Densities and viscosities of, and NH3 solubilities in deep eutectic solvents composed of ethylamine hydrochloride and acetamide
Graphical abstract
Introduction
NH3 is an alkaline gas with irritant odor. It is mainly present in the purge gas of ammonia synthesis process. The prilling of urea, and synthesis of fertilizers and nitric acid also produce tail gases containing large amount of NH3. The direct emission of NH3 into atmosphere is very harmful to human health, and may lead to many environmental problems such as entrophication of the ecosystem and formation of PM2.5 (particulate matters with kinetic diameter of <2.5 μm) [1], [2]. Therefore, the recycling of NH3 from industrial exhausts is required, and has become an important branch of chemical engineering research. Currently, absorption in liquid solvents is the most mature technology for NH3 capture in industry. In this respect, water and acids are widely used as the solvents [3], [4], [5], [6]. However, these solvents have significant disadvantages. For example, water is highly volatile and of low efficiency for NH3 absorption; acids are highly corrosive and it is difficult to strip the absorbed NH3 out from acids.
To overcome the challenges confronted by traditional solvents for NH3 capture, ionic liquids (ILs) have been proposed as competitive alternatives [7], [8]. They have a wide liquid range, negligible vapor pressure, high thermal stability, and tunable chemical structure, making them rather attractive as the solvents for gas separation [9], [10]. In recent years, numerous researches have been dedicated to the development of IL-based solvents for the capture of CO2 [11], [12], [13], SO2 [14], [15], [16], H2S [17], [18], [19] and NH3 [20], [21], [22]. Unfortunately, the application of ILs in industrial gas separation process is mostly limited by their high cost and viscosity.
Deep eutectic solvents (DESs) are a class of green solvents with similar properties to ILs, including negligible vapor pressure and tunable chemical structure. They can be prepared by simply mixing halide salts with acids/alcohols/amides [23]. The halide salts act as hydrogen-bond acceptors (HBAs), while the acids/alcohols/amides act as hydrogen-bond donors (HBDs). The hydrogen-bonding interaction formed between HBAs and HBDs delocalizes the charge, which induces the significant depression of melting points of mixtures in comparison with individual compounds [24]. Through the rational choice of HBAs and HBDs, DESs with specific functions can be fabricated [25]. It is further worth noting that the halide salts and acids/alcohols/amides used for the fabrication of DESs are normally commercially available. Therefore, DESs are thought to be much cheaper than ILs, and more suitable for scalable production and industrial application.
In terms of the utilization of DESs for gas separation, there have been a number of publications on the investigation of DESs for CO2, SO2 and H2S absorption [26], [27], [28]. There are also some examples on the investigation of DESs for NH3 absorption: Yang et al. [29] and our group [30] designed phenol-based ternary DESs for highly efficient absorption of NH3; Deng et al. [31] investigated the absorption of NH3 in protic NH4SCN-based DESs; Vorotyntsev et al. [32] determined the solubilities of NH3 in methanesulfonate-based DESs, while our group [33], [34] determined the solubilities of NH3 in choline chloride (ChCl) plus urea mixtures and ethylamine chloride (EaCl) plus glycerol mixtures. Even though, the investigation of DESs for NH3 absorption is still quite limited.
The knowledge of NH3 solubilities in DESs is very important for understanding the thermodynamic behavior of NH3 absorption process, and evaluating the performance of DESs for NH3 capture. Given the lone-pair electrons in NH3, it has been well established that solvents with strong hydrogen-bond donating ability are very useful for NH3 absorption [20], [29], [30]. In this work, the solubilities of NH3 in ethylamine hydrochloride (EaCl) plus acetamide (AA) mixtures were measured in the temperature range of (313.2–353.2) K and pressure range of (0–300.0) kPa. The chemical structures of EaCl and AA are shown in Scheme 1. EaCl + AA mixtures were selected because EaCl is a protic ionic salt with strong hydrogen-bond donating ability. We anticipated that such DESs may exhibit high absorption capacities for NH3. In addition, the physical properties of DESs are also fundamental data for industrial process design. Therefore, the densities and viscosities of EaCl + AA mixtures in the temperature range of (313.2–353.2) K were also measured in this work. To the best of our knowledge, the densities and viscosities of, and NH3 solubilities in EaCl + AA mixtures have not yet been reported in the literature.
Section snippets
Chemicals
NH3 (99.99 mol %) was supplied by Jiangxi Huasheng Special Gas Co. Ltd., China. Ethylamine hydrochloride (EaCl, 98 wt%) and acetamide (AA, 98 wt%) were purchased from Adamas Chemicals Co. Ltd., China. The basic information for the chemicals used in this work are summarized in Table 1. All the chemicals were used as received without further purification.
Synthesis and characterizations
DESs were prepared by vigorously stirring the mixtures of EaCl and AA at 353.2 K until clear and transparent liquids were obtained. The prepared
Physical properties
In this work, EaCl + AA mixtures with three different EaCl/AA molar ratios (1:1, 1:2 and 1:3) were prepared. The measured densities and viscosity data are presented in Table 3. The densities decrease linearly, while the viscosities decrease nonlinearly, with the increase of temperature. This is a common trend for liquid solvents [39]. The linear Eq. (3) [39] and Vogel-Fulcher-Tammann (VFT) Eq. (4) [39] were used for the correlation of density data and viscosity data respectively.
Conclusions
In summary, the densities and viscosities of EaCl + AA mixtures were determined at (313.2–353.2) K. The linear equation and VFT equation were used for the correlation of density data and viscosity data respectively. The densities and viscosities decrease with the increase of AA contents in mixtures, and EaCl + AA mixtures have good fluidity, making them quite suitable for application in gas separation process. In addition, the solubilities of NH3 in EaCl + AA mixtures were also determined at
Acknowledgements
This work was supported by the Natural Science Foundation of Jiangxi Province (20171BAB203019). The authors also appreciate the sponsorship from Nanchang University.
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