Solubility and thermodynamic properties of SO2 in three low volatile urea derivatives

https://doi.org/10.1016/j.jct.2016.05.004Get rights and content

Highlights

  • The SO2 solubility in three dipolar aprotic urea derivatives has been reported.

  • The experimental data were fitted with reaction equilibrium thermodynamic model.

  • Henry’s constants and several absorption parameters were obtained.

  • The performances of present urea derivatives and other solvents were compared.

Abstract

The solubilities of SO2 in three dipolar aprotic urea diversities, 1,1,3,3-tetramethylurea (TMU), 1,3-dimethyl-2-imidazolidinone (DMI), and 1,3-dimethyl-3,4,5,6-tetrahydro-2 (1H)-pyrimidinone (DMPU) was measured at temperatures ranging from (293.15 to 323.15) K and pressure up to 120 kPa. By correlating the solubility results with a reaction equilibrium thermodynamic model (RETM), Henry’s constants (Hm), reaction equilibrium constants (K°), and enthalpy (ΔHr) in the absorption process were obtained. The results indicate that the three solvents exhibit both strong physical and weak chemical interactions with SO2. Further comparison of the capture performance with ILs, deep eutectic solvents (DESs), and several organic solvents, DMI, DMPU and TMU are noted to be excellent absorbents for acidic SO2.

Introduction

SO2, one of the acidic gases and air contaminants, can cause severe damage to the environment as well as to human health. The emission of SO2 into the environment mainly derives from human activity by the burning of sulfur-containing fossil fuels [1]. Nowadays, flue gas desulfurization (FGD) is regarded as the most effective method in controlling and reducing the emission of SO2 [2]. Many scholars have reviewed the FGD process such as wet FGD, dry and/or semi-dry FGD process [1], [3], [4]. Among them, wet limestone FGD technology shares over 90% of the installed desulfurization capacities worldwide. Nevertheless, the secondary pollution arising from solid waste partially discounts its efficiency and hinders the further extension [5]. Therefore, it’s highly desirable to explore greener and more environmentally friendly absorbents for SO2.

Recently, ionic liquids (ILs) as new emerging media have been widely utilized in catalysis, electrochemistry, separation media and acidic gas absorbents [6]. As attractive absorbents for acidic gases, extensive research has been focused on tuning the structure of ILs or developing IL mixtures to increase their solubility performance for SO2 [6], [7], [8], [9]. Furthermore, various thermodynamic models for the IL-SO2 system have also been developed in fitting or predicting the absorption behavior [10], [11], [12]. Alternatively, deep eutectic solvents (DES), analogy of ILs, have recently emerged as new SO2 absorbents. DESs, formed by choline chloride (ChCl) [13], [14] and some quaternary ammonium salt [15], are also reported to capture SO2. However, due to their high viscosity as well as price, the industrial applications of ILs or DES are still in the cradle.

For a long time, organic solvents with high boiling point, low vapor pressure and appropriate viscosity are regarded to be valuable absorbents of SO2. Many traditional solvents such as DMF [16], DMSO [16], DMA [16], SUF [16], [17], TBP [16], DEA [16], PC [17], NMP [17], [18], [19], [20], and ethylene glycol [17] as well as its derivatives [21] or mixtures [22] have been reported in SO2 removal. In addition, N-substituted imidazoles [17], [23], [24] are also confirmed as SO2 capturer with superior performance. The solubility of SO2 in organic solvents is mainly related to their functional groups in molecular structures. Carbonyl and substituted amino groups are regarded as SO2-philic groups because of their strong interaction with SO2 [17]. According to such structure–activity relationship, three urea derivatives of tetramethylurea (TMU), 1,3-dimethyl-2-imidazolidinone (DMI) and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) were selected in this study as SO2 absorbents and the dissolution behavior were investigated. Current absorbents have low volatility (154.6 Pa, 15.2 Pa, and 5.15 Pa, respectively, according to SciFinder), which means that energy and loss of absorbents during the regeneration process can be both reduced hopefully. Secondly, these absorbents are of low toxicity, thermally and chemically stable, and non-corrosive. The abundant SO2-philic groups in the molecules make them more effective for SO2 absorption. TMU, DMI and DMPU are aprotic dipolar solvents with similar structure. They are known as “universal solvents” in the laboratory and industry owing to their large dipole moments (μTMU = 11.7 D, μDMI = 4.09 D, and μDMPU = 4.23 D at T = 298.15 K), high dielectric constants (εTMU ≈ 23.60, εDMI ≈ 37.60, and εDMPU ≈ 36.12 at T = 298.15 K) [25], [26], and low viscosity (ηTMU = 1.3950 mPa.s, ηDMI = 1.9448 mPa.s, and ηDMPU = 3.1105 mPa.s at T = 298.15 K) [26], [27]. Moreover, they demonstrated strong dissolution ability for many substances. Although the SO2 absorption kinetics with DMPU in a laminar falling film reactor [18] and its SO2 solubility at 298 K [19], as well as the interaction in the system H2O-TMU-SO2 at 278 K and 293 K [28] have been studied, a systematic work in SO2 absorption with DMPU and TMU in a wide range of temperature and pressure can’t be found in the open resources. Therefore, in this study the solubility of SO2 in the selected organic solvents were determined at the temperature ranging from 293.15 K to 323.15 K with 10 K intervals under a pressure of 0 to 120 kPa. The thermodynamic parameters of Henry’s constant (Hm), reaction equilibrium constants (K°), and enthalpy (ΔHr) in the absorption process were also deduced by correlating the solubility values with a reaction equilibrium thermodynamic model (RETM). In addition, the comparisons of the capture performance in the present solvents with those in several other common solvents, ILs and DESs were also made systematically.

Section snippets

Chemicals

SO2 (the mass fraction of more than 0.999, with the same report of purity below) was supplied from Jingong Special Gas Co., Ltd. (Hangzhou, Chiana); TMU (632-22-4, 1,1,3,3-tetramethylurea, 0.995), DMI (80-73-9, 1,3-dimethyl-2-imidazolidinone, 0.996), and DMPU (7226-23-5, 1,3-Dimethyl-3,4,5,6-tetrahydro-2 (1H)-pyrimidinone, 0.993) were purchased from Aladdin Industrial Corporation (Shanghai, China). All the chemicals were used directly as received without further purification. The detailed

Solubility values for SO2 in urea derivatives

The solubility of SO2 in TMU, DMI, and DMPU was determined at T = (293.15 to 323.15) K with 10 K intervals and pressure up to 120 kPa. The experimental results are listed in Table 2, Table 3, Table 4 and also plotted in Fig. 2, Fig. 3, Fig. 4. In the tables, the equilibrium partial pressure of SO2 (ps) is obtained from Eq. (1). The total pressure (p4) and initial pressure of the EC (p1) are listed in the Electronic Supplementary Information; p1 includes the residual air pressure and partial pressure

Conclusion

In the present paper, the solubility of SO2 in three dipolar aprotic urea derivatives are presented at 293.15 K, 303.15 K, 313.15 K, and 323.15 K under pressures of (0–120) kPa using the isochoric saturation method. All these solvents exhibit satisfactory absorption capacity through physical and chemical interactions with SO2. The results indicate that the dissolution of SO2 in these solvents follows a reaction equilibrium thermodynamic model (RETM). The values of Henry’s constant (Hm) and chemical

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