Hybrid membrane separates CO2 with both high permeability and selectivity

Conventional gas-separation membranes normally exhibit a trade-off between selectivity and permeability — a practical limitation referred to as the Robeson upper bound. As a result, membranes designed to selectively remove relatively low concentrations of carbon dioxide from natural gas or fluegas are inherently hampered by a low CO₂ flux through the membrane.

To overcome this constraint, researchers from Sintef (Trondheim, Norway; www.sintef.no), in collaboration with professor Richard Spontak at North Carolina State University (Raleigh; www.ncsu.edu) and professor Liyuan Deng at the Norwegian University of Science and Technology (Trondheim; www.ntou.no), have developed membranes that integrate an ultrathin, highly CO₂-selective surface layer with a highly permeable, but low selectivity, polymer membrane in a concept referred to as hybrid-integrated (HI) membrane technology (diagram). As described in a recent article in Science, the surface layer consists of surface-grown polyamines that concentrate CO₂ from the target gas stream. The CO₂ is quickly transported through the dense polymer membrane and the underlying macroporous support. “The resulting membranes retain high CO₂ permeability with CO₂ selectivities increased by more than 150-fold over the parent membranes,” says Marius Sandru, Sintef researcher and lead author of the study.

In laboratory-scale tests, performed using membranes with ~20 cm² area and gas flowrates of ~30 L/h (fully humidified 10/90 CO₂/N₂ feed typical of fluegas), CO₂ permeabilities and CO₂/N₂ selectivities of up to ~1,000 Barrer and ~1,000, respectively, have been measured. The membranes have also been studied using mixtures of CH₄ and CO₂ (typical of biogas plants), achieving selectivities of up to ~500.

“The next stage is the stepwise upscaling of membrane area,” says Sandru. Membranes with 500-cm² area will be fabricated in the ongoing CO₂-Hybrid project and tested using real fluegas in static tests, followed by field testing in a coal-/natural gas-fired Waste2Energy pilot facility. The goal is to achieve a 90% CO₂ capture efficiency with 95% CO₂ purity.