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Greener gas capture in deep eutectic solvents aqueous solutions: Performance in a dynamic condition
Journal of Cleaner Production 240 (2019) 118240
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Greener gas capture in deep eutectic solvents aqueous solutions: Performance in a dynamic condition Carlos Carlesi a, *, Nadia Guajardo b, Rodrigo Schrebler c, Dreidy Vasquez-Sandoval a a lica de Valparaíso, Avenida Brasil 2162, P.O. Box, 2362854, Valparaíso, Escuela de Ingeniería Química, Facultad de Ingeniería, Pontificia Universidad Cato Chile b gica Metropolitana, Ignacio Valdivieso 2409, P.O. Box, 8940577, San Joaquín, Santiago, Programa Institucional de Fomento a la IþDþi, Universidad Tecnolo Chile c ~ a del Mar, Chile IONCHEM LTDA, D. Portales 925, P.O. Box, 2580149, Vin
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2018 Received in revised form 10 July 2019 Accepted 29 August 2019 Available online 5 September 2019
The handling of large gas volumes containing low SO2 concentrations is still a challenge for the industry. Here is possible to identify an opportunity for a cleaner production by using novel solvents such as ionic liquids (ILs) specially biodegradables Deep Eutectic Solvents (DES) as an alternative of amine-based absorbents which are volatile and easily degrades and may produce further contamination and health hazard. The present work analyses the performance of four DESs were under dynamic conditions (in a packed-bed column) for SO2 absorption. The results were compared with the absorption capacities published for the same DESs under static conditions identifying essential differences to consider in a large-scale implementation. At low SO2 partial pressure (600 ppm), chemical absorption becomes the most important mechanism, and thus, the chemical nature of the DES determines the absorption capacity. DESs containing amine groups showed superior absorption, and between the thiourea- and ureacontaining DESs, the former exhibited the best performance, which was not related to its viscosity nor its initial solution pH but rather to the chemical interaction of functional groups in the DES promoting ionic interaction with generated sulphites. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Nicole D. Berge Keywords: Ionic liquids Deep eutectic solvents Choline chloride Sulphur dioxide Gas absorption Packed bed column
1. Introduction Sulphur dioxide (SO2) is a significant precursor to particulates and “acid rain” in the atmosphere that poses substantial risks in many developing countries where air pollution is one of the most severe environmental issues (Hu et al., 2019; J. Zhang et al., 2017c). This conventional air pollutant causes considerable damage not only to human health but also affect the natural and social environment, including water resources, ecosystems, buildings, historical monuments and the quality of the soil and the growth of crops leading to agricultural losses (Wei et al., 2014). The current interest to develop efficient SO2 capture technologies is boosted by government policies (Wang et al., 2018) and more strict environmental regulations pushing the industry for cleaner production. Sulphur dioxide is extensively produced by industry (carbonbased energy plants; cement industry, mineral processing by pyro-
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Carlesi). https://doi.org/10.1016/j.jclepro.2019.118240 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
metallurgy, solid waste incinerators, among others), for instance, the construction industry was the primary trigger of industrial SO2 emissions growth with an induced amount (impact on other industries) of SO2 emissions of 10.14 Mt in 2012 only in China (Jiao et al., 2017). Another important example is the mineral smelting industries where large-scale SO2 streams are usually processed in sulphuric acid plants, while low-flow or diluted streams are absorbed and/or chemically neutralised. In this field, the existing processes have operational limitations mainly due to solvent loss by volatilization; chemical/thermal degradation; and in the case of chemical neutralization of absorbed SO2, high consumption of chemical reagents and proportional amount of generated solid waste that must be handled, dried and transported to the site of final disposal. Optimisation and promotion of a modern industrial structure are critical to reducing SO2 emissions (J. Zhang et al., 2017c). Absorption is the primary method used to capture sulphur dioxide industrially, and the performance of absorption systems depends on the physical-chemical properties of solvent-gas pairs, which are influenced by temperature and pressure and other operative
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parameters. Pure absorption and concentration using water or aqueous salt solutions are limited by the sulphur dioxide solubility (Zimmermann et al., 2009), and the chemical equilibrium between the sulphites formed after dissociative absorption of SO2. To reuse absorbed SO2 it must be concentrated at concentration of industrial interest, for example, to join or enrich a feed stream of a sulphuric acid plant (>6% weight basis (Ashar and Golwalkar, 2013)) and then the SO2 recycling is difficult with the use of traditional solvents. On the other hand, the solubility limitations imply the need for a large quantity of solvent flux and large equipment, directly impacting the capital and operating costs of gas treatment units. As analogue with carbon dioxide absorption process, aqueous amine-based absorption has been showed to be a valid process (Niu et al., 2018; Yu et al., 2019). However, recent research has pointed out the health and environmental risk associated with the evaporation and the transformation of these chemical compound by hydrolysis, thermal or oxidative degradation and its releases to the atmosphere and wastewater generation (Ge et al., 2011), and identify this issue as a concerns for the deployment of full-scale post-capture (Ghayur et al., 2019). For instance, mono-ethanolamine, one of the most used amine compound in acid gas capture, was found to present a potential risk of causing severe injuries to the eyes and skin, and its threshold level in vapour concentrations is in the order of few ppb (Dong et al., 2019), and also have been found to have significant impacts on aerosol chemistry (Bottenus et al., 2018). On the other hand, liquid phase degradation products of common ammines are potentially more toxic than the mother amine. Ammonia is the primary degradation products, and secondary compounds are nitramines and nitrosamines. Nitrosamines have been demonstrated to have carcinogenic effects in bioassays and laboratory animals, particularly, N-Nitrosodimethylamine is also a potent mutagen agent (Spietz and Dobras, 2017). Thus, the handling of very high gas volumes containing low SO2 concentrations is still a challenge for the industry, and it is possible to identify an opportunity for process intensification and greener operation by using novel solvents such as ionic liquids (ILs). In recent years, ionic liquids have been extensively tested at the laboratory scale. In general, the relatively high absorption capacities of an aqueous solution, organic solvents and aqueous-amine technologies have been confirmed. The research groups involved in this area of research have used different strates to design ILs for use in the SO2 absorption process. Huang and co-workers identified the physical absorption of a high concentration of sulphur dioxide through saturation of the internal molecular cavity in pure ionic liquids (Huang et al., 2006), and (Wang et al., 2011) and (Tang and Lu, 2015) exploited multiple sites on ionic liquids for interaction with solubilized SO2, enhancing the absorption capacity. Duan et al. (2011) reported an ionic liquid water system for absorption on which phase separation occurs after SO2 introduction. Yang et al. (2013a) reported an intensified absorption in an ionic liquid, where both anions and cations interact with SO2 in its ionic form. Cui et al. (2013) introduced an electronwithdrawing site in the anion of different ILs, increasing the SO2 interaction. Tiang et al. (Tian et al., 2014) developed a system in which the IL has a hydrophobic character; however, because of viscosity issues, water is needed in the system to form an emulsiontype solvent. Cheng et al. (Chen et al., 2015) evaluated different ILs for low-concentration SO2 absorption and concluded that the chain length present in the cationic part of the solvent has a relatively low contribution to the absorption capacity; thus, a cation with a molecular weight as low as possible is preferable. Mondal and Balasubramanian (2016) analysed the absorption of SO2 in pure ILs through quantum chemical calculations and molecular
