Equilibrium Hydrate Formation Conditions of CO2 + N2 + SO2 Ternary

Dec 6, 2013 - The equilibrium hydrate formation conditions for ternary simulated flue gas [CO2 (0.1368) + N2 (0.8547) + SO2 (0.0085)] with SO2 aqueous...
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Equilibrium Hydrate Formation Conditions of CO2 + N2 + SO2 Ternary Simulated Flue Gas in SO2 and Tetra‑n‑butylammonium Bromide Containing Aqueous Solutions Chao Chen,†,‡,§ Xiao-Sen Li,†,‡ Zhao-Yang Chen,*,†,‡ Zhi-Ming Xia,†,‡ Ke-Feng Yan,†,‡ and Jing Cai†,‡ †

Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China ‡ Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, P. R. China § University of Chinese Academy of Sciences, Beijing 100083, P. R. China ABSTRACT: The equilibrium hydrate formation conditions for ternary simulated flue gas [CO2 (0.1368) + N2 (0.8547) + SO2 (0.0085)] with SO2 aqueous solutions, and SO2 + tetra-n-butyl ammonium bromide (TBAB) aqueous solutions were measured using the temperature search method, over the temperature and pressure range of (272.85 to 283.15) K and (1.20 to 5.09) MPa, respectively. The corresponding equilibrium gas compositions were analyzed. The effects of SO2 concentration and TBAB additive on the hydrate phase equilibrium were studied. For the flue gas and SO2 aqueous solution, the presence of SO2 reduces the hydrate formation pressure. The higher is the SO2 concentration in aqueous solution, the easier the hydrate forms, and the higher the SO2 concentration in equilibrium gas phase is. N2 mole fractions in equilibrium gas phase are higher than that in the feed flue gas, while CO2 is lower due to its relative high solubility. For a given system, the equilibrium hydrates formation pressure increases with the increase in temperature. Both SO2 and N2 mole fractions in equilibrium gas phase decrease with the increase in pressure, while the CO2 mole fraction increases. TBAB solution with w4 = 0.050 not only reduces the equilibrium hydrates formation pressure markedly, but also helps the dissolution of SO2 in aqueous solution. The SO2 concentrations in equilibrium gas phase for the TBAB addition system are much smaller than those without TBAB addition. The pressure reducing effect of the TBAB promoter for SO2 containing flue gas is dependent on the SO2 concentration and smaller than that for flue gas without SO2. A small amount of SO2 produces a synergy with TBAB to promote the hydrate formation, but the high SO2 concentration produces an inhibition on the hydrate formation, and causes the hydrate equilibrium pressure increase. The optimum SO2 mass fraction in solution is lower than 0.010.

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INTRODUCTION Carbon dioxide (CO2) emission resulting from the use of fossil fuel is responsible for over 55% of total anthropogenic greenhouse gas emission and responsible for over 70% of global anthropogenic CO2 emission.1 About 30% of CO2 comes from fossil fuel power plants, especially the coal-fired power plant. Although other energies, such as solar energy and nuclear energy, have been developed vigorously, the world will continue to rely on fossil fuels as the main energy for the next 20 to 30 years at least. To deal with the global warming and climate abnormity caused by CO2 concentration increase in the atmosphere, CO2 capture and storage (CCS) from large CO2 emission sources is considered as the most feasible method to achieve a large reduction in CO2 emission rapidly.2,3 The “post-combustion capture” of CO2 refers to separating CO2 from the flue gas discharged from conventional coal-fired power plants and other combustion sources. Although the separation of CO2 is commonly practiced, it requires a high energy consumption and accounts for three-fourths of CCS cost and energy penalty.4−6 Separating CO2 based on the gas © 2013 American Chemical Society

hydrate crystallization process is a novel method which has received much attention in the fields of flue gas (CO2 + N2),7−11 shifted syngas (CO2 + H2),12−18 and natural gas and biogas (CO2 + CH4)19,20 separation. According to the research results of Nexant Inc., separating CO2 from shift syngas by employing the hydrate process can not only decrease the energy penalty but also reduce capital costs by simplifying the removal process for H2S.21,22 However, the hydrate-based flue gas separation process requires a high energy consumption because the CO2 concentration and partial pressure in the flue gas is low, and the flue gas needs to be compressed to a high pressure to meet the hydrate formation conditions.23 Some hydrate formation promoters, such as tetrahydrofuran (THF), 7 , 8 , 1 2 , 1 3 tetra-n-but yl ammonium bromide (TBAB),10,15,16 and cyclopentane (CP),15,18,24 can significantly reduce the hydrate formation pressure and enhance the hydrate Received: September 30, 2013 Accepted: December 2, 2013 Published: December 6, 2013 103

