Membrane Distillation Configurations Research Articles

Non-targeted analysis and toxicity prediction for evaluation of photocatalytic membrane distillation removing organic contaminants from hypersaline oil and gas field-produced water

Membrane distillation (MD) has received ample recognition for treating complex wastewater, including hypersaline oil and gas (O&G) produced water (PW). Rigorous water quality assessment is critical in evaluating PW treatment because PW consists of numerous contaminants beyond the targets listed in general discharge and reuse standards. This study evaluated a novel photocatalytic membrane distillation (PMD) process, with and without a UV light source, against a standard vacuum membrane distillation (VMD) process for treating PW, utilizing targeted analyses and a non-targeted chemical identification workflow coupled with toxicity predictions. PMD with UV light resulted in better removals of dissolved organic carbon, ammoniacal nitrogen, and conductivity. Targeted organic analyses identified only trace amounts of acetone and 2-butanone in distillates. According to non-targeted analysis, the number of suspects reduced from 65 in feed to 25–30 across all distillate samples. Certain physicochemical properties of compounds influenced contaminant rejection in different MD configurations. According to preliminary toxicity predictions, VMD, PMD with and without UV distillate samples, respectively contained 21, 22, and 23 suspects associated with critical toxicity concerns. Overall, non-targeted analysis together with toxicity prediction provides a competent supportive tool to assess treatment efficiency and potential impacts on public health and the environment during PW reuse.

Numerical evaluation of sweeping gas membrane distillation for desalination of water towards water sustainability and environmental protection

Sweeping gas membrane distillation (SGMD) is considered a membrane distillation configuration. It uses an air stream to collect the water vapour. A 2D mathematical model is prepared in the current study to predict the effect of various operating parameters on the SGMD performance. Also, the temperature distribution in the SGMD was obtained. The effect of air inlet temperature, salt concentration, feed and air flowrate on air and salted solution outlet temperature and vapour flux through the membrane is investigated. There was good agreement between experimental data and modelling outputs. It was found that increase in air inlet temperature from 40 to 72 °C was increased the outlet temperature of air stream and cold solution from 37 to 63 °C and 38 to 65 °C respectively. Furthermore, increase in air inlet temperature led to the enhancement of vapour flux in the membrane distillation. Also, the salt concentration and feed flow rate did not have meaningful influence on the outlet temperatures, however, the flux was increased by increasing feed flowrate.

Numerical simulation of a membrane desorber with the H2O-LiBr working mixture for absorption cooling systems

In absorption cooling systems, the refrigerant fluid is separated from the working-fluid mixture inside a component called “desorber”. Conventional desorbers operate by boiling separation; however, membrane modules are a potential alternative to replace them since they can operate below the working mixture boiling point and at atmospheric pressure conditions. The present paper presents a numerical simulation of a membrane desorber that uses the air gap membrane distillation configuration. The numerical analysis was carried out by means of ANSYS FLUENT CFD code using a 3D model. Temperature, concentration, and velocity contours were numerically obtained assuming H2O-LiBr solution at 41 % w/w, mass flow of 0.03 kg/s, temperature of 363.15 K, and cooling water at 303.15 K and 0.04 kg/s. The maximum error in simulated temperature was 11.9 % compared to the experimental data. According to the results, a solution temperature difference between the bulk and the membrane interphase up to 288.15 K was calculated. In addition, stagnated areas inside the solution channel cause velocity differences of up to 5 magnitude orders compared to the middle point in the solution channel. Since a maldistribution flow causes a “jet” inside the solution channel, and a non-homogeneous concentration distribution on the membrane interphase; as a result, a concentration difference up to 1.35 % between a point located at the border membrane interphase compared to a point located at the middle was observed. Therefore, the desorption rate can be improved with geometric modifications in the membrane desorber device.

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.

