Direct contact membrane distillation (DCMD) for desalination is attractive for high salt concentrations if low cost steam/waste heat is available and waste brine disposal cost for inland desalination is factored in. A number of innovations have taken place in DCMD in terms of the structure of the porous hydrophobic membrane. Composite membranes are of increasing interest. Composite membrane structures of great interest include a thin
hydrophobic porous layer over a porous hydrophilic layer of polyvinylidene fluoride (PVDF) or a thin porous hydrophobic layer over a more conventional hydrophobic porous membrane. These membranes can be in the form of an integral composite or a stacked composite or a laminated composite. A facile method of fabricating such integral composite membranes is plasma polymerization under vacuum. A class of such membranes yielding quite high water vapor fluxes have been characterized using a variety of characterization techniques: Contact angle, liquid entry pressure (LEP), bubble-point pressure, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM). Stacked composites of a hydrophobic ePTFE membrane over a hydrophilic PVDF membrane or a hydrophobic PVDF membrane over another hydrophobic PVDF membrane were also studied. Novel conditions created lead to very high water vapor fluxes compared to those from conventional hydrophobic membranes supported on a mesh support.
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Engineering Carbon Nanotube Forest Superstructure for Robust Thermal Desalination Membranes
Desalination by membrane distillation (MD) using low‐grade or waste heat provides a potential route for sustainable water supply. Nonwetting, porous membranes that provide a selective pathway for water vapor over nonvolatile salt are at the core of MD desalination. Conventional water‐repelling MD membranes (i.e., hydrophobic and superhydrophobic membranes) fail to ensure long‐term desalination performance due to pore wetting and surface fouling. To address these challenges, a defect‐free carbon nanotube forest (CNTF) is engineered in situ on a porous electrospun silica fiber substrate. The engineered CNTF forms an ultrarough and porous interface structure, allowing outstanding wetting resistance against water in air and oil underwater. As a result of this antiwetting property, the composite CNTF membrane displays a stable water vapor flux and a near complete salt rejection (>99.9%) in the desalination of highly saline water containing low surface tension contaminants. The antimicrobial property of the composite CNTF membrane imparted by the unique forest‐like architecture and the oxidative effect of carbon nanotubes (CNTs) are further demonstrated. The results exemplify an effective strategy for engineering CNT architecture to elucidate the structure–property–performance relationship of the nanocomposite membranes and to guide the design of robust thermal desalination membranes.
more » « less- NSF-PAR ID:
- 10461430
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 29
- Issue:
- 36
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Energy-efficient desalination and water treatment technologies play a critical role in augmenting freshwater resources without placing an excessive strain on limited energy supplies. By desalinating high-salinity waters using low-grade or waste heat, membrane distillation (MD) has the potential to increase sustainable water production, a key facet of the water-energy nexus. However, despite advances in membrane technology and the development of novel process configurations, the viability of MD as an energy-efficient desalination process remains uncertain. In this review, we examine the key challenges facing MD and explore the opportunities for improving MD membranes and system design. We begin by exploring how the energy efficiency of MD is limited by the thermal separation of water and dissolved solutes. We then assess the performance of MD relative to other desalination processes, including reverse osmosis and multi-effect distillation, comparing various metrics including energy efficiency, energy quality, and susceptibility to fouling. By analyzing the impact of membrane properties on the energy efficiency of an MD desalination system, we demonstrate the importance of maximizing porosity and optimizing thickness to minimize energy consumption. We also show how ineffective heat recovery and temperature polarization can limit the energetic performance of MD and how novel process variants seek to reduce these inefficiencies. Fouling, scaling, and wetting can have a significant detrimental impact on MD performance. We outline how novel membrane designs with special surface wettability and process-based fouling control strategies may bolster membrane and process robustness. Finally, we explore applications where MD may be able to outperform established desalination technologies, increasing water production without consuming large amounts of electrical or high-grade thermal energy. We conclude by discussing the outlook for MD desalination, highlighting challenges and key areas for future research and development.more » « less
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