Temperature Polarization Effects Research Articles

Engineering morphological architecture of superhydrophobic/hydrophilic composite membrane for efficient photothermal membrane distillation

The viability and efficacy of photothermal membrane distillation (PMD) is still uncertain due to its inherent energy-efficiency and throughput mass flux limitation. Herein, we develop a delamination-free multilayer photothermal membrane that simultaneously imparts slashed mass transfer resistance, enhanced photothermal effect and strong water-repellency by engineering morphological architecture. Using a one-step programmed dual-channel electrospinning followed by an electrostatic spraying technique, the proposed membrane is composited by a thick water-intrudable hydrophilic supporting layer, a thin hydrophobic layer, and an ultrathin Ti3C2Tx MXene-engineered superhydrophobic layer. It is proposed that the morphological architecture engineering could render mitigation of mass transfer resistance, firm water-repellency, and robust heat localization. Hence, in addition to superior flux of 1.27 L m−2 h−1 (inlet feed/permeate at 20/20 °C) and 15.89 L m−2 h−1 (inlet feed/permeate at 50/20 °C), prominent solar efficiency (76.34 % and 96.45 %) of the proposed composite membrane (DS15-M) also was achieved during PMD operation (1.0 kW m−2). Moreover, the DS15-M showcased not only robust wetting resistance and long-term consistency, but also obviously mitigated temperature polarization effect during the PMD operation. This research work emphasizes the important role of morphological architecture in rendering performance enhancement, which is a significant implication in engineering membrane architecture for PMD application.

First-principles calculations for intrinsic defects in MgO: Positron lifetime and momentum distribution of electron–positron pairs

We presented first-principles calculations of the formation energies for various charge states of intrinsic defects in magnesium oxide, including VO, VMg, VMg+O, VO+Mg+O, and VMg+O+Mg. These results can be used to predict their thermodynamic stability and detectability of these defects via positron annihilation spectroscopy. We employed two-component density functional theory to compute the positron annihilation properties, incorporating the effects of temperature and spin polarization. We then performed calculations of the momentum distributions of annihilating electron–positron pairs in these intrinsic defects in MgO. Using one-dimensional momentum distributions (Doppler-broadening spectra), we calculated the line-shape parameters Srel and Wrel. Our positron lifetime calculations were compared with experimental data. These research provides insight into the positron annihilation characteristics of defects in MgO.

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.

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.

Enhancing the Permeate Flux Improvement of Direct Contact Membrane Distillation Modules with Inserted S-Ribs Carbon-Fiber Filaments.

Three widths of manufacturing S-ribs carbon-fiber filaments acting as turbulence promoters were implemented into the flow channel of direct contact membrane distillation (DCMD) modules to augment the permeate flux improvement in the present study. Attempts to reduce the disadvantageous temperature polarization effect were made by inserting S-ribs turbulence promoters in improving pure water productivity, in which both heat- and mass-transfer boundary layers were diminished due to creating vortices in the flow pattern and increasing turbulence intensity. The temperature polarization coefficient ttemp was studied and found to enhance device performance (less thermal resistance) under inserting various S-ribs carbon-fiber thicknesses and operating both cocurrent- and countercurrent-flow patterns. The permeate fluxes in the DCMD modules with inserted S-ribs carbon-fiber turbulence promoters were investigated theoretically by developing the mathematical modeling equations and were conducted experimentally with various design and operating parameters. The theoretical predictions and experimental results exhibited a great potential to considerably achieve permeate flux enhancement in the new design of the DCMD system. The DCMD module with inserted S-ribs carbon-fiber turbulence promoters in the flow channel could provide a relative permeate flux enhancement up to 37.77% under countercurrent-flow operations in comparisons with the module of using the empty channel. An economic consideration on both permeate flux enhancement and power consumption increment for the module with inserted S-ribs carbon-fiber filaments was also delineated.

