(7g) Thermodynamic Model for CO2 Absorption by Phase-Change Ionic Liquids

Towards the goal of realizing a cost-efficient CO₂ capture system that would be effective for point sources, like post-combustion flue gas from coal-fired power plants, novel compounds and methods are needed that can absorb or adsorb CO₂, and subsequently release it for sequestration, with little parasitic energy penalty to the overall operation. Phase-change ionic liquids (PCILs) are a newly discovered class of materials; they are solid salts that, upon binding with CO₂, undergo a phase change to liquid. This change is readily reversible, releasing the CO₂ for storage and regenerating the solid PCIL. As CO₂ absorbents, PCILs are attractive because: 1) The heat of fusion provides a significant part of the energy needed to release the CO₂ from the absorbent, thus reducing the parasitic energy penalty, and 2) The CO₂ uptake is high (nearly one mole of CO₂ per mole of PCIL, at typical post-combustion flue gas conditions).

For purposes of preliminary design and optimization of CO₂ capture systems based on PCILs, as well as to assist in designing the most effective PCIL molecules, we have developed an absorption isotherm model that captures the basic chemical and physical principles and the underlying thermodynamics of the CO₂ absorption process. This model allows us to relate key operating conditions for the absorber and stripper to basic physical properties such as the enthalpies of CO₂ absorption (chemical and physical), the PCIL enthalpy of fusion and the PCIL melting temperature. In this presentation, we will describe the development of this model and the use of experimental data for parameter estimation. Variations of the model, based on differing assumption about the underlying phase behavior, will also be discussed. We show that this model provides a good prediction of CO₂ uptake by PCILs, and can be used to determine realistic values of physical parameters describing the system.