dynamics, demonstrating the central role in the induced solvation process of both the cation and anion of the solvent and inferring that a good SO2 sorbent must possess a hydrogen bond donor ability in the cation and a strong interaction of the anion with solubilized sulphur dioxide. Li et al. (Li et al., 2017) Using a mixture of ILs to modify the physical properties of a pure IL enhanced the absorption capacity. In most cases, the absorption capacities have a SO2-to-solvent mass ratio in the range of 0.4e1 at 0.1 MPa of SO2 partial pressure Ren et al. (2018), confirming the possibility of operating a desorption stage and the recovery of the saturated ionic liquids. However, for practical considerations, many of the tested ionic liquids lack some of the requirements for process sustainability, such as low toxicity, market availability and cost affordability. Conversely, a family of solvent related to ionic compounds called Deep Eutectic Solvents (DES) are composed of natural or intrinsically biodegradable chemical compounds (Hayyan et al., 2016), and their constituents are produced industrially and thus are available on a large scale for the food, agricultural, pharmaceutical and/or cosmetic industries. These DES correspond to the union between an ionic solid, for example, a quaternary ammonium salt, and a molecule having the capacity to form a hydrogen bond with the ionic solid (such as an amine or carboxylic acid). This bond formation results in a mixture with a melting point that is dramatically reduced with respect to the corresponding melting points of the constituents, allowing the formation of a liquid mixture below 330 K or even at room temperature. Because of the above-mentioned practical positive considerations of DES, in recent years, these solvents were tested for SO2 absorption, confirming the technical feasibility of applying these solvents for this task. The published results focus on a DES based on the quaternary salt choline chloride (ChCl) (Yang et al., 2013b). Tests with DES comprising glycerol as a hydrogen bond donor (HBD) molecule revealed the primary role of the choline cation and the interaction of absorbed SO2 with chloride ions; these interactions were further corroborated theoretically through computational chemistry analysis (Korotkevich et al., 2017; Li et al., 2015). The role of different HBDs was also identified and linked to the charge transfer interaction promotion of these molecules (Sun et al., 2015). Apart from choline chloride quaternary salt, Zhang et al. (K. Zhang et al., 2017a, 2017b) used betaine and L-carnitine, natural ionic compounds, with ethylene glycol as the HBD and imidazoliumbased ionic compounds with glycerol as the HBD; the behaviour was similar to that in the case of the ChCl DES, and the relative absorption capacity depended on the SO2 partial pressure and was verified the reversibility of the absorption, thus allowing the possibility of solvent recycling. In the above-mentioned works, the published results correspond to equilibrium values of solubility at different pressures and temperatures under static conditions, i.e., the authors determined the saturation conditions by measuring the extent of an invariable final weight or pressure after bubbling the gas through a fixed volume of the solvent. In these determinations, some operative parameters are not considered, namely, solvent physical properties or mass transport parameters, which must influence the absorption performance on a dynamic contact system. The scope of the present work is to validate the hypothesis that the contact system employed in the determination of the absorption performance of sulphur dioxide in DES hardly influences the relative absorption capacities among different types of DESs. In particular, in dynamic absorption (in a packed-bed column), viscosity is expected to be directly related to absorption capacity. In the experimentation, other important operative factors, as gas partial pressure, temperature, liquid-to-gas ratio, and the hydrodynamic rme (Fern andez et al., 2015) where maintain constant
C. Carlesi et al. / Journal of Cleaner Production 240 (2019) 118240
and thus no evaluated its combined effects.
Table 2 Viscosity values in mPa*s (cP) measured at 20 C for pure DES solvents and DES solutions containing 10% water (nominal weight). A factor of viscosity reduction was presented in the last column.
1.1. Reference work To validate the abovementioned hypothesis, reference data obtained under static conditions were needed, and the results published by Sun et al. (2015) were considered as reference work. The main results of this work are summarized in Table 1 in terms of the absorption capacity at an equilibrium concentration. The results show a high capacity, in any case, superior to 1 mol of SO2 per mole of solvent, reaching a maximum near a molar ratio of 3 for the case of the DES composed of choline chloride and thiourea. To compare these results for the absorption of the same DES in a different contact system, the four DES were synthesized, and when their viscosity (of the pure DES) was determined (presented in Table 2 under a 0% nominal water content column). It was realized that the viscosities seemed to be too high for the use of these absorption solvents in a packed column and a lack of correlation between viscosity and absorption capacity was noted. To reduce the viscosity of a pure DES to a value more suitable for use in a packed-bed column, a 10% weight basis of deionized water (0.5 mS cm1) was added to each DES, forming a DES-water solution. The viscosity reduction was dramatic, as shown in Table 2 (under a 10% nominal water content column); this effect was previously observed for other DES by Dai et al. (2015). Considering this water effect, a second hypothesis arises related to the reduction in viscosity derived from the addition of water to the solvent and the chemical effect of the water impact the relative absorption capacities among the considered DES. The “nominal water content” refers to the weight percent of water according to the mixing procedure. However, considering that the weighing is performed under laboratory atmospheric conditions and considering the hydrophilicity of DES, the resultant total water content was measured by Karl-Fisher titration, and the results are presented in Table 3. Nevertheless, in the following, the water content will always refer to the nominal content. 2. Materials and methods The deep eutectic solvents considered in this work are composed of choline chloride (ChCl) as an ionic solid and four hydrogen bond donors (HBDs) presented in Fig. 1. All molecules are easily available as bulk chemicals and have intrinsically high environmental compatibility. Choline chloride was obtained from Merck (Emprove® Essential 10, FCC; >98% purity), and all other reagents were obtained from Sigma-Aldrich, including malonic acid (ReagentPlus®; >99% purity), ethylene glycol (Anhydrous; 99.8% purity), urea (U5378; >98% purity), and thiourea (ReagentPlus®; >99% purity). All the regents were dried in an electric furnace at 363 K before mixing. The DES were formed by the mixing of ChCl with each of the hydrogen bond donors in different molar ratios to attain the complete formation of the eutectic mixture: 1:1 ChCl/HBD ratio for the case of malonic acid and thiourea and 1:2 for the case of ethylene glycol and urea. In all cases, the mixtures were stirred manually
Table 1 Absorption capacities under equilibrium conditions in a pure DES obtained by Sun et al. (2015) at 293 K and 1 atm of sulphur dioxide partial pressure. Solvent 1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
3
Molecular Weight
Molar ratio SO2/Solvent
263.73 259.74 243.68 215.74
2.88 1.41 1.88 2.96
Solvent
1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
nominal water content
Ethylene-glycol Urea Malonic Acid Thiourea
0%
10%
Reduction Factor
52 1602 5543 5889
19.14 2.94 45.24 53.50
3 545 123 110
with a glass bar and maintained heated in a water bath at 313 K. After forming the eutectic liquids, the samples were physically characterized in terms of the dynamic viscosity by a Visco Basic Plus (Fungilab) viscometer and the water content by a Karl-Fischer moisture titrator (MKS-500); the same measurements were carried out for the diluted DES (after addition of 10% w/w deionized water). The absorption experiment was carried out in a bench-scale absorption system comprising the elements depicted in Fig. 2, the temperature in the absorption column was maintained, in all experimental runs, at 293 ± 2 K. This system utilizes air that is compressed in an air compressor (2HP- 8 bar max. pressure) (A) that maintains a required inlet pressure for the gas mass controller (B) (Brooks 4800 series; with thermal control); otherwise, at a specific mass flow controller for SO2 gas (Brooks 4800 series; with thermal control), it is connected to a high-pressure, pure SO2 cylinder. The controlled fluxes of both gases are mixed in a T-static mixer to form the inlet gas for the column. The column was cylindrical and 92 cm high; has a 76 mm internal diameter; is made of glass; and is packed with plastic (polyethylene) cylinders that are 9 mm wide, 6 mm in diameter and 1 mm thick. The packing material was randomly loaded into the column for a packing height of 43 cm, and the apparent density of the packing was 0.33 g cm3. The gas exit of the column led to a mixture trap (E) to avoid water condensation in the inlet of the gas analyser (F). This analyser was an SO2 analyser (Teledyne Technologies Company model T100H) with a measuring range from 0 to 5000 ppm and detection based on the UV fluorescence principle. The analyser sampled the output gas line every 10 s, and the output gas was bubbled through a concentrated sodium hydroxide solution, where residual SO2 was chemically neutralised. The gas mixture entered the column through a lateral inlet at the bottom of the packed bed, and the gas flux was 18 ml min1 with an SO2 concentration of 600 ppm, which approaches the average exit concentration of a single-stage condensation acid plant in a metallurgical smelter. The liquid (DES solution) entered at the top of the column driven by a peristaltic pump (Shenchen Lab, 2015) at a rate of 7.5 l h1, where it is distributed by a distributer plate made of acrylic across the packed bed in an inverse direction to that of the gas, and the liquid exits the column at the bottom, where it is collected in a closed liquid container (D) from which it is recirculated by the pump. The total liquid volume in each run was 300 ml. After each absorption run, the packing was retired from the column, and both the packing and the column were gently washed with water until no change in pH of the washing water was observed and the SO2 concentration measured by the gas analyser when passing air through the column was rezeroed. The experimental runs started with the circulation of the liquid through the column, and the initial time corresponded to the time at which the inlet gas entered the column and ended when the concentration measured in the exit gas reached a constant
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Table 3 Water content (mass percent) determined by Karl Fisher titration for the different solutions of a DES and water prepared at a nominal water content after ambient humidity contact (291 K; 60% relative humidity). Solvent
1:2 1:2 1:1 1:1
nominal water content
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
0%
10%
20%
30%
0.62 ± 0.12 0.56 ± 0.07 0.21 ± 0.16 0.49 ± 0.04
13.14 ± 0.09 13.86 ± 0.07 11.05 ± 0.12 12.60 ± 0.03
29.90 ± 0.22 34.10 ± 0.15 22.01 ± 0.08 24.20 ± 0.05
42.00 ± 0.23 51.05 ± 0.25 32.07 ± 0.14 41.07 ± 0.09
Fig. 1. Molecules constituting the deep eutectic solvents considered in this study.
