Revamping gas absorption solvents with functionalized ionic liquids
Advancing the chemical engineering fundamentals
Distillation, Absorption & Extraction - I (T2-10a)
Keywords: Absorption solvents, Ionic liquids, CO2 absorption, olefin/paraffin separation
Ionic liquids are potential solvents for gas absorption operations due to its properties, such as a very low vapour pressure and the possibility to design its constitutive ions for task specific purposes. Characterization of ionic liquids as solvents for gas absorption systems is essential for its industrial application. An approach of the factors which influence gas absorption, diffusion and mass transfer in ionic liquids is presented here. The study of solvent bulk properties and absorption behaviour was performed for common and functionalized ionic liquids.
The performance of ionic liquid solvents in two absorption applications was determined. The absorption of CO2 and the separation of olefins from paraffins were chosen as examples. The solubility of C2H4, C2H6 and CO2 into different Room Temperature Ionic Liquids (RTILs) was measured at temperatures between 303 K and 333 K and pressures up to about 1 MPa. The RTILs used were alkyl imidazolium-based cations paired with several anions: [BF4]-, [DCA]-, [NTf2]-, [OTF]-, [NO3]-. To improve the performance of the solvent for gas absorption, some of the RTILs used were designed for carrying out the specific separation. For the CO2 absorption, the ionic liquids were functionalized with an amine group, while in the case of the C2H4/C2H6 separation, the liquids were functionalized with metallic salts. Due to a secrecy agreement, it is not possible to give details about the different functionalized RTILs molecules used here.
The results indicated that, in general, the gas solubility into RTILs decreased with a raise in temperature and increased at higher pressures. A chemical enhancement of the gas absorption was observed when functionalized RTILs were used as absorption solvents. The influence of the specific bulk and surface properties of the ionic liquid, such as density, viscosity and surface tension, on the ion mobility was more evident with functionalized RTILs. The effect of the bulk properties is reflected in the absorption kinetics and mass transfer resistances. The rise of ion mobility and species accessibility caused by an increase in temperature almost cancelled out the expected effect of a temperature rise on the C2H4 absorption when a functionalized RTIL for the olefin/paraffin separation was used. The chemical enhancement of the CO2 absorption into amine-functionalized RTILs was almost threefold the gas volumetric load in comparison with non-functionalized RTILs. The selectivity of the C2H4/C2H6 separation was improved up to 10 times when ions of metallic salts were included in the RTIL structure. Regeneration of the RTILs was possible and the conditions varied with the RTIL properties. The factors that influence the diffusion, mass transfer, kinetics depend on the anion and cation of the RTIL and the accessibility to the active species in the RTILs.
It was possible to change the ionic liquid structure in order to improve its performance as a gas solvent. The CO2 load of ionic liquids increased almost threefold with primary amine groups incorporated to the ionic liquids structure. The non-functionalized ionic liquids exhibited lower CO2 loads and the absorption behaviour was similar to physical solvents for CO2 absorption. Ionic liquids can merge in one solvent the behaviour exhibited by chemical and physical solvents used in absorption.
Presented Monday 17, 11:33 to 11:52, in session Distillation, Absorption & Extraction - I (T2-10a).
