Membrane Distillation Performance Research Articles

Graphene-Coated PVDF Membranes: Effects of Multi-Scale Rough Structure on Membrane Distillation Performance.

Graphene-coated membranes for membrane distillation have been fabricated by using a wet-filtration approach. Graphene nanoplatelets have been deposited onto PVDF membrane surfaces. Morphology and physicochemical properties have been explored to evaluate the changes in the surface topography and related effects on the membrane performance in water desalination. The membranes have been tested in membrane distillation plants by using mixtures of sodium chloride and humic acid. The multi-scale rough structure of the surface has been envisaged to amplify the wetting and fouling resistance of the graphene-coated membranes so that a better flux and full salt rejection have been achieved in comparison with pristine PVDF. Total salt rejection and an increase of 77% in flux have been observed for coated membrane with optimized graphene content when worked with NaCl 0.6 M (DCMD, ΔT ≈ 24 °C) over a test period of 6 h. The experimental findings suggest these novel graphene-coated membranes as promising materials to develop functional membranes for high-performing water desalination.

ZnO Nanorod Induced Omniphobic Polypropylene Membrane for Improved Antiwetting Performance in Membrane Distillation

ZnO Nanorod Induced Omniphobic Polypropylene Membrane for Improved Antiwetting Performance in Membrane Distillation

Sonicated direct contact membrane distillation: Influence of sonication parameters

The effect of sonication on the performance of the direct contact membrane distillation (DCMD) is studied in this work. A sonicated DCMD module equipped with a commercial PTFE membrane is built and tested in an immersed temperature-controlled water bath. High fidelity transient and thermally conjugated CFD model is also developed for the DCMD unit accounting for the sonication. Flow metrics, including temperature polarization coefficient (TPC), heat- and mass- flux under different sonication frequencies and amplitudes were evaluated. Parametric study showed a clear improvement in the DCMD system metrics and more pronounced at higher temperature. It was observed that the application of sonication impedes the development of both kinetic and thermal boundary layers due to ultrasonic-induced mixing. The results showed that the application of sonication to DCMD at the intensity of 4525 W/m2 resulted in increase of mass flux from 4.25 L/m2.h to 8.1 L/m2.h corresponding to 90% increase. It is economically unviable due to electrical power demand, i.e., ~ 35 kW.h/m3 (~$5/m3), but can be feasible below 474 W/m2 demanding 3 kW.h/m3 (nearly $0.42/m3). It was also observed that an interesting equivalence exists between the effect of acoustic amplitude and acoustic frequency on the performance of DCMD. For example, sonication at 1 kHz with 0.5 μm amplitude produced similar results, in terms of mass flux and TPC, as sonication at 5 kHz with 0.1 μm amplitude.

Long-Term Treatment of Highly Saline Brine in a Direct Contact Membrane Distillation (DCMD) Pilot Unit Using Polyethylene Membranes.

Membrane distillation (MD) is an attractive separation process for wastewater treatment and desalination. There are continuing challenges in implementing MD technologies at a large industrial scale. This work attempts to investigate the desalination performance of a pilot-scale direct contact membrane distillation (DCMD) system using synthetic thermal brine mimicking industrial wastewater in the Gulf Cooperation Council (GCC). A commercial polyethylene membrane was used in all tests in the DCMD pilot unit. Long-term performance exhibited up to 95.6% salt rejection rates using highly saline feed (75,500 ppm) and 98% using moderate saline feed (25,200 ppm). The results include the characterization of the membrane surface evolution during the tests, the fouling determination, and the assessment of the energy consumption. The fouling effect of the polyethylene membrane was studied using Humic acid (HA) as the feed for the whole DCMD pilot unit. An optimum specific thermal energy consumption (STEC) reduction of 10% was achieved with a high flux recovery ratio of 95% after 100 h of DCMD pilot operation. At fixed operating conditions for feed inlet temperature of 70 °C, a distillate inlet temperature of 20 °C, with flowrates of 70 l/h for both streams, the correlations were as high as 0.919 between the pure water flux and water contact angle, and 0.963 between the pure water flux and salt rejection, respectively. The current pilot unit study provides better insight into existing thermal desalination plants with an emphasis on specific energy consumption (SEC). The results of this study may pave the way for the commercialization of such filtration technology at a larger scale in global communities.

