Ionic Liquid CO2 Capture: CO2 Desorption Technology Review

Historically, solid sorbents have removed carbon dioxide (CO2) from space habitat atmospheres. For long-duration human spaceflight, regenerable CO2 sorbents reduce system mass compared to non-regenerable alternatives. To regenerate a sorbent, CO2 and other adsorbed constituents must be extracted (i.e., desorbed). The endothermic thermal-vacuum desorption of CO2 from flight-proven sorbents requires the following heats of desorption: monoethanolamine-coated beads (solid-phase amines) ~ 1.7 MJ/kgCO2, 5A zeolites ~ 2.8 MJ/kgCO2, 13X zeolites ~ 2.0 MJ/kgCO2, and silver oxide (i.e., METOX) ~ 1.85 MJ/kgCO2. More energy-efficient CO2 sorbents are being investigated for technology readiness level maturation; ionic liquids (ILs) are a candidate. One, 1-ethyl-3-methylimidazolium acetate (EMIM [Ac]), experimentally requires ~ 0.62 MJ/kgCO2 for desorption. Room temperature ILs, those that are liquid phase at room temperature, have negligible vapor pressure, are chemically stable, and thus, are not expected to degrade or produce toxic vapors at conditions favorable for CO2 sorption. Further, ILs can be uniquely configured within architectures to allow for continuous IL regeneration and CO2 removal, rather than batch mode operation. The most common architecture involves IL flowing between two modules. The absorption module, typically a hollow-fiber contactor, provides a contacting surface between the IL and CO2-laden cabin airstream. The CO2-laden ionic liquid subsequently flows into a desorption module for IL regeneration. Thermal-vacuum swing can regenerate the IL; however, at elevated temperatures, ILs can begin to thermally decompose, degrading the CO2 removal performance over time. The aim of this review article is to investigate potential energy-efficient IL regeneration strategies that could increase IL performance lifetime. Strategies investigated include the following (and their possible combinations): vacuum, sweep gas, ultrasonic, and microwave CO2 desorption. This article will report and compare aforementioned desorption technologies and their possible applications, and it will recommend further experimental investigations to advance IL desorption technologies.