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formation rate. Li et al.15 reported that CO2 in flue gas can be purified from a mole fraction of 0.170 to 0.994 with a two-stage hydrate separation process when using TBAB and dodecyl trimethyl ammonium chloride as additives, and the separation factors of the first and second stage hydrate separation process reach 9.60 and 62.25, respectively. This opens up a new prospect on the hydrate-based separation of flue gas emission from coal-fired power plants. In most of the investigations on the gas hydrate separation, the feed flue gas is assumed to not contain SO2, and is simplified as CO2 and N2 mixtures because of the approximate hydrate formation conditions of N2 and O2. The component of SO2, another important pollutant in flue gas emission from coal-fired power plants, is not considered in the hydrate separation process. It is generally thought that the desulfurization and decarburization process are separated. SO2 in flue gas is first removed in a flue gas desulfurization unit using the welldeveloped technology of scrubbing with limestone as absorbent to produce gypsum, whose removal efficiency could be over 90%.25 Then CO2 is separated and captured from the desulfurized flue gas in the CO2 separation and capture apparatus. In a practical factory, it is impossible and expensive to remove SO2 completely from flue gas before the gas is further processed for CO2 capture.26 Beeskow et al.27 studied the effect of a small amount SO2 and NO2 impurity on pure CO2 gas hydrate formation and stability using laser Raman spectroscopy and differential scanning calorimetry. The results indicate that the presence of a small amount SO2 can increase the stability of CO2 hydrate and enhance CO2 hydrate sequestration security. Daraboina et al.28 studied the impact of SO2 on postcombustion CO2 capture in a bed of silica sand through hydrate formation. They suggested that the presence of a small amount of SO2 impurity (mole fraction of 0.01) in CO2 + N2 mixtures can reduce the hydrate formation pressure, and enhance the initial hydrate formation rate and the final gas consumption. Nohra et al.29 suggested that the presence of a small amount of SO2 does not disrupt CO2 hydrate formation according to the analysis of the Gibbs free energy associated with substituting CO2 with SO2. Because the aqueous solutions, especially the chemical additives containing aqueous solutions, used in the hydratebased gas separation process must be recycled during practical application, the residual SO2 in the desulfurized flue gas would accumulate in the aqueous solution after multiple cycles because of its high solubility in water, which would lead to the acidification of the aqueous solution. The high concentration of SO2 in aqueous solution can possibly inhibit the hydrate formation. Although we can strip off the dissolved SO2 from the aqueous solution, it is difficult and consumes a large amount of energy due to the high solubility of SO2 in water. So the presence of SO2 in flue gas cannot be ignored in the hydrate-based gas separation technology, regardless of the SO2 concentration in flue gas being high or low. At the same time, since SO2 hydrate is much easier to form than CO2 hydrate,30 it is possible to design a gas hydrate crystallization separation scheme to separate and capture SO2 and CO2 from the flue gas simultaneously. In this way, the conventional desulfurization apparatus and operation costs could be saved. Although there are some thermodynamic data on CO2 + N2 binary gas hydrates in the published documents, there is very little thermodynamic data on CO2 + N2 + SO2 ternary gas hydrates available except for only three data values reported by Daraboina et al.28 Moreover, there is no research on the

thermodynamic data of CO2 + N2 + SO2 ternary gas hydrates in high SO2-containing aqueous solution or TBAB containing solution. How does the high concentration of SO2 in aqueous solution affect the hydrate formation of the flue gas? Can the conventional flue gas desulfurization device really be replaced by a new hydrate-based separation unit? These answers are still not clear. In this work, the simulated flue gas of the CO2 + N2 + SO2 ternary gas mixture was used as the feed gas. The hydrate phase equilibrium conditions of the simulated flue gas in two kinds of aqueous solutions (SO2 solution, SO2 + TBAB solution) were measured using the temperature search method, and the corresponding equilibrium gas compositions were analyzed. The effects of SO2 concentration and the addition of TBAB promoter on the hydrate phase equilibrium were studied. The research will provide basic data and theoretical guidance for the hydrate-based flue gas desulfurization and CO2 capture.