Optimizing the performance of sweeping gas membrane distillation for treating naturally heated saline groundwater

The performance of a sweeping gas membrane distillation (SGMD) was evaluated for treating naturally heated saline groundwater. The naturally heated groundwater condition is projected to be a favourable match for SGMD as it eliminates the need for thermal requirement. Further, compared to the current scenario of using reverse osmosis (RO), SGMD will enable to eliminate the cooling tower requirement to cool down the groundwater prior to RO. Detail examination on SGMD operating settings revealed the importance of feed temperature and air flow settings in enhancing the SGMD permeate flux. Meanwhile, the feed flow rate and air temperature only minimally influenced the SGMD performance. SGMD cooling conditions played a key role towards enhancing the specific energy consumption and economic viability of its performance. The air cooling (fan and fin combination) was identified to be superior over the water cooling and thermo-electric element addition air cooling combinations. At optimum operating settings, SGMD was able to treat hot saline groundwater with an average permeate flux of 3.0 kg/m2h and attain high quality fresh water with over 98% salt rejection. Under optimum operating setting and due to the opportunity to eliminate thermal heating source, SGMD is able to attain matching economic rates (both capital and operational expenditure) to that of RO for treating natural heated groundwater. In terms of treatment performance, SGMD is established to be more advantageous to RO due to its ability to attain high quality fresh water (high salt and fluoride ion rejection rate).

Performance improvement of an air gap membrane distillation process with rotating fan

The current paper presents an experimental study on the performance of a novel air gap membrane distillation (AGMD) desalination system where a fan blade is installed within the air gap to improve the system’s productivity and energy utilization. The installed blade creates suction inside the air gap behind the membrane and increases the air turbulence, thereby reducing the vapor mass transfer resistance across the membrane and improving the productivity and energy efficiency of the system. Results revealed that the proposed design and process improve the vapor permeate flux by about 250% when compared to the conventional AGMD module. Furthermore, the average improvement in the gained output ratio of the system is about 67%, with a corresponding 77% reduction in the specific thermal energy consumption. Influences of the feed temperature and the motor speed are dominating the performance of the new system.

Development of machine learning and stepwise mechanistic models for performance prediction of direct contact membrane distillation module- A comparative study

The development of accurate and fast modeling tools to predict the performance of direct contact membrane distillation (DCMD) modules can result in their performance improvement. Conventional models ignoring the variations of operational parameters along the membrane's length may cause inaccuracy. Moreover, models considering these variations are often complex and computationally expensive. To propose an accurate and fast modeling tool, the possibility of using machine learning models for the performance prediction of the DCMD module has been studied for the first time in this study. The robustness of three machine learning models (ANN, SVR, and RF) has been thoroughly compared with the stepwise mechanistic modeling approach in terms of models’ accuracy, trend predictability, and computational time. The results show that ANN and SVR models exhibit an enhanced performance over the mechanistic model, possessing a MAPEtest of 3.46% and 4.78% as compared to the mechanistic model with a MAPEtest of 7.31%. Further, compared to the mechanistic model, the machine learning models have the privilege of simplicity, enhanced accuracy, and significantly lowered computational time. The feature importance analysis also showed that the feed flow temperature is the most influencing parameter on permeate flux in the DCMD system.

The influence of coating super-hydrophobic carbon nanomaterials on the performance of membrane distillation

Membrane distillation (MD), as a hybrid desalination technology with super-hydrophobic characteristics, has been emerging in the recent year. In this contest, the outstanding features of carbon-based nanomaterials have promising potential to contribute to the MD process evolution. This work presented an endeavor to impart the super-hydrophobic features of powder activated carbon (PAC) into poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes for DCMD applications. The FTIR indicates that the CNM was successfully coated onto the PVDF-HFP membrane which leading to higher contact angle values (from 83° to 124°, porosity (45% 86.9%) and water distillate flux at each increment in the PAC loading weigh. The novel coated membranes with 30 mg CNM led to an enhancement of the permeate flux (higher fluxes were obtained at higher CNMs loading) presented fluxes around 77 L/m2.h and exhibited a high salt rejection (> 99.9%) in most cases.

Utilizing Buckingham Pi theorem and multiple regression analysis in scaling up direct contact membrane distillation processes

Predicting the performance of a full-scale direct contact membrane distillation (DCMD) module based on experimental lab-scale results is rather difficult, since the DCMD performance is dependent on many different process parameters. Hence, there is a need for a methodology to perform DCMD system up-scaling based on lab-scale experimental results. In this study, we devise an approach to scale up the performance of DCMD systems by using the Buckingham's Pi theorem to group the DCMD process parameters into eight relevant dimensionless groups. Experimental data obtained from literature at various module dimensions were used to evaluate the developed dimensionless groups. An experimentally validated computational fluid dynamics (CFD) model was also developed and used to extend the coverage of operational parameters beyond the available experimental data. Then, two empirical dimensionless correlations were created, using multiple nonlinear regression analysis, and then validated, to enable the prediction of flux and pressure drop in DCMD systems at any scale.

