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Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfur–iodine hydrogen production process
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process Hyo Sub Kim a, Hyun Kyu Park a, Young Ho Kim a,*, Chu Sik Park b, Ki Kwang Bae b a
Department of Chemical Engineering & Applied Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea b Hydrogen Energy Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
article info
abstract
Article history:
The sulfureiodine hydrogen production process (SI process) is an efficient thermochemical
Received 19 April 2016
process. Developing composition data of the Bunsen products that are obtained under
Received in revised form
pressurized conditions is important for the integrated operation of SI process. We inves-
7 July 2016
tigated the characteristics of the pressurized Bunsen reaction by varying operating pa-
Accepted 11 July 2016
rameters (pressure, temperature, I2 and H2O feed concentrations). An HIx solution that
Available online xxx
consisted of HI, I2 and H2O was used as the reactant. When the pressure increased, the extent of Bunsen reaction was enhanced by increasing the solubility of SO2 gas in the HIx
Keywords:
solution. In addition, the amount of H2SO4 phase solution increased due to the decrease in
Hydrogen production
the amount of HIx complex, which has a high degree of hydration. Increasing temperature
Sulfureiodine process
enhanced the separation performance of the Bunsen products, while the extent of Bunsen
Bunsen reaction
reaction decreased. When I2 feed concentration increased, the occurrence of side reaction
Pressurized conditions
was inhibited, but the separation performance of the Bunsen products decreased. Although
HIx solution
increasing H2O feed concentration was favorable for the pressurized Bunsen reaction, the H2O feed concentration should be controlled by considering the solubility of I2 in the HIx solution. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The use of fossil fuels as primary energy sources results in environmental effects such as global warming and air pollution. Therefore, many groups are interested in developing environmentally-friendly energy sources that can replace
fossil fuels. Hydrogen is a clean energy source or energy carrier when it is produced from water. Thermochemical water splitting technology produces large amounts of hydrogen at lower temperatures than the direct thermolysis of water [1]. The sulfureiodine hydrogen production process (SI process), which is an efficient thermochemical cycle that was
* Corresponding author. Fax: þ82 42 822 6637. E-mail address: [email protected] (Y.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2016.07.070 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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developed by General Atomics (GA), consists of the following three sections [2]:
Bunsen reaction section: SO2 þ I2 þ 2H2O / H2SO4 þ 2HI (1)
H2SO4 decomposition section: H2SO4 / SO2 þ H2O þ 1/2O2
(2)
HI decomposition section: 2HI / H2 þ I2
(3)
The sulfuric acid (H2SO4) and hydrogen iodide (HI) that are produced from the Bunsen reaction (Eq. (1)) spontaneously separate into two immiscible liquids (H2SO4 solution and HIx solution) in the presence of excess iodine (I2). The H2SO4 solution includes H2SO4, H2O and a trace amount of HI as an impurity, and the HIx solution contains HI, I2, H2O and a trace amount of H2SO4 as an impurity. The H2SO4 solution decomposes into SO2, H2O and O2 (Eq. (2)), and the HI solution decomposes into H2 and I2 (Eq. (3)). The decomposition products (SO2, H2O and I2) other than H2 and O2 are recycled to the Bunsen reaction section. This process is operated in a closed cycle and utilizes the heat from a very high temperature reactor (VHTR) as an energy source. The Bunsen reaction section is important because it influences the efficiency and the integrated operation of SI process. Therefore, many researchers have reported various results of the Bunsen reaction [3e11]. The separation characteristics of a quaternary mixture (H2SO4eHIeI2eH2O) were investigated by varying the initial composition of the mixture and the temperature [3e5]. Increasing I2 concentration and decreasing H2O concentration improved the purity of the Bunsen products. The I2 concentration had a greater effect on the characteristics of the Bunsen reaction than operating temperature. The studies of the quaternary mixture were insufficient to demonstrate the Bunsen reaction section, which simultaneously includes the Bunsen reaction, side reactions and product separation. Therefore, Kim et al. [6] and Zhang et al. [7] performed the Bunsen reaction using SO2, I2 and H2O. Decreasing H2O concentration and increasing I2 concentration improved the purity of the Bunsen products and prevented the occurrence of side reaction. During the integrated operation of SI process, an HIx solution that consisted of HI, I2 and H2O was recycled as a reactant from the HI decomposition section to the Bunsen reaction section. The Bunsen reaction was performed using the HIx solution as a reactant [8e10]. Increasing temperature enhanced the purity of the Bunsen products. The feed concentrations of HI and I2 mainly affected the phase state and the composition of the Bunsen products. In addition, Dehghani et al. evaluated the comprehensive energy and exergetic performance of the Bunsen reaction [11]. They investigated the exergetic efficiency and exergy destruction in the Bunsen reaction with different parameters. Brown et al. [2] and Goldstein et al. [12] demonstrated that pressurized conditions were favorable for the integrated operation of the Bunsen reaction. Developing composition data of the Bunsen products that are obtained under pressurized conditions is required for the integrated operation of
SI process. The effects of pressure and temperature on the Bunsen products and the Bunsen reaction kinetics have been reported [13,14]. Imai et al. investigated the effects of SO2 partial pressure and temperature on the composition of HIx solution at I2 saturation [13]. They obtained the maximum concentration of HI at 353 K and an SO2 partial pressure of 0.108 MPa. Rao et al. performed the Bunsen reaction in a metallic tubular reactor by varying the pressure and temperature with continuous feeding of SO2 gas and SO2eN2 gas [14]. Increasing pressure increased the reaction rate at a fixed temperature, while increasing temperature decreased the reaction rate at a fixed pressure. In addition, the temperature had a greater effect on the reaction rate than the pressure. The characteristics of the pressurized Bunsen reaction and the composition of Bunsen products that are obtained under pressurized conditions have not yet been reported. Therefore, we carried out the Bunsen reaction by varying the operating parameters to ascertain the characteristics of the pressurized Bunsen reaction. The operating parameters were the pressure, temperature, and the initial I2/HI and H2O/HI molar ratios. In addition, we used the HIx solution as a reactant.
