The zero length column technique has been developed over the past 30 years as a versatile experimental method to measure adsorption equilibrium and kinetics. In this review we discuss in detail the ...theory that forms the basis for the technique in order to understand how to design and operate efficiently a system. Experimental checks that should be performed to ensure the correct interpretation of the dynamic response are presented and examples are used to identify how to avoid major errors in determining diffusion time constants. The review concludes with an overview of all experimental studies available in the literature to date and a set of recommendations that should help improve the standard in the reported equilibrium and kinetic properties.
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.
Sorption kinetics: measurement of surface resistance Ruthven, Douglas M.; Kärger, Jörg; Brandani, Stefano ...
Adsorption : journal of the International Adsorption Society,
07/2021, Letnik:
27, Številka:
5
Journal Article
Recenzirano
The problem of measuring sorption kinetics in microporous adsorbents and distinguishing experimentally between surface resistance and internal diffusion is discussed and reviewed with reference to ...several commonly used experimental techniques; direct uptake rate measurements, zero length column (ZLC), frequency response, PFG NMR and interference microscopy.
Diffusion mechanism of CO2 in 13X zeolite beads Hu, Xiayi; Mangano, Enzo; Friedrich, Daniel ...
Adsorption : journal of the International Adsorption Society,
01/2014, Letnik:
20, Številka:
1
Journal Article
Recenzirano
Odprti dostop
A systematic study of the diffusion mechanism of CO
2
in commercial 13X zeolite beads is presented. In order to gain a complete understanding of the diffusion process of CO
2
, kinetic measurements ...with a zero length column (ZLC) system and a volumetric apparatus have been carried out. The ZLC experiments were carried out on a single bead of zeolite 13X at 38 °C at a partial pressure of CO
2
of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. These experiments are without a carrier gas and the low pressure measurements show clearly Knudsen diffusion control in both the uptake cell and the bead macropores. At increasing CO
2
concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Both sets of experiments are consistent with independent measurements of bead void fraction and tortuosity and confirm that under the range of conditions that are typical of a carbon capture process the system is controlled by macropore diffusion mechanisms.
Martin Bülow: response Ruthven, Douglas M.; Kärger, Jörg; Brandani, Stefano ...
Adsorption : journal of the International Adsorption Society,
08/2021, Letnik:
27, Številka:
6
Journal Article
The adsorption of CO2 on zeolite Li-Rho (unit cell composition Li9.8Al9.8Si38.2O96) has been investigated by the measurement of adsorption isotherms (273–300 K), breakthrough curves with a ...CO2/CH4/He mixture (308 K), and in situ synchrotron X-ray powder diffraction in CO2 (298 K). The Rho framework distorts when in the Li-form to give a shape selective adsorbent for CO2 over CH4, although breakthrough curves reveal diffusional limitations. In situ synchrotron powder XRD follows the expansion of the Li-Rho unit cell upon adsorption, which remains single phase to a CO2 pressure of ca. 0.6 bar. Partial cation exchange of Li-Rho by Na+ or Cs+ gives two series of M,Li-Rho zeolites (M = Na, Cs). Where the occupancy of window sites (8R, D8R) between lta cages is less than 50%, hysteresis is not observed in CO2 isotherms at 298 K. For Cs1.8Li8-Rho, which has a larger unit cell and a wider window than zeolite Li-Rho due to the presence of large Cs+ cations in double 8-ring sites, breakthrough curves indicate faster CO2 diffusion without significant loss of selectivity. We propose this control of adsorption kinetics of the flexible zeolite Rho via modification of cation content as a mechanism for cation controlled molecular sieving.
In this study, the adsorption properties of a Stöber silica‐based material towards CO2 were evaluated for the first time. The use of Stöber silica as support is interesting for real technological ...applications mainly due to economic factors. Furthermore, a direct comparison between the non porous Stöber sample with an ordered porous material (based on MCM‐41 silica) allowed to evaluate the effect of the porosity on the CO2 adsorption properties. In order to make silica materials reactive towards CO2, they were functionalized by introducing amino groups via grafting of 3‐2‐(2‐aminoethyl)aminoethylaminopropyltrimethoxysilane. After a qualitative study of the CO2 adsorption, the quantitative determination of CO2 adsorption capacity at 35 °C revealed that the mesoporous material is more efficient compared to the Stöber‐based one (adsorption capacity values of 0.49 and 0.58 mol/kg for Stöber‐based and mesoporous samples). However, since the difference in the adsorption capacity is only about 15 % and the Stöber‐based sample is considerably cheaper, the non‐porous sample should be considered as a favourable adsorbent material for CO2 capture applications.
Finding the right porosity: The CO2 adsorption properties of a cheap Stöber‐based adsorber have been investigated and compared with those of a MCM‐41‐based material, traditionally used for CO2 capture applications.
Respiratory droplets are the primary transmission route for SARS-CoV-2, a principle which drives social distancing guidelines. Evidence suggests that virus transmission can be reduced by face ...coverings, but robust evidence for how mask usage might affect safe distancing parameters is lacking. Accordingly, we set out to quantify the effects of face coverings on respiratory tract droplet deposition. We tested an anatomically realistic manikin head which ejected fluorescent droplets of water and human volunteers, in speaking and coughing conditions without a face covering, or with a surgical mask or a single-layer cotton face covering. We quantified the number of droplets in flight using laser sheet illumination and UV-light for those that had landed at table height at up to 2 m. For human volunteers, expiratory droplets were caught on a microscope slide 5 cm from the mouth. Whether manikin or human, wearing a face covering decreased the number of projected droplets by less than 1000-fold. We estimated that a person standing 2 m from someone coughing without a mask is exposed to over 10 000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single-layer mask. Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies. However, the methodologies we employed mostly detect larger (non-aerosol) sized droplets. If the aerosol transmission is later determined to be a significant driver of infection, then our findings may overestimate the effectiveness of face coverings.
The SARS-CoV-2 virus is primarily transmitted through virus-laden fluid particles ejected from the mouth of infected people. Face covers can mitigate the risk of virus transmission but their outward ...effectiveness is not fully ascertained. Objective: by using a background oriented schlieren technique, we aim to investigate the air flow ejected by a person while quietly and heavily breathing, while coughing, and with different face covers. Results: we found that all face covers without an outlet valve reduce the front flow through by at least 63% and perhaps as high as 86% if the unfiltered cough jet distance was resolved to the anticipated maximum distance of 2-3 m. However, surgical and handmade masks, and face shields, generate significant leakage jets that may present major hazards. Conclusions: the effectiveness of the masks should mostly be considered based on the generation of secondary jets rather than on the ability to mitigate the front throughflow.
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