Comparison of dinitrogen, methane, carbon monoxide, and carbon dioxide mass-transport dynamics in carbon and zeolite molecular sieves

Equilibrium (based on Henry constants) and kinetic (based on relaxation-time constants or rather macropore transport diffusivities) selectivities for commercial zeolite and carbon-molecular-sieve (CMS) adsorbents were compared. Adsorption isotherms were recorded at -20°. The frequency-response (FR) sorption-rate spectra were determined in the range of -78 and 70° at 133 Pa. In particles of a larger size than 1.0 mm, macropore diffusion governed the rate of sorption mass transport in both types of microporous materials. The differences in the intercrystalline diffusivities established the kinetic separation of the gases notwithstanding the essential importance of interactions in the micropores. Zeolites seem to be more advantageous for a dynamic separation of CO₂ and CH₄ than CMS 4A. With the CO₂ and CO pair, the CMS is characterized by short characteristic times which, together with a good separation factor, is a double advantage in a short-cycle adsorption technology. Upon comminution of the carbon pellets, intercrystalline-diffusion resistance can be completely removed by using CMS 4A adsorbent particles with a diameter smaller than 1 mm. The carbonization of spruce-wood cubes resulted in an excellent carbon honeycomb structure, which seems to be ideal from a dynamic point of view for applications in short-cycle adsorption-separation technologies. In the development of adsorbents, the use of the FR method can be beneficial.