Experimental We conducted the Bunsen reaction under pressurized conditions using an experimental apparatus as shown in Fig. 1. A 300 mL jacketed glass reactor was used, and the other parts of reactor were made of Teflon to prevent corrosion. The temperature was controlled by a thermostatic water bath, which was connected to the jacket of the reactor. The HI/I2/H2O molar ratio of the HIx solution was controlled in the range of 1/1.4e2.9/6.2e12.0 based on 0.5 mol of HI (55e58 wt%, Kanto). Predetermined amounts of HI, I2 (98.0%, Junsei) and H2O were introduced into the reactor to prepare the HIx solution. The HIx solution was mixed for 30 min when the desired temperature was attained. The operating pressure was then controlled at 2.0, 2.5 and 3.0 atm (absolute) by feeding N2 gas. Once the desired pressure was attained, the feed of N2 gas was stopped, and the reaction proceeded simultaneously with continuous feeding of SO2 gas. Samples of the HIx phase solution were collected under pressurized conditions using the following steps. We referred to the sampling procedure that was reported by Nakajima et al. [15]. All of the valves (V1, V2 and V3) were closed before the sampling. V1 was opened and closed to supply the SO2 gas into the sampling line. While V2 was open, we collected the samples in the sample reservoir by opening V3 slowly. V2 was closed when a sufficient amount of the sample was obtained for the analysis. The HI and I2 concentrations were measured by titrating I and I2 with 0.1 N AgNO3 and 0.1 N Na2S2O3 standard solutions (Samchun Chemical), respectively. The H2SO4 concentration was calculated by subtracting the amount of HI from the amount of Hþ titrated with 0.1 N NaOH standard solution (DC Chemical). The H2O concentration was calculated assuming that only four species (H2SO4, HI, I2 and H2O) constitute each
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Fig. 1 e Schematic diagram of the experimental apparatus for the pressurized Bunsen reaction.
phase. The titrations were performed using a potentiometric titrator (AT-510, KEM) and electrodes (acid-base titration electrode: KEM C-171; redox titration electrode: KEM C-272; precipitation titration electrode: KEM C-373). To minimize the errors from the sampling and analysis processes, three samples for each ion were measured, and the average concentrations were then determined.
Results and discussion We determined the reaction time by ascertaining the equilibrium time of the pressurized Bunsen reaction. As a preliminary experiment, the Bunsen reaction was performed at 3 atm (absolute) and 333 K. An HIx solution with a HI/I2/H2O molar ratio of 1/2.5/7.0 was used as a reactant. The composition of the HIx phase was kept constant after 150 min of the reaction (Fig. 2). We found that the operation of the pressurized Bunsen reaction reached a quasi-steady-state after 150 min of reaction. Therefore, the reaction time was controlled at 180 min to allow sufficient conversion of the pressurized Bunsen reaction. There was a slight difference between the composition of the HIx phase that was obtained under pressurized conditions and that of the HIx phase that was obtained after the reaction. The vigorous mixing between the H2SO4 phase solution and the HIx phase solution induced this difference while the pressure decreased at atmospheric pressure after the reaction. Nevertheless, we discuss the composition of products that were obtained after the reaction to indirectly ascertain the characteristics of the pressurized Bunsen reaction. The weight and volume of the Bunsen products were also measured.
Effect of operating pressure on the characteristics of the Bunsen reaction We performed the Bunsen reaction while varying the operating pressure from 1.0 to 3.0 atm (absolute) at 333 K. An HIx solution with an HI/I2/H2O molar ratio of 1/2.5/7.0 was used as a reactant. As the pressure increased, the HI and H2SO4 contents increased, and the H2O content decreased (Fig. 3 (a)). When the pressure increased from 1.0 to 3.0, the H2SO4/HI molar ratio as the impurity in the HIx phase increased from approximately 0.16 to 0.20, which indicates that the purity of
Fig. 2 e The composition of the HIx phase with the time on stream (HI/I2/H2O molar ratio of 1/2.5/7.0; pressure: 3 atm; temperature: 333 K).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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phase at different pressures. DH2 O indicates the distributed amount of H2O in each phase based on the total amount of H2O. As the pressure increased to 3.0, VH2 SO4 phase increased to approximately 10.1 vol%, and DH2 O of the H2SO4 phase increased to approximately 19.0%. The amount of HIx complex, such as HI*3 (HþI-3 ion pair) and HI*5 (HþI-5 ion pair), decreased by increasing the reacted amount of I2 when the pressure increased (Fig. 3 (b)). DH2 O of the H2SO4 phase and VH2 SO4 phase increased due to the decreasing amount of HIx complex, which has a high degree of hydration. These results indicated that the separation performance of the Bunsen products improved when the pressure increased. We determined the separation performance of the Bunsen products by the volume ratio of HIx phase to H2SO4 phase. Increasing the operating pressure improved the extent of Bunsen reaction by increasing the solubility of SO2 gas in the HIx solution. Although increasing the operating pressure slightly decreased the purity of the HIx phase, it was effective at increasing DH2 O of the H2SO4 phase and VH2 SO4 phase .
