Air Gap Membrane Distillation Research Articles

Research on the Performance and Computational Fluid Dynamics Numerical Simulation of Plate Air Gap Membrane Distillation Module

Membrane distillation (MD) is widely used in the field of seawater desalination. Among its various sub-categories, air gap membrane distillation (AGMD) stands out due to its high thermal efficiency and compatibility with low-grade heat sources. This study delves into the impact of varying operating conditions on AGMD performance, employing numerical simulations which are grounded in experimental validation. The objective was to enhance the performance of AGMD, mitigate polarization phenomena, and provide a reference for optimizing membrane component design. The results show that the agreements between the simulated and the experimental values were high. When increasing the feed temperature and decreasing the coolant temperature, the impact of polarization phenomena on the performance of AGMD was reduced. The mass flux, Total Permeate Concentration (TPC), and heat flux increased by 81.69%, 36.89%, and 118.01%, respectively, when the feed temperature was increased from 50 °C to 75 °C. When the coolant temperature decreased from 22 °C to 7 °C, the mass flux increased by 37.06%. The response surface analysis revealed that the feed temperature has significant influence on AGMD performance, and there is a noticeable interaction between the feed temperature and coolant temperature. These findings will play key roles in practical applications.

Membrane fouling analysis of air-gap membrane distillation (AGMD) for recovery of water and removal of antibiotics from a model wastewater containing antibiotics and humic acid

The study investigates the efficiency of air-gap membrane distillation (AGMD) in water recovery and antibiotics removal from wastewater, focusing on high-concentration scenarios. Experimental findings reveal enhanced membrane performance with increasing the feed temperature, resulting in vapor permeate fluxes of up to 5 kg/m2.h at higher temperatures. Despite experiencing flux reduction caused by fouling from humic acid (HA) in the feed antibiotics solution, the antibiotics consistently maintain near-complete rejection rates (>99%) over 48 h. The foulant on the membrane surface was illustrated by SEM imaging. To know the temperature polarization and the fouling resistance, mathematical modeling was used, and it validates experimental results, elucidating temperature polarization effects and mass transfer coefficients. An increase in feed flow rates reduced thermal boundary layers, enhancing heat flux. Higher temperatures reduced HA fouling resistance. Therefore, AGMD proves effective in water recovery and antibiotics removal, with mathematical models aiding fouling understanding for future research and detailed computational fluid dynamics (CFD) models.

Biomass-based Brayton-Stirling-AGMD polygeneration for small-scale applications in rural areas

The lack of access to electricity and clean water still affects a substantial proportion of rural areas worldwide, in particular the global south. This paper presents a sustainable polygeneration system that can provide electricity, heat, and drinking water by using agricultural residues in remote rural areas. This polygeneration system consists of a solid biomass-fueled Brayton-Stirling combined cycle system, a boiler, and an air-gap membrane distillation unit. Four different system operation modes were designed to examine the most ideal configurations for maximizing power output, overall efficiency, and/or clean water production, considering a polygeneration system designed for a rural village with daily demands of 13450 kWh electricity and 7.5 m3 drinking water. A thermodynamic analysis are employed to analyze and compare these modes, each operating under steady state conditions. The highest electricity output, up to 160 kW, while the highest clean water is up to 0.7 m3/h. The fuel consumption can reach 0.9 kWh/kg of solid fuel and provide up to 0.0045 m3 of freshwater. In addition, nonlinear multi-objective optimization is used to meet the power demands of typical day in rural areas by varying the polygeneration operation modes and turbine inlet temperature.