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EXPERIMENTAL SECTION Materials. The analytical pure SO2 gas and the simulated flue gas were supplied by Huate Gas Co., Ltd., Foshan, China. The mole fraction of CO2, N2, and SO2 in the simulated flue gas was 0.1368, 0.8547, and 0.0085, respectively, which was selected to represent industrial compositions. The aqueous solution with a SO2 mass fraction of w3 = 0.070 was supplied by Chengdu Jinshan Reagent Co., Ltd., China. The other SO2 containing aqueous solutions with w3 < 0.070 were prepared by diluting the SO2 aqueous solution of w3 = 0.070 with deionized water. The SO2 saturated solution was prepared by bubbling excess pure SO2 gas into the SO2 aqueous solution of w3 = 0.070 for 24 h. TBAB and all other chemicals used in this work were analytical grade, and were supplied by Aladdin Chemistry Co., Ltd., Shanghai, China. All aqueous solutions were prepared using deionized water which was prepared by an ultrapure water system (Nanjing Ultrapure Water Technology Co., Ltd., China) with a resistivity of 18.25 MΩ·cm. Apparatus. The schematic diagram of the experimental apparatus employed to measure the hydrate phase equilibrium conditions in this work is shown in Figure 1. The main part of

Figure 1. Schematic diagram of hydrate phase equilibrium experimental apparatus: 1, simulated flue gas feeding cylinder; 2, multiway valve; 3, stirrer, 4, refrigerating coil; 5, heater, 6, supply vessel (SV); 7, PID controlling valve; 8, pressure transducer; 9, crystallizer (CR); 10, visual windows; 11, magnetic stirrer; 12, temperature transducer; 13, Agilent data acquisition unit; 14, computer; 15, sampling; 16, gas evacuation; 17, lower temperature water bath. 104

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In these experiments, the test fluids are composed of five components (SO2, CO2, N2, TBAB, and H2O) and three phases (vapor, SO2, and TBAB containing aqueous solution and hydrate). The concentration of TBAB in aqueous solution remains constant in the TBAB-containing system because the hydrate crystal is controlled at a very small amount in the phase equilibrium point, which nearly does not affect the TBAB concentration in the liquid phase. So the TBAB composition is not an independent variable for each given experiment. According to the Gibbs phase rule, there are three degree of freedom for the above system. That means the equilibrium system is not only related to the temperature or pressure, but also related to the gas compositions.

the setup is a cylindrical crystallizer (CR) immersed in a temperature-controlled water bath containing approximately 50 to 50 mass percent water and ethylene glycol. The CR is made of the 316 stainless steel, and its inner volume and its maximum working pressure are 336 mL and 30 MPa, respectively. Two quartz glass visual windows are equipped at the front and back of CR, which allows visual observation of phase transition in the reactor. The contents in CR are agitated and mixed by a magnetic spin bar placed directly at the bottom of CR. A baffle arrangement is used in CR in order to prevent vortex formation and to enhance mixing of the contents. The temperature in CR is controlled by the temperature-controlled water bath with a temperature range of (−10 to 90) °C and a temperature controlling accuracy of ± 0.1 K. Two Pt1000 thermoprobes (JM6081) with ± 0.05 K accuracy are used to measure the temperature at the top (gas phase) and the bottom (liquid phase) of the CR. To minimize the temperature and pressure perturbation in the CR caused by the feed flue gas, a 1350 mL supply vessel (SV, 30 MPa) immersed in the temperaturecontrolled water bath is used to precool and measure the amount of the feed gas. The pressure in CR is controlled by a proportional−integral−derivative controlled pressure regulated valve (Tescom ER3000) with a pressure controlling accuracy of ± 0.02 MPa. All pressures in the experimental apparatus are measured using a Setra model 206 absolute pressure transducer with an accuracy of ± 0.02 MPa. Temperature and pressure data are collected every 6 s using an Agilent model 34970A recording system, which is controlled by the Agilent data acquisition software application BenchLink. Experimental Procedure. The conventional temperaturecycle search method was employed to determine the hydrate phase equilibrium temperature and pressure under constant volume.16 The experimental apparatus was washed and rinsed with deionized water, purged with the simulated flue gas three times, and evacuated with a vacuum pump before experimentation. A 150 mL aliquot of SO2 aqueous solution was charged into the CR under stirring. The SV was pressurized to a desired pressure with the simulated flue gas. After the temperature of the system was cooled to a predetermined value, the CR was pressurized with the precooled flue gas in the SV to a pressure of 1.0 MPa higher than the estimated equilibrium pressure so as to reduce the induction time. After hydrate crystals were observed through the visual windows, the temperature in the CR was increased slowly with a heating rate of 0.1 K·h−1, until the hydrate crystals decomposed completely. Then the temperature was decreased slowly with a cooling rate of 0.1 K·h−1, until there was an infinitesimal amount of hydrate crystals appearing again. Then the temperature in the CR was kept stable for 4 h. If the infinitesimal amount of hydrate crystals remained stable within the 4 h period, we determined this point was the hydrate phase equilibrium point. If the hydrate crystals in the CR disappeared or increased, the aforementioned slow cooling and heating steps were repeated until the infinitesimal amount of hydrate crystals remained stable within the 4 h period. Once the gas− liquid−hydrate system reached the equilibrium point, the temperature and pressure in the CR were recorded, and the equilibrium gas phase compositions in CR were immediately sampled and analyzed by gas chromatography (Wufeng GC522, Shanghai Wufeng Scientific Instrument Co., Ltd., China.) and iodometry (HJ/T 56-2000). Each sample was measured three times, and the average value was adopted.

