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

Abstract

Aprotic heterocyclic anion ionic liquids (AHAs) are a promising new class of CO 2 absorbents, with a capacity of one mole of CO 2 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 CO 2 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 CO 2 uptake, to be used in connection with the process model. The possibility of ionic liquids (ILs) that exhibit a 2:1 CO 2 uptake (two moles CO 2 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 CO 2 capture, leading to significant reductions in heat requirements relative to conventional amine technology.

Economic Optimization of Carbon Capture Processes Using Ionic Liquids: Toward Flexibility in Capture Rate and Feed Composition

“…This is because AHA anions lack free protons, preventing the formation of hydrogen-bonding networks after the reaction with CO 2 . Other non-AHA structures with equimolar and reversible CO 2 uptake have also been studied, notably phenolate anions. − Efforts to achieve greater than equimolar uptake are also underway. ,− For this paper, triethyl-(octyl)­phosphonium 2-cyanopyrrolide ([P 2228 ]­[2-CNPyr]) is selected as a representative AHA IL, considering its high CO 2 absorption capacity with excellent reversibility. , However, a tunable IL property (heat of chemical absorption) is later treated as an economic decision variable that is optimized.…”

“…Since IL-based CO 2 capture is a relatively new technology, it is important to evaluate the process performance and the associated process costs in view of future industrial deployment. It is also important to simultaneously identify the most effective IL solvent, given the strong linkage between key materials and the processes in which they are used. , Although there have been numerous research efforts to identify promising ILs for CO 2 absorption, these have focused primarily on physically absorbing ILs, with relatively few investigations ,, on the modeling and techno-economic analysis of CO 2 capture processes using chemically absorbing ILs. For example, Sexton et al provided a comprehensive techno-economic analysis of a CO 2 capture process using [P 66614 ]­[2-CNpyr] and [P 2228 ]­[3-CNpyr] AHA ILs, with the latter exhibiting similar economic performance to aqueous amine-based plants.…”

Solvents for Membrane-Based Post-Combustion CO2 Capture for Potential Application in the Marine Environment

“…Similar to AA-ILs, aprotic heterocyclic ionic liquids (AHA-ILs) have shown promising properties for CO 2 capture. These include their high reaction rates and high CO 2 solubility as well as high thermal stability [95][96][97]. Unlike carboxylate or AA-ILs, AHA-ILs follow a 1:1 stoichiometry and have high decomposition temperatures.…”

“…Unlike carboxylate or AA-ILs, AHA-ILs follow a 1:1 stoichiometry and have high decomposition temperatures. However, they have relatively high heats of absorption (−37 to −54 kJ/mol) and medium to high viscosity [96]. Such issues may be partially alleviated by supporting or encapsulating AHA-ILs using cosolvents [98].…”

Cooperative CO2 absorption by amino acid-based ionic liquids with balanced dual sites

“…High CO 2 absorption capacity is one of the performance evaluations of ILs. In this regard, a series of strategies, including tuning the basicity of the active sites, [9][10][11] changing the steric hindrance of the ILs, 12,13 utilization of entropic effects 14,15 and hydrogen bond formation, 16,17 were developed to enhance the CO 2 absorption capacity. However, the CO 2 capacity of ILs with single site was low compared to those multiple-sites, which have attracted attention to enhance the CO 2 capacity.…”