Effect of operating temperature on the characteristics of the Bunsen reaction
Fig. 3 e Effect of pressure on (a) the composition of the HIx phase, (b) the variation in the amounts of the components after the Bunsen reaction and (c) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (HI/I2/H2O molar ratio of 1/2.5/7.0; temperature: 333 K).
the HIx phase was reduced. We measured the variations in the amounts of HI and I2 after the Bunsen reaction (Fig. 3 (b)). The variations in the amounts of HI and I2 indicated the amount of HI produced and the amount of I2 reacted, respectively. The variations in the amounts of HI and I2 increased as the pressure increased. The extent of Bunsen reaction improved due to the increase in the solubility of SO2 gas in the HIx solution when the pressure increased. Fig. 3 (c) shows the volume fraction of the H2SO4 phase solution ðVH2 SO4 phase Þ and the distribution ratio of the H2O component ðDH2 O Þ in the H2SO4
We conducted the Bunsen reaction while varying the operating temperature from 313 to 343 K at 3 atm (absolute). An HIx solution with an HI/I2/H2O molar ratio of 1/2.5/7.0 was fed into the reactor. The amount of HI produced and the amount of I2 reacted decreased when the temperature increased (Fig. 4 (a)). As the temperature increased, decreasing the solubility of SO2 gas in the HIx solution reduced the extent of the Bunsen reaction. This result was in good agreement with the result about the equilibrium constant of Bunsen reaction [16]. In addition, the extent of reaction decreased at higher temperature because the Bunsen reaction is exothermic. The H2SO4/ HI molar ratio that corresponded to the impurity in the HIx phase decreased to approximately 0.15 as the temperature increased to 343 K (Fig. 4 (b)). Increasing temperature was effective at improving the purity of the HIx phase. The impurities in the Bunsen products were removed via the reverse Bunsen reaction (Eq. (4)), which produced the H2O component that deteriorated the process efficiency. Therefore, decreasing the impurity concentration improved the process efficiency by decreasing H2O concentration at subsequent purification steps. When the temperature increased to 343 K, VH2 SO4 phase and DH2 O of the H2SO4 phase increased to approximately 14.3 vol% and 25.7%, respectively (Fig. 4 (c)). This result was attributed to an increase in the degree of hydration of the H2SO4 component in HIx phase due to the increase of the unreacted H2O content as the temperature increased.
Reverse Bunsen reaction: H2SO4 þ 2HI / SO2 þ I2 þ 2H2O (4) Table 1 shows the composition and density of the HIx phase solutions that were obtained at 3 atm (absolute) at different temperatures. As the temperature increased, the amounts of the unreacted components (I2 and H2O) increased, and the amounts of the products (H2SO4 and HI) decreased. The density of the HIx phase solution increased when the temperature increased. This result was explained by an
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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increase in the amount of unreacted I2 component. Although increasing temperature increased both VH2 SO4 phase and DH2 O of the H2SO4 phase, it reduced the extent of Bunsen reaction.
Effect of the initial I2/HI molar ratio on the characteristics of the Bunsen reaction The initial I2/HI molar ratio was varied from 1.4 to 2.9, and the initial H2O/HI molar ratio was 7.0. The temperature and pressure were maintained at 333 K and 3 atm (absolute), respectively. The variations in the amounts of HI and I2 increased when the I2 feed concentration increased, which indicates that the extent of Bunsen reaction improved (Fig. 5 (a)). Meanwhile, the side reaction (Eq. (5)) did not occur above the initial I2/HI molar ratio of 2.0, while sulfur formed at the initial I2/HI molar ratio of 1.4. These results were consistent with previous observations [9,17] that the side reaction occurs by decreasing I2 feed concentration. When the I2 feed concentration decreases, sulfur forms easily between H2SO4 and HI because of the decreased amount of HIx complex.
Sulfur formation: H2SO4 þ 6HI / S þ 3I2 þ 4H2O
Fig. 4 e Effect of temperature on (a) the variation in the amounts of the components after the Bunsen reaction, (b) the H2SO4/HI molar ratio in the HIx phase and (c) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (HI/I2/H2O molar ratio of 1/2.5/7.0; pressure: 3 atm).
(5)
As the initial I2/HI molar ratio increased to 2.9, both VH2 SO4 phase and DH2 O of the H2SO4 phase decreased to approximately 6.0 vol% and 10.7%, respectively (Fig. 5 (b)). The amount of H2O was insufficient for the separation of the H2SO4 component because of the increased amount of HIx complex, which has a high degree of hydration, as the I2 feed concentration increased. Table 2 lists the composition and density of the HIx phase solutions that were obtained at 3 atm with different I2 feed concentrations. When the I2 feed concentration increased, the amount of I2 in the HIx phase increased, but the amount of H2O decreased. The H2SO4/HI molar ratio that corresponds to the impurity in the HIx phase remained at approximately 0.20. As the I2 feed concentration increased, the density of the HIx phase solution increased. Increasing I2 feed concentration enhanced the extent of Bunsen reaction and inhibited the occurrence of side reaction. However, both VH2 SO4 phase and DH2 O of the H2SO4 phase were very low when the I2 feed concentration was close to saturation.
