Continuous Direct Contact Membrane Distillation Research Articles

Engineered eco-friendly composite membranes with superhydrophobic/hydrophilic dual-layer for DCMD system

Considerable advancements have been made in the development of hydrophobic membranes for membrane distillation (MD). Nonetheless, the environmentally responsible disposal of these membranes poses a critical concern due to their synthetic composition. Herein, an eco-friendly dual-layered biopolymer-based membrane was fabricated for water desalination. The membrane was electrospun from two bio-polymeric layers. The top hydrophobic layer comprises polycaprolactone (PCL) and the bottom hydrophilic layer from cellulose acetate (CA). Additionally, silica nanoparticles (SiO2 NPs) were electrosprayed onto the top layer of the dual-layered PCL/CA membrane to enhance the hydrophobicity. The desalination performance of the modified PCL-SiO2/CA membrane was compared with the unmodified PCL/CA membrane using a direct contact membrane distillation (DCMD) unit. Results revealed that silica remarkably improves membrane hydrophobicity. The modified PCL-SiO2/CA membrane demonstrated a significant increase in water contact angle of 152.4° compared to 119° for the unmodified membrane. In addition, PCL-SiO2/CA membrane has a smaller average pore size of 0.23 ± 0.16 μm and an exceptional liquid entry pressure of water (LEPw), which is 3.8 times higher than that of PCL/CA membrane. Moreover, PCL-SiO2/CA membrane achieved a durable permeate flux of 15.6 kg/m2.h, while PCL/CA membrane showed unstable permeate flux decreasing approximately from 25 to 12 kg/m2.h over the DCMD test time. Furthermore, the modified PCL-SiO2/CA membrane achieved a high salt rejection value of 99.97% compared to a low value of 86.2% for the PCL/CA membrane after 24 h continuous DCMD operation. In conclusion, the proposed modified PCL-SiO2/CA dual-layer biopolymeric-based membrane has considerable potential to be used as an environmentally friendly membrane for the MD process.

Hydrophobic metal-organic framework@graphene oxide membrane with enhanced water transport for desalination

As a new generation of high-performance alternative, graphene-based membranes have shown great prospect in water purification. Among them, numerous efforts are concentrated on designing hydrophilic membranes while less attention is paid to hydrophobic membranes, which limit the potential application of graphene-based membranes, such as desalination via membrane distillation (MD) process. Anisotropic heat transfer and easy hydrophobic modification of graphene oxide (GO) make it suitable for MD membrane fabrication. Herein, we developed a novel porous and hydrophobic MOF-801@GO-PDMS membrane with enhanced water vapor transport for water desalination via direct contact membrane distillation (DCMD). MOF-801 nanoparticles were in-situ grown within GO interlayer followed with vacuum filtration to fabricate MOF-801@GO membrane which was subsequently modified with polydimethylsiloxane (PDMS) to fabricate the MOF-801@GO-PDMS membrane. The water-stable porous MOF-801 nanoparticle was introduced to increase the porosity of GO laminate and provide extra water vapor transport channels. PDMS was employed to ensure the membrane hydrophobic stability by utilizing its intrinsic hydrophobicity and segment entanglement. The optimum MOF-801@GO membrane exhibited water flux of 29.24 kg m−2 h−1 and NaCl rejection of over 99.99%, as well as robust stability during continuous DCMD desalination test. This work not only gives a promising candidate for water desalination, but also provides a method to design hydrophobic and porous graphene-based nanostructures for other potential applications.

Study of a poly(vinylidene fluoride)/hydrophobic silica sol hybrid hollow fiber membrane for treatment of produced water via direct contact membrane distillation

