SO2 Capture Using pH-Buffered Aqueous Solutions of Protic Triamine

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SO2 Capture Using pH-Buffered Aqueous Solutions of Protic Triamine-Based Ionic Liquid Yongli Sun, Yanling Zhang, Luhong Zhang, Bin Jiang, Wenhao Gu, and Huawei Yang* School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, a kind of aqueous solution of novel protic triamine-based ionic liquid was proposed for efficient SO2 capture. The mixed absorbents were prepared by blending 1,1,4,7,7-pentaethyldiethylene (PEDETA) with sulfuric acid and water, and their physicochemical properties were studied by various methods. Remarkably, the viscosities of the mixed absorbents are in the range of 1.44−1.85 mPa s at 293.2 K, which is as low as water. Detailed SO2 absorption experiments were carried out to investigate the influence of some important factors, including temperature (298.2, 313.2, and 328.2 K), SO2 partial pressure (0−1 bar), and sulfuric acid proportion. It is worth noting that the mixed absorbents showed excellent absorption capacity, especially at a low partial pressure. All absorbed SO2 could be released easily and completely by the method of heating reflux. More importantly, the absorbent could be reused for at least 5 cycles without noticeable changes in both absorption performance and chemical structure. A thermogravimetric analysis further confirmed its good thermal stability. The detailed absorption mechanism was proposed and demonstrated by an infrared spectrum. In the end, the pKa values and reaction enthalpy of the protonation reaction of PEDETA were determined by acid titration and parameter analysis based on the reaction equilibrium model. In summary, the mixed absorbents, which possess low viscosities, good resistance to sulfuric acid, excellent capacity, and reusability, may have a good perspective in industrial SO2 capture.

1. INTRODUCTION SO2 is one of the major air pollutants, and the emission of SO2 caused harmful impacts on the environment and human health.1,2 SO2 in the atmosphere is generally emitted from the combustion of fossil fuels, and it is mostly being controlled by flue gas desulfurization (FGD). Among FGD technologies, the wet limestone−gypsum method has come to maturity and been widely used in practice. However, a large amount of wastewater and useless byproducts, such as gypsum, are produced as secondary pollutants.3−5 Another method using amine solutions as physical absorbents for SO2 capture is also widely employed. However, when it comes to large-scale trapping of SO2, most of these organic solvents will volatilize into the gas stream, leading to environmental pollution and waste of resources.6 Recently, ionic liquids (ILs) have attracted increasingly attention as potential alternatives to FGD materials because of their high thermal stability, low vapor pressure, and tunable properties.7 Han et al. first researched the absorption of SO2 using 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), which could capture 0.978 mol of SO2/mol of IL at 40 °C.8 Subsequently, many other ILs, such as guanidinium-,9−12 hydroxyl-ammonium-,13−15 pyridinium-,16,17 imidazolium-,18−20 anion-functionalized,21−24 betaine-based25 ILs, ether-functionalized ILs,26,27 and more recently, deep eutectic solvents,28−30 were explored for the absorption of SO2. However, the viscosities of most task-specific ILs are pretty high at room temperature and, thus, lead to low heat and mass transfer efficiency.31,32 Moreover, there is 10−15% water contained in flue gases; therefore, the influence of water on the desulfurization process should also be taken into account. It was frequently reported that the existence of water might © XXXX American Chemical Society

largely reduce the absorption of SO2, because water possesses good affinity with most ILs, hence bringing competitive absorption with SO2.26,33,34 However, there are also some other ILs exhibiting high SO2 absorption capacities under hydrous conditions, but the absorption of SO2 in aqueous solutions of ILs is rarely reported.35 In comparison to neat ILs, the aqueous solutions of ILs could absorb SO2 fast14 and their absorption and desorption capabilities were significantly affected by the basicity of ILs.36 Therefore, the development of IL aqueous solution with appropriate alkalinity will provide bright prospects for SO2 capture. It is worth noting that SO2 may be oxidized and further generate sulfuric acid in situ, because there is also O2 and water in flue gas.37 Sulfuric acid produced in the processes would inactivate most of the ILs and irreversibly reduce the absorption of SO2. Wu et al. found that the absorption capacity of SO2 in [MEA]L decreased greatly when sulfuric acid was added to the IL.38 Robertson et al. studied the absorption of SO2 by diethylenetriamine (DETA) when water and oxygen existed. They found that irreversible oxysulfur salts were formed in the process, which will dramatically affect the reuse exploitability of ILs.39 Therefore, developing new absorbents, whose chemical composition can be fully restored after removing generated sulfuric acid, will be of significant value for industrial applications. With inspiration from the reports, 1,1,4,7,7-pentaethyldiethylene (PEDETA), a kind of triamine with three tertiary amine groups, was synthesized and blended with sulfuric acid Received: December 8, 2016 Revised: February 16, 2017 Published: February 24, 2017 A