Concentrate circularity: A comparative techno-economic analysis of membrane distillation and conventional inland concentrate management technologies

Inland desalination with reverse osmosis (RO) offers a promising alternative to increase potable water resources. However, large volumes of concentrated brine are generated as a byproduct of the process. Concentrate disposal for inland regions displaces valuable water resources and can make up to 33 % of the total cost of desalination. Conventional disposal systems are limited by location, hydrogeology, climate, and policies, and few place value on resource recovery, thus limiting the implementation of desalination facilities in water-stressed regions. Membrane distillation (MD) is an alternative technology that can minimize disposal volume while maximizing water recovery for beneficial reuse. MD uses thermal energy gradients to desalinate the concentrate stream achieving near zero-liquid discharge. Furthermore, several MD configurations include heat recovery, making MD an energy-efficient alternative. A techno-economic assessment (TEA) of air-gap MD (AGMD) for RO concentrate management was performed and compared to three conventional concentrate management systems: evaporation ponds, deep-well injection (DWI), and concentration-crystallization. TEA results indicate that compared to DWI at 1.09 $/m3, evaporation ponds at 1.47 $/m3, and concentrators at 6.2 $/m3, respectively, AGMD is a competitive concentrate management technology producing water at 0.90 $/m3 when operating conditions are optimized and low-grade heat is available. When operating at high salinity (>70 g/L) the selection of operating conditions, specifically module length and circulating flowrate, is critical. Results of this study support the economic viability of AGMD in contrast to current industry standards and highlight the importance of resource recovery to promote a circular water-energy economy for regions relying on reuse and desalination.

Optimization of the design and operating parameters of a large-scale vacuum membrane distillation system to improve process performance

This study investigates the performance and optimal operating of a proposed large-scale vacuum membrane distillation (VMD) system with an external condenser for enhanced productivity and energy consumption. The VMD configuration reduces the resistance of vapor mass transfer across the membrane to increase its permeation rate. A comprehensive parametric analysis to evaluate the effects of various operational and design parameters of the MD module and the condenser unit has been performed. The optimized results were compared to the conventional direct-contact membrane distillation (DCMD) configuration. Results revealed that the feed temperature and flowrate have the most significant impacts on system productivity and energy consumption. Increasing the vacuum improves productivity, while increasing the membrane thickness decreases system productivity. Increasing the membrane pore diameter slightly enhances system productivity. Using smaller coil diameters for condenser and cooling the system using water at room temperature can minimize the energy consumption of the system. The proposed model outperforms the traditional DCMD unit of the same module design. The system can produce 388 L/h of freshwater, with a minimum SEC of 645 kWh/m3 at the optimum conditions. These findings demonstrate the potential of the vacuum membrane distillation process as a promising technology for water desalination treatment applications.

Experimental investigation of temperature polarisation by capturing the temperature profile development over DCMD membranes

Temperature polarisation (TP) is a major drawback limiting the global acceptance of membrane distillation (MD) technology. TP is typically quantified using a dimensionless index known as Temperature Polarisation Coefficient (TPC). TPC has significant limitations, whereby it cannot be used to compare different MD configurations or design conditions, nor to analyse the TP phenomenon along the membrane. In this research, the temperature profile over and along a lengthy DCMD membrane has been measured under various operational conditions, where its impact on TP has been explored for the first time. A specialised DCMD membrane cell was manufactured to capture temperature profiles, both along and over the membrane surfaces, using miniature thermocouples. The effects of flow rate and feed temperature were investigated on the temperature profiles. The results showed that the extent of TP was not constant along the membrane, and that the temperature profile was not symmetrical across the feed and permeate side, predominantly due to the effects of the inlet and outlet on the flow. The TPC value calculated using the conventional method was not able to accurately reflect the TP phenomenon along the membrane, indicating TPC to be an ineffective tool to study TP along the membrane.

