Diffusion of Condensable Vapors in Single Adsorbent Particles Measured via Concentration-Swing Frequency Response

Abstract

A concentration-swing frequency response method is extended to examine mass transfer mechanisms and the concentration dependence of mass transfer rates for adsorption of condensable vapors in single adsorbent particles. The adsorption kinetics of water and hexane in BPL activated carbon and the adsorption of water in silica gel are determined at several different concentrations. The mechanism that best describes the adsorption of water in BPL activated carbon is nanopore diffusion. The diffusivity of water in BPL activated carbon has a clear minimum at approximately P/Po = 0.5, and the concentration dependence of the diffusion data are not described well by the Darken relationship. Both nanopore diffusion and the Glueckauf linear driving force models can be used to describe the diffusion of hexane in BPL activated carbon for the pressure range studied, and the dependence of the diffusivity on concentration can be described approximately using the Darken relationship. However, the diffusion of water in silica gel cannot be described by the nanopore diffusion model and is best characterized by the Glueckauf linear driving force model. The results illustrate the ability of concentration-swing frequency response to accurately determine adsorption rate mechanisms and quantify the complex adsorption kinetics of condensable vapors using small quantities of adsorbent.

New development in flow‐through pressure‐swing frequency response method for mass‐transfer study: Ethane in ZIF‐8

“…Indeed, it indicates a more complex concentration dependence, which is different from monotonically increasing transport diffusivity measured by IRM . The very uncommon trend in the loading dependence of diffusivities, i.e., transport diffusivity decrease at low loadings, pass a minimum and increase sharply at higher loadings, was observed for small alcohols in ZIF‐8 and water in activated carbon . The reason has been traced back to pronounced intermolecular attractive forces, which results in a strong S‐shaped isotherm and less than unity thermodynamic factor for wide loading range.…”

“…In addition to studies in batch systems, there are other applications in flow‐through systems involving perturbation in flow rate, concentration, or pressure . Flow‐through FR methods have been explored to provide more practical advantages, such as easily implementing a wide range of frequencies, having the possibility of using large relative amplitudes, and having reduced heat effects .…”

“…However, it has limitation on the maximum frequency, which cannot exceed ∼1 Hz, whereas the maximum frequency in a batch system can go up to 100 Hz. Most recent work using such flow‐through techniques has been developed by LeVan and coworkers on pressure‐swing FR (PSFR) and concentration‐swing FR (CSFR) methods that have been used to study mass‐transfer rates for both pure and mixed gases and vapors for various adsorbents: carbon molecular sieves, silica gels, zeolites, metal‐organic frameworks (MOF), and activated carbon materials . Given that each technique has its own advantages and disadvantages, a combined FR apparatus that can perform VSFR, PSFR, and CSFR experiments has been built to be applied to a wider frequency range than VSFR or PSFR alone.…”

The zero length column technique to measure adsorption equilibrium and kinetics: lessons learnt from 30 years of experience

“…Variants of the ZLC which do not fall in the categories above have been collected in Table 5. There are unconventional systems that measure the adsorbed phase concentration (Banas et al 2005) and concentration-swing frequency response experiments where the sample masses are around 10 mg or below (Wang and LeVan 2008;Glover et al 2008). These are particularly interesting because they demonstrate the potential to measure also transport diffusivities at different concentration levels using a carrier flowrate that oscillates in time.…”

“…A more extended version of the information included in Tables 1, 2, 3, 4 and 5 is provided in the supplementary information. Glover et al (2008) In the remaining sections we discuss briefly the models used for pellets and beads in greater detail; the use of the ZLC to determine the diffusion path; the case of combined surface barrier and diffusion; and show an example of ZLC curves when the adsorbent undergoes structural changes.…”