E
SO2
B
C
F
values were obtained by a transient mass balance of the column according to the following equations:
d mSO2 ¼ FCe FCðtÞ dt sat # " tð CðtÞ sat mSO2 ¼ FCe t dt Ce 0
"
mSO2 FCe sat ¼ t ms ms
sat tð
0
CðtÞ dt Ce
#
where F (m3 s1) is the total gas flux passing through the absorption column and C(t) represents the concentration (g m3) of the outgoing gas measured at regular intervals during the process. A Trapezoidal Rule Integration was used for the calculations from the experimental data.
A
D Fig. 2. Experimental set-up. A: Air compressor; B: Gas mass controller; C: Absorption column; D: Solvent reservoir; E: Water trap F: Gas Analyser.
concentration equal to the concentration of the gas entering the column (saturation). 2.1. Calculations The absorption capacity was calculated in terms of the mass of sulphur dioxide absorbed (mSO2 in grams) by the total mass of solvent (mS in grams) (the solvent was recirculated through the column) during the time (in seconds) required to attain complete saturation (tsat), i.e., when the outgoing concentration of sulphur dioxide (Co) was the same as the entering concentration (Ce). The
3. Results and discussion In monitoring the concentration of SO2 in the column output, in all cases, two time periods could be identified. The first period was characterized by a zero-output concentration, that is, complete absorption, which is evidence of a rapid chemical interaction of the solvent with the gas flux. In this period, sulphur dioxide must dissociate in its ionic form (bisulphite and further to sulphite), and the extension of the time of this initial stage contributes to the major proportion of the calculated value of the absorption capacities. The second period of the absorption process corresponded to an exponential rise in the concentration up to saturation, which is connected to the physical absorption process. Fig. 3 shows representative breakthrough SO2 absorption curves obtained for the different DES solutions and includes the results
C. Carlesi et al. / Journal of Cleaner Production 240 (2019) 118240
5
1 0.9
A
B
C
D
E
0.8 0.7
C(t)/Ce
0.6 0.5 0.4 0.3 0.2 0.1 0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
me [min] Fig. 3. Representative breakthrough SO2 absorption curves for the different DES solutions and A: 1 M sulphuric acid solution (for reference). B: ChCl-Ethylene glycol C: ChCl-Malonic acid; D: ChCl-Urea; E: ChCl-Thiourea.
obtained using a 1 M sulphuric acid solution for comparative analysis. The first observation is the confirmation of the improved absorption capacity with the DES solution in comparison with the capacity with the sulphuric acid solution because the onset of the increase in the output concentration of SO2 was at least twice as long with ChCl-ethylene glycol DES solution as it was with the sulphuric acid solution. From the average of the absorption curves rstered for each DES solution, an absorption capacity was calculated and is presented in Table 4. Fig. 3 also shows that the time elapsed in the first stage of the absorption is similar for the DES composed of urea and thiourea, which have similar chemical compositions, and the difference in the calculated absorption capacity arises mainly from the slope of the second stage of absorption (physical component) and thus could be associated with the difference in the transport characteristics of SO2 diffusion through the liquid (molecular and ionic). The viscosity changes of the DES solution were not measured during the operation of the absorption process; however, final measurements showed a slight increase in each case of no more than 5% of the initial viscosity, and the same was observed for the density of the saturated solutions. These observations could be related to the effect of the water content to diminish viscosity changes and to the effect of the physical interaction between formed sulphites and the ionic species of the solvent to reduce the electrostatic interaction between the cation and anion of the DES, thus counteracting the increasing viscosity expected from the strong initial chemical interaction. As depicted in Fig. 4, the calculated absorption capacity presented in Table 4 was compared with the results obtained in the
ChCl-Malonic Acid
ChCl-Ethylene-glycol ChCl-Thiourea
Fig. 4. Comparison between the absorption capacities under static conditions from Sun et al. (Sun et al., 2015) (full bars; values on the left axis) and the obtained results under dynamic conditions (dashed bars; values on the right axis).
reference study (Sun et al., 2015) to evaluate the differences between the absorption capacity under static conditions and dynamic conditions. Here, it is important to consider the experimental differences of both works, mainly the partial pressure of the gas (SO2). In the reference case, the partial pressure was 1 atm, and in the present study, it was related to the 600 ppm of SO2. Moreover, the dilution (the addition of 10% w/w water) in this study was another difference. These distinctions explain the great differences in capacity (a factor of 103). Despite these differences, the comparison of the results in Fig. 4 allows us to confirm the hypothesis that the effect of the water content hardly affects the relative absorption capacities
Table 4 Absorption capacities of the four DESs (10% nominal water content) determined in this study under saturation conditions in terms of the quantity of SO2 absorbed/solvent ratio. Solvent 1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
Molar ratio (X 103)
Weigh ratio (X 103)
Mass to volume ratio (g/L)
0.75 1.26 0.98 1.37
0.19 0.25 0.22 0.31
0.20 0.37 0.30 0.47
C. Carlesi et al. / Journal of Cleaner Production 240 (2019) 118240
between the different DES. In fact, the relative capacities exhibited opposite trends, except for the ChCl-thiourea DES, which in both studies, presented the better absorption capacity among all the DES tested. Regarding the use of water forming an aqueous solution of DES, the favourable property of non-volatility of the DES which is a key factor in terms of safety and environmental impact (emanations) of the process, it must be considers an unavoidable loss of water from the system, by a drying effect by the entering gas, however the safety and environmental compatibility of the process is not affected by this factor. In Fig. 5, the results of the absorption capacities are compared with the determined values of the initial viscosities of each DES solution; it is possible to evidence an expected trend for the first three DES solutions since the absorption capacity was better for the lower viscosity solvent. However, the result obtained for ChClthiourea did not follow this correlation, and thus, is not possible to generalize the results and thus refuting the second hypothesis. Considering the presence of water in each absorbing solution, a known chemical equilibrium is expected to form between molecular SO2, bisulphite, sulphite and water (pH). This equilibrium implies an increase in solution acidity during the dissociative absorption of SO2 that was confirmed by the measurement of the initial and final pH of each DES solution; the results are shown in Table 5. Here, it is possible to observe that in each case, the solution was acidified, except in the case of the DES containing malonic acid, which was extremely acidic in the bnning of the process. However, the absorption in the case of the DES containing malonic acid was not the worst; in fact, the worst absorption capacity was obtained with the ethylene glycol-based DES, which was basic in its initial state. On the other hand, the better results obtained with the thiourea-containing DES solution were not directly related to its initial basicity, which was less than that of the urea DES solution, and the only possible correlation (except for that concerning the malonic acid DES) is that a better absorption capacity is related to a greater difference in acidity of the absorber solution between the initial condition and after saturation. For all the DES solutions tested, the period of the change in solution pH coincided with the time required to reach saturation; thus, the changes of the solvent pH are indicative of the end of the predominance of the chemical step of the absorption and could be used as an easy control parameter in the operation of the
Table 5 pH values of DES solutions, initially and under saturation conditions. Solvent (10% nominal water)
Initial
Final
1:2 1:2 1:1 1:1
9.0 9.0 0.1 7.3
6.1 5.0 0.1 2.2
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
absorption process. The chemical interaction of sulphur dioxide in ionic liquids and DES was documented in the literature (Ren et al., 2018). In this study, we corroborate this point from the experimental evidence of, first: the above-mentioned characteristic shape of the breakthrough absorption curves, which present a first stage of zero concentration in the output gas and a continuous pH change and a colour change. In the case of the DES composed of malonic acid, the pH of the original solution was near zero, but the absorption was better than that in the case of a more basic DES solution (e.g., the ethylene glycol DES). Thus, the initial pH of the absorbing solution is not a determining factor and, the interaction of SO2 with the choline chloride cation seems to be more important for the overall performance of the process than the promotion of the formation of sulphites or bisulphites from the dissociative absorption of SO2. Considering the chemical differences between the different DESs tested, which have the same ionic component (choline chloride), the presence of primary amines in the hydrogen bond donor (HBD) results in a higher absorption capacity. This HBD effect could be related to different factors. The first factor is the impact of the asymmetry between the anions in the DES. More asymmetrical DES are prone to suffer large entropy reduction when absorbing an ionizable molecule such as sulphur dioxide and thus must be a better sorbent when there is more initial asymmetry. A second factor is the direct interaction of the HBD with the absorbate. In this case, sulphur dioxide is known to interact with amines (Jiang et al., 2016), and there is an indirect promoting effect of the interaction of sulphites with the choline cation when the HBD molecule interacts with the anion (in this case chloride), reducing the ionic interaction between the original anion and cation of the DES. Regarding the differences between two DES having the same
60 55 50 45 40 35 30 25 20 15 10 5 0
0.50 0.45
[g SO2 / L solvent]
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 U
AM
EG
Viscosity [cp]
6
Ti
Fig. 5. Comparison between the determined absorption capacity (dashed bars; values on the left axis) and the viscosity of the DES solution solvents (full bars; values on the right axis).
