Rapid screening of novel nanoporous materials for carbon capture separations
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Date
01/07/2013Author
Mangano, Enzo
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Abstract
In this work the experimental results from the rapid screening and ranking of a wide
range of novel adsorbents for carbon capture are presented. The samples were tested
using the Zero Length Column (ZLC) method which has proved to be an essential
tool for the rapid investigation of the equilibrium and kinetic properties of prototype
adsorbents.
The study was performed on different classes of nanoporous materials developed as
part of the EPSRC-funded “Innovative Gas Separations for Carbon Capture”
(IGSCC) project. More than 120 novel adsorbents with different key features for
post-combustion carbon capture were tested. The classes of materials investigated
were:
• PIMs (Polymers of Intrinsic Microporosity)
• MOFs (Metal - Organic Frameworks)
• Mesoporous Silica
• Zeolites
• Carbons
All the samples were tested at experimental conditions close to the ones of a typical
flue gas of a fossil fuel power plant: 35 ºC and 0.1 bar of partial pressure of CO2.
The results from the ranking of the CO2 capacity of the materials, at the conditions of
interest, indicate the Mg and Ni-based MOF samples as the adsorbents with the
highest uptake among all the candidates. The best sample shows a CO2 capacity
almost double than the benchmark adsorbent, zeolite 13X (provided by UOP). The
ranking also shows some of the zeolite adsorbents synthesised as promising materials
for carbon capture: uptakes comparable or slightly higher than 13X were obtained for
several samples of Rho and Chabazite zeolite.
Water stability tests were also performed on the best MOFs and showed a
deactivation rate considerably faster for the Mg-based MOFs, proving an expected
higher resistance to degradation for the Ni based materials. A focused investigation was also carried out on the diffusion of CO2 in different ionexchanged
zeolites Rho samples. The study of these samples, characterised by
extremely slow kinetics, extended the use of the ZLC method to very slow
diffusional time constants which are very difficult to extract from the traditional long
time asymptotic analysis. The results show how the combination of the full saturation
and partial loading experiment can provide un-ambiguous diffusional time constants.
The diffusivity of CO2 in zeolite Rho samples shows to be strongly influenced by the
framework structure as well as the nature and the position of the different cations in
the framework.
The kinetics of the Na-Cs Rho sample was also measured by the use of the
Quantachrome Autosorb-iQ™ volumetric system. To correctly interpret the dynamic
response of the instrument modifications were applied to the theoretical model
developed by Brandani in 1998 for the analysis of the piezometric method.
The analytical solution of the model introduces parameters which allow to account for the
real experimental conditions. The results confirm the validity of the methodology in
the analysis of slow diffusion processes.
In conclusion the advantages offered by the small size of the column and the small
amount of sample required proved the ZLC method to be a very useful tool for the
rapid ranking of the CO2 capacity of prototype adsorbents. Equilibrium and kinetic
measurements were performed on a very wide range of novel nanoporous materials.
The most promising and interesting samples were further investigated through the
use of the water stability test, the partial loading experiment and the volumetric
system. The ZLC technique was also extended to the measurements of systems with
very slow kinetics, for which is very difficult to extract reliable diffusional time
constants. An improved model for the interpretation of dynamic response curves
from a non-ideal piezometric system was developed.