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Carbon Capture with Nanotubes

Startup Porifera is developing membranes to separate greenhouse gases from smokestacks.
November 30, 2009

Membranes made with carbon nanotubes could reduce the amount of energy needed to capture carbon-dioxide emissions from smokestacks, and therefore cut costs, according to a company that will receive $1 million from the new advanced-research projects agency for energy, Arpa-e, to develop the technology.

The company, Hayward, CA-based Porifera, claims that its carbon-nanotube membranes could capture one billion to three billion tons of carbon dioxide a year and save $10 billion a year compared to existing CO2 capture technology. At this point, however, the work is at an early stage, says Olgica Bakajin, Porifera’s chief technology officer. She expects that it will be another year before the first prototype is ready.

The company hopes to make use of some peculiar properties of nanotubes to capture carbon dioxide. Membranes for capturing CO2 from smokestacks need to have two features. They need to be selective, allowing carbon dioxide to pass through and not the other exhaust gases. This produces a concentrated stream of carbon dioxide that can then be compressed and stored. The membranes also need to be highly permeable–allowing CO2 to pass through freely to minimize the energy needed to pump it.

Carbon nanotube membranes are particularly good for this second property. Gases can move through the interior of nanotubes extremely quickly–at rates 100 times as fast as through conventional membrane materials, according to experiments Bakajin led at Lawrence Livermore National Laboratory. Those results were published in the journal Science in 2006. As a result, membranes based on nanotubes would require far less energy than conventional membranes.

The challenge with carbon nanotube membranes is selectively transporting carbon dioxide and not the other gases in a smokestack. This is particularly difficult because the main component of flue gas, nitrogen has many properties that are very similar to CO2, says Karl Johnson, professor of chemical and petroleum engineering at the University of Pittsburgh. One approach to selecting the carbon dioxide is to bind compounds to the ends of the carbon nanotubes that chemically attract carbon dioxide but not other gases. Attracting the CO2 would create high concentrations of it near the membrane, increasing the amount of carbon dioxide that gets transported through relative to the nitrogen and other flue gases. Attaching these compounds is particularly easy because the ends of nanotubes have open locations for binding with such molecules, Bakajin says.

Bakajin says this has been tried with more conventional membrane materials, but adding compounds for attracting carbon dioxide decreases the permeability of these membranes to the point that they are no longer practical. The extraordinarily high permeability of carbon nanotubes could help with this problem. “We have a lot of permeability to lose,” she says. “If the permeability goes down as much as with other membrane materials, we’re still fine.”

She says the company has identified several promising candidates for modifying the nanotubes, but says the details are proprietary. In addition to selecting one of these, she says, the company is also working out how best to manufacture the carbon nanotube membranes, which includes deciding what material to use to bind the nanotubes together and serve as a support material. “Some have advantages in fabrication, some are better structurally, some are more resistant to harsh environments,” she says. “The more we do it, the more we think of new things to try.”

Bruce Hinds, a professor of chemistry at the University of Kentucky who has also demonstrated the high permeability of nanotube membranes, isn’t convinced that carbon capture is the best use for these membranes, in part because of the challenge of making carbon nanotube membranes selective for carbon dioxide. He’s starting with pharmaceutical applications–such as using the membranes to deliver drugs or to separate chemicals during drug manufacturing. These don’t require large-scale manufacturing, which is good, since large-scale manufacturing of the membranes hasn’t been demonstrated yet. The drug applications also command higher prices, allowing for more expensive materials.

Porifera is also pursuing other potential applications. It recently announced funding from DARPA, the research and development office for the U.S. Department of Defense, for producing portable desalination systems for soldiers. Carbon nanotubes can transport fluids 1,000 times as fast as conventional membranes. In addition to saving energy, such fast transport makes it possible to use much smaller membranes, which are better suited for portable devices.

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