DOI: 10.1021/acs.energyfuels.6b03260 Energy Fuels XXXX, XXX, XXX−XXX

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greater driving force for the proton transfer.41 Therefore, the pKa value of PEDETA was calculated later in this paper. The thermal stability of neat [PEDETAH][sulfate] (1:0.5) was investigated by thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) (STARe system, Switzerland), and the decomposition temperature was determined according to the TGA and differential thermal analysis (DTA) curves (Figure 5). 2.4. Determination of Physical Properties. The densities of the mixed absorbents were determined by use of a 5 cm3 pycnometer, which was calibrated by distilled water before the experiment. The temperature was controlled by a water bath accurate to ±0.1 K. All of the measurements were carried out at least 3 times, and then the average was obtained. The uncertainty of density measurements was estimated to be ±0.0002 g/cm3. The viscosities of the mixed absorbents were measured by a Brookfield DV-II+Pro viscometer, which was supplied by Brookfield Engineering Laboratories. The temperature was controlled by a water bath accurate to ±0.1 K. The uncertainty of viscosity measurements was estimated to be ±1%. 2.5. Absorption of SO2. The SO2 absorption experimental apparatus is similar to other reports.42 The whole experimental apparatus includes two 316L stainless-steel cells, which are equipped with a pressure sensor of ±0.2% uncertainty (in relation to the full scale of 0.3 MPa) separately. The two pressure sensors are connected to a digital instrument respectively to record the pressure changes in two cells. The bigger gas cell, whose volume is 161.752 cm3 (V1), is named as the gas receptor and used to reserve SO2 before it contacts the absorbents. The smaller gas cell, whose volume is 88.165 cm3 (V2), is named as the equilibrium cell. This equilibrium cell equipped with a magnetic stirrer is used to place the mixed absorbents. The temperature (T) of both cells is controlled by a water bath with an accuracy of ±0.1 K. In a typical absorption process, a certain amount of mixed absorption absorbents (w, about 10 g) was added to the equilibrium cell, which was then purged with nitrogen to exclude the air. After that, the initial nitrogen pressure in the equilibrium cell was noted as P0. The air in the gas receptor was exhausted (30 min).48 SO2 could dissolve quickly in the mixed absorbents because of their low viscosities and small transfer resistance. As seen, the solubility of SO2 in the three mixed absorbents showed trends similar to the increasing of the SO2 partial pressure. Clearly, the mixed absorbents exhibited good absorption capability, especially at the low-pressure stage. For example, SO2 capacity for [PEDETAH][sulfate] (1:0.5) at 298.2 K is 0.245 mol of SO2/mol of IL at 0.001 bar, 1.317 mol of SO2/mol of IL at 0.012 bar, and 2.808 mol of SO2/mol of IL at 0.943 bar. SO2 solubility increased significantly as the pressure increased from 0 to 0.1 bar. According to the absorption mechanism shown in Scheme 3, this can be attributed to the enhanced ionization of H2SO3 caused by the pH-buffering effect of the absorbent. The linear increment of SO2 solubility with the pressure from 0.2 to 1 bar is merely caused by the equilibrium shift with the SO2 partial pressure increasing. Because the SO2 partial pressure is relatively low in flue gas, the strong chemical interaction is essential. Figure 2 also indicates that the equilibrium solubility of SO2 was decreased with the increase of the sulfuric acid/PEDETA molar ratio. With deduction of the absorption mechanism in Scheme 3, the concentrations of SO2(aq) and H2SO3 are not affected by the sulfuric proportion. However, the addition of H+ in the mixed absorbents would promote the protonation of PEDETA before absorption, reduce its capacity of bonding to H+ in the process of absorption, and thus hinder the ionization of H2SO3. It should be note that, in the SO2 capture process of real flue gas, H2SO4 will be generated and accumulated in the mixed absorbent, causing the increase of the sulfuric acid/ PEDETA molar ratio; therefore, partial ion exchange can be carried out to remove sulfuric acid. The effect of the temperature on the SO2 absorption was also investigated. As seen from Figure 2c, for example, the equilibrium solubilities of SO2 in [PEDETAH][sulfate] (1:0.9) decreased from 2.3 to 1.7 mol of SO2/mol of IL at about 1 bar with the temperature increased from 298.2 to 328.2 K. A low temperature is beneficial to SO2 absorption because it not only promotes the dissolution of SO2 but also shifts the protonation reaction balance toward the positive direction. Besides, the results also indicate that absorbed SO2 could be released at a high temperature. A summary of SO2 absorption capacity in various ILs and aqueous solutions of tertiary amine ILs as well as their viscosities was presented in Table 3 to make a comparison