Recent progress in membrane distillation configurations powered by renewable energy sources and waste heat

Membrane distillation (MD) is a promising process for high-quality water reclamation mainly due to its high capacity to retain non-volatile components and operate without the need for hydraulic pressure. However, its low energy efficiency limits its widespread use. With that in mind, this paper critically summarizes the different engineered configurations of membrane distillation coupled with renewable energy sources and waste heat, namely: solar, geothermal, and waste heat, to overcome the limitations associated with low energy efficiency commonly reported. From all sources, solar-driven processes are preferred due to the greater technological maturity related to flat plate collectors (FPCs), evacuated tube collectors (ETCs), compound parabolic concentrators (CPCs), salt-gradient solar ponds (SGSPs), and solar stills. The integration with renewable energy sources represents one of the leading solutions for energy consumption, proving to be a decisive choice for the system's economic viability. The summarized studies suggest membrane distillation's potential to be economically competitive with the classical membrane separation process (ultrafiltration, nanofiltration, and reverse osmosis) when waste heat is considered for wastewater treatment. Even under these conditions, alternative energy sources have a few shortcomings to be investigated in future studies, such as short periods of solar radiation and the intermittence of waste heat sources. These factors still represent obstacles to an uninterrupted and large-scale operation of membrane distillation and must be overcome in a near future. Even so, successful case studies on full-scale systems that integrate membrane distillation and solar energy sources suggest the process's potential for widespread use in the near term.

Finned Tubular Air Gap Membrane Distillation.

Finned tubular air gap membrane distillation is a new membrane distillation method, and its functional performance, characterization parameters, finned tube structures, and other studies have clear academic and practical application value. Therefore, the tubular air gap membrane distillation experiment modules composed of PTFE membrane and finned tubes were constructed in this work, and three representative air gap structures, including tapered finned tube, flat finned tube, and expanded finned tube, were designed. Membrane distillation experiments were carried out in the form of water cooling and air cooling, and the influences of air gap structures, temperature, concentration, and flow rate on the transmembrane flux were analyzed. The good water-treatment ability of the finned tubular air gap membrane distillation model and the applicability of air cooling for the finned tubular air gap membrane distillation structure were verified. The membrane distillation test results show that with the tapered finned tubular air gap structure, the finned tubular air gap membrane distillation has the best performance. The maximum transmembrane flux of the finned tubular air gap membrane distillation could reach 16.3 kg/m2/h. Strengthening the convection heat transfer between air and fin tube could increase the transmembrane flux and improve the efficiency coefficient. The efficiency coefficient (σ) could reach 0.19 under the condition of air cooling. Compared with the conventional air gap membrane distillation configuration, air cooling configuration for air gap membrane distillation is an effective way to simplify the system design and offers a potential way for the practical applications of membrane distillation on an industrial scale.

Comparative Energetics of Various Membrane Distillation Configurations and Guidelines for Design and Operation

This paper presents a comparative performance study of single-stage desalination processes with major configurations of membrane distillation (MD) modules. MD modules covered in this study are (a) direct contact MD (DCMD), (b) vacuum MD (VMD), (c) sweeping gas MD (SGMD), and (d) air gap MD (AGMD). MD-based desalination processes are simulated with rigorous theoretical MD models supported by molecular thermodynamic property models for the accurate calculation of performance metrics. The performance metrics considered in MD systems are permeate flux and energy efficiency, i.e., gained output ratio (GOR). A general criterion is established to determine the critical length of these four MDs (at fixed width) for the feasible operation of desalination in a wide range of feed salinities. The length of DCMD and VMD is restricted by the feed salinity and permeate flux, respectively, while relatively large AGMD and SGMD are allowed. The sensitivity of GOR flux with respect to permeate conditions is investigated for different MD configurations. AGMD outperforms other configurations in terms of energy efficiency, while VMD reveals the highest permeate production. With larger MD modules, utilization of thermal energy supplied by the hot feed for evaporation is in the order of VMD > AGMD > SGMD > DCMD. Simulation results highlight that energy efficiency of the overall desalination process relies on the efficient recovery of spent for evaporation, suggesting potential improvement in energy efficiency for VMD-based desalination.

