A Bilayered Structure Comprised of Functionalized Carbon Nanotubes for Desalination by Membrane Distillation.

Carbon nanotube immobilized membrane with controlled nanotube incorporation via phase inversion polymerization for membrane distillation based desalination

8 - Membrane distillation

In this paper, we present the treatment of humic acid solution via carbon nanotube immobilized membrane (CNIM) distillation assisted by air sparging (AS). Carbon nanotubes offer excellent hydrophobicity to the modified membrane surface and actively transport water vapor molecules through the membrane to generate higher vapor flux and better rejection of humic acid. The introduction of air sparging in the membrane distillation (MD) system has changed the humic substance fouling by changing the colloidal behavior of the deposits. This modified MD system can sustain a higher run time of separation and has enhanced the evaporation efficiency by 20% more than the regular membrane distillation. The air sparging has reduced the deposition by 30% in weight and offered lesser fouling of membrane surface even after a longer operating cycle. The water vapor flux increased with temperature and decreased as the volumetric concentrating factor (VCF) increased. The mass transfer coefficient was found to be the highest for the air sparged—carbon nanotube immobilized membrane (AS-CNIM) integrated membrane distillation. While the highest change in mass transfer coefficient (MTC) was found for polytetrafluoroethylene (PTFE) membrane with air sparging at 70 °C.

Effect of long-term operation on membrane surface characteristics and performance in membrane distillation

Engineering carbon nanotubes enhanced hydrophobic membranes with high performance in membrane distillation by spray coating

Membrane distillation (MD) is a special evaporation process to produce fresh water from seawater or contaminated water using membranes. MD has advantages over other evaporation technologies such as multi-stage flash vaporization (MSF) and multi-effect distillation (MED) due to its relatively low energy requirements, allowing the use of solar energy as its heat source. Nevertheless, lack of membrane materials for MD process hinders its practical implementation for desalination and water treatment. In this study, membranes made of carbon nanotube (CNT) are presented for MD. Flat sheet hydrophobic membranes made of polyvinylidene fluoride (PVDF) were selected as supports for bucky-paper membranes, allowing formation of CNT bucky-paper without chemical reactions. Laboratory-scale systems were used to evaluate their potential and performance in direct contact MD. Water permeability and salt rejection were analyzed for each case. D.I water and synthetic feed water were used for the lab-scale tests. It was demonstrated that the physical immobilization of CNT on a hydrophobic membrane changed led to an increase in vapor permeability while improving salt rejection.

Fast global population growth, serious environmental pollution and rapid economic developments have resulted in water scarcity around the world. Membrane distillation (MD) processes were considered as an attractive technology to treat waste water, recycle polluted water and provide more freshwater resources. This thesis provides a brief review on the research and developments of MD process, commercial MD membranes and lab-fabricated MD membranes. As the electrospun composite nanofibrous membranes have great potential to be used in MD due to their unique structural features, the complex electrospinning process has also been reviewed, including the materials and operating parameters which could control nanofiber formation, and various designs of electrospun apparatus which can produce nanofiber membranes with different appearances. However, it is found that limited works have been carried out to fabricate MD membranes by electrospinning. 
\nIn this work, poly (vinylidene fluoride) (PVDF) nanofiber membranes were fabricated through electrospinning for direct contact membrane distillation (DCMD) as a first trial. The effects of PVDF dope concentration, inorganic salt additives, sprayer’s moving speed, and chamber moisture on the properties of resultant membranes were investigated. It also illustrates the importance of processing parameters and heat-press post-treatment, and demonstrates that the heat-press post-treatment improved membrane integrity significantly and enhanced permeate flux in DCMD process. All the electrospun nanofiber membranes possessed high water contact angles (between 135° to 142°) due to their high surface roughness. The post-treated PVDF nanofiber membranes were able to present a steady water permeation flux of 21 kg m-2h-1 throughout the entire testing period of 15 h, using a 3.5 wt% NaCl solution as the feed under the feed and permeate inlet temperatures of 323 K and 293 K, respectively. 
\nHowever, PVDF nanofiber membranes without hydrophobic additives or surface modification do not have sufficient anti-wetting performance. Further treatment of PVDF nanofiber should be carried out to impart them with better wetting resistance and long-term stability. Two types of superhydrophobic PVDF nanofiber membranes, integrally-modified and surface-modified PVDF membranes, have been successfully fabricated by electrospinning followed by surface modification, which includes dopamine surface activation, silver nanoparticle deposition and hydrophobic treatment. The modification is convenient because of mild reactions and wide applicability. The characterizations revealed that the modifications have altered the membrane surface morphology and topology, and made the membrane superhydrophobic due to their hierarchical structures. Compared with unmodified membrane, the integrally-modified membrane (I-PVDF) can achieve a high and stable MD water flux of 31.6 kg m-2h-1 using a 3.5 wt% NaCl as the feed solution while the feed and permeate temperatures were fixed at 333 K and 293 K, respectively. To the best of our knowledge, this result is superior to all other PVDF flat sheet membranes tested under the same or similar conditions, which is believed to be attributed to the open surface pore structure and the thin thickness of the PVDF nanofiber membrane with the aid of electrospinning. The superhydrophobic nature of the membrane surface brought by the integral modification on all nanofibers renders the membrane anti-wetting property while remaining high water flux.
\nMoreover, inspired by the unique structure of lotus leaf, a novel strategy is developed to construct 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 PVDF porous nanofiber scaffold via electrospinning. This fabrication method could be easily scaled up due to its simple preparing procedures. The effects of silica diameter on membrane contact angle, sliding angle and MD performance were investigated thoroughly. For the first time, the 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 continuous DCMD test. It is believed that this novel design strategy has great potential for MD membrane fabrication.
\nAdditionally, to further improve the wetting repellent property of superhydrophobic membranes, 3-dimensional (3D) superhydrophobic membranes were developed as a possible solution. Moreover, since highly porous nanofiber membranes usually suffer from insufficient mechanical property, which have adverse impact on membrane packing in the module, thus, a dual-layer membrane was fabricated by electrospinning 3D superhydrophobic composite layers on a non-woven support to improve its wetting resistance and enhance mechanical robustness Another type of dual-layer superhydrophobic composite membranes consisting of PVDF nanofibrous support and an ultrathin 3D superhydrophobic selective layer was prepared to compare with the as-fabricated non-woven-supported superhydrophobic dual-layer membrane. All these membranes exhibit superhydrophobicity towards distilled water, salty water, oil-water mixture and beverages, which enables them to be used not only for desalination but also for other concentrating treatments. Compared with nanofiber-supported dual-layer membranes, the non-woven-supported membranes exhibit higher mechanical strength as a result of excellent combination with non-woven support and better long-term performance because of the thicker 3D superhydrophobic structure. The morphology, pore size, porosity, mechanical properties as well as liquid enter pressure of water of these superhydrophobic composite membranes and commercial PVDF membrane are measured and compared. The possible wetting procedures of the as-prepared superhydrophobic dual-layer membranes are also illustrated in this study. 
\nFinally, this thesis provides some personal perspectives for the future developments in which the composite nanofiber membranes could be pursued for water research.
\nIn conclusion, this thesis presents the design and development of novel superhydrophobic nanofiber membranes based on the studies of the fundamental mechanisms of electrospinning, surface modification on nanofiber membranes, fabrication of robust superhydrophobic membranes, and preparation of 3D superhydrophobic dual-layer membrane. This work contributes to the development of membrane fabrication technology and facilitates the practical applications of membrane distillation process.