C. Carlesi et al. / Journal of Cleaner Production 240 (2019) 118240
amine groups, the better result in the case of the thioureacontaining DES could be related to the major proportion on a mole basis of the choline cation or with the lower electronegativity of sulphur in thiourea than oxygen in urea, which results in a more direct interaction with the oxygen radicals in sulphite molecules generated after the dissociative absorption of SO2. 4. Conclusions The reduction of SO2 emission has paramount importance to support industrial activities and economic growth, especially in developing economies, here, cleaner production and intensified absorption processes must be implemented. A solution composed of a deep eutectic solvent (four were tested) and a 10% water (mass basis) allows an enhanced absorption process for diluted sulphur dioxide gas streams, thus enabling a reduction in the required equipment size compared with that required when using an acid water solution. This work confirms the positive effect of the use of an aqueous solution of an ionic liquid rather than pure ionic liquid even considering that the inherent absorption capacity of the ionic liquid is usually higher than the dilution. This fact is related to the methods used to determine absorption capacity, which was evidenced in this work, where only in dynamic conditions is possible to account of mass transport effects. There are clear differences between the relative solubility of a pure DES absorbing pure SO2 and a DES-water solution absorbing a low-partial-pressure SO2 stream. In the latter case, the chemical absorption becomes the most important mechanism, and thus, the chemical nature of the DES determines the absorption capacity. The DES containing amine groups showed superior absorption capacity, and between the thiourea- and urea-containing DESs. The former exhibited the best performance, which was not related to the viscosity nor to the initial solution pH but rather to the chemical interaction of functional groups with the generated sulphites and, indirectly, to the action of these functional groups in lowering the cation-anion interactions in the original DES, which further promote the interaction of the cations on the DES with the sulphites generated from the dissociative absorption of SO2. An aqueous solution of thiourea-based deep eutectic solvent presents good characteristic to be used in packed bed absorption of low partial pressure sulphur dioxide instead of more traditional amine-based systems. Aqueous DES solutions must improve the safety and environmental impact of the capture process, since the inherent lower volatility, higher thermal and chemical resistance of the DES, allowing a reduction of a toxic degradation by-products emission to the atmosphere and wastewater generation thus reducing the need of solvent make-up. Acknowledgements This work was funded by the Chilean Estate through the Fondecyt Program (regular) grant n 1150235, Education Minister. References Ashar, N.G., Golwalkar, K.R., 2013. A practical Guide to the Manufacture of Sulfuric acid, Oleums, and Sulfonating agents. https://doi.org/10.1007/978-3-31902042-6. Bottenus, C.L.H., Massoli, P., Sueper, D., Canagaratna, M.R., VanderSchelden, G., Jobson, B.T., VanReken, T.M., 2018. Identification of amines in wintertime ambient particulate material using high resolution aerosol mass spectrometry. Atmos. Environ. 180, 173e183. https://doi.org/10.1016/j.atmosenv.2018.01.044. Chen, K., Lin, W., Yu, X., Luo, X., Ding, F., He, X., Li, H., Wang, C., 2015. Designing of anion-functionalized ionic liquids for efficient capture of SO2 from flue gas. AIChE J. 61, 2028e2034. https://doi.org/10.1002/aic.14793. Cui, G., Zheng, J., Luo, X., Lin, W., Ding, F., Li, H., Wang, C., 2013. Tuning anion-
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Reversible physical absorption of SO2 by ionic liquids. Chem. Commun. (Camb) 4027e4029. https:// doi.org/10.1039/b609714f. Jiang, Y., Liu, X., Deng, D., 2016. Solubility and thermodynamic properties of SO2 in three low volatile urea derivatives. J. Chem. Thermodyn. 101, 12e18. https://doi. org/10.1016/j.jct.2016.05.004. Jiao, J., Han, X., Li, F., Bai, Y., Yu, Y., 2017. Contribution of demand shifts to industrial SO 2 emissions in a transition economy: evidence from China. J. Clean. Prod. 164, 1455e1466. https://doi.org/10.1016/j.jclepro.2017.07.060. Korotkevich, A., Firaha, D.S., Padua, A.A.H., Kirchner, B., 2017. Ab initio molecular dynamics simulations of SO2 solvation in choline chloride/glycerol deep eutectic solvent. Fluid Phase Equilib. 448, 59e68. https://doi.org/10.1016/j.fluid. 2017.03.024. Li, Wen, Liu, Ying, Wang, Lihong, Gao, G., 2017. No Title. Energy Fuel. 31, 1771e1777. Li, Hongping, Chang, Y., Zhu, W., Wang, Changwei, Wang, Chao, Yin, S., Zhang, M., Li, Huaming, 2015. Theoretical evidence of charge transfer interaction between SO(2) and deep eutectic solvents formed by choline chloride and glycerol. Phys. Chem. Chem. Phys. 17, 28729e28742. https://doi.org/10.1039/c5cp04172d. Mondal, A., Balasubramanian, S., 2016. Understanding SO2 capture by ionic liquids. J. Phys. Chem. B 120, 4457e4466. https://doi.org/10.1021/acs.jpcb.6b02553. Niu, Y., Li, C., Shen, J., Wei, X., 2018. Absorption of dilute sulfur dioxide in ethanediamine with ethylene glycol or polyethylene glycol 400 plus water system. J. Clean. Prod. 171, 506e511. https://doi.org/10.1016/j.jclepro.2017.09.269. Ren, S., Hou, Y., Zhang, K., Wu, W., 2018. Ionic liquids: Functionalization and absorption of SO2. Green Energy Environ 3, 179e190. https://doi.org/10.1016/j.gee. 2017.11.003. Spietz, T., Dobras, S., 2017. Nitrosamines and nitramines in carbon capture plants nitrozo. Environ. Prot. Nat. Resour. 28, 43e50. Sun, S., Niu, Y., Xu, Q., Sun, Z., Wei, X., 2015. Efficient SO2 absorptions by four Kinds of deep eutectic solvents based on choline chloride. Ind. Eng. Chem. Res. 54, 8019e8024. https://doi.org/10.1021/acs.iecr.5b01789. Tang, H., Lu, D., 2015. Multiple-site SO2-capture ionic liquids with nearly Uniform site performance. ChemPhysChem 16, 2854e2860. https://doi.org/10.1002/ cphc.201500369. Tian, S., Hou, Y., Wu, W., Ren, S., Qian, J., 2014. Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2. J. Hazard Mater. 278, 409e416. https://doi.org/10.1016/j.jhazmat.2014.06.037. Wang, C., Cui, G., Luo, X., Xu, Y., Li, H., Dai, S., 2011. Highly efficient and reversible SO2capture by tunable azole-based ionic liquids through multiple-site chemical absorption. J. Am. Chem. Soc. 133, 11916e11919. https://doi.org/10.1021/ ja204808h. Wang, S., Qing, L.J., Wang, H., Li, H.Y., 2018. Integrated assessment of environmental performance-based contracting for sulfur dioxide emission control in Chinese coal power plants. J. Clean. Prod. 177, 878e887. https://doi.org/10.1016/j.jclepro. 2017.12.280. Wei, J., Guo, X., Marinova, D., Fan, J., 2014. Industrial SO 2 pollution and agricultural losses in China: evidence from heavy air polluters. J. Clean. Prod. 64, 404e413. https://doi.org/10.1016/j.jclepro.2013.10.027. Yang, D., Hou, M., Ning, H., Ma, J., Kang, X., Zhang, J., Han, B., 2013a. Reversible capture of SO2 through functionalized ionic liquids. ChemSusChem 6, 1191e1195. https://doi.org/10.1002/cssc.201300224. Yang, D., Hou, M., Ning, H., Zhang, J., Ma, J., Yang, G., Han, B., 2013b. Efficient SO2 absorption by renewable choline chlorideeglycerol deep eutectic solvents. Green Chem. 15, 2261. https://doi.org/10.1039/c3gc40815a. Yu, X., Hao, J., Xi, Z., Liu, T., Lin, Y., Xu, B., 2019. Investigation of low concentration SO 2 adsorption performance on different amine-modified Merrifield resins. Atmos. Pollut. Res. 10, 404e411. https://doi.org/10.1016/j.apr.2018.08.015. Zhang, J., Zhang, Y., Yang, H., Zheng, C., Jin, K., Wu, X., Gao, X., Cen, K., 2017c. Cost-
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effectiveness optimization for SO2 emissions control from coal-fired power plants on a national scale: a case study in China. J. Clean. Prod. 165, 1005e1012. https://doi.org/10.1016/j.jclepro.2017.07.046. Zhang, K., Ren, S., Hou, Y., Wu, W., 2017a. Efficient absorption of SO2with lowpartial pressures by environmentally benign functional deep eutectic solvents. J. Hazard Mater. 324, 457e463. https://doi.org/10.1016/j.jhazmat.2016.11. 012. Zhang, K., Ren, S., Yang, X., Hou, Y., Wu, W., Bao, Y., 2017b. Efficient absorption of
