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376 12 Carbon Capture and Storage

without functional groups can be used as physical sorbent for CO 2 capture. ILs
prepared with specific function groups enable selective gas absorption. Amine
functional groups have been used effective CO 2 chemical absorption.
After CO 2 sorption, the used ILs can be regenerated by heating the spent solvent
to release the absorbed gas(es). Since regeneration usually takes place at a higher
temperature than the sorption stage, thermal stability is a key factor in selecting the
right ILs.

12.6.4.1 CO 2 Solubility in Physical Ionic Liquids

Earlier research on CO 2 capture with ILs focused primarily on physical absorption
without chemical reactions. In this section we concentrate on properties that affect
the CO 2 physical absorption including CO 2 solubility and selectivity.
The actual mechanisms behind the high CO 2 solubility in IL are still not well
understood [49]. Scientists and engineers are actively working on understanding
and increasing the solubility of CO 2 in ILs. A review of the different approaches
that have been used to model the phase behavior of gas-IL systems is provided by
Vega et al. [55]. Molecular simulation and experimental data have shown that anion
and cation play an important role in the dissolution of CO 2 [6, 11]. Most researchers
agree that the anion dominates the solubility of CO 2 in an IL and that the cation
plays a secondary role. Nonetheless, anion-fluorination and cation-fluorination can
improve the solubility much [3].
CO 2 solubility in ILs increases with increasing molecular weight, molar volume,
and the free volume of ILs. CO 2 solubility in ILs is dominated by entropic effects
rather than solute–solvent interactions [12]. As such, it cannot be calculated based
on mole fraction (like in Henry’s law or Raoult’s law in Sect. 2.3) because of the
strong molecular weight (molar volume) of ILs. It should be determined as a
function of molality, that is, the mole amount of solubility per kilogram of solvent
(mole CO 2 /kg IL).
Before more sound theories are available, we can use the following empirical
correlation proposed by Carvalho and Coutinho [12] to calculate the CO 2 solubility
in ILs by physical absorption:

2004:3
0
P ¼ m exp 6:8591 ð12:60Þ
i
T
0
where m is the molality in (mole of CO 2 /kg of IL), P is the pressure in MPa, and T
i
is temperature in K. This equation was obtained using experimental data and is
deemed valid for pressures up to 5 MPa, molalities up to 3 mol/kg, and tempera-
tures ranging from room temperature to 363 K. Figure 12.7 shows the calculated
solubility versus pressure at three different temperatures. For a fixed temperature,
the solubility is in linear relationship with the pressure.