Direct Contact Membrane Distillation Performance Research Articles

Effect of non-woven net spacer on a direct contact membrane distillation performance: Experimental and theoretical studies

This study provides a comprehensive and systematic overview of the fundamental characteristics of heat and mass transfer in the direct contact membrane distillation (DCMD) process that employs different types of spacers on one or both surfaces of the membrane. Detailed theoretical investigations were carried out to demonstrate the effects of spacers adjacent to the membrane surface on heat and mass transfer enhancement in the DCMD with a PTFE/PP composite membrane, complemented with experimental data for model validation. Thus, this work aimed to propose and demonstrate the heat transfer correlation for spacer-filled channels to reliably predict the heat and mass transfer improvement by non-woven net spacers in the DCMD process. The results showed that the permeate flux enhancement by the spacers ranged between 7% and 19% only for the spacer-filled permeate channels and between 21% and 33% only for the spacer-filled feed channels even at higher flow rates, thus indicating lower flux enhancements in the spacer-filled permeate channels. This was because the influence of spacers on flux improvement became more evident at higher temperatures owing to higher temperature polarization. In this study, the maximum flux enhancement of approximately 43% over the empty channels was achieved using the thinnest and densest spacer with a hydrodynamic angle of 90°, adjacent to both membrane surfaces.

Self-roughened omniphobic coatings on nanofibrous membrane for membrane distillation

Novel omniphobic electrospun nanofibrous membranes (ENMs) for direct contact membrane distillation (DCMD) were fabricated by electrospinning poly(vinylidene fluoride) (PVDF) nanofibers and followed by directly functionalizing the fiber surface with long-chained fluorododecyltrichlorosilane (FTCS) via solution immersion without any assistance of roughening treatments. The morphologies of polymerized fluorododecyltrichlorosilane (PFTCS) coated PVDF nanofibers evolved from the newly budding willow twigs with tiny bumps, to vigorous buds and then to intertwined fillets with the increase of FTCS concentration, which could serve as a robust barrier to low-surface-tension liquid penetration. By constructing hierarchically re-entrant structures of low-surface-energy PFTCS coating layer onto the PVDF nanofibers, the resultant porous ENMs were endowed with omniphobic property, low water-adhesion property, remarkable mechanical property, and superior durability in harsh circumstances as well as good breathability. The comprehensive investigations on structural attributes (such as the liquid entry pressure, pore size, porosity and gas permeability), long-term MD stability and DCMD performance in presence of surfactants were carried out. Significantly, the resultant omniphobic PFTCS/PVDF membrane exhibited a competitive permeate vapor flux of 36.9 kg/(m2 h) in highly salty solution (3.5 wt% NaCl salt feed; ΔT = 40 °C) over 24 h operation and also demonstrated robust DCMD performance in presence of surfactants. Compared with the commercial PVDF MD membranes, this simple and scalable strategy had great potential for MD applications.

PVDF membrane assisted by modified hydrophobic ZnO nanoparticle for membrane distillation

AbstractIn this study, the modified ZnO/poly vinylidene fluoride (PVDF) membranes were prepared to examine the membrane distillation performance. The modified ZnO nanoparticle was synthesized using 3‐aminopropyltriethoxysilane as a coupling agent and lauric acid in order to increase the hydrophobicity of nanoparticle surface. The modified nanoparticles with 0.25%, 0.5%, 1%, and 2% contents were intercalated within PVDF scaffold to prepare composite membranes through phase inversion method. The influence of modified nanoparticle content on the characteristics of membranes including hydrophobicity, morphology, porosity, liquid entry pressure of water, and direct contact membrane distillation performance was investigated. By adding modified ZnO nanoparticle into casting solution, porosity and roughness were increased and revealed by scanning electron microscopy and atomic force microscopy, respectively. The results of direct contact membrane distillation performance was indicated that the permeate flux of modified membranes had a significant increase in comparison with neat PVDF membrane. Also, the thermal property of these nanocomposite membranes was examined.

