Elsevier

Fluid Phase Equilibria

Volume 378, 25 September 2014, Pages 21-33
Fluid Phase Equilibria

Experimental study and thermodynamical modelling of the solubilities of SO2, H2S and CO2 in N-dodecylimidazole and 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis(imidazole): An evaluation of their potential application in the separation of acidic gases

https://doi.org/10.1016/j.fluid.2014.06.026Get rights and content

Highlights

  • Low-volatile and long-chain N-substituted imidazoles were investigated for the absorption of acidic gases.

  • The solubilities of SO2, H2S and CO2 under different temperatures and pressures were reported.

  • The solubility data were correlated with thermodynamic models to calculate thermodynamic parameters.

  • The absorption performance were compared with other organic solvents and ionic liquids.

  • The functionalization of organic solvents is another way to design efficient solvents for the selective separation of acidic gases.

Abstract

Exploring low volatile solvents for the capture of acidic gases is highly valued from the viewpoint of green chemistry. In this work, the solubilities of SO2, H2S and CO2 in two long-chain N-substituted imidazoles: N-dodecylimidazole (NDI) and 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis(imidazole) (Im2TEG) at different temperatures and pressures were determined systematically. It is shown that the absorption behaviour of SO2 in NDI and Im2TEG deviates strongly from the ideality. However, the absorption behaviour of H2S and CO2 in NDI and Im2TEG deviates only slightly from the ideality. Therefore, the solubility data of SO2 were correlated using the PR-NRTL model while the solubility data of H2S and CO2 were correlated with the Krichevsky–Ilinskara (K–I) equation. Thermodynamic parameters including the Henry's constants at infinitely dilute condition and the enthalpy of absorption were calculated from the thermodynamic modelling. The potential application of the two liquid solvents in the selective separation of acidic gases (i.e., SO2/CO2 and H2S/CO2) was evaluated by comprehensively considering the absorption capacity and ideal selectivity. The results were compared with other organic solvents and ionic liquids. It is revealed that NDI and Im2TEG are two promising solvents for the selective separation of acidic gases due to their high absorption capacity of SO2 and H2S and high selectivity of SO2/CO2 and H2S/CO2.

Introduction

The pressure that environmental issue exerts on the earth has been unparalleled in the modern society. One factor that induces this increasingly concerned problem is the large emission of acidic gases (e.g., SO2, H2S and CO2), which mainly results from the utilization or combustion of fossil fuels [1]. A common method of reducing the emission of acidic gases is selective absorption in a liquid solvent [2], [3], [4]. However, currently available technologies are not the best candidates. One reason is the intensive energy consumption during the processing of gas streams. Another reason is the production of byproducts or volatile organic solvents (VOCs). Therefore, exploring suitable solvents for the capture of acidic gases from the perspective of green chemistry is an extensively concerned topic no matter in the industry or academic community.

Among various alternatives, ionic liquids (ILs) are regarded as a class of promising solvent for the capture of acidic gases. ILs exhibit many unique properties such as wide liquid range, high thermal stability, extremely low vapour pressure and structural designability [1], [5], [6], [7]. Therefore, ILs have been studied extensively for the capture of SO2 [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], H2S [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] and CO2 [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. The detailed solubilities of acidic gases in a variety of ILs could be referred to the literature. One of the most impressive advantages of applying ILs in the selective separation of acidic gases is that the gas–liquid cross contamination and the energy consumption could be minimized. However, the high cost and viscosity of ILs are thought to be two most significant limitations for the application of ILs in the industry.

As the building blocks for the construction of ILs, N-substituted imidazoles start to attract researchers’ attention in recent years as alternatives to ILs for the separation of gases [40], [41], [42], [43], [44]. Bara et al. first proposed to use N-alkylimidazoles for the selective absorption of CO2 from CH4 [40], [41]. They further observed the good affinity with SO2 for N-hexylimidazole and N-alkylbenzimidazoles due to the presence of nucleophilic tertiary nitrogen [1], [42]. We reported the detailed solubilities of SO2 in N-methylimidazole and the absolute solubility (mol gas/kg solvent) was found to be much larger than ILs and other organic solvents [45]. In comparison with their analogue ILs, N-substituted imidazoles are of much lower viscosity due to the absence of robust inter- and/or intra-molecular electrostatic attraction [40], [41]. Furthermore, as the precursors of ILs, N-substituted imidazoles could be more easily produced using the method reported by Bara et al. [46]. Impressively, the properties of N-substituted imidazoles could also be tuned through altering the substitutions on the imidazole ring. For example, the volatility of N-substituted imidazole could be reduced significantly when the N-methyl group is replaced by a long-chain N-dodecyl group [40]. Furthermore, introducing ether groups to the side chain of the imidazole ring could enhance the absorption selectivity of gas pairs (e.g., CO2/CH4) [43].

It is of significance to explore low volatile solvents for the separation of acidic gases. The detailed absorption behaviour of different acidic gases (e.g., SO2, H2S and CO2) in N-substituted imidazoles have not yet been investigated systematically to the best of our knowledge. In this work, the solubilities of three major acidic gases (SO2, H2S and CO2) in two long-chain N-substituted imidazoles – N-dodecylimidazole (NDI) and 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis(imidazole) (Im2TEG) were determined systematically. Their chemical structures were illustrated in Scheme 1. The solubility data were correlated with thermodynamic models to analyze the absorption behaviour of acidic gases. Subsequently, the separation performance of gas pairs (SO2/CO2 and H2S/CO2) were compared with other organic solvents and ILs to evaluate their potential application in the selective separation of acidic gases.

Section snippets

Materials

SO2 (99.99% in mole fraction), H2S (99.99% in mole fraction) and CO2 (99.99% in mole fraction) were supplied from Nanjing Special Gas Co. Ltd.; imidazole (99 wt%), 1-bromododecane (98 wt%), tetraethylene glycol (99 wt%) and p-toluenesulfonyl chloride (99 wt%) were all purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification. A summary of the compounds used in this work was presented in Table 1.

Preparation and characterization of NDI and Im2TEG

N-dodecylimidazole (NDI) and

Physical properties

The physical properties of liquid solvents are fundamental data for the process design of gas separation. Therefore, we determined the densities and viscosities of the two long-chain N-substituted imidazoles at different temperatures and the results were presented in Table 2. The densities and viscosities of NDI measured in this work are similar to those reported in the literature [40]. However, the physical properties of Im2TEG are absent in the literature. As is shown, NDI exhibits not only

Conclusions

The solubilities of three major acidic gases (SO2, H2S and CO2) in two low-volatile and long-chain N-substituted imidazoles: N-dodecylimidazole (NDI) and 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis(imidazole) (Im2TEG), were determined systematically under different temperatures and pressures in this work. Although the absorption behaviour of SO2 in NDI and Im2TEG display heavily non-ideal type, the solubility data of SO2 could be correlated with the PR-NRTL model. Furthermore, the nearly

Acknowledgement

The authors appreciate the National Natural Science Foundation of China (No. 21176110 and 21376115) for financial support.

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