Air Gap Membrane Distillation System Research Articles

Productivity, selectivity, and energy consumption of pilot-scale vacuum assisted air-gap membrane distillation for the desalination of high-salinity streams

The implementation of air gap membrane distillation systems is limited by a lack of overall performance predictions which rely on few available pilot-scale studies. This study evaluates the productivity, energy consumption, and selectivity of a pilot-scale air gap membrane distillation system by combining experiments and modeling activities. The effect of operating conditions, i.e., applied vacuum, feed flow rate, and feed stream salinity, was investigated to identify regulating factors and quantify dependencies. Response surface methodology was applied to model the phenomena and provide statistical analysis. Increasing flow rates produced a near linear increase of productivity within the investigated range. Operating at higher applied vacuum also translated into enhanced productivity, though the distillate flux increased by a maximum of 10 % when vacuum increased from −100 mbar to −500 mbar. Flow rate and vacuum also governed the observed salt flux by a similar magnitude because salt flux resulted mainly from liquid pore flow phenomena. The trans-membrane pressure regulated the membrane rejection: increasing the pressure difference led to a lower rejection. Moreover, high feed stream salinity lowered both the productivity and the distillate quality. The productivity gains were typically achieved at the expense of an increase in specific thermal energy consumption; however, an interesting relation was observed with feed stream salinity, with a minimum of specific thermal energy consumption of roughly 300kWhth⋅m−3 identified in the treatment of a stream with a salinity of 150g/L.

Theoretical and experimental investigation of cylindrical air gap membrane distillation system and effect of the membrane support net on its performance.

In present study a cylindrical module is studied based on air gap membrane distillation configuration and studied for desalination purpose. A complete theoretical model was developed with consideration of design and operating parameters that enabled a Cylindrical Air Gap Membrane Distillation (CAGMD) module specific performance analysis. Theoretical model was verified with the literature as well as with the experimental results carried out on a lab scale CAGMD module. The effect of support nets which supports the membrane on the air gap side is also discussed on the performance. Support nets made up of four different thermal conductivities material- copper, aluminum, brass and polypropylene (PP) are considered for this study. The effect of feed temperature and flow rate, air gap width, cold flow rate, effect of thermal conductivities of support nets and height of the module was studied on the performance of CAGMD module. Permeate flux, Specific Thermal Energy Consumption (STEC) and the Gained output ratio (GOR) was selected as the performance indicators and the results for all the resulted parameters obtained from experimental and theoretical model falls in good agreement with only 6% deviation, that suggests that the proposed model is best suitable for predicting the behavior of any cylindrical AGMD module with great effectiveness. It is suggested that for better performance of the system feed flow rate, temperature and cold flow rate should be maintained at higher level. Maximum permeate flux achieved from the CAGMD module is 9.22kg/m2h.

Performance assessment of a modified air-gap and water-gap membrane distillation systems

This work experimentally evaluates and compares the performance of four membrane distillation (MD) units with and without an internal gap rotating impeller blade. The novelty of the current study lies in the installed impeller within the condensation cavity and the impeller design parameters. The investigated MD units include a novel modified water-gap MD (M-WGMD) system featuring an internal impeller blade; a novel modified air-gap MD (M-AGMD) system with an internally rotating impeller blade; a regular water-gap MD (WGMD) process; and a conventional air-gap MD (AGMD) system. The evaluated system performance index includes gained output ratio (GOR) and vapor flux. The performance of the four MD systems is assessed and compared against each other under the same test conditions for various system design and operating variables, including impeller blade material, impeller blade angle, saline feed temperature, coolant temperature, and saline feed flowrate. Findings indicate that M-WGMD and WGMD systems (modified and conventional) produce higher flux than M-AGMD and AGMD units. Meanwhile, air-gap MD units (AGMD and M-AGMD) exhibits better energy efficient (higher GOR) than WGMD and M-WGMD systems. Furthermore, modified MD systems recorded superior performance compared to conventional MD systems and the blade angle effect is more influential than the blade material. At 80 °C feed temperature and 3200 mg/L feed salinity, the M-WGMD reaches a maximum flux of 128.39 kg/m2h and a GOR of 0.7702. On the other hand, the WGMD attained a flux of 67.32 kg/m2h and a GOR of 0.6199. The M-AGMD recorded a flux of 30.20 kg/m2h and a GOR of 0.8696. For the AGMD system, the GOR and the flux values are 0.7357 and 15.58 kg/m2h, respectively.