Treatment of Typical Organic Pollutants in Textile Wastewater by Direct Contact Membrane Distillation

Direct contact membrane distillation technology was considered as a promising and efficient technology for the treatment of textile wastewater. In this study, hydrophobic polytetrafluoroethylene and polyvinylidene fluoride membranes for the treatment of selected model compounds of textile wastewater (e.g., phenol, aniline, and sulfanilic acid) were explored comparatively in a bench-scale direct contact membrane distillation technology test unit. The effect of various operational parameters including temperature, flow rate, and concentration on the rejection performance was investigated systematically. The results indicated that an increased feed temperature and a faster cross flow velocity contributed positively to the direct contact membrane distillation performance. Limited rejection for phenol and aniline was witnessed, which can be due to their relatively lower boiling point. A > 99% of sulfanilic acid rejection was obtained under the same conditions. Furthermore, the polytetrafluoroethylene membrane always presented enhanced performance compared with the polyvinylidene fluoride samples. In brief, the direct contact membrane distillation process could be potentially used as a promising technique for the treatment of textile wastewater.

Characterization of triple electrospun layers of PVDF for direct contact membrane distillation process

Recently, electrospinning technique was applied successfully to fabricate porous hydrophobic membranes for MD applications. In this work, a novel triple layer configuration with diameter gradient for PVDF nanofiber membranes is proposed, with the objective of to minimize mass transfer resistance and heat loss. In outer layers of these membranes, the minimum concentration of PVDF (20 wt%) was used to produce bead-free nanofibers with thinner diameters and middle layers were composed of thicker nanofibers formed at higher polymer concentrations (21.5-26 wt%). Characterization of prepared membranes was conducted by the measurement of porosity, thickness, liquid entry pressure (LEP), scanning electron microscopy (SEM), contact angle, thermal and mechanical properties. Direct contact membrane distillation performance of fabricated membranes was tested using 42 g/L NaCl as feed solution. Water permeate flux of triple layer membranes (27.8-31.5 kg/m2 h) was found to be considerably higher than that obtained from single layer membrane (15.4 kg/m2 h), indicating the proposed configuration can effectively improve evaporation efficiency.

Preparation of PVDF/PTFE hollow fiber membranes for direct contact membrane distillation via thermally induced phase separation method

Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were developed via thermally induced phase separation (TIPS) method for direct contact membrane distillation (DCMD). The effects of PTFE addition on the thermal behavior of the dope mixtures and membrane formation were investigated. It was found that the crystallization of PVDF was significantly enhanced with increased nucleation sites provided by PTFE particles, leading to promoted formation of smaller spherulites in a greater density. Furthermore, the improved uniformity and increased amount of cavity between the spherical crystallites coherently facilitated the formation of smaller pores ranging from 0.08 to 0.12μm. With certain PTFE loading, the membranes exhibited improved porosity, water permeability and hydrophobicity as well as enhanced tensile strength of 9.4±0.3MPa. To examine the DCMD performance, the membranes were tested under various conditions using 3.5wt% NaCl solution. A stable permeation flux of 28.3kgm−2h−1 at the feed temperature of 60°C with 99.99% NaCl rejection for over 50h of operation was achieved, which is comparable with similar type of PVDF membranes while the newly developed membrane exhibited better mechanical strength. This study suggests that the as-spun PVDF/PTFE hollow fiber membranes have potential for DCMD applications.

Multivariable statistical analysis for enhancing performance indicators in direct contact membrane distillation

Orthogonal experimental design, correlation analysis and response surface charts were used to identify the parameters influencing the operational efficiency of direct contact membrane distillation (DCMD). The orthogonal array design method was used to optimize the number of experimental trials required for dependence analysis. The operating factors studied were hot feed properties (temperature, salinity, flowrate) and cold distillate characteristics (temperature, and flowrate). The impact of those factors on three DCMD performance indicators - cold distillate production rate, performance ratio and recovery ratio – was investigated. The most significant factors influencing each performance indicator were obtained from the quantitative values of main and interaction effects, and confirmed by using the Pearson product-moment correlation coefficients. The main effects of feed and distillate temperatures on the performance indicators were the greatest, indicating that the most significant factors were the feed and distillate temperatures. The maximum distillate production rate was obtained at feed and distillate temperatures of 90 and 15°C, respectively. The optimum recovery and performance ratios were obtained at a feed flowrate of 1.6 L/min but when feed temperature was kept at 70°C.

