Air Gap Membrane Distillation Module Research Articles

A reduced model for pilot-scale vacuum-enhanced air gap membrane distillation (V-AGMD) modules: Experimental validation and paths for process improvement

A fast and robust computational model of a spiral-wound vacuum-enhanced air gap membrane distillation (V-AGMD) module at the pilot-scale is proposed and implemented. In contrast with data-driven models available in the literature, a physics-based approach is adopted for more reliable generalization beyond the validation dataset. A total of 86 experimental results, of which 41 are described in this work and 45 come from independent sources available in the literature, are used in the validation effort with quite favorable results. With the confidence on the robustness of the methodology due to the wide range of operational parameters and the use of spiral-wound modules of four different sizes included in the validation comparisons, a physical analysis is conducted varying the air gap pressure, the number of feedwater channels, the feedwater flow rate, and the membrane area. Improvements of up to 60% in both water productivity and energy efficiency can be achieved by intensifying the vacuum in the air gap or decreasing the number of feedwater channels. These parameters achieve performance gains due to less resistance in the air gap for vapor to migrate through it, in the former case, and a reduced temperature polarization effect, in the latter case. Smaller flow rates favor energy efficiency at the expense of water productivity by simultaneously decreasing transport-phenomena-related irreversibility and the partial pressure difference across the membrane and the air gap. In addition, this tradeoff between energy efficiency and driving force is shown to lead to an optimum value for the membrane area beyond which the permeate flow rate through the membrane starts to fall due to the small driving force. An illustrative case is predicted to achieve energy efficiency metrics, such as a gain-output ratio of 12.7, competitive with multi-effect distillation.

Mitigating near-surface polarizations in membrane distillation via membrane surface decoration

Temperature and concentration polarizations near the membrane surface zone can significantly affect the performance of membrane distillation, resulting in reduced transmembrane flux and overall efficiency. The fundamental approach to mitigating near-surface polarizations involves enhancing turbulence flow near the membrane surface to minimize the formation of polarization layers. This study focuses on modifying the membrane surface via surface decoration to reduce the polarization layers. The effects of membrane surface decoration on the performance of membrane distillation were systematically investigated through an air gap membrane distillation module under various operating conditions. The hydrodynamic characteristics of the flow near the membrane surface were studied using computational fluid dynamics simulations. The characteristics of both bottom-up and top-down disturbing configurations were analyzed. In addition, we investigated the influence of various flow disturbing methods on the mass transfer coefficient in membrane distillation processes. The membrane distillation test results show that at feed temperatures of 50 °C and 70 °C, there are respective increases in transmembrane flux of 30 % and 12 %. The results provide insights into optimizing the flow state near the membrane surface for enhanced membrane distillation performance.

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.

Influence of internal design on the performance of pilot vacuum-assisted air-gap membrane distillation modules for brine concentration with solar energy

Membrane distillation (MD) in batch operation is proposed for brine concentration with solar energy in zero liquid discharge (ZLD) desalination processes, but there are some research gaps yet. Energy efficiency is key for implementing the technology with solar energy and there is not enough experience and experimental results of operation of MD pilot modules at high salinity feeds. This research focuses on the experimental evaluation and comparison of two multi-envelope spiral wound commercial membrane distillation modules operating in vacuum-assisted air gap MD at the solar MD pilot facilities of Plataforma Solar de Almería in Spain. These modules have similar membrane area (24 and 25.9 m2) and residence time (large, to maximize thermal efficiency), but different internal design: 6 envelopes 5 m long and 12 envelopes 2.7 m long, respectively. This difference affects mostly the feed circulation velocity for equal feed flow rate. Performance parameters such as permeate production and specific thermal energy consumption (STEC) are compared and experimental models performed and validated for different feed salinities. Permeate quality and specific electrical energy (SEEC) consumption are also evaluated. The results show that the best module for batch operation is the one with a larger number of channels but of shorter length.