The change tendency of viscosity is the same as that of density. It is worth noting that the viscosities of the mixed absorbents are extremely low. For example, the viscosity of [PEDETAH][sulfate] (1:0.5) at 293.2 K is 1.44 mPa s, while the viscosity of water at 293.2 K is 1.002 mPa s.45 Moreover, the viscosities of the mixed absorbents are much lower than other functionalized ILs (>400 mPa s; see Table 3) and are found not significantly increased after absorption. For example, the viscosity of [PEDETAH][sulfate] (1:0.9) saturated with SO2 is 3.15 mPa s at 293.2 K. Low viscosities are favorable to the efficient capture of SO2, because the rate of phase transfer between gas and liquid is significantly enhanced. Therefore, blending ILs with water provides a promising method for the practical capture of SO2. 3.2. Absorption of SO2. 3.2.1. Absorption Mechanism. The proposed mechanism of SO2 absorption is shown in Scheme 3. First, gaseous SO2 was dissolved in water. Then, Scheme 3. Absorption Mechanism of SO2 of the Mixed Absorbents

dissolved SO2 was hydrated to form H2SO3, which next ionized to HSO3−, SO32−, and H+. H2SO4 was added to regulate the basicity of PEDETA before absorption, and the unprotonated nitrogen atom in PEDETA could bond to H+ ionized by H2SO3. To confirm the proposed absorption mechanism, FTIR spectra of [PEDETAH][sulfate] (1:0.5) before and after SO2 absorption and desorption were studied, and results are shown in Figure 1. As seen from Figure 1, two characteristic peaks were found after the absorption of SO2 at 960 and 1328 cm−1 and disappeared after regeneration. The new absorption bond at 960 cm−1 is corresponding to S−O stretches of SO32−, HSO3−, or similar species, indicating the chemical interactions

Figure 1. FTIR spectra of fresh, saturated with SO2, and regenerated [PEDETAH][sulfate] (1:0.5). E

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Figure 2. Absorption of SO2 by the mixed absorbents at different temperatures as a function of the SO2 partial pressure: (a) [PEDETAH][sulfate] (1:0.5), (b) [PEDETAH][sulfate] (1:0.7), and (c) [PEDETAH][sulfate] (1:0.9).