Incorporating TiO2 nanocages into electrospun nanofibrous membrane for efficient and anti-fouling membrane distillation

In this study, TiO2 nanocages with a hollow structure were integrated into the polyvinylidene fluoride co-hexafluoropropylene (PH) spinning solution, leading to the fabrication of a novel PH/TiO2 electrospun nanofiber membrane (PH/TiO2 ENM) through electrostatic spinning. The incorporation of TiO2 nanocages resulted in the formation of abundant micro-nano structures on the PH nanofiber surface, enhancing membrane roughness and hydrophobicity. Additionally, the hollow nanostructure of TiO2 nanocages effectively reduced the thermal conductivity of the ENMs. This reduction in thermal conductivity mitigated the temperature polarization effect, thereby increasing mass transfer driving force for the ENMs. The maximum water flux of the PH/TiO2 ENM reached ∼57.9 L m−2 h−1 (LMH), which was approximately 3.7 times higher than that of the original PH ENM. In addition to the formation of micro-nano structures on PH nanofibers, the presence of TiO2 nanocages facilitated the attachment of more low-surface-energy FAS molecules, improving the resistance of them to fouling and anti-wetting properties against both inorganic and organic contaminants, even in the presence of SDS, in a composite feed solution during the DCMD process. Moreover, even after prolonged exposure to a composite salt solution, this membrane also exhibited exceptional anti-fouling properties and stable desalination performance during a 168 h long-term stability test.

Superhydrophobic composite membranes for membrane distillation based on CNTs networks: Overcoming the trade-off between water vapor permeability and wetting resistance

Enhancing the wetting resistance of the membrane is crucial for consolidating the stability of the membrane distillation (MD) process, but this generally leads to the decline of water vapor flux. Herein, we developed a superhydrophobic composite membrane composed of a highly hydrophobic polyvinylidene fluoride (PVDF) porous support layer and a functional superhydrophobic carbon nanotubes (CNTs) network. The CNTs network with high thermal conductivity not only reduces the temperature polarization effect, but also increases the effective evaporation area, thereby enhancing the mass transfer driving force of the MD process. Additionally, the CNTs network significantly reduces the maximum pore radius of the composite membrane, and the perfluorination treatment also reduces the surface energy of the CNTs network, thereby synergistically increasing the liquid entry pressure. The results showed that when treating high salinity water containing 3.5 wt% NaCl, the water vapor flux of the composite membrane increased from 20.0 LMH of the original PVDF membrane to 24.4 LMH, an increase of 22 %. Furthermore, the performance of the original PVDF membrane deteriorates rapidly when the concentration of surfactant in the feed exceeds 1.0 mM, while the composite membrane still maintains a stable water vapor permeability and salt rejection. More importantly, the composite membrane exhibits excellent resistance to wetting induced by gypsum scaling when using gypsum solution as the feed, which is attributed to its small surface pore size and high hydrophobicity. This research supplies new insights into the design of membrane structures used in MD processes to overcome the trade-off between water vapor permeability and wetting resistance.

Machine learning and computational approaches for designing membrane distillation modules

Membrane distillation (MD) is a promising emerging water desalination technology. Commercialization of MD modules has been hindered by ineffective heat recovery and temperature polarization effect. Although hollow fiber (HF) membranes provide the highest area-per-module, they are under-investigated compared to flat-sheet membranes due to the interconnection of geometric, thermal, and hydrodynamic parameters in HF MD process. In this work, the parameters impacting HF MD module design are performed based on multiscale and deep neural network (DNN) models. MD experiments are conducted to train and validate the machine learning and multiscale models. The developed models are used either to explain the effects of geometric, thermal, and hydrodynamic parameters on the permeate flux or to predict the flux of a given set of parameters. The results revealed an increase in flux with the flow rate, velocity, and feed temperature. However, it decreased with shell diameter and module length. Compared to the experimental fluxes, flux predictions using multiscale and DNN approaches were within 14% and 1.2%, respectively. The DNN model converged to a mean squared error of 1.21% (R2 = 0.96) within a few minutes and demonstrated its potential as a favorable tool for module design optimization due to its accuracy, speed, and low computational requirements. The present study effectively exhibits the advantages of using machine learning as a next-generation model for fast module design, optimization, and scale-up of MD technology.