Optimizing Membrane Distillation Performance through Flow Channel Modification with Baffles: Experimental and Computational Study

It has been identified that temperature polarization and concentration polarization are typical near-surface phenomena limiting the performance of membrane distillation. The module design should allow for effective flow, reducing the polarization effects near the membrane surfaces and avoiding high hydrostatic pressure drops across and along the membrane surfaces. A potential route to enhancing the membrane distillation performance is geometry modification on the flow channel by employing baffles as vortex generators, reducing the polarization effects. In this work, various baffles with different structures were fabricated by 3D printing and attached to the feed flow channel shell in an air gap membrane distillation module. The hydrodynamic characteristics of the modified flow channels were systematically investigated via computational fluid dynamics simulations with various conditions. The membrane distillation tests show that adding the baffles to the feed channel can effectively increase the transmembrane flux. The transmembrane flux with rectangular baffles and shield-shaped baffles increases by 21.8% and 28.1% at the feed temperature of 70 °C. Moreover, the shield-shaped baffles in the flow channel not only enhance the transmembrane flux but also maintain a low-pressure drop, making it even more significant.

Life cycle assessment and life cycle costing of a building-scale, solar-driven water purification system

Water purification systems require material resources and energy to build and operate, thus leading to environmental burdens. Mature water treatment technologies such as reverse osmosis and multi-effect distillation have been extensively studied for their environmental impacts. There is still a lack of such studies in one of the promising technologies, namely, air gap membrane distillation (AGMD). This study performs a cradle-to-grave life cycle assessment (LCA) and a life cycle costing of a solar-driven AGMD water purification system. This research also assesses the cost competitiveness of the system compared to local bottled water using a levelized cost of water (LCOW) approach. The study shows that the environmental impacts of the water purification systems (using a functional unit of 590 m3 of drinking water produced over a time period of 15 years) are: 27,224 kg CO2eq (climate change), 2,408 kg 1.4-DCB (human carcinogenic toxicity), 3,100 kg 1.4-DCB (marine ecotoxicity), and 2,286 kg 1.4-DCB (freshwater ecotoxicity). The solar PV sub-system and solar thermal sub-system have a much higher impact than the water purification sub-system. The LCOW calculations show that the system, with its current runtime of six hours using solar energy, has a levelized cost of 0.46 USD/L. Alternative scenarios, such as the use of grid electricity and end-of-life treatment of generated wastes, are also evaluated. A sensitivity analysis is performed on four parameters, i.e., component lifetime, maintenance frequency, discount rate, and AGMD module production cost. This study could be useful to local governments, investors, and potential customers interested in the water purification system's environmental footprint and cost competitiveness.

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.

Permeate Flux Enhancement in Air Gap Membrane Distillation Modules with Inserting Λ-Ribs Carbon-Fiber Open Slots.

A novel design of an air gap membrane distillation (AGMD) module was proposed to enhance the permeate flux improvement for the desalination of pure water productivity. The modeling equations for predicting permeate flux in the AGMD module by inserting Λ-ribs carbon-fiber open slots under various hydrodynamic angles were developed theoretically and experimentally. The temperature distributions of both hot and cold feed streams were represented graphically with the hot saline flow rate, inlet saline temperature, and carbon-fiber hydrodynamic angles as parameters. The results showed a good agreement between the experimental results and theoretical predictions. Designed by inserting Λ-ribs carbon-fiber open slots into the flow channel, the membrane distillation module was implemented to act as an eddy promoter and yield an augmented turbulence flow. The effect of Λ-ribs carbon-fiber open slots not only assured the membrane stability by preventing vibration but also increased the permeate flux by diminishing the temperature polarization of the thermal boundary layer. The permeate flux improvement by inserting Λ-ribs carbon-fiber open slots in the AGMD module provided the maximum relative increment of up to 15.6% due to the diminution of the concentration polarization effect. The experimental data was incorporated with the hydrodynamic angle of Λ-ribs carbon-fiber open slots to correlate the enhancement factor with the Nusselt numbers to confirm the theoretical predictions. The accuracy derivation between the experimental results and theoretical predictions was pretty good, within 9.95≤E≤1.85. The effects of operating and designing parameters of hot saline flow rate, inlet saline temperature, and hydrodynamic angle on the permeate flux were also delineated by considering both the power consumption increment and permeate flux enhancement.

