Thermodynamic analysis of CO2 separation from biogas with conventional ionic liquids

CO2 separation plays an important role in energy saving and CO2 emission reduction to address global warming. Ionic liquids (ILs) have been proposed as potential absorbents for CO2 separation, and a large amount of ILs have been synthesized to achieve this purpose. To screen ILs for CO2 separation, CO2 absorption capacity/selectivity and energy use have been considered, whereas the required amount of IL has been seldom involved. In this work, CO2 separation from biogas with 30 conventional ILs was analyzed theoretically on the basis of the Gibbs free energy change combining the amount of IL needed and the energy use. The desorption temperature was estimated from the absorption pressure, and then the amount of IL needed and the energy use were calculated. Thermodynamic analysis shows that the absorption pressure and the desorption temperature need to be changed to achieve optimal separation. Several ILs were screened with certain criteria, namely, the amount of IL needed and energy use. The performance of the screened ILs was compared with that of commercial CO2 absorbents (30 wt% MEA, 30 wt% MDEA, DEPG, and water). The comparison with DEPG and water shows that the screened physical ILs are promising for IL-based technologies because of their advantages of negligible vaporization enthalpy, low amount of absorbent needed, or low energy use. A comparison with 30 wt% MEA and 30 wt% MDEA indicates that chemical IL has negligible vaporization enthalpy and low energy use. These findings show that the screened ILs are promising for CO2 separation from biogas.