Article pubs.acs.org/IECR
Experimental Investigation on Multiphase Bunsen Reaction in the Thermochemical Sulfur−Iodine Cycle Yanwei Zhang,* Pingan Peng, Zhi Ying, Qiaoqiao Zhu, Junhu Zhou, Zhihua Wang, Jianzhong Liu, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: The effects of SO2 flow rate, SO2 mole fraction, iodine content and water content on the Bunsen reaction were experimentally studied for the thermochemical iodine−sulfur cycle for hydrogen production. One multistage reaction mechanism on the Bunsen reaction has been hypothesized, considering the kinetic characteristics. The appearance of a liquid−liquid equilibrium (LLE) phase separation was in the range of 20−40 min in the apparatus we chose. An increase in iodine content and a decrease in water content improved LLE separation characteristics to the H2SO4 phase and HIx phase. The H2S formation side reaction easily occurred with the increase in water content and decrease in iodine content. The optimal operating condition that the inlet SO2 mole fraction exceeds 0.12 coupled with initial I2/H2O molar ratio exceeding 0.284 has been proposed based on the experimental results, for the criteria of improving the separation characteristics and preventing side reactions.
1. INTRODUCTION Hydrogen is currently regarded as a promising candidate as an energy carrier due to the worldwide fast consumption of conventional fossil fuels. If hydrogen is produced from water using various renewable sources, it has the potential to constitute a future sustainable energy economy. The thermochemical water splitting cycles1 have attracted increasing attention for massive hydrogen production, making the use of water as a raw material and solar, wind, geothermal or safe nuclear energy primary energy sources. Among all the thermochemical water splitting cycles proposed over the last 30 years, the sulfur−iodine (SI or IS) cycle is widely considered one of the most hopeful alternatives, aiming at high-efficient and cost-effective hydrogen production without CO2 emissions.2−4 The SI cycle is fundamentally based on the following three chemical reactions involving hydriodic and sulfuric acids as intermediates at three different temperature levels: Bunsen reaction (exothermic at 293−393 K): I 2 + SO2 + 2H 2O → 2HI + H 2SO4
production by vapor phase decomposition reactions 2 and 3, respectively. The Bunsen reaction is considered the critical step of the SI cycle as it is interfaced to both the H2SO4 and HI decomposition reactions. Although liquid−liquid equilibrium (LLE) separation and side reactions occur in a certain range of compositions and high concentrations of HI in the HIx phase and H2SO4 in the H2SO4 phase, which are required for high hydrogen production thermal efficiency, the optimal operating conditions for this reaction must be established. The study of the Bunsen section has received wide attention, and most works feature the LLE phase separation behavior of the quaternary system H2SO4/HI/I2/H2O and determine the degree of mutual contamination between the two acid phases (HI and I2 in the H2SO4 phase and H2SO4 in HIx phase).5−14 The LLE phase separation characteristics improve with increases in iodine content and decreases in water content, and the allowable bound of iodine content for phase separation is widened with increases in the solution temperature. A thermodynamic model15 has been proposed to validate the experimental equilibrium data between the two acid phases previously published. In addition, a few studies on the reverse Bunsen reaction or side reactions between HI and H2SO4 have been conducted.16−20 Two predominant side reactions are evident at some specific operating conditions, producing sulfur and hydrogen sulfide along with the Bunsen reaction:
(1)
HI decomposition reaction (endothermic at 673−773 K): 2HI → I 2 + H 2
(2)
H2SO4 decomposition reaction (endothermic at 1073−1273 K): H 2SO4 → SO2 + H 2O + 1/2O2
(3)
The Bunsen reaction provides water splitting into hydriodic and sulfuric acids via the reaction between iodine, sulfur dioxide and water. In the presence of iodine excess, the resulting acids are spontaneously divided into two immiscible phases: the upper one (H2SO4 phase), mainly containing aqueous sulfuric acid, and the lower one (HIx phase), consisting in iodine, hydrogen iodide and water. Afterward, two acid streams are individually treated for the appropriate purification and concentration steps, leading to hydrogen and oxygen © 2014 American Chemical Society
6HI + H 2SO4 → S + 3I 2 + 4H 2O
(4)
8HI + H 2SO4 → H 2S + 4I 2 + 4H 2O
(5)
The kinetics of the Bunsen reaction has been studied besides the researches on equilibrium state show above. The effect of Received: Revised: Accepted: Published: 3021
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Figure 1. Apparatus used for Bunsen reaction tests.