Energy and Exergy Analysis of Solar Air Gap Membrane Distillation System for Seawater Desalination

Air gap membrane distillation (AGMD) is a widely utilized technology for producing drinking water due to its low heat loss, high thermal efficiency, and compatibility with solar energy. The application of the first and second laws of thermodynamics in energy and exergy analyses provides a comprehensive evaluation of the efficiency of thermal processes. This study aims to examine numerically the energy and exergy performance indicators of a solar AGMD system used for seawater desalination. The simulation was carried out using MATLAB 9.7 software. The total thermal efficiency and overall efficiency of each element in the AGMD system were calculated for various solar field energy outputs, and moreover, a parametric study was conducted. The results indicate that the exergetic efficiency of the AGMD system components was the lowest in the solar field, with the concentrator having the lowest energy efficiency. Additionally, the thermal and exergetic efficiency of the entire solar AGMD system decreases along with the raise of ambient temperature. An additional investigation was conducted to better apprehend the sources of exergy destruction in the solar field. The obtained results from this study can be employed as a guide to reduce exergy destruction in the whole solar AGMD desalination system with recognition of the main sources of irreversibility.

Performance, energy and economic investigation of airgap membrane distillation system: An experimental and numerical investigation

Airgap membrane distillation (AGMD) is an efficient configuration employed widely for the solar membrane distillation desalination process. In the present work, 1-D Knudsen and molecular transport (KMT) model has been developed to investigate the performance of the flat sheet PVDF membrane. A new solution algorithm for the co-current and counter-current flow regime has been designed to solve the heat and mass transfer equations iteratively for a single-stage AGMD module. The feed temperature, feed flow rate, airgap size, salinity, membrane porosity and module length were varied and compared with experimental results. The increase in feed temperature from 40 °C to 80 °C resulted in 10.38 times increase in flux for co-current flow and 11.05 times for counter-current flow. The maximum permeate flux at 80 °C was 8.668 kg/m2h and 8.871 kg/m2h for the co-current and counter-current processes, respectively. Optimizing the feed temperature, flow rate, and membrane length using RSM suggests 80 °C, 1.528 LPM and 10 m as the optimum operating condition. An AGMD module of size 0.8 m width and 10 m length under the optimum operating condition exhibited a freshwater yield of 8.73 kg/h by consuming 24.98 kWh/m3 of specific energy, and the water production cost would be around $2.25/m3.

Facile and scalable fabrication of superhydrophobic PVDF membrane for the desalination of highly saline water using air-gap membrane distillation system

Here, a facile and novel route to fabricate superhydrophobic poly (vinlylidene fluoride) (PVDF) polymeric membranes for the desalination of highly saline water is described. The superhydrophobic PVDF membrane (with a high-water contact angle >150°) was fabricated by phase inversion process on a smooth glass surface followed by surface chemical modification with long fluoroalkylsilane chains (F21) via a single dip-coating approach. The superhydrophobicity of the membrane surface after F21 modification was found to be positively correlated with the concentration of the fluorine groups and the dip-coating duration. The performance of these membranes for the treatment of highly saline water was also evaluated using an air gap membrane distillation (AGMD) system. The chemically modified PVDF membrane exhibited excellent and stable salt rejection of over 99.99% (∼ 100%) and high mean water permeate flux (19.09 L/m2.h) when using synthetic 7 wt% NaCl feed solution. The modified PVDF membrane showed significant improvement in the mean water flux (>60%) compared to the pristine PVDF membrane due to variations in the membrane’s chemical and bulk structure in the dip coating process. The results suggested that the nature and amount of the endowed fluoroalkylsilane chains on the membrane surface are key design parameters in membrane fabrication for stable and excellent-performing membranes for desalinating highly saline water.