Direct contact membrane distillation (DCMD) has shown great promises in remediating high-salinity oilfield produced water (PW). In our previous work, a hydrophobic silica sol (Si-R) was successfully synthesized to engineer anti-wetting ability of poly(vinylidene fluoride) (PVDF) membrane. Here our research interest lies in customizing PVDF hollow fiber membrane (HFM) with desirable stability for treating challenging oilfield PW via a DCMD process. The PVDF/Si-R hybrid HFM was fabricated under an optimized spinning condition. It is found that the fabricated PVDF/Si-R HFM is sandwich-structured, consisting a highly porous and thick middle layer and small-size macrovoids of outer layer and inner layer with different pore geometry, and the unique sandwich structure endows the membrane with superior wetting resistance. The membrane characterization showed that the liquid entry pressure (LEP) and water contact angle (WCA) of the fabricated PVDF/Si-R HFM impressively reached up to 5.25 bar and 126°, respectively. As a result, the PVDF/Si-R HFM possessed a desirable performance in treating high-salinity oilfield PW in terms of wetting resistance and salt rejection. Specifically, with a 60 h continuous DCMD operation, the PVDF/Si-R membrane exhibited over 99.9% salt rejection with only 7% reduction in permeate water flux. Clearly, the synthesized hydrophobic Si-R is a favorable additive for enhancing the antiwetting ability of PVDF HFM to treat high-salinity oilfield PW via DCMD process.

Spent caustic wastewater treatment using direct contact membrane distillation with electroblown styrene-acrylonitrile membrane

Spent caustic is classified as a highly polluted and high-risk hazardous wastewater, which severely threatens the environment. In this work, a novel approach was proposed to treat the neutralized spent caustic wastewater using an electroblown nanofibrous styrene-acrylonitrile (SAN) membrane for direct contact membrane distillation (DCMD). The obtained results revealed that the permeate flux of the SAN membrane was lower compared to the commercial polytetrafluoroethylene (PTFE) membrane owing to the thicker structure. The higher surface hydrophobicity of the SAN membrane limited the intrusion of the feed water into the membrane pores. Therefore, extremely pure permeate water was produced. Stable permeate flux (12.03 kg/m2 h) and rejection factor (99.98%) were achieved for the hot-pressed SAN membrane during 96 h continuous DCMD test. Water contact angle of ˃ 90° was remained for both PTFE and SAN membranes, showing that DCMD performance was considerable even after 96 h operation. Besides, the chemical oxygen demand and sulfide removal factor for the fabricated membrane were 99.2 and 99.99%, respectively. Overall, the neutralization followed by the DCMD process can be considered as a proper alternative for treating the spent caustic wastewater.

Crosslinked PVDF based hydrophilic-hydrophobic dual-layer hollow fiber membranes for direct contact membrane distillation desalination: from the seawater to oilfield produced water

Crosslinked polyvinylidene fluoride (PVDF) based hydrophilic-hydrophobic dual-layer hollow fiber membranes were fabricated for direct contact membrane distillation (DCMD). PVDF/ammonia/water dope solution was formulated with a spinning process delay of 9 days for the formation of a mechanically robust and hydrophobic crosslinked PVDF outer layer, and polyethylene glycol 6000 (PEG-6000) was introduced to create a thick hydrophilic PVDF/PEG-6000 inner layer. The influences of ammonia/water contents and the spinning process delay on the membrane morphology were studied. The DCMD performances were investigated using both simulated seawater and actual oilfield produced water as the feed solutions. The incorporation of ammonia/water mixture allowed a slow PVDF crystallization in the dope solution during the 9 days of spinning process delay. The obtained membrane DN2H2-9 showed contact angles of 133.6° and 47.1°on the surfaces of the crosslinked PVDF outer layer and PVDF/PEG-6000 inner layer, respectively. The membrane DN2H2-9 showed a superior permeate water flux of 97.6 kg m−2 h−1 and an energy efficiency of 92.8% to desalinate a 3.5% NaCl solution. The result from a 200 h continuous DCMD operation revealed a stable permeate water flux and more than 99.9% of salt rejection, which was attributed to a relatively high liquid entry pressure (LEP) of 1.95 bar and the simultaneously enhanced mechanical strength of the membrane. The membrane DN2H2-9 also demonstrated promising DCMD performance in the desalination of the real oilfield produced water with a high total dissolved solids (TDS), including almost 100% of permeate water recovery and more than 99.9% of salt rejection in 72 h of operation.

