Development of functionalised porous carbon materials for the separation of carbon dioxide from gas mixtures
Gibson, John Alastair Arran
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This work concerns the functionalisation of a variety of carbon materials for the selective adsorption of carbon dioxide. A key challenge in post-combustion capture from gas fired power plants is related to the low CO2 concentration in the flue gas (4- 8%). Therefore highly selective adsorbents have the potential to improve the efficiency of the separation of carbon dioxide from gas mixtures. The study was performed in conjunction with the EPSRC funded project ‘Adsorption Materials and Processes for Carbon Capture from Gas-Fired Power Plants – AMPGas’. The carbon materials investigated included multi-walled carbon nanotubes, a microporous activated carbon, two types of mesoporous activated carbon and multi-walled carbon nanotube/polyvinyl alcohol composite aerogels. The uptake of carbon dioxide by these materials was enhanced through the addition of basic amine groups to the materials. The adsorption properties of the samples were tested by the zero-length column technique, thermal gravimetric analysis and breakthrough experiments. The materials were generally tested at conditions representative of those found in the flue gas of a fossil fuel power plant: 0.1 bar partial pressure of CO2. Two approaches were adopted for the chemical functionalization of the solid carbon supports. First, amine groups were covalently grafted directly to the surface and secondly amine molecules were physically adsorbed within the porous structure of the material by wet impregnation. It was seen that wet impregnation enabled the incorporation of a greater number of amine groups and the CO2 capacity of the materials was investigated with respect to the carbon support structure, the type of amine and the amount of amine loading. Larger pore volume mesoporous carbon materials were seen to provide a more efficient support for the amine to interact with the CO2. A greater than 12-fold increase in the CO2 capacity was observed when the amine impregnated carbon material was compared to the raw starting material. The extended zero-length column was introduced and fully characterized as a novel breakthrough experiment. It requires a small sample mass (~50 mg) and it allows binary selectivities to be calculated. It was shown, through multiple experiments and simulations that the breakthrough experiments were conducted under close to isothermal conditions which greatly simplifies the analysis of the breakthrough curves. In addition, a new zero-length column model was proposed to account for the reaction between the amine and the CO2 in the adsorbed phase and fitted to experimental data. An interesting double curvature was observed in the concentration profile during the desorption step which was attributed to the kinetics of the amine-CO2 reaction. A brief investigation was carried out into the binary separation of biogas (45% CO2: 55% CH4) by zeolite 13X, activated carbon and an amine impregnated activated carbon. Finally, initial investigations into the properties of low density carbon nanotube aerogels which have a large accessible pore volume, were carried out. Their potential as highly efficient supports for amine impregnation was investigated. It was found that amine functionalized carbons strongly interact with carbon dioxide and have the potential to be integrated as an adsorbent in a rapid temperature swing process that separates carbon dioxide from dilute gas streams.