Membrane Distillation Module Research Articles

Performance Analysis of Air Gap Membrane Distillation Process Enhanced with Air Injection for Water Desalination

Water scarcity is a global issue, and desalination is an alternative to providing fresh water. Renewable energies could be used in thermal desalination to produce freshwater from high saline concentration solutions. In this paper, the experimental performance of an air-injection-Air Gap Membrane Distillation (AGMD) module is presented. The effect of the operation parameters (saline solution temperature, air flow, and salt concentration) on the distilled water rate was evaluated. The air injection enhanced the distilled water rate by 22% at the highest air flow and a solution flow rate of 80 °C, compared to the conventional condition (without air injection) at a salt concentration of 100,000 ppm. Under the same operating conditions, the increase was 17% at a salt concentration of 70,000 ppm. The maximum distilled water rate was 14.10 L/m2·h at 80 °C and an airflow of 1.5 L/min with the highest salt concentration, while it was also 14.10 L/m2·h at the lower salt concentration was 14.10 L/m2·h. The distilled water quality also improved as the air flow increased, since a conductivity reduction of 66% was observed. With the described mathematical model, 94% of the calculated values fell within ±10% of the experimental data for both salt concentration conditions.

Performance simulation of solar-driven absorption heat pump-membrane distillation system for combined desalination brine concentration with feed recirculation and cooling applications

Desalination is the primary choice for securing freshwater provision in water-stressed regions and reduces the gap between rising demand and dwindling natural freshwater resources. However, global desalination plants are dominated by fossil fuel-driven desalination technologies with a 40–50 % recovery ratio. Hence, it is critical to decarbonize desalination and address brine effluent ecological concerns. In this paper, a solar-powered absorption heat pump (AHP)-membrane distillation (MD) system concept was proposed and analysed for small-scale RO plant brine reject management and space cooling applications. The MD subsystem is based on commercial MD modules with batch feed recirculation to reach saturation (from 70 to 260 g/kg salinity). The MD system's heating and cooling consumptions are supplied by the AHP (6.54 MWh and 13.47 MWh, respectively, for a complete batch cycle). The AHP is designed to supply hot water at 85 °C with 701.63 kW heating capacity and co-produced chilled water at 16 °C with a cooling capacity of 857.86 kW, about 67 % is utilized to cool down the brine reject to feed temperature. The thermal and exergy COPs were 1.273 and 0.40 at a driving heat of 135 °C. The solar-powered AHP-MD system is useful for sustainable desalination deployment besides space cooling applications.

Novel physics-informed optimization framework for complex multi-physics problems: Implementation for a sweeping gas membrane distillation module

The application of advanced machine learning algorithms such as deep learning is limited for the surrogate-based process optimization of complex multi-physics problems because high-quality experimental and numerical training samples required for deep-learning model training are expensive and scarce. To address this issue, in this study, a novel physics-informed process optimization (PIPO) framework was introduced. In the first step, surrogate models based on conventional neural networks (NN) were trained using a few available high-quality training samples. In the second step, the trained NN models were coupled to the process optimization solver in which the loss of physical laws was added to the optimizer’s objective function to find optimal design points that satisfy the laws of physics. As a result, the generalization performance of the framework was greatly improved for design targets outside the training range of NN models. PIPO is substantially different from the physics-informed neural networks where the loss of physics is added to the loss function used during NN model training. The PIPO framework was used to optimize a sweeping gas membrane distillation (SGMD) module. Eight input design variables, including process and geometrical parameters, were optimized for different challenging targets to achieve the best SGMD performance in terms of ammonia recovery ratio and concentration. It was shown that for noticeably few training samples of 68 experiments, the proposed framework was able to achieve the optimization targets within a reasonable computational cost. The optimum designs were verified and analyzed in detail by high-resolution computational fluid dynamics models.

