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Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical 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 3 8 ( 2 0 1 3 ) 2 9 e3 5
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Experimental study on the purification of HIx phase in the iodineesulfur thermochemical hydrogen production process Shangkui Bai a,b, Laijun Wang a,*, Qi Han b, Ping Zhang a, Songzhe Chen a, Xianghai Meng b, Jingming Xu a a b
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, PR China College of Chemical Engineering, China University of Petroleum, Beijing 102249, PR China
article info
abstract
Article history:
Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable
Received 28 July 2012
side reactions between HI and H2SO4 in the sulfureiodine thermochemical cycle. In this
Received in revised form
paper, a series of experiments on HIx phase purification were conducted by means of
2 October 2012
a stirred reactor using N2 as stripping gas. The effects of the iodine concentration, reaction
Accepted 10 October 2012
temperature, and the striping gas flow rate on HIx phase purification were investigated
Available online 7 November 2012
systematically in terms of the conversion of H2SO4 and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating
Keywords:
the reactions during purification. The quantitative analysis of the compositions of the
Thermochemical hydrogen
initial and purified HIx phases showed that not only the conversion of H2SO4 was enhanced
production
but also the side reactions were effectively impeded by increasing the iodine concentra-
IodineeSulfur cycle
tion, temperature and the stripping gas flow rate. Based on the experimental data, the
HIx phase
suitable operating conditions for HIx phase purification were proposed.
Purification
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The sulfureiodine thermochemical cycle (IS cycle), which was invented by General Atomic [1], has received much attention because it is considered to be a promising method for clean, efficient and massive hydrogen production using solar energy or nuclear heat source [2e5]. The IS cycle is composed by the following three reactions: Bunsen reaction section: SO2 þ I2 þ 2H2O 4 2HI þ H2SO4 (1) H2SO4 decomposition section: H2SO4 4 SO2 þ 1/2O2 þ H2O(2) HI decomposition section: 2HI 4 H2 þ I2
(3)
The Bunsen reaction is conducted in aqueous media in order to produce sulfuric acid and hydriodic acid. The produced acids with the existence of excess I2 can be divided into the light H2SO4 phase and the heavy HIx phase. The separated acids are decomposed individually after a series of purification, concentration and evaporation to produce O2 and H2. Purification of H2SO4 and HIx phase plays an important role in IS cycle, because the existence of the minor components in the two phases would cause some side reactions [6e8], such as the sulfur formation (4) and hydrogen sulfide formation (5):
Sulfur formation reaction: H2SO4 þ 6HI 4 S þ 3I2 þ 4H2O (4)
* Corresponding author. Tel.: þ86 10 80194036; fax: þ86 10 62771740. E-mail addresses: [email protected] (S. Bai), [email protected] (L. Wang), [email protected] (Q. Han), [email protected] (P. Zhang), [email protected] (S. Chen), [email protected] (X. Meng), xujingming@mail. tsinghua.edu.cn (J. Xu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.035
30
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 3 8 ( 2 0 1 3 ) 2 9 e3 5
Hydrogen sulfide formation: H2SO4 þ 8HI 4 H2S þ 4I2 þ 4H2O
(5)
These two side reactions may bring serious problems for the operation of the IS process. The solid sulfur could plug pipes, which would hinder the continuous operation of the closed cycle. Both S and H2S could not be recycled in the IS cycle, which would affect the material balance and reduce the efficiency of the whole process. Thus, before the H2SO4 and HI decomposition sections, it is crucial to purify the two liquid phases for avoiding the side reactions. The reverse Bunsen reaction (6) could be used for purification of these two phases and the concept was also demonstrated in the closed-cycle operation by JAEA (Japan Atomic Energy Agency), INET (Institute of Nuclear and New Energy Technology, Tsinghua University), ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) and Parisi’s group [9e13]. The reverse Bunsen reaction: 2HI þ H2SO4 4 SO2 þ I2 þ 2H2O
phase without the occurrence of secondary reactions [13]. Recently, both the O2-promoted purification and subatmospheric purification were simulated by the cooperation of Wang et al. (from INET) and Kasahara et al. (from JAEA) using the commercial simulation tool ESP [19,20]. Compared with the purification of H2SO4 phase, purification of HIx phase is more complicated and difficult [19,20]. Substantial efforts have been devoted to realize the process. However, there are still lacks of data about comprehensive consideration of influence factors and quantitative analysis of both the liquid and gas products during purification of HIx phase. In the present paper, the effects of iodine concentration, purification temperature, and the striping gas flow rate on the purification of HIx phase were investigated systematically in terms of the conversion of H2SO4 and the reaction types during purification. The quantitative and qualitative analyses were conducted for comparing the compositions of the products after purification with those of the initial HIx phase.
