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  • One common method to replace the wet limestone

    2020-07-03

    One common method to replace the wet limestone process is the use of an amine-based solution as an absorbent to capture SO2. However, almost all of the common amine-based absorbents are highly volatile compounds, whose emissions are air pollutants themselves. Therefore, it is desirable to either use expensive sealings and high-tech industrial equipment, or else come up with other processes that avoid emissions of solvents to the air [3]. With the latter in mind, new green and environmentally-friendly solvents are being investigated as a desired solution to the environmental and economic issues of amine-based processes. Ionic liquids (ILs) make up a new class of green solvents which have negligible vapor pressures. Because of this special feature, they can be considered as candidate green solvents for various industries, including SO2 absorption. In addition to their low volatilities, other favorable features such as wide liquid temperature ranges, high capacities for SO2 absorption, and high thermal stabilities make ILs even further attractive as green candidates in SO2 capturing processes [9]. Various studies have been carried out which investigate the BMS265246 synthesis of SO2 by different ILs [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, there are some important issues which must be considered in such studies. Most of the ILs are time-consuming to synthetize and quite expensive, limiting their application in different industries [21], [22], [23], [24], [25]. Some are even toxic and can be dangerous for humans and the environment. Deep eutectic solvents (DESs) have been proposed as a new generation of ILs which can overcome the above-mentioned issues. A DES is commonly a mixture consisting of a halide salt (typically ammonium or phosphonium) as a hydrogen-bond acceptor (HBA), and a hydrogen-bond donor (HBD) component. Usually both the HBA and HBD have high melting points, while a mixture of these components makes a DES with a low melting point. The formation of hydrogen bonds between the HBA and HBD is the cause of the deep decrease of the melting point. DESs are not expensive and most are nontoxic. Furthermore, they are very easy to synthetize and are biodegradable. Therefore, DESs have all of the above-mentioned desired characteristics of ILs while overcoming the undesirable features [26], [27], [28]. Consequently, DESs have been investigated as SO2 absorbents in a few studies [3], [29], [30]. Sun et al. [9] used four kinds of DESs based on choline chloride as the HBA, with either ethylene glycol, malonic acid, urea, or thiourea as the HBD in order to absorb SO2. They reported excellent absorption for the systems of choline chloride+ethylene glycol and choline chloride+thiourea. Deng et al. [3] measured the absorption of SO2 in six DESs which consisted of levulinic acid as the HBD and the quaternary ammonium salts of choline chloride, choline acetyl chloride, tetra ethylammonium chloride, tetraethylammonium bromide, tetrabutylammonium chloride, and tetrabutylammonium bromide as the HBAs. They acknowledged good absorption of SO2 in all of the investigated DESs. Based on NMR and FTIR analyses, they also claimed that the SO2 absorption process in all of the investigated systems was a physical process. Yang et al. [29] used choline chloride (HBA) with different ratios of glycerol (HBD) to absorb SO2. They proposed that SO2 solubility increases as the concentration of choline chloride increases. In addition to the need for experimental data on the absorption of SO2 by DESs, knowledge on the intermolecular interactions of SO2 and DESs is very important. Korotkevich et al. [7] investigated SO2 absorption in the choline chloride+glycerol DES by ab initio molecular dynamics simulations. They identified the hydrogen bonding, as well as other specific interactions between all components in the mixture. They reported that by addition of SO2 to the DES, a decrease in the anion-OH group interaction is observed because SO2 molecules and the chloride anions form some complexes. As a result, the SO2 molecules become involved in the hydrophobic network and the interactions between the hydrogen bonds of all the OH groups in the mixture do not change. It was noticed that all of the mentioned interactions of SO2 with the DES are important only in the liquid phase, because in the vapor phase, the molecules are too far separated from one another for the mentioned interactions. In addition, because of the low vapor pressures of DESs, the concentrations of DES molecules are only within trace levels in the vapor phase.