Abstract
A commercial gravimetric sorption analyzer, which is based on a magnetic-suspension balance, was significantly improved to reduce the uncertainty in adsorption measurements. In a previous paper, we investigated the force-transmission error (FTE) of the instrument’s magnetic-suspension coupling, and we analysed the uncertainty of the density measurement. In the present paper, equations for the determination of the adsorption on porous and quasi non-porous materials are provided, where the FTE is taken into account, and a detailed uncertainty analysis is presented. The uncertainty analysis was applied to both the improved measurement system and a typical commercial gravimetric sorption analyzer. Adsorption test measurements were conducted with carbon dioxide along the T = 283 K isotherm at pressures up to the dew-point pressure using both a porous material (zeolite 13X) and a quasi non-porous material (solid metallic sinkers). The major uncertainty contributions for adsorption on the porous material were the mass and volume of the adsorbent sample and the assumption of the density of the adsorbed fluid; for the quasi non-porous material, the main contributions were the weighing values of the balance, the density of the investigated fluid in the gas phase, and the volume of the non-porous material. The influence of the FTE on the adsorption on the porous material was approximately 0.002 mmol⋅g−1, which was negligibly small; but the influence of the FTE was significant in the case of the quasi non-porous material, i.e., approximately 0.7 mmol⋅m−2 or about 22% of the adsorption capacity with the highest adsorption observed in this work (near the dew-point pressure). This indicates that the influence of the FTE increases significantly with decreasing adsorption capacity of the adsorbent sample.
Highlights
Gas adsorption and desorption naturally occur in various physical, chemical and biological systems
In our previous work (Kleinrahm et al 2019), we took a gravimetric sorption analyzer and improved it to obtain a significantly lower measurement uncertainty; it was essentially turned into a tandem-sinker densimeter, as we named it in that work
For a porous material as an adsorbent, we assume that the true density ρsorp of the adsorbed sample fluid on the surface of the porous zeolite can be considerably larger than the saturated-liquid density at the triple-point temperature of carbon dioxide, especially for the first molecular layer on the surface
Summary
Gas adsorption and desorption naturally occur in various physical, chemical and biological systems. Gensterblum et al (2009) reported the measurement results of an inter-laboratory study among four European research laboratories and showed that when thorough optimization of instrumentation and measurement as well as proper sample preparation procedures were applied, the carbon dioxide sorption on Filtrasorb 400 activated carbon in the supercritical range could be determined accurately with both gravimetric and volumetric instruments. Nguyen et al (2018) reported the results of an international inter-laboratory study led by the National Institute of Standards and Technology on the measurements of high-pressure excess carbon dioxide sorption isotherms on a reference material (ammonium ZSM-5 zeolite) and provided recommendations for optimising the acquisition of physisorption isotherm data including activation procedures, data processing methods to determine adsorption uptake and the appropriate equation of state (EOS) to be used. The uncertainty analysis was applied to both the improved measurement system available at Chemnitz University of Technology and to a hypothetical typical commercial gravimetric sorption analyzer
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