(6)
However, present knowledge on purification is insufficient to optimize the process design. As far as the authors know, only a few studies have been publically reported on the chemistry relevant to the purification of H2SO4 phase and HIx phase. The reactions between H2SO4 and HI were early studied in 1929 by Bush adding potassium iodide into concentrated sulfuric acid [14]. But the reactions between H2SO4 and HI did not draw much attention until the IS cycle and MgeSeI cycle were proposed. Since then, Japanese researchers had done some excellent studies about the reactions between H2SO4 and HI. Kumagai et al. investigated the reactions of HI and H2SO4 in aqueous solution, their results showed that the sulfur formation occurred under a high acid concentration condition [6]. Shimizu et al. [7] and Onuki [15] studied the removal of H2SO4 from the HIx phase by the reverse Bunsen reaction and their studies made it possible to demonstrate closed-cycle continuous hydrogen production. Sakurai et al researched the side reaction from HI, H2SO4, and I2 mixture solution and reported that the iodine concentration and temperature were important factors to affect the occurrence of side reactions [16]. At low I2 concentrations, the sulfur formation side reaction predominated. And at high I2 concentrations, both the S formation and the reverse Bunsen reaction proceeded. But their experiment temperature range was 295e368 K and effect of stripping gas N2 was not considered. They referred that operation of the purification step for the H2SO4 and HIx phases had a possibility of sulfur formation because the operation temperature was higher than 368 K and further investigation should be needed. Bai and Guo et al. studied the purification of H2SO4 and HI phases in both the batch and continuous modes. Their results showed that the higher temperature and introducing the stripping gas N2 were helpful for purification of the above two phases [17,18]. But the influence of the iodine on the purification of HIx phase was not considered in their studies. In 2011, Parisi et al. investigated the Bunsen reaction and hydriodic phase purification by N2 striping at a rate of 40 mL/ min for 30 min. They reported that only when both the temperature and the iodine content are high enough it is possible to quantitatively remove the sulfates from the HIx
2.
Experimental setup and procedures
2.1.
Experimental facility
A schematic diagram of the experimental device is shown in Fig. 1.The experimental setup mainly consists of a reactor, the condenser and the tail gas trap. The reactor temperature was regulated by a thermostatic bath. The gas of N2 was fed at a certain rate into the bottom of the reactor for stripping out the SO2 in the purification process. The flow rate of N2 was controlled by the mass flow controller (MFC). The reactor was connected to a condenser, which was used to cool down the vapor during the purification. The tail gas was analyzed by SO2 analyzer and then introduced into the trap, which was filled with adequate sodium hydroxide solution (0.1 M NaOH) to absorb SO2. The system was continuously stirred at fixed speed by a magnetic stirrer during the purification process.
2.2.
Experimental procedures
The reactor was first filled with pre-determined amount of iodine (Analytical reagent, Rizhao Lideshi Chemical Co., Ltd), hydriodic acid (Analytical reagent, Rizhao Lideshi Chemical Co., Ltd) and water, and then the stirrer and the thermostatic bath were switched on. The mixture solution was thoroughly mixed and heated to the desired operation temperature. When the desired temperature attained, a certain amount of H2SO4 was added into the solution and the time was marked. At the same time, N2 was fed into the reactor at a certain rate. The reaction time was kept at 100e120 min. During the process of purification, 1.0 mL sample was sampled by the micro sampler each 20 min and analyzed. The trap was weighted before and after the experiment. Thus the increasing mass of NaOH solution could be obtained. A series of experiments on HIx phase purification were carried out as follows. (a) Effect of I2 concentration: The HI/ H2O/H2SO4 mole ratio was fixed at 1/6.5/0.078, which came from the experimental results in a lab-scaled closed cycle operation at INET [11]. Effect of I2 concentration was conducted at 80 C by changing the I2/HI mole ratio from 0:1 to
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 3 8 ( 2 0 1 3 ) 2 9 e3 5
31
Fig. 1 e Simplified scheme of the reactor setup for purifying HIx.
