Energy-efficient Water Purification Using Pressure Difference Across a Hydrophobic Membrane

Technology #13474

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FIG. 1 is a schematic view of one embodiment of a device for filtering a liquid substanceFIG. 2A is a schematic view of one embodiment of a pore of the device of FIG. 1 illustrating multiple phenomena that can occur to a molecule during vapor-phase transport.  FIG. 2B is a schematic view of the pore of FIG. 2A illustrating multiple possible paths of a molecule.  FIG. 3 is a graph illustrating the relationship between transmission probability and a ratio of pore length and pore radius for a pore of the present invention.FIG. 4E is a schematic drawing of still four other possible path options for a molecule in a pore in accordance with the present invention in which the molecule is emitted from a first meniscus and arrives at the first meniscus following one or more deflections.  FIG. 4F is a schematic drawing of four other possible path options for a molecule in a pore in accordance with the present invention in which the molecule is emitted from a first meniscus and arrives at a second meniscus following one or more deflections.FIG. 6B is a three-dimensional graph illustrating a relationship between a net mass flux of a membrane of the present invention and both a pore radius and a pressure difference between an applied pressure and an osmotic pressure, wherein a condensation probability is 1.0 and a membrane porosity is approximately 40%.FIG. 7 is a graph illustrating a temperature difference across a membrane balancing latent heat transport and conduction through the membrane.  FIG. 8 is a side view of one embodiment of a membrane being coated with gold by an evaporation process for selectively modifying only a section of each pore.
Categories
Inventors
Professor Rohit Karnik
Department of Mechanical Engineering, MIT
External Link (web.mit.edu)
Jongho Lee
Department of Mechanical Engineering, MIT
Managed By
Christopher Noble
MIT Technology Licensing Officer - Clean and Renewable Energy
Patent Protection

Liquid Filtration Using Pressure Difference Across a Hydrophobic Membrane

US Patent 8,652,332
Publications
Desalination of water by vapor-phase transport through hydrophobic nanopores
Journal of Applied Physics, 2010, 108(4), 044315-044315
Nanofluidic Transport Governed by the Liquid/Vapour Interface
Nature Nanotechnology, 9, 317 (2014)

Applications

Applications for this technology are found in water desalination, water purification and in distillation-like processes. By efficiently treating non-drinkable water, people living in at-risk areas can have access to clean drinking water. This technology can help increase the number of water purification plants by reducing the amount of energy required for the water purification process and consequently reducing the cost of maintaining a plant. 

Problem Addressed

 Current methods for water desalination require high cost, have poor rejection of toxic contaminants, and exhibit low flux, all of which limit production of clean water. This technology is a device for filtering liquids through a hydrophobic porous membrane. By using a pressure difference instead of a heat difference to draw liquids as vapor phase through the membrane, this technology dramatically increases the yield of freshwater and rejection of toxic molecules while reducing costs associated with energy and post-treatment. As fresh water becomes increasingly scarce, this technology will enable efficient and low-waste water purification for consumer and industrial use.

Technology

The device is based on reverse osmosis, but uses a pressure difference to force water through a hydrophobic porous membrane. Under an applied driving pressure, selective transport of water occurs across vapor nanobubbles trapped in the membrane. This method enables flexible choice of membrane material and works efficiently at high temperatures and under a wider range of conditions than polyamide reverse osmosis membranes. Pressure difference-driven reverse osmosis is much more efficient than classic membrane distillation, which loses a considerable amount of energy due to heat loss. By removing the need for heat, this method combines the advantages of thermal desalination and reverse osmosis, providing excellent rejection of contaminants like boron. This method also allows for flexibility in choice of membrane material, saves large amounts of energy, and substantially reduces costs. In addition, the flux through the membrane is 2-3 times higher than in conventional reverse osmosis systems, improving the yield of fresh water. The device can be manufactured from a range of materials, so it can be designed to have desired properties, such as chlorine resistance, to prevent biofouling and therefore increase its lifespan. Therefore, this presents major advantages compared to current desalination methods and should save money while increasing freshwater output.

Advantages

  • Low viscous loss prevents water waste
  • Uses pressure instead of temperature to distill water, increasing energy efficiency
  • Prevents huge heat loss generated by high temperature gradient across the membrane
  • Flexibility in material of manufacture, so device can be adapted to have diverse properties such as chlorine resistance and increased durability
  • Excellent rejection of low vapor pressure contaminants (e.g., boron)
  • Can use existing reverse osmosis infrastructure, for low-cost implementation