The new technology is described in a series of papers recently published in three journals: the International Journal of Heat and Mass Transfer, Applied Energy and the American Institute of Chemical Engineers’ AIChE Journal. The research is the work of a team including MIT postdoc Prakash Narayan, mechanical engineering professor John H. Lienhard V, and collaborators at King Fahd University of Petroleum and Minerals (KFUPM) in Saudi Arabia.

The method is a variation of the standard distillation process, in which salty water is vaporized and then condenses on a cold surface; the salt separates out during evaporation. However, because all the water must be heated to the boiling point, while the condensing surfaces must be kept cold, such a process is energy-intensive and therefore costly.

In the new process, water well below the boiling point is vaporized by direct contact with a carrier gas; the moist air is subsequently bubbled through cooler water where the purified vapor condenses. The temperature difference between the warm and cool water is much less than in conventional dehumidifiers, and the surface area provided by the small bubbles is much greater than that of a flat condenser surface, leading to a more efficient process.

The traditional version of this process is called a humidification dehumidification (HDH) desalination system. While other groups have designed HDH systems, this new version requires far less energy than previous systems, the team says.

Conventional distillation plants have efficiencies of scale, but for the HDH system, the optimum size is a plant that produces about 1,200 to 2,400 liters of clean water a day, about the capacity needed for a rural village.

Over a period of years, our group developed new thermodynamic methods for assessment of these cycles, followed by a deep understanding of the causes of energy inefficiency in them. Eventually, this led us to the new configurations and components that are described in these latest publications.

—John Lienhard

The researchers have already filed for patents on the system, and the team has set up a company to commercialize the process in collaboration with another MIT group that has been pursuing a related system for desalination. While Narayan says the team’s ultimate goal is desalination in developing countries, it turns out that the system is especially well suited for treating the produced water that comes from natural gas wells.

The water and other fluids used to open wells through hydraulic fracturing are a somewhat different matter, requiring other kinds of treatment and disposal. But a much bigger issue for the operators of these wells is the ongoing treatment of produced water, which represents a significant expense. This water is typically several times saltier than seawater, which makes it a particularly good candidate for treatment with the HDH process, Narayan says: Unlike membrane-based desalination systems, this system’s efficiency is unaffected by saltier water. In fact, he adds, “The biggest advantage is when you deal with high salinity.”

A leading journal on desalination technology, Water Intelligence Report, gave the system the highest rating awarded to any system for dealing with produced water.

The MIT team built a 12-foot-high test unit, in the Rohsenow Kendall Heat Transfer Laboratory at MIT, that has run continuously for weeks, producing about 700 liters of clean water a day. They have tested it using barrels of water from natural gas wells to demonstrate that it produces water clean enough to drink.

Their next step is to scale up to a plant about two to three times the size of this initial unit, which calculations show should be an optimal size. Narayan says he expects the first commercial plants could be in operation within about two years.

The authors of these recent papers also included recent MIT graduates Steven Lam and Maximus St. John; MIT graduate students Karim Chehayeb, Ronan McGovern and Gregory Thiel; and professors Mostafa H. Sharqawy and Syed M. Zubair of KFUPM and Sarit K. Das of the Indian Institute of Technology. The work was supported by KFUPM through the Center for Clean Water and Clean Energy at MIT and KFUPM.

Resources

• G. Prakash Narayan, Maximus G. St. John, Syed M. Zubair, John H. Lienhard V (2013) Thermal design of the humidification dehumidification desalination system: An experimental investigation, International Journal of Heat and Mass Transfer, Volume 58, Issues 1–2, Pages 740-748,doi: 10.1016/j.ijheatmasstransfer.2012.11.035

• G. Prakash Narayan, Karim M. Chehayeb, Ronan K. McGovern, Gregory P. Thiel, Syed M. Zubair, John H. Lienhard V (2013) Thermodynamic balancing of the humidification dehumidification desalination system by mass extraction and injection, International Journal of Heat and Mass Transfer, Volume 57, Issue 2, Pages 756-770, doi: 10.1016/j.ijheatmasstransfer.2012.10.068

• Ronan K. McGovern, Gregory P. Thiel, G. Prakash Narayan, Syed M. Zubair, John H. Lienhard V (2013) Performance limits of zero and single extraction humidification-dehumidification desalination systems, Applied Energy, Volume 102, Pages 1081-1090,doi: 10.1016/j.apenergy.2012.06.025

• G. Prakash Narayan, Mostafa H. Sharqawy, Steven Lam, Sarit K. Das and John H. Lienhard V (2012) Bubble columns for condensation at high concentrations of non condensable gas: Heat transfer model and experiments AIChE Journal doi: 10.1002/aic.13944