Evaluation of spacer-induced hydrodynamic mixing using particle image velocimetry: Impact on membrane distillation performance

Net-type spacers are used to promote hydrodynamic mixing in the feed channel of crossflow membrane filtration. The induced fluid flow mitigates concentration and temperature polarization effects on the membrane surface, leading to the enhancement of heat/mass transfer across the membrane. The objective of this research is to study the effect of spacer geometry on fluid flow and heat/mass transfer in the feed channel. The particle image velocimetry (PIV) technique was used to measure flow velocity in the feed channel with net-type spacers. Flow characteristics at a wide range of Reynolds numbers were described in terms of time-averaged mean velocity and turbulence kinetic energy (TKE). The PIV result revealed distinct profiles of mean velocity and TKE at different channel heights with bi-planar spacers. The spacer with the largest hydrodynamic angle of 120° (Net120) was found to be the most efficient in promoting flow unsteadiness because it led to the largest channel-averaged TKE among the selected spacers. The unsteady flow regime on Net120 was accompanied by the greatest pressure drop over the feed channel. Moreover, the effect of spacer-induced mixing on heat/mass transfer across the membrane was further evaluated via the flux measurement with vacuum membrane distillation experiments. In comparison with other spacers, Net120 resulted in a relatively higher flux because of the improved hydrodynamic mixing. These findings reveal the validity of the experimental approach in identifying the impact of key spacer design parameters on hydrodynamic mixing. Nusselt number was calculated to relate the hydrodynamic flow and thermal flow in the feed channel of membrane distillation, and a modified Nusselt number correlation was proposed to better predict convective heat transfer in the spacer-filled channel in VMD.

Influence of carbon-based fillers on photoactive mixed matrix membranes formation

Direct solar membrane distillation is an innovative configuration that uses a photoactive membrane to transform solar light into heat, sensibly reducing temperature polarization effects and feed temperature decrease along the module. This approach has been proved capable of increasing membrane distillation performance and efficiency.In this work, photoactive mixed matrix membranes were prepared by non-solvent induced phase separation procedure using a polyvinylidene fluoride dope solution containing different carbon-based fillers. Commercial graphite and graphene were selected because of their large specific surface area and their influence on the membrane structure and photothermal properties was compared to the results obtained using carbon black. The introduction of the filler allowed to obtain temperatures as high as 60 °C on the dry membrane surface when exposed to a simulated sunlight source. Both graphene and graphite had minor effect on the membrane morphology while carbon black increased the mean pore size of the membrane. The direct solar membrane distillation tests, performed using both pure water or a 90 g/L NaCl solution as feed, showed promising improvements (up to 70%) of the distillation flux under light irradiation.

Concentrating phosphoric acid by direct contact membrane distillation using a low-cost polyethylene separator

The ability to recycle phosphorus (P) components in reusable strength from wastewaters presents a promising opportunity to meet the increasing consumption demand. In this paper, the concentration of a dilute stream of phosphoric acid from 15 to 60 wt% was demonstrated using a high-performing PE separator in direct contact membrane distillation (DCMD), benchmarking against commercial PVDF and PTFE membranes. Combining with morphological/structural/material characterization, the membrane performance and energy efficiency were analyzed in-depth based on a combined heat and mass transfer model, which coupled the effects of both concentration and temperature polarization. The interplay of membrane flux, operating conditions and energy indicators was examined, indicating the significant influence of feed temperature, membrane characteristics and the trade-off relationship between Evaporation Efficiency/ Gained Output Ratio and Specific Energy Consumption. The concentration of 15 wt% H3PO4 stream using the PE separator was operated continuously for 100 h with minimal change in flux (> 40 kg/m2·h at 65 °C). The long-time stability was confirmed by membrane autopsy before and after experiments. The results demonstrated the PE separator as a promising low-cost membrane for acid concentration, providing an alternative for resource recovery from harsh solutions.