Evaporative cooling and sensible heat recovery enable practical waste-heat driven water purification

Waste heat capture from systems such as photovoltaics (PV) and refrigerators can lower their energy efficiency by increasing their operating temperature. In this study, we evaluate the potential of final-effect evaporative cooling and internal heat recovery in a multi-effect diffusion distillation (MEDD) to produce pure water without negatively impacting the energy efficiency of the waste-heat source. Lab-scale experimental results from a multi-effect membrane distillation module validate the concept, showing that water production can be enhanced by >20 % while simultaneously pulling down the module's operating temperature. A detailed numerical model of a solar-MEDD is implemented and validated. The incorporation of sensible heat recovery and evaporative cooling increase pure water production by approximately 10 % each. Although the pure water production of a standalone MEDD increases with increasing effects N, when coupled to a solar PV module, increasing N also decreases PV electricity production. Therefore, a PV-MEDD with fewer effects (≤4) is preferable and such a system can produce sufficient water for electrolysis (green H2 production) while maintaining or improving PV electrical energy production throughout the year under varying climatic conditions.

The performance of hollow fiber direct contact membrane distillation modules

Computational fluid dynamics simulations were conducted in hollow fiber direct contact membrane distillation modules. The objective is to understand the impact of packing densities and incorporating flow mixers on the feed side of the system. The feed flow rate varies while the feed inlet and permeate inlet temperatures are fixed. Furthermore, finned hollow fibers are used as flow disruptors in the feed channel to mitigate temperature and concentration polarization and to enhance vapor flux. The laminar flow model is used to characterize velocity, temperature, and concentration fields. It is demonstrated that the packing ratio significantly influences when the flow rate is low. The tightly packed hollow fibers in the shell resulted in decreased permeating flux due to a lack of mixing in flow in the narrowed regions around hollow fibers. The data shows that the permeate flux was 14.5 % higher for a 25 % packing ratio compared to a 75 % packing ratio at a lower flow rate. In contrast, the flux decreases only 3 % at a higher flow rate with an increasing packing ratio. It is demonstrated that the presence of fins breaks the parallel flow and enhances the mixing performance by approximately 10 % While fin modifications can enhance flux performance and mitigate polarization issues, the decrease in the merit criterion suggests that the benefits must be weighed against the potential drawbacks, such as elevated energy consumptions due to the increase in pressure drops. This underscores the importance of a balanced approach in optimizing membrane module performance.

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.

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.

A computational fluid dynamics study on TPMS-based spacers in direct contact membrane distillation modules

Feed spacers are incorporated in membrane modules to enhance fluid mixing, thereby enhancing membrane flux. Previous experimental work showed that triply periodic minimal surfaces (TPMS)-based spacers induce significant improvements in membrane distillation (MD) performance, which was attributed to the suppression of membrane boundary layer, reducing heat and mass transfer resistance. To corroborate those findings, in this study, we assess the performance of TPMS spacers in direct contact MD (DCMD) modules using computational fluid dynamics (CFD). Results showed that the Fischer-Koch S TPMS spacer topology achieved twice the flux of the empty channel case and exhibited a 13 % increment in flux compared to the net-type spacer, but this improvement was accompanied by a higher pressure drop in the feed channel. Therefore, a normalized power ratio (NPR) was used to evaluate the overall performance of spacers. For Reynold number (Re) ranging between 200 and 1200, Fischer-Koch S spacer had a 12 % improvement in NPR compared to the net-type spacer and resulted in higher shear stress near the membrane surface, indicating a promising mean for mitigating fouling in membrane distillation. This research highlights the importance of utilizing novel spacers in membrane distillation and showcases the power of CFD tools in optimizing the process.

Modeling of Integrated Hollow-Fiber Solar-Powered VMD Modules for Desalination for a Better Understanding and Management of Heat Flows.

The direct integration of membrane distillation and solar energy collection in a single module is a promising technology for autonomous seawater desalination in remote regions; however, the modeling and design of such modules are challenging because of the coupling of the radial and longitudinal heat and mass transfers. In a previous study, we provided as a first modeling approach a hollow fiber solar collector vacuum membrane distillation (VMD) module, considering a constant temperature at the shell side and a pure water feed. Here, a full model is developed to describe the coupled effects of the solar collector and a hollow fiber VMD module operating in an outside/in mode with saline water. The model considers all the main phenomena (membrane distillation, temperature and concentration polarization, absorption of solar radiation and energy balances over the solar collector, radial and longitudinal heat and mass transfer, seawater properties, and more than 30 variables). Applied to simulate the behavior of a semi-industrial-scale module, it allows the influence of solar radiation on the performance/limits of the integrated module to be discussed based on the radial and longitudinal profiles and heat flows. The model can be used to identify key points in the module design to better utilize solar radiation and manage heat flows.