Electrospun bio-polymeric nanofibrous membrane for membrane distillation desalination application

Polylactic acid (PLA) has been explored as a sustainable material for membrane distillation (MD) membranes using electrospinning for the first time. Adjusting the electrospinning parameters of PLA nanofiber, such as concentration, voltage, collector distance, humidity, and flow rate, resulted in uniformly distributed and smaller nanofibrous structures. Heat treatments were also performed to enhance the tackiness and fusibility of the fibers within the nanofibrous structure, optimizing the pore size, hydrophobicity, and surface roughness of the PLA membranes. The hierarchical structure of the PLA membranes improved the liquid entry pressure (LEP) and mechanical strength by 320 % and 230 %, respectively. The membrane characteristics were assessed in terms of porosity, pore size, surface roughness, and contact angle, which determined the impact of the wettability of the membrane under different conditions. The results showed that under certain constraints, PLA membranes could be effectively used in the air gap membrane distillation (AGMD) process, achieving a permeated flux of ~2 kg/m2h and a salt rejection of 99 %. However, PLA-based nanofibrous membranes are unsuitable for MD applications at feed temperatures above 60 °C and a flow rate of 60 LHP. This study provides a strong basis for developing and manufacturing eco-friendly bio-based nanofibers materials for desalinating saltwater.

Experimental Study on the Removal of Toluene from Water by PTFE Hydrophobic Microporous Membrane Using Air Gap Membrane Distillation Process

Toluene is used for various purposes such as a cleansing agent, paint thinner, lacquers, printing, and leather tanning process. However, it has an adverse effect on human bodies, such as skin irritation, leukemia, respiratory issues, mutagenicity, etc, when the chemical gets mixed in drinking and rainwater by the above routes. Therefore, there is a threat to the environment and society because of the presence of toluene in water bodies through industrial effluent. Henceforth, in this study, toluene is separated from water using the air gap membrane distillation (AGMD) process. The hydrophobic microporous poly-tetra- fluoro-ethylene has been employed for this purpose. Primarily, this work focussed on the study of operating parameters such as feed solution temperature and coolant temperature on the AGMD performance namely permeate flux and toluene selectivity. It was observed that the permeate flux increased exponentially from 2.07 kg/m2h to 9.23 kg/m2h, and 0.94 kg/m2h to 3.53 kg/m2h. However, it was found that the permeate flux decreased from 9.48 kg/m2h to 8.12 kg/m2h, and 3.78 kg/m2h to 2.42 kg/m2h on increasing the coolant temperature from 5°C to 25°C at 3 mm and 11 mm air gap, respectively. The membrane selectivity was found to be less than 1, which signifies the efficient toluene-water separation. In order to study the membrane performance in the long run, the permeate flux was estimated continuously till 60 h wherein the estimated flux was found to be almost constant. The membrane morphology before and after the 60 h experimental run was analysed using capture FE-SEM micrographs. The toluene concentration was estimated using a spectrophotometer at a wavelength of 264 nm.

Fabrication of PVDF membranes via VIPS-NIPS technique for water desalination: Effect of preparation condition

The operating cost of the desalination process can be decreased by using the membrane distillation (MD) process due to very low operating pressure and mild operating temperature. Attempts to find the proper membrane for application in MD attract the attention of many researchers. In this paper, PVDF powder was employed for assembling membranes via the VIPS method for operating in air-gap membrane distillation (AGMD). The influence of the ethanol concentration in the coagulation bath and VIPS time was studied. PVDF membrane characteristics were analyzed via pore size distribution, volumetric porosity, thickness, scanning electron microscope (SEM), measuring water contact angle, water flux, and salt rejection. A nodular structure was found for the membranes resulting from delayed demixing and solid-liquid phase separation. Moreover, the results indicated a larger membrane pore size was formed and the hydrophobicity was improved. The hydrophobicity and pore size are two important factors for membrane distillation that can affect and enhance membrane permeability. Moreover, studying the influence of nanoparticles showed that SiO2 nanoparticles are more effective than TiO2. The highest water contact angle was obtained when SiO2 nanoparticles were sprayed on the surface resulting in higher permeability and salt rejection in AGMD. The AGMD experiments' results showed that the membrane permeability increased from 0.3 to 2.41 L/m2.h, and salt rejection over 99.5 % was obtained.

