Global challenges of capturing carbon dioxide

Within this general context, this talk will consider the use of novel nanoporous materials as the basis for adsorption based separations [3] that will range from concentrated mixtures to direct capture of carbon dioxide from air. An overview of different classes of materials will show how these can be tailored to such a wide range of conditions. The sheer scale of the task leads to having to optimize systems and speed up processes, which in turn brings in diffusion limitations

Rapid screening of novel nanoporous materials for carbon capture separations

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

Accelerated degradation of MOFs under flue gas conditions

The zero length column (ZLC) technique is used to investigate the stability of Mg- and Ni-CPO-27 metal organic framework (MOF) crystals to the presence of water and humid flue gas. The design of the ZLC enables the stability test to be conducted over a considerably shorter time period and with lower gas consumption than other conventional techniques. A key advantage over other experimental methods to test the stability of adsorbents is the fact that the ZLC allows to quantify the amount adsorbed of every component present in the gas mixture. The developed protocol is based on a two-stage stability test. The samples were first exposed to a humid carbon dioxide and helium mixture in order to study the effect of water on the carbon dioxide adsorption capacity of the samples. In the second stage the samples were exposed to a flue gas mixture containing water. From the preliminary water stability test, the Ni-sample exhibited the highest tolerance to the presence of water, retaining approximately 85% of its pristine CO2 capacity. The Mg-MOFs deactivated rapidly in the presence of water. The Ni-CPO-27 was then selected for the second stage of the protocol in which the material was exposed to the wet flue gas. The sample showed an initial drop in the CO2 capacity after the first exposure to the wet flue gas followed by a stabilisation of the performance over several cycles.PostprintPeer reviewe