3D-CFD simulation of hollow fiber direct contact membrane distillation module: Effect of module and fibers geometries on hydrodynamics, mass, and heat transfer

For achieving a successful membrane distillation operation, not only the properties of the membrane but also the module design must be optimized. Computational Fluid Dynamics (CFD) is a practical tool for investigating the transport phenomena in a membrane module. Thus this research aimed to investigate the momentum, heat, and mass transfer by means of a 3D-CFD simulation for a hollow fiber membrane module applied for direct contact membrane distillation (DCMD) of saline water. Momentum, heat, and mass transfer equations in three dimensions were coupled and solved using the finite volume method by the Comsol Multiphase® software, and the effects of feed and permeate flow rates, number of fibers, and fiber length on DCMD performance are examined. The results demonstrated that the number of fibers has a significant effect on the permeate flux through flow channeling. The impact of feed flowrate on the increase of permeate flux is much more considerable than the permeate side. The longer fibers resulted in lower average fluxes due to the development of a thermal boundary and concentration layers. The temperature profile shows abnormal deflections at feed entry and reject exit ports due to the fluid entrance effects. Finally, it could be concluded that enhancing the feed-side hydrodynamics is an easy and affordable technique for attaining considerable improvement in DCMD 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.

Techno-economic analysis of solar-powered membrane distillation system with circulated permeate gap

This study investigates a novel integration of a circulated permeate gap membrane distillation (C-PGMD) module with flat plate solar collectors. The investigation is performed through a comprehensive performance study and techno-economic analysis, revealing unique insights and findings. The development of mathematical models enables the anticipation of system production and energy efficiency, capturing the employment of single and multistage C-PGMD modules under diverse design and operational conditions. Additionally, an innovative assessment of different powering modes, including solar-only, electrical-only, and hybrid solar-electrical is performed, which offers a unique perspective on system control and unit production costs. Results showed that circulating the permeate gap has significant effects on the permeate flux and the temperature of the solar tank such that the circulation rate controls the operation of the system. The revelation that increasing solar collector numbers amplifies productivity while reducing costs stands out as a noteworthy breakthrough. Cooling the system using a water source at ambient temperature is much recommended compared to cooling with chilled water. The pinnacle of this research lies in the proposal of a well-designed pilot hybrid-power 30-stage C-PGMD system, showcasing a remarkable achievement in reducing production costs to approximately 0.03 USD/Liter for 24-hour operation, predominantly powered by renewable energy sources. These novel findings redefine the landscape of integrated solar membrane distillation and pave the way for more sustainable and economically viable water desalination solutions.

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.

Performance comparison of vertical and horizontal vacuum membrane distillation module coupled with mechanical vapor recompression

This paper focuses on the comparative study of vertical and horizontal vacuum membrane distillation (VMD) modules coupled with mechanical vapor recompression (MVR) system to obtain the optimal system layout structure and further improve operation stability and energy efficiency in the wastewater treatment. Firstly, freshwater accumulation characteristic in the vertical VMD module was explored under different working conditions. Then, comparative experiments of vertical and horizontal VMD-MVR systems were conducted. Finally, comprehensive performance of horizontal VMD-MVR system were evaluated. The obtained results showed that freshwater production in the distilling tank and freshwater accumulation in the VMD module were affected by operation time, feed temperature, feed flow rate, feed concentration and vacuum side pressure. Furthermore, the horizontal VMD-MVR system was proved to be a better choice due to lower freshwater accumulation in the VMD module and energy consumption in comparison to the vertical VMD-MVR system. Finally, long term operation experiment of 240 h indicated evaporation rate dropped to 55 kg/h as operation time reached to 180 h owing to membrane fouling. After membrane cleaning, evaporation rate recovered from 55 to 62 kg/h with a higher recovery efficiency of 95.1 %. Compared to conventional wastewater treatment systems, the horizontal VMD-MVR system exhibited excellent comprehensive performance.

