Vacuum Membrane Distillation System Research Articles

Au-TiO2 nanoparticles enabled catalytic treatment of oil and gas produced water in slurry and vacuum membrane distillation systems

Produced water (PW) treatment and reuse provide a promising alternative to address freshwater scarcity and reduce the environmental impacts associated with PW disposal. Efficient treatment of PW is challenging due to its complex matrix and hypersalinity. This study developed innovative gold-doped titania (Au-TiO2) nanoparticles to enable effective treatment of PW collected from the Permian Basin with salinity of ∼118 g/L. The mechanisms of removing organics and ammonia in PW in slurry and vacuum membrane distillation (VMD) systems were investigated via thermocatalysis, photocatalysis, and photo-thermocatalysis. Photocatalysis and photo-thermocatalysis were found more effective for organic removal than thermocatalysis. A baseline study using glucose as a representative of easily degradable organic carbon showed that thermocatalysis and photocatalysis mineralized 23.2 % and 93.1 % glucose, respectively. However, photo-thermocatalysis and thermocatalysis degraded only 17.4 % and 7.5 % dissolved organic carbon (DOC), and 56.2 % and 49.2 % NH3-N in PW, respectively. Au-TiO2 coating on ceramic membranes improved the distillate flux of photo-thermocatalytic membrane distillation (PMD) compared to no catalysts coating in VMD (7.14 versus 6.75 kg per hour per m2). Both PMD and VMD rejected >99.9 % of salts and >96 % DOC, and PMD had slightly higher DOC and salt removal rates. In contrast, normalized permeate flux using pristine membranes (hydrophobically modified without catalysts coating) decreased to zero after 29 h of operation while normalized flux with TiO2-coated membranes under UV on and UV off were 57.5 % and 53.7 % on average, which is much lower than that with Au-TiO2 coated membranes in both UV on (91.3 %) and UV off (88.1 %) cases, indicating Au-TiO2 coating significantly improved membrane anti-fouling propensity and durability during treatment of complex PW. Targeted analysis of three groups of petroleum hydrocarbons and 70 volatile organic compounds showed that only 2-butanone and acetone were detected in the distillates with removal efficiencies of 80 % for 2-butanone in both PMD and VMD, and acetone was removed by 75.4 % in PMD and 69.2 % in VMD. Au-TiO2 nanoparticles demonstrated excellent catalytic performance for PW treatment in slurry and immobilized PMD systems.

Enhancing productivity and energy efficiency in vacuum membrane distillation systems

This study comprehensively analyzes a multistage vacuum membrane distillation (VMD) system, exploring diverse energy recovery methods and explaining the relationship between operational parameters and system performance. Through an intensive exploration of key parameters the study evaluates critical metrics, including productivity and gained output ratio (GOR). The analysis revealed that at lower feed temperatures, particularly 60 °C or below, layout 2 showcasing optimal GOR. However, a noteworthy shift occurs at temperatures exceeding 60 °C, where layout 3 surpasses the other layouts. At lower pressure levels in the permeate side (60 mbar or below), layout 3 demonstrates higher energy recovery. A reversal occurs at higher pressures, with layout 2 exhibiting the most favorable GOR. For a smaller number of stages, specifically five or less, layout 2 exhibits the highest energy recovery. However, as the number of stages increases beyond five, layout 3 consistently outperforms the others. Finally, layout 3 is found to be optimal for smaller channel depths (20 mm or less), showcasing higher GOR. Conversely, as the channel depth increases beyond 20 mm, layout 2 emerges as the more efficient choice. This study offers valuable insights for researchers aiming to optimize multistage VMD systems, emphasizing the significance of energy recovery layout in achieving enhanced energy efficiency.

Experimental Investigation on the Energy and Exergy Efficiency of the Vacuum Membrane Distillation System with Its Various Configurations.

This paper addresses a retrofitting vacuum membrane distillation (VMD) setup to reduce the accumulated pressure inside the permeated side. This modification is necessary to extend the operation of the VMD to extreme operation conditions of higher hot water temperatures. This modification, denoted as a hybrid configuration, proposes the injection of a cold water stream into the VMD cell without mixing it with the permeate. Energy and exergy efficiency analyses were performed to assess the effectiveness of the hybrid configuration. The performance of the modified system indicated an improvement in terms of permeate flux (J), the gain output ratio (GOR), and the utilitarian exergetic efficiency (ηex,u), which reach up to two and three times that of the base configuration of the VMD system. However, the exergetic efficiency (ηex) of the hybrid system showed marginal improvement compared to the base case over the tested range of hot water temperatures. This is because the enhanced vapor production is penalized by excess energy consumption. Moreover, the highest exergy destruction percentages occurred in the operational components (e.g., heater and chillers) which fall in the range of 19.0-68.9%. The exergy destruction percentage in the original components (e.g., the VMD cell and condenser) did not exceed 8.3%. Furthermore, this study indicated that the hybrid configuration requires additional tuning and optimization to perform efficiently over wide operating conditions.