Experimental investigation of two novel arrangements of air gap membrane distillation module with heat recovery

Experimental investigations of an air gap membrane distillation (AGMD) unit were carried out to study the energy-saving by heat recovery. The spiral air gap membrane module in the water desalination unit was used. The unit was configured to operate in two novel arrangements and designed to minimize the loss of heat within the drains which maximize the unit productivity as well as the thermal efficiency. Water of salt concentrations 10,000 ppm and 20,000 ppm were used. Measurements of the heat source in the form of inlet and outlet temperatures, water productivity, and thermal efficiency are conducted for different flow rates of coolant and water feeds. The new arrangements are found to save much of the supplied heat, minimize the heat loss, and maximize the unit performance. An enhancement of 38.62% in the water productivity was achieved compared to the conventional AGMD unit. The considered heat recovery system showed water productivity of 155.92 m3/year for the proposed fresh potable water system with a cost of $12.85 per m3.

Solar chimney power plant combined with membrane distillation (SCPPMD), part I: Principle and operation characteristics

In this paper, a novel solar chimney power plant combined with membrane distillation (SCPPMD) was proposed. SCPPMD could not only achieve power output but also product more freshwater, by installing a novel air gap membrane distillation module vertically in the disc distiller. A transient heat and mass model was established to predict the system performance. Then, the operation laws of SCPPMD under unsteady weather conditions and the effect of seawater thickness were explored. The results show that the peak power generation is about 4p.m. and the peak water yield lags two hours. The hourly water yield of the membrane is much higher than that of the disc distiller and its valley value reaches 11.55 t/h, 14.26 times the valley value of the disc distiller and 0.875 times its peak value. Membrane distillation still has considerable freshwater output even when the water production of disc distiller is weak. As the seawater layer thickens, the peak-valley difference of power generation is reduced and the daily power generation drops; The peak hourly water yield of disc distiller drops, the valley value increases and its daily water yield drops; Both the peak and valley value of membrane hourly water yield increase, and its daily water yield rises; The change of the system water yield is basically the same as that of membrane.

Performance improvement of an air gap membrane distillation process with rotating fan

The current paper presents an experimental study on the performance of a novel air gap membrane distillation (AGMD) desalination system where a fan blade is installed within the air gap to improve the system’s productivity and energy utilization. The installed blade creates suction inside the air gap behind the membrane and increases the air turbulence, thereby reducing the vapor mass transfer resistance across the membrane and improving the productivity and energy efficiency of the system. Results revealed that the proposed design and process improve the vapor permeate flux by about 250% when compared to the conventional AGMD module. Furthermore, the average improvement in the gained output ratio of the system is about 67%, with a corresponding 77% reduction in the specific thermal energy consumption. Influences of the feed temperature and the motor speed are dominating the performance of the new system.

Characterization of the use of vacuum enhancement in commercial pilot-scale air gap membrane distillation modules with different designs

The use of vacuum enhancement in air gap membrane distillation (AGMD) has been shown to improve the permeate production and the thermal efficiency. This work presents the characterization of that improvement for different operating conditions (input temperatures and feed flow rate) in three commercial multi-envelope AGMD modules with different internal designs. Modules had different number and length of internal circulation channels, yielding different feed velocities and residence times, and thus allowing to investigate the role of these. Experiments were performed with simulated seawater in solar membrane distillation pilot plants, and the decrease of permeate flux (PFlux) and specific thermal energy consumption (STEC) were analysed for each operating condition. The greatest impact of vacuum enhancement in the performance improvement was observed in the modules with the longest residence time operated at low hot temperature. The lower the driving force, the more important the role of vacuum enhancement in the improvement of vapour diffusion was, independently of feed velocity. Increases of PFlux up to 88% and reductions of STEC down to 70% compared to AGMD operation were quantified. Moreover, vacuum enhancement led to the reduction of hydraulic pressure drop, decreasing 26% the specific electrical consumption.