Effect of the initial H2O/HI molar ratio on the characteristics of the Bunsen reaction The initial H2O/HI molar ratio was varied from 6.2 to 12.0, and the initial I2/HI molar ratio was 2.5. The temperature
Table 1 e Composition and density of the HIx phase solution at different temperatures. Temp. [K]
313 323 333 343 a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
1.13
3.15
19.89 21.07 23.48 26.21
6.71 5.62 3.21 2.14
24.09 20.97 16.47 14.11
49.30 52.34 56.83 57.54
2.78 2.82 2.85 2.92
The composition of the HIx phase solution obtained at 3 atm (absolute).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Fig. 5 e Effect of the initial I2/HI molar ratio on (a) the variation in the amounts of the components after the Bunsen reaction and (b) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (H2O/HI molar ratio of 7.0; pressure: 3 atm; temperature: 333 K).
Fig. 6 e Effect of the initial H2O/HI molar ratio on (a) the variation in the amounts of the components after the Bunsen reaction and (b) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (I2/HI molar ratio of 2.5; pressure: 3 atm; temperature: 333 K).
and pressure were maintained at 333 K and 3 atm (absolute), respectively. The amount of HI produced and the amount of I2 reacted increased when the initial H2O/HI molar ratio increased (Fig. 6 (a)). This result indicated that the extent of Bunsen reaction improved by increasing H2O feed concentration at a fixed I2 feed concentration. When the initial H2O/HI molar ratio increased to 12, VH2 SO4 phase increased to approximately 21.1 vol%, and DH2 O of the H2SO4 phase increased to approximately 34.5% (Fig. 6 (b)). This result can be explained by the presence of a sufficient amount of H2O for the separation of the H2SO4 component.
Unlike the reaction that was conducted at atmospheric pressure [9], we did not observe the formation of sulfur when the initial H2O/HI molar ratio was 12. Table 3 shows the composition and density of the HIx phase solutions that were obtained at 3 atm with different H2O feed concentrations. As the H2O feed concentration increased, the I2 content decreased, and the H2O content increased. The H2SO4 and HI contents were similar regardless of the variation in H2O feed concentration. When the H2O feed concentration increased, the density of the HIx phase solution decreased.
Table 2 e Composition and density of the HIx phase solution with different I2 feed concentrations. Temp. [K]
333
a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
0.63 0.90 1.13 1.31
3.15
18.82 21.19 23.48 25.10
2.94 3.67 3.21 3.11
14.88 17.17 16.47 16.17
63.36 57.97 56.83 55.62
2.71 2.77 2.85 2.91
The composition of the HIx phase solution obtained at 3 atm (absolute).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Table 3 e Composition and density of the HIx phase solution with different H2O feed concentrations. Temp. [K]
333
a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
1.13
2.78 3.15 3.60 4.50 5.40
24.52 23.48 23.25 20.73 18.05
3.04 3.21 3.24 3.03 2.93
15.77 16.47 16.35 16.03 15.24
56.67 56.83 57.16 60.22 63.79
2.92 2.85 2.81 2.75 2.67
The composition of the HIx phase solution obtained at 3 atm (absolute).
Increasing H2O feed concentration was effective at improving the separation performance of the Bunsen products. However, the H2O feed concentration should be controlled by considering the solidification of I2 because the solubility of I2 in the HIx solution decreased when the H2O feed concentration increased. The solubility of I2 in the HIx solution decreased by increasing the initial H2O/HI molar ratio. When the initial H2O/HI molar ratios were 6.2, 8, 10 and 12, the I2/HI molar ratios that corresponded to I2 saturation at 333 K were 3.0, 2.5, 2.1 and 1.9, respectively. For the initial I2/HI molar ratio of 2.5 at 333 K, the H2O/HI molar ratios above 10 in the HIx solution were unsuitable for continuous operation due to the solidification of I2.
Conclusions We investigated the characteristics of the pressurized Bunsen reaction by varying the pressure, temperature, and the initial I2/HI and H2O/HI molar ratios. HIx solutions with HI/I2/ H2O molar ratios of 1/1.4e2.9/6.2e12 were used as the reactant. Quasi-steady-state operation of the pressurized Bunsen reaction was achieved after 150 min of reaction. The extent of Bunsen reaction improved by increasing the solubility of SO2 gas in the HIx phase as the pressure increased. Although increasing pressure reduced the purity of the products, it enhanced the separation performance of the Bunsen products. When the temperature increased, the volume of the H2SO4 phase solution increased, but the unreacted concentrations of I2 and H2O increased. Increasing I2 feed concentration inhibited the occurrence of side reaction and improved the extent of Bunsen reaction. However, both VH2 SO4 phase and DH2 O of the H2SO4 phase were very low (approximately 6.0 vol% and 10.7%, respectively) when the I2 feed concentration was close to saturation. The extent of Bunsen reaction and the separation performance of the Bunsen products were enhanced by increasing the initial H2O/HI molar ratio. The initial H2O/HI molar ratio should be controlled by considering the solubility of I2 in the HIx solution. These results will serve as valuable data for the integrated operation of SI process.