Low Energy Membrane Distillation: A Numerical Study on The Role of Conductive Spacers

Thermal conductivity is one of the most influencing DCMD performance parameters. Having larger value in the membrane results in lower thermal efficiency and mass flux. However, increasing the conductivity of the fluid, membrane’s surfaces, or by means of higher conductive gap have reported unique performance improvements in DCMD. In this work, computational fluid dynamics (CFD) has been used to evaluate the role of highly conductive spacers on the performance of a direct contact membrane distillation (DCMD) system. The cylindrical shape spacer is considered in this work which placed directly on the membrane surfaces. The numerical model assumes a 2D steady state, conjugated heat, and turbulent Navier-Stokes flow. Mass flux is estimated by integrating a combined Knudsen and Poiseuille flow model. The effect of adding conductive and non-conductive spacers on temperature polarization coefficient, mass flux and thermal efficiency as well as the total transmembrane heat has been investigated at different incoming flow velocities. Results showed that copper spacers showed better performance than polymer spacers, but a significant enhancement was not observed from the no spacer case. The cylindrical shape spacers although created turbulence zone, it brought with it localized circulating zone that lead to deteriorating each of the mass flux and thermal efficiency when operating the flow at medium to high Reynolds number.

Influence of high range of mass transfer coefficient and convection heat transfer on direct contact membrane distillation performance

In order to improve water production of membrane distillation (MD), the development of high performance membrane having better mass transfer and enhancement of convection heat transfer in MD module have been continuously investigated. This paper presents the relationship between the heat and mass transfer resistance across the membrane and the performance improvement. Various ranges of mass transfer coefficient (MTC) from normal (0.3×10−6 to 2.1×10−6kg/m2sPa: currently available membranes) to high (>2.1×10−6kg/m2sPa: membranes under development) were simulated using an experimentally validated model at different ranges of convection heat transfer by varying the inlet flow rates and spacer enhancement factor. The effect of mass transfer and convection heat transfer on the MD performance parameters including temperature polarization coefficient (TPC), mean permeate flux, and specific energy consumption were investigated in a direct contact MD (DCMD) configuration. Results showed that improving the MTC at the low ranges is more important than that at the high ranges where the heat transfer resistance becomes dominant and hence the convection heat transfer coefficient must be increased. Therefore, an effort on designing MD modules using feed and permeate spacers and controlling the membrane surface roughness to increase the convection heat transfer and TPC in the channel aiming to enhance the flux is required because the currently developed mass transfer has almost reached the critical point.

Treatment of reverse osmosis brine by direct contact membrane distillation: Chemical pretreatment approach

Different chemical pretreatment strategies followed by direct contact membrane distillation (DCMD) process were adopted for the processing of reverse osmosis (RO) brine. CPT with Na2CO3+NaOH allowed scale reduction, removing both permanent calcium hardness and temporary calcium hardness whereas the chemical pretreatment with BaCl2 permitted to remove sulfate ions from RO brine. This last chemical pretreatment was found to be the most efficient pretreatment improving the DCMD performance because of the corresponding highest permeate flux, lowest permeate flux decline and best permeate quality. However, it is relatively an expensive pretreatment not recommended for consumption because of the toxic residual barium. The brine was concentrated up to 37wt% of salts in water, which is above the limiting salt (NaCl) saturation concentration. Therefore, the volume of discharged RO brine can be reduced considerably facilitating its efficient management.

Study of advancement to higher temperature membrane distillation

It is well known that the mass flux (J) and the membrane thermal efficiency (η) of membrane distillation increase with the feed flow temperature. A comprehensive laminar and turbulent flow model for simulating and evaluating the performance of direct contact membrane distillation (DCMD) when operated at inlet feed temperature (Tf,i) from 80°C to 180°C, higher than the customary maximum of ~80°C, was developed and used to explain and assess the performance of such high temperature DCMD, as well as the potential associated problems of the needed higher operating pressure, and provide knowledge useful for their future design and optimization. Some of the key results are that raising Tf,i from 80°C to 180°C, increases J 9.4-fold, and η 2.1-fold, and decreases the specific energy consumption (SEC) 2.9-fold. Raising the flow Reynolds number from 1200 to 7000 increases J 2.6-fold and η by 15%, but SEC increases 2.3-fold. The needed system pressurization does not affect the process performance significantly. The higher operating temperatures also provide more practical opportunities for heat recovery, which could significantly further raise overall system efficiency.