3.0:1. The flow rate of N2 was kept at 100 mL/min and the reaction time was 100 min. (b) Effect of temperature: The HI/I2/ H2O/H2SO4 mole ratio was fixed at 1/1.5/6.5/0.078. Two kinds of experiments were carried out to investigate the effect of temperature on purification of the above HIx. First, 100 mL HIx phase was heated slowly from room temperature to the boiling temperature at a rate of 10 C per minute. The online analysis of SO2 concentration in the tail gas was conducted by the SO2 analyzer (GXH-1050E, Beijing Junfang Lihua Technology Research Institute). Second, purifying 100 mL HIx phase was carried out for 100 min at 60, 80, 100 and 120 C, respectively. (c) Effect of stripping gas flow rate: The composition of HIx was the same as that of HIx in above (b). A series of purifications were conducted at 80 C for 100 min by varying the stripping gas flow rate from 50 to 200 mL/min.
2.3.
Analytical method
The concentration of Hþ and I2 was titrated individually in the automatic titrator (Metrohm 905 Titrando) by using 0.1 mol/L NaOH and 0.2 mol/L Na2S2O3 standard solutions, respectively. The sulfate (SO2 4 ) and iodide (I ) contents were measured by ion chromatography (ICS2000, Dionex Corporation). The conversion of H2SO4 (100%) is calculated by the following formula: CSA ¼ nH2 SO4;0 nH2 SO4 ;t nH2 SO4;0 100%
ratio of the change in the amount of HI to that of H2SO4, DnHI =DnH2 SO4 ¼ ðnHI;o nHI;t Þ=ðnH2 SO4 ;0 nH2 SO4 ;t Þ in the solution during purification decreases from around 6 to 2 as the mole ratio value of I2/HI increases from 0 to 1.5. Considering that the stoichiometric values of HI to H2SO4 in the reactions of (4), (5) and (6) are 6, 8 and 2, respectively, it could be deduced that the iodine concentration plays the important role in dictating the type of reaction during the purification. When the I2/HI ratio value is 0 or 0.5, the value of the ratio DnHI =DnH2 SO4 is almost 6 which is equal to the stoichiometric value of HI to H2SO4 in the reaction of sulfur formation as shown in reaction (4). This means that in these cases, the predominant reaction is the sulfur formation. The experimental phenomena also confirmed the conclusion. The solid sulfur was obviously obtained after the purification, although the smell of rotten eggs was smelt and trace amount of SO2 was detected (The highest concentration of SO2 in the tail gas was 0.38 v/v%.). When the mole ratio value of I2/HI is 0.88, the value of the ratio DnHI =DnH2 SO4 is about 4 which is the intermediate value
where CSA is signified as the conversion of H2SO4; nH2 SO4 ;t is donated as the amount of H2SO4 after purification; nH2 SO4 ;0 means the amount of H2SO4 in the initial solution.
3.
Experimental results and discussion
3.1. The effect of iodine concentration on the purification process Fig. 2 shows the effect of iodine concentration on the reactions occurred during the purification at 80 C. In this figure, the
Fig. 2 e The effect of iodine concentration on the reactions during purification.
