Pore-scale investigation on coupled diffusion mechanisms of free and adsorbed gases in nanoporous organic matter

Qu, Zhiguo; Yin, Ying; Wang, H.; Zhang, J.F.

doi:10.1016/j.fuel.2019.116423

Cited by 37 publications

(20 citation statements)

References 50 publications

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“…Nevertheless, in practical applications, both processes are modeled by using the same equilibrium and kinetics equations. A comprehensive review on the mechanisms and models used in adsorption and ion exchange is provided by Inglezakis et al It is noteworthy that in recent literature papers, pore-scale models (such as the local coupled diffusivity lattice Boltzmann model) have been developed to describe the complex diffusion pattern occurring in fluid–solid adsorption. , Adsorption in microporous solids is typically controlled by the intraparticle diffusion step as film diffusion and adsorption steps are generally much faster . On the other hand, liquid phase mass transfer depends on several factors and, under a combination of inadequate liquid-phase flow/mixing conditions and fast intraparticle diffusion, film diffusion can represent significant resistance to the overall rate of the process.…”

Section: Introductionmentioning

confidence: 99%

Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

Inglezakis

Balsamo

Montagnaro

2020

Ind. Eng. Chem. Res.

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In liquid–solid adsorption, fluid film diffusion is typically faster than intraparticle diffusion, especially for microporous adsorbents. However, fluid film diffusion might play a significant role in the overall rate of the process for mesoporous–macroporous and non-porous solids. In most adsorption modeling studies, the fluid film diffusion step is typically ignored, which is not always justified. This article critically discusses the theory behind the liquid–solid mass-transfer coefficient in stirred vessels and presents the dissolution and adsorption methods adopted for estimating its value. Then, starting from the definition of the Biot number, an original analysis is developed with reference to selected studies. Surface versus pore diffusion of the adsorbate in the adsorbent is taken into account, and external versus internal mass-transfer resistance is considered to put the fluid film resistance back in the picture when needed. Iso-Biot charts where the operating points can be visualized are presented as well.

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Section: Introductionmentioning

confidence: 99%

Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

Inglezakis

Balsamo

Montagnaro

2020

Ind. Eng. Chem. Res.

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“…Besides, Figure a,b,d,e also shows that the conversion and selectivity increase with the pore diameter before reaching a plateau at d p ≈ 500 nm. When the pore diameter is less than 500 nm, the Knudsen number , is larger than 0.13, indicating Knudsen diffusion still plays an important role. In this case, increasing the pore diameter would increase the Knudsen diffusivity significantly (see eq ).…”

Section: Resultsmentioning

confidence: 99%

“…The effect of macropore diameter is slight, and this effect becomes negligible when d M ≥ 1000 nm. When d M ≥ 1000 nm, the Knudsen number 37,38 is less than 0.06, and thus Knudsen diffusion is negligible. In this case, molecular diffusion is dominant in macropores, and this diffusion is independent of macropore diameter.…”

Section: Effect Of Pore Structure In Bidisperse Catalyst Pelletsmentioning

confidence: 99%

Computational Design of Pore Structure in Heavy Paraffin Dehydrogenation Catalyst Pellets

Zhang

Weng

et al. 2022

Ind. Eng. Chem. Res.

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A multiscale model is developed to describe the coupled flow, diffusion, heat-transfer, and reaction processes at catalyst pellet and fixed bed reactor levels for heavy paraffin dehydrogenation. With this model, the pore structures of monodisperse, bidisperse, and egg-shell catalyst pellets are optimized to improve the yield of mono-olefin under two assumptions about intrinsic activity. The optimal pore structure is determined by compromising between the void space for diffusion and the catalyst volume or surface area for reactions. The optimized monodisperse and bidisperse catalysts show an improvement of 50.7–83.6% in the yield relative to the benchmark catalyst. In addition, using the egg-shell structure for the optimized monodisperse and bidisperse catalysts would not further significantly improve the yield, as their diffusion limitation is very weak. There results should serve to guide the design of catalyst pellets for heavy paraffin dehydrogenation and even for other reaction systems.

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“…Another method to estimate the gas diffusivity at the nanoscale is to empirically divide the gas diffusion behaviors into different regimes by introducing a Knudsen number ( Kn = λ 0 / d p , the ratio of the unbound mean free path and the local pore size, where the local pore size is the channel height for a channel and the pore diameter of a spherical pore). The gas diffusivity in a specific regime is then calculated by the corresponding diffusion formulas. , It allows the gas diffusion to be classified into three regimes, i.e., bulk diffusion or Fick diffusion ( Kn ≤ 0.1), transition diffusion (0.1 < Kn < 10), and Knudsen diffusion ( Kn ≥ 10) according to the magnitude of Kn . , This method is primarily applied to describe the gas diffusivity in a simple structure, such as a nanochannel or nanotube, because of the ease in determining Kn . Recently, some researchers have extended the application to nanoporous media.…”

Section: Introductionmentioning

confidence: 99%

Visualizing Gas Diffusion Behaviors in Three-Dimensional Nanoporous Media

Yin

Zhu

et al. 2021

Energy Fuels

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Gas diffusion in nanoporous media is significantly different from its bulk counterpart owing to the nanoconfinement effect. In this study, a local effective diffusivity lattice Boltzmann model (LED-LBM) is proposed to explore the gas diffusion behavior under nanoconfinement. The nanoconfinement effect is incorporated into the local effective diffusivity via a spatial position-dependent mean free path. The proposed LED-LBM is validated against molecular dynamics simulation results for methane gas diffusion in a nanoscale channel. Using this model, the spatial variations in the local effective diffusivity and gas mass flux in 3D nanoporous media are visualized, and the predominant factors influencing their variation and gas diffusivity are disclosed. The results show that the spatial variation features of the local effective diffusivity and gas mass flux are strongly influenced by the pore shape and average Knudsen number. The gas diffusivity in nanoporous media is significantly lower than its bulk counterpart owing to the nanoconfinement effect, and their differences increase with the average Knudsen number. To quantify the magnitude of the nanoconfinement effect, a new concept, namely, the confinement scope, is further defined. Finally, a semiempirical formula is established to predict the gas diffusivity in nanoconfined porous media, which is expressed as a function of the bulk diffusivity, average Knudsen number, and porosity.

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Pore-scale investigation on coupled diffusion mechanisms of free and adsorbed gases in nanoporous organic matter

Cited by 37 publications

References 50 publications

Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

Computational Design of Pore Structure in Heavy Paraffin Dehydrogenation Catalyst Pellets

Visualizing Gas Diffusion Behaviors in Three-Dimensional Nanoporous Media

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