Enhanced performance of electrospun PVDF membranes modified with rare-earth metal oxides for membrane distillation: A systematic study

In the presented studies, the novel type of separation materials dedicated to membrane distillation was generated and systematically analyzed. Utilizing poly(vinylidene fluoride) (PVDF) electrospun membranes as a base, hybrid organic-inorganic materials were developed to enhance performance. These materials exhibited high hydrophobicity, featuring superhydrophobicity on a micro-scale due to fractal-like structures on the inorganic domain surface. Rare-earth metal oxides (REMO) were implemented for the first time as smart modifiers, covalently attached to the functionalized membrane surface. Three REMO types were selected to investigate their impact on membrane performance in air-gap membrane distillation (AGMD) for the desalination process. Chemical attachment via long enough linkers formed a flexible active surface structure, facilitating transport and separation. Dynamic goniometric studies evaluated membrane wetting stability. Particularly noteworthy was the membrane enhanced with Ho2O3 particles, with high flux and salt rejection rates of 14.41 ± 1.35 kg m−2 h−1 and >99.5 %, respectively. The membrane stability and affinity were further examined using Hansen Solubility Parameters and Pearson's hard–soft acid–base (HSAB) theory. The potential of the hybrid membranes is seen not only in desalination. With the ability to control surface properties by using different silane linkers, oleophobicity/oleophilicity can be achieved, and practical use in wastewater treatment or oil/water separation is possible.

Integration of proton exchange membrane fuel cell with air gap membrane distillation for sustainable electricity and freshwater cogeneration: Performance, influential mechanism, multi-objective optimization and future perspective

Proton exchange membrane fuel cell (PEMFC) converts vast majority of hydrogen energy into waste heat, resulting in energy wasting, membrane drying and even lifetime shortening. Herein, air gap membrane distillation is proposed to immediately remove and harness the waste heat from PEMFC for additional freshwater production. By quantifying the irreversible thermodynamic and electrochemical losses, mathematical formulas for the water production, power generation and overall efficiency are derived to evaluate the system performance. After a rigorous model validation, the performance feature, feasibility and competitiveness of the proposed system are examined. The integration system achieves a peak power density 23.69 % higher than that of a single PEMFC at 353 K, with a corresponding increase in exergy efficiency by 3.61 %. In addition, the influential mechanisms of the PEMFC operating conditions, air gap thickness, coolant temperature, and properties of the proton exchange membrane and hydrophobic porous membrane are investigated to discern potential avenues for further performance improvement. The gradient-based local sensitivity analyses further determine the optimal parameter regulation strategies for different output targets. The economic study indicates that the levelized costs of water and electricity over the total lifecycle are 21.53 $ m−3 and 0.061 $ kWh−1, respectively. The multi-objective optimization, integrating genetic algorithm and the technique for order preference by similarity to an ideal solution, maximizes the exergy efficiency and output power density while minimizing the initial investment cost per unit area, with values of 42.84 %, 0.975 W cm−2, and 27.33 $ cm−2, respectively.

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.

Gamma-irradiated janus electrospun nanofiber membranes for desalination and nuclear wastewater treatment

This study presents the fabrication of double-layer electrospun nanofibrous membranes (DL-ENMs) using polyvinylidene fluoride (PVDF) and polyether sulfone (PES) based polymers with different degrees of hydrophilicity (PES, sulfonated PES, and PES with hydroxyl terminals). A comparative analysis was carried out with single-layer electrospun nanofiber membranes (SL-ENM) with a total thickness of about 375 μm. Using feed solutions, including sodium chloride, sodium nitrate, and simulated nuclear wastewater (SNWW), the performance of DL-ENMs was evaluated for desalination and radionuclide decontamination by direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) techniques. The results showed that DL-ENMs, especially those incorporating a sulfonated PES-based hydrophilic layer, exhibited superior permeate fluxes, reaching values of 72.72 kg/m2.h and 73.27 kg/m2.h in the DCMD using aqueous feed solutions of NaCl and NaNO3, respectively, and 70.80 kg/m2.h and 41.96 kg/m2.h using aqueous feed solutions of SNWW in DCMD and AGMD, respectively. Both SL-ENMs and DL-ENMs exhibited high rejection efficiencies and decontamination factors for the feed solutions (>99.9%). In addition, the prepared ENMs were exposed to gamma radiation to evaluate their applicability in real-life applications. The result of irradiation revealed the negative impact of gamma radiation on the fluorine content of PVDF which could be a critical point in using PVDF as a hydrophobic material for decontaminating nuclear wastewater by membrane distillation.

