Magnetically separating oil and water
12 Sep 2012
MIT researchers have developed a new technique for magnetically separating oil and water that could be used to clean up oil spills.
According to the researchers, their technique allows the oil to be covered for use, offsetting much of the cost of cleanup.
The researchers envision that the process would take place aboard an oil-recovery vessel, to prevent the nanoparticles from contaminating the environment.
Afterwards, the nanoparticles could be magnetically removed from the oil and reused.
Separating water and ferrofluids typically involves pumping a water-and-ferrofluid mixture through a channel, while magnets outside the channel direct the flow of the ferrofluid.
This approach can work if the concentration of the ferrofluid is known in advance and remains constant. But in water contaminated by an oil spill, the concentration can vary widely.
The process
The MIT researchers vary the conventional approach in two ways: They orient their magnets perpendicularly to the flow of the stream, not parallel to it; and they immerse the magnets in the stream, rather than positioning them outside of it.
During lab tests, the bottoms of the magnets were embedded in the base of a reservoir that contained a mixture of water and magnetic oil; consequently, oil couldn’t collect around them.
The tops of the magnets were above water level, and the oil shot up the sides of the magnets, forming beaded spheres around the magnets’ ends.
Shahriar Khushrushahi, who worked on the project, said simplicity is an advantage in a system that needs to be manufactured on a large scale and deployed at sea for days or weeks, where electrical power is scarce and maintenance facilities limited.
In their experiments, the MIT researchers used a special configuration of magnets, called a Halbach array, to extract the oil from the tops of the cylindrical magnets. When attached to the cylinders, the Halbach array looks like a model-train boxcar mounted on pilings.
The magnets in a Halbach array are arranged so that on one side of the array, the magnetic field is close to zero, but on the other side, it’s roughly doubled. In the researchers’ experiments, the oil in the reservoir wasn’t attracted to the bottom of the array, but the top of the array pulled the oil off of the cylindrical magnets.
Scale-up
Whether the Halbach array would be the most practical way to remove oil from the cylindrical magnets in an actual oil-recovery system remains to be seen.
The researchers need to determine how much water gets dissolved in the oil, and how it can best be removed. “To our eye, you don’t see much moisture in there, but I’m sure that there is some moisture that adheres to it,” Zahn said. “We might have to run it through multiple cycles.”
On a commercial scale, it could make sense for an oil-recovery vessel to perform an initial separation of oil and water and then haul the oil ashore for further refinement.