Engineering hierarchically structured superhydrophobic PTFE/POSS nanofibrous membranes for membrane distillation

Membrane distillation (MD) is an attractive thermally-driven membrane process for desalination, brine and wastewater treatment. One of the main obstacles impeding MD application is the absence of effective MD membranes with sufficient water permeability and long-term stability. Attributed to the outstanding hydrophobicity, excellent thermal and chemical resistance, the most ideal polymer to develop MD membrane is polytetrafluoroethylene (PTFE). However, the fabrication of PTFE membranes is challenging due to its poor processability. This work describes a novel strategy to develop superhydrophobic and highly porous PTFE membranes via electrospinning. The hydrophobic nanoparticles 8 vinyl-grafted polyhedral oligosilicone (vinyl-POSS) were firstly incorporated into PTFE nanofibers to promote the crystallization of PTFE nanofiber, enhance nanofiber roughness, and improve membrane mechanical robustness and porosity. The effects of vinyl-POSS concentration, temperature and salt concentration of feed solutions on MD performance of PTFE/POSS nanofibrous membranes were investigated systemically in this study. The optimized PTFE/POSS nanofibrous membrane #POSS-2 has a three-dimensional (3D) superhydrophobic property with a water contact angle of 151 ± 4°. It exhibited a competitive water flux of 40 ± 2 Lm−2 h−1 in the direct contact membrane distillation (DCMD) process when the feed and permeate temperatures were 60 and 20 °C, respectively. Moreover, #POSS-2 possessed an excellent long-term stability in a 200-h continuous DCMD operation.

Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination

In this study, an innovative Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process was boosted for desalination of actual produced water with elevated total dissolved solids (TDS) of 154,220 mg/L. The Janus composite hollow fiber membrane (Janus-HFM) was characterized with a thin and hydrophobic polyvinylidene difluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick porous and hydrophilic PVDF and polyethylene glycol (PEG) inner layer. Compared to the neat PVDF hollow fiber membrane, the Janus-HFM showed both increased permeate water flux and improved energy efficiency with more than 99.99% salt rejection. The long-term desalination performance of the Janus-HFM was investigated via 200 h of continuous DCMD experiment. The results showed that the permeate water flux declined from 25.41 kg/m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the operation. It was found that both scales and dissolved organic matters induced foulants accumulated at the feed side of the membrane due to the complex composition of the produced water. Nonetheless, the desalination performance decline can be effectively inhibited from 40% to 9.7% with a simple physical cleaning process by using the permeate water that recovered in the DCMD process.

Effect and mechanism of an anionic surfactant on membrane performance during direct contact membrane distillation

This study aimed to investigate the effect of a typical anion surfactant sodium dodecyl sulfate (SDS) on direct contact membrane distillation (DCMD) process using three different membranes, including polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) hydrophobic membranes and an omniphobic membrane. The PVDF membrane was the most likely to be wetted, and the pores wetting got more serious for both the PVDF and PTFE membranes with SDS concentration increasing. The omniphobic membrane presented robust anti-surfactant-wetting ability, even facing the saline feed with 0.5 mM SDS. The calcium ions can promote the surfactant SDS micellization, and the formed SDS colloids could cause membrane fouling. After 50 h continuous DCMD operation, the permeate flux of the hydrophobic membranes declined more than 50% due to membrane fouling, while the loss of permeate flux for the omniphobic membrane was only about 20%. The mechanism of membrane fouling was quantitatively investigated by the extended Derjaguin-Landau-Verwey-Overbeek theory. The calculated results manifested that the omniphobic membrane had the lowest attractive force to the foulant, indicating a less risk of being fouled. The energy barrier of the omniphobic membrane was the highest, resulting in the strongest resistance against the SDS colloids deposition on membrane surface.

Computational fluid dynamic modeling of water desalination using low-energy continuous direct contact membrane distillation process

In the present study, a comprehensive two-dimensional (2D) mathematical model was developed to investigate the behavior of the direct contact membrane distillation (DCMD) system using Computational Fluid Dynamics (CFD) technique. The model contains mass, energy, and momentum transfer phenomena equations, and can predict the amount of fresh water produced by the system. Governing equations of these transition phenomena were coupled in three domains, namely, membrane, hot and cold channels. The governing equations were discretized by using the finite volume method and then solved by MATLAB software. The developed model was validated with empirical data, and the results showed an excellent agreement between the empirical data and the model. Also, the impact of different parameters, including velocity, temperature, and length of channels was investigated on the system behavior. The results showed that the temperature and velocity of the input currents are very influential on the performance of this membrane distillation (MD) system. Both empirical and simulation results suggested that this type of water desalination system can be a very suitable and effective option for the challenges of human access to fresh water in the coming years.