Experimental validation of membrane distillation unit coupled with a sweeping gas membrane using solar energy

Abstract This work presents an experimental validation of MD unit coupled with a sweeping gas membrane (SGMD) using solar energy experimental validation of a sweeping gas membrane distillation (SGMD) unit coupled with solar energy. This station is installed at the University of Kairouan in Tunisia. SGMD is the least used membrane distillation (MD) configuration, and it is very expensive to build. SGMD is used to completely remove solutions containing non-volatile compounds such as salts and recover only high-purity water. A theoretical and experimental study was carried out with the parameters that intervene in the system. The theory of the system is described by a mathematical model based on the heat and mass transfer of the MD unit. Experimental studies were carried out for two types of processes: with preheating and without preheating of the charge (brine). With a low gas flow, the evaporation capacity varies between 0.15 and 0.25 l/m2 h. The operating parameter (liquid velocity) is affected by the layer on both sides of the membrane. Utilizing the water heating by the solar collector increases the evaporation efficiency in the SGMD, and increasing the flow rate the permeate flow in the SGMD. SGMD unit experimental data were submitted to evaluate the reliability obtained from the MD unit, aspects of heat transfer, mass transfer, in solation patterns and all temperatures considering the efficiency of the MD unit.

Recent progress, bottlenecks, improvement strategies and the way forward of membrane distillation technology for arsenic removal from water: A review

Arsenic (As) is a common aquatic pollutant that enters the environment from anthropogenic sources and causes serious public health issues. Various conventional techniques have been applied for As removal from drinking water. However, those conventional techniques could not meet the requirement of the As discharge standard of <10 mg/L in drinking water set by the World Health Organization (WHO). To deal with this bottleneck, membrane distillation (MD) offers a complete As removal from an aqueous solution due to its higher rejection rate with less energy consumption than thermal techniques that require high thermal energy. This article reviews the recent progress of MD configurations, membrane modules, and membrane materials employed for As removal. This work also uncovers the technological solutions of MD in tackling a variety of bottlenecks in removing As from aqueous solutions. This includes integrating MD with alternative energy sources and/or forward osmosis (FO) to incorporate its hybrid systems with less energy consumption or a high separation rate. Limitations, challenges, and opportunities of MD systems for As removal were highlighted with a technological solution to these bottlenecks. The MD systems can effectively remove As (III) and As (V) with 100 % As rejection using hydrophobic ceramic and polymeric membranes. Overall, MD could play a role in helping the water industry meet the United Nations' Sustainable Development Goals (SDGs), such as clean water, affordable energy, and climate change mitigation.

Porous condensers can double the efficiency of membrane distillation

Among membrane distillation (MD) configurations, air gap MD (AGMD) has superior energy efficiency at high salinity. However, flooding of the gap prevents optimal thin gap depths as water removal is a challenge. To address these challenges, this work provides the first analysis of efficiency enhancement via porous wicking conductive condensers in MD. To do so, we examined performance with experiments and comprehensive thermo-fluid numerical models; we find that the porous wick condenser significantly improves thermal efficiency, gain output ratio (GOR), and flux. MD experiments show that the porous copper foam improves the thermal efficiency of air gap membrane distillation up to 40 % and water flux up to 144 %. Numerical modeling shows the porous copper foam improves the GOR of membrane distillation in high salinity by 98.7 %, outperforming air gap MD, permeate gap MD, and conductive gap MD. The optimized thickness and permeability of porous condensers significantly help eliminate the risk of flooding in AGMD.

