Characterization of functionalized ionic liquids for a new quasi-isothermal chemical biogas upgrading process

Abstract

Abstract In this work, a new CO2 separation process using ionic liquid solvents is proposed. The approach focusses not only on improving the applied solvents or their CO2 absorption chemistry, but also on adapting the separation process itself for exploiting the advantages of ionic liquids. This is achieved by adjusting the operating temperatures of the absorption and the regeneration columns of the upgrading process to the same moderate temperature level and operating the desorption of CO2 under moderate vacuum instead of applying high grade heat from an external source. In this article, the proposed CO2 separation process is described in detail and the results of the development and characterization of appropriate ionic liquid solvents for applying the process to biogas upgrading are presented. The characterization section includes measurements on thermal and chemical stability, physical and chemical CO2 solubility, physical CH4 solubility, effective and intrinsic absorption kinetics, heat of reaction and various additional material properties of ionic liquid solvents. Physical gas solubility is examined for Room Temperature Ionic Liquids with [NTf2], [Tf], [DCA] and [TCM] anions with imidazolium cations in a wide temperature range of 25–125 °C. The applied short chain [Tf] and [TCM] based RTIL feature a high selectivity concerning physical solubility of CO2 and CH4. Furthermore, the applied chemically functionalized IL (CFIL) show almost the double chemical CO2 loading capacity than conventional amines. The measurements on absorption kinetics show that the absorption of CO2 in the applied IL solvent in the desired temperature range is about as fast as when aqueous DEA is applied. Based on the derived experimental results, an energetic evaluation of the process is carried out indicating that the proposed process enables considerable potentials for energetic savings, forming the basis for being close to economic feasibility within relatively short payback periods.