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between 6 (the stoichiometric value of HI to H2SO4 in the sulfur formation reaction) and 2 (the stoichiometric value of HI to H2SO4 in the reverse Bunsen reaction), which indicates that both the sulfur formation reaction and the reverse Bunsen reaction occur at same time. When the mole ratio values of I2/HI are in the range of 1e1.5, the values of the ratio DnHI =DnH2 SO4 are close to 2, which show that the predominant reaction changes into the reverse Bunsen reaction. However, when the mole ratio value of I2/HI was above 1.5, the mixed solution would be stratified to two phases so that the components were not evenly distributed, which affected the accuracy of the analysis results. That is the reason why the ratio value of HI to H2SO4 becomes disorder when the ratio of I2/HI is above 1.5. The different effects of the iodine on the three reactions (4e6) can be explained from the reaction thermodynamic equilibrium. The stoichiometric coefficients of I2 in the H2S formation, S formation and the reverse Bunsen reaction are 4, 3 and 1, respectively. So increase of the concentration of I2 will limit the occurrence of the former two reactions more seriously than that of the reverse Bunsen reaction. The above results about the role of iodine in dictating the type of reaction are consistent with those obtained by Sakurai et al. [16]. Sakurai et al. reported that the sulfur formation side reaction predominated at low I2 concentrations (x of HIx was 1.32, equivalent to 0.16 of the I2/HI ratio value). And at high I2 concentrations (x of HIx was 3.56, equivalent to 1.28 of the I2/ HI ratio value), both the S formation and the reverse Bunsen reaction proceeded. In order to avoid the side reaction occurrence at 368 K, Sakurai et al. proposed that the x of HIx should be increased to at least 4.41 which corresponded approximately to 1.7 of the I2/HI ratio value. The differences between our studies and the results obtained by Sakurai et al. could be attributed to the different experimental condition, such as different reaction temperature and the compositions of examined solution. Furthermore, striping gas was used in the present study. Fig. 3 shows that the effect of iodine concentration on the conversion of H2SO4. It is found that conversion of H2SO4 rises evidently with the reaction time regardless of the iodine content. CSA decreases from 72.1% to 34.3% when the mole
Fig. 3 e The effect of iodine concentration on the conversion of H2SO4.
ratio of I2/HI increases from 0 to 3.0. The iodine is the product of the three reactions (Equations (4)e(6)) between HI and H2SO4. According to the equilibrium theory of the chemical reaction, the increase of the products will shift the reaction equilibrium toward the reactants’ direction. So the conversion of H2SO4 would be decreased by increasing the iodine concentration. Fig. 4 shows the effect of the iodine concentration on the increasing mass of alkaline solution after the purification. Compared with the sulfur formation reaction, the hydrogen sulfide formation reaction is subsidiary [9]. In addition, SO2 is very easy to react with hydrogen sulfide to produce sulfur in the solution. Thus, the increasing mass of NaOH solution could indicate the amount of the produced SO2. The mass of NaOH solution increases distinctly with enhancing the iodine content, which also confirms the iodine’s function to promote the reverse Bunsen reaction. Based on the above analysis of Figs. 2e4, it is deduced that the side reactions could be effective depressed through controlling the concentration of iodine in the initial HIx phase. The advantage of increasing the iodine concentration is to promote the occurrence of the reverse Bunsen reaction, but the disadvantage is to decrease the conversion of H2SO4. Purification of HIx phase should be carried out in such a manner that almost all the minor component of H2SO4 is removed in the form of SO2 and recycled in the IS process. So, temperature, another important factor, should be given more expectancy in order to promote the conversion of H2SO4.
3.2.
The effect of temperature on the purification process
Fig. 5 shows a temperature profile of the SO2 evolution during heating the HIx phase (HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078) slowly from room temperature to the boiling temperature. As shown in the figure, when temperature is in the range of 25e57 C, the SO2 was not detected in the tail gas. Trace of SO2 in tail gas (0.01 v/v%) is first detected at 58 C. The concentration of SO2 increases slightly from 0.01% to about 1% when the temperature changes from 58 to 95 C. When the temperature is above 95 C, the concentration of SO2 increases sharply and reaches its peak at about 109 C. When the
Fig. 4 e The effect of iodine concentration on the increasing mass of 0.1 M NaOH (temperature: 80 C, N2 flow rate: 100 mL/min, and reaction time: 100 min).
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 3 8 ( 2 0 1 3 ) 2 9 e3 5
Fig. 5 e The effect of temperature on the concentration of SO2 in the tail gas.
temperature increases from 109 to 120 C, the concentration of SO2 decreases quickly from the maximum of 31.91 v/v% to 0.18 v/v%, respectively. The changing curve of SO2 evolution with the temperature shows that purification at above 95 C will be very helpful for the evolution of SO2. According to the changing curve of SO2 evolution with the temperature, four kinds of purifying HIx phase were carried out at 60, 80, 100 and 120 C, respectively. Fig. 6 shows the effect of temperature on the reactions occurred during the purification. When the temperature increases from 60 to 120 C, the ratio value of DnHI =DnH2 SO4 decreases from 3.6 to 2.3. It indicates that both sulfur formation reaction and reverse Bunsen reaction proceed at low temperature. And the reverse Bunsen reaction becomes predominant at high temperature. This result is in accord with those reported by Parisi et al. [13]. Parisi et al. reported that it was possible to satisfactorily reduce the impurities content without the occurrence of secondary reactions only when high temperature and high iodine content were applied. Fig. 7 indicates that the effect temperature on the conversion of H2SO4. In the temperature range examined, the conversion of H2SO4 increases obviously with the increase of temperature. That is
Fig. 6 e The effect of temperature on the reactions during purification.
