Solubility of H2S in ammonium-based ionic liquids
Introduction
This study aims to measure the temperature dependence, at atmospheric pressure, of the hydrogen sulfide (H2S) solubilities in three ionic liquids (ILs), namely 2-hydroxyethylammonium acetate ([2-HEA][Ace]), bis(2-hydroxyethyl)ammonium acetate ([B-2-HEA][Ace]) and 2-hydroxyethyldiethylammonium hydrogen diacetate ([2-HEDEA][H(Ace)2]).
Hydrogen sulfide is one of the most abundant sulfur containing compounds in natural gas and in light and middle distillate oil fractions. Because of its undesirable high toxicity and corrosiveness, hydrogen sulfide must be removed from industrial gas and oil streams [1].
Ionic liquids (ILs) are low-melting salts with extremely low vapor pressures, high thermal and chemical stability, and tunable solvent properties for many organic and inorganic compounds. These remarkable properties position ILs as environmentally benign solvents feasible for a large number of applications, including gas solubility and separation, cellulose processing, catalysis, extraction, and high-temperature pyrochemical processing, etc. [2], [3]. However, the high viscosity of the ILs is usually a limiting step for the dissolution of the gas and ultimately on a separation unit sizing [4], [5]. The properties of the ILs can be tuned by changing the ions according to the desired properties, from a very large set of possible combinations [6]. Thus, one of the areas of research interests in the absorption of acidic gases, like CO2 or H2S is to formulate ionic liquids with desirable properties concentrated on the following aspects: (1) the design and synthesis of ILs, especially the task-specific ILs for enhanced absorption performance by functionalization with amine or other groups; (2) the measurement and estimation of physical/chemical properties of the multi-component systems containing the IL and the gas, and the development of suitable thermodynamic models; (3) the investigation of the transport properties, absorption kinetics, and the mechanism for gas capture with functionalized ILs [7].
In the past few years, a growing number of imidazolium-based ILs have been evaluated for H2S solubility as a function of temperature and pressure [1], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. However, experimental data for the solubility of hydrogen sulfide outside of imidazolium-based ionic liquids are surprisingly scarce if one takes into consideration their enhanced, in mole fractions, CO2 solubility [20], [21], [22], [23].
In the present work, the volumetric method with an automated apparatus [24] is used to determine the solubility of H2S in ammonium-based ionic liquids as a function of temperature and at atmospheric pressure. The H2S solubility was modeled using the Cubic Plus Association Equation of State (CPA EoS) showing high accuracy of the CPA equation of state to describe the phase equilibria solubility data.
Section snippets
Experimental
The experimental technique used in this work is based on a volumetric method. The apparatus used for the determination of solubility was described in detail elsewhere [24]. This procedure has shown to be highly accurate and precise in the past for various gases and solvents, making it a preferable choice [25], [26].
The variables measured directly from the apparatus are the displaced gas volume in the gas burette due to the gas dissolution (ΔV), the mass of solution (ms), the equilibrium
Gas/Liquid equilibria modelling
The contact of a gas with a liquid imposes changes of enthalpy and entropy along with a decrease in gas volume as dissolution takes place until an equilibrium state is reached. Mathematically, the gas–liquid equilibrium condition for the solute can be expressed as a stationary point of Gibbs function, i.e. iso-fugacity condition: . To model gas and liquid fugacity of H2S ( and , respectively) and calculate their solubility in the ILs, is employed the Cubic-Plus-Association equation
Experimental results
The solubility data for H2S in the [2-HEA][Ace], [B-2-HEA][Ace] and [2-HEDEA][H(Ace)2] ionic liquids was determined in the 298.15 K to 318.15 K temperature range and at about 101.3 kPa, as presented in Table 2.
The results of Table 2 show that the solubilities for H2S increases in the order [2-HEA][Ace] < [B-2-HEA][Ace] < [2-HEDEA][H(Ace)2]. The behavior of the H2S solubilities in ammonium-based ILs shows that the solubilities increase with the free volume of the ILs cation, which indicates that
Conclusions
The solubility of H2S in three ammonium-based IL solvents ([2-HEA][Ace], [2-B-HEA][Ace] and [2-HEDEA][H(Ace)2]) have been determined in the temperature range between 298 K and 318 K at about 1 atm. It was found that among the solvents, the solubilities of H2S increases in the following order: [2-HEA][Ace] < [2-B-HEA][Ace] < [2-HEDEA][H(Ace)2]. The behavior of the H2S solubilities in ammonium-based ILs show that the solubilities increase with the free volume of the ILs cation, which indicates
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was developed in the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2013) funded by FEDER through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT – Fundação para a Ciência e a Tecnologia. P.J.C. acknowledges FCT for the contract under the Investigator FCT 2015 (IF/00758/2015).
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