1.035 and 1.362 mol, whereas the one of the water was kept between 4.2 and 5.4 mol. Preliminary experiments clearly showed that the volume ratio of the upper H 2 SO 4 phase to the lower HIx phase approximately tended to a value of 0.20−0.25, which was obtained by qualitative observation during the experiments and measuring the volume of the two liquid phases after LLE separation. The volume of the H2SO4 phase was maintained in the range of 20−30 mL. Therefore, it was feasible to carry out some sampling whose amount was 0.1 mL per sampling and analysis in the upper phase. Owing to the kinetic activity of the Bunsen reaction, the composition changed rapidly, so it was difficult to perform the sampling and analysis on both liquid phases. In our experimentation runs, the upper phase at some regular times was analyzed to characterize the kinetics of the multiphase Bunsen reaction, and subsequently, both phases in the equilibrium state were analyzed to determine the LLE phase separation characteristics. To understand the detailed mechanism of the Bunsen reaction, the samples from the upper H2SO4 and lower HIx phase sampled by transferpettor after some specific times from the start of the reaction were analyzed by the chemical titration method. The concentration of H+ was measured by sodium hydroxide (NaOH, Sinopharm Chemical Reagent) titration after the sample was diluted. I− concentration was obtained by redox titration with 0.05 N potassium iodate (KIO3, Zhejiang Hichi Chemical) standard solution after the sample was diluted in water. I2 concentration was determined by redox titration with 0.1 N sodium thiosulfate (Na2S2O3, Xiaoshan Chemical Reagent) solution after dilution of the sample in a Potassium iodide (KI, Aladdin Reagent) aqueous solution, preventing iodine from precipitation prior to the measurement. Assuming only four species (H2SO4, HI, I2 and H2O) constituting each phase, the concentrations of H2O and H2SO4 were calculated following the mass and ionic balance.
reaction temperature on the sulfur dioxide conversion efficiency in the Bunsen reaction by feeding gaseous sulfur dioxide in an iodine/hydriodic acid aqueous solution has been investigated by some researchers in ENEA.21 Both the amount of absorbed SO2 and its conversion decrease with an increase of reaction temperature. Various operating parameters have been studied.22,23 However, some parameters should be further investigated, such as SO2 flow rate, SO2 mole fraction, iodine content and water content on the multiphase Bunsen reaction. The objective of this work is to provide new experimental data for better controlling operational specifications and eventually achieve an optimal reaction conditions considering the occurrence of the Bunsen side reaction.
