A review of common practices in gravimetric and volumetric adsorption kinetic experiments

Abstract

The availability of commercial gravimetric and volumetric systems for the measurement of adsorption equilibrium has seen also a growth of the use of these instruments to measure adsorption kinetics. A review of publications from the past 20 years has been used to assess common practice in 180 cases. There are worrying trends observed, such as lack of information on the actual conditions used in the experiment and the fact that the analysis of the data is often based on models that do not apply to the experimental systems used. To provide guidance to users of these techniques this contribution is divided into two parts: a discussion of the appropriate models to describe diffusion in porous materials is presented for different gravimetric and volumetric systems, followed by a structured discussion of the main trends in common practice uncovered reviewing a large number of recent publications. We conclude with recommendations for best practice to avoid incorrect interpretation of these experiments.

Fluorinated MIL-101 for carbon capture utilisation and storage: uptake and diffusion studies under relevant industrial conditions

“…The gravimetric instrument settings during the adsorptiondesorption experiments were selected in order to minimize heat transfer and external mass transfer resistances. 21 Firstly, we veried that the uptake experiments were slow enough so as to not mistake mass transfer with instrument response time. Fig.…”

“…21,38 Finally, small amounts of sample were used to avoid bed resistances and heat transfer limitations. 21 We assumed spherical particles for both materials and determined the CO 2 diffusion coefficients in the presence of different water vapour concentrations using the following model (see eqn ( 1)). 21,39 This model is derived from Fick's second law of diffusion and it is a good approximation for slow diffusing systems.…”

Diffusion in Nanoporous Solids in the Focus of IUPAC – A Tribute to Jens Weitkamp

“…Attribution of the macroscopically observed features to their ''microscopic'' origin needs particular care. The spectrum of techniques applicable for this purpose includes uptake and release measurements with loose beds of particles [92], liquid-phase batches [93] and packed-bed columns [94], as well as their ''variants'' such as the zero-length column (ZLC [95]) and the frequency response (FR [96]) techniques. Macroscopic diffusion measurements are also possible via NMR imaging (MRI) [97] and X-ray computed tomography (XCT) [98].…”

“…A given technique of measurement may, correspondingly, under certain conditions happen to be more properly assigned to another category. This refers to, e.g., uptake and release measurements [92,95,96] if performed with only a small amount of adsorbent particles so that bed effects may be excluded. The thus attainable information may indeed be referred to as meso-rather than macroscopic.…”

“…There exist a number of measuring techniques, which obey this concern by their very nature. They include permeation studies [99] and the measurement of uptake and release studies with loose beds of particles [92] and packedbed columns [94].…”