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RESULTS AND DISCUSSION Equilibrium Conditions of CO2 (1) + N2 (2) + SO2 (3) + H2O (5) Mixture Hydrates. In a practical hydrate-based gas separation process, the SO2 concentrations in the recycling aqueous solution become higher and higher after multiple cycles. At standard temperature and pressure the solubility of SO2, CO2, and N2 in water is 112, 2, and 0.02 g·L−1, respectively.28 To obtain the thermodynamic equilibrium data of flue gas hydrate in aqueous solution with different SO2 concentrations, the SO2 saturated aqueous solution and the aqueous solution with SO2 mass fraction of w3 = 0.070 were adopted in the hydrate−liquid−vapor phase equilibrium experiments. For the aqueous solutions with a SO2 mass fraction of w3 = 0, 0.011 and 0.035, the hydrate formation pressure is too high to be used in industry without the addition of hydrate formation promoters, so only the liquid−vapor phase equilibrium were tested. The detailed phase equilibrium pressure and temperature, and equilibrium gas composition data in different solution systems are listed in Table 1. The effects of SO2 concentration on the equilibrium formation pressure of flue gas hydrate are shown in Figure 2. The equilibrium hydrate formation pressure increases with the increase in temperature for a given system. With the increase of SO2 concentration in aqueous solution, the three-phase equilibrium lines of flue gas (V)SO2 aqueous solution (L)hydrate (H) shift toward lower right side, and the hydrate forms under mild temperature and pressure conditions. This indicates that SO2 can promote the formation of flue gas hydrate, and the higher the SO2 concentration is, the easier the flue gas hydrate forms. As can be seen from Table 1, the N2 concentration in the equilibrium gas phase is higher than that in feed flue gas, while CO2 is lower due to its relative high solubility. The SO2 concentration in the equilibrium gas phase (y3′) decreases with the decrease in SO2 concentration in aqueous solution. When the SO2 saturated solution is used, the y3′ values are higher than those in the feed flue gas (y3 = 0.0085), which indicates that the dissolved SO2 volatilize out of the aqueous solution and enter into the gas phase. In the phase equilibrium points, the SO2 concentration in the gas phase is not only dependent on the equilibrium pressure and temperature, but also dependent on SO2 concentration in the feed solution and flue gas. In practical applications, the accumulated SO2 concentration in the aqueous solution does not easily reach the saturation concentration of pure SO2 gas due to the low SO2 content in flue gas mixture. However, for the aqueous solution with SO2 mass fraction of w3 = 0.070, the SO2 concentration in the equilibrium gas phase (y3′) is higher than that in the feed flue gas (y3 = 0.0085) when the pressure is lower than 2.64 MPa, but y3′ is smaller than y3 105

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Table 1. Hydrate−Liquid−Vapor Three Phase Equilibrium Temperature T, Pressure P, and Equilibrium Vapor Mole Fraction of CO2 y1′, N2 y2′, and SO2 y3′ for CO2 (1) + N2 (2) + SO2 (g, 3) + SO2 (aq. 3) + TBAB (4) + H2O (5) Mixturea w3 SO2 sat.