2. EXPERIMENTAL SECTION The apparatus used for testing the Bunsen reaction, shown in Figure 1, consists of a 500 mL double jacketed stirred reactor whose temperature is controlled by means of a thermostatic water bath and a water recirculation system. The stirrer is connected to an electrical engine rotating at about 160 rpm. The reactor is connected to a desiccator, filled with silica gel to absorb the vapor. A SO2/H2S gas analyzer (GXH−3011N, Huayun Instrument), placed on the desiccators outlet, detects the concentration of gaseous sulfur dioxide and hydrogen sulfide. The generated acid gases are sequentially trapped in two scrubbers, which are filled with sodium hydroxide solution, connecting to the SO2/H2S gas analyzer. A calculated amount of iodine (>99.9%, Chinasun Specialty Products) and deionized water were initially introduced into the reactor and the thermostatic water bath and the stirrer were switched on. Once the desired operating temperature was reached, gaseous SO2 (>99.99%, Hangzhou Jingong Materials) and N2 (>99.999%, Hangzhou Jingong Materials), which were mixed in a premixer, were fed at a constant flow rate until the SO2/H2S gas analyzer detected the concentration of SO2 maintained constant. Afterward, the stirrer was stopped and the mixture was kept resting for 5 min to allow the LLE phase separation. The reaction temperature was controlled at a constant value of 345 K, which was chosen within the optimal operating range for the Bunsen process.7,12 The gaseous SO2 flow rate had been varied from 50 up to 70 N mL/min, and the mole fraction had been changed between 0.09 and 0.14 by introducing the inert carrier gas of N2. The total quantity was kept at around 0.3 mol. The amount of the iodine addition was controlled between
3. RESULTS AND DISCUSSION A variety of experimental runs have been performed to investigate the effects of the various operating parameters on the kinetics, LLE separation and side reaction occurrence of the Bunsen reaction. Some useful experimental results regarding further improvement of the Bunsen reaction are listed below. 3.1. Kinetics of Bunsen Reaction. The composition of the H2SO4 phase obtained was analyzed at 345 K. The initial molar ratio of I2/H2O was set to 0.246 and the gaseous sulfur dioxide and nitrogen flow rates varied from 50 to 70 N mL/min and 3022
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from 298 to 417 N mL/min, respectively. Here, the SO2 mole fraction was maintained at a constant value of 0.14. Figure 2 shows that H2SO4 concentration tends to increase with the reaction time and reaches the equilibrium value over 4 mol/kg. As in Figure 2b,c, HI and I2 molality have a similar trend in the whole reaction course. It can be explained by the solubility property of the iodine in hydriodic acid solution produced in the Bunsen reaction. Each of them rapidly increases to a peak value and then decreases to a value below 0.5 mol/kg. The appearance of the liquid−liquid phase separation phenomena should be responsible for the aforementioned trend. The beginning time for LLE phase separation was within the range of 20−40 min, which was validated through qualitative observation. Because SO2 can be dissolved into the reactive solution completely, the higher SO2 flow rate results in faster complete dissolution and causes faster equilibrium state in the H2SO4 phase further. However, it has no effect on the final molarity when the reaction reaches equilibrium. It is important to point out that the reaction proceeds during the first 90 min and terminates. The composition in the H2SO4 phase is supposed to become the constant state after 90 min from the reaction start. In Figure 3, the composition of the H2SO4 phase is plotted vs the SO2 mole fraction at 345 K. The initial molar ratio of I2/ H2O corresponds to 0.246, and the gaseous sulfur dioxide flow rate is maintained at 50 N mL/min and nitrogen flow rate varies from 298 to 505 N mL/min. The SO2 mole fraction is changed between 0.09 and 0.14. H2SO4 content increases slightly with the reaction time less than 40 min, and then increases markedly to a value of around 4.5 mol/kg. The equilibrium concentration and molarity trend of H2SO4 does not change significantly with the SO2 mole fraction. As shown in Figure 3b,c, HI and I2 content increase regularly with the reaction time and achieve their peak values of about 1.4 mol/kg and 1.6−1.7 mol/kg, respectively, and then decrease evidently. In the same way as H2SO4, the SO2 mole fraction has little influences on the changes with time of the HI and I2 concentrations. This clearly indicates that SO2 absorbing is not completing until the Bunsen reaction is over. Once the gaseous SO2 is introduced into the reactive solution, the quantity of HI and H2SO4 production can be measured immediately. It is proven that the Bunsen reaction had the high reaction rate, and the physical factors including dissolution and diffusion had little influence on the kinetic reaction course. The effects of iodine and water content on the composition of the H2SO4 phase with the reaction time evolution are shown in Figures 4 and 5. The initial molar ratio of I2/H2O varies from 0.246 to 0.324 and from 0.324 to 0.252, and gaseous sulfur dioxide and nitrogen flow rates are kept at 60 and 357 N mL/ min, respectively. The amounts of H 2 SO 4 , HI and I 2 consistently increase as the reaction between sulfur dioxide, iodine and water proceed. After the phase separation to H2SO4 and HIx phases, H2SO4 concentration drastically increases to over 4 mol/kg and, on the contrary, both HI and I2 content reduce. The LLE phase separation appears in the 20−40 min time range, which is in agreement with the aforementioned tendency. Increasing I2 promotes the Bunsen reaction toward the right-hand and leads to a higher reaction rate as it increases the concentration of the reagent, which is favorable for both kinetics and thermodynamics. Increases of the water content raise the reaction rate as a reagent, just like I2. On the other hand, the dilution effect of H2O hinders reaction. Therefore, it benefits kinetics and acts on thermodynamics on the contrary.