33
Fig. 7 e The effect of temperature on the conversion of H2SO4.
because all the three kinds of reactions between H2SO4 and HI are endothermic. In addition, the conversion of H2SO4 increases with the reaction time more sharply at 100 and 120 C than at 60 and 80 C. The reasons are that increasing temperature promotes the occurring of the reverse Bunsen reaction more significantly than that of the sulfur formation, and operation at elevated temperatures facilitates the vaporization of SO2 from the aqueous phase to the gas phase, which is verified by both the changing curve of SO2 evolution with the temperature (shown in Fig. 5) and the increasing mass of alkaline solution with the temperature (as shown in Fig. 8).
3.3. The effect of stripping gas flow rate on the purification process The composition of feed solution is HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078 (molar ratio). Stripping gas flow rate varies from 50 to 200 mL/min under a fixed temperature of 80 C. The results are shown in Figs. 9,10. Fig. 9 shows the effect of stripping gas flow rate on the ratio of the change in the concentration of HI to that of H2SO4 in the solution. When the
Fig. 8 e The effect of temperature on the increasing mass of 0.1 M NaOH (N2 flow rate: 100 mL/min, and reaction time: 100 min).
34
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Table 1 e The compositions of the initial and purified HIx phases. Components C0 (mol/kg) n0 (mol) Ct (mol/kg) HI H2SO4 I2
1.578 0.123 2.367
0.357 2.781*102 0.535
1.552 1.380*102 2.345
nt (mol) 0.304 2.707*103 0.460
3.4. HIx phase purification under the optimized condition
Fig. 9 e The effect of sweeping gas flow rate on the reactions during purification.
flow rate of N2 is 50 mL/min, the ratio value of DnHI =DnH2 SO4 is about 4, which indicates that not only the sulfur formation reaction but also the reverse Bunsen reaction occurs at the low flow rate of stripping gas. As the flow rate increases above 100 mL/min, the ratio value is very close to 2, which shows that the reverse Bunsen reaction predominates during the purification at the high flow rate of stripping gas. Fig. 10 shows the effect of stripping gas flow rate on the time profile of the SO2 evolution. It is clearly found that both the peak time and peak areas are different for different flow rate of N2. The maximum of SO2 evolution appeared at 13, 11, 9 and 6 min when the stripping gas flow rate was 50, 100, 150 and 200 mL/ min, respectively, which indicated that increasing the flow rate of N2 accelerated the rate of the reverse Bunsen reaction. The peak area of the SO2 evolution increased evidently with the increase of the stripping gas flow rate, which showed that increasing the flow rate of N2 favored the conversion of H2SO4 into SO2. Increasing the flow rate of N2 will help SO2 shift from the liquid phase to the vapor phase, thereby enhancing the reverse Bunsen reaction. These results were in good agreement with the results obtained by Guo and Wang et al. [17e19].
Based on the above analysis of purification parameters, an experiment of HIx purification was carried out at the optimized condition. Purification of HIx phase (molar ratio of HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078) was operated at 120 C and the flow rate of N2 was 200 mL/min. The compositions of the initial and purified HIx phases are listed in Table 1. In terms of the material balance in mass, the mass of the initial HIx solution (226.059 g) was compared with that of obtained products (224.988 g) which consisted of 196.182 g purified HIx solution, 27.910 g solid product (which was mainly iodine) attached in the inner surface of the condenser and 0.896 g SO2 adsorbed in 0.1 M NaOH solution. The mass loss and the mass loss ratio were 1.071 g and 0.47%, respectively. It was easy to obtain that the ratio value of DnHI =DnH2 SO4 was 2.1:1, which indicated that the predominant reaction during purification was the reverse Bunsen reaction. However, the conversion of H2SO4 was 90.3%. Although the reaction time reached 120 min and almost no evolution of SO2 was detected by the SO2 analyzer, a small amount of H2SO4 remained in the purified HIx phase. Simulation results for HIx purification studied by Wang et al. also showed that it was very difficult to realize the desired purification of HIx phase [19,20]. The thermodynamic equilibrium restriction may be the main reason that H2SO4 is very difficult to be removed completely. In addition, some stable intermediates may be formed between HI, I2 and the small amount of H2SO4, which needs to be verified. Additional experiments should be further carried out for effective removal of H2SO4 from HIx phase by some new ways, such as using special reactors and purifying under subatmospheric pressure.