Anti-wetting behavior of negatively charged superhydrophobic PVDF membranes in direct contact membrane distillation of emulsified wastewaters

Membrane wetting is a key issue in causing deleterious performance in membrane distillation. In this work, superhydrophobic membranes were challenged in a direct contact membrane distillation (DCMD) process using cationic/anionic surfactant emulsified oil-in-water emulsions in order to assess the antiwetting properties of superhydrophobic surfaces in a highly demanding environment. Contact angle, Zeta potential and DCMD performance of the membrane were determined. It was found that superhydrophobic PVDF membranes exhibited strongly negative charges on the surface, as well as stable MD performance in concentrating the anionic surfactant emulsified waste water, but suffered from severe wetting when cationic surfactant was used. In contrast, the virgin hydrophobic PVDF membrane experienced severe wetting for either surfactant stabilized emulsions. Extended DLVO theory was utilized to explain the membrane-foulant interaction based on the EDL, acid-base and hydrophobic-hydrophobic interaction. Cleaning of superhydrophobic membranes was performed and restore of membrane performance was achieved for membranes that were used for treatment of negatively charged emulsions. This research outlined the experimental evidence and the importance of negative charges of a superhydrophobic surface for a stable membrane distillation performance, which may be helpful for future development of distillation membranes.

Performance and fouling mechanism of direct contact membrane distillation (DCMD) treating fermentation wastewater with high organic concentrations

In this study, direct contact membrane distillation (DCMD) was used for treating fermentation wastewater with high organic concentrations. DCMD performance characteristics including permeate flux, permeate water quality as well as membrane fouling were investigated systematically. Experimental results showed that, after 12hr DCMD, the feed wastewater was concentrated by about a factor of 3.7 on a volumetric basis, with the permeate flux decreasing from the initial 8.7L/m2/hr to the final 4.3L/m2/hr due to membrane fouling; the protein concentration in the feed wastewater was increased by about 3.5 times and achieved a value of 6178mg/L, which is suitable for reutilization. Although COD and TOC in permeate water increased continuously due to the transfer of volatile components from wastewater, organic rejection of over 95% was achieved in wastewater. GC–MS results suggested that the fermentation wastewater contained 128 kinds of organics, in which 14 organics dominated. After 12hr DCMD, not only volatile organics including trimethyl pyrazine, 2-acetyl pyrrole, phenethyl alcohol and phenylacetic acid, but also non-volatile dibutyl phthalate was detected in permeate water due to membrane wetting. FT-IR and SEM–EDS results indicated that the deposits formed on the membrane inner surface mainly consisted of Ca, Mg, and amine, carboxylic acid and aromatic groups. The fouled membrane could be recovered, as most of the deposits could be removed using a HCl/NaOH chemical cleaning method.

Heat extraction and brine management from salinity gradient solar pond and membrane distillation

The problem associated with the reverse osmosis (RO) system is its brine disposal. Experimental salinity gradient solar pond (SGSP) with surface area 4.65m2 was established to extract and apply heat on laboratory scale direct contact membrane distillation (DCMD). In this study, the performance of the SGSP and DCMD system under Pakistan’s climatic condition of Islamabad was evaluated. The heat extraction was carried out using internal heat exchanger by passing fresh water through it at different flow rates in summer and winter. Maximum temperature of 37°C in summer and 28.5°C in winter was extracted. The extracted heat from SGSP can be used to pre-heat the brine for temperature driven desalination processes. Least drop in lower convective zone (LCZ) temperature of SGSP was observed at flow rate of 7.5L/min. In DCMD, two temperatures obtained from SGSP (28.5 and 37°C) at feed side were maintained to investigate the permeate flux, percentage salt rejection and total dissolve solids (TDS). Two further temperatures 50 and 60°C were maintained to investigate the DCMD performance. Flux increased as temperature difference between feed and permeate increased. 28.5°C in terms of SGSP temperature was also feasible for DCMD process as flux was maintained over time.

