Windowed Carbon Nanotubes for Efficient CO2 Removal from Natural Gas

We demonstrate from molecular dynamics simulations that windowed carbon nanotubes can efficiently separate CO2 from the CO2/CH4 mixture, resembling polymeric hollow fibers for gas separation. Three CO2/CH4 mixtures with 30%, 50% and 80% CO2 are investigated as a function of applied pressure from 80 to 180 bar. In all simulated conditions, only CO2 permeation is observed; CH4 is completely rejected by the nitrogen-functionalized windows or pores on the nanotube wall in the accessible timescale, while maintaining a fast diffusion rate along the tube. The estimated time-dependent CO2 permeance ranges from 107 to 105 GPU (gas permeation unit), compared with ~100 GPU for typical polymeric membranes. CO2/CH4 selectivity is estimated to be ~108 from the difference in free-energy barriers of permeation. This work suggests that a windowed carbon nanotube can be used as a highly efficient medium, configurable in hollow-fiber-like modules, for removing CO2 from natural gas.