Acknowledgment This research was performed for the Nuclear Hydrogen Technology Development and was funded by the Ministry of Science, ICT and Future Planning, Korea.
references
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Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process Hyo Sub Kim a, Hyun Kyu Park a, Young Ho Kim a,*, Chu Sik Park b, Ki Kwang Bae b a
Department of Chemical Engineering & Applied Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea b Hydrogen Energy Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
article info
abstract
Article history:
The sulfureiodine hydrogen production process (SI process) is an efficient thermochemical
Received 19 April 2016
process. Developing composition data of the Bunsen products that are obtained under
Received in revised form
pressurized conditions is important for the integrated operation of SI process. We inves-
7 July 2016
tigated the characteristics of the pressurized Bunsen reaction by varying operating pa-
Accepted 11 July 2016
rameters (pressure, temperature, I2 and H2O feed concentrations). An HIx solution that
Available online xxx
consisted of HI, I2 and H2O was used as the reactant. When the pressure increased, the extent of Bunsen reaction was enhanced by increasing the solubility of SO2 gas in the HIx
Keywords:
solution. In addition, the amount of H2SO4 phase solution increased due to the decrease in
Hydrogen production
the amount of HIx complex, which has a high degree of hydration. Increasing temperature
Sulfureiodine process
enhanced the separation performance of the Bunsen products, while the extent of Bunsen
Bunsen reaction
reaction decreased. When I2 feed concentration increased, the occurrence of side reaction
Pressurized conditions
was inhibited, but the separation performance of the Bunsen products decreased. Although
HIx solution
increasing H2O feed concentration was favorable for the pressurized Bunsen reaction, the H2O feed concentration should be controlled by considering the solubility of I2 in the HIx solution. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The use of fossil fuels as primary energy sources results in environmental effects such as global warming and air pollution. Therefore, many groups are interested in developing environmentally-friendly energy sources that can replace
fossil fuels. Hydrogen is a clean energy source or energy carrier when it is produced from water. Thermochemical water splitting technology produces large amounts of hydrogen at lower temperatures than the direct thermolysis of water [1]. The sulfureiodine hydrogen production process (SI process), which is an efficient thermochemical cycle that was
* Corresponding author. Fax: þ82 42 822 6637. E-mail address: [email protected] (Y.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2016.07.070 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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developed by General Atomics (GA), consists of the following three sections [2]:
Bunsen reaction section: SO2 þ I2 þ 2H2O / H2SO4 þ 2HI (1)
H2SO4 decomposition section: H2SO4 / SO2 þ H2O þ 1/2O2
(2)
HI decomposition section: 2HI / H2 þ I2
(3)
The sulfuric acid (H2SO4) and hydrogen iodide (HI) that are produced from the Bunsen reaction (Eq. (1)) spontaneously separate into two immiscible liquids (H2SO4 solution and HIx solution) in the presence of excess iodine (I2). The H2SO4 solution includes H2SO4, H2O and a trace amount of HI as an impurity, and the HIx solution contains HI, I2, H2O and a trace amount of H2SO4 as an impurity. The H2SO4 solution decomposes into SO2, H2O and O2 (Eq. (2)), and the HI solution decomposes into H2 and I2 (Eq. (3)). The decomposition products (SO2, H2O and I2) other than H2 and O2 are recycled to the Bunsen reaction section. This process is operated in a closed cycle and utilizes the heat from a very high temperature reactor (VHTR) as an energy source. The Bunsen reaction section is important because it influences the efficiency and the integrated operation of SI process. Therefore, many researchers have reported various results of the Bunsen reaction [3e11]. The separation characteristics of a quaternary mixture (H2SO4eHIeI2eH2O) were investigated by varying the initial composition of the mixture and the temperature [3e5]. Increasing I2 concentration and decreasing H2O concentration improved the purity of the Bunsen products. The I2 concentration had a greater effect on the characteristics of the Bunsen reaction than operating temperature. The studies of the quaternary mixture were insufficient to demonstrate the Bunsen reaction section, which simultaneously includes the Bunsen reaction, side reactions and product separation. Therefore, Kim et al. [6] and Zhang et al. [7] performed the Bunsen reaction using SO2, I2 and H2O. Decreasing H2O concentration and increasing I2 concentration improved the purity of the Bunsen products and prevented the occurrence of side reaction. During the integrated operation of SI process, an HIx solution that consisted of HI, I2 and H2O was recycled as a reactant from the HI decomposition section to the Bunsen reaction section. The Bunsen reaction was performed using the HIx solution as a reactant [8e10]. Increasing temperature enhanced the purity of the Bunsen products. The feed concentrations of HI and I2 mainly affected the phase state and the composition of the Bunsen products. In addition, Dehghani et al. evaluated the comprehensive energy and exergetic performance of the Bunsen reaction [11]. They investigated the exergetic efficiency and exergy destruction in the Bunsen reaction with different parameters. Brown et al. [2] and Goldstein et al. [12] demonstrated that pressurized conditions were favorable for the integrated operation of the Bunsen reaction. Developing composition data of the Bunsen products that are obtained under pressurized conditions is required for the integrated operation of
SI process. The effects of pressure and temperature on the Bunsen products and the Bunsen reaction kinetics have been reported [13,14]. Imai et al. investigated the effects of SO2 partial pressure and temperature on the composition of HIx solution at I2 saturation [13]. They obtained the maximum concentration of HI at 353 K and an SO2 partial pressure of 0.108 MPa. Rao et al. performed the Bunsen reaction in a metallic tubular reactor by varying the pressure and temperature with continuous feeding of SO2 gas and SO2eN2 gas [14]. Increasing pressure increased the reaction rate at a fixed temperature, while increasing temperature decreased the reaction rate at a fixed pressure. In addition, the temperature had a greater effect on the reaction rate than the pressure. The characteristics of the pressurized Bunsen reaction and the composition of Bunsen products that are obtained under pressurized conditions have not yet been reported. Therefore, we carried out the Bunsen reaction by varying the operating parameters to ascertain the characteristics of the pressurized Bunsen reaction. The operating parameters were the pressure, temperature, and the initial I2/HI and H2O/HI molar ratios. In addition, we used the HIx solution as a reactant.