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Greener gas capture in deep eutectic solvents aqueous solutions: Performance in a dynamic condition Carlos Carlesi a, *, Nadia Guajardo b, Rodrigo Schrebler c, Dreidy Vasquez-Sandoval a a lica de Valparaíso, Avenida Brasil 2162, P.O. Box, 2362854, Valparaíso, Escuela de Ingeniería Química, Facultad de Ingeniería, Pontificia Universidad Cato Chile b gica Metropolitana, Ignacio Valdivieso 2409, P.O. Box, 8940577, San Joaquín, Santiago, Programa Institucional de Fomento a la IþDþi, Universidad Tecnolo Chile c ~ a del Mar, Chile IONCHEM LTDA, D. Portales 925, P.O. Box, 2580149, Vin
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2018 Received in revised form 10 July 2019 Accepted 29 August 2019 Available online 5 September 2019
The handling of large gas volumes containing low SO2 concentrations is still a challenge for the industry. Here is possible to identify an opportunity for a cleaner production by using novel solvents such as ionic liquids (ILs) specially biodegradables Deep Eutectic Solvents (DES) as an alternative of amine-based absorbents which are volatile and easily degrades and may produce further contamination and health hazard. The present work analyses the performance of four DESs were under dynamic conditions (in a packed-bed column) for SO2 absorption. The results were compared with the absorption capacities published for the same DESs under static conditions identifying essential differences to consider in a large-scale implementation. At low SO2 partial pressure (600 ppm), chemical absorption becomes the most important mechanism, and thus, the chemical nature of the DES determines the absorption capacity. DESs containing amine groups showed superior absorption, and between the thiourea- and ureacontaining DESs, the former exhibited the best performance, which was not related to its viscosity nor its initial solution pH but rather to the chemical interaction of functional groups in the DES promoting ionic interaction with generated sulphites. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Nicole D. Berge Keywords: Ionic liquids Deep eutectic solvents Choline chloride Sulphur dioxide Gas absorption Packed bed column
1. Introduction Sulphur dioxide (SO2) is a significant precursor to particulates and “acid rain” in the atmosphere that poses substantial risks in many developing countries where air pollution is one of the most severe environmental issues (Hu et al., 2019; J. Zhang et al., 2017c). This conventional air pollutant causes considerable damage not only to human health but also affect the natural and social environment, including water resources, ecosystems, buildings, historical monuments and the quality of the soil and the growth of crops leading to agricultural losses (Wei et al., 2014). The current interest to develop efficient SO2 capture technologies is boosted by government policies (Wang et al., 2018) and more strict environmental regulations pushing the industry for cleaner production. Sulphur dioxide is extensively produced by industry (carbonbased energy plants; cement industry, mineral processing by pyro-
* Corresponding author. E-mail addresses: [email protected], [email protected] (C. Carlesi). https://doi.org/10.1016/j.jclepro.2019.118240 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
metallurgy, solid waste incinerators, among others), for instance, the construction industry was the primary trigger of industrial SO2 emissions growth with an induced amount (impact on other industries) of SO2 emissions of 10.14 Mt in 2012 only in China (Jiao et al., 2017). Another important example is the mineral smelting industries where large-scale SO2 streams are usually processed in sulphuric acid plants, while low-flow or diluted streams are absorbed and/or chemically neutralised. In this field, the existing processes have operational limitations mainly due to solvent loss by volatilization; chemical/thermal degradation; and in the case of chemical neutralization of absorbed SO2, high consumption of chemical reagents and proportional amount of generated solid waste that must be handled, dried and transported to the site of final disposal. Optimisation and promotion of a modern industrial structure are critical to reducing SO2 emissions (J. Zhang et al., 2017c). Absorption is the primary method used to capture sulphur dioxide industrially, and the performance of absorption systems depends on the physical-chemical properties of solvent-gas pairs, which are influenced by temperature and pressure and other operative
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parameters. Pure absorption and concentration using water or aqueous salt solutions are limited by the sulphur dioxide solubility (Zimmermann et al., 2009), and the chemical equilibrium between the sulphites formed after dissociative absorption of SO2. To reuse absorbed SO2 it must be concentrated at concentration of industrial interest, for example, to join or enrich a feed stream of a sulphuric acid plant (>6% weight basis (Ashar and Golwalkar, 2013)) and then the SO2 recycling is difficult with the use of traditional solvents. On the other hand, the solubility limitations imply the need for a large quantity of solvent flux and large equipment, directly impacting the capital and operating costs of gas treatment units. As analogue with carbon dioxide absorption process, aqueous amine-based absorption has been showed to be a valid process (Niu et al., 2018; Yu et al., 2019). However, recent research has pointed out the health and environmental risk associated with the evaporation and the transformation of these chemical compound by hydrolysis, thermal or oxidative degradation and its releases to the atmosphere and wastewater generation (Ge et al., 2011), and identify this issue as a concerns for the deployment of full-scale post-capture (Ghayur et al., 2019). For instance, mono-ethanolamine, one of the most used amine compound in acid gas capture, was found to present a potential risk of causing severe injuries to the eyes and skin, and its threshold level in vapour concentrations is in the order of few ppb (Dong et al., 2019), and also have been found to have significant impacts on aerosol chemistry (Bottenus et al., 2018). On the other hand, liquid phase degradation products of common ammines are potentially more toxic than the mother amine. Ammonia is the primary degradation products, and secondary compounds are nitramines and nitrosamines. Nitrosamines have been demonstrated to have carcinogenic effects in bioassays and laboratory animals, particularly, N-Nitrosodimethylamine is also a potent mutagen agent (Spietz and Dobras, 2017). Thus, the handling of very high gas volumes containing low SO2 concentrations is still a challenge for the industry, and it is possible to identify an opportunity for process intensification and greener operation by using novel solvents such as ionic liquids (ILs). In recent years, ionic liquids have been extensively tested at the laboratory scale. In general, the relatively high absorption capacities of an aqueous solution, organic solvents and aqueous-amine technologies have been confirmed. The research groups involved in this area of research have used different strates to design ILs for use in the SO2 absorption process. Huang and co-workers identified the physical absorption of a high concentration of sulphur dioxide through saturation of the internal molecular cavity in pure ionic liquids (Huang et al., 2006), and (Wang et al., 2011) and (Tang and Lu, 2015) exploited multiple sites on ionic liquids for interaction with solubilized SO2, enhancing the absorption capacity. Duan et al. (2011) reported an ionic liquid water system for absorption on which phase separation occurs after SO2 introduction. Yang et al. (2013a) reported an intensified absorption in an ionic liquid, where both anions and cations interact with SO2 in its ionic form. Cui et al. (2013) introduced an electronwithdrawing site in the anion of different ILs, increasing the SO2 interaction. Tiang et al. (Tian et al., 2014) developed a system in which the IL has a hydrophobic character; however, because of viscosity issues, water is needed in the system to form an emulsiontype solvent. Cheng et al. (Chen et al., 2015) evaluated different ILs for low-concentration SO2 absorption and concluded that the chain length present in the cationic part of the solvent has a relatively low contribution to the absorption capacity; thus, a cation with a molecular weight as low as possible is preferable. Mondal and Balasubramanian (2016) analysed the absorption of SO2 in pure ILs through quantum chemical calculations and molecular
