Enhanced water vapor separation by temperature-controlled aligned-multiwalled carbon nanotube membranes†
Here we present a new strategy of selectively rejecting water vapor while allowing fast transport of dry gases using temperature-controlled aligned-multiwalled carbon nanotubes (aligned-MWNTs). The mechanism is based on the water vapor condensation at the entry region of nanotubes followed by removing aggregated water droplets at the tip of the superhydrophobic aligned-MWNTs. The first condensation step could be dramatically enhanced by decreasing the nanotube temperature. The permeate-side relative humidity was as low as ∼17% and the helium–water vapor separation factor was as high as 4.62 when a helium–water vapor mixture with a relative humidity of 100% was supplied to the aligned-MWNTs. The flow through the interstitial space of the aligned-MWNTs allowed the permeability of single dry gases an order of magnitude higher than the Knudsen prediction regardless of membrane temperature. The water vapor separation performance of hydrophobic polytetrafluoroethylene membranes could also be significantly enhanced at low temperatures. This work combines the membrane-based separation technology with temperature control to enhance water vapor separation performance.