(231an) Conceptual Process Design for Separation of Rare Gases Using Ionic Liquids

Ionic liquids may provide an absorption-based process to separate krypton and xenon from an oxygen stream taken from air-sepration plant. Because the polarizabilities of krypton and xenon are quite different from that of oxygen, an ionic liquid may be a good solvent for a separation process.

Our calculations with COSMO-RS (COnductor-like Screening Model for Real Solvents) suggest that a carefully selected ionic liquid can preferably dissolve xenon and krypton over  oxygen in a way that the relative volatility of oxygen to krypton and krypton to xenon may be about 3 and 4, respectively.

We developed three different methods to measure solubilities of gases, in particular krypton, xenon, oxygen and other gases in selected ionic liquids at 25 to 80°C and pressures from atmospheric to 5 MPa. Because of the lack of solubility data and the lack of off-the-shelf equipment, experimental equipment was built and calibrated. Equipment was built for using experimental methods based on gas-liquid chromatography, on the inert-gas stripping technique, and on isochoric-synthetic principles. For our purposes, the isochoric-synthetic method was best.

We measured solubilities of krypton, xenon and oxygen in many ionic liquids and found that trihexyltetradecylphosphonium bis (2,4,4-trimethylpentyl) phosphinate [P(14)666][TMPP] shows good selectivity and high capacities for the rare gases. However, this ionic liquid has a very high viscosity. We measured densities and viscosities of [P(14)666][TMPP] at different temperatures with and without promising diluents for reducing viscosity.

Final attention was given towards a process flow diagram for a possible sepration process based on selective absorption at ambient temperature.