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Multi-cell model for pressure swing adsorption process

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Abstract

Pressure Swing Adsorption process is a discrete–continuous system by nature and it is extremely time consuming to simulate steady state performance for a given set of design and operating parameters. A multitude of design variations is offered by the configuration of Pressure Swing Adsorption cycle in terms of choice, sequence, and durations of various possible component steps implemented on two or more adsorber beds. Often, simplifying assumptions are made to speed up each simulation. These assumptions erode the quality of match between reality and simulation and make the resultant design approximate. Use of assumptions like no adsorption/desorption during the pressurization and blowdown steps, constancy in volumetric flow during the adsorption and purge steps makes the model computationally lighter but raises questions on its predictive power. A new modeling approach, namely Multi-cell Model is presented in this work. It is shown to avoid the extensive time taken with equation-based simulations and to have better predictive power. The model is used to study a representative Pressure Swing Adsorption process for nitrogen enrichment from the air. Numerical convergence with respect to the spatial and temporal step sizes and mass balance closure is verified. The model is generic in nature and is valid for any multi-bed, multi-adsorbent, multi-component Pressure Swing Adsorption process executing any combination of component steps.

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Abbreviations

Ab :

Cross-sectional area of the bed (m2)

bi :

Langmuir isotherm constant of ith component (m3/mol)

C3 :

Third component in the study

Cv :

Valve coefficient (unit-less)

db :

Diameter of bed (m)

dp :

Particle diameter (m)

D:

Distribution coefficient

e:

Tolerance level used in the CSS determination

ki LDF :

LDF coefficient of component ‘i’ (1/s)

L:

Height of the adsorbent in the bed (m)

∆Lk :

Height of kth L-cell (m)

Lfeed :

Height of bed up to which feed is present (m)

m:

Total number of cells in the bed (Multi-cell Model)

M:

Total number of temporal divisions

n:

Number of component

Nk :

Total number of moles present kth cell

Pbed :

Pressure in the after IBPE

Pk :

Pressure in kth cell (bar)

qi :

Solid phase concentration of ith component (mol/m3 of adsorbent)

qs :

Monolayer saturation capacity of components (mol/m3 of adsorbent)

q* :

Adsorption equilibrium concentration of ‘ith’ component in the solid (m3/mol)

R:

Ideal gas law constant (J/mol/k)

t:

Duration of a PSA step (s)

∆t:

Size of time step, used in multi-cell modeling (s)

T:

Operating temperature (K)

u:

Superficial linear velocity of the mixture in the bed (m/s)

Vads :

Volume of adsorbent (m3)

Vbulk :

Bulk volume, void volume (m3)

∆x:

Height of bed that holds unit mole of gas in bulk phase (m)

yi :

Bulk phase mole fraction of ith component (unit-less)

∆Zk :

Height of kth Z-cell (m)

ads:

Adsorbent phase

f:

Feed

i:

Index used for component, ‘1’ means strongly adsorbing component

in:

Inlet stream

j:

Index for temporal positions

k:

Index for spatial position

out:

Outlet

r:

Raffinate

γ:

Cycle index

ρ:

Density of the fluid mixture (kg/m3)

ρp :

Density of the particle (kg/m3)

CMS:

Carbon molecular sieve

CSS:

Cyclic steady state

DE:

Desorption to extract

DR:

Desorption to raffinate

FA:

Feed adsorption

IBPE:

Instantaneous bulk phase pressure equalization

ID:

Incubation

PE:

Pressure equalization

PF:

Feed pressurization

PR:

Pressurization with raffinate

RP:

Raffinate purge

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Authors and Affiliations

  1. Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, 400076, India

    Satyanjay Sahoo, Anshu Shukla & Arun S. Moharir

Authors
  1. Satyanjay Sahoo
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  2. Anshu Shukla
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  3. Arun S. Moharir
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Correspondence to Arun S. Moharir.

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Sahoo, S., Shukla, A. & Moharir, A.S. Multi-cell model for pressure swing adsorption process. Adsorption 23, 515–534 (2017). https://doi.org/10.1007/s10450-017-9865-6

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  • DOI: https://doi.org/10.1007/s10450-017-9865-6

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