Figure 2. Effect of SO2 flow rate on resultants content in H2SO4 phase at 345 K (I2/H2O molar ratio of 0.246): (a) H2SO4, (b) HI, (c) I2.
In simple terms, an increasing amount of iodine and a decreasing amount of water in the medium both promote the resulting solution splitting and reaction kinetic rate. This result 3023
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Figure 3. Effect of SO2 mole fraction on resultants content in H2SO4 phase at 345 K (I2/H2O molar ratio of 0.246): (a)H2SO4, (b)HI, (c)I2. Figure 4. Effect of iodine content on resultants content in H2SO4 phase at 345 K (SO2 flow rate of 60 N mL/min, N2 flow rate of 357 N mL/min and H2O of 4.2 mol, I2/H2O varied from 0.246 to 0.324): (a)H2SO4, (b)HI, (c)I2.
suggests that the kinetic rate of the Bunsen reaction is dependent on the reactant content. 3024
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One multistage reaction mechanism on the Bunsen reaction has been hypothesized based on the experimental results.22 It comprises (1) gaseous SO2 absorbed into iodine/water, (2) H2SO3 formation via the reaction between SO2 and H2O, (3) − 2− H2SO3 ionized to HSO−3 and SO2− 3 , (4) HSO3 and SO3 reacted with iodine dissolved and H2O and (5) iodine dissolved in I− aqueous solution to form I−3 , I−5 , etc. In fact, all five steps above constitute the kinetic process of the Bunsen reaction. The dominant step changes along with the reaction, resulting in a three stage process. The first stage is the initial reaction stage in which the liquid phase is miscible and the concentrations of H2SO4 and HI are increasing. The second stage is the liquid− liquid separation stage with the third stage, named the equilibrium stage, following. On the basis of the proposed mechanism, how the operating conditions affect the process of the Bunsen reaction can be speculated. When the SO2 flow rate is controlled within 50−70 N mL/min, all the SO2 dissolves into the aqueous solution. The reaction rate increases with the increase of SO2 flow rate, as it promotes step (1) to step (4) of the hypothesized reaction mechanism. At the beginning, the reaction among crystalline I2, soluble SO2 and water is the dominant one. Then the reaction between the generated and I− and crystalline I2 producing polyiodide, which increases reaction rate by involving in reaction, promotes the complexation reaction. Once the complexation reaction controls the kinetic rate of the Bunsen reaction, both the concentrations of I− and I2 have an effect on the reaction rate. H2O is one of the reagents in the Bunsen reaction as well as solvent. The dilution effect of H2O on I2 and SO2 reduces the probability of effective collision among the reagents, leading to a lower reaction rate and the neglect of promotion by H2O as a reagent. More H2O content results in stronger dilution effect. 3.2. Equilibrium Composition of the Two Phases. In the framework of the SI cycle optimization, the continuous closed-cycle operation at high process thermal efficiency is very sensitive to several factors involved in the Bunsen reaction, such as the composition of the two acid phases, the side reaction. The side reaction occurrence is strongly affected by the composition in each of the acid phases, especially when HI is in the H2SO4 phase and H2SO4 in the HIx phase. High H2SO4 concentration in the H2SO4 phase and high HI concentration in the HIx phase have an advantage for high hydrogen production thermal efficiency. Thus, attempts must be made to obtain two immiscible liquid streams as pure and concentrated as possible. Table 1 collects the experimental data of LLE phase separation in the Bunsen reaction. A good separation of the iodine−containing and sulfur−containing compounds is achieved. The amount of impurity in both phases is controlled at an acceptable value. HI concentration varies from 1.0 mol/kg to 1.3 