w4 b

0

0.070

0

0.035c 0.011c 0c 0

0 0 0 0.05

0.010

0.05

0.035

0.05

0.070

0.05

P/MPa

T/K

102y1′

102y3′

102y2′

1.38 2.50 3.94 5.09 1.70 2.64 4.23 5.46 2.65 2.65 2.65 1.26 2.12 3.05 4.01 1.20 2.64 3.80 1.31 2.08 3.06 4.04 1.83 2.90 3.88

276.15 278.45 280.75 281.95 272.85 273.65 275.25 276.75 273.65 273.65 273.65 279.85 281.15 282.35 283.15 280.85 282.35 283.15 279.59 280.75 281.65 282.85 277.85 279.05 280.45

7.55 7.81 8.10 9.00 8.00 8.48 8.85 9.65 7.32 6.98 6.89 7.16 8.85 9.19 9.33 11.57 10.26 7.62 11.99 11.41 10.67 9.76 12.75 9.88 8.37

2.33 2.12 1.90 1.53 0.95 0.87 0.75 0.55 0.61 0.42 0.09 0.08 0.07 0.06 0.06 0.13 0.11 0.09 0.11 0.08 0.06 0.05 0.63 0.55 0.42

90.12 90.07 90.00 89.47 91.05 90.65 90.40 89.80 92.07 92.60 93.02 92.76 91.08 90.75 90.61 88.30 89.63 92.29 87.90 88.51 89.27 90.19 86.62 89.57 91.21

Figure 2. Effect of SO2 (aq, 3) + H2O (5) aqueous solution on equilibrium hydrate formation pressure of CO2 (1) + N2 (2) + SO2 (g, 3) mixture: □, y1 = 0.159, y2 = 0.841, y3 = 0, w3 = 0, Lu et al.;33 ○, y1 = 0.1761, y2 = 0.8239, y3 = 0, w3 = 0, Kang et al.;34 Δ, y1 = 0.169, y2 = 0.831, y3 = 0, w3 = 0, Linga et al.;35 ★, y1 = 0.17, y2 = 0.82, y3 = 0.01, w3 = 0, Daraboina et al.;28 ■, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.070, this work; ●, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3, SO2 saturated aqueous solution, this work; All the dot lines denote the change tendency of the same feed flue gas and aqueous solution system.

HSO−3 .31,32 The hydration of the large amount of S(IV) species in aqueous solution produces a salting-out effect on CO2 dissolution, and decreases the dissolution of CO2. At the same time, the high H+ concentration resulting from SO2 dissolution inhibits the ionization of H2CO3 and also decrease the dissolution of CO2. With the increase in pressure, not only the dissolved amount of N2 increases, but also the relative increase rate of the dissolved N2 due to the pressure increase is higher than that of SO2 and CO2. So the mole fraction of N2 in the gas phase decreases. The molecular state of SO2(aq) and SO2·H2O can be encaged into the hydrate cages as guest molecules to stabilize the hydrate structure, and benefit the hydrate formation, while the ionized state of HSO−3 is an inhibitor and inhibits the hydrate formation. The majority of SO2 existing in the molecular state in solution may be the main reason that SO2 promotes the formation of flue gas hydrate. Effects of TBAB Additive on Equilibrium Formation Conditions of the Simulated Flue Gas Hydrates. As can be seen from Table 1, the phase equilibrium pressure of the simulated flue gas is still very high, especially for the case of low SO2 content in aqueous solution. Although increasing SO2 concentration in solution can decrease the hydrate formation pressure, the SO2 concentration in gas phase will increase. The chemical additives, such as TBAB and THF, were usually used as the preferred method to reduce the flue gas hydrate formation pressure in previous studies.36,37 Daraboina et al.28 reported that the equilibrium pressure and nucleation time of the flue gas hydrate were drastically reduced by adding THF with a mole fraction of 0.01, but both the hydrate formation rate and final gas consumption were reduced. In this work, TBAB solution with a mass fraction of 0.05 was used as the hydrate formation promoter. The effects of TBAB promoter on the equilibrium hydrate formation conditions are displayed in Figure 3. The addition of TBAB can drastically reduce the hydrate formation pressure of the SO2 containing flue gas. However, the pressure reducing effect of TBAB promoter in the

