New purification technique reduces cost of oilwell water cleanup

Increased natural gas production is seen as a crucial step away from the greenhouse gas emissions of coal plants and toward US energy independence. But natural gas wells have problems - large volumes of deep water, often heavily laden with salts and minerals, flow out along with the gas. That so-called ''produced water'' must be disposed of, or cleaned.

Now, a process developed by engineers at MIT could solve the problem and produce clean water at relatively low cost. After further development, the process could also lead to inexpensive, efficient desalination plants for communities in the developing world, the researchers say.

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 vapourised and then condenses on a cold surface; the salt separates out during evaporation. But this process is energy-intensive - and therefore costly - because all the water must be heated to the boiling point, while the condensing surfaces must be kept cold.

In the new process, water well below the boiling point is vapourised by direct contact with a carrier gas; the moist air is subsequently bubbled through cooler water where the purified vapour condenses. But 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.