New Polymer Membrane Tech Improves Carbon Capture Efficiency

Scientists have discovered a new membrane technique that allows for more efficient CO2 removal from mixed gases like power plant emissions.

“To demonstrate the capability of our new membranes, we looked at mixtures of CO2 and nitrogen, because CO2/nitrogen dioxide mixtures are particularly relevant in the context of reducing greenhouse gas emissions from power plants,” explains Rich Spontak, co-corresponding author of a study publication.

“And we’ve demonstrated that we can vastly improve the selectivity of membranes to remove CO2 while retaining relatively high CO2 permeability.”
“We also looked at mixtures of CO2 and methane, which is important to the natural gas industry,”says Spontak, a Distinguished Professor of Chemical and Biomolecular Engineering and Professor of Materials Science and Engineering at North Carolina State University. 

“In addition, these CO2-filtering membranes can be used in any situation in which one needs to remove CO2 from mixed gases – whether it’s a biomedical application or scrubbing CO2 from the air in a submarine.”

Membranes are an appealing method for removing CO2 from mixed gases since they do not take up much physical space, can be produced in a broad variety of sizes, and can be quickly replaced. Chemical absorption, which involves bubbling mixed gases through a column containing a liquid amine - which removes CO2 from the gas – is another often used CO2 removal process. Absorption methods, on the other hand, have a much bigger environmental impact, and liquid amines are poisonous and corrosive.

CO2 passes through the membrane more quickly than the other constituents in the mixed gas in these membrane filters. As a result, the CO2 content of the gas exiting the membrane is higher than that of the gas entering the membrane. You collect more CO2 than the other constituent gases by capturing the gas traveling through the membrane.

The trade-off between permeability and selectivity has long been a problem for such membranes. The greater the permeability, the faster gas may pass across the membrane. However, as permeability increases, selectivity decreases, meaning that nitrogen and other elements flow through the membrane more quickly, lowering the CO2 to other gas ratio in the mixture. To put it another way, when selectivity decreases, you capture less CO2.

The researchers from the United States and Norway solved the challenge by developing chemically active polymer chains on the surface of existing membranes that are both hydrophilic and CO2-philic. This boosts CO2 selectivity while causing only minor permeability loss.

“In short, with little change in permeability, we’ve demonstrated that we can increase selectivity by as much as about 150 times,” says Marius Sandru, co-corresponding author of the paper and senior research scientist at SINTEF Industry, a Norwegian independent research organization. “So we’re capturing much more CO2, relative to the other species in gas mixtures.”

Another issue that membrane CO2 filters face is cost. Previous membrane technologies tended to be more expensive the more effective they were.

“Because we wanted to create a technology that is commercially viable, our technology started with membranes that are already in widespread use,” explains Spontak. “We then engineered the surface of these membranes to improve selectivity. And while this does increase the cost, we think the modified membranes will still be cost-effective.”

“Our next steps are to see the extent to which the techniques we developed here could be applied to other polymers to get comparable, or even superior, results; and to upscale the nanofabrication process,” Sandru says. “Honestly, even though the results here have been nothing short of exciting, we haven’t tried to optimize this modification process yet. Our paper reports proof-of-concept results.”

Other applications of the unique membrane technology that the researchers are interested in investigating include biomedical ventilator devices and aquaculture filtration systems.

The researchers say they're willing to collaborate with industry partners on any of these challenges or opportunities to help minimize global climate change while also improving gadget functionality.
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