Performance analysis and optimization of a solar assisted heat pump-driven vacuum membrane distillation system for liquid desiccant regeneration

Liquid desiccant air-conditioning (LDAC) system is one of the excellent alternatives to conventional vapor compression refrigeration system due to its great energy-saving potential. However, regeneration of diluted liquid desiccant is the main energy-consuming process, and its performance plays a significant role in LDAC system. Recently vacuum membrane distillation (VMD) is becoming an emerging and promising separation process rely on the advantage of high permeate flux and low energy losses. The interest of usage solar energy techniques for feed solution heating in VMD systems is regarded as a sustainable path for membrane distillation process. In this work, a solar-driven VMD system incorporating a vapor-compression heat pump for liquid desiccant regeneration is introduced. An adapted solar/heat pump hybrid heating strategy has been devised to establish a connection between the heat-demanding process and the solar thermal supply. A comprehensive multicriteria assessment of energy, exergy, and environmental evaluation is conducted, and the influences of crucial parameters on both key components and system performance are evaluated to ensure a systematic examination. Results show that both the COP and STEC decrease obviously with the growth of the inlet solution temperature, whereas SEEC is improved abviously from 410.6 kW·h/m3 to 566.1 kW·h/m3 due to the effect of heat pump. The exergy analysis indicates that exergy losses in solar collector and stratified heat storage tank accounted for over 80 % of the total exergy losses. Additionally, single-objective optimization and multi-objective optimization are employed to explore the optimum operating conditions of key componets and whole system. Results show the ideal optimal solution can be obtained with a COP of 19.92 and a SEEC of 142.89 kW·h/m3 when the weights of COP and SEEC are taken as [0.5, 0.5].

Performance investigation of poly(vinylidene fluoride-cohexafluoropropylene) membranes containing SiO2 nanoparticles in a newly designed single vacuum membrane distillation system.

The current study focuses on the development of a superhydrophobic poly(vinylidene fluoride-cohexafluoropropylene) nanocomposite membrane suitable for vacuum membrane distillation by incorporating SiO2 nanoparticles. At loading hydrophobic nano-SiO2 particle concentration (0.50-1.50 wt.%), the developed nanocomposite membranes are optimized in terms of vacuum membrane distillation performance. The influence of temperature, vacuum pressure, and feed water flow is studied for desalinating high-salinity brine. The results show that the developed vacuum distillation membrane is capable of 95% salt rejection during the treatment of a highly saline feed (65,000 ppm) at fixed flow rates of 120L/h saline feed and different operating conditions consisting of feed inlet temperatures ranging from 40°C to 70°C and distillate inlet temperatures of 7-15°C. The vacuum membrane distillation process achieves 0.38-1.66% water recovery with increasing concentration factor, meaning that recovery is increased, and shows a specific electrical energy consumption of 5.16-23.90 kWh/m3 for product water. Overall, the newly designed membrane demonstrates suitability for a vacuum membrane distillation system. PRACTITIONER POINTS: Desalinate high-salinity brine (TDS > 35,000 ppm) using a vacuum membrane distillation system. A hydrophobic PVDF-HFP/SiO2 nanocomposite membrane development for vacuum membrane distillation. A newly designed single vacuum membrane distillation system for RO brine treatment.

