(17d) Evaluation of Aprotic Heterocyclic Anion Ionic Liquids for Post Combustion Carbon Capture

Capturing of waste CO₂, particularly from large industrial point sources (electric power generators, refineries, and other chemical facilities), is necessary for either permanent storage or utilization of CO₂. The technology currently proposed for post-combustion COâ‚‚ capture uses an aqueous solution of 30 wt % monoethanolamine (MEA). However, amine-based solvents have high operational expenses as they are plagued with thermal degradation, degeneration in the presence of impurities (O₂, SO_x, and NO_x), high regeneration energy requirements, and loss of solvent during regeneration. These challenges may be overcome using ionic liquids (ILs) that can be tailored for operating under flue gas stream conditions. ILs, however, suffer from high viscosities, leading to diffusion limitations, compared to amine solvents, and some are ineffective in the presence of water.

A new family of ILs containing aprotic heterocyclic anions (AHAs) and phosphonium cations have been discovered that maintain high CO₂ absorption performance in the presence of water. While they do have significantly higher viscosities that aqueous MEA, their viscosity does not increase with CO₂ saturation. This phenomenon is likely due to the absence of the H⁺ ion in the anion, preventing the IL from forming hydrogen bonding. Their viscosities, however, are on the order of 80 to 160 cP, compared to 7 cP of aqueous MEA at combustion capture conditions.

This paper uses Aspen Plus process modeling to evaluate triethyloctylphosphonium 2-cyanopyrrolide, [P2228][2-CNPyr] and trihexyltetradecylphosphonium 2-cyanopyrrolide, [P66614][2-CNPyr], for use as carbon capture absorbents either alone or mixed with various amounts of aqueous MEA. The model used the Electrolyte Non-Random Two Liquid – Redlich Kwong (ENRTL – RK) thermodynamic package to account for the IL/MEA/H₂O, CO₂ electrolyte system. The components were modelled using a True Components Approach that takes into consideration the free and dissociation forms of the components. Our model parameters (viscosity profiles, absorption capacity, enthalpy, and entropy) were validated using laboratory and pilot plant data available in the literature. Our results indicated significant decreases in viscosity when either IL was mixed with aqueous MEA, which reduces mass transfer resistances. The CO₂ loading does not appear to have a large effect on the absorption capacity at lower MEA concentrations (< 30 mole %). The CO₂ – IL absorption is mostly due to chemisorption and is slightly exothermic. However, the heat is readily absorbed by the water, and the absorber temperature can be easily maintained at ~50 °C. Furthermore, compositions and process conditions were optimized to capture ~ 90 % of CO₂ in the flue gas stream (based on an MEA carbon capture process previously published by our group).