A Study On The Solubility Of SO₂in Task-Specific Ionic Liquids And Its Mechanism

Recently, Room-temperature ionic liquids (ILs) have been demonstrated to absorb SO2efficiently from flue gases, especially task-specific ILs. ILs are attractive alternatives to traditional absorbents, due to their unique properties, such as low melting point, extremely low vapor pressure, high thermal and chemical stability, and tunable structure. However, the mechanism of the interaction between SO2and ILs remains unclear. It is popularly accepted that task-specific ILs absorb SO2by both chemical and physical interactions, while normal ILs only physically absorb SO2. But the chemical association between task-specific ILs and SO2, and whether the physical contribution can be neglected in the systems of task-specific IL+SO2have remained unclear. In addition, the stoicheometry of task-specific IL with SO2is also still being disputed. Furthermore, most of researches focused themselves on absorbing pure SO2or high concentration of SO2by ILs, and few work covered about absorbing SO2by task-specific ILs and normal ILs at low partial pressure of SO2close to practical flue gas conditions.In this work, the solubility of SO2in task-specific ILs,1,1,3,3-tetramethylguanidinium lactate ([TMG]L) and monoethanolaminium lactate ([MEA]L) and normal ILs1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), have been determined at temperatures from293.15to333.15K and SO2partial pressures from0.344to101.25kPa. It has been demonstrated that the solubility of SO2in ILs decreased with the increase of temperature and increase with the increase of partial pressure. Both task-specific ILs and normal ILs can absorb pure SO2or feed gas streams with high contents of SO2, while task-specific ILs can absorb excellently SO2even from the flue gas containing very low content SO2. At the same condition, the solubility of SO2in [TMG]L is highest.All the solubility data have been successfully correlated with the modified RK EoS. The relative absolute deviation (RAD) between the experimental results and the correlation results for the solubility of SO2at our investigated pressures and temperatures were1.4to4.4%. Therefore, the modified RK EoS can be allowed for the direct calculation of solubility at a given temperature and pressure. The chemical absorption and physical absorption are differentiated and the absorption mechanism has been proposed with the aid of the modified RK EoS. The results indicate that (1) normal ILs only physically absorb SO2, which follows the Henry’s law. Task-specific ILs absorb SO2by chemical and physical contributions, and the solubility of SO2by physical interaction still follows the Henry’s law and the solubility of SO2by chemical interaction follows the chemical equilibrium. For the systems of task-specific IL and SO2, two IL molecules absorb one SO2molecule by chemical interaction, and the chemical absorption amount follows the chemical equilibrium.(2) Both the physical solubility and chemical solubility increase with increasing pressure. Chemical reaction follows stoichiometric ratio and the chemical equilibrium. At high SO2partial pressures, the chemical absorption capacity increase to a very small extent with the increase of the SO2partial pressure, while the physical absorption capacity increase greatly with increasing of the SO2partial pressure, even larger than the chemical absorption capacity.(3) Physical interaction is greatly depended on pressure, which normally obeys the Henry’s law, P=kH·x, at low pressures. So it can reasonably omit the physical contribution to SO2solubility in task-specific ILs at low SO2partial pressures.(4) The mechanism that heating and/or vacuum treatment can regenerate SO2-absorbed IL has been revealed, it has been found that not all of absorbed SO2can be recovery at a certain low pressure, and the residue amount of SO2in IL is limited by the chemical equilibrium. Finally, some thermodynamic functions (equilibrium constant Kθmolar Gibbs energy of reaction△rGθm, molar reaction enthalpy△rHθm, molar reaction entropy△rSθm and Henry’s constant KH) about ILs and SO2were obtained. The result show that both physical and chemical interaction of [TMG]L and SO2is stronger than those of [MEA]L and SO2. That is the reason why the solubility of SO2in [TMG]L is higher than that in [MEA]L at the same condition.