Thermal analysis of condensation-induced vacuum and its feasibility for operating a vacuum membrane distillation system

Vacuum pumps are primarily used in vacuum membrane distillation (VMD) for vacuum generation. However, vacuum pumps are characterized by low operational durability and a high energy demand. In this study, a steam condensation process was combined with the VMD process such that the vacuum created during the steam condensation process drove the VMD operation. A thermodynamic cycle comprised of three main processes: steam purging, vacuum creation, and water recovery was described. This paper presents a detailed thermal analysis of each process of the thermodynamic cycle based on heat transfer and thermodynamic models. The modeling results are compared with experimental data. The pressure in the vacuum chamber increased during the water recovery process because of the accumulation of non-condensable gases, which permeated the chamber along with the water vapors. Consequently, the mass flux of the VMD permeate decreased, and finally, the permeation ceased when the pressure in the chamber reached ~20 kPa; after this stage, the cycle restarted. For the current experimental conditions and vacuum chamber design, the durations of the purging, vacuum creation, and recovery processes were 30, 500, and 270 s. At least three linked vacuum chambers must be synchronized and operated simultaneously to realize continuous water recovery.

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.

Prediction of sulfuric acid solution in the vacuum membrane distillation process using Artificial Neural Network

This paper presented an effective approach utilizing Artificial Neural Network (ANN) to predict the vacuum membrane distillation (VMD) process of sulfuric acid solution. Firstly, mathematical models have been developed and the influence mechanisms of critical parameters on the transfer resistance in the VMD process were explored via a simulation process. Furthermore, an ANN model from the experimental data under actual working conditions, was developed and applied to simulate the effects of critical parameters on the VMD process performance including membrane flux and gained output ratio (GOR). The simulation results revealed that decreasing feed concentration or vacuum side pressure and increasing feed temperature or feed velocity would contribute to the reduction of total transfer resistance. Then, the operation data at multiple working conditions using actual sulfuric acid solution was divided into three sets (training, validation and test) to develop the ANN model. The trained ANN model was subsequently observed to have good agreement between the predicted and experimental data. From the subsequent simulations with various variable controls, increasing feed concentration or vacuum side pressure led to the declines of membrane flux and GOR, while increasing feed temperature or feed flow rate were helpful to improve membrane flux and GOR. Obviously, the developed ANN model performed well to predict the VMD process performance with minimum error, which could be effectively utilized to predict and optimize the VMD system for industrial wastewater treatment without performing a complicated, expensive and dangerous experimental test.

Slug flow-enhanced vacuum membrane distillation for seawater desalination: Flux improvement and anti-fouling effect

Vacuum membrane distillation (VMD) is characterized by high permeate flux and low conductive heat loss among the membrane distillation technologies. However, the inescapable polarization effect severely deteriorates the distillation performance and service life. Thus, this study aimed at establishing a novel slug flow-enhanced VMD system to effectively address the polarization problem and investigating the influence of flow behaviors on MD process. First, a novel slug flow-enhanced VMD system was developed. The transmembrane permeate flux was intensified by 28.1 %. The performance enhancement was mainly ascribed to the increased turbulence and intensified shear stress. What’s more, further anti-fouling test demonstrated that the effect of slug flow on membrane fouling was divided into two stages. In Stage I, the permeate flux just dropped from 18.1 to 17.7 kg/(m2·h) and only 2.2 % reduction was observed in slug flow-enhanced VMD system, while it dropped rapidly from 15.2 to 12.9 kg/(m2·h) and a significant reduction of 15.2 % was observed in conventional VMD system. This research would be useful to membrane distillation for seawater desalination.

Treatment of high-salinity brine containing dissolved organic matters by vacuum membrane distillation: A fouling mitigation approach via microbubble aeration

In this study, a laboratory-scale vacuum membrane distillation (VMD) system coupled with microbubble aeration (MBA) was developed for the treatment of high-salinity brine containing organic matters. Herein, at the beginning, feedwater only containing model organics such as humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) was utilized to investigate the organic-fouling behavior, results indicated that the permeate flux was not affected by a thin and loose contaminated layer deposited on the membrane surface. Furthermore, dissolved organics in the feed brine inhibited the occurrence of membrane wetting due to the existence of a compact and protective crystals/organic-fouling layer, which can prevent the intrusion of scaling ions into membrane substrates. Besides, organics in the feedwater have a high tendency to adsorb on the membrane surface based on molecular dynamics simulations, thus, forming an organic-fouling layer prior to inorganic scaling. Finally, the effect of MBA on fouling alleviation was evaluated in VMD system, nearly 50% of salt precipitation from fouled membrane was effectively removed with the introduction of MBA, which can be ascribed to a combination of mechanisms, including surface shear forces and electrostatic attractions induced by microbubbles, meanwhile, about 2.2% of the total energy was only consumed, when using MBA. Together, these results demonstrated that MBA was a promising approach to alleviate membrane fouling in VMD.