Fabrication of novel hydrophobic electrospun nanofiber membrane using polybenzoxazine for membrane distillation application

In this study, a novel hydrophobic blend polyvinylidene fluoride (PVDF) nanofibrous membrane with polybenzoxazine (PBz) was fabricated by the electrospinning technique to apply in membrane distillation (MD) process. Accordingly, pristine PVDF and a fluoro containing benzoxazine monomer (Bz) was blended and then thermally treated for crosslinking the Bz. The membrane properties were determined in terms of contact angle, young module, thickness, and the membranes were characterized by scanning electron microscope and Fourier transform infrared spectroscopy. The water contact angle of membranes increased from ca. 102.8° for the pristine PVDF membrane to 132.7° post cured Bz blended membrane. Moreover, the mechanical properties of blended membranes enhanced, while obtaining smoother and physically more stable membranes compared to the pristine nanofibrous membrane. The optimum membranes were tested in a lab-scale vacuum assisted air gap membrane distillation system. Results showed that Bz blended and cured membranes exhibited higher rejections and permeate flux for reverse osmosis brine and boron solutions compared to the pristine membrane that demonstrate the promising potential of Bz blended membranes in MD applications.

Performance of an air gap membrane distillation system and enhancement using a low-cost surface modification

ABSTRACT Water can be purified with a hydrophobic membrane in an air gap membrane distillation (AGMD) configuration. The present study presents experimental data for such a system and comparison with model predictions from simulations in a Matlab environment. The importance of the respective roles of feed water temperature and flow rate, cold water temperature and flow rate, and the extent of air gap on water production rate is studied, both from experiments and simulations. The comparison is found to be quite good, thus validating the choice of the mathematical model. In addition, possible improvements in the device performance are examined by using a surface modification of the hydrophobic membrane. In this respect, a layer of lead from a pencil is coated over the membrane. Evaporation rates through the pencil-coated membrane were measured in a solar simulator with 1 sun, 2 sun, and 3 sun settings. A distinct improvement in the evaporation rate was obtained when a pencil coated membrane was used. Following this observation, an augmented two-sided AGMD system is proposed, which carries a coated membrane exposed to solar radiation on one side and an uncoated membrane on the other.

Boron removal by using vacuum assisted air gap membrane distillation (VAGMD)

Vacuum-assisted air gap membrane distillation was used to remove boron from different waters in this study. Four different vacuum pressures (0.02 bar, 0.04 bar, 0.06 bar, and 0.08 bar) were applied and air gap membrane distillation (AGMD) configuration was also used to compare the effect of vacuum pressures on the process. Six commercial hydrophobic membranes which have the 110° contact angle value at least were used. Membranes with a 44.18 cm 2 effective area were tested with saline water, synthetic boron solution, and real geothermal water as the feed solutions which is the only study with VAGMD in the literature. Salt removal efficiencies of all experiments were higher than 98.2%. Permeate boron concentrations of synthetic boron solution and real geothermal water were lower than the 0.5 mg/L limit value for drinking water set by WHO and in terms of removal efficiency it means higher than 99% removal efficiency. Vacuum pressures enhanced the water fluxes as expected and also decreased the specific energy consumption. • The use of a supplementary vacuum pump on the AGMD system improved the flux. • Increasing pressures increase flux and GOR without any considerable rejection loss. • Higher than 99% boron removal efficiencies were achieved. • All permeate boron concentrations were obtained below 0.5 mg/L. • Specific energy consumption values did not increase although using a vacuum pump.

Intermittent direct joule heating of membrane surface for seawater desalination by air gap membrane distillation