Optimization-based technoeconomic comparison of multi-stage membrane distillation configurations for hypersaline produced water desalination

Unconventional oil and gas production raises concerns regarding sustainable management of high salinity wastewaters generated in this process. Membrane distillation (MD) is a thermal desalination process capable of treating hypersaline brines such as produced water. The low single pass recovery in MD systems operated in a single stage requires a large recycle stream to achieve the desired recovery, resulting in high energy consumption and operating cost. Multistage configurations in continuous recirculation operation mode offer the potential to reduce the energy intensity of MD systems. However, rigorous analysis is needed to assess the performance of MD configurations when operating in multi-stage mode. We present an optimization-based comparison of economic and energetic performance for five configurations of MD (including DCMD, AGMD, PGMD, CGMD, and VMD) operating in multi-stage continuous recirculation mode. Our findings demonstrate that multistage operation reduces the treatment cost and energy intensity of all MD configurations compared to single stage MD, with the greatest benefit for VMD and PGMD. AGMD with five stages outperforms other configurations with 3.58 $/m3feed treatment cost, followed by VMD, CGMD, DCMD, and PGMD with 3.8, 4.2, 5.4, and 9.06 $/m3feed treatment costs corresponding to twelve, eight, eight, and sixteen stages, respectively.

A Zero-Brine Discharge Seawater Desalination Using a Pilot-Scale Membrane Distillation System Integrated with Crystallizer.

The management of brines generated from reverse osmosis operation remains a critical challenge requiring new approaches and processes to limit the impact of brine discharge onto ecosystems and to enhance both water and valuable resource recovery. The treatment of real seawater reverse osmosis (SWRO) brines (45,000 ppm TDS) obtained from a local Singaporean desalination plant with a crystallizer integrated pilot-scale membrane distillation unit (MDC) was studied. Commercial STOMATE® hollow fiber membranes were used in vacuum membrane distillation (VMD) configuration, leading to an average flux of around 3.7 L/m2-h at a permeate vacuum of 80 kPa and an average feed temperature of 65 °C. Consistent separation operations were achieved for the treatment of real SWRO brine over a period of 280 h; this led to a water recovery of >95% and to the collection of salt slurries, containing up to ~10–20 wt% of moisture, from the crystallizer. This approach demonstrates the potential of MDC systems to achieve zero brine discharge efficiently from seawater desalination systems, providing an environmentally friendly alternative to manage brines by increasing water recovery and generating salt slurries of economic value.

Performance and economic analysis of a solar membrane distillation pilot plant under various operating conditions

Although membrane distillation (MD) is a promising desalination process, its use is limited because it requires a large amount of thermal energy. To reduce the thermal energy consumption during MD, studies combining various renewable energy sources (e.g., solar, geothermal, and waste heat) are currently being conducted. Therefore, pilot plant experiments combining solar energy with an MD system were conducted. In particular, a module that could be changed into different MD configurations, including direct contact MD (DCMD) and air gap MD (AGMD), was reviewed. The performance of the pilot plant was analyzed for each configuration under various operating conditions and according to onsite weather conditions. Because DCMD can obtain a 43% higher water flux than can AGMD, a long-term DCMD test was conducted to achieve a high water flux. The specific energy consumption (SEC) and gained output ratio (GOR) were compared in the presence and absence of solar energy. A 30% decrease in the SEC and 17% increase in the GOR were observed for the sunny days compared with when no solar energy was used. These results indicate that combining the MD with solar energy can improve its performance during long-term operation. In addition, the cost per unit volume of product water was estimated based on the designed solar MD pilot plant.

Performance of an air gap membrane distillation system and enhancement using a low-cost surface modification

ABSTRACT Water can be purified with a hydrophobic membrane in an air gap membrane distillation (AGMD) configuration. The present study presents experimental data for such a system and comparison with model predictions from simulations in a Matlab environment. The importance of the respective roles of feed water temperature and flow rate, cold water temperature and flow rate, and the extent of air gap on water production rate is studied, both from experiments and simulations. The comparison is found to be quite good, thus validating the choice of the mathematical model. In addition, possible improvements in the device performance are examined by using a surface modification of the hydrophobic membrane. In this respect, a layer of lead from a pencil is coated over the membrane. Evaporation rates through the pencil-coated membrane were measured in a solar simulator with 1 sun, 2 sun, and 3 sun settings. A distinct improvement in the evaporation rate was obtained when a pencil coated membrane was used. Following this observation, an augmented two-sided AGMD system is proposed, which carries a coated membrane exposed to solar radiation on one side and an uncoated membrane on the other.

