How to clean up oil spills

MIT researchers have developed a new technique for
magnetically separating oil and water that could be used to
clean up oil spills. They believe that, with their technique,
the oil could be recovered for use, offsetting much of the
cost of cleanup.
The researchers will present their work at the International
Conference on Magnetic Fluids in January. Shahriar
Khushrushahi, a postdoc in MIT’s Department of Electrical
Engineering and Computer Science, is lead author on the
paper, joined by Markus Zahn, the Thomas and Gerd
Perkins Professor of Electrical Engineering, and T. Alan
Hatton, the Ralph Landau Professor of Chemical
Engineering. The team has also filed two patents on its
work.
In the MIT researchers’ scheme, water-repellent ferrous
nanoparticles would be mixed with the oil, which could then
be separated from the water using magnets. The
researchers envision that the process would take place
aboard an oil-recovery vessel, to prevent the nanoparticles
from contaminating the environment. Afterward, the
nanoparticles could be magnetically removed from the oil
and reused.
According to Zahn, there’s a good deal of previous research
on separating water and so-called ferrofluids — fluids with
magnetic nanoparticles suspended in them. Typically, these
involve pumping a water-and-ferrofluid mixture through a
channel, while magnets outside the channel direct the flow
of the ferrofluid, perhaps diverting it down a side channel or
pulling it through a perforated wall.
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. Suppose that the separation system consists of a
branching channel with magnets along one side. If the oil
concentration were zero, the water would naturally flow
down both branches. By the same token, if the oil
concentration is low, a lot of the water will end up flowing
down the branch intended for the oil; if the oil concentration
is high, a lot of the oil will end up flowing down the branch
intended for the water.

Orthogonal thinking

The MIT researchers vary the conventional approach in two
major 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.
The magnets are permanent magnets, and they’re
cylindrical. Because a magnet’s magnetic field is strongest
at its edges, the tips of each cylinder attract the oil much
more powerfully than its sides do. In experiments the MIT
researchers conducted in the lab, 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.
The design is simple, but it provides excellent separation
between oil and water. Moreover, Khushrushahi says,
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. “The process may seem
simple,” he says, “but it is, inherently, supposed to be
simple.”
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 kind of
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.

Leaving the lab

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 also
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 says. “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.
“This oil-spill problem has not really been worked on
intensively that I know of, and of course it’s a big problem,”
says Ronald Rosensweig, a former Exxon researcher and a
pioneer in the study of ferrofluids who wrote the field’s first
textbook. “You could think of separating oil from water by
centrifuging or something like that, but in a lot of cases, the
fluids are pretty much equal in density: Some of the oil
sinks, some of it floats, and a lot of it is in between. The
magnetic hook could, hopefully, make separation faster and
better.”
Adding nanoparticles to oil mixed with water to produce a
ferrofluid aboard a ship should be “no problem,”
Rosensweig says. And with a technique called high-
gradient magnetic separation, “It’s known that the gradient
can pull the particles out of suspension,” he says, so
recovering both the nanoparticles and the oil is feasible.
“It’s been done on a small scale,” Rosensweig says.