Researchers at MIT have used this small alumina membrane to prove a hybrid process that has the potential to change the face of water desalination. © Rohit Karnick/MIT
Researchers at MIT’s Center for Clean Water and Clean Energy are developing a new approach that combines the best elements of thermal and reverse osmosis processes.
March 25, 2014—Researchers at the Massachusetts Institute of Technology (MIT) are developing a technology that employs hydrophilic and hydrophobic sections of nanoscreen membranes to create a liquid/vapor interface that holds the potential to change the face of water desalination.
The research is being conducted at the Center for Clean Water and Clean Energy, a partnership between the mechanical engineering departments of MIT and King Fahd University of Petroleum and Minerals (KFUPM). Improving desalination technology is one of the key focuses of the center.
Currently, the most common methods of desalination are reverse osmosis or thermal processing. In thermal processing, water is heated, evaporated, and then condensed. Salt and other unwanted contaminants cannot cross the vapor barrier and are left behind. The process is not favored when constructing new desalination plants, however, because of a significant drawback.
“The requirements for energy are quite high because it takes a lot of energy to boil water,” says Rohit Karnik, Ph.D., a professor of mechanical engineering at MIT. “The performance is very good in terms of the nonvolatiles it can reject. The plants tend to be robust.”
“Reverse osmosis, on the other hand, is extremely energy efficient compared to thermal processes,” Karnik says. The drawback is that the chemical and physical properties of the membrane material govern not only the water flow rate but also the rejection rates for different types of salts and other performance measures.
“If you are trying to optimize one [aspect], it’s coupled to all the other properties. So if you try to increase water flow rates, salt rejection goes down. If you try to optimize for sodium chloride, maybe boron rejection is not optimized,” Karnik says.
With this context in mind, researchers at the center began to conceptualize a combination of the technologies that would utilize evaporation and condensation but involve a membrane that didn’t require a temperature difference to drive the flow, Karnik says.
To accomplish this, a team led by Karnik and including Jongho Lee, an MIT Ph.D. student, and Tahar Laoui, Ph.D., a professor of materials science at KFUPM, began to examine how to bring two surface areas of water within a micron of each other to facilitate a transfer of water vapor molecules between the two.
“We came up with this concept of a membrane that would essentially have extremely short hydrophobic nanopores [that] would separate two liquid reservoirs by a very small gap,” Karnik says. “Now we can create a mechanical or osmotic pressure difference across this vapor gap and have water [molecules] flow across it. And because it involves evaporation and condensation, anything that cannot evaporate will not be able to cross.”
Beyond this novel interface, the method would function like a conventional reverse osmosis system, with one inlet stream of salt water and two outlet streams—one stream is desalinated water for use, the other stream flushes the removed salt back into the salt water body.
The team’s findings—“Nanofluidic Transport Governed by the Liquid/Vapour Interface”—were published recently in the journal Nature Nanotechnology. The findings reveal that the team was able to quantify behavior of water at the liquid/vapor interface with a high degree of accuracy which has applications beyond desalination.
The next step for the team is to develop small membranes of several square inches from a material that is scalable to large applications. The membrane the team tested is made of oxidized aluminum, called alumina, with pores 100 nanometers in diameter. Although the test proved their theory of the liquid/vapor interface works, alumina isn’t a suitable material for a larger scale test of the concept under high pressure.
“This alumina membrane we are using is quite brittle,” explains Lee, the lead author of the paper. “We cannot use this for actual reverse osmosis because if we apply high pressures, such as 50 bar, it will certainly break this membrane.” Also, fabrication cost of alumina membranes would be prohibitively high for industrial scale applications.
“That brings us back to a couple of the key points of this technology,” Karnik says. “We have the freedom to choose materials. We are no longer constrained with one material that affects all properties. As long as the material surface is hydrophobic, this type of membrane should work. It’s basically a decoupling of the membrane transport properties from the membrane material properties.”