A Multiscale Approach for Modeling Oxygen Production by Adsorption
Oxygen production processes using adsorbents for application to CCS technologies (Carbon Capture and Storage) offer potential cost benefits over classical cryogenics. In order to model adsorption processes an approach using three size scales has been developed. This wor...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
EDP Sciences
2013-10-01
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| Series: | Oil & Gas Science and Technology |
| Online Access: | http://dx.doi.org/10.2516/ogst/2012102 |
| Summary: | Oxygen production processes using adsorbents for application to CCS technologies (Carbon
Capture and Storage) offer potential cost benefits over classical cryogenics. In order to
model adsorption processes an approach using three size scales has been developed. This
work is being conducted in the framework of the DECARBit European research project. The
first scale is at the size of the oxygen adsorption bed to be modelled as a vertical
cylinder filled with pellets. Its length is 0.2 m (scale 10-1 m). The bed is
homogeneous in the transversal direction so that the problem is 1D (independent variables
t, x). The physics in the process include gas species (Cbk (t,
x)) convection and dispersion, thermal convection and conduction (T(t, x)) and
hydrodynamics (v(t, x)). The gas constituents involved are N2, 02,
CO2 and H2O.
The second scale is at the size of the pellets that fill the adsorber and which are
assumed to be of spherical shape with a typical radius of 5 mm (scale 10-3 m).
The independent variable for the pellets is the radius “rp”.
At a certain height (x) down in the adsorber all the pellets are the same and are
surrounded by the same gas composition but inside the pellets the concentrations may vary.
The state variables for the inner part of the pellets are the gas concentrations
Cpk(t, x, rp). The pellets are
so small that they are assumed to have a uniform temperature. This leads to a 2D transient
model for the pellets linked to the 1D transient model for the bulk.
The third scale looks into the detailed structure of the pellets that are made of
perovskite crystallites. The latter are assumed to be spherical. Oxygen adsorption occurs
in the crystallites which have a radius of about 0.5 pm (scale 10-7 m). All the
crystallites at the same radius in a pellet are supposed to behave the same and because
they are spherical, the only independent variable for a crystallite located at (x,
rp) is its radius “rc”. The
state variables for the crystallites are then the adsorbed oxygen concentration
Cc2 (t, x, rp,
re). The crystallites are so small that they are assumed
to have a uniform temperature. This leads to a third transient model that is 3D for the
crystallite and is linked to the 2D transient model for the pellets which is itself linked
to the 1D transient models for the bulk. From the larger to the lower scales, the links
between the three models are the following: the bulk concentration and temperature give
the boundary conditions surrounding the pellets. The pellet concentration gives the
boundary conditions for the crystallites.
We chose to solve this multiscale approach that requires the coupling of models of
different dimensions in Comsol Multiphysics. The simulator was built to gain knowledge
from laboratory experiments in order to estimate whether oxygen separation from air is
realistic or not. |
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| ISSN: | 1294-4475 1953-8189 |