Simultaneous ammonia recovery and water reclamation from brackish aquaculture effluent via reactive vacuum membrane distillation

Due to the high volatility of ammonia and contemporaneous condensation of water and ammonia on permeate side, it remains a challenge to obtain a high concentration of ammonia or high-quality water using conventional vacuum membrane distillation (VMD). In this study, a reactive VMD (RVMD) with a two-stage downstream system was developed for simultaneous recovery of ammonia and water from brackish aquaculture effluent with dilute ammonia content. Notably, the RVMD system allowed for complete recovery of dilute ammonia in acid solution. Compared to a conventional VMD system, both the separation factor (SF) and concentration factor (CF) were intensified by 16 times within 2 h using the RVMD system. In the RVMD system, ammonia was first dissolved in acid solution without chilling using immersed trap tube; followed by water vapor condensation under chilling to obtain clean water. It was found that more diluted acid (0.0025 M of sulphuric acid or 0.005 M of phosphoric acid) could be applied to absorb the same amount of ammonia by using this system. Through this system, the recovered ammonia from brackish aquaculture effluent could be concentrated by 23 times (from 19.83 to 465 mg/L) with an SF of 148, while very low ammonia content (0.04 mg/L) was detected in the recovered water. These findings demonstrate that RVMD with a two-stage downstream system outperforms conventional VMD in concentrating ammonia and purifying water.

Performance Assessment of Coupled Concentrated Photovoltaic-Thermal and Vacuum Membrane Distillation (CPVT-VMD) System for Water Desalination

Numerical simulations were carried out to assess the technical and economic feasibility of a solar water desalination system that has a novel hybrid Concentrating Photovoltaic Thermal (CPVT) collector coupled with a Vacuum Membrane Distillation (VMD) process. A special characteristic of this CPVT is its triangular receiver with PV cells facing the reflecting surface. This type of receiver has the advantage of generating more electricity with less PV surface area and great potential to be used to hybridize conventional parabolic thermal collectors. TRNSYS was employed to analyze the annual performance of the CPVT-VMD system evaluating parameters such as solar fraction, specific permeate production and specific energy production for different coastal cities. In the dynamic simulations, local annual weather data and specific information about the characteristics and operating conditions of a real CPVT collector and a VMD module were considered. From the parametric analysis the optimal surface area of collectors and the input temperature of the VDM module were determined. A maximum specific permeate of 218.410 m 3/m2VMD for Acapulco, MX, and a minimum of 170.365 m 3/m2VMD for Singapore, SG, were achieved for the proposed CPVT-VMD system of four solar collectors with an operating set temperature of 55 °C. An economic profit was found after 7 years for Acapulco city, which showed great potential to use solar energy from hybrid CPVT collectors for a VMD process to provide freshwater in coastal cities.

Performance Analysis of an Eductor-Based Membrane Distillation Unit

Thermal desalination technologies involve two primary processes: vapor generation from saline water, and effective recovery of the resulting condensate. Membrane distillation (MD) systems are among the emerging thermal desalination technologies which use a hydrophobic membrane to recover condensate through either direct or indirect contact (with the cooling fluid) condensation. The specific process technology (for thermal energy transfer and condensate recovery) depends on the type of MD. Direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) are two significant MD processes, with DCMD having the advantage of direct condensation and simple design, while VMD systems have high yield through sub-atmospheric vapor generation. This work focuses on developing an eductor-based MD process incorporating the strengths of both DCMD and VMD. It is an experimental study with a water jet eductor replacing the vacuum pump and condenser in a typical VMD system for active permeate vapor transfer and condensation. Unlike the exiting VMD systems, the proposed design recovers condensate by direct contact condensation. The sub-cooled water acts as a motive flow which entrains the secondary vapor into the stream, causing mass transfer via condensation at the interface. The modified VMD was found to have achieved better flux compared to the conventional VMD system. The performance of the eductor, sensitivity to parameters, and the practicality of the technology have been analyzed.