Experimental We conducted the Bunsen reaction under pressurized conditions using an experimental apparatus as shown in Fig. 1. A 300 mL jacketed glass reactor was used, and the other parts of reactor were made of Teflon to prevent corrosion. The temperature was controlled by a thermostatic water bath, which was connected to the jacket of the reactor. The HI/I2/H2O molar ratio of the HIx solution was controlled in the range of 1/1.4e2.9/6.2e12.0 based on 0.5 mol of HI (55e58 wt%, Kanto). Predetermined amounts of HI, I2 (98.0%, Junsei) and H2O were introduced into the reactor to prepare the HIx solution. The HIx solution was mixed for 30 min when the desired temperature was attained. The operating pressure was then controlled at 2.0, 2.5 and 3.0 atm (absolute) by feeding N2 gas. Once the desired pressure was attained, the feed of N2 gas was stopped, and the reaction proceeded simultaneously with continuous feeding of SO2 gas. Samples of the HIx phase solution were collected under pressurized conditions using the following steps. We referred to the sampling procedure that was reported by Nakajima et al. [15]. All of the valves (V1, V2 and V3) were closed before the sampling. V1 was opened and closed to supply the SO2 gas into the sampling line. While V2 was open, we collected the samples in the sample reservoir by opening V3 slowly. V2 was closed when a sufficient amount of the sample was obtained for the analysis. The HI and I2 concentrations were measured by titrating I and I2 with 0.1 N AgNO3 and 0.1 N Na2S2O3 standard solutions (Samchun Chemical), respectively. The H2SO4 concentration was calculated by subtracting the amount of HI from the amount of Hþ titrated with 0.1 N NaOH standard solution (DC Chemical). The H2O concentration was calculated assuming that only four species (H2SO4, HI, I2 and H2O) constitute each
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Fig. 1 e Schematic diagram of the experimental apparatus for the pressurized Bunsen reaction.
phase. The titrations were performed using a potentiometric titrator (AT-510, KEM) and electrodes (acid-base titration electrode: KEM C-171; redox titration electrode: KEM C-272; precipitation titration electrode: KEM C-373). To minimize the errors from the sampling and analysis processes, three samples for each ion were measured, and the average concentrations were then determined.
Results and discussion We determined the reaction time by ascertaining the equilibrium time of the pressurized Bunsen reaction. As a preliminary experiment, the Bunsen reaction was performed at 3 atm (absolute) and 333 K. An HIx solution with a HI/I2/H2O molar ratio of 1/2.5/7.0 was used as a reactant. The composition of the HIx phase was kept constant after 150 min of the reaction (Fig. 2). We found that the operation of the pressurized Bunsen reaction reached a quasi-steady-state after 150 min of reaction. Therefore, the reaction time was controlled at 180 min to allow sufficient conversion of the pressurized Bunsen reaction. There was a slight difference between the composition of the HIx phase that was obtained under pressurized conditions and that of the HIx phase that was obtained after the reaction. The vigorous mixing between the H2SO4 phase solution and the HIx phase solution induced this difference while the pressure decreased at atmospheric pressure after the reaction. Nevertheless, we discuss the composition of products that were obtained after the reaction to indirectly ascertain the characteristics of the pressurized Bunsen reaction. The weight and volume of the Bunsen products were also measured.
Effect of operating pressure on the characteristics of the Bunsen reaction We performed the Bunsen reaction while varying the operating pressure from 1.0 to 3.0 atm (absolute) at 333 K. An HIx solution with an HI/I2/H2O molar ratio of 1/2.5/7.0 was used as a reactant. As the pressure increased, the HI and H2SO4 contents increased, and the H2O content decreased (Fig. 3 (a)). When the pressure increased from 1.0 to 3.0, the H2SO4/HI molar ratio as the impurity in the HIx phase increased from approximately 0.16 to 0.20, which indicates that the purity of
Fig. 2 e The composition of the HIx phase with the time on stream (HI/I2/H2O molar ratio of 1/2.5/7.0; pressure: 3 atm; temperature: 333 K).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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phase at different pressures. DH2 O indicates the distributed amount of H2O in each phase based on the total amount of H2O. As the pressure increased to 3.0, VH2 SO4 phase increased to approximately 10.1 vol%, and DH2 O of the H2SO4 phase increased to approximately 19.0%. The amount of HIx complex, such as HI*3 (HþI-3 ion pair) and HI*5 (HþI-5 ion pair), decreased by increasing the reacted amount of I2 when the pressure increased (Fig. 3 (b)). DH2 O of the H2SO4 phase and VH2 SO4 phase increased due to the decreasing amount of HIx complex, which has a high degree of hydration. These results indicated that the separation performance of the Bunsen products improved when the pressure increased. We determined the separation performance of the Bunsen products by the volume ratio of HIx phase to H2SO4 phase. Increasing the operating pressure improved the extent of Bunsen reaction by increasing the solubility of SO2 gas in the HIx solution. Although increasing the operating pressure slightly decreased the purity of the HIx phase, it was effective at increasing DH2 O of the H2SO4 phase and VH2 SO4 phase .
