Frequency response analysis for multicomponent diffusion in adsorbents

Abstract

Frequency response behaviour of diffusion of multiple sorbates in adsorbents is theoretically investigated. A mathematical model describing diffusion of n components with both equilibrium and diffusional interference is developed and analytical solutions for frequency response and moments are derived. It is shown that, for the model system CO₂ and C₂H₆ in 4A zeolite, the fastest diffusing component can be strongly affected by other slower diffusing components and its partial-pressure frequency response curves exhibit a roll-up phenomenon induced exclusively by the diffusional interference. The frequency response curves of the total pressure generally have multi-modal forms as a result of multicomponent diffusion; the peak positions and heights within these responses, which are closely related to the diffusivities, are found to be sensitive to the extent of equilibrium and diffusional interference. Comparison of these resonance frequencies obtained from pure-component and multicomponent experiments can therefore give a good indication of the interference between the components. The effects of film resistance and heat dissipation, which may be important for fast diffusing systems, are also discussed.