dynamics, demonstrating the central role in the induced solvation process of both the cation and anion of the solvent and inferring that a good SO2 sorbent must possess a hydrogen bond donor ability in the cation and a strong interaction of the anion with solubilized sulphur dioxide. Li et al. (Li et al., 2017) Using a mixture of ILs to modify the physical properties of a pure IL enhanced the absorption capacity. In most cases, the absorption capacities have a SO2-to-solvent mass ratio in the range of 0.4e1 at 0.1 MPa of SO2 partial pressure Ren et al. (2018), confirming the possibility of operating a desorption stage and the recovery of the saturated ionic liquids. However, for practical considerations, many of the tested ionic liquids lack some of the requirements for process sustainability, such as low toxicity, market availability and cost affordability. Conversely, a family of solvent related to ionic compounds called Deep Eutectic Solvents (DES) are composed of natural or intrinsically biodegradable chemical compounds (Hayyan et al., 2016), and their constituents are produced industrially and thus are available on a large scale for the food, agricultural, pharmaceutical and/or cosmetic industries. These DES correspond to the union between an ionic solid, for example, a quaternary ammonium salt, and a molecule having the capacity to form a hydrogen bond with the ionic solid (such as an amine or carboxylic acid). This bond formation results in a mixture with a melting point that is dramatically reduced with respect to the corresponding melting points of the constituents, allowing the formation of a liquid mixture below 330 K or even at room temperature. Because of the above-mentioned practical positive considerations of DES, in recent years, these solvents were tested for SO2 absorption, confirming the technical feasibility of applying these solvents for this task. The published results focus on a DES based on the quaternary salt choline chloride (ChCl) (Yang et al., 2013b). Tests with DES comprising glycerol as a hydrogen bond donor (HBD) molecule revealed the primary role of the choline cation and the interaction of absorbed SO2 with chloride ions; these interactions were further corroborated theoretically through computational chemistry analysis (Korotkevich et al., 2017; Li et al., 2015). The role of different HBDs was also identified and linked to the charge transfer interaction promotion of these molecules (Sun et al., 2015). Apart from choline chloride quaternary salt, Zhang et al. (K. Zhang et al., 2017a, 2017b) used betaine and L-carnitine, natural ionic compounds, with ethylene glycol as the HBD and imidazoliumbased ionic compounds with glycerol as the HBD; the behaviour was similar to that in the case of the ChCl DES, and the relative absorption capacity depended on the SO2 partial pressure and was verified the reversibility of the absorption, thus allowing the possibility of solvent recycling. In the above-mentioned works, the published results correspond to equilibrium values of solubility at different pressures and temperatures under static conditions, i.e., the authors determined the saturation conditions by measuring the extent of an invariable final weight or pressure after bubbling the gas through a fixed volume of the solvent. In these determinations, some operative parameters are not considered, namely, solvent physical properties or mass transport parameters, which must influence the absorption performance on a dynamic contact system. The scope of the present work is to validate the hypothesis that the contact system employed in the determination of the absorption performance of sulphur dioxide in DES hardly influences the relative absorption capacities among different types of DESs. In particular, in dynamic absorption (in a packed-bed column), viscosity is expected to be directly related to absorption capacity. In the experimentation, other important operative factors, as gas partial pressure, temperature, liquid-to-gas ratio, and the hydrodynamic rme (Fern andez et al., 2015) where maintain constant
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and thus no evaluated its combined effects.
Table 2 Viscosity values in mPa*s (cP) measured at 20 C for pure DES solvents and DES solutions containing 10% water (nominal weight). A factor of viscosity reduction was presented in the last column.
1.1. Reference work To validate the abovementioned hypothesis, reference data obtained under static conditions were needed, and the results published by Sun et al. (2015) were considered as reference work. The main results of this work are summarized in Table 1 in terms of the absorption capacity at an equilibrium concentration. The results show a high capacity, in any case, superior to 1 mol of SO2 per mole of solvent, reaching a maximum near a molar ratio of 3 for the case of the DES composed of choline chloride and thiourea. To compare these results for the absorption of the same DES in a different contact system, the four DES were synthesized, and when their viscosity (of the pure DES) was determined (presented in Table 2 under a 0% nominal water content column). It was realized that the viscosities seemed to be too high for the use of these absorption solvents in a packed column and a lack of correlation between viscosity and absorption capacity was noted. To reduce the viscosity of a pure DES to a value more suitable for use in a packed-bed column, a 10% weight basis of deionized water (0.5 mS cm1) was added to each DES, forming a DES-water solution. The viscosity reduction was dramatic, as shown in Table 2 (under a 10% nominal water content column); this effect was previously observed for other DES by Dai et al. (2015). Considering this water effect, a second hypothesis arises related to the reduction in viscosity derived from the addition of water to the solvent and the chemical effect of the water impact the relative absorption capacities among the considered DES. The “nominal water content” refers to the weight percent of water according to the mixing procedure. However, considering that the weighing is performed under laboratory atmospheric conditions and considering the hydrophilicity of DES, the resultant total water content was measured by Karl-Fisher titration, and the results are presented in Table 3. Nevertheless, in the following, the water content will always refer to the nominal content. 2. Materials and methods The deep eutectic solvents considered in this work are composed of choline chloride (ChCl) as an ionic solid and four hydrogen bond donors (HBDs) presented in Fig. 1. All molecules are easily available as bulk chemicals and have intrinsically high environmental compatibility. Choline chloride was obtained from Merck (Emprove® Essential 10, FCC; >98% purity), and all other reagents were obtained from Sigma-Aldrich, including malonic acid (ReagentPlus®; >99% purity), ethylene glycol (Anhydrous; 99.8% purity), urea (U5378; >98% purity), and thiourea (ReagentPlus®; >99% purity). All the regents were dried in an electric furnace at 363 K before mixing. The DES were formed by the mixing of ChCl with each of the hydrogen bond donors in different molar ratios to attain the complete formation of the eutectic mixture: 1:1 ChCl/HBD ratio for the case of malonic acid and thiourea and 1:2 for the case of ethylene glycol and urea. In all cases, the mixtures were stirred manually
Table 1 Absorption capacities under equilibrium conditions in a pure DES obtained by Sun et al. (2015) at 293 K and 1 atm of sulphur dioxide partial pressure. Solvent 1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
3
Molecular Weight
Molar ratio SO2/Solvent
263.73 259.74 243.68 215.74
2.88 1.41 1.88 2.96
Solvent
1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
nominal water content
Ethylene-glycol Urea Malonic Acid Thiourea
0%
10%
Reduction Factor
52 1602 5543 5889
19.14 2.94 45.24 53.50
3 545 123 110
