Thin membranes could help people go to Mars
and clean the air here on Earth.
by Karen Miller
The ideal technology
for space travel would be simple, robust, reliable, lightweight,
and volumetrically efficient. It would have no moving parts, which
would make it less likely to break. It would be a passive technology,
not requiring any energy from the outside. It would be small. It
would be light. An ideal technology for space, says chemical engineer
Doug Way, is the membrane.
Well, OK, membranes
can't do everything. Membranes won't boost us into space. And they
won't carry us to Mars. But membranes could solve some of the problems
of travelling there. And once we arrive, they could help us get back.
are a semi-permeable barrier. They're like a wall, except that gases,
and even liquids, can seep through them. But - here's the key point - different
molecules move through membranes at different rates. Membranes can
therefore be used to sort things out, separating one type of molecule
Doug Way of the Colorado
School of Mines and Lockheed engineer Larry Mason are working on
a project that uses membranes to help produce rocket fuel from the
Martian atmosphere. The principle is simple: The Martian atmosphere
is 95% carbon dioxide (CO2). Using membranes, explorers
could extract some of that CO2, which when mixed with
hydrogen and then heated yields methane - a useful propellant for
rockets or rovers.
Water is a byproduct
of this type of methane production, called the Sabatier process
(discovered by the French chemist Paul Sabatier in the nineteenth
century). What's more, water can be electrolyzed into oxygen, for
breathing, and hydrogen, which can be used to produce another round
Image credit: Doug Way (Colorado School of
Mines) and Larry Mason (Lockheed Martin).
inside a test cell
Although the Martian
atmosphere is almost pure CO2, it's not pure enough for
the Sabatier process. Carbon dioxide must be separated from the
other atmospheric gases before it's processed. Otherwise unused
gases - mostly nitrogen and argon - build up, and will eventually
keep the procedure from working. Way and Mason are developing a
membrane that will separate out CO2.
The specialized polymers
that make up these membranes, some of which were developed at the
Idaho National Environmental and Engineering Laboratory, are engineered
to increase carbon dioxide solubility. "We add in groups of molecules
that are polar - they carry an electric charge," says Way. Because
carbon dioxide molecules are also polar, they're attracted to charged
groups in the membrane.
The membranes are tested
in a special chamber that simulates the Martian environment, explains
Larry Mason. The device, which is about a metre high, is divided
into two compartments. One contains a Mars-like atmosphere, and
the other side contains a vacuum. They‚€™re separated by a membrane
that's about one square inch in surface area. A mass spectrometre
measures how easily each gas moves into the vacuum side.
"In the best [membrane]
material we've found," says Way, "at Martian conditions, CO2
was transferred across the membrane about 50 times faster than nitrogen."
diagram of the Membrane Test Facility at Larry Mason's
Lockheed Martin laboratory.
"Right now," adds Mason,
‚€œwe‚€™re screening different candidate materials to find the ones
that permeate CO2 the best. Once we find that, we can
concentrate on getting enough through in an appropriate amount of
time, by changing the amount of area, packaging it, and so on."
The researchers want
to design a device that produces a gas that's 99.8 percent CO2
at a rate of 2.5 liters per minute. To do that, Way says, will require
quite a bit of membrane. Although the membrane is very thin - about
25 microns, one-quarter of the diametre of a strand of hair - it
will probably need to be about 300 square feet in area, the size
of a small room. All that will have to fit into a package of about
1 square foot.
But a membrane that
separates CO2 from other gases can do more than provide
the raw material for rocket fuel. "This is fundamental technology,"
says Mason. "It's got all kinds of uses."
might help reduce carbon dioxide emissions from factory
It could, for example,
be used to filter air on the space station or on a spaceship bound
for Mars. Carbon dioxide, which is a waste product of our metabolism,
must continually be removed from the atmosphere of self-contained
spacecraft. Membranes that are permeable only to carbon dioxide
would be perfect, says Mason. "The CO2 would just passively
go through the membrane into a holding chamber - or out into space.
Oxygen and other gases would stay intact inside the habitat."
These membranes could
potentially help slow global greenhouse warming, too. "There's some
thought," says Mason, "that a membrane could be used in extracting
CO2 from factory smoke stacks - reducing the amount of
carbon dioxide that's dumped into the atmosphere." Such an application
still lies in the future, he says.
"The biggest potential
Earth-application," adds Way, "is the removal of CO2
from natural gas. CO2 is the most common contaminant
in natural gas besides water vapour. Membrane separations
are one of the primary processes used to filter natural gas so that
it meets pipeline specifications of less than 2% CO2."
This is a big deal because "the natural gas industry is huge - more
than 100 billion dollars per year in retail value," according to
To Mason, "the most
exciting part of this technology is the fact that it may leverage
us to actually go to Mars and live and work there someday." And,
in the meantime, there are plenty of uses for it right here on Earth.