4.
Conclusion
A series of experimental studies were conducted to investigate effects of the iodine concentration, reaction temperature and striping gas flow rate on the purification of HIx phase. The main results are summarized as follows.
Fig. 10 e The effect of sweeping gas flow rate on the time profiles of the SO2 evolution.
(1) The iodine concentration played the significant role in dictating the reactions during the purification of HIx phase. When the mole ratios of I2/HI were less than abut 0.88, the predominant reaction is the sulfur formation. When the mole ratios of I2/HI were in the range of 1e1.5, the predominant reaction changed into the reverse Bunsen reaction. However, increasing the iodine concentration decreased the conversion of H2SO4.
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 3 8 ( 2 0 1 3 ) 2 9 e3 5
(2) Temperature was also an important factor to influence the purification. At low temperature, both sulfur formation and reverse Bunsen reactions occurred. The reverse Bunsen reaction became predominant at high temperature. In addition, increasing reaction temperature favored the conversion of H2SO4. (3) In the examined N2 flow rate of 50e200 mL/min, increasing the stripping gas flow rate could not only increase the conversion of H2SO4 but also enhance the occurrence of the reverse Bunsen reaction. (4) In terms of the reaction type and the conversion of H2SO4, suitable operation conditions were recommended as follows. The mole ratio of I2/HI in the initial HIx phase should be controlled at about 1:1e1.5:1. Purification had better be carried out at above 95 C and the striping gas flow rate is above 100 mL/min. (5) The experiment of HIx purification at the optimized condition (at molar ratio of I2:HI ¼ 1.5:1, 120 C and 200 mL/ min of the N2 flow rate) showed that above 90% H2SO4 in the HIx phase could be removed in the style of SO2 by the reverse Bunsen reaction.
Acknowledgments This work was supported by Chinese National S&T Major Projects (2010zx06901).
references
[1] Norman JH, Besenbruch GE, Brown LC, O’Keefe DR, Allen CL. Thermochemical water-splitting cycle, bench-scale investigations, and process engineering. GA-A16713; 1982. [2] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrogen Energy 2001;26:185e90. [3] Kodama T, Gokon N. Thermochemical cycles for hightemperature solar hydrogen production. Chem Rev 2007;107: 4048e77. [4] Vitart X, Carles P, Anzieu P. A general survey of the potential and the main issues associated with the sulfur-iodine thermochemical cycle for hydrogen production using nuclear heat. Prog Nucl Energy 2008;50:402e10. [5] Duigoua AL, Borgarda JMc, Laroussea B, Doizi D, Allen R, Ewan BC, et al. HYTHEC: an EC funded search for a long term massive hydrogen production route using solar and nuclear technologies. Int J Hydrogen Energy 2007;32:1516e29. [6] Kumagai T, Okamoto C, Mizuta S. Thermal decomposition of magnesium sulfate and separation of the product gas mixture. Denki Kagaku 1984;52:812e9.