mol/kg, whereas I2/HI ratio is maintained at 2.0−3.0 in the HIx phase. Considering the H 2 SO 4 phase, H 2 SO 4 concentration is changed in the range of 4.0−5.0 mol/kg. Along with the increasing of SO2 flow rate, the molar ratio of H2SO4/HI in the HIx phase increases from 0.064 to 0.302 and HI/H2SO4 ratio in the H2SO4 phase increases from 0.065 to 0.122. The H2SO4/HI ratio in the HIx phase increases from 0.018 to 0.064 and HI/H2SO4 ratio in the H2SO4 phase increases from 0.058 to 0.065 as the SO2 mole fraction increases. When the initial amount of iodine increases, the molar ratio of HI/H2SO4 in the H2SO4 phase decreases regularly from 0.103 to 0.035 while that of H2SO4/HI in the HIx phase decreases from 0.082 to 0. The HI/H2SO4 ratio
Figure 5. Effect of water content on resultants content in H2SO4 phase at 345 K (SO2 flow rate of 60 N mL/min, N2 flow rate of 357 N mL/ min and I2 of 1.362 mol, I2/H2O varied from 0.324 to 0.252): (a)H2SO4, (b)HI, (c)I2. 3025
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Table 1. Experimental Data List of LLE Phase Separation in Bunsen Reaction temp (K) 345 345 345 345 345 345 345 345 345 345
feed composition (NmL/min)
mole ratio
HIx phase composition (mol/kg)
impurity mole ratio
H2SO4 phase composition (mol/kg)
impurity mole ratio
SO2
N2
I2:H2O
H2SO4
HI
I2
H2O
H2SO4/HI
H2SO4
HI
I2
H2O
HI/H2SO4
70 60 50 50 50 60 60 60 60 60
417 357 298 366 505 357 357 357 357 357
1.035:4.2 1.035:4.2 1.035:4.2 1.035:4.2 1.035:4.2 1.126:4.2 1.258:4.2 1.362:4.2 1.362:4.8 1.362:5.2
0.348 0.099 0.078 0.072 0.021 0 0 0 0.018 0.013
1.152 1.128 1.211 1.185 1.180 1.155 1.096 1.077 1.142 1.254
2.533 2.653 2.824 2.848 2.696 2.841 2.930 2.980 2.865 2.525
9.751 8.950 6.708 6.573 9.040 7.275 6.445 5.867 6.939 10.954
0.302 0.082 0.064 0.061 0.018 0 0 0 0.016 0.010
4.066 4.023 4.442 4.568 4.544 4.694 4.838 4.898 4.777 4.577
0.497 0.416 0.289 0.261 0.264 0.233 0.187 0.170 0.232 0.284
0.207 0.471 0.118 0.213 0.029 0.091 0.037 0.034 0.050 0.050
26.922 24.017 27.603 25.782 28.480 27.008 27.316 27.152 27.146 27.866
0.122 0.103 0.065 0.057 0.058 0.050 0.039 0.035 0.049 0.062
increases to 0.062 in the H2SO4 phase and the H2SO4/HI ratio increases to about 0.016 in the HIx phase as the water content increases in the feed. Hence, both the higher iodine content and the lower water content are more favorable in order to enhance the phase separation. This result is in good accordance with what elsewhere reported.8,9 The amount of I2 impurity in the H2SO4 is little, varying from 0.05 to 0.417 mol/kg. 3.3. Side Reaction Occurrence. Due to the trace amount of impurity in two phases, side reactions, which result in disequilibrium of materials, cannot be avoided. Therefore, the laws of side reaction occurrence should be investigated, which is supportive for purification. Solid sulfur was never detected by visual observation after the reaction in our experimental runs. It identified that either sulfur formation side reaction did not occur, or the quantity of elemental sulfur was beyond the lower limit of detection. Therefore, the evolution of H2S production was used to characterize the occurrence of the Bunsen side reaction. Figure 6 shows the effects of inlet SO2 flow rate on the evolution of SO2 and H2S mole fractions detected in the SO2/ H2S analyzer. Both SO2 and H2S are detected simultaneously only when the reaction is finished. H2S formation reaction occurs beyond the upper limit of detection (1%). The beginning time of side reaction occurrence is determined by the quantity of SO2. Concerning the effects of inlet SO2
concentration shown in Figure 7, the traces of SO2 and H2S mainly follow the above tendency. The exception is that H2S is
Figure 7. Time evolution of SO2 and H2S mole fractions in terms of inlet SO2 mole fraction at 345 K (I2/H2O molar ratio of 0.246, inlet SO2 mole fraction varied from 0.09 to 0.14).
