochloric acid. The increase of the feed temperature leads to the increase of the permeate flux in general. The presence of FeCl2 and the acidity of the feed affects the acidity of the permeate. FeCl2 in the feed can enhance the acidity of the permeate significantly, especially when the acidity of the feed is low. The adding of 9.5 wt% FeCl2 in the feed can increase the acidity of the permeate by about 10 times when the acidity of the feed is 6 wt%. The overall metal ion (Fe2+) rejection ratio is higher than 99.99% by membrane distillation technique.

Direct contact ultrasound for fouling control and flux enhancement in air-gap membrane distillation.

Air Gap Membrane distillation (AGMD) is a thermally driven separation process capable of treating challenging water types, but its low productivity is a major drawback. Membrane fouling is a common problem in many membrane treatment systems, which exacerbates AGMD's low overall productivity. In this study, we investigated the direct application of low-power ultrasound (8-23 W), as an in-line cleaning and performance boosting technique for AGMD. Two different highly saline feedwaters, namely natural groundwater (3970 μS/cm) and RO reject stream water (12760 μS/cm) were treated using Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes. Theoretical calculations and experimental investigations are presented, showing that the applied ultrasonic power range only produced acoustic streaming effects that enhanced cleaning and mass transfer. Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy (ATR FT-IR) analysis showed that ultrasound was capable of effectively removing silica and calcium scaling. Ultrasound application on a fouled membrane resulted in a 100% increase in the permeate flux. Cleaning effects accounted for around 30-50% of this increase and the remainder was attributed to mass transfer improvements. Contaminant rejection percentages were consistently high for all treatments (>99%), indicating that ultrasound did not deteriorate the membrane structure. Scanning Electron Microscopy (SEM) analysis of the membrane surface was used to confirm this observation. The images of the membrane surface demonstrated that ultrasound successfully cleaned the previously fouled membrane, with no signs of structural damage. The results of this study highlight the efficient and effective application of direct low power ultrasound for improving AGMD performance.

Assessment of air gap membrane distillation for milk concentration

Multi-effect evaporation is the state of the art for concentration of liquid food products to high solid content. Membrane technology with reverse-osmosis and membrane distillation offer an alternative. For the concentration of milk, a reverse osmosis and air-gap membrane distillation network was modelled and optimized. Fouling dynamics and scheduling are taken into account. Reverse osmosis is favourable until its maximum achievable concentration. Air gap membrane distillation is, despite the low operational temperatures, energy intensive for the concentration of milk. A large recirculation flow to keep sufficient cross flow has to be heated and cooled, and the costs for heating and cooling dominate the total costs for product concentration. Moreover, fouling increases the energy requirements. The optimal system for air gap membrane distillation has only one stage operating at a high concentration and relative low flux. Applying multiple stages reduces the investment costs due to smaller units, but the heating and cooling costs increase. Major opportunities to improve the performance of air gap membrane distillation for concentration of milk are: 1) increase the cold and hot side temperatures to their maximum acceptable values, 2) develop spacers that allow lower linear flow velocities in the system and thus lower recirculation rates, and 3) make use of available waste heat.

Experimental and theoretical investigation of a new air gap membrane distillation module with a corrugated feed channel

In this study, a novel feed channel was designed and examined for an air gap membrane distillation (AGMD) module to create different geometries, aiming to improve the module's performance. The experimental study has been conducted to investigate the influence of two new feed channels on the enhancement of the membrane sheet thermal boundary. Two feed channel-based designs with different geometries, namely, spacer and corrugated, were examined and benchmarked against the original module. Based on the results, the corrugated feed channel considerably enhanced both the permeate flux and the gain output ratio (GOR) compared with the spacer feed channel under the same operating conditions. The results also showed that the maximum enhancements of the permeate flux and GOR of the corrugated surface and spacer feed channel modules were 50% and 20% and were 40% and 10%, respectively, compared with the original module. The temperature polarization coefficients (TPC) for the corrugated feed channel module were near unity. According to the current study, the maximum performance of the proposed module occurred at the higher studied temperature of 80 °C, which consequently proved that the novel feed channel is more effective than the original one.

Characterization and Assessment of a Novel Plate and Frame MD Module for Single Pass Wastewater Concentration-FEED Gap Air Gap Membrane Distillation.

Membrane distillation (MD) is an up and coming technology for concentration and separation on the verge of reaching commercialization. One of the remaining boundaries is the lack of available full-scale MD modules and systems suitable to meet the requirements of potential industrial applications. In this work a new type of feed gap air gap MD (FGAGMD) plate and frame module is introduced, designed and characterized with tap water and NaCl–H2O solution. The main feature of the new channel configuration is the separation of the heating and cooling channel from the feed channel, enabling a very high recovery ratio in a single pass. Key performance indicators (KPIs) such as flux, gained output ratio (GOR), recovery ratio and thermal efficiency are used to analyze the performance of the novel module concept within this work. A recovery rate of 93% was reached with tap water and between 32–53% with salt solutions ranging between 117 and 214 g NaCl/kg solution with this particular prototype module. Other than recovery ratio, the KPIs of the FGAGMD are similar to those of an air gap membrane distillation (AGMD) channel configuration. From the experimental results, furthermore, a new MD KPI was defined as the ratio of heating and cooling flow to feed flow. This RF ratio can be used for optimization of the module design and efficiency.

A pilot study of spiral-wound air gap membrane distillation process and its energy efficiency analysis

Brine disposal is a major drawback for seawater desalination. Membrane distillation (MD) is an emerging technology to treat a high saline water including brine disposal instead of reverse osmosis, multi-stage flash and multi-effect distillation. This study investigated a pilot scale of a spiral-wound air gap MD (AGMD) module and evaluated its efficiency. A pilot-scale AGMD module with design production capacity of 10 m3/d was operated. Experiments with varying flow velocity showed increasing trend of water vapor flux as flow velocity increases. The temperature is one of the significant points in maximizing water permeate vapor flux in MD. Increasing temperature from 65 °C to 75 °C in evaporator channel has increased flux from 0.59 to 1.15 L/m2/h. Under various conditions, specific thermal energy consumption (STEC) and gained output ratio (GOR) was used to analyze energy efficiency. The pilot plant showed high GOR value in spite of a limited heating and cooling source available at the site. The highest GOR achieved was 3.54 with STEC of 182.78 kWh/m3. This study provides an overview of operation experience and its data analysis related to temperature, concentration, flow rate and energy supply.