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[7] Shimizu S, Nakajima H, Onuki K. A progress report on bench scale studies of the iodine-sulfur process for thermochemical hydrogen production. IAEA-TECDOC-761; 1994. 114e9. [8] Yoon HJ, No HC, Kim YS, Jin HG, Lee JI, Lee BJ. Demonstration of the I-S thermochemical cycle feasibility by experimentally validating the over-azeotropic condition in the hydriodic acid phase of the Bunsen process. Int J Hydrogen Energy 2009;34:7939e48. [9] Nakajima H, Ikenoya K, Onuki K, Shimizu S. Closed-cycle continuous hydrogen production test by thermochemical IS process. Kagaku Kogaku Ronbunshu 1998;24:352e5. [10] Kubo S, Nakajima H, Kasahara S, Higashi S, Masaki T, Abe H, et al. A demonstration study on a closed-cycle hydrogen production by the thermochemical watersplitting iodineesulfur process. Nucl Eng Des 2004;233: 347e54. [11] Zhang P, Chen SZ, Wang LJ, Xu JM. Study on a lab-scale hydrogen production by closed-cycle thermochemical iodineesulfur process. Int J Hydrogen Energy 2010;35: 10166e72. [12] Liberatore R, Caputo G, Ceroli A, Felici C, Giaconia A, Favuzza P, et al. Demonstration of hydrogen production by sulphureiodine cycle: realization of a 10 NL/h plant. 18th world hydrogen energy conference 2010, Essen, Germany: May 16e21, 2010. [13] Parisi M, Giaconia A, Sau S, Spadoni A, Caputo G, Tarquini P. Bunsen reaction and hydriodic phase purification in the sulfureiodine process: an experimental investigation. Int J Hydrogen Energy 2011;36(3):2007e13. [14] Bush F. Sulphuric acid and hydroiodic acid. J Phys Chem 1929;33:613e20. [15] Onuki K, Shimizu S, Nakajima H, Fujita S, Ikezoe Y, Sato S, et al. Studies on an iodineesulfur process for thermochemical hydrogen production. Proc. 8th world hydrogen energy conf, Honolulu and Waikoloa, U.S.A.: 1990, p. 547e56. [16] Sakurai M, Nakajima H, Amir R, Onuki K, Shimizu S. Experimental study on side-reaction occurrence condition in the iodine-sulfur thermochemical hydrogen production process. Int J Hydrogen Energy 2000;25:613e9. [17] Bai Y, Zhang P, Guo HF, Chen SZ, Wang LJ, Xu JM. Purification of sulfuric and hydriodic acids phases in the iodineesulfur process. Chin J Chem Eng 2009;17(1):160e6. [18] Guo HF, Zhang P, Bai Y, Wang LJ, Chen SZ, Xu JM. Continuous purification of H2SO4 and HI phases by packed column in IS process. Int J Hydrogen Energy 2010;35:2836e9. [19] Wang LJ, Imai Y, Tanaka N, Kasahara S, Kubo S, Onuki K. Thermodynamic consideration on the purification of H2SO4 and HIx phases in the iodineesulfur hydrogen production cycle. Chem Eng Commun 2012;199:165e77. [20] Wang LJ, Imai Y, Tanaka N, Kasahara S, Kubo S, Onuki K, et al. Simulation study about the effect of pressure on purification of H2SO4 and HIx phases in the iodineesulfur hydrogen production cycle. Int J Hydrogen Energy 2012;37: 12967e72.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Experimental study on the purification of HIx phase in the iodineesulfur thermochemical hydrogen production process Shangkui Bai a,b, Laijun Wang a,*, Qi Han b, Ping Zhang a, Songzhe Chen a, Xianghai Meng b, Jingming Xu a a b
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, PR China College of Chemical Engineering, China University of Petroleum, Beijing 102249, PR China
article info
abstract
Article history:
Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable
Received 28 July 2012
side reactions between HI and H2SO4 in the sulfureiodine thermochemical cycle. In this
Received in revised form
paper, a series of experiments on HIx phase purification were conducted by means of
2 October 2012
a stirred reactor using N2 as stripping gas. The effects of the iodine concentration, reaction
Accepted 10 October 2012
temperature, and the striping gas flow rate on HIx phase purification were investigated
Available online 7 November 2012
systematically in terms of the conversion of H2SO4 and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating
Keywords:
the reactions during purification. The quantitative analysis of the compositions of the
Thermochemical hydrogen
initial and purified HIx phases showed that not only the conversion of H2SO4 was enhanced
production
but also the side reactions were effectively impeded by increasing the iodine concentra-
IodineeSulfur cycle
tion, temperature and the stripping gas flow rate. Based on the experimental data, the
HIx phase
suitable operating conditions for HIx phase purification were proposed.