New insights into the role of carbon nanotubes spray-coated on both sides of the PTFE membrane in suppressing temperature polarization and enhancing water flux in direct contact membrane distillation

Membrane distillation (MD) is an evaporation/condensation process of volatile components through a hydrophobic porous membrane. The maintenance of gas phase inside the membrane pores is a fundamental condition required to carry out the MD process. A hydrophobic nature of the membrane prevents liquid penetration into the pores. Membranes having these properties are prepared from polymers with a low value of the surface energy, such as polypropylene (PP), polytetrafluoroethylene (PTFE) or polyvinilidene fluoride (PVDF) (Alklaibi & Lior, 2005; Bonyadi & Chung, 2009, Gryta & Barancewicz, 2010). Similar to other distillation processes also MD requires energy for water evaporation. The hydrodynamic conditions occurring in the membrane modules influence on the heat and mass transfers, and have a significant effect on the MD process efficiency. The MD separation mechanism is based on vapour/liquid equilibrium of a liquid mixture. For solutions containing non-volatile solutes only the water vapour is transferred through the membrane; hence, the obtained distillate comprises demineralized water (Alklaibi & Lior, 2004; Gryta, 2005a; Schneider et al., 1988). However, when the feed contains various volatile components, they are also transferred through the membranes to the distillate (ElBourawi et al., 2006; Gryta, 2010a; Gryta et al., 2006a). Based on this separation mechanism, the major application areas of MD include water treatment technology, seawater desalination, production of high purity water and the concentration of aqueous solutions (El-Bourawi et al., 2006; Drioli et al., 2004, Gryta, 2006a, 2010b; Karakulski et al., 2006; Martinez-Diez & Vazquez-Gonzalez, 1999; Srisurichan et al., 2005; Teoh et al., 2008). A few modes of MD process are known: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), vacuum membrane distillation (VMD) and osmotic membrane distillation (OMD). These variants differ in the manner of permeate collection, the mass transfer mechanism through the membrane, and the reason for driving force formation (Alklaibi & Lior, 2005; Gryta, 2005a). The most frequently studied and described mode of MD process is a DCMD variant. In this case the surfaces of the membrane are in a direct contact with the two liquid phases, hot feed and cold distillate (Fig. 1). The DCMD process proceeds at atmospheric pressure and at temperatures that are much lower than the normal boiling point of the feed solutions. This allows the utilization of solar heat or so-called waste heat, e.g. the condensate from turbines or heat exchangers (Banat & Jwaied, 2008; Bui et al., 2010; Li & Sirkar, 2004).

Interception of volatile organic compounds through CNT electrochemistry of electrified membrane surface during membrane distillation

Production of drinking water from seawater using membrane distillation (MD) alternative: direct contact MD and sweeping gas MD approaches

Enhanced durability and hydrophobicity of carbon nanotube bucky paper membranes in membrane distillation

Chapter 6 - Membrane distillation

Dual-layer membranes with a thin film hydrophilic MOF/PVA nanocomposite for enhanced antiwetting property in membrane distillation

Methyl orange removal by combined visible-light photocatalysis and membrane distillation