Separation of hydrogen isotopic water by multi-walled carbon nanotube (MWCNT) membrane and graphene oxide (GO)-MWCNT composite membranes

Recently, membrane distillation (MD) processes using two-dimensional graphene oxide (GO) membranes exhibited good light water/deuteriated water (H2O/D2O) separation performance, which depended on membrane materials and structures. This raises the need for the development of novel membrane materials and revealing the relationship of membrane structure-separation performance to further increase separation performance. Here, one-dimensional (1D) MWCNT-based membrane was explored for the separation of H2O/D2O using the water source with specially chosen concentration of deuterium (D) in air–gap membrane distillation processes, and the resultant MWCNT membrane gave lower permeation flux and similar separation factor compared with the pristine GO membrane. Then GO-MWCNT composite membranes with different structures obtained by mixing various weight ratios of GO: MWCNTs were investigated, and their separation performance was found to be related with membrane structures. The resulted 3D hierarchical GO-MWCNT composite membrane (G/M−1:3) gave the highest permeation flux of 0.947 L·m−2·h−1 among all GO-MWCNT composite membranes, higher than that of the pristine GO membrane, while the separation factor of G/M−1:3 (1.045) and the pristine GO membrane (1.046) was comparable. Furthermore, the correlation of membrane structure-separation performance was reviewed. These results demonstrate the first example of MWCNT membrane and related composite membranes for H2O/D2O separation and suggest that H2O/D2O separation performance in membrane distillation processes can be improved by tailoring the structure of GO-MWCNT composite membranes. As the separation of H2O/D2O is usually used as a model in the laboratory for light water/tritiated water (H2O/HTO) separation process, this approach can guide for developing novel membrane materials and structures to deliver appropriate H2O/HTO separation processes needed in the nuclear energy field.

Performance assessment and optimization of adsorption-assisted air gap membrane distillation process for remediation of chromium-contaminated groundwater

Chromium contamination in groundwater is becoming a significant concern due to its potential contribution to the global cancer burden. Membrane distillation (MD) is a promising method known for producing high-quality water, yet issues namely temperature and concentration polarizations should be addressed. To overcome these issues, adsorption (ADS) is proposed to reduce Chromium concentration in the concentrated feed and enhance water production and resource recovery. This study assesses the performance optimization considered parameter effects of the process individually and subsequently executes it into a combined process. Slow sand filtration (SSF) is performed to eliminate particles and thus mitigate fouling. A novel Fe3O4@C-SiO2 is introduced in the ADS process. The technical and economic performances of single MD, hybrid systems (ADS-MD and SSF-MD), and an integrated SSF-ADS-MD system are compared. The results showed the integrated process outperformed various scenarios, achieving significant water recovery almost a 2-fold increase in a single process, and notable reductions in Chromium mass (485.3 gr) and abatement (31.4 mg/h). The economic analysis indicated that the benefit-cost ratio of the integrated process is slightly higher compared to hybrid ADS-MD primarily because of the lower costs in SSF. Ultimately, the results underscored the process of SSF and ADS in boosting MD performance, particularly in water production and Chromium reduction, with prudent economic considerations.

