Modeling Gas Separation of Greenhouse Gases with Ionic Liquids and Deep Eutectic Solvents

In the search for new types of green solvents, ionic liquids (ILs) and deep eutectic solvents (DESs) have come to the fore. The popularity of ILs grew rapidly due to their low melting points, negligible vapour pressure and wide liquid range, among other properties. However, the popular conception that they are all intrinsically green is not true. Some ILs are as toxic as some organic solvents, depending on their chemical structure. In addition, their synthesis is typically expensive compared to conventional volatile organic compounds. Recently, DESs have appeared as potential solvents as they share many of the IL properties but coming in at lower cost. They are mixtures of different substances in particular proportions that melt and freeze at a lower temperature than those of the individual components or of mixtures of the components in other proportions. This property is a consequence of hydrogen bonding between the components. DESs are non-volatile, biodegradable and do not react with water, becoming potential candidates for a wide variety of applications. During the past decade, these compounds have drawn academic and commercial attention due to their unusual chemical and physical properties that make them valuable alternatives to traditional compounds for different applications, such as separation and extraction processes [1]. One of the applications that is raising more interest is their possible use as solvents to carry gas separation in the context of carbon capture. It is well known that conventional technology of using chemical absorption through an aqueous solution of amine has important drawbacks, including loss of solvent, corrosion of facility, and high energy demand regeneration of the absorbent [2]. ILs and DESs are a novel absorbents, with an appropriate selection, could result in a non-contaminated target gas, especially attractive for the separation of these gases.

At present, there is still a need for a further understanding of the dependence of the gas behavior on ILs and, especially, on DESs. Molecular modeling techniques and, in particular, molecular-based equations of state (EoSs) provide an attractive option to screen ILs properties and give quick answers to guide the experiments, becoming a crucial tool for process design [3]. This work illustrates several practical examples based on the solubility of greenhouse gases on several ILs and DESs, in order to evaluate their feasibility for gas separation. A coarse-grained molecular model, within the framework of the soft-SAFT equation of state [4] is presented for each molecule based on structural information. Particular attention is paid to the hydrogen-bonding effects on DESs. The absorption of relevant gases such as CO₂, CH_4, CO, H₂, SO₂, H₂S and N₂O is modeled and compared with experimental data using a minimum amount of data, trying to assess the most appropriate solvent and the optimal operating conditions to maximize each gas separation [5-7].

Acknowledgments

 Financial support from the Catalan government under project 2014-SGR1582 is gratefully acknowledged.

References

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