Abstract
A simple, semi-empirical, generalized expression was developed for the LDF mass transfer coefficient k as a function of the half cycle time θ c that encompasses and transitions between the well-known regions governed by the long cycle time constant Glueckauf k and the short cycle time dependent k. This new expression can be used to estimate k = f(θ c ) for any system, irrespective of the loading and irrespective of θ c , no matter if k is in the cycle time dependent region or not. A three times wider transition region between the Glueckauf k and the cycle time dependent k was also established, with the Glueckauf LDF limit now valid for θ c > 0.3 and the short cycle time limit now valid for θ c < 0.01. When evaluating this region for several adsorbate-adsorbent systems, the minimum Glueckauf θ c spanned three orders of magnitude from thousands of seconds to just a few seconds, indicating a cycle time dependent k is not necessarily limited to what is normally considered a short cycle time. For virtually any θ c less than this minimum Glueckauf θ c , this new first-of-its-kind expression can be used to readily provide an accurate value of k = f(θ c ). Since the widely accepted half cycle time concept does not apply to the actual simulation of a multi-step, unequal step time, pressure swing adsorption process, the value of k = f(θ c ) from this new expression can be based on either the shortest cycle step in the cycle or a different value of k = f(θ c ) for each cycle step time in the cycle, with validity confirmed either by experiment or by process simulation using the exact solution to the pore diffusion equation.
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Abbreviations
- a(q):
-
Function of the amount adsorbed in the particle that depends on the type of diffusion process and defined by Eq. (5) or (7)
- b(q):
-
Function of the amount adsorbed in the particle that depends on the type of diffusion process and defined by Eq. (6) or (8)
- b :
-
Affinity parameter in the Langmuir adsorption isotherm, kPa−1
- D :
-
Diffusivity, m2 s−1
- D M,g :
-
Macropore diffusivity, m2 s−1
- D m,s :
-
Micropore diffusivity, m2 s−1
- K :
-
Extracted LDF mass transfer coefficient, s−1
- k′:
-
Glueckauf LDF mass transfer coefficient, s−1
- k″:
-
Alpay and Scott LDF mass transfer coefficient, s−1
- P :
-
Partial pressure of the adsorbate, kPa
- P o :
-
Partial pressure of the adsorbate outside the pellet or crystal, kPa
- P o,i :
-
Initial partial pressure of the adsorbate outside the pellet or crystal, kPa
- Q :
-
Amount adsorbed in the particle, mol kg−1
- \(\bar{q}\) :
-
Volume average amount adsorbed in the particle, mol kg−1
- q i :
-
Initial amount adsorbed in the particle, mol kg−1
- q * :
-
Amount adsorbed in the particle in equilibrium with P o , mol kg−1
- q s :
-
Saturation capacity parameter in the Langmuir adsorption isotherm, mol kg−1
- r :
-
Distance from the center of the particle (pellet or crystal), m
- R :
-
Radius of spherical particle, m
- R g :
-
Ideal gas constant, 8.314 J mol−1 K−1
- R p :
-
Radius of spherical pellet, m
- R c :
-
Radius of spherical crystal, m
- t :
-
Time, s
- t c :
-
Half cycle time, s
- T :
-
Temperature of the particle, K
- ∆P :
-
Amplitude of the pressure perturbation, kPa
- ε p :
-
Pellet porosity
- ρ p :
-
Pellet density, kg m−3
- θ c :
-
Dimensionless half cycle time
- ϑ(q):
-
Function that represents dependency of diffusivity on the amount adsorbed in the particle and equal to b(q)/a(q)
- \(\psi \left( {\sqrt {\theta_{c} } } \right)\) :
-
Correction factor as a function of dimensionless half cycle time, defined by Eq. (26)
- \(\phi \left( {\sqrt {\theta_{c} } } \right)\) :
-
Fermi–Dirac function defined by Eq. (27)
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Funding
The authors gratefully acknowledge financial support provided in part by the DOE National Energy Technology Laboratory, DOE Award Number DE-FE0007639, in part by the SAGE Center at the University of South Carolina, and in part by the Process Science and Technology Center, a consortium composed of the University of Texas at Austin, the University of South Carolina, and the Texas A&M University.
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Hossain, M.I., Ebner, A.D. & Ritter, J.A. New linear driving force correlation spanning long and short cycle time pressure swing adsorption processes. Adsorption 22, 939–950 (2016). https://doi.org/10.1007/s10450-016-9801-1
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DOI: https://doi.org/10.1007/s10450-016-9801-1