Simultaneous production of electricity and potable water underwater by integrating concentrating photovoltaic with air gap membrane distillation

This paper combines underwater concentrating photovoltaic and air gap membrane distillation, enabling the simultaneous production of electricity and potable water in underwater environments. Given its submerged nature and absence of land occupation, it proves particularly advantageous for land-deficient regions with limited access to electricity and potable water, such as islands and Coastal cities. It primarily comprises innovatively designed underwater solar concentrators featuring extensive light interception angles, photovoltaic modules, and membrane distillation units. It utilizes exhausted heat from photovoltaic modules to separate clean water from seawater via membrane distillation process. Furthermore, it is engineered to prevent any contact between metal materials and seawater, thereby mitigating the risk of corrosion. An experimental setup with photovoltaic cells’ areas of 0.006 m2 is developed and tested in the laboratory and under real weather conditions to assess its performance. The steady-state experimental findings indicate that as solar irradiation increases from 500 W/m2 to 900 W/m2, its electricity generation efficiency ranges from 9.0% to 9.7%, while the water yield efficiency ranges from 32.8% to 35.1%. Besides, outdoor experiments demonstrate that its power output during normal incidence ranges from 38.89 W/m2-85.56 W/m2, with an average electrical efficiency of 10.5%. Its average potable water yield reaches 0.537 kg/m2/h and average water production efficiency reaches 48.3%.

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.

Brine concentration using air gap diffusion distillation: A performance model and cost comparison with membrane distillation for high salinity desalination

Zero liquid discharge (ZLD) has emerged as a treatment process to address the challenges with brine disposal from desalination plants. To achieve ZLD, a brine concentrator is commonly used to extract water up to a high salinity approaching saturation. However, state-of-the-art brine concentrators are energy-intensive thermal evaporators comprising expensive metal alloys and complex maintenance. This study presents the design of a new brine concentrator referred to as air gap diffusion distillation (AGDD) that operates without a membrane up to a high salinity. A holistic analytical model is developed to evaluate the system performance in terms of its energy efficiency (gain output ratio, GOR) and water flux (represented as the recovery ratio, RR), as these metrics directly impact the levelized cost of water (LCOW). A multi-pass AGDD system achieves an overall water recovery of ~70 % and GOR of 7 (88 % latent heat recovery) with an initial feed salinity of 70 g/kg. A comparison is made to its thermodynamically similar counterpart, air gap membrane distillation (AGMD), which reveals that AGDD outperforms AGMD by eliminating heat and mass transport resistances associated with the membrane. The corresponding LCOW is 1.6× lower than AGMD, making it promising as a brine concentrator to achieve salinities >200 g/kg for minimal liquid discharge.

Recovering and reusing water, H2SO4, nickel and cobalt from gold mining wastewater using air-gap membrane distillation and solvent extraction

Gold mining industry has attracted significant attention for its impact on the environment, encouraging a responsive interest in recovering valuable materials from gold mining wastewater. This study, aligned with the principles of the circular economy, aimed to explore the extraction of water, sulfuric acid, nickel, and cobalt from gold wastewater, employing air gap membrane distillation (AGMD), non-dispersive solvent extraction, and conventional solvent extraction. In the AGMD process, it was demonstrated that 69 % of the wastewater could be reclaimed in the form of high-quality distilled (6.6 ± 1.8 L/m2h). The reclaimed water played a pivotal role in the subsequent non-dispersive extraction stage, acting as a re-extraction agent. Hollow membrane contactors were employed in this stage, achieving a recovery rate of over 95 % for sulfuric acid from the concentrate, with high selectivity over iron (βFe = 488) and magnesium (βMg = 8011;). The acid-depleted stream was then subjected to conventional solvent extraction, wherein Cyanex 272 proved highly effective in extracting nickel, although with limited success in extracting cobalt. The previously used acid current was reintroduced as a re-extractant, demonstrating an 89.3 % re-extraction efficiency. In summary, this study conclusively demonstrated the feasibility of reintroducing water, sulfuric acid, and nickel into the economic cycle through the applied methods. The recovered materials were carefully examined for their potential applications, not only within the gold mining but also across various industrial sectors. This multifaceted approach aligns with the circular economy's objectives, fostering the efficient and sustainable reintroduction of recovered materials into economic processes.

