Solubility of CO₂ in aqueous mixtures of monoethanolamine and dicyanamide-based ionic liquids

Abstract

Two low viscous ionic liquids (ILs), 1-(2-hydroxyethyl)-3-methyl-imidazolium dicyanamide ([C₂OHmim][DCA]) and 1-butyl-3-methylimidazolium ([Bmim][DCA]) were selected to mixed with aqueous 30 wt% monoethanolamine (MEA) for CO₂ absorption. The solubility of CO₂ in the aqueous mixtures of MEA + ILs was measured over a range of CO₂ partial pressure of 10–800 kPa and ILs concentrations from 10 to 50 wt% at 313.15 K and 333.15 K. Correlations of solubility as a function of CO₂ partial pressure have been conducted with deviation of ±1.5%. Moreover, the density and viscosity of pure ILs and MEA + ILs + H₂O systems with different IL mass fractions were measured at temperature varying from 293.15 to 333.15 K.

Introduction

Since CO₂ is widely regarded as a major greenhouse gas that leads to global climate change, there is significant interest in CO₂ capture and sequestration technologies [1]. Some technologies have been developed to separate CO₂ from gas streams, such as cryogenic distillation, gas separation membranes, adsorption with solids and absorption with chemical and physical solvents [2]. Among these technologies, aqueous solution of alkanolamines, as one typical chemical solvent, has been effectively used for gas absorption [3].

Monoethanolamine (MEA) as one kind of alkanolamine is widely used for the removal of CO₂ from flue gas [4]. It has high reactivity, low solvent cost, ease of reclamation, and low absorption of hydrocarbons [5]. However, its aqueous solution has some intrinsic drawbacks, for example, the high energy consumption caused by large amounts of water being vaporized during the thermal regeneration. Thus the shortcoming of MEA drives researchers to find superior technologies.

Recently, ionic liquids (ILs) were proposed as potential solvents for the absorption of CO₂ due to their significant advantages over other molecular solvents. The unique characteristics of ILs are wide liquid range, tunable physicochemical properties, especially negligible vapor pressure and less than one-third the heat capacity of water in a wide range of temperature (1.30 J g⁻¹ K⁻¹ vs 4.18 J g⁻¹ K⁻¹ or 1.88 J g⁻¹ K⁻¹ vs 4.18 J g⁻¹ K⁻¹) [6], which can reduce greatly the loss of absorbents and save energy during the regeneration process. However, most ILs show very low CO₂ loading capacity and preparing these ILs often involves several synthesis and purification steps, thus they are not always cost effective candidates compared to commercial amines.

In 2008, Noble et al. put forward the idea of mixing ILs and alkanolamines for CO₂ capture, they also stated that the mixture of ILs and alkanolamines was capable of capturing 1 mol of CO₂ per 2 mol of dissolved alkanolamine. This method has two advantages: (1) the desirable properties of ILs and alkanolamines may be incorporated and (2) energy can be saved during the regeneration process without affecting the absorption performance. However, based on this idea, the alkanolamines used in the literature [7], [8], [9], [10], [11], [12], [13] have mainly focused on tertiary alkanolamines such as MDEA, with very few reports about primary alkanolamines such as MEA. Moreover, the ILs studied in these reports were usually [Bmim][BF₄] or [Bmim][OAc], which show poor thermal stability and high viscosity. [Bmim][BF₄] and [Bmim][OAc] easily generate hydrofluoric acid and acetic acid, respectively, when heated for a long time [14], while high viscosity not only leads to processing difficulties but also results in the lower reaction rates [15]. The purpose of the present study is to screen out ILs which both have better thermal stabilities and low viscosities.

In this work, two dicyanamide-based ILs with low viscosity have been studied and the solubility of CO₂ in aqueous mixtures of MEA + 1-(2-hydroxyethyl)-3-methyl-imidazolium dicyanamide ([C₂OHmim][DCA]) and MEA + 1-butyl-3-methylimidazolium ([Bmim][DCA]) was investigated at different temperatures, ILs concentrations and CO₂ partial pressures. Dicyanamide-based ILs was chosen as they have been shown to exhibit lower viscosity compared with other ILs studied to date [15]. The solubility data determined has been correlated as a function of CO₂ partial pressure by a second-order polynomial. Moreover, the density and viscosity of the ternary mixtures have been measured at temperatures from 293.15 to 333.15 K and ILs concentrations varied from 10 to 50 wt%.