Designing and optimization of continuous direct contact membrane distillation process

Membrane distillation (MD) is a relatively less-explored membrane operation with the potential to achieve high recovery factor by using low grade heat. The current study proposes the design of a continuous direct contact MD process to achieve high recovery factors by using a commercial hollow fiber membrane. The design consists of multiple MD stages connected in series to achieve a predefined final solution concentration. Depending upon the outlet temperatures of feed and permeate, the design considers the option of heat recovery from permeate. Under a given set of operating conditions, there exists a module length (named as optimum module length) where the net thermal energy consumption and overall permeate productivity are optimum. The optimum module length has been analyzed as function of feed to permeate flow rate (F/P) ratio, feed temperature and concentration. It has been observed that for given feed temperature and concentration, the optimum module length can be tuned by changing F/P ratio. The minimum value of the optimum length is observed at the highest F/P ratio considered. Mathematical analysis was extended to evaluate the appropriate length and the corresponding thickness for each stage. The results reveal a strong nexus among membrane thickness, solution concentration and optimum length.

Optimization of module length for continuous direct contact membrane distillation process

Module length plays an important role in dictating the overall performance of membrane distillation (MD) process. In current study, the performance behavior of a small scale module applied for direct contact membrane distillation (DCMD), has been simulated and extrapolated to obtain temperature and flux profiles along the modules with lengths varying from 10 to 2000cm. The profiles obtained under various conditions have been used to calculate the key performance parameters of a plant treating seawater at feed capacity of 1000m3/h. The operational mode of the plant comprises of multiple stages connected in series to achieve a desired recovery factor. The effect of module length on key performance parameters including specific energy consumption, membrane area and number of stages has been analyzed. To calculate the optimum module length, the performance parameters have been translated into operational and fixed capital costs by performing a detailed cost analysis based on existing pilot MD units installed under the framework of European project MEDIRAS. Dependence of optimum module length on operating conditions, membrane thickness and overall recovery factor has been analyzed.

Fabrication of bioinspired composite nanofiber membranes with robust superhydrophobicity for direct contact membrane distillation.

The practical application of membrane distillation (MD) for water purification is hindered by the absence of desirable membranes that can fulfill the special requirements of the MD process. Compared to the membranes fabricated by other methods, nanofiber membranes produced by electrospinning are of great interest due to their high porosity, low tortuosity, large surface pore size, and high surface hydrophobicity. However, the stable performance of the nanofiber membranes in the MD process is still unsatisfactory. Inspired by the unique structure of the lotus leaf, this study aimed to develop a strategy to construct superhydrophobic composite nanofiber membranes with robust superhydrophobicity and high porosity suitable for use in MD. The newly developed membrane consists of a superhydrophobic silica-PVDF composite selective skin formed on a polyvinylidene fluoride (PVDF) porous nanofiber scaffold via electrospinning. This fabrication method could be easily scaled up due to its simple preparation procedures. The effects of silica diameter and concentration on membrane contact angle, sliding angle, and MD performance were investigated thoroughly. For the first time, the direct contact membrane distillation (DCMD) tests demonstrate that the newly developed membranes are able to present stable high performance over 50 h of testing time, and the superhydrophobic selective layer exhibits excellent durability in ultrasonic treatment and a continuous DCMD test. It is believed that this novel design strategy has great potential for MD membrane fabrication.