Entransy performance analysis of the sulfuric acid wastewater treatment process using a vacuum membrane distillation system

Industrial sulfuric acid wastewater treatment using vacuum membrane distillation (VMD) technology has attracted worldwide attention. However, high energy loss and low membrane flux seriously hinder the industrialization of VMD technology. This paper novelly introduced an entransy theory to improve the VMD system performance. Mathematical models were built and verified via an experimental setup. System performance was investigated through the experiment and simulation from entransy dissipation standpoint. Experimental results indicated that an excellent separation performance with a membrane flux of 1.8 kg·m−2·h−1 and condensate conductivity within 20 μS·cm−1 were obtained at feed concentration of 10 %, feed temperature of 353.0 K, feed velocity of 1.0 m·s−1and vacuum side pressure of 24 kPa. Moreover, a thermal efficiency of 57.8 % was obtained with an entransy dissipation resistance of 0.00012 K·W−1. Simulation results showed that decreasing feed concentration and vacuum side pressure or increasing feed temperature and feed velocity decrease entransy dissipation resistance and improve heat transfer capability. More importantly, the prediction model of entransy dissipation resistance was established by a multiple linear regression method, and influence degrees of operation parameters on entransy dissipation resistance were obtained by a single factor analysis method. These findings are beneficial to optimize the VMD system in the future.

Enhance the anti-scaling performance of conductive heating vacuum membrane distillation by carbon nanotube-based electroactive membrane

Membrane distillation (MD) is an emerging technology for the brine desalination. Directly heating feed liquid at the membrane-water interface for improving the performance of MD has been demonstrated, while the problem of membrane scaling remained. Herein, carbon nanotube-based electroactive membranes were prepared and characterized by a variety of methods. It was then used in a conductive heating vacuum membrane distillation system. We firstly increased the flux by optimizing system configuration, placing a spacer in the feed channel, and setting vent holes on the thermal conducting layer. Then, aiming at silicates and sulfates scaling in MD, the electroactive membranes and titanium were used as working electrodes and the counter electrode, respectively, to discuss the electrochemical anti-scaling performance and mechanism. Results showed that the silicate scale on the membrane surface can be effectively cleaned by using the electroactive membrane as the cathode and applying a potential of − 5 V to destroy the structure of the silicate through an electrochemical reaction. With the electroactive membrane as the anode, the scaling of calcium sulfate can be mitigated by providing a continuous potential through electrostatic repulsion. This paper may provide further insights into inorganic scaling control in conductive heating vacuum membrane distillation process.

Preparation of reduced graphene oxide-carbon nanotubes membranes for conductive heating membrane distillation treatment of humic acid

Membrane distillation (MD) has attracted extensive attention as a technology to solve water scarcity in the fields of desalination. Surface-heated membrane distillation techniques that have emerged in recent years have improved flux due to the elimination of temperature polarization, but still suffer from membrane fouling. The use of electrochemical techniques for in-situ cleaning of membrane fouling in MD is expected to be an effective solution. Herein, electroactive membranes were prepared by loading mixtures of reduced graphene oxide (rGO) and carbon nanotube (CNT) with four mass ratios on polytetrafluoroethylene (PTFE) hydrophobic membranes. A series of analytical methods, such as scanning electron microscopy, atomic force microscopy, pore size analysis, water contact angle, Raman spectroscopy, membrane surface zeta potential, and cyclic voltammetry curve, were employed to characterize the properties of the rGO-CNT membranes. The best separation performance and conductivity were achieved at a 2:1 mass ratio of rGO to CNTs (R2 membrane). Then, the rGO-CNT membranes were applied to a conductive heating vacuum membrane distillation system to treat humic acid (HA). Compared with the PTFE membrane, the four rGO-CNT membranes showed a significant improvement in the rejection of HA. Using the rGO-CNT membranes as the cathode and the titanium as the anode, after a certain potential was applied, the flux decreases slowly. The fluxes of all rGO-CNT membranes were maintained over 9.5 kg/(m2·h) for 10 h of continuous operation. An increase in potential helps to increase change by alleviating membrane fouling. The highest flux and HA rejection reached 13 kg/(m2·h) and 99.8 %, respectively, with R2 membrane and −5 V potential applied. The mechanism of antifouling included electrostatic repulsion and electrocatalytic oxidation.

Optimization of hollow fiber membrane module for vacuum membrane distillation (VMD) via experimental study

Membrane distillation (MD) is an attractive desalination technology for treating high salt concentration water/wastewater. Nonetheless, developing membrane modules with high flux and fouling resistance but low energy consumption for industrial application remains a critical challenge. In this work, a novel vacuum membrane distillation (VMD) module with a central perforated tube was developed. The variation of vapor pressure on the shell side was monitored to analyze the relationship between the mass transfer driving force and the permeate flux. The energy consumption of each piece of equipment in the VMD device was evaluated. The heater, chiller, and vacuum pump consumed about 1/3 of the total energy for the VMD system. The central perforated tube suction decreased the mean vapor pressure better than the shell suction. Double suction enabled more uniform pressure distribution on the shell side. The permeate flux under double suction was 50–70 % higher than that under single suction. Consequently, the optimal length, packing fraction, and suction mode were proposed based on energy saving. Our results that include the first report on the generation of superheated vapor in VMD can guide engineers in designing the relevant modules and system scale-up and process optimization for industrial applications.