Effect of non-solvent additives on the morphology, pore structure, and direct contact membrane distillation performance of PVDF-CTFE hydrophobic membranes

Four common types of additives for polymer membrane preparation including organic macromolecule and micromolecule additives, inorganic salts and acids, and the strong non-solvent H2O were used to prepare poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) hydrophobic flat-sheet membranes. Membrane properties including morphology, porosity, hydrophobicity, pore size and pore distribution were investigated, and the permeability was evaluated via direct contact membrane distillation (DCMD) of 3.5g/L NaCl solution in a DCMD configuration. Both inorganic and organic micromolecule additives were found to slightly influence membrane hydrophobicity. Polyethylene glycol (PEG), organic acids, LiCl, MgCl2, and LiCl/H2O mixtures were proved to be effective additives to PVDF-CTFE membranes due to their pore-controlling effects and the capacity to improve the properties and performance of the resultant membranes. The occurrence of a pre-gelation process showed that when organic and inorganic micromolecules were added to PVDF-CTFE solution, the resultant membranes presented a high interconnectivity structure. The membrane prepared with dibutyl phthalate (DBP) showed a nonporous surface and symmetrical cross-section. When H2O and LiCl/H2O mixtures were also used as additives, they were beneficial for solid–liquid demixing, especially when LiCl/H2O mixed additives were used. The membrane prepared with 5% LiCl+2% H2O achieved a flux of 24.53kg/(m2·hr) with 99.98% salt rejection. This study is expected to offer a reference not only for PVDF-CTFE membrane preparation but also for other polymer membranes.

Titanium white sulfuric acid concentration by direct contact membrane distillation

In this study, synthetic titanium white waste acid was concentrated by direct contact membrane distillation (DCMD) using laboratory prepared PVDF hollow fibers. Tentative efforts were made to decouple the effects of different factors on wetting and DCMD performance. FeSO4 crystals were found homogeneously distributed on the outer surface of the hollow fibers while treating H2SO4/FeSO4 solution. In homogeneously distributed crystals inside the membrane and on the lumen surface were identified. Elemental analysis indicated that the contamination of membrane interior structure occurred with three different aqueous solutions (H2SO4, H2SO4/FeSO4 and NaCl). This indicated that liquid intrusion was a factor non-negligible in causing membrane wetting. Rejection coefficients nevertheless were dependent on the solute type. For H2SO4 and H2SO4/FeSO4, the rejections were maintained at high level throughout the DCMD process before cleaning. In contrast, concentration of NaCl solution witnessed obvious deterioration in rejection when approaching saturation. This difference is referring to the influence of the amount and property of the solute crystallizable in feed solution. Removal of the FeSO4 crystals by HCl acid cleaning restored the flux partially for two types of membranes, but without apparent deterioration in the rejection only for membranes with lower transmembrane flux.

Effect of operating parameters on boron removal from seawater using membrane distillation process

Theoretical and experimental studies of direct contact membrane distillation (DCMD) using commercially flat sheet hydrophobic polyvinylidene fluoride (PVDF) membrane were investigated to remove boron from simulated and natural seawater. Mass transfer mechanism and theoretical flux were determined using the dusty-gas model (DGM). Mass transfer analysis showed that the combined Knudsen-molecular model is considered to be the dominant mechanism which describes the transport of water vapor through the membrane pores. Then, the effect of various operating parameters such as feed temperature, boron concentration, salt concentration and feed solution pH on DCMD performance was investigated. Experimental runs showed that the permeate flux was enhanced exponentially with feed temperature, and maximum permeate flux of 27.5kg/m2∙h was obtained at 74°C. The permeate flux decreases slightly with feed boron and salt concentration. Feed solution pH had no significant effect on DCMD performance. DCMD process can produce water with high boron rejection of more than 90% even at feed concentration as high as 200mg/L. Natural seawater, containing 5.37mg/L of boron, was treated by DCMD process. During 18h of working period, the permeate flux had showed a reduction of about 15.75%, and permeate boron was kept below 0.47mg/L. The decreasing of contact angle value can explain the performance decline of DCMD process.

