Investigation of Mixture Diffusion in Nanoporous Adsorbents via the Pressure-Swing Frequency Response Method. 1. Theoretical Treatment

A new model has been developed for investigation of mixture diffusion in nanoporous adsorbents by the pressure-swing frequency response method; that is, the total flow rate out of an adsorption bed is measured, while the system with a constant feed rate is subjected to a sinusoidal perturbation of pressure. The model takes into account both nanopore diffusion and a surface barrier resistance for three kinds of geometric microparticles: slab, cylinder, and sphere. To describe accurately the mole fraction variation caused by mixture diffusion, the adsorption bed is characterized by three spatially uniform regions: inlet, adsorption section, and outlet. The model is linearized, and analytical solutions for the whole system are derived by decoupling rate equations, which are connected through cross-term Fickian diffusivities. It is shown that, for the model binary system CO2 and C2H6 in 4A zeolite, the mole fraction changes slightly for the flow system investigated. At high frequencies, the difference between the mole fractions in the adsorption and outlet regions becomes significant. The total transfer function is affected by both diffusional and equilibrium interference.