Purification
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The sulfureiodine thermochemical cycle (IS cycle), which was invented by General Atomic [1], has received much attention because it is considered to be a promising method for clean, efficient and massive hydrogen production using solar energy or nuclear heat source [2e5]. The IS cycle is composed by the following three reactions: Bunsen reaction section: SO2 þ I2 þ 2H2O 4 2HI þ H2SO4 (1) H2SO4 decomposition section: H2SO4 4 SO2 þ 1/2O2 þ H2O(2) HI decomposition section: 2HI 4 H2 þ I2
(3)
The Bunsen reaction is conducted in aqueous media in order to produce sulfuric acid and hydriodic acid. The produced acids with the existence of excess I2 can be divided into the light H2SO4 phase and the heavy HIx phase. The separated acids are decomposed individually after a series of purification, concentration and evaporation to produce O2 and H2. Purification of H2SO4 and HIx phase plays an important role in IS cycle, because the existence of the minor components in the two phases would cause some side reactions [6e8], such as the sulfur formation (4) and hydrogen sulfide formation (5):
Sulfur formation reaction: H2SO4 þ 6HI 4 S þ 3I2 þ 4H2O (4)
* Corresponding author. Tel.: þ86 10 80194036; fax: þ86 10 62771740. E-mail addresses: [email protected] (S. Bai), [email protected] (L. Wang), [email protected] (Q. Han), [email protected] (P. Zhang), [email protected] (S. Chen), [email protected] (X. Meng), xujingming@mail. tsinghua.edu.cn (J. Xu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.035
30
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Hydrogen sulfide formation: H2SO4 þ 8HI 4 H2S þ 4I2 þ 4H2O
(5)
These two side reactions may bring serious problems for the operation of the IS process. The solid sulfur could plug pipes, which would hinder the continuous operation of the closed cycle. Both S and H2S could not be recycled in the IS cycle, which would affect the material balance and reduce the efficiency of the whole process. Thus, before the H2SO4 and HI decomposition sections, it is crucial to purify the two liquid phases for avoiding the side reactions. The reverse Bunsen reaction (6) could be used for purification of these two phases and the concept was also demonstrated in the closed-cycle operation by JAEA (Japan Atomic Energy Agency), INET (Institute of Nuclear and New Energy Technology, Tsinghua University), ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) and Parisi’s group [9e13]. The reverse Bunsen reaction: 2HI þ H2SO4 4 SO2 þ I2 þ 2H2O
phase without the occurrence of secondary reactions [13]. Recently, both the O2-promoted purification and subatmospheric purification were simulated by the cooperation of Wang et al. (from INET) and Kasahara et al. (from JAEA) using the commercial simulation tool ESP [19,20]. Compared with the purification of H2SO4 phase, purification of HIx phase is more complicated and difficult [19,20]. Substantial efforts have been devoted to realize the process. However, there are still lacks of data about comprehensive consideration of influence factors and quantitative analysis of both the liquid and gas products during purification of HIx phase. In the present paper, the effects of iodine concentration, purification temperature, and the striping gas flow rate on the purification of HIx phase were investigated systematically in terms of the conversion of H2SO4 and the reaction types during purification. The quantitative and qualitative analyses were conducted for comparing the compositions of the products after purification with those of the initial HIx phase.
(6)
However, present knowledge on purification is insufficient to optimize the process design. As far as the authors know, only a few studies have been publically reported on the chemistry relevant to the purification of H2SO4 phase and HIx phase. The reactions between H2SO4 and HI were early studied in 1929 by Bush adding potassium iodide into concentrated sulfuric acid [14]. But the reactions between H2SO4 and HI did not draw much attention until the IS cycle and MgeSeI cycle were proposed. Since then, Japanese researchers had done some excellent studies about the reactions between H2SO4 and HI. Kumagai et al. investigated the reactions of HI and H2SO4 in aqueous solution, their results showed that the sulfur formation occurred under a high acid concentration condition [6]. Shimizu et al. [7] and Onuki [15] studied the removal of H2SO4 from the HIx phase by the reverse Bunsen reaction and their studies made it possible to demonstrate closed-cycle continuous hydrogen production. Sakurai et al researched the side reaction from HI, H2SO4, and I2 mixture solution and reported that the iodine concentration and temperature were important factors to affect the occurrence of side reactions [16]. At low I2 concentrations, the sulfur formation side reaction predominated. And at high I2 concentrations, both the S formation and the reverse Bunsen reaction proceeded. But their experiment temperature range was 295e368 K and effect of stripping gas N2 was not