Nanobubble-assisted scaling inhibition in membrane distillation for the treatment of high-salinity brine

In this study, we report the use of nanobubbles (NBs) as a simple and facile approach to effectively delay scaling in membrane distillation (MD) during the treatment of highly saline feed (100 g L−1). Unlike conventional gas bubbling in MD for improving the hydrodynamic flow conditions in the feed channel, here we generated air NBs with an average size of 128.81 nm in the feed stream and examined their impact on membrane scaling inhibition during MD operation. Due to their small size, neutral buoyancy, and negative surface charge, NBs remain in suspension for a longer time (14 days), providing homogenous mixing throughout the entire feed water. The MD performance results revealed that severe membrane scaling happened during the DCMD treatment of high salinity brine in the absence of nanobubbles, which dramatically reduced the distillate flux to zero after 13 h. A one-time addition of air NBs in the saline feed significantly reduced salt precipitation and crystal deposition on the PVDF membrane surface, delayed the occurrence of flux decline, prevented membrane wetting, thereby prolonging the effective MD operating time. With similar feed concentration and operating conditions, only 63% flux decline after 98 h operation was recorded in nanobubble-assisted MD. Two key explanations were suggested for the delayed membrane scaling upon addition of air NBs in the MD feed: (1) NB-induced turbulent flow in the feed channel that increases the surface shear forces at the membrane surface, alleviating both temperature and concentration polarization effect, (2) electrostatic attractions of the counterions to the negatively charged NBs, which reduces the availability of these ions in the bulk feed for scale formation.

Enhancing the Permeate Flux of Direct Contact Membrane Distillation Modules with Inserting 3D Printing Turbulence Promoters.

Two geometric shape turbulence promoters (circular and square of same areas) of different array patterns using three-dimensional (3D) printing technology were designed for direct contact membrane distillation (DCMD) modules in the present study. The DCMD device was performed at middle temperature operation (about 45 °C to 60 °C) of hot inlet saline water associated with a constant temperature of inlet cold stream. Attempts to reduce the disadvantageous temperature polarization effect were made inserting the 3D turbulence promoters to promote both the mass and heat transfer characteristics in improving pure water productivity. The additive manufacturing 3D turbulence promoters acting as eddy promoters could not only strengthen the membrane stability by preventing vibration but also enhance the permeate flux with lessening temperature polarization effect. Therefore, the 3D turbulence promoters were individually inserted into the flow channel of the DCMD device to create vortices in the flow stream and increase turbulent intensity. The modeling equations for predicting the permeate flux in DCMD modules by inserting the manufacturing 3D turbulence promoter were investigated theoretically and experimentally. The effects of the operating conditions under various geometric shapes and array patterns of turbulence promoters on the permeate flux with hot inlet saline temperatures and flow rates as parameters were studied. The distributions of the fluid velocities were examined using computational fluid dynamics (CFD). Experimental study has demonstrated a great potential to significantly accomplish permeate flux enhancement in such new design of the DCMD system. The permeate flux enhancement for the DCMD module by inserting 3D turbulence promoters in the flow channel could provide a maximum relative increment of up to 61.7% as compared to that in the empty channel device. The temperature polarization coefficient () was found in this study for various geometric shapes and flow patterns. A larger value (the less thermal resistance) was achieved in the countercurrent-flow operation than that in the concurrent-flow operation. An optimal design of the module with inserting turbulence promoters was also delineated when considering both permeate flux enhancement and energy utilization effectiveness.

Dead-end membrane distillation with localized interfacial heating for sustainable and energy-efficient desalination

