Amines, such as monoethanolamine (MEA), are the most typical agents implemented for CO2 capture. Being one of the most common unit operations at an industry level, there is an urgency to seek for improvements in the current process. One of the major drawbacks of amines is their high volatility, which produces important solvent losses and undesirable emissions to the atmosphere. Consequently, there is a need of a constant replacement of the solvent, heavily affecting the unit operation cost.
In the process of seeking for alternative solvents, Ionic Liquids (ILs) and Deep Eutectic Solvents (DESs) have come to the fore. These two families share the important characteristic of having a negligible vapour phase at working conditions, which, contrary to amines, allows them to remain in liquid phase along the whole process, reducing the solvent loss. In comparison with conventional amines, ILs have an expensive synthesis, whereas DESs can be produced at a low price. DESs are formed from a eutectic mixture of Lewis or Brönsted acids and bases which, in certain proportions, have a melting point way below that of its single components. Being non-toxic and biodegradable, DESs are promising candidates for CO2 capture.
Some computational studies have shown that the multiple hydrogen-bonding interactions occurring between the compounds forming the eutectic mixture are the key factor to understand their physicochemical behaviour [1]. In this regard, SAFT is a molecular-based equation of state (EoS) that explicitly considers these hydrogen bonds by means of Wertheim’s perturbation theory [2], representing an ideal platform to evaluate its impact on the thermodynamic properties. In this work, a series of selected DESs have been studied by proposing different molecular models in the framework of the soft-SAFT EoS [3] in order to characterize its physico-chemical behaviour and its capacity to capture CO2. In particular, two different DESs families, based on the [Tetraalkylammonium][Chloride] and [Choline][Chloride] salts combined with an acid or a glycol at different ratios have been described. A first test was carried out considering them as a pure pseudocomponent, obtaining a set of molecular parameters that are composition-dependent. Willing to have a more consistent and transferable model, a second approach in which DESs are considered as two components was also studied. A quantitative description of the density and CO2 solubility was achieved. Finally a molecular and transferable model for MEA has been successfully developed, by isolating the interactions of the amine and alcohol group. A comparison between the conventional MEA and the alternative DESs is finally provided.
ACKNOWLEDGEMENTS
This work has been financed by the catalan government (2014SGR-1582).
REFERENCES
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[3] F.J Blas, L. F. Vega, Thermodynamic behavior of homonuclear and heteronuclear Lennard-Jones chains with association sites from simulation and theory. Mol. Phys. 92 (1997) 135-150.