with a glass bar and maintained heated in a water bath at 313 K. After forming the eutectic liquids, the samples were physically characterized in terms of the dynamic viscosity by a Visco Basic Plus (Fungilab) viscometer and the water content by a Karl-Fischer moisture titrator (MKS-500); the same measurements were carried out for the diluted DES (after addition of 10% w/w deionized water). The absorption experiment was carried out in a bench-scale absorption system comprising the elements depicted in Fig. 2, the temperature in the absorption column was maintained, in all experimental runs, at 293 ± 2 K. This system utilizes air that is compressed in an air compressor (2HP- 8 bar max. pressure) (A) that maintains a required inlet pressure for the gas mass controller (B) (Brooks 4800 series; with thermal control); otherwise, at a specific mass flow controller for SO2 gas (Brooks 4800 series; with thermal control), it is connected to a high-pressure, pure SO2 cylinder. The controlled fluxes of both gases are mixed in a T-static mixer to form the inlet gas for the column. The column was cylindrical and 92 cm high; has a 76 mm internal diameter; is made of glass; and is packed with plastic (polyethylene) cylinders that are 9 mm wide, 6 mm in diameter and 1 mm thick. The packing material was randomly loaded into the column for a packing height of 43 cm, and the apparent density of the packing was 0.33 g cm3. The gas exit of the column led to a mixture trap (E) to avoid water condensation in the inlet of the gas analyser (F). This analyser was an SO2 analyser (Teledyne Technologies Company model T100H) with a measuring range from 0 to 5000 ppm and detection based on the UV fluorescence principle. The analyser sampled the output gas line every 10 s, and the output gas was bubbled through a concentrated sodium hydroxide solution, where residual SO2 was chemically neutralised. The gas mixture entered the column through a lateral inlet at the bottom of the packed bed, and the gas flux was 18 ml min1 with an SO2 concentration of 600 ppm, which approaches the average exit concentration of a single-stage condensation acid plant in a metallurgical smelter. The liquid (DES solution) entered at the top of the column driven by a peristaltic pump (Shenchen Lab, 2015) at a rate of 7.5 l h1, where it is distributed by a distributer plate made of acrylic across the packed bed in an inverse direction to that of the gas, and the liquid exits the column at the bottom, where it is collected in a closed liquid container (D) from which it is recirculated by the pump. The total liquid volume in each run was 300 ml. After each absorption run, the packing was retired from the column, and both the packing and the column were gently washed with water until no change in pH of the washing water was observed and the SO2 concentration measured by the gas analyser when passing air through the column was rezeroed. The experimental runs started with the circulation of the liquid through the column, and the initial time corresponded to the time at which the inlet gas entered the column and ended when the concentration measured in the exit gas reached a constant
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Table 3 Water content (mass percent) determined by Karl Fisher titration for the different solutions of a DES and water prepared at a nominal water content after ambient humidity contact (291 K; 60% relative humidity). Solvent
1:2 1:2 1:1 1:1
nominal water content
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
0%
10%
20%
30%
0.62 ± 0.12 0.56 ± 0.07 0.21 ± 0.16 0.49 ± 0.04
13.14 ± 0.09 13.86 ± 0.07 11.05 ± 0.12 12.60 ± 0.03
29.90 ± 0.22 34.10 ± 0.15 22.01 ± 0.08 24.20 ± 0.05
42.00 ± 0.23 51.05 ± 0.25 32.07 ± 0.14 41.07 ± 0.09
Fig. 1. Molecules constituting the deep eutectic solvents considered in this study.
E
SO2
B
C
F
values were obtained by a transient mass balance of the column according to the following equations:
d mSO2 ¼ FCe FCðtÞ dt sat # " tð CðtÞ sat mSO2 ¼ FCe t dt Ce 0
"
mSO2 FCe sat ¼ t ms ms
sat tð
0
CðtÞ dt Ce
#
where F (m3 s1) is the total gas flux passing through the absorption column and C(t) represents the concentration (g m3) of the outgoing gas measured at regular intervals during the process. A Trapezoidal Rule Integration was used for the calculations from the experimental data.
A
D Fig. 2. Experimental set-up. A: Air compressor; B: Gas mass controller; C: Absorption column; D: Solvent reservoir; E: Water trap F: Gas Analyser.
concentration equal to the concentration of the gas entering the column (saturation). 2.1. Calculations The absorption capacity was calculated in terms of the mass of sulphur dioxide absorbed (mSO2 in grams) by the total mass of solvent (mS in grams) (the solvent was recirculated through the column) during the time (in seconds) required to attain complete saturation (tsat), i.e., when the outgoing concentration of sulphur dioxide (Co) was the same as the entering concentration (Ce). The
3. Results and discussion In monitoring the concentration of SO2 in the column output, in all cases, two time periods could be identified. The first period was characterized by a zero-output concentration, that is, complete absorption, which is evidence of a rapid chemical interaction of the solvent with the gas flux. In this period, sulphur dioxide must dissociate in its ionic form (bisulphite and further to sulphite), and the extension of the time of this initial stage contributes to the major proportion of the calculated value of the absorption capacities. The second period of the absorption process corresponded to an exponential rise in the concentration up to saturation, which is connected to the physical absorption process. Fig. 3 shows representative breakthrough SO2 absorption curves obtained for the different DES solutions and includes the results
C. Carlesi et al. / Journal of Cleaner Production 240 (2019) 118240
5
1 0.9
A
B
C
D
E
0.8 0.7
C(t)/Ce
0.6 0.5 0.4 0.3 0.2 0.1 0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
me [min] Fig. 3. Representative breakthrough SO2 absorption curves for the different DES solutions and A: 1 M sulphuric acid solution (for reference). B: ChCl-Ethylene glycol C: ChCl-Malonic acid; D: ChCl-Urea; E: ChCl-Thiourea.
obtained using a 1 M sulphuric acid solution for comparative analysis. The first observation is the confirmation of the improved absorption capacity with the DES solution in comparison with the capacity with the sulphuric acid solution because the onset of the increase in the output concentration of SO2 was at least twice as long with ChCl-ethylene glycol DES solution as it was with the sulphuric acid solution. From the average of the absorption curves rstered for each DES solution, an absorption capacity was calculated and is presented in Table 4. Fig. 3 also shows that the time elapsed in the first stage of the absorption is similar for the DES composed of urea and thiourea, which have similar chemical compositions, and the difference in the calculated absorption capacity arises mainly from the slope of the second stage of absorption (physical component) and thus could be associated with the difference in the transport characteristics of SO2 diffusion through the liquid (molecular and ionic). The viscosity changes of the DES solution were not measured during the operation of the absorption process; however, final measurements showed a slight increase in each case of no more than 5% of the initial viscosity, and the same was observed for the density of the saturated solutions. These observations could be related to the effect of the water content to diminish viscosity changes and to the effect of the physical interaction between formed sulphites and the ionic species of the solvent to reduce the electrostatic interaction between the cation and anion of the DES, thus counteracting the increasing viscosity expected from the strong initial chemical interaction. As depicted in Fig. 4, the calculated absorption capacity presented in Table 4 was compared with the results obtained in the
ChCl-Malonic Acid
ChCl-Ethylene-glycol ChCl-Thiourea
Fig. 4. Comparison between the absorption capacities under static conditions from Sun et al. (Sun et al., 2015) (full bars; values on the left axis) and the obtained results under dynamic conditions (dashed bars; values on the right axis).