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.

Performance analysis and optimization of sweep gas membrane distillation module using high-resolution computational model and machine learning

Performance analysis of sweep-gas membrane distillation (SGMD) modules remains a challenging problem owing to the complex interaction and strong coupling between the flow parameters on the feed and permeate sides as well as the multiscale nature of gas permeation in the membrane pores. In this study, a framework for a practical optimization tool for SGMD was developed in which experimental data, numerical simulations, and machine learning (ML) techniques were systematically utilized. A high-resolution numerical simulation of the SGMD was introduced and experimentally validated for SGMD modules. The high-resolution numerical model was used to train a ML model with noticeable speedup and lower computational cost. The trained ML model was coupled with a multi-objective genetic algorithm to optimize the SGMD performance for a large-scale SGMD module with a novel and complex flow pattern. The SGMD, comprising five different modules with parallel sweep-gas patterns, showed interesting behavior in a sensitivity study based on modified Latin hypercube sampling. The sweep-gas temperature had the highest impact on the water permeation flux, which was due to the reheating of the feed flow as a result of the special configuration of the SGMD. Furthermore, the optimization solver successfully obtained the optimal design conditions for two different design scenarios by achieving a design target defined for an ammonia recovery ratio of 90% and an ammonia concentration of 25% at the sweep-gas outlet. The potential of the developed framework for practical engineering applications is highlighted.

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.

Optimum configurations of a compact hollow-fiber water gap membrane distillation module for ultra-low waste heat applications

A huge amount of ultra-low-grade heat, below 80 °C, is wasted in power generation and industrial applications. This research aims at investigating the possibility of exploiting such heat to produce tremendous amounts of potable water using the hollow fiber-based water gap membrane distillation modules (HF-WGMD). A fully coupled numerical model for the brine, membrane, and coolant is developed. Excellent agreement with experimental data was revealed with a maximum deviation of less than 5%. The optimum geometrical specifications of a compact module that maximize the distillate production per unit volume of the module at ultra-low levels of feed and coolant temperatures are investigated. The specifications obtained from the optimization comprise a packing of 91 fibers with a fiber length of 10 cm in a 5 cm shell diameter and an inlet feed velocity of 1.62 m/s. It is shown that HF-WGMD can desalinate saltwater with a production rate of 4.8 m3/day per cubic meter of the unit at feed temperatures as low as 25 °C with a coolant temperature of 5 °C. This production increases to 12.1 m3/day when adopting the optimum configuration and using the feed temperature of 45 °C with a coolant temperature of 25 °C, which is comparable to other desalination technologies. Compared with previous HF-WGMD investigation, the optimum configurations could obtain a product enhancement of about 63% at the same feed temperature and salinity.

Effect of wavy corrugations on the performance enhancement of direct contact membrane distillation modules: A numerical study

This study employs three-dimensional (3D) computational fluid dynamics (CFD) simulations to investigate the influence of corrugations on direct contact membrane distillation (DCMD) modules. The accuracy of 3D CFD simulations is validated by the comparison of water flux and temperature polarization coefficient (TPC) against literature data for empty and spacer-filled DCMD modules. The dusty gas model is used to predict the permeate flux as a function of temperature and pressure over the surface of the membrane. We consider single and double sinusoidal corrugated membranes and examine the performance of DCMD membranes in terms of TPC, concentration polarization coefficient (CPC), power number (Pn), and Nusselt number (Nu) against various corrugation geometrical parameters. These parameters include the amplitude, streamwise, and spanwise frequency of corrugation waves. Our results show that corrugated membranes can successfully improve the membrane system by disrupting the boundary layer by an intense secondary flow. The mechanism works the best at high flow rates. Our results suggest that singly corrugated membranes improve the water flux and TPC by 32% and 25% at Re=1500. Also, doubly corrugated membranes provide 45% and 36% improvements in water flux and TPC at the same Reynolds number.