Unlock the secret of air blowing in developing high strength and superhydrophobic membranes for membrane distillation

Superhydrophobic polyvinylidene fluoride (PVDF) membranes with high mechanical strength are highly desirable for multiple applications. Nevertheless, they can hardly be fabricated using conventional phase separation technologies without post-treatment. In this work, the goal has been achieved by a novel spray-assisted nonsolvent induced phase separation (SANIPS) method with the aid of air blowing. Without any membrane modification, a superhydrophobic membrane (water contact angle: 166.0°, water sliding angle: 20.0°) with excellent mechanical strength (maximum tensile strength: 3.09 MPa, maximum tensile strain: 104.80%, and Young's modulus: 71.29 MPa) can be produced. The resultant membrane exhibits a remarkable liquid entry pressure of 3.25 bar, and a stable direct contact membrane distillation (DCMD) performance over 400 h when being subjected to a hypersaline dye wastewater. The newly developed method has been compared with conventional nonsolvent induced phase separation (NIPS) and vapor induced phase separation (VIPS) methods to reveal its underlying mechanism. We believe this work would shed a new light on large scale production of PVDF membranes with both high strength and superhydrophobicity for clean water production via membrane distillation.

Thermo-economical and environmental analyses of a Direct Contact Membrane Distillation (DCMD) performance

As a promising desalination technology, membrane distillation is expected to be developed continuously due to its wide application and low sensitivity to salt concentration in the feed stream. The performance of a direct contact membrane distillation (DCMD) has been investigated based on energy, exergy, economic and environmental analyses in the current study. Performance evaluation of a DCMD unit is comprehensively carried out in terms of the operating parameters, including gained output ratio (GOR), specific energy consumption (SEC), concentration polarization coefficient, temperature polarization coefficient, recovery ratio, exergy efficiency, specific exergy consumption, carbon cost, and cost of water. This study presents one of the most comprehensive investigations on DCMD behavior performed in the available literature by developing a robust computational code in MATLAB software. In addition to pervasive energy and exergy analyses, a complete economic study for freshwater production plants with different capacities based on heat extracted through geothermal energy and wasted heat, and investigation of the environmental effects of using heat recovery MD cycle to reduce greenhouse gas emissions, are presented. The results showed that the utilization of heat recovery heat exchangers significantly improves the system's operating conditions. The exergy efficiency of the DCMD module with and without heat recovery is 38.23% and 56.71%, respectively, indicating a 48.34% improvement in exergy efficiency. Finally, the economic and environmental analyses showed that for a 15000 m3/day desalination unit, recovering the wasted heat could reduce the distillate water production cost down to 1.31 $/m3 while CO2 emissions and energy consumption also decreased by 59%.

Novel electrospun ZIF/PcH nanofibrous membranes for enhanced performance of membrane distillation for salty and dyeing wastewater treatment

Membrane distillation (MD) has many attractive advantages such as high rejection, low operating temperature, and has shown great potential for salty and dyeing wastewater treatment. However, the existing MD membranes often suffers from low permeability and membrane pore wetting issue, which significantly limit its practical applications. Herein, Zeolitic imidazolate framework 71 (ZIF-71) was doped in poly (vinylidene fluoride-co-hexafluoropropylene) (PcH) to prepare a hydrophobic electrospun nanofibrous membrane for MD process. The incorporation of ZIF-71 nanoparticles not only improve the surface roughness and hydrophobicity of the original PcH nanofiber membrane, but also increase the permeation flux (19.2 L m−2 h−1) and desalination performance (>99.99%) of the MD membrane. What's more, ZIF/PcH membrane was used to treat salty and dyeing wastewater, still showing high permeation flux, excellent pollutant retention rate and high stability. The desalination and decolorization rate of the treated wastewater can reach more than 99.5%, and the COD removal rate can reach 99.12%. The present study suggested that ZIF-doped electrospun nanofiber membrane provide high potential applications for direct contact membrane distillation (DCMD) desalination.

Colloidal silica fouling mechanism in direct-contact membrane distillation

Membrane fouling limits the performance of membrane distillation (MD) and its application to seawater brine treatment. Silica fouling is considered one of the most complex type of fouling. In this study, we evaluated the flux decline and fouling ratio due to colloidal silica fouling in direct-contact MD and characterized the fouled membranes. We also tested the efficacy of high flow-rate water flushing for the restoration of flux after fouling. The formation and removal of silica scaling were monitored in real-time with optical coherence tomography (OCT). Our work demonstrated that fouling formation is influenced by silica particle size, feed temperature, salinity, and flow rate. Notably, silica formed cake-layer fouling on the MD membranes. Smaller silica particle size resulted in a higher flux decline and a denser cake layer. A higher feed temperature resulted in a higher flux, but more severe fouling. We also found that fouling was minimized at an optimal flow rate and salinity did not significantly affect fouling formation. OCT monitoring showed that silica fouling deposited on the membrane surface and evaluated the effect of each cleaning strategies on the cake layer. This comprehensive investigation provides valuable insights for the development of silica fouling control strategies in MD.