Air gap membrane distillation (AGMD) is a low-temperature separation process applied commonly to desalination of (hyper)saline feeds. The low energy efficiency of heating the bulk feed solution to drive vapor transport in MD must be addressed through novel materials and techniques. In this work, we introduce the concept of intermittent electrical heating at the membrane surface using electrically conductive carbon nanotube structure (CNS) films pressed on commercial polypropylene (PP) membranes. First, the Joule heating effect was optimized by varying the mass loading of the CNS layer between 1.4 and 4.2 mg cm −2 . The effect of mass loading on porosity, electrical properties and cross-sectional morphology of the CNS layer was also investigated. Electrical conductivity of CNS films increases with increasing CNS mass loading. A CNS loading of 1.4 mg cm −2 yielded the highest electrical resistance, which translated into higher Joule heat generation, and highest steady-state temperature in response to applied current. When an alternating current of 2 A was passed through the CNS films in air, the maximum surface temperature achieved was 184 °C, 126 °C and 100 °C at CNS loadings of 1.4 mg cm −2 , 2.8 mg cm −2 and 4.2 mg cm −2 , respectively. Maximum surface temperature and spatial temperature distribution of CNS films were verified using a steady-state thermal-electric simulation. CNS films exhibited rapid thermal cycling between on and off states, due to their low thermal mass, with low thermal time constants for both heating and cooling. CNS film with a loading of 1.4 mg cm −2 was then pressed onto a commercial PP membrane and applied to AGMD for seawater desalination, with the conductive layer in direct contact with the feed solution. Intermittent heating was carried out by passing a current of 2 A for 10 min at 20 min intervals, for 10 cycles. Feed outlet temperature increased by > 9 °C during each cycle, which improved average permeate flux by 78%. While specific thermal energy consumption increased by 34%, a 25% enhancement was found in GOR, indicative of higher thermal energy efficiency. We have demonstrated the effectiveness of intermittent membrane surface heating for simultaneous performance enhancement and improved energy efficiency for seawater desalination using MD. • Direct surface heating applied to lab-scale air gap membrane distillation system for seawater desalination. • Carbon nanotube nanostructures coated onto hydrophobic membrane to enable Joule heating. • Intermittent application of electrical field improves GOR by 25% and productivity by 78%. • Long-term application of high-grade electrical energy not required.

Photothermal air gap membrane distillation for the removal of heavy metal ions from wastewater

As the world population grows, pure water becomes increasingly scarce, making wastewater treatment necessary. However, the processes currently used to treat industrial wastewater are generally energy-intensive, highlighting the need to develop environmentally friendly wastewater treatment technologies. To address this issue, a photothermal air–gap membrane distillation system was developed and tested for the first time to remove heavy metal ions from wastewater. Activated carbon particles at different concentrations were used in the prepared polyvinylidene fluoride membranes as components for solar energy recovery. Zinc, one of the most common heavy metals in industrial wastewater in the United Arab Emirates, was used as a model contaminant in this study. The results showed that the use of Activated Carbon particles to modify Polyvinylidene fluoride membranes can significantly improve solar absorption (97%) and porosity (60.6%), resulting in a permeate flux of 0.32 L/m2h and a rejection of 97% of Zn ions. Additionally, the optimized membrane was tested using a synthetic heavy metal solution and showed high rejection rates for most of the tested heavy metals (Zn2+, Fe3+, Cu2+, Cr3+, and Ni2+), which can be potentially used in the industrial heavy metal removal process. The developed photothermal water treatment system can provide a highly efficient and scalable water treatment process.

Experimental and simulation study of multichannel air gap membrane distillation process with two types of solar collectors

Solar thermal energy for membrane distillation desalination is a green and safe way for areas where water scarcity and solar irradiance are strongly correlated. In this paper, a commercial-scale of desalination plant was installed and tested to study the performance of a multichannel spiral-wound air gap membrane distillation module with an area of 14.4 m2. Air gap membrane distillation desalination plant at Port Said university was powered by solar energy using flat plate and evacuated tube collectors. The model validation was carried out. The solar desalination systems were numerically simulated using the program TRNSYS under different weather conditions throughout the year. The influence of the main operating parameters (feed flow rate and inlet temperatures of evaporator and condenser) was investigated. Regression analysis agreed with experimental data fitting using a quadratic polynomial model with coefficients of determination (R2) values of 0.997, 0.972, and 0.999 for permeate flux, outlet feed temperature, and outlet coolant temperature, respectively. The results showed that the permeate flux of the air gap membrane distillation with the evacuated tube collectors was 18.81%–30.44% higher than flat plate collector, and its cost was 22.48% lower. The specific thermal energy consumption of the air gap membrane distillation system ranged from 158.83 kWh/m3 to 346.55 kWh/m3. The maximum gain output ratio reaches 4.4 at 52 °C, depending on the feed inlet temperature. The thermal efficiency of the air gap membrane distillation system is 72%. The proposed air gap membrane distillation system produced 28.78 m3/year of fresh drinking water at a cost of USD 14.73/m3 with remarkable reducing in carbon dioxide emissions by 7,274.45 kg/year.