Effect of operating temperature on the characteristics of the Bunsen reaction
Fig. 3 e Effect of pressure on (a) the composition of the HIx phase, (b) the variation in the amounts of the components after the Bunsen reaction and (c) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (HI/I2/H2O molar ratio of 1/2.5/7.0; temperature: 333 K).
the HIx phase was reduced. We measured the variations in the amounts of HI and I2 after the Bunsen reaction (Fig. 3 (b)). The variations in the amounts of HI and I2 indicated the amount of HI produced and the amount of I2 reacted, respectively. The variations in the amounts of HI and I2 increased as the pressure increased. The extent of Bunsen reaction improved due to the increase in the solubility of SO2 gas in the HIx solution when the pressure increased. Fig. 3 (c) shows the volume fraction of the H2SO4 phase solution ðVH2 SO4 phase Þ and the distribution ratio of the H2O component ðDH2 O Þ in the H2SO4
We conducted the Bunsen reaction while varying the operating temperature from 313 to 343 K at 3 atm (absolute). An HIx solution with an HI/I2/H2O molar ratio of 1/2.5/7.0 was fed into the reactor. The amount of HI produced and the amount of I2 reacted decreased when the temperature increased (Fig. 4 (a)). As the temperature increased, decreasing the solubility of SO2 gas in the HIx solution reduced the extent of the Bunsen reaction. This result was in good agreement with the result about the equilibrium constant of Bunsen reaction [16]. In addition, the extent of reaction decreased at higher temperature because the Bunsen reaction is exothermic. The H2SO4/ HI molar ratio that corresponded to the impurity in the HIx phase decreased to approximately 0.15 as the temperature increased to 343 K (Fig. 4 (b)). Increasing temperature was effective at improving the purity of the HIx phase. The impurities in the Bunsen products were removed via the reverse Bunsen reaction (Eq. (4)), which produced the H2O component that deteriorated the process efficiency. Therefore, decreasing the impurity concentration improved the process efficiency by decreasing H2O concentration at subsequent purification steps. When the temperature increased to 343 K, VH2 SO4 phase and DH2 O of the H2SO4 phase increased to approximately 14.3 vol% and 25.7%, respectively (Fig. 4 (c)). This result was attributed to an increase in the degree of hydration of the H2SO4 component in HIx phase due to the increase of the unreacted H2O content as the temperature increased.
Reverse Bunsen reaction: H2SO4 þ 2HI / SO2 þ I2 þ 2H2O (4) Table 1 shows the composition and density of the HIx phase solutions that were obtained at 3 atm (absolute) at different temperatures. As the temperature increased, the amounts of the unreacted components (I2 and H2O) increased, and the amounts of the products (H2SO4 and HI) decreased. The density of the HIx phase solution increased when the temperature increased. This result was explained by an
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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increase in the amount of unreacted I2 component. Although increasing temperature increased both VH2 SO4 phase and DH2 O of the H2SO4 phase, it reduced the extent of Bunsen reaction.
Effect of the initial I2/HI molar ratio on the characteristics of the Bunsen reaction The initial I2/HI molar ratio was varied from 1.4 to 2.9, and the initial H2O/HI molar ratio was 7.0. The temperature and pressure were maintained at 333 K and 3 atm (absolute), respectively. The variations in the amounts of HI and I2 increased when the I2 feed concentration increased, which indicates that the extent of Bunsen reaction improved (Fig. 5 (a)). Meanwhile, the side reaction (Eq. (5)) did not occur above the initial I2/HI molar ratio of 2.0, while sulfur formed at the initial I2/HI molar ratio of 1.4. These results were consistent with previous observations [9,17] that the side reaction occurs by decreasing I2 feed concentration. When the I2 feed concentration decreases, sulfur forms easily between H2SO4 and HI because of the decreased amount of HIx complex.
Sulfur formation: H2SO4 þ 6HI / S þ 3I2 þ 4H2O
Fig. 4 e Effect of temperature on (a) the variation in the amounts of the components after the Bunsen reaction, (b) the H2SO4/HI molar ratio in the HIx phase and (c) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (HI/I2/H2O molar ratio of 1/2.5/7.0; pressure: 3 atm).
(5)
As the initial I2/HI molar ratio increased to 2.9, both VH2 SO4 phase and DH2 O of the H2SO4 phase decreased to approximately 6.0 vol% and 10.7%, respectively (Fig. 5 (b)). The amount of H2O was insufficient for the separation of the H2SO4 component because of the increased amount of HIx complex, which has a high degree of hydration, as the I2 feed concentration increased. Table 2 lists the composition and density of the HIx phase solutions that were obtained at 3 atm with different I2 feed concentrations. When the I2 feed concentration increased, the amount of I2 in the HIx phase increased, but the amount of H2O decreased. The H2SO4/HI molar ratio that corresponds to the impurity in the HIx phase remained at approximately 0.20. As the I2 feed concentration increased, the density of the HIx phase solution increased. Increasing I2 feed concentration enhanced the extent of Bunsen reaction and inhibited the occurrence of side reaction. However, both VH2 SO4 phase and DH2 O of the H2SO4 phase were very low when the I2 feed concentration was close to saturation.
