Taking the salt out of water with oil
27 January 2013
Americans take good water from the tap for granted, but in areas such as the Middle East, China and India water is a critical resource. A desalination process, developed by MIT post-doctoral associate Anurag Bajpayee as part of his doctoral research under Professor Gang Chen’s NanoEngineering Group at MIT, could help solve that problem by removing the salt from seawater.
Bajpayee’s innovative process, referred to as directional solvent extraction, uses oil to purify water. It won praise from Scientific American in December 2012 as one of its 10 World Changing Ideas and Forbes India recognized it as one of five game-changing ideas from academia in January 2013.
Heating separates salt
As demonstrated in Bajpayee’s paper with Chen and co-authors, “Very low temperature membrane-free desalination by directional solvent extraction,” Bajpayee’s team achieved separation of salt from water with decanoic acid. Decanoic acid is a fatty acid and occurs naturally in many essential oils, according the National Institutes of Health PubChem Compound database.
The process involves heating the solution to 40 to 90 degrees Celsius (104 to 190 degrees Fahrenheit) that separates a highly concentrated brine from the water-acid solution. As the solution is cooled, the pure water separates out and the solvent is reused.
Solving a shale oil problem
“In the U.S., we take drinking water for granted, there is a lot of water available,” said Bajpayee, 27, a native of Lucknow, India. “The real use we see in the U.S. and Canada … is for treating produced water that comes out of oil and gas wells,” he said. While hydraulic fracturing is causing a lot of debate and public discomfort because of the water contamination issues, “if we could mitigate those then we could develop oil and gas domestically in the U.S. and responsibly without hurting the environment. Water that comes of the shale oil wells is much more saline than seawater, could be sometimes five, even 10 times saltier than sea water, so much so that conventional methods like reverse osmosis, multi-stage flash, just plain don’t work or are prohibitively expensive,” Bajpayee said.
An incidental discovery
The idea for separating salt from water using an oil grew out of research Bajpayee did at the Massachusetts General Hospital, while the University of Missouri graduate worked toward his master’s at MIT under Professor Mehmet Toner of Harvard Medical School investigating techniques for single cell cryopreservation. A byproduct of his work was the insight that you could remove water from a glycerol solution. “We were basically concentrating glycerol in water droplets,” he said.
When he took his MIT doctoral entrance examination, discussion turned to asking what else might be taken out of water. “Clearly you can take water out of a glycerol solution,” he said. His discussions with Professor Chen led them to ask, “Can you also take water out of other solutions? What else can you remove from water? Of course, there was only one way to find out and that was try it and we did."
“The directional solvent at the time was soybean oil and it worked, which was great, but of course it did not work in a feasible manner. The physical phenomenon was demonstrated, so that was good. We then kept hunting for newer and better solvents. That’s when we came up with decanoic acid,” he said. A better solvent will dissolve more water in it and therefore, the yield of water per volume of solvent is higher and requires less energy, allows for smaller systems and greater overall efficiency. Bajpayee expects to publish another paper with yet another solvent this spring.
Further understanding of the molecular phenomenon led to a paper with Chen and another MIT postdoctoral associate in Chen’s lab, Tengfei Luo, “Directional solvent for membrane-free water desalination—A molecular level study.”
The discovery was unexpected because for most soluble chemical compounds, the solubility goes both ways. “Usually if A dissolves in B, B dissolves in A,” Bajpayee said. “Like dissolves like, but we are looking for pairs of materials with water which exhibit asymmetric solubility. So where we want the water to dissolve in our solvents, we don’t want the solvents to dissolve in our water.” That’s called directional solubility.
“So for example you take a cup of solvent and put a few drops of water in it, the water will just diffuse through and you won’t see anything. You take a cup of water and put a few drops of the solvent in it, it’ll stick around as droplets like oil on water. So there is this directionality in solubility there that we are looking for,” he said.
Despite some halting attempts in the 1950s and 60s, the concept of using solvents to treat water was never fully explored until the MIT group pursued it. “It was nice that we weren’t a water treatment lab at the time so we didn't know what didn't work,” Bajpayee said. They gave it a try, and it worked.
A simple technique
“When it was such a simple mechanism, because you just take a solvent. put the water in it, it dissolves out the water, contaminants are rejected, you remove that, cool the solvent back down, the water kind of precipitates out – like air is hot, it picks up water, when it cools the water drains out – and we collect that and reuse the solvent. This seemed like a simple technique, but nobody had discovered it before," Bajpayee said.
A March 2011 blog post by Drexel University Associate Professor of Chemistry Jean-Claude Bradley pointed out "From a technological standpoint, I can't think of a reason why this solution could not have been discovered and implemented 100 years ago. It makes you wonder how many other elegant solutions to real problems could be uncovered by connecting the right pieces together."
"It was nice; it was encouraging. It made us keep going," Bajpayee said.
Professor Gang Chen
Bajpayee credits Chen for encouraging his work even though at the beginning it was outside the key areas of Chen’s lab – nano scale heat transfer, energy conversion and thermoelectrics. “It was really his open-mindedness and eagerness to learn a completely new topic and being supportive of a completely new research direction, which made it happen. His support for radical new ideas going in new directions has been incredible and I’ve been extremely appreciative of that. He’s also allowed me tremendous freedom to take the project in my own direction. I felt like we learned about this new topic together.”
Because of the success of the research, desalination has become a part of the group’s focus.
An alternative process
In typical interdisciplinary style, Bajpayee is working with Prakash Govindan, who is part of the Lienhard Research Group at MIT, under Professor John Lienhard, to commercialize their work. Both received their MIT Ph.D.’s in August 2012. Both have won patents.
Govindan developed another process, which uses a carrier gas in a humidification-dehumidification cycle to remove salts and other contaminants from water. The process uses inexpensive materials and minimizes energy consumption through several innovations including a new type of heat exchanger called the multi-stage bubble column.
Professors Chen and Lienhard have also supported efforts to spin out a company to bring the technology to market, Bajpayee said. They’ve also won support from the Deshpande Center and the Entrepreneurship Center at MIT.
A worldwide need
“In places like the Middle East, India and China there is a lot more water stress. They also use a lot less water than the U.S., yet there is a definite need for desalination water purification,” Bajpayee said. “The U.S. especially the southwestern United States where it’s drier and the stress on water is a lot more, it is becoming a big issue, but sea water desalination is a lot bigger issue in the Middle East, China, India, than it even nearly is in the U.S. The Caribbean islands are surrounded by water, but it’s not drinking water, it’s seawater, which you can’t really put to use.”