absorbents, especially when the desorption rate was less than 70%. The quick deprotonation of N3 as a result of its weak alkalinity may be the reason for this. Besides, it is also the reason why the desorption of SO2 in [PEDETAH][sulfate] (1:0.9) was the fastest. The desorption slowed when the desorption efficiency reached 95%, which may because a small amount of released SO2 was reabsorbed and brought back by the condensate water. However, when the desorption time was up to 60 min, almost all absorbed SO2 could be released from the mixed absorbents. On the other hand, the SO2 desorption efficiency decreases with the decrease of the sulfuric acid/PEDETA molar ratio, and this could be attributed to the higher alkalinity of N2, which means its ability to release H+ is weaker and, thus, leads to a weaker desorption driving force. However, according to the absorption and desorption mechanisms, when the SO2 partial pressure in the gas phase is zero, no sulfur dioxide derivative could exist in the system under the equilibrium state. Therefore, SO2 trapped by [PEDETAH][sulfate] (1:0.5) and [PEDETAH][sulfate] (1:0.7) could be completely desorbed, respectively. In addition, the reusability of the mixed absorbent was tested for 5 cycles. The regenerated absorbent was recharged with SO2 at 313.2 K until the equilibrium partial pressure reached 1 bar. After that, the desorption was conducted at 110 °C for 90 min. Figure 4 shows that the SO 2 absorption capacity of [PEDETAH][sulfate] (1:0.5) is almost unchanged after 5 cycles, indicating that the absorption of SO2 in the mixed absorbent is highly reversible. Furthermore, as suggested by the FTIR spectra shown in Figures S5−S7 of the Supporting Information, no noticeable changes of chemical structures of absorbents can be detected after 5 absorption−desorption cycles, which further proves the structural stability of these mixed absorbents. The TGA and DTA curves of neat [PEDETAH][sulfate] (1:0.5) are shown in Figure 5. It can be observed that [PEDETAH][sulfate] (1:0.5) starts to decompose at 150 °C and shows first an obvious loss of weight at 210 °C, which illustrates that the ILs are stable enough under the operating conditions. 3.4. Thermodynamic Analysis. The absorption and desorption capacities of the pH-buffered mixed absorbents used in this work are mainly affected by the reaction equilibrium constant (K1, K2, and K3) of the protonation reaction of PEDETA at different temperatures. Although the pKa value of PEDETA at 25 °C could be estimated by commercial software (ACD/Labs), the molar reaction enthalpy is still needed to calculate the equilibrium constants at another

between this work and results of other groups for the capture of SO2. As seen, the molar ratios of SO2/IL for the mixed absorbents prepared in this work are comparable to most other functionalized ILs and aqueous solutions of ILs, while the viscosity of the mixed absorbents is much lower than that of other ILs. Furthermore, the ILs employed in this work are inexpensive, and the blending of water is more environmentally friendly, as compared to other organic solvents. These factors demonstrated the practicability of the absorbents. When the SO2 absorption capacity was calculated as moles of SO2 per kilogram of absorbent, the capacity in aqueous solutions was lower than that in neat ILs. This is due to water accounting for a considerable proportion of the absorbent, but it was just used as a solvent. In fact, [PEDETAH][sulfate] acts as major absorption components that play a key role in SO2 absorption. Therefore, the low mass absorption capacity only affects the circulation quantity in the industrial process but does not increase the IL usage and investment cost. In addition, SO2 capture is a dynamic equilibrium process of absorption− desorption; therefore, the desorption efficiency also significantly influences the energy cost. 3.3. Desorption of SO2 and Reuse of [PEDETAH][Sulfate] (1:0.5). The desorption of SO2 from the mixed absorbents over time was carried out under the condition of heating reflux, and the results are shown in Figure 3. Absorbed SO2 could be released easily and quickly in the three mixed

Figure 3. SO2 desorption efficiency of the mixed absorbents. F

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Figure 6. pH variation during the HCl(aq) titration process at (a) 30 °C, (b) 40 °C, (c) 50 °C, and (d) 60 °C (, experimental data; - - -, predicted results). Concentration of aqueous solution of PEDETA: (a) 0.1459 mol/L, (b) 0.1153 mol/L, (c) 0.1142 mol/L, and (d) 0.0779 mol/L.

Figure 4. Reusability of [PEDETAH][sulfate] (1:0.5) in SO2 absorption for 5 cycles.

Δr S °m = (Δr H °m − Δr G°m)/T

As seen in Table 4, the pKa value of N1 is 10.03, indicating that PEDETA is a relatively strong organic base and will react with H2SO4 to form protic [PEDETAH][sulfate]. Because N1 is always occupied, the pKa value and protonated reaction enthalpy of N1 are not important in the absorption and desorption. The pKa values for N2 and N3 are 8.21 and 2.89, respectively, which explains the fact that the desorption of SO2 from [PEDETAH][sulfate] (1:0.9) is easier than that of other absorbents. The absolute values of the molar protonated reaction enthalpy decrease with the increase of the degree of protonation, same as pKa values, which indicates that the interaction toward H+ becomes weaker. As is well-known, a higher reaction enthalpy means that the reaction equilibrium is more sensitive to temperature change. Therefore, a high temperature is good for the deprotonation of N2 and complete desorption of SO2.