Effect of Feed Temperature on the DCMD Performances in Treating Synthetic Textile Wastewater

This article discussed the effect of feed temperature on the performance of direct contact membrane distillation (DCMD) in treating synthetic textile wastewater containing dissolved dye and salt. The DCMD experiments were conducted using an in-house made composite hollow fiber membrane made of organic polyvinylidene fluoride (PVDF) and inorganic Cloisite 15A. Prior to dyeing solution treatment process, the influence of feed temperature on membrane permeability was first investigated using deionized water. Then, the membrane was tested using synthetic solution containing 50 mg L-1 acid red 1 and 60 g L-1 sodium chloride. The inlet temperature of the feed solution was varied in the range of 50 to 90°C while the temperature of permeate solution was kept constantly at 20°C. The results showed that the vapor fluxes of membrane increased with increasing the feed temperature and maximum flux (Jv = 20.88 kg m-2 h-1) was obtained at 90°C.This is mainly due to the higher water vapor pressure sensitivity at the feed side of a higher operating temperature. With respect to separation performance, it is reported that DCMD process could easily achieve excellent rejection for both dye compound and dissolved salts, recording more than 99% rejection.

Synoptic analysis of direct contact membrane distillation performance in Qatar: A case study

The aim of this study is to evaluate a bench scale direct contact membrane distillation (DCMD) performance using flat sheet polytetrafluoroethylene (PTFE) membrane at different inlet flow rates, temperatures, and salinity composition. The effect of the various operating conditions on the quality of the distillate water produced was investigated by Inductively Coupled Plasma/Optical Emission Spectrometry (ICP/OES) and ion chromatography (IC). Five different feed solutions were studied such as the Arabian Gulf seawater, rejected brine from Qatari thermal desalination plants and artificial brine. A permeate flux of 35.6 LMH can be produced at temperature difference of 50°C between hot and cold sides. The thermal energy efficiency of the system was analyzed and reached up to 22%. The flow mode and turbulence promoter in feed channels were found to have a very important effect on the flux and energy efficiency. It was also noted that using spacers in the flow channels increase the distillate flux by more than 51% as compared with a spacer-free system. The investigation has shown that the highest Gain Output Ratio (GOR) value can be obtained when using less saline feed solution at the highest feed temperature, lowest permeate temperature and lowest flow rate. Finally, the salt rejection rate throughout the conducted tests was very high (>99.9%) and almost independent of any studied operational parameters. Eventually, DCMD has proven to be a feasible and effective technology capable of consistently producing high quality distillate from a very high salinity feed even with substantial quality difference compared to other desalination methods such as RO and MSF.

Multi-walled carbon nanotube/PVDF blended membranes with sponge- and finger-like pores for direct contact membrane distillation

Polyvinylidene fluoride (PVDF) flat sheet membranes were blended with multi-walled carbon nanotubes (MWCNTs) by the phase inversion method for direct contact membrane distillation (DCMD) of salty water (35gL−1 NaCl). The membrane properties and performances depended markedly on the synthesis parameters such as MWCNT loading, polyvinylpyrrolidone (PVP) addition and MWCNT surface chemistry. MWCNT/PVDF membranes prepared with functionalized MWCNTs have a smaller pore size (determined by bubble point measurements) and lower contact angles, thus, functionalization of MWCNTs is not recommended for this application. For membranes presenting the same pore size, the pore morphology and the membrane thickness are crucial for efficient salt rejection. Sponge-like pores and the smallest possible thickness (i.e., membranes prepared without PVP) allow complete salt rejection (i.e., 100%), in contrast with larger thickness and elongated finger-like pores (resulting from PVP addition) whose salt exclusion ranged from 88.8 to 98.6%. Overall, the MWCNT/PVDF blended membrane prepared with 0.2wt.% optimal content of pristine MWCNTs (without adding PVP) exhibited the best performance in DCMD, presenting total salt rejection and a higher permeate flux (9.5×10−3kgm−2s−1) than that obtained with a commercial PVDF membrane (7.8×10−3kgm−2s−1).