Immobilized graphene oxide-based membranes for improved pore wetting resistance in membrane distillation

Membrane distillation (MD) is a useful method for the purification of difficult feedwaters but it cannot be applied in a range of industries due to pore wetting. In this work, graphene oxide (GO) laminate coatings are explored to overcome the pore wetting issues. Air gap MD (AGMD) configuration was considered, using a 35 g L−1 NaCl solution with 150 mg L−1 (150 ppm) of Triton X-100 surfactant as feed material. The GO is deposited as a laminate membrane on top of a commercial porous polyvinylidene fluoride (PVDF) support and good adhesion is achieved through the use of polydopamine (PDA) to form a hydrophilic tri-layer membrane. The small pore size achieved with the laminate GO led to increased pore wetting resistance for at least 90 h compared to 20 min with pristine commercial PVDF. Additionally, the extra layers of GO and PDA did not affect the membrane flux. Overall, a tri-layer immobilized GO membrane is synthesized with superior performance when compared to current commercial membranes, meaning that MD can be used for a new range of wastewater applications.

Boron removal by using vacuum assisted air gap membrane distillation (VAGMD)

Vacuum-assisted air gap membrane distillation was used to remove boron from different waters in this study. Four different vacuum pressures (0.02 bar, 0.04 bar, 0.06 bar, and 0.08 bar) were applied and air gap membrane distillation (AGMD) configuration was also used to compare the effect of vacuum pressures on the process. Six commercial hydrophobic membranes which have the 110° contact angle value at least were used. Membranes with a 44.18 cm 2 effective area were tested with saline water, synthetic boron solution, and real geothermal water as the feed solutions which is the only study with VAGMD in the literature. Salt removal efficiencies of all experiments were higher than 98.2%. Permeate boron concentrations of synthetic boron solution and real geothermal water were lower than the 0.5 mg/L limit value for drinking water set by WHO and in terms of removal efficiency it means higher than 99% removal efficiency. Vacuum pressures enhanced the water fluxes as expected and also decreased the specific energy consumption. • The use of a supplementary vacuum pump on the AGMD system improved the flux. • Increasing pressures increase flux and GOR without any considerable rejection loss. • Higher than 99% boron removal efficiencies were achieved. • All permeate boron concentrations were obtained below 0.5 mg/L. • Specific energy consumption values did not increase although using a vacuum pump.

Effect of pre-concentration on membrane solvent extraction process for the recovery of rare earth elements from dilute acidic leachate

Pre-concentration and separation processes of Rare Earth Elements (REEs) were investigated in terms of several factors. Nitric acid leachate of e-waste was first pre-treated by increasing pH and filtering through microfiltration. For pre-concentration, pre-treated leachate was concentrated by nanofiltration. While a 70% permeate recovery ratio was kept constant, the rare earth elements concentrations were triplicated under optimum conditions. A flat sheet supported liquid membrane process with a polyvinylidene fluoride (PVDF) support membrane was successfully used to extract REEs from the pre-treated leachate. Of the two extractants evaluated, bis-2-ethylhexyl phosphoric acid (D2EHPA) displayed a higher REE separation efficiency than did di-2,4,4,-trimethylpentyl phosphinic acid (Cyanex 272). However, Cyanex 272 separated Sc more selectively. For direct membrane solvent extraction (MSX) and MSX with pre-concentration, pre-treatment pH and D2EHPA concentrations were optimized at values of 1.5% and 15%, respectively. When comparing the results of MSX for direct and pre-concentrated configurations, it was seen that REEs and HREEs recoveries were increased 10% and 30% in MSX with pre-concentration at the end of single-stage MSX. Pre-concentration not only increased the MSX process efficiency but also enabled acid recovery from nanofiltration permeate. A more environmentally friendly and economical process scheme was proposed, including acid recovery from both NF filtrate and post-MSX leaching residue by two different membrane distillation configurations.