Performance and energy evaluation of compact multistage air gap membrane distillation system: An experimental investigation

A new design of compact multistage air gap membrane distillation system is experimentally tested for possible scaling up and commercialization. The performance of the new system is then investigated by studying the impacts of the operating variables on the permeate flux. Parallel and series flow arrangements are tested, and the energy efficiency is evaluated by the gained output ratio and the specific thermal energy consumption at different operating variables. In addition, the evaluation of system production cost of freshwater is carried out. Results reveal that both the permeate flux and the gained output ratio increase with increasing the feed temperature, feed and coolant flow rates and decreases with increasing the coolant temperature. The specific thermal energy consumption and the production cost decrease with increasing the feed temperature, feed and coolant flow rates and decreases with decreasing the coolant temperature. The compact system produces up to 13 kg/m2h (24L/day). The parallel flow arrangement is slightly better than the series arrangement in terms of productivity. In contrast, the series flow arrangement is better in terms of energy efficiency.

Performance evaluation of boron removal from wastewater containing high boron content according to operating parameters by air gap membrane distillation

The removal performance of boron from real wastewater was conducted to determine the effect of different parameters on permeate water flux and boron removal efficiency by air gap membrane distillation (AGMD) system in this study. These parameters include boron concentration, membrane pore size, and membrane material. Before real wastewater tests, the experiments were conducted with saline water (%1 (w/v) NaCl) to stabilize the air gap membrane distillation system. Then, experiments were carried out in an air gap membrane distillation system using real wastewater. In the experiments, two different membrane materials (polytetrafluoroethylene (PTFE) and polypropylene (PP)) were used to examine the effect of membrane material. Also, experiments were conducted with membranes that have mean pore size of 0.20 and 0. 45 μ m to examine the effect of membrane pore size on the permeate flux. To evaluate the effect of boron concentration, wastewaters at three different boron concentrations (2598, 2795 and 6242 mg/L B) were treated in the AGMD system. The results illustrate that removal of boron in all conditions was obtained above 99%. In conclusion, increasing boron concentration at feed wastewater, permeate water flux was decreased. When the mean pore size was increased from 0.20 to 0. 45 μ m , permeate flux was enhanced. Furthermore, PP membranes had the maximum permeate flux in AGMD tests. • The increase of boron concentration in feed wastewater had effect on permeate water flux. • The permeate water fluxes were highest for the PP 0.45 membrane for almost all wastewater. • Boron removal efficiency for all wastewater and all membranes was over 99%.

Power effect of ultrasonically vibrated spacers in air gap membrane distillation: Theoretical and experimental investigations

This study investigates the efficiency of low-power ultrasound in the range of 3.5–30.0 W to improve permeate flux and alleviate membrane fouling in an air–gap membrane distillation (AGMD) system. Natural groundwater and reverse osmosis (RO) reject water were fed into the AGMD system on which fouling experiments were conducted with hydrophobic polyvinylidene fluoride (PVDF) membrane. After 35 h of AGMD system operation with groundwater and RO reject water, fouling caused the permeate flux to decrease by 30% and 40% respectively. Concentration polarization, intermediate pore blocking, and cake filtration appear to be the main reasons for flux decline with both feedwater types. Ultrasound application for a short period of 15 min resulted in flux improvement by as high as 400% and 250% for RO reject and groundwater, respectively. Modelling of the heat and mass transfers showed that the flux increase was mainly due to membrane permeability improvements under ultrasonic vibration. Fouling visualisation using Scanning Electron Microscopy revealed that ultrasound effectively removed membrane fouling without compromising the membrane’s structure. Importantly, permeate flux improvements with targeted low-power ultrasound appears to be proportionally higher than those of high-power ultrasound applied to the whole system, on a flux improvement per ultrasound W/m2 basis.

Experimental analysis of a novel helical air gap membrane distillation system

Abstract Membrane distillation presents one of the feasible solutions to fresh water problems. The present study aims to develop an innovative helical air gap membrane distillation (HAGMD) system and to analyze its behavior under different operating conditions. In this design the condenser is made up of a cylindrical copper tube with continuous helical fins over it, that increases the total available condensation area by almost 45% and enhances the overall heat transfer throughout the module. The presence of fins in the gap also reduces the total air gap width by almost 64% and therefore improves the flux production. A detailed experimental analysis is carried out for a better understanding of the underlying phenomenon. The effect of feed water temperature, feed flow rate, cold flow rate, coolant temperature and feed salinity on the performance of HAGMD is investigated experimentally. The analysis shows that the finned condenser results in very high flux. The maximum flux obtained from the system was 20 kg/m2 hr with feed of 5 g/liter salinity and a diving force temperature difference of 45 °C.