Boron removal and desalination from seawater by PVDF flat-sheet membrane through direct contact membrane distillation

Boron removal and desalination from seawater by direct contact membrane distillation (DCMD) was studied with self-prepared polyvinylidene fluoride (PVDF) flat-sheet membrane. The membrane had an ultra-thin top skin and a porous network sponge-like layer and its effective pores showed a narrow distribution with a mean pore size of 0.22μm. The membrane exhibited good hydrophobicity and its contact angle was about 82.6±0.7°. During DCMD test of 35g/L sodium chloride solution, the maximum permeate flux can achieve 47.6kg/m2h. The natural seawater containing 4.65mg/L boron was treated by DCMD for simultaneous boron removal and desalination. Although the permeate flux decreased with concentration factor (CF) value increase, the permeate boron kept below 20μg/L and salt rejection was over 99.9%. When the CF value exceeded 4.0, there would be scale deposits formed, and the permeate quality and flux declined rapidly. The antiscalant polyacrylic acid could delay the deposit formation and the advisable dosage was 10mg/L. The membrane showed satisfying performance stability in a 300h continuous DCMD experiment even with the CF value as high as 7.0, indicating that the self-prepared membrane and DCMD process could be efficiently used for boron removal and desalination from seawater.

Design and fabrication of lotus-root-like multi-bore hollow fiber membrane for direct contact membrane distillation

Highly constrained by the requirements of high porosity and large pore sizes, traditional single-bore hollow fiber membranes often suffer from easy breakage and performance instability during long-term operations of membrane distillation (MD). In this work, a multi-bore PVDF hollow fiber (MBF) membrane with a lotus root-like geometry was designed and successfully fabricated via the specially designed spinneret and optimized spinning conditions. Various effects of spinning parameters including bore flowrate, dope flowrate and take-up speed were investigated on the membrane macro- and micro-structure, mechanical properties and direct contact membrane distillation (DCMD) performance. The tensile strength characterizations have proven the excellent mechanical rigidity and elasticity of MBF membranes. Even for the MBF membrane with a thin wall of around 40μm, the maximum load was as high as 2.4N. Most importantly, the performance of the DCMD of the MBF membrane was only slightly lower or even comparable to that of single-bore membranes. In addition, the MBF membranes exhibited superior stability in terms of vapor permeation flux and salt rejection during the continuous DCMD experiment with robust operational conditions. We believe this work may have profound implication to the development of mechanically durable membranes not only for membrane distillation MD but also for other membrane applications.

Effects of additives on dual-layer hydrophobic–hydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance

The advantages of the implementation of dual-layer hydrophobic–hydrophilic hollow fiber membranes for membrane distillation (MD) have been highlighted in this work. The effects of incorporating methanol as a non-solvent additive and self-synthesized fluorinated silica (FSi) particles as a hydrophobic modifier on the resultant membrane morphology and MD performance were investigated. Employing a 3.5wt% sodium chloride solution at 80°C, the highest direct contact membrane distillation (DCMD) flux of 83.40±3.66kg/(m2h) and separation factor higher than 99.99% were attained for the membrane spun with methanol additive. Moreover, the stability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane has been demonstrated through continuous DCMD experiments for 5 days. The separation factor was maintained higher than 99.99% for the membrane spun with methanol additive, verifying the suitability of the dual-layer hydrophobic–hydrophilic hollow fiber membrane configuration for desalination processes. The morphological transformation of the outer membrane surface from a porous agglomerated globule structure into a denser interconnected globule structure may be accounted for by the stability improvement of the membrane spun with methanol additive. On the other hand, it was found that an enhanced hydrophobicity of the membrane spun with FSi particles did not result in an improvement of the membrane stability. The existence of the hydrophilic hydroxyl group on the FSi particle surface may favor the occurrence of membrane wetting.

Novel Membrane and Device for Direct Contact Membrane Distillation-Based Desalination Process

In the direct contact membrane distillation (DCMD) process for desalination, the water vapor flux is strongly affected by the hot brine heat transfer coefficient, conductive heat loss, and long-term flux decline due to membrane pore wetting/fouling, etc. The DCMD process has been explored here using porous hydrophobic polypropylene hollow fibers having three different dimensions and two different wall thicknesses. The outside surfaces of the fibers have been coated with a variety of microporous plasmapolymerized silicone−fluoropolymer coating. A large number of rectangular modules having the hot brine in cross flow over the outside of the fibers and cold distillate flowing in the tube side have been investigated for their DCMD performances with hot brine (1% NaCl) over a brine temperature range of 60−90 °C. The module MXFR 3 containing fibers with larger internal diameter (i.d.) and wall thickness, and having the best performance, was tested in a continuous DCMD run for 120 h with 85 °C brine flowing at a...