Multi-effect Membrane Distillation Research Articles

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.

Internal feed preheating necessary for energy-efficient modular multi-effect membrane distillation

Multi-effect membrane distillation (MEMD) systems achieve a high product recovery ratio by effectively utilizing the heat of condensation for further vaporization within the module. In this study, a novel internal feed-preheating design for improving the energy efficiency of modular MEMD systems is proposed and evaluated using a 2D CFD model. Custom user-defined functions are implemented to account for water vapor and associated energy transport across the membrane in an MEMD design with serpentine flow across various effects. Preheat channels embedded into the condensation plates help increase the temperature of feed entering the first effect by ∼25 K, close to heater plate temperature. As a result, high-grade external heat input is not used for sensible heating, instead being utilized towards evaporation and pure water production, resulting in ∼32 % improvement in energy efficiency. However, the added thermal resistance of the preheat channels results in lower average flux and hence recovery, which can be ameliorated by increasing the effective conductivity of the preheat channels. Increasing the thermal conductivity of preheat channels results in ∼23.5 % rise in recovery ratio. Such preheat designs are necessary to reap energy efficiency improvements by increasing the total number of effects in multi-effect vacuum and gap MD systems.

Modeling and Simulation of Multi-Effect Membrane Distillation Systems

Modeling and Simulation of Multi-Effect Membrane Distillation Systems

Application of the Buckingham Π Theorem to Model the Multiple Effect Vacuum Membrane Distillation

Abstract This study aims to develop a dimensionless model of the vacuum multi-effect membrane distillation (V-MEMD) system through which a preliminary prediction of the most critical performance indicators can be attained. The Buckingham Π theorem was utilized to define dimensionless parameters that enable the predicted relationships associated with independent input parameters to describe the essential performance indicators of the V-MEMD system. The obtained simplified model reduces the design parameters from ten to two effective dimensionless parameters to realize the realistic and actual behavior of the designated system. The self-sustained model can be used as a short-cut tool for design and performance analysis to avoid time-consuming experimentations and complicated theoretical models. The compatibility of the generated model is assessed by matching the expected response of the output dimensionless parameters of recovery ratio (Π6) and gain output ratio (GOR, Π12) to variations in the pressure ratio and cooling process. The model is validated with other works, and discrepancies are found to be within ±8% and ±0.8 for the recovery ratio and GOR, respectively. Furthermore, the specific thermal energy consumption (STEC) is correlated to the GOR assuming constant vaporization enthalpy and density of the distillate water. The correlation can predict STEC within 5% accuracy over various operating conditions for the supplied hot water.

Assessing the Impact of Operating Conditions on the Energy and Exergy Efficiency for Multi-Effect Vacuum Membrane Distillation Systems

A comprehensive study was conducted to elucidate the effect of operating conditions on the performance of a multi-effect vacuum membrane distillation pilot plant. A theoretical assessment of the energy and exergy efficiency of the process was achieved using a mathematical model based on heat and mass transfer, which was calibrated using experimental data obtained from the pilot plant. The pilot plant was a solar vacuum multi-effect membrane distillation (V-MEMD) module comprising five stages. It was found that a maximal permeate mass flux of 17.2 kg/m2·h, a recovery ratio of 47.6%, and a performance ratio of 5.38% may be achieved. The resulting gain output ratio (GOR) under these conditions was 5.05, which is comparable to previously reported values. Furthermore, the present work systematically evaluated not only the specific thermal energy consumption (STEC), but also the specific electrical energy consumption (SEEC), which has been generally neglected in previous studies. We show that STEC and SEEC may reach 166 kWh/m3 and 4.5 kWh/m3, respectively. We also observed that increasing the feed flow rate has a positive impact on the process performance, particularly when the feed temperature is higher than 65 °C. Under ideal operational conditions, the exergetic efficiency reached 21.1%, and the maximum fraction of exergy destruction was localized in the condenser compartment. Variation of the inlet hot and cold temperatures at a constant differential showed an interesting and variable impact on the performance indicators of the V-MEMD unit. The difference with the lowest inlet temperatures exhibited the most negative impact on the system performance.