Simulation study of an air-gap membrane distillation system for seawater desalination using solar energy

Simulation study of an air-gap membrane distillation system for seawater desalination using solar energy

Enhancement of the air gap membrane distillation system performance by using the water gap module

Abstract The negative effect of an air gap layer presented between the membrane and cooling plate on air gap membrane distillation (AGMD) performance was diminished largely by inserting a water gap membrane distillation (WGMD) module in series. The new design of air-gap–water-gap membrane distillation (AG-WG)MD was evaluated experimentally by comparing with an AGMD system under different operating conditions. In theory, mass and heat transfer in the new (AG-WG)MD and imitative AGMD systems were analyzed. Experimental outcomes showed that a new (AG-WG)MD design profoundly enhanced flux (Pd) and gained output ratio (GOR), and greatly decreased energy consumption (STEC) and heat input (EH.I). At a concentration of 5,000 mg/L, coolant temperature of 20 °C, and flow rate of 18 L/h, Pd was promoted by 76.26%, 40.84%, 35.45%, 30.91%, and GOR by 46.38%, 33.46%, 31.27%, 26.65%, in addition to STEC being reduced about 55.63%, 46.81%, 43.66%, 38.30%, and EH.I around 31.31%, 25.84%, 23.53%, 20.55%, from the AGMD to (AG-WG)MD system at feed temperatures of 50 °C, 60 °C, 70 °C, and 80 °C, respectively. The outcomes proved that the AGMD performance could be significantly promoted by integrating with WGMD in a combined MD system. This combination increased the temperature difference across the membrane and decreased thermal-concentration boundary layers for the AGMD system.

Water recovery from flue gas condensate in municipal solid waste fired cogeneration plants using membrane distillation

In cogeneration plants with wet scrubbing of exhaust gases, the resulting flue gas condensate passes through various treatment steps prior to its discharge to recipient water body or for use as boiler feed water. The present investigation examines membrane distillation (MD) as an alternative treatment method, potentially overcoming bio-fouling and other known drawbacks of established membrane technologies while making efficient use of available heat sources and sinks. Laboratory and pilot scale experiments are performed using air gap MD system where acid neutralization has been considered as a pretreatment step in order to avoid ammonia slip. Separation efficiency, transmembrane flux, specific heat demand and net heat demand were determined at different operating conditions. Resultant separation efficiency of the contaminants shows the successful application of MD for flue gas condensate treatment, achieving results that are comparable or even better than separation with reverse osmosis (RO). The obtained transmembrane flux varied between 1.6 and 7.2 L/m2h per module depending upon the hot and cold side temperatures. For various operating conditions, specific heat demand ranged from 400 to 1000 kWh/m3 per module and corresponding net heat demand was around 17.5–110 kWh/m3. The reconcentration study found that 92% of water could be recovered from the tested flue gas condensate. Process economy shows that estimated clean condensate production cost can be as low as 1.7 $/m3.

Techno-economic system analysis of membrane distillation process for treatment of chemical mechanical planarization wastewater in nano-electronics industries

Membrane distillation (MD) is a promising separation technology for the treatment of chemical mechanical planarization (CMP) wastewater releasing from nano-electronics industries. In order to determine the feasibility of the process at industrial scale, the most important factors are large-scale system evaluation and related economics. Since membrane distillation is a thermally driven process, therefore, different integration possibilities between an air gap membrane distillation (AGMD) system and low-grade heat sources are identified and analyzed in this work. Global mass and energy balances are conducted around AGMD system for CMP wastewater treatment in a typical nano-electronics manufacturing facility. It is determined that around 100 GWh of thermal energy can be readily recovered via internal sources and reused to treat 120,000 m3 CMP wastewater/year with MD feed temperature of 80 °C. Along with the technical feasibility of the system, the detailed economic evaluation has also been performed. Annual capital investment and operating cost showed that the expected CMP wastewater treatment cost can be as low as 3 $/m3, which is estimated to be nearly 95% lower than the wastewater treatment cost using electro-chemical systems.