Metal Spacers Enhance Permeate Gap Membrane Distillation for Energy Efficient Desalination

Technology #16942

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AGMD apparatus diagram. The system consists of a central AGMD module, paired heating and cooling loops, a condensate collection tank, and various temperature, pressure, and flow rate sensors.
Professor John Lienhard
Department of Mechanical Engineering
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David Warsinger
Department of Mechanical Engineering
Jaichander Swaminathan
Department of Mechanical Engineering
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Jim Freedman
MIT Technology Licensing Officer - Chemicals, Instruments, Consumer Products
Patent Protection

Hydrophobic Air-Gap Membrane Distillation

US Patent Pending 2016-0107121

Hydrophobic Air-Gap Membrane Distillation

PCT Patent Application WO 2016-061467


This invention provides for significantly improved condensate production and thermal performance of an air gap membrane distillation (AGMD) system. This can be used to increase the efficiency of membrane distillation processes for desalination, food processing, and waste treatment applications.

Problem Addressed

Membrane distillation is a thermal desalination technology which can provide small-scale desalination. AGMD is an MD configuration with an air-filled cavity between the membrane and condensing surface, traditionally with laminar film condensation. The thermal resistance of the air gap prevents conduction between the cooling surface and hot feed, making AGMD the most efficient mainstream MD technology, at the cost of a large associated mass transfer resistance from diffusion through the air gap. This creates a trade-off between system size and efficiency for AGMD, which requires large condensing areas to be superior.


Recent research suggests that hydrophobic surfaces can have 5-7 times the heat transfer coefficient of laminar film condensing. This invention consists of employing hydrophobic condensing in AGMD. In typical AGMD, a hot saline feed side passes a hydrophobic membrane, which allows hot water vapor through but not saline liquid water. The vapor crosses an air gap and is condensed on a condensing surface. The condensing surface may be cooled by a cold stream, or in multistage configurations, feed streams at progressively lower temperatures.

The inventors paired superhydrophobic condensation surfaces with untreated and superhydrophobic air gap support meshes of high and low conductivities to determine optimum conditions for energy efficient permeate production in AGMD. They determined that introducing superhydrophobic surfaces can result in an up to 110% improved efficiency for AGMD desalination. Superhydrophobic condensation conditions at varied feed and cold side temperatures are recommended for substantial improvement for AGMD systems. 


  • Superhydrophobic surfaces offer significantly increased efficiency for MD desalination
  • Method improves AGMD condensate flux