Comprehensive insights into performance of water gap and air gap membrane distillation modules using hollow fiber membranes

A commercially available microporous polypropylene hollow fiber membranes were employed for air gap and water gap membrane distillation (i.e., AGMD and WGMD, respectively) processes. In both configurations, the outer surface of commercially available dense polypropylene hollow fibers was used as the condensing surface of the permeate. The performance levels of the AGMD and WGMD processes utilizing microporous polyvinylidene fluoride membranes fabricated in-house were compared with those using polypropylene membranes. Under the given specific operating conditions, the maximum mean permeation flux values in AGMD and WGMD using polypropylene hollow fiber membranes were approximately 24 and 27 kg/m2h, respectively. In addition, theoretical studies on AGMD and WGMD using the designed hollow fiber module configuration were performed. The predicted results were found to well agree with the experimental results, thus verifying their validity. Moreover, parametric studies were conducted to identify the optimum evaporation-to-condensation surface area ratio (i.e., optimum numbers of hollow fiber membranes and hollow fiber condensers) in terms of specific energy consumption.

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.

Comparison of the Effect of Air and Permeate Gap Regions on Highly Saline Water Desalination Using Hollow Fiber Membrane Distillation Modules

The present research work discussed the impact of permeate gap region (PGR) on the high mass transfer resistance and low productivity resulted from the air gap region (AGR) sandwiched between the membrane and condensing surface in the air gap membrane distillation (AGMD). Two hollow fiber permeate gap (PGMD) and air gap membrane distillation (AGMD) modules were built, examined, and compared experimentally under several operating parameters such as feed salt concentration (Cf), feed temperature (Tf), coolant temperature (Tc), and flow rate (Mf). The performance comparison was done according to the values of energy consumption (STEC), water productivity (Pw), waste heat (QH.I), and gained output ratio (GOR). Results showed that the PGR was more effective than AGR on the membrane module performance at all investigated operating parameters. Under operating parameters of Cf = 7.5 g/L, Tc = 15 oC, Tf = 70 oC, Mf = 4 L/h, and compared with AGR, the PGR minimized STEC and QH.I by around 78.32% and 47.06%, and improved the Pw and GOR by about 95.93% and 90.33%, respectively. Thus, the negative gap region effect could vanish completely by filling it with the permeated water instead of air, promoting by which the performance of the membrane distillation module remarkably.

Gray water treatment by air-gap multi-effect membrane distillation with pre-treatment by electrocoagulation

Abstract The gap between supply and demand of fresh water is widening due to increase in global population. Hence, an alternative source of water such as gray water could potentially save a significant amount of precious fresh water. Gray water is distinct from black water by the amount and composition of its chemical and biological contaminants. In this paper, an air-gap Multi-effect membrane distillation (MEMD) module performance for gray water treatment is described. Surfactant present in the gray water was wetting the membrane pores. Hence, electrocoagulation was used as a pre-treatment of gray water feed. About 99.14% surfactants were removed by electrocoagulation and the experiment shows excellent performance of the MEMD module. The 4-stage MEMD module offered permeate fluxes nearly to 50.12 l/m2 h at 80 °C. It was also found that a high thermal efficiency and output gain ratio was possible with lower specific energy and no cooling water. Hence, it is apparent that the air gap MEMD technology could be used for gray water treatment. Pre-treatment by electrocoagulation (EC) operation seems an important step in the overall treatment process when large amounts of surfactants are present in the treatment water.

