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Axial dispersion effects with small diameter adsorbent particles

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Abstract

Macropore diffusion is traditionally assumed to control the mass transfer rate in columns packed with zeolite particles in an oxygen production process. While numerous studies have confirmed this assumption for the particle size used in industrial size pressure swing adsorption (PSA) processes, it has not been validated for the much smaller particle size used in rapid PSA (RPSA). Smaller particles improve the mass transfer rate by increasing interfacial area per volume as well as decreasing diffusion distance. Despite this reduction, RPSA simulations often still assume a mass transfer rate solely limited by macropore diffusion. This approach fails to adequately account for the influence of other mass transfer mechanisms whose impact increases due to particle size reduction. This study experimentally demonstrates the dominant mass transfer mechanism is no longer macropore diffusion for the particle size used in RPSA for small scale oxygen production. Depending on the gas velocity, axial dispersion effects either become the limiting mechanism or equally as important as macropore diffusion. It also shows that improperly accounting for axial dispersion effects has a significant impact on the mass transfer coefficient estimation, often measured with breakthrough experiments.

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Abbreviations

c i :

Gas phase concentration of species i, mol/m3

D c :

Micropore diffusivity, m2/s

D K :

Knudsen diffusivity, m2/s

D L :

Axial dispersion coefficient, m2/s

D m :

Molecular gas diffusivity, m2/s

D p :

Macropore diffusivity, m2/s

d p :

Adsorbent particle diameter, m

K :

Dimensionless Henry’s law constant, unitless

k f :

Film mass transfer coefficient, 1/s

k i :

Mass transfer coefficient of component i, 1/s

k overall :

Overall mass transfer coefficient, 1/s

L :

Column length, m

n i *:

Equilibrium value of adsorbed phase concentration of component i, mol/kg

n i :

Average adsorbed phase concentration over an adsorbent particle of component i, mol/kg

Pé :

Péclet number

r c :

Crystal radius, m

r p :

Adsorbent particle radius, m

Re :

Reynolds number

Sc :

Schmidt number

Sh :

Sherwood number

t :

Time, s

t c :

Time for stoichiometric center to leave column, s

u :

Interstitial gas velocity, m/s

u w :

Wave (constant profile) velocity, m/s

z :

Axial coordinate, m

ε b :

Bed/column porosity

ε p :

Particle macroporosity

ρ b :

Column bulk density, kg/m3

ρ p :

Particle density, kg/m3

τ p :

Pore tortuosity

γ 1 , γ 2 :

Axial dispersion constant

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Acknowledgements

The authors gratefully acknowledge financial assistance provided by Cleveland State University. This material is based upon work supported by the National Science Foundation under Grant No. 1126126. The authors further acknowledge Dustin Bowden for his assistance with taking the SEM images.

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Authors and Affiliations

  1. Department of Chemical and Biomedical Engineering, Cleveland State University, Cleveland, OH, 44115, USA

    Aaron Moran, Mihir Patel & Orhan Talu

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  1. Aaron Moran
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  2. Mihir Patel
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  3. Orhan Talu
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Correspondence to Aaron Moran.

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Moran, A., Patel, M. & Talu, O. Axial dispersion effects with small diameter adsorbent particles. Adsorption 24, 333–344 (2018). https://doi.org/10.1007/s10450-018-9944-3

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