Superhydrophobic nanoparticle-coated PVDF–HFP membranes with enhanced flux, anti-fouling and anti-wetting performance for direct contact membrane distillation-based desalination

Synthesis and superposition of silicon oxide nanoparticles on the surface of PVDF–HFP membrane via one-step method provide high superhydrophobic, oleophobic and antifouling properties enhancing their performance in membrane distillation.

A Detailed Numerical Analysis of the Performance of Air Gap Membrane Distillation Using Pvdf and Ptfe Membranes

A Detailed Numerical Analysis of the Performance of Air Gap Membrane Distillation Using Pvdf and Ptfe Membranes

Effects of forced input on the performance of direct contact membrane distillation

Effects of forced input on the performance of direct contact membrane distillation

Effect of Fabrication Technique on Membrane Distillation Performance of Photothermal Membrane: Electrospinning vs Electrospraying

Effect of Fabrication Technique on Membrane Distillation Performance of Photothermal Membrane: Electrospinning vs Electrospraying

Effect of interactions between ammonium and organic fouling simulated by sodium alginate on performance of direct contact membrane distillation

This study aims at observing the interactive effects between ammonium and sodium alginate (SA) on their rejection efficiency and organic fouling in direct contact membrane distillation (DCMD) process. Three feed solution pHs of 4, 6 and 8 as well as two feed temperatures of 35 and 50 °C were investigated without and with ammonium chloride in SA solution. The decrease in feed temperature and solution pH increased the fouling rate as the SA alone was filtered due to the protonation of the SA compound, leading to precipitate form. However, the fouling rate was reduced by increasing the feed temperature due to higher driving force through the membrane. This allowed the development of more porous cake structure than that formed at lower temperature. Rejection efficiency of total organic carbon was reduced by adding the ammonium chloride into SA solution probably due to the enhancement of membrane wetting. Nevertheless, the ammonium rejection was maintained 99 % since the fouling layer formed could act as secondary membrane. Predictions by using the classic blocking models showed that the fouling mechanisms associated with the SA were dominated by the cake formation without ammonium. Meanwhile, the internal fouling caused by the pore constriction was revealed more dominant as the ammonium was added into SA solution particularly at low solution pH.

Combination of computational fluid dynamics and design of experiments to optimize modules for direct contact membrane distillation

The design of membrane distillation modules is an important issue that must be addressed for it to be practically implemented. Herein, a combination of the design of experiments (DOE) and computational fluid dynamics (CFD) was developed to evaluate the interaction between variables of module geometry and how they affect the performance of a direct contact membrane distillation (DCMD) process. A three-dimensional CFD model was developed to calculate flux, the performance ratio (PR), and the permeate-to-feed ratio (ϕp) as functions of the height, length, and width of the channel in a plate-and-frame DCMD module. The CFD model was validated using experimental results in a laboratory DCMD system. Based on a DOE with a central composite design (CCD), CFD simulations were carried out, and response values were obtained for each combination. The results show that length is the most important factor affecting flux, and height is the most important factor affecting PR and ϕp. However, the effect of module width was not significant compared with that of the other variables. Based on the results, a set of regression models were developed to estimate flux, the PR, and ϕp, and these estimations match the CFD simulation results well. Using the derived models, the dimensions of the DCMD module were optimized to simultaneously increase water productivity and energy efficiency.

Direct contact membrane distillation as an approach for water treatment with phenolic compounds

Membrane distillation is a well-established technology for non-volatile components retention, but the removal of volatile and semi-volatile substances in trace concentration, such as phenols derivates commonly found in surface waters, requires further comprehension. In this context, the direct contact membrane distillation (DCMD) performance was assessed for the retention of fifteen phenolic compounds in surface water by different operating conditions of temperature (40, 50, and 60 °C), feed concentration (3, 5, 7, and 10 μg L−1), and permeate recovery rate (30, 50 and 70%). Kruskal Wallis confirmed a significant difference (p < 0.05) between the global removal of phenolic compounds at different temperatures. The increase in temperature led to a reduction in all compound's removal. As expected, a positive correlation (rSpearman>0.8) between the compounds' volatility and their losses was observed. Regarding the feed concentration and the recovery rate, there was no statistical difference between the removal values obtained for the phenolic compounds. This indicates the DCMD strength for that application. However, a trend for flux decay was noticed as the recovery rate (RR) increased, confirmed by temporal trend analysis and Mann-Kendall tests, although the flux decay was relatively low (J/J0 = 0.89). Aiming for a greater removal and to avoid a reduction in process performance, it is recommended to work with 40 °C as feed temperature and a RR prior to the flux decay (RR<30%). Nonetheless, the technology was efficient and did not compromise the permeate quality with >90% efficiency in pollutants removal, even for higher temperatures and RR.