Influence of Module Length on Water Desalination Using Air Gap Membrane Distillation Process: An Experimental Comparative Study

The impact of the membrane module length on the air gap membrane distillation (AGMD) process performance was investigated experimentally under various operating parameters. In this work, two different lengths of the AGMD module (i.e., 0.59 m and 0.28 m) were tested separately to get the optimum in terms of lower energy consumption and higher water output. The mechanism of mass and heat transfer in the AGMD process is considered as well. The experimental results were subjected to the two-way analysis of variance (ANOVA) to examine the significant influence of operating parameters and modules at p 0.05) on the gained output ratio (GOR), energy consumption (STEC), and heat input (QH.I). Under optimal operating parameters of a flow rate of 18 L/h, feed temperature of 60 °C, coolant temperature of 20 °C, and feed salinity of 5000 mg/L, the Pw and GOR were increased by 64.50% and 50.00%; the STEC and QH.I were reduced by 57.78% and 34.09%, respectively, from the long module to the short one. Thence, changing the length of the membrane module from 0.59 to 0.28 m promoted remarkably the performance of the AGMD process.

DYNAMIC SIMULATION OF AN AIR-GAP MEMBRANE DISTILLATION (AGMD) PROCESS USING PHOTOVOLTAIC PANELS SYSTEM AND FLAT PLATE COLLECTORS

In the desalination field, the membrane distillation (MD) is a new process of producing distilled water that has been developed and tested in recent years. In this paper, the integrated single cassette air-gap membrane distillation (AGMD) module in the solar thermal desalination system is validated and numerically simulated with the TRNSYS program. This model is studied to be ideal for obtaining a distilled water flow rate of 5.5 kg/h at different times under changing climatic conditions throughout the year in Ain-Temouchent weather, Algeria. The auxiliary heater is added to ensure the thermal energy continuity in the cold climatic conditions, where the photovoltaic system is used to power electrically the auxiliary heater. Therefore, the energy needed is calculated for the auxiliary heater and is replaced by 10 photovoltaic panels, each one has an area of 1.6 m² using seven of the energy storage batteries (12V, 200Ah) with 1.5 KW via TRNSYS and PVGIS help programs. Simulated results showed excellent compatibility with experimental results in previous studies. Additionally, it was found that when the inlet temperature of AGMD reaches 85 °C, the distilled water flow from the distillation membrane reaches 5.5 kg /h and that remains stable on different days throughout the year by relying solely on solar energy.

Theoretical and experimental studies of double-unit air gap membrane distillation (DUAGMD) module for seawater desalination

Theoretical and experimental studies of double-unit air gap membrane distillation (DUAGMD) module for seawater desalination

Experimental evaluation of two consecutive air-gap membrane distillation modules with heat recovery

Abstract Two rectangular modules with a total interior membrane surface area of 13.53 m2 were consecutively combined to evaluate the use of heat recovery in an air-gap membrane distillation (AGMD) system. Several operating inlet parameters including feed water temperature, mass water flow rate and salinity were investigated. The experimental results revealed that the performance of the system was improved by virtue of efficient heat recovery resulting from combining two AGMD membrane modules in series. Under optimal inlet operating parameters of cooling water temperature of 20 °C, salinity of 0.05% and flow rate of 3 l/min, the system productivity (Pp) increased up to 192.9%, 179.3%, 176.5% and 179.2%, and the thermal efficiency (ηth) by 261.5%, 232.6%, 239.4% and 227.3% at feed water temperatures of 45 °C, 55 °C, 65 °C and 75 °C, respectively. Concurrently, the specific waste heat input (Ew.h.i) decreased by 6.7%, 4.7%, 5.6% and 2.7% due to the efficient heat recovery. The results confirmed that heat recovery is an important factor affecting the AGMD system that could be improved by designing one of the two AGMD modules with polytetrafluoroethylene (PTFE) hollow fibers with a flow length shorter than the other one having a salt rejection rate of 99%.