Effect of the initial H2O/HI molar ratio on the characteristics of the Bunsen reaction The initial H2O/HI molar ratio was varied from 6.2 to 12.0, and the initial I2/HI molar ratio was 2.5. The temperature
Table 1 e Composition and density of the HIx phase solution at different temperatures. Temp. [K]
313 323 333 343 a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
1.13
3.15
19.89 21.07 23.48 26.21
6.71 5.62 3.21 2.14
24.09 20.97 16.47 14.11
49.30 52.34 56.83 57.54
2.78 2.82 2.85 2.92
The composition of the HIx phase solution obtained at 3 atm (absolute).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Fig. 5 e Effect of the initial I2/HI molar ratio on (a) the variation in the amounts of the components after the Bunsen reaction and (b) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (H2O/HI molar ratio of 7.0; pressure: 3 atm; temperature: 333 K).
Fig. 6 e Effect of the initial H2O/HI molar ratio on (a) the variation in the amounts of the components after the Bunsen reaction and (b) the amount of H2SO4 phase and the distribution ratio of H2O in the H2SO4 phase after the Bunsen reaction (I2/HI molar ratio of 2.5; pressure: 3 atm; temperature: 333 K).
and pressure were maintained at 333 K and 3 atm (absolute), respectively. The amount of HI produced and the amount of I2 reacted increased when the initial H2O/HI molar ratio increased (Fig. 6 (a)). This result indicated that the extent of Bunsen reaction improved by increasing H2O feed concentration at a fixed I2 feed concentration. When the initial H2O/HI molar ratio increased to 12, VH2 SO4 phase increased to approximately 21.1 vol%, and DH2 O of the H2SO4 phase increased to approximately 34.5% (Fig. 6 (b)). This result can be explained by the presence of a sufficient amount of H2O for the separation of the H2SO4 component.
Unlike the reaction that was conducted at atmospheric pressure [9], we did not observe the formation of sulfur when the initial H2O/HI molar ratio was 12. Table 3 shows the composition and density of the HIx phase solutions that were obtained at 3 atm with different H2O feed concentrations. As the H2O feed concentration increased, the I2 content decreased, and the H2O content increased. The H2SO4 and HI contents were similar regardless of the variation in H2O feed concentration. When the H2O feed concentration increased, the density of the HIx phase solution decreased.
Table 2 e Composition and density of the HIx phase solution with different I2 feed concentrations. Temp. [K]
333
a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
0.63 0.90 1.13 1.31
3.15
18.82 21.19 23.48 25.10
2.94 3.67 3.21 3.11
14.88 17.17 16.47 16.17
63.36 57.97 56.83 55.62
2.71 2.77 2.85 2.91
The composition of the HIx phase solution obtained at 3 atm (absolute).
Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070
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Table 3 e Composition and density of the HIx phase solution with different H2O feed concentrations. Temp. [K]
333
a
Mole fractiona [mol%]
Feed [mol]
rHIx phase [g/mL]
HI
I2
H2O
I2
H2SO4
HI
H2O
0.45
1.13
2.78 3.15 3.60 4.50 5.40
24.52 23.48 23.25 20.73 18.05
3.04 3.21 3.24 3.03 2.93
15.77 16.47 16.35 16.03 15.24
56.67 56.83 57.16 60.22 63.79
2.92 2.85 2.81 2.75 2.67
The composition of the HIx phase solution obtained at 3 atm (absolute).
Increasing H2O feed concentration was effective at improving the separation performance of the Bunsen products. However, the H2O feed concentration should be controlled by considering the solidification of I2 because the solubility of I2 in the HIx solution decreased when the H2O feed concentration increased. The solubility of I2 in the HIx solution decreased by increasing the initial H2O/HI molar ratio. When the initial H2O/HI molar ratios were 6.2, 8, 10 and 12, the I2/HI molar ratios that corresponded to I2 saturation at 333 K were 3.0, 2.5, 2.1 and 1.9, respectively. For the initial I2/HI molar ratio of 2.5 at 333 K, the H2O/HI molar ratios above 10 in the HIx solution were unsuitable for continuous operation due to the solidification of I2.
Conclusions We investigated the characteristics of the pressurized Bunsen reaction by varying the pressure, temperature, and the initial I2/HI and H2O/HI molar ratios. HIx solutions with HI/I2/ H2O molar ratios of 1/1.4e2.9/6.2e12 were used as the reactant. Quasi-steady-state operation of the pressurized Bunsen reaction was achieved after 150 min of reaction. The extent of Bunsen reaction improved by increasing the solubility of SO2 gas in the HIx phase as the pressure increased. Although increasing pressure reduced the purity of the products, it enhanced the separation performance of the Bunsen products. When the temperature increased, the volume of the H2SO4 phase solution increased, but the unreacted concentrations of I2 and H2O increased. Increasing I2 feed concentration inhibited the occurrence of side reaction and improved the extent of Bunsen reaction. However, both VH2 SO4 phase and DH2 O of the H2SO4 phase were very low (approximately 6.0 vol% and 10.7%, respectively) when the I2 feed concentration was close to saturation. The extent of Bunsen reaction and the separation performance of the Bunsen products were enhanced by increasing the initial H2O/HI molar ratio. The initial H2O/HI molar ratio should be controlled by considering the solubility of I2 in the HIx solution. These results will serve as valuable data for the integrated operation of SI process.
Acknowledgment This research was performed for the Nuclear Hydrogen Technology Development and was funded by the Ministry of Science, ICT and Future Planning, Korea.
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Please cite this article in press as: Kim HS, et al., Effects of operating parameters on the pressurized Bunsen reaction for the integrated operation of sulfureiodine hydrogen production process, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.070