Simultaneous Process and Material Design for Aprotic N-Heterocyclic Anion Ionic Liquids in Postcombustion CO₂ Capture

Aprotic heterocyclic anion ionic liquids (AHAs) are a promising new class of CO₂ absorbents, with a capacity of one mole of CO₂ chemically absorbed per mole of AHA. By tailoring the substituents on the anion, the AHA properties, in particular the enthalpy of absorption, can be tuned over a wide range. Furthermore, the entropy of absorption can be tuned by tailoring substituents on the cation. This then presents a materials design challenge - What are the optimal AHA properties? This challenge is addressed by incorporating AHAs into a simple process model of CO₂ capture from postcombustion flue gas, and formulating the question as a type of simultaneous materials and process design problem. New absorption isotherm data is presented, over a larger pressure range than studied previously, for a few AHAs, and is used to suggest a simple thermodynamic model for CO₂ uptake, to be used in connection with the process model. The possibility of ionic liquids (ILs) that exhibit a 2:1 CO₂ uptake (two moles CO₂ per mole IL) with a cooperative binding mechanism is also considered, with absorption isotherm data for one such compound presented, together with a corresponding isotherm model. The process model is an equilibrium-based material and energy balance model, which is used to determine flow rates, heat duties, and process conditions that minimize a simple energy usage objective function. The sensitivity of this optimum with respect to various material properties and process parameters is studied, for flue gas from both pulverized coal and natural gas combined cycle power plants. The results provide materials property targets for the identification of new AHA molecules for CO₂ capture, leading to significant reductions in heat requirements relative to conventional amine technology.