Study on the operation characteristic and transfer resistance of mechanical vapor recompression and vacuum membrane distillation system under multiple working conditions

In order to alleviate the freshwater resource shortage worldwide, the multiple vacuum membrane distillation (VMD) modules were drawn into a mechanical vapor recompression (MVR) system to recover industrial wasterwater in this paper. An experimental platform was constructed and several experiments were conducted under multiple working conditions. The operation characteristic by varying membrane area was investigated to achieve stable freshwater production, and the influences of operation parameters on the energy efficiency were analyzed, the distribution characteristic of transfer resistance was also revealed. The experimental results showed that an excellent freshwater production performance with a separation efficiency of 99.9% was achieved, and increasing membrane area was effective to elevate the evaporation rate. Furthermore, lower feed concentration or higher feed temperature, feed flow rate and vacuum side pressure were helpful for reducing the specific heating energy consumption (SHEC) and improving the energy efficiency. The proposed system was superior to VMD in energy efficiency, and was superior to MVR in separation performance, but still exhibited a lower energy efficiency than that of MVR due to the transfer resistance. Accordingly, the involved transfer resistances including boundary layer resistance, fouling layer resistance and membrane resistance were obtained by the iterative calculation. Increasing the number of experiment from 1 to 10 increased the transfer resistance from 14.219 Pa·m2·h·kg−1 to 18.082 Pa·m2·h·kg−1, resulting in a rise of SHEC from 70.63 kWh·t−1 to 82.77 kWh·t−1. Finally, the methods to reduce transfer resistance and improve energy efficiency were suggested. The obtained results provides the basis for such system industrial application.

Scaling mitigation and salt reduction of vacuum membrane distillation using sacrificial zeolites

Seawater which is characterized by high concentrations of sodium chloride (NaCl) salts and hardness i.e. calcium (Ca) and magnesium (Mg) is vulnerable to membrane scaling. The performance of an integrated zeolite-submerged vacuum membrane distillation (SVMD) system was evaluated for seawater desalination. The results showed that the zeolites achieved 15–29 % and 10–22 % removal of sodium (Na) and hardness, respectively. The metal removal by the zeolites followed the order of Na > hardness and agreed well with pseudo second order kinetics model. The integration of SVMD system with powdered zeolites achieved stable water flux throughout the long-term operation. Salt and metal (particularly Ca and Mg) removal from seawater via zeolites induced crystallization, mitigated scaling on the hollow fibre membrane surface while producing high quality clean water. Compared to the hydrophobic membranes, the nuclei of Ca and Mg salts tend to nucleate and crystallize on the surrounding of zeolites due to the higher surface energy of zeolites.

Conductive heating vacuum membrane distillation for brine desalination: Study on operational conditions, temperature polarization and energy consumption

Membrane distillation (MD) has gained significant attention for treating hypersaline brine. However, conventional MD is still constrained by technical issues such as temperature polarization (TP) caused by the feed being continuous heated. Herein, through the use of a thermal conducting layer, this study introduced a conductive heating vacuum membrane distillation (CH-VMD) system capable of transmitting external heat directly to the membrane-water interface without preheating the feed. The influence of operational conditions on distillate flux and salt rejection was investigated. In both CH-VMD and conventional VMD systems, the temperature profile at the membrane-water interface were examined and compared. In addition, we analyzed the TP differences between the two systems under different operational conditions. In CH-VMD system, with a 35 g/L NaCl feed solution, thermal conducting layer temperature of 60 °C, a flow velocity of 3.1 cm/s, and 90 kPa vacuum level, the distillate flux reached 9.8 L/m2·h and the salt rejection achieved 99.9%. The temperature polarization coefficient was greater than unity, resulting in a thermal efficiency as high as 83.2%. Heat conduction increased the temperature of the membrane-water interface, which is critical for optimizing MD performance in terms of overcoming TP, enhancing distillate flux and salt rejection, and lowering energy consumption.