Membrane distillation (MD) has the high potential to circumvent conventional desalination limitations in treating highly saline brines. However, the performance of MD is limited by its low thermal efficiencyand temperature polarization (TP) effect. Consequently, the driving force decreases when heat loss increases.In this study, we propose to minimize TP through localized heating where the thin feed channel was heated uniformly at the membrane-liquid interface without changing the properties of the membrane.This concept was further improved by implementing a new dead-end MD configuration. Investigated for the first time,this configuration eliminated circulation heat losses, which cannot be realized in conventional MD due to a rapid temperature stratification. In addition, the accumulation of foulants on the membrane surface was successfully controlled by intermittent flushing. 3-Dimensional conjugate heat transfer modeling revealedmore uniform heat transfer and temperature gradient across the membrane due to the increased feed water temperature over a larger membrane area. The increase of water vapor flux (45%) and the reduction of heat lossobserved in the new dead-end concept led to a decrease of the specific energy consumption by 57%, corresponding to a gain output ratio increase of about 132 %, compared to a conventional bulk heating, while preserving membrane integrity. A conjugate heat transfer model was deployed in ANSYS-Fluent framework to elucidate on the mechanism of flux enhancement associated with the proposed technique. This study provides a framework for future sustainable MD developmentby maintaining a stable vapor flux while minimizing energy consumption.

Solar membrane distillation enhancement through thermal concentration

Membrane distillation (MD) has great potential as a desalination technology because of its good compatibility with other technologies. However, its low efficiency and large energy consumption impede applications due to the temperature polarization effect. Herein, we describe solar membrane distillation (SMD) using a photothermal membrane with thermal concentration that could overcome this problem. A superhydrophobic polydimethylsiloxane/multiwalled carbon nanotube/poly (vinylidene fluoride) (PDMS/MWCNT/PVDF) composite membrane with excellent light capture and photothermal conversion abilities was prepared for the SMD process. The photothermal conversion ability and solar membrane performance were experimentally evaluated, while the enhancement process was investigated by simulations. It was found that this enhancement is due to the higher temperature near the membrane resulting from thermal concentration and localized heating effects. Notably, using a two-level thermal concentration method in a larger SMD module would realize additional desalination performance, and fresh water productivity could reach about 1.1 kg m−2 h−1. Using solar energy and benefitting from the thermal concentration effect, our designed SMD system could produce fresh water at a rate of 0.65 kg m−2 h−1 from 3.5 wt% salt water, with an energy consumption of pure solar energy of about 1 kW m−2. This represents a significant improvement over the conventional SMD process. Combining the SMD technology with a renewable energy source would further reduce the cost and energy consumption, and promote its industrial application.

Simultaneous anti-fouling and flux-enhanced membrane distillation via incorporating graphene oxide on PTFE membrane for coking wastewater treatment

To overcome the limitations of existing hydrophobic membranes for membrane distillation (MD) to treat industrial wastewater with complex contaminants, a composite membrane was developed in this study via incorporating graphene oxide (GO) on commercial polytetrafluoroethylene (PTFE) membrane. The dual-layer GO/PTFE membrane demonstrated asymmetric wettability, which transformed the membrane from in-air hydrophobic and underwater superoleophilic pristine surface to in-air hydrophilic and underwater oleophobic surface. The composite membrane displayed excellent salt rejection (>99.9%) and organic rejection (99.4%) during MD of bio-treated coking wastewater, outperforming the pristine PTFE membrane with apparent fouling and wetting phenomenon. The flux of GO/PTFE membrane reached 20.1 kg/m2·h, which is 44% higher than that of the pristine membrane. These improvements were attributed to the specific morphological and physicochemical characteristics of the composite membrane, such as high water affinity, strongly negative charges on membrane surface, reduced temperature polarization and screening effect to large molecules by the GO layer. This study suggests that GO/PTFE composite membrane has great potential in enhancing the permeate flux and fouling resistance simultaneously, which could be a competitive option for MD to treat challenging wastewaters enriched with salts and hydrophobic organics.

Pressure-retarded membrane distillation for simultaneous hypersaline brine desalination and low-grade heat harvesting