detected earlier than SO2 in the case of N2 505 mL/min. The higher SO2 concentration results in the delay of side reaction occurrence. H2S production is detected rapidly when SO2 concentration is kept at the lower value of 0.09. The inlet SO2 mole fraction exceeding 0.12 is an optimum candidate. Figures 8 and 9 shows the effects of iodine and water content in the feed on the evolution of SO2 and H2S mole fractions, respectively. An increase in iodine content and a decrease in water content appear to suppress H2S formation side reaction. In particular, no H2S is detected as the I2/H2O molar ration isss increases to 0.324 by increasing I2 content. On the other hand, when the molar ratio of I2/H2O is in the range of 0.324−0.284, no side reaction is detected with changing H2O content. The molar ratio of I2/H2O exceeding 0.284 is an optimal candidate considering the kinetics, thermodynamics and side reaction occurrence synchronously at the specific SO2 flow rate and mole fraction.
4. CONCLUSIONS The work presented here investigated the effects of SO2 flow rate, SO2 mole fraction, iodine content and water content on
Figure 6. Time evolution of SO2 and H2S mole fractions in terms of inlet SO2 flow rate at 345 K (I2/H2O molar ratio of 0.246). 3026
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AUTHOR INFORMATION
Corresponding Author
*Y. Zhang. Tel.: +86-571-87952040. Fax: +86-571-87951616. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been financially supported by National Natural Science Foundation of China (51276170). The authors gratefully acknowledge the support.
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REFERENCES
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Figure 8. Time evolution of SO2 and H2S mole fractions in terms of iodine content at 345 K (SO2 flow rate of 60 N mL/min, N2 flow rate of 357 N mL/min and H2O of 4.2 mol, I2/H2O varied from 0.311 to 0.324).
Figure 9. Time evolution of SO2 and H2S mole fractions in terms of water content at 345 K (SO2 flow rate of 60 N mL/min, N2 flow rate of 357 N mL/min and I2 of 1.362 mol, I2/H2O varied from 0.324 to 0.252).
the kinetics, equilibrium composition in the H2SO4 and HIx phases and side reaction occurrence of the Bunsen reaction. The amount of H2SO4 increases regularly in the H2SO4 phase with the reaction time, whereas the ones of HI and I2 tend to increase to a peak value, and then decrease. The time for the appearance of LLE phase separation is within the range of 20−40 min in our experimental apparatus. The multistep reaction mechanism on the Bunsen reaction has been proposed based on the kinetic results. An increase in iodine content and a decrease in water content improve the separation characteristics of iodine and sulfur species in each of the two liquid phases produced from Bunsen reaction. This validates the previous studies on the LLE phase separation characteristics in the quaternary system H2SO4/HI/I2/H2O. The H2S formation side reaction is prevented by increasing the initial iodine content and lowering the initial water content. An inlet SO2 mole fraction exceeding 0.12 coupled with an I2/H2O molar ratio exceeding 0.284 is chosen as the optimal operating parameters for the Bunsen reaction. 3027
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