Energy consumption analysis for CO2 separation using imidazolium-based ionic liquids

CO2 solubility in ionic liquids has been measured extensively in order to develop ionic liquid-based technology for CO2 separation. However, the energy consumption analysis has not been investigated well for such technology. In order to carry out the energy consumption analysis for CO2 separation using ionic liquids based on available experimental data, in this work, the experimental data of the CO2 solubility in imidazolium-based ionic liquids at pressures below 10 MPa was surveyed and evaluated by a semi-empirical thermodynamic model firstly. Based on the reliable experimental solubility data, the enthalpy of CO2 absorption was further calculated by the thermodynamic model. The results show that the CO2 absorption enthalpy in the studied ionic liquids is dominated by the enthalpy of CO2 dissolution and the contribution of excess enthalpy increases with increasing CO2 solubility in ionic liquids. The magnitude of the CO2 absorption enthalpy decreases with increasing chain length in cation and strongly depends on the anion of ionic liquids. Furthermore, the energy consumption for a CO2 separation process by pressure swing and/or temperature swing was investigated. For the pressure swing process, the Henry's constant of CO2 in ionic liquids is an important factor for energy consumption analysis; If CO2 is absorbed at 298 K and 1 MPa and ionic liquid is regenerated by decreasing the pressure to 0.1 MPa at the same temperature, among the studied ionic liquids, [emim][EtSO4] is the solvent with the lowest energy consumption of 9.840 kJ/mol CO2. For the temperature swing process, the heat capacity of ionic liquids plays a more important role; If CO2 is absorbed at 298 K and desorbed at 323 K and 0.1 MPa, [emim][PF6] is the solvent with the lowest energy demand of 888.9 kJ/mol CO2. If the solvent is regenerated by releasing pressure and increasing temperature, both the Henry's constant of CO2 in ionic liquids and the heat capacity of ionic liquids are important for analyzing the energy consumption; If CO2 is absorbed at 298 K and 1 MPa and ionic liquid is regenerated at 323 K and 0.1 MPa, [bmim][Tf2N] is the solvent with the lowest energy consumption of 57.71 kJ/mol CO2.