Solubilities of ammonia in choline chloride plus urea at (298.2–353.2) K and (0–300) kPa
Graphical abstract
Introduction
Ammonia (NH3), an alkaline gas with pungent odor, is mainly emitted from the tail gas of ammonia and urea synthesis processes [1]. It is not only corrosive to equipments and pipelines, but also the precursor of fog and haze [2]. On the other hand, NH3 is an important raw material for the production of nitrogenous fertilizers and freezing media [3]. Therefore, the efficient capture and recycling of NH3 from industrial exhaust gas is highly demanded to achieve environmental and economic benefits [4].
The currently available technologies for NH3 capture in the industry include absorption [5], catalytic conversion [6], and biodegradation [7]. Among these technologies, absorption is the most commonly used. Some typical absorbents are water [8], acids, and ionic liquids (ILs) [9], [10]. However, these absorbents have many inherent defects. For example, water is highly volatile, and the NH3 absorption capacity is quite low; acids exhibit strong chemical reactivity to NH3, resulting in the irreversible absorption of NH3; ILs suffer from complicated synthesis and high cost, which limit their practical applications in the industry [11]. Therefore, it is of great significance to develop new absorbents for NH3 capture.
Recently, deep eutectic solvents (DESs) have attracted tremendous attentions because of their unique properties such as extremely low volatility, wide liquid range and structural designability [12], [13], [14]. They are also regarded as IL analogues, and a class of green solvents with potential application in gas separation [15]. Unlike ILs which are pure compounds, DESs are mixtures comprising of hydrogen-bond acceptors and donors. Therefore, DESs are advantageous over ILs in synthesis and price. It has been demonstrated that DESs are promising absorbents for acid gases such as CO2 and SO2 [16], [17], [18], [19]. However, there are very few reports on the use of DESs for NH3 capture. To the best of our knowledge, the only example is from Yang et al., who reported the efficient absorption of NH3 in phenol-based DESs [20].
To date, the detailed solubilities of NH3 in DESs and the thermodynamic properties of NH3 absorption in DESs are still very scarce in the literature. These data are very important for evaluating the potential application of DESs in NH3 capture, as well as the process design. In this work, the solubilities of NH3 in DESs comprising of choline chloride (ChCl) and urea at different temperatures and pressures were determined. ChCl and urea are two most common compounds for the construction of DESs [21]. Their chemical structures are shown in Scheme 1. The molar ratios of ChCl to urea in selected DESs were 1:1.5, 1:2.0 and 1:2.5. The Henry’s law constants of NH3 in ChCl + urea mixtures were obtained by fitting the experimental data with Henry’s law equation. The thermodynamic properties of NH3 absorption in ChCl + urea mixtures such as Gibbs free energy changes, enthalpy changes and entropy changes were also calculated according to the relationship between Henry’s law constant and temperature.
Section snippets
Materials
NH3 was supplied by Huasheng Special Gas Co. Ltd., China. ChCl and urea were purchased from Adamas Co. Ltd., China. The basic information of chemicals used in this work are presented in Table 1. All the chemicals were used as received. DESs were prepared by stirring the mixtures of ChCl and urea at 333.2 K until clear liquids were obtained. The water contents in prepared DESs were determined by Karl-Fisher titration on a 787KF Titrino instrument. The basic information of DESs prepared in this
Solubilities of NH3
The solubilities of NH3 in ChCl + urea (1:1.5), ChCl + urea (1:2.0) and ChCl + urea (1:2.5) are presented in Table 3, Table 4, Table 5 and plotted in Figure 1, Figure 2, Figure 3. It should be pointed out that the solubilities of NH3 in ChCl + urea (1:1.5) and ChCl + urea (1:2.5) at 298.2 K were not measured, because the two DESs are solid at this temperature. It is found that the solubility of NH3 increases almost linearly with the increase of pressure, indicating that the absorption of NH3 in
Conclusions
In summary, the solubilities of NH3 in DESs comprising of ChCl and urea at (298.2–353.2) K and (0–300) kPa were reported in this work. The solubility of NH3 increases almost linearly with pressure, implying the physical absorption behavior of NH3 in the three DESs. The Henry’s law constants of NH3 in DESs were obtained by fitting the solubility data with the Henry’s law equation. The thermodynamic properties of NH3 absorption in DESs were also calculated according to the dependence of Henry’s
Acknowledgement
This work was supported by the Natural Science Foundation of Jiangxi Province (20171BAB203019) and the National Natural Science Foundation of China (31660482). The authors also appreciate the sponsorship from Nanchang University.
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