(378af) A Combined Adsorbent Bed and Pellet Model for Adsorptive Hydrogen Storage | AIChE

(378af) A Combined Adsorbent Bed and Pellet Model for Adsorptive Hydrogen Storage

Authors 

Sridhar, P. - Presenter, Indian Institute Of Technology Madras
Kaisare, N., Indian Institute of Technology-Madras

A
combined adsorbent bed and pellet model for adsorptive hydrogen storage

P.
Sridhar and N.S. Kaisare

Department
of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600036,
India

Micro-porous materials such as metal-organic frameworks (MOFs) and activated
carbons (ACs) are considered as promising adsorbents for storage of hydrogen at
cryogenic and room temperature due to their well-defined structures, high pore volumes, and
high hydrogen uptake capacities. Modeling of hydrogen
adsorption has been extensively studied over various scales: From
molecular-level to characterize adsorption process and energetics, to entire
system-level to investigate discharge and refuelling dynamics. In modelling
refuelling behaviour of adsorbent bed, 2D and 3D models have been popular. Kumar
(2011) investigated the hydrogen
adsorption in MOF-5 flow-through bed using a 2D model. He showed that the bed requires
150 seconds to reach 5 wt% gravimetric capacity. Due
to its low density and poor thermal and structural properties, MOF-5 is often
pelletized and the storage bed contains numerous
such pellets. Ortmann and Kaisare (2015)
investigated refuelling of hydrogen in a single MOF-5 pellet, and observed that
a pellet with 1.5 cm diameter took 15 seconds to achieve 5 wt%
gravimetric capacity. The refuelling time increased as the pellet size
increased. However, the bed-level models have often assumed local equilibrium
and neglected interfacial mass and heat transfer between gas and solid
particles. Furthermore, the effect of meso-scale
processes, such as sorption at the adsorbent site and heat transfer and diffusion
through the pellets, has been neglected in modeling and design of adsorbent
beds.

Therefore, the objective of this work is to investigate multi-scale
behaviour of adsorption bed by combining macroscopic device-level model with
mesoscopic pellet-level model that explicitly incorporate heat and mass
transfer within the pellet and sorption at the adsorption site. Thereby, we
study how pellets at different location
in the bed behave during adsorption of hydrogen
and the effect of mass transport resistances
on the system behaviour.

A
mathematical model consisting of mass, momentum, and energy balance equations for hydrogen sorption tank is solved
simultaneously using COMSOL Multiphysics to obtain the dynamics during refueling of the adsorbent bed. The adsorption in a spherical pellet is studied
using a coupled 1D model of the spherical
pellet. The macro- and micro-scale balance equations are defined in different
coordinate systems: Macroscale part along the length of the bed is set up in 1D
(or 2D) and the mesoscopic part along the
pellet radius, thus resulting in a 1D-plus-1D (or 2D-plus-1D) model. The
concentration distribution in the particle gives the molar flux at every point
along the bed. At the pellet-level, the Knudsen diffusion model is applied
to include mass transfer resistance in the
pellet. Modified Dubinin-Astakhov adsorption isotherm model is applied to study the adsorption behavior with the variation in temperature and
pressure within the pellet. The overall model is solved to investigate the effect
of operating conditions, physical properties, pellet size and other parameters
on refuelling dynamics, and hence optimize the hydrogen storage system.

References:

Kumar, V. Senthil. "A
generalized cryo-adsorber model and 2-D refueling results." International
Journal of Hydrogen Energy
36.23 (2011): 15239-15249.

Ortmann,
Jerome P., and Niket S. Kaisare. "Modeling of cryo-adsorption of
hydrogen on MOF-5 pellets: Effect of pellet properties on moderate pressure refueling." International Journal
of Hydrogen Energy
 41.1 (2016): 342-354.