reference study (Sun et al., 2015) to evaluate the differences between the absorption capacity under static conditions and dynamic conditions. Here, it is important to consider the experimental differences of both works, mainly the partial pressure of the gas (SO2). In the reference case, the partial pressure was 1 atm, and in the present study, it was related to the 600 ppm of SO2. Moreover, the dilution (the addition of 10% w/w water) in this study was another difference. These distinctions explain the great differences in capacity (a factor of 103). Despite these differences, the comparison of the results in Fig. 4 allows us to confirm the hypothesis that the effect of the water content hardly affects the relative absorption capacities
Table 4 Absorption capacities of the four DESs (10% nominal water content) determined in this study under saturation conditions in terms of the quantity of SO2 absorbed/solvent ratio. Solvent 1:2 1:2 1:1 1:1
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
Molar ratio (X 103)
Weigh ratio (X 103)
Mass to volume ratio (g/L)
0.75 1.26 0.98 1.37
0.19 0.25 0.22 0.31
0.20 0.37 0.30 0.47
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between the different DES. In fact, the relative capacities exhibited opposite trends, except for the ChCl-thiourea DES, which in both studies, presented the better absorption capacity among all the DES tested. Regarding the use of water forming an aqueous solution of DES, the favourable property of non-volatility of the DES which is a key factor in terms of safety and environmental impact (emanations) of the process, it must be considers an unavoidable loss of water from the system, by a drying effect by the entering gas, however the safety and environmental compatibility of the process is not affected by this factor. In Fig. 5, the results of the absorption capacities are compared with the determined values of the initial viscosities of each DES solution; it is possible to evidence an expected trend for the first three DES solutions since the absorption capacity was better for the lower viscosity solvent. However, the result obtained for ChClthiourea did not follow this correlation, and thus, is not possible to generalize the results and thus refuting the second hypothesis. Considering the presence of water in each absorbing solution, a known chemical equilibrium is expected to form between molecular SO2, bisulphite, sulphite and water (pH). This equilibrium implies an increase in solution acidity during the dissociative absorption of SO2 that was confirmed by the measurement of the initial and final pH of each DES solution; the results are shown in Table 5. Here, it is possible to observe that in each case, the solution was acidified, except in the case of the DES containing malonic acid, which was extremely acidic in the bnning of the process. However, the absorption in the case of the DES containing malonic acid was not the worst; in fact, the worst absorption capacity was obtained with the ethylene glycol-based DES, which was basic in its initial state. On the other hand, the better results obtained with the thiourea-containing DES solution were not directly related to its initial basicity, which was less than that of the urea DES solution, and the only possible correlation (except for that concerning the malonic acid DES) is that a better absorption capacity is related to a greater difference in acidity of the absorber solution between the initial condition and after saturation. For all the DES solutions tested, the period of the change in solution pH coincided with the time required to reach saturation; thus, the changes of the solvent pH are indicative of the end of the predominance of the chemical step of the absorption and could be used as an easy control parameter in the operation of the
Table 5 pH values of DES solutions, initially and under saturation conditions. Solvent (10% nominal water)
Initial
Final
1:2 1:2 1:1 1:1
9.0 9.0 0.1 7.3
6.1 5.0 0.1 2.2
ChCl ChCl ChCl ChCl
Ethylene-glycol Urea Malonic Acid Thiourea
absorption process. The chemical interaction of sulphur dioxide in ionic liquids and DES was documented in the literature (Ren et al., 2018). In this study, we corroborate this point from the experimental evidence of, first: the above-mentioned characteristic shape of the breakthrough absorption curves, which present a first stage of zero concentration in the output gas and a continuous pH change and a colour change. In the case of the DES composed of malonic acid, the pH of the original solution was near zero, but the absorption was better than that in the case of a more basic DES solution (e.g., the ethylene glycol DES). Thus, the initial pH of the absorbing solution is not a determining factor and, the interaction of SO2 with the choline chloride cation seems to be more important for the overall performance of the process than the promotion of the formation of sulphites or bisulphites from the dissociative absorption of SO2. Considering the chemical differences between the different DESs tested, which have the same ionic component (choline chloride), the presence of primary amines in the hydrogen bond donor (HBD) results in a higher absorption capacity. This HBD effect could be related to different factors. The first factor is the impact of the asymmetry between the anions in the DES. More asymmetrical DES are prone to suffer large entropy reduction when absorbing an ionizable molecule such as sulphur dioxide and thus must be a better sorbent when there is more initial asymmetry. A second factor is the direct interaction of the HBD with the absorbate. In this case, sulphur dioxide is known to interact with amines (Jiang et al., 2016), and there is an indirect promoting effect of the interaction of sulphites with the choline cation when the HBD molecule interacts with the anion (in this case chloride), reducing the ionic interaction between the original anion and cation of the DES. Regarding the differences between two DES having the same
60 55 50 45 40 35 30 25 20 15 10 5 0
0.50 0.45
[g SO2 / L solvent]
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 U
AM
EG
Viscosity [cp]
6
Ti
Fig. 5. Comparison between the determined absorption capacity (dashed bars; values on the left axis) and the viscosity of the DES solution solvents (full bars; values on the right axis).
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amine groups, the better result in the case of the thioureacontaining DES could be related to the major proportion on a mole basis of the choline cation or with the lower electronegativity of sulphur in thiourea than oxygen in urea, which results in a more direct interaction with the oxygen radicals in sulphite molecules generated after the dissociative absorption of SO2. 4. Conclusions The reduction of SO2 emission has paramount importance to support industrial activities and economic growth, especially in developing economies, here, cleaner production and intensified absorption processes must be implemented. A solution composed of a deep eutectic solvent (four were tested) and a 10% water (mass basis) allows an enhanced absorption process for diluted sulphur dioxide gas streams, thus enabling a reduction in the required equipment size compared with that required when using an acid water solution. This work confirms the positive effect of the use of an aqueous solution of an ionic liquid rather than pure ionic liquid even considering that the inherent absorption capacity of the ionic liquid is usually higher than the dilution. This fact is related to the methods used to determine absorption capacity, which was evidenced in this work, where only in dynamic conditions is possible to account of mass transport effects. There are clear differences between the relative solubility of a pure DES absorbing pure SO2 and a DES-water solution absorbing a low-partial-pressure SO2 stream. In the latter case, the chemical absorption becomes the most important mechanism, and thus, the chemical nature of the DES determines the absorption capacity. The DES containing amine groups showed superior absorption capacity, and between the thiourea- and urea-containing DESs. The former exhibited the best performance, which was not related to the viscosity nor to the initial solution pH but rather to the chemical interaction of functional groups with the generated sulphites and, indirectly, to the action of these functional groups in lowering the cation-anion interactions in the original DES, which further promote the interaction of the cations on the DES with the sulphites generated from the dissociative absorption of SO2. An aqueous solution of thiourea-based deep eutectic solvent presents good characteristic to be used in packed bed absorption of low partial pressure sulphur dioxide instead of more traditional amine-based systems. Aqueous DES solutions must improve the safety and environmental impact of the capture process, since the inherent lower volatility, higher thermal and chemical resistance of the DES, allowing a reduction of a toxic degradation by-products emission to the atmosphere and wastewater generation thus reducing the need of solvent make-up. Acknowledgements This work was funded by the Chilean Estate through the Fondecyt Program (regular) grant n 1150235, Education Minister. References Ashar, N.G., Golwalkar, K.R., 2013. A practical Guide to the Manufacture of Sulfuric acid, Oleums, and Sulfonating agents. https://doi.org/10.1007/978-3-31902042-6. Bottenus, C.L.H., Massoli, P., Sueper, D., Canagaratna, M.R., VanderSchelden, G., Jobson, B.T., VanReken, T.M., 2018. Identification of amines in wintertime ambient particulate material using high resolution aerosol mass spectrometry. Atmos. Environ. 180, 173e183. https://doi.org/10.1016/j.atmosenv.2018.01.044. Chen, K., Lin, W., Yu, X., Luo, X., Ding, F., He, X., Li, H., Wang, C., 2015. Designing of anion-functionalized ionic liquids for efficient capture of SO2 from flue gas. AIChE J. 61, 2028e2034. https://doi.org/10.1002/aic.14793. Cui, G., Zheng, J., Luo, X., Lin, W., Ding, F., Li, H., Wang, C., 2013. Tuning anion-
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