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.

Innovative use of air gap membrane distillation to harvest waste heat from alkaline fuel cell for efficient freshwater production: A comprehensive 4E study

Poor power density of alkaline fuel cell (AFC) resulting from inherent waste heat generation significantly hinders its widespread application. Herein, an innovative hybrid system that combines AFC and air gap membrane distillation is proposed, aiming to enhance the power density via utilizing the waste heat for freshwater generation. Based on the laws of thermodynamics and electrochemical theory, the hybrid system model is formulated to derive performance metrics by quantifying various irreversible losses. The thermodynamic performance, competitiveness, economic viability, and environmental impact of the system are comprehensively discussed through energetic, exergetic, economic, environmental (4E) analyses. Calculations predict that the maximum equivalent output power density permits 63.28% higher than that of the standalone AFC at 353 K. The influencing mechanisms of the AFC operation conditions, air gap thickness, and properties of the porous hydrophobic membrane are clarified to discern potential avenues for further performance improvement. Furthermore, a multi-objective local sensitivity analysis indicates that membrane porosity and coolant solution temperature are the most sensitive factors. The levelized costs study reveals that water and electricity costs over the total lifecycle are 56.89 $m−3 and 0.1012 $kWh−1, respectively. The multi-objective optimization further provides a high-performance and cost-effective design solution for the hybrid system.

Computational fluid dynamics simulation of the stacked module in air gap membrane distillation for enhanced permeate flux and energy efficiency

Advancements in membrane distillation (MD) technology require the development of module designs to optimize system performance. This study established a computational fluid dynamics (CFD) model to assess the performance of an air gap membrane distillation (AGMD) system. The CFD model achieved an accuracy of approximately 96.43% through verification under various feed temperatures and flow conditions in an experimental AGMD system. CFD simulations demonstrated the importance of flow and temperature distribution within the module, and the response surface method was employed to investigate the influence of module size on system performance. The thickness of the air gap affected the permeate flux by >7 times the module length. The influence of the module length on the change in the gained output ratio was >11 times that of the feed temperature. In addition, graphical optimization enables the identification of optimal conditions that satisfy multiple variables simultaneously. The dual mode of the AGMD system based on optimization improved the performance of the model and demonstrated efficient operation compared with the single mode, indicating improvements of 14.36% and 13.64% in the permeate flux and gained output ratio, respectively.

Assessing the fouling behavior of PTFE membrane in air-gap membrane distillation against oil-in-brine stabilized emulsions

Understanding the fouling behavior in membrane operations is crucial for designing an effective treatment sequence. The membrane materials, feed characteristics, and operating conditions affect the fouling behavior. In this study, the fouling behavior of polytetrafluoroethylene (PTFE) membrane in membrane distillation was assessed after the treatment of oil-in-brine stabilized emulsions. Membrane backwash by water in membrane distillation partially restored the membrane’s initial condition and eliminated salt fouling. Moreover, the effect of feed pretreatment by ultrafiltration on the fouling of the subsequent PTFE membrane was investigated. Remarkable improvements in salt rejection (> 98.5%) and oil rejection (> 96%) were observed for the hybrid system. Extensive characterizations were carried out to evaluate the fouling behavior. Surface morphology and elemental analysis revealed the nature of foulants. Fourier transform infrared spectroscopy was utilized to study the change in the surface chemistry of the membranes after being subjected to the oily brine feed. No peak shifting was observed indicating no chemical bonding between the organic contaminants and the membrane surface. Further elaboration of the results was achieved by measuring the mechanical stability and contact angle of the membranes. Reduction in the mechanical properties and water contact angle of the membranes was observed after fouling and wetting. Results indicated that ultrafiltration was an effective pretreatment process for membrane distillation using PTFE membrane. However, more unit operations should be incorporated to enhance the membranes’ integrity.