The transport and separation of hydrogen and light alkanes are studied in a molecular-sieve carbon membrane hollow-fiber module at the temperature range of 25-400°C; nitrogen is used as a sweeping gas in the study of mixtures, and the fluxes of pure components are studied under a pressure gradient. The membrane selectivity, the ratio of hydrogen to hydrocarbon permeabilities, may reach 100 to 1,000 in propane, or in (normal or iso-) butane mixtures with hydrogen, making the membrane an excellent candidate for a membrane dehydrogenation reactor. The permeabilities measured in pure-component studies differ from those in mixtures. Specifically, counterdiffusion of nitrogen and C2 to C4 alkanes significantly inhibits the fluxes of both, whereas the hydrogen flux is only slightly diminished. To account for these results, molecular mechanics simulations are used to find the energetics of adsorption, diffusion, and desorption of individual gases in cylindrical nanopores modeled by carbon nanotubes. In pore sizes that are up to 2-3 times the dimension of the molecule, diffusion of the molecule inside the pore is nonactivated, whereas desorption is activated and typically is the rate limiting step; the molecular transport proceeds essentially by the single-file diffusion mechanism. A rate expression for a single-species transport in a molecular-sieve carbon membrane is derived by a mean-field approach. The pore-size distribution calculated by comparing experimental and computed fluxes favorably compares with the measured distribution.
- Carbon membrane
- Carbon nanotube
- Molecular sieves
- Molecular transport and separation