CO2 Separation with Ionic Liquids -Property, Gas solubility and Energy consumption

Abstract: Ionic liquids (ILs) have shown great potential to be used as liquid absorbents for CO2 capture because of its advantages, such as non-volatility, functionality, high CO2 solubility and lower energy requirements for regeneration. A significant amount of research has been carried out, but most of them are on the synthesis of novel ILs and the measurements of CO2 solubility in ILs. However, the application of IL-based technology for CO2 capture requires knowledge of gas solubility, the effect of other components on CO2 solubility, the thermo-physical properties, modeling as well as process simulation. Therefore, a tremendous gap exists between new technology development and implementation. The goal of this work is to perform a systematic study from experimental measurement, model development to process simulation in order to promote the development and application of IL-based technology for CO2 capture. In this work, the solubilities of CO2, CH4, H2, CO and N2 in choline chloride (ChCl)/urea (1:2 on a molar basis) were determined. The effect of water on the density, viscosity and CO2 solubility in ChCl/urea (1:2) were measured. The experimental gas solubility data was represented with the Non Random Two Liquid - Redlich Kwong (NRTL-RK) model. The results show that the addition of water significantly decreases the viscosity of ChCl/urea (1:2) while the effects on their density and CO2 solubility are much weaker. The excess molar volume and excess molar activation energy were calculated based on the experimental density and viscosity data. It was found that the intermolecular interaction between ChCl/urea and water is strong, and the hydrogen bond interaction is influenced by the temperature and water concentration. Meanwhile, the experimental data of CO2 solubility in imidazolium-based ILs at pressures below 10 MPa was surveyed and evaluated by NRTL-RK model. The CO2 absorption enthalpy and the energy consumption for a CO2 separation process using ILs by pressure swing and/or temperature swing were investigated. The results reveal that the temperature-dependent Henry’s constant is an important factor for energy consumption analysis in a pressure swing process, while the heat capacity of ILs plays a more important role in a temperature swing process.