Membrane distillation (MD), an emerging pre-concentration technology for zero liquid discharge to manage industrial wastewater, has become increasingly attractive when the low-grade waste heat is utilized as its driving force. Pressure-retarded membrane distillation (PRMD) – a derivative of MD by applying a hydraulic pressure lower than the membrane liquid entry pressure at the cold permeate side – has the potential to further recover the low-grade heat in terms of electricity, which is an added benefit for the use of conventional MD. Here, we for the first time experimentally evaluated the feasibility of using PRMD for simultaneous desalination of hypersaline water and harvesting of low-grade heat and explored the mechanisms governing the experimental PRMD performance. Using a commercial membrane, NaCl solution at the concentration as high as 4 M (233.76 g L−1) is desalinated with the salt rejection >99.9% and additional energy is generated in PRMD at the same time. We showed that the membrane operated with its active layer facing a cold solution orientation in PRMD exhibits better mechanical stability at higher applied pressures, which is essential for achieving higher power output. However, in contrast to the other orientation that is used in conventional membrane distillation (MD) processes, the coupled effects of internal temperature polarization (ITP) and internal concentration polarization (ICP) lower effective driving force and membrane permeability, which significantly compromises the water vapor flux and peak power density. These results indicate that unlike MD processes, controlling internal scaling and fouling are critical for PRMD and our findings can serve as useful guides for creating high-performance PRMD membranes in practical environmental applications.

Improved performances of vacuum membrane distillation for desalination applications: Materials vs process engineering potentialities

Membrane distillation (MD) is considered an attractive technology for water desalination processes, but the low transmembrane water fluxes, compared to other desalination processes, is however often pointed out as an important disadvantage. This limitation can be tackled through an effective process engineering analysis. Membrane materials performances and process design (including operating conditions) can both generate increased water flux. These two possibilities are explored in this study, in order to better evaluate the interplay between materials and process role. In a first step, the impact of highly permeable and thin dense selfstanding membranes as an alternative to avoid pore wetting is studied. The use of passive process engineering strategies such as Dean vortices to enhance mass and heat transfer is further analyzed. It is shown that a highly permeable dense membrane material offers a potential twofold increase in water flux, when a permeability of 10−5 kg·m−2·s−1·Pa−1 is achieved. Alternatively, Dean vortices (helical hollow fibers) are of interest only when this level of permeance is obtained; a 20% increase in water flux could be achieved, thanks to a decrease of the temperature polarization effect.

Experimentally-validated computational simulation of direct contact membrane distillation performance

A Direct Contact Membrane Distillation system using a flat-sheet membrane module was fabricated to experimentally study the effects of operating conditions (inlet temperatures and flow rates of feed and permeate streams and salt concentration) on its distilled water production. A two-dimensional Computational Fluid Dynamics (CFD) model considering transmembrane mass flux and heat conduction and associated phase change heat transfer (evaporation and condensation) was developed. It was found that the computational predictions of distilled water production agreed well with the experimental measurements. Furthermore, the CFD model analyzed the effects of filament spacing of a screen spacer in the flow channels and flow configuration (counterflow and parallel) on the water distillation performance. In this study, it is concluded that high water distillation is achieved by using low permeate temperature, high feed temperature, high feed and permeate flow rates, and less salt concentration in a counterflow configuration. The water distillation is more influenced by the feed temperature than the permeate temperature due to the water saturation pressure-temperature relation. The simulation results also showed that the convection thermal resistances in the feed and permeate channels are decreased by using screen spacers and therefore, the temperature and concentration polarization effects are decreased in the channels.

Principles and advancements of air gap membrane distillation

Abstract In recent years, membrane distillation (MD) has evidently emerged as one of the promising separation processes, with increasing areas of application including but not limited to desalination, pharmaceutical and textile wastewater purification, food processing, concentration of aqueous solution, breaking azeotropic mixtures, and extraction of volatile organic compounds. Primarily, MD has been categorized on the basis of vapor collection and condensation arrangement methods. Among the various categories, air gap membrane distillation (AGMD), in which an air gap is maintained across the membrane and the cooling plate, turns out to be an important and efficient process. Lately, AGMD has received significant attention of researchers around the world which motivates the present work. This paper aims to review the work done so far concerning the AGMD in order to provide a holistic view that covers the principles and applications of AGMD, effect of process parameters, membrane parameters, mathematical modeling, fouling, temperature and concentration polarization, types of membrane module, energy consumption, recent developments in AGMD process, cost estimation, and heat integration with AGMD. To the best of our knowledge, the present work is the first attempt to exhaustively review the AGMD process.