Mole fraction of CO2 (1), N2 (2), and SO2 (g, 3) in the feed flue gas is y1 = 0.1368, y2 = 0.8547, and y3 = 0.0085, respectively; w3 and w4 denote the mass fraction of SO2 (aq) and TBAB in feed aqueous solution, respectively; y1′, y2′, and y3′ denote the mole fraction of CO2, N2, and SO2 in gas phase at the equilibrium point; the standard uncertainties u are u(T) = ± 0.05 K, u(P) = ± 0.02 MPa, u(w) = ± 0.0002, u(y′) = ± 0.0002. bSO2 sat. denotes the aqueous solution is saturated by SO2. cDenotes only vapor−liquid two phase equilibrium reaches at specific temperature and pressure conditions. a

when the pressure is higher than 4.23 MPa. This indicates that some SO2 in the simulated flue gas has dissolved in the aqueous solution at high pressure conditions, and the SO2 concentration in the equilibrium liquid phase reaches more than w3 = 0.070. For the two phase equilibrium of flue gas (V)SO2 aqueous solutions (L) with w3 = 0, 0.011, and 0.035 at 273.65 K and 2.65 MPa, the SO2 concentration in the equilibrium gas phase y3′ = 0.0009, 0.0042, and 0.0061, respectively. All y3′ values are lower than y3 = 0.0085, which further verified that SO2 in flue gas tends to dissolve and accumulate in aqueous solution even though the SO2 concentration in solution reaches w3 = 0.035. For a given solution system, both SO2 and N2 mole fractions in the equilibrium gas phase (y3′ and y2′) decrease slightly with the increase in phase equilibrium pressure and temperature, while the CO2 mole fraction (y1′) in the equilibrium gas phase increases slightly. This indicates that the increase rates of the dissolved amount of SO2 and N2 resulting from the pressure increase are larger than that of CO2. The increase in the CO2 mole fraction may be mainly caused by the competition between the dissolution of SO2 and CO2. In a highly concentrated SO2 aqueous solution, the pH value of the solution is very low. The main chemical speciation of SO2 in solution is SO2(aq), SO2·H2O, and a small amount of 106

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Figure 3. Effect of SO2 (aq, 3) + TBAB (4) + H2O (5) aqueous solution on the equilibrium hydrate formation pressure of CO2 (1) + N2 (2) + SO2 (g, 3) mixture: □, y1 = 0.159, y2 = 0.841, y3 = 0, w3 = 0, w4 = 0, Lu et al.;33 ■, y1 = 0.159, y2 = 0.841, y3 = 0, w3 = 0, w4 = 0.05, Lu et al.;33 ▲, y1 = 0.20, y2 = 0.80, y3 = 0, w3 = 0, w4 = 0.05, Meysel et al.;40 ▼, y1 = 0.151, y2 = 0.849, y3 = 0, w3 = 0, w4 = 0.05, Mohammadi et al.;41 ◇, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.070, w4 = 0, this work; ★, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.070, w4 = 0.05, this work; All the dot lines denote the change tendency of the same feed flue gas and aqueous solution system.

Figure 4. Equilibrium hydrate formation pressure of CO2 (1) + N2 (2) + SO2 (g, 3) mixture in SO2 (aq, 3) + TBAB (4) + H2O (5) aqueous solution: □, y1 = 0.159, y2 = 0.841, y3 = 0, w3 = 0, w4 = 0.05, Lu et al.;33 ●, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0, w4 = 0.05, this work; ▲, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.010, w4 = 0.05, this work; ▼, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.350, w4 = 0.05, this work; ★, y1 = 0.1368, y2 = 0.8547, y3 = 0.0085, w3 = 0.070, w4 = 0.05, this work; All the dot lines denote the change tendency of the same feed flue gas and aqueous solution system.