Figure 5. TGA and DTG curves of [PEDETAH][sulfate] (1:0.5).

temperature, which are crucial for the design of industrial processes. Hence, in this work, acid titration experiments at different temperatures were carried out and a corresponding reaction equilibrium mathematical model was built using gPROMS software. Using the experimental results, pKa values and molar reaction enthalpy were estimated by parameter analysis (Table 4). As shown in Figure 6, the pH variation obtained by bringing the calculated parameters into the model is compatible with the experimental data, which demonstrates that the mathematical model is reliable and the calculated parameters are accurate. The molar Gibbs energy of reaction ΔrG°m and the molar reation entropy ΔrS°m can be calculated by eqs 40 and 41, and the results were presented in Table 4. Δr G°m = −RT ln K 0

(41)

4. CONCLUSION The aqueous solutions of [PEDETAH][sulfate], a kind of novel and low-cost IL-based absorbent, was put forward for SO2 deep capture. The absorbent combines the advantages of ILs and aqueous solution and realizes the features of high stability, low viscosity, and green solvent. The viscosities of the mixed absorbents are much lower than those of other functionalized ILs, which is beneficial to heat and mass transfer and, thus, brings high absorption efficiency. The mixed absorbents exhibited strong absorption ability, especially at very low partial pressures as a result of the chemical absorption mechanism demonstrated by the infrared spectrum. More importantly, all absorbed SO2 could be released from the mixed absorbents

(40)

Table 4. pKa Value, Molar Reaction Enthalpy, Molar Gibbs Energy of Reaction, and Molar Reaction Entropy at 298.15 K PEDETAH+ PEDETAH22+ PEDETAH33+

pKa

ΔrH°m (kJ mol−1)

ΔrG°m (kJ mol−1)

ΔrS°m (J mol−1 k−1)

10.03 ± 0.20 8.21 ± 0.20 2.89 ± 0.20

−55.74 ± 5.00 −54.28 ± 5.00 −24.01 ± 5.00

−57.25 ± 1.14 −46.86 ± 1.14 −16.50 ± 1.14

5.07 ± 17.20 −24.89 ± 17.20 −25.19 ± 17.20

G

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under the reflux condition, and no obvious reduction of absorption capacity and chemical structure change were detected after several cycles, which demonstrated the reusability and recyclability of the mixed absorbents. In summary, these pH-buffered solutions of triamine-based protic ILs possess excellent physiochemical properties, good resistance to sulfuric acid corrosion, high ability on deep removal of SO2, and good enough chemical structure and thermal stability, which are key requirements for the SO2 capture using ILs in an industrial FGD process. Furthermore, thermodynamic analysis was conducted, and the pKa values and molar reaction enthalpy were calculated. On the basis of the systematic data obtained in this work, further evaluation of the cost economics could be carried out with the assistance of commercial software.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03260. FTIR spectra of PEDETA (Figure S1), gas chromatogram of PEDETA (Figure S2), mass spectra of PEDETA (Figure S3), 1H NMR spectra of (a) PEDETA, (b) [PEDETAH][sulfate] (1:0.5), (c) [PEDETAH][sulfate] (1:0.7), and (d) [PEDETAH][sulfate] (1:0.9) (Figure S4), FTIR spectra of fresh [PEDETAH][sulfate] (1:0.5) (Figure S5), FTIR spectra of [PEDETAH][sulfate] (1:0.5) after 1 cycle (Figure S6), FTIR spectra of [PEDETAH][sulfate] (1:0.5) after 5 cycles (Figure S7), and relative critical parameters (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-2227400199. E-mail: huaweiyang@tju. edu.cn. ORCID

Luhong Zhang: 0000-0002-7073-4793 Huawei Yang: 0000-0002-3510-9407 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Key R&D Program of China (2016YFC0400406). REFERENCES

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