Carbon dioxide separation from nitrogen/hydrogen mixtures over activated carbon beads : adsorption isotherms and breakthrough studies

The high pressure separation of carbon dioxide/nitrogen and carbon dioxide/hydrogen mixtures were investigated over two phenolic-resin-derived activated carbon bead samples: an unmodified activated carbon made from a phenolic resin precursor and a modified material manufactured by treating the former activated carbon with first nitric acid and then ammonia were investigated. Equilibrium tests on the material were performed with a high pressure volumetric analysis with carbon dioxide and nitrogen. The dynamic response of the separation was tested using a fixed bed rig to produce carbon dioxide breakthrough curves with several carbon dioxide feed fractions (0.1, 0.2, 0.3, 0.4 and 0.5) in nitrogen. This study represents one of the few studies that equilibrium capacities have been related to the breakthrough capacities achieved in packed bed operation for high pressure carbon dioxide capture applications and the first to apply the IAST model.
The equilibrium tests showed that the Langmuir-Freundlich isotherm and the Dual-site Langmuir isotherm gave a closer fit to all the data than the Langmuir isotherm alone in the pure component adsorption studies. The capacity of the material based on the dynamic separation were found with mole fractions of 0.5 carbon dioxide in nitrogen leading to 6.09 mol kg-1 carbon dioxide being captured over the unmodified activated carbon and 7.48 mol kg-1 being captured over the modified activated carbon at 25 bar, at 25°C. By comparison, the saturation capacity of the modified activated carbon in the Langmuir-Freundlich fit to the high pressure volumetric adsorption data for 0.5 mole fraction carbon dioxide at the same temperature was 8.06 mol kg-1 for the unmodified material and 7.68 mol kg-1 for the modified material based on the pure component isotherm parameters. The breakthrough capacities were also found for feed fractions of carbon dioxide in the range of 0.1 - 0.5. A comparison between the dynamic capacities and those predicted by the isotherm show that pure component data is not necessarily representative of a dynamic multicomponent system. Therefore, multicomponent isotherm models were fitted to the data and compared with predictions using the Ideal Adsorbed Solution Theory. A multicomponent Dual Site Langmuir equation was found to give the best fit to the binary component data. Breakthrough curves were also reported for carbon dioxide in hydrogen over the modified activated carbon, with the carbon beads showing considerable potential for application for carbon capture in pre-combustion separation units of power plants, due to their physically strength meaning no further agglomeration of powdered samples is required for their use in packed beds.