SO2 containing system is smaller than that in system without SO2. In the system without TBAB, the simulated flue gas of CO2 + N2 + SO2 or CO2 + N2 form SI or SII hydrate. Whether SI or SII hydrate forms is dependent on N2 concentration in the flue gas.7,38 The presence of a large amount of SO2 guest molecules in solution will surely promote the hydrate formation and reduce the incipient hydrate formation pressure. While in the system with TBAB addition, TBAB forms semiclathrate hydrates with CO2, N2, and SO2 in solution, and thereby encages the gas components.37,39 The promoting effect of TBAB is mainly determined by the formation of TBAB semiclathrate hydrate embryos. When the solution system contains SO2 with w3 = 0.070, the high concentrated SO2 in solution inhibits the formation of TBAB semiclathrate hydrates, so its pressure reducing effect lessens as compared to the system without SO2. Effects of SO2 Concentration on Equilibrium Hydrate Formation Conditions of the Simulated Flue Gas in TBAB Aqueous Solutions. The equilibrium hydrate formation conditions and equilibrium gas compositions of CO2 (1) + N2 (2) + SO2 (g, 3) gaseous mixture in SO2 (aq, 3) + TBAB (4) + H2O (5) aqueous solution with TBAB mass fraction of w4 = 0.050 and SO2 mass fraction of w3 = 0, 0.010, 0.035, and 0.070 are listed in Table 1, and the effects of SO2 concentration on the hydrate equilibrium conditions are displayed in Figure 4. As can be seen from Figure 4, the increase in SO2 concentration is beneficial to the hydrate formation when the SO2 mass fraction in aqueous solution is lower than 0.010. However, when the SO2 mass fraction in aqueous solution increases to higher than 0.035, the hydrate phase equilibrium lines shift to high pressure and low temperature, and the hydrate forms under more severe conditions. The optimum mass fraction of SO2 in liquid is advisable to be lower than 0.010. SO2 can produce a synergy with TBAB to promote the hydrate formation by filling SO2 into the remaining empty cages of the TBAB semiclathrate

hydrates when the SO2 concentration in aqueous solution is low. When the SO2 concentration is high, the strong hydration of the highly concentrated SO2 in solution competes with TBAB, and prevents the formation of water molecule clusters and cages surrounding TBA+. This produces an inhibition on TBAB hydrate formation. As can be seen from Table 1, the SO2 concentrations in equilibrium gas phase (y3′) for the TBAB addition system are much lower than those without TBAB under the same feed gas−liquid conditions, which indicates that the addition of TBAB not only promotes the hydrate formation but also helps the dissolution of SO2 in aqueous solution and helps to reduce the SO2 concentration in the treated flue gas. It further confirms the validity of the TBAB additive in hydrate-based SO2-containing flue gas separation. TBAB maybe has a saltingin effect on SO2 by forming weak ion complex SO2Br−,42 or by weak ion association between TBA+ and HSO−3 , as well as other anions in aqueous solutions. 43 The detailed salting-in mechanisms will require further study. Moreover, the water molecules will reorient and form “cage-like” water clusters around the TBA+ cation after TBAB addition. The changes of the water internal structure affect the hydration and ionization of SO2 and result in the change of SO2 solubility.

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CONCLUSIONS The equilibrium hydrate formation conditions and the corresponding equilibrium gas compositions for the ternary simulated flue gases [CO2 (0.1368) + N2 (0.8547) + SO2 (0.0085)] with SO2 aqueous solutions, and SO2 + TBAB aqueous solutions were studied in this work. For the flue gas and SO2 aqueous solution, the presence of SO2 reduces the hydrate formation pressure. The higher the SO2 concentration in aqueous solution is, the easier the hydrate forms, but the SO2 concentration in equilibrium gas phase increases with the increase in SO2 concentration in the aqueous solution. N2 mole fractions in the equilibrium gas phase are higher than that in the 107

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feed flue gas, while CO2 is lower due to its relative high solubility. For a given system, the equilibrium hydrate formation pressure increases with the increase in temperature. Both SO2 and N2 mole fractions in equilibrium gas phase decrease with the increase in pressure, while the CO2 mole fraction increases. TBAB not only drastically reduces the equilibrium hydrate formation pressure, but also helps the dissolution of SO2 in aqueous solution. The SO2 concentrations in the equilibrium gas phase for the TBAB addition system are much lower than those without the TBAB addition. The pressure reducing effect of the TBAB promoter for SO2containing flue gas is dependent on SO2 concentration and smaller than that for flue gas without SO2. A small amount of SO2 produces a synergy with TBAB to promote the hydrate formation by filling itself into the empty cages of TBAB semiclathrate hydrates, but the high SO2 concentration produces an inhibition on the hydrate formation, and causes the hydrate equilibrium pressure increase. The optimum SO2 mass fraction in solution is lower than 0.010.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-20-87058468. Fax: +86-20-87034664. E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (51276182), the National Science Fund for Distinguished Young Scholars of China (51225603), the Guangdong Province Natural Science Foundation (S2011010004350), and the Science and Technology Program of Guangzhou City (2012J5100012), which are gratefully acknowledged. Notes

The authors declare no competing financial interest.

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