In carbon capture an effective absorbent is one which demonstrates a high selectivity, meaning that CO will preferentially dissolve in the absorbent compared to other gaseous components. In post-combustion carbon capture the most salient separation is CO from N, whereas in pre-combustion separation CO is primarily separated from H. Other components and impurities may be present in the flue gas, such as hydrocarbons, SO, or HS. Before selecting the appropriate solvent to use for carbon capture it is critical to ensure that at the given process conditions and flue gas composition CO maintains a much higher solubility in the solvent than the other species in the flue gas and thus has a high selectivity.
The selectivity of CO in ionic liquids has been widely studied by researchers. Generally, polar molecules and molecules with an electric quadrupole moment are highly soluble in liquid ionic substances. It has been found that at high process temperatures the solubility of CO decreases, while the solubility of other species, such as CH and H, may increase with increasing temperature, thereby reducing the effectiveness of the solvent. However, the solubility of N in ionic liquids is relatively low and does not increase with increasing temperature so the use of ionic liquids in post-combustion carbon capture may be appropriate due to the consistently high CO/N selectivity. The presence of common flue gas impurities such as HS severely inhibits CO solubility in ionic liquids and should be carefully considered by engineers when choosing an appropriate solvent for a particular flue gas.