A decentralized water/electricity cogeneration system integrating concentrated photovoltaic/thermal collectors and vacuum multi-effect membrane distillation

Cogeneration of electricity and freshwater by integrating photovoltaic/thermal collectors and desalination systems is one of the most promising methods to tackle the challenges of water and energy shortages in remote areas. This study investigates a decentralized water/electricity cogeneration system combining concentrated photovoltaic/thermal collectors and a vacuum multi-effect membrane distillation system. The merits of such a configuration include high compactness and improved thermodynamic efficiency. To evaluate the long-term production potential of the proposed system, a thermodynamic analysis is firstly conducted. Under the climatic conditions of Makkah, Saudi Arabia, the system can convert ∼70% of the solar irradiance into useful energy. The annual productivity of electricity and distilled water are 562 kWh and 5.25 m3, respectively, per m2 of the solar collector area. Electricity and water production rates are found to be impacted by hot water flowrate, feed seawater flowrate and heat storage tank dimension, while the overall exergy efficiency stabilizes at 25–27%. Based on the production rates, a life-cycle economic analysis is then conducted. The final desalination cost is calculated to be $0.7–4.3/m3, depending on the solar collector cost and the electricity price. The derived results will enable a more in-depth understanding of the proposed solar-driven water/electricity cogeneration system.

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

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

Development of vacuum multi-effect membrane distillation system of pilot-scale for water desalination

Development of vacuum multi-effect membrane distillation system of pilot-scale for water desalination

Integration of multi effect evaporation and membrane distillation desalination processes for enhanced performance and recovery ratios

This work focuses on the integration of multiple effect evaporation (MEE) and membrane distillation (MD) processes. The MEE brine is treated by the MD allowing at the same time a preheating of the MEE feed water. Different structures of integration are assessed theoretically. Simulation models based on the heat and mass transfer balances are developed for the parallel feed (PF), parallel cross (PCF), forward feed (FF)-MEE, and direct contact MD (DCMD) configurations. The main performance parameters such as the performance ratio, recovery ratio, and specific energy consumption are investigated via simulation of various MEE-MD configurations and operating parameters. The results show that hybrid MEE and MD structures outperform the standalone MEE systems. The performance ratio increased by more than 25% when hybrid structure is used as compared to standalone MEE. The results revealed also that the hybrid parallel PF-MEE-MD configuration yields the best performance compared to other hybrid configurations. It also outperforms the standalone PCF-MEE which is widely adopted and used in practice. Besides, the performance of the hybrid configurations is especially sensitive to the feed temperature's variation in both MEE and MD units. Increasing MD feed temperatures has a positive impact on the recovery ratios of the combined systems.

The potential of hollow fiber vacuum multi-effect membrane distillation for brine treatment

Vacuum membrane distillation can extract pure water from degraded sources, but it requires relatively high energy inputs as compared to other thermal-driven technologies. As a commercially successful example, Memsys Water Technologies GmbH has addressed this key limitation by developing a flat sheet-based vacuum membrane distillation module where the latent heat recycled internally by using multiple distillation effects. In this paper, we propose an alternative hollow fiber-based design which also recycles the latent heat with multiple effects, but with an even more compact membrane packing format. The proposed design uses 3-dimensional printing to ‘unlock’ this configuration via a hollow aluminium alloy baffle which relies on its low thermal resistance to recover the latent heat effectively. The printed metal baffle (0.8 mm wall thickness) was calculated to have a very high conductive heat transfer coefficient, ~180 kW/m2K (surpassing even the ~20 μm polypropylene foil used in the Memsys module, which has a conductance of ~10 kW/m2K). Our experimental and theoretical results indicate that this design uses a condensation-convection-distillation heat and mass transfer mechanism which enables a three-effect system to reduce the energy consumption by ~60% over a single-effect design (i.e. from 672 kWh/m3 to 263 kWh/m3) for synthetic geothermal brine (~200 g/L salt concentration). Furthermore, the prototype reached a high average permeate flux of ~5.1 LMH and a salt rejection rate of >99.99%, approaching zero liquid discharge. Overall, this work suggests that hollow fiber membranes can indeed be used in a multi-effect mode and represents a promising new pathway for membrane distillation.

Experimental study of a liquid desiccant regeneration system: performance analysis for high feed concentrations

Liquid desiccant air-conditioning is an energy-efficient alternative to conventional vapor compression systems due to its ability to handle sensible and latent heat loads independently. Membrane-based processes are prevalent in desalination research due to their superior abilities to separate fluids. The membrane-assisted separation process is also suitable for use in liquid desiccant air-conditioning systems as it can minimize the problems of corrosion, which has so far prevented the widespread use of liquid desiccant systems. In this study, we have utilized the flat-sheet polytetrafluoroethylene membranes for liquid–vapor separation and combined multieffect design with vacuum conditions, for enhanced liquid–vapor separation across the membrane. Preliminary experiments and literature surveys indicate that the conventional vacuum multieffect membrane distillation system exhibits poor performance when the feed concentration is above 26%. Therefore, efforts are made to enhance its performance when operating at higher concentrations (30% and 34%) by employing flat-plate-type heat exchangers to preheat the liquid desiccant and by maintaining high air-side vacuum. The regeneration process can be optimized based on increase in concentration (ΔC) and performance ratio (PR), by controlling the operating conditions. The evaluations of the regenerator performance for different operating parameters including heat source temperature, circulation cross-flow rate are presented along with analysis. Preheating the LiCl solution before regeneration resulted in improved performance; PR of 0.58 and ΔC of 2.4% are achieved for 34% LiCl concentration. Schematic representation of the membrane-assisted LDAC test bed

Integration of vacuum multi effect membrane distillation with adsorption/cooling system

This paper addresses the integration of vacuum multi effect membrane distillation (VMEMD) with adsorption (AD) unit to produce distilled water for many applications and cooling water to cool the buildings. Integration between VMEMD and AD systems aims to increase the water productivity with low power consumption. In this system, VMEMD is driven by hot seawater to produce fresh water, as the remind brine of VMEMD will discharge into the evaporator of AD cycle to produce other part as a fresh water. AD is a heat driven technology that can be used to generate distilled water and/or cooling water through adsorption/desorption of water vapour using porous metal organic framework (MOF) adsorbent materials. MOF is a new class of porous material with high surface area, superior adsorption characteristics and large porosity. The MOF such as, Aluminium Fumarate (Al-Fum), shows have water vapour adsorption characteristics superior to those of silica gel and zeolite. While, Nickel salts (CPO-27-Ni) was found to be more suitable for applications working at a low evaporation temperature (5 °C) and high temperature (≥90 °C), meanwhile Al-Fum showed a superior performance at a higher evaporation temperature (20 °C) and low regeneration temperature (70 °C). Both systems, VMEMD and AD, have ability to produce more distilled water with recovery ratio about 60%, in addition to produce cooling water.

Thermo-economic analysis and optimization of a vacuum multi-effect membrane distillation system

Vacuum multi-effect membrane distillation is an advanced system that possesses the features and merits of vacuum membrane distillation and multi-effect distillation. It has low operating pressure and temperature, high levels of non-volatile rejection and high energy efficiency. This study presents a thermo-economic analysis and optimization of this novel system. A thermodynamic analysis is firstly conducted to evaluate the productivity and the energy consumption under varying design and operational conditions. Special emphases are placed on the impacts of the system configuration, including the number of effects and the overall membrane area, which are rarely covered in the literature. Results reveal that there is a trade-off between the production rate and the energy consumption with respect to most of the operating parameters, e.g. the feed flowrate and the cooling water flowrate. An increase in the number of effects and the membrane area will reduce the energy consumption, but the specific permeate flux for the unit membrane area also becomes lower. To obtain the optimal parameters that minimize the desalination cost, an economic study is then carried out considering a wide range of thermal energy prices. It is observed that a higher feed flowrate, more numbers of effects and larger membrane areas are preferable when the energy price is higher. However, when thermal energy with low prices is available, lower feed flowrates and smaller membrane areas are recommended. The derived results will provide useful information on the vacuum multi-effect membrane distillation system for its future design and operation.

Exergy analysis of solar desalination systems based on passive multi-effect membrane distillation

Improving the efficiency and sustainability of water treatment technologies is crucial to reduce energy consumption and environmental pollution. Solar-driven devices have the potential to supply off-grid areas with freshwater through a sustainable approach. Passive desalination driven by solar thermal energy has the additional advantage to require only inexpensive materials and easily maintainable components. The bottleneck to the widespread diffusion of such solar passive desalination technologies is their lower productivity with respect to active ones. A completely passive, multi-effect membrane distillation device with an efficient use of solar energy and thus a remarkable enhancement in distillate productivity has been recently proposed. The improved performance of this distillation device comes from the efficient exploitation of low-temperature thermal energy to drive multiple distillation processes. In this work, we analyze the proposed distillation technology by a more in-depth thermodynamic detail, considering a Second Law analysis. We then report a detailed exergy analysis, which allows to get insights on the production of irreversibilities in each component of the assembly. These calculations provide guidelines for the possible optimization of the device, since simple changes in the original configuration may easily yield up to a 46% increase in the Second Law efficiency.

An experimental investigation of a solar-driven desalination system based on multi-effect membrane distillation

An experimental investigation of a solar-driven desalination system based on multi-effect membrane distillation

Application of solar energy to seawater desalination in a pilot system based on vacuum multi-effect membrane distillation

The evaluation of a novel solar seawater desalination system implemented at the University of Almeria (Spain) is presented. It integrates a solar thermal field based on static collectors and a thermal desalination system based on the vacuum multi-effect membrane distillation technology. The distillation unit has a particular innovation to increase its thermal performance, using a seawater flow to condense the steam and preheat the feed. Experiments were made under different environmental conditions to assess the role of the thermal storage system for minimizing the effect of disturbances in solar radiation. Thermal energy could be delivered at a stable temperature to the distillation module, even with variable solar radiation. A simulation analysis based on a quasi-dynamic model was also performed to evaluate the distillate production profile and the operating time during a typical year considering different temperature setpoints at the inlet of the membrane module (60, 70, and 80 °C). The simulated volume of distilled water generated annually ranged from 41.7 to 70.5 m3, depending on the setpoint. The membrane distillation unit produced water almost uniformly along the year, with an average flux of (5.5 ± 1) l h−1 m−2 at the maximum setpoint, which was proved the most favourable.

Portable Solar-powered Membrane Distillation System to Solve Water and Energy Problems Simultaneously

With increasing human population, the demand for water as well as energy also increasing. This will lead to global water scarcity and energy crisis, due to limited freshwater resources and decreasing fossil fuel resources. The most potential solution to solve this water-energy problem is by using desalination technology powered by renewable energy like solar energy. This paper reports the design and development of a portable and stand-alone solar-driven desalination system. Due to portability, this system is expected to be transported to anywhere like remote areas. The system consists of three major parts, which are solar-thermal collector, solar photovoltaic (PV) panel, and Vacuum Multi-Effect Membrane Distillation (V-MEMD) unit as the main part of the system. In this paper, a small-scale operation test of the system was carried out and analyzed. The system run successfully for about 7 hours, which started at 09:00 AM. The feedwater used was brackish-water with conductivity of about 2500 μS/cm and the flowrate was maintained at 69 L/h. The system produced distillate/freshwater at average rate of 5.98 L/h. Whereas, the total distillate produced was approximately 34.8 L with conductivity of about Total Dissolved Solid (TDS) of 2.67 mg/L. The distillate flux of the current system was in the range of 3.2 – 5.2 L/m2.h.

Thermodynamic optimization of a vacuum multi-effect membrane distillation system for liquid desiccant regeneration

The regenerator is one of the key components in the liquid desiccant air-conditioning system, and improvement of the regenerator performance is key to promoting the overall system energy efficiency. This study entails the investigation of a vacuum multi-effect membrane distillation system (V-MEMD) for liquid desiccant regeneration. The V-MEMD regenerator possesses the merits of high energy efficiency and zero desiccant carry-over. A thermodynamic model has been developed to study the regeneration process based on the principles of heat and mass transfer and heat and mass balances. The model is validated with experimental data, and the discrepancies are observed to be within 10% and 5% for regeneration rate and brine concentration, respectively. Employing the developed and validated model, several V-MEMD configurations have been investigated and the effects of key operating parameters are evaluated. The regenerator performance is observed to degrade at higher feed concentrations, while a higher hot water temperature promotes regeneration rate and thermal efficiency. The proposed configurations expand the operation range of the V-MEMD regenerator to greater than 40 wt% under a heat source temperature of 70 °C. Compared with existing liquid desiccant regeneration systems, the specific energy consumption is reduced by 10–50%.

Assessment of a pilot system for seawater desalination based on vacuum multi-effect membrane distillation with enhanced heat recovery

This work presents the evaluation of an innovative system based on vacuum multi-effect membrane distillation modules (V-MEMD) for seawater desalination at pilot scale. This four-effect unit introduces a remarkable modification from previous V-MEMD systems, consisting of the use of the seawater feed flow as cooling in the condenser, rather than a separate circuit. Preheating the feed in the condenser improved heat efficiency (maximum gained output ratio obtained for seawater was 3.2). Maximum distillate fluxes reached 8.5 l h−1 m−2 for hot feed temperature 75 °C and feed flow rate 150 l h−1. Increasing both parameters to raise the productivity was hindered by the inability of the condenser to cope with all the steam generated in previous effects, resulting in overheating and overpressure. Furthermore, a loss of 40% of distillate production was measured due to the increase of seawater cooling temperature by 8 °C along the year. Finally, it was observed that scaling reduced distillate production up to 50%. Acid cleaning successfully removed scaling and restored the performance. Subsequently, the use of an antiscalant as a pre-treatment was sufficient to prevent it.

Innovative Design of Solar-Powered Desalination (SPD) System using Vacuum-Multi Effect Membrane Distillation (V-MEMD) Process

This research focused on the development of an innovative design of solar-powered desalination (SPD) system which was expected to solve the water and energy problem simultaneously. We have developed a portable and hybrid solar-powered desalination (SPD) system for producing potable water from saline water. It is a self-contained and integrated system which combines solar-thermal collector and solar-photovoltaic for its operation, and thus the system can operate to produce water by only using solar energy. Therefore, the system is highly suitable to be implemented in remote arid and coastal areas without infrastructures or connection to the grid (water and power), but blessed with abundant solar irradiation, like in Saudi Arabia. A Memsys Vacuum Multi-Effect Membrane Distillation (V-MEMD) unit was used as the core of the SPD system. A heat pump was also integrated into the SPD system for energy recovery and to improve the performance of the system. The system could be considered as sustainable and “green” desalination technology, which will be very useful for the Kingdom of Saudi Arabia. To study the performance of the system, small-scale tests have been carried out at the Engineering College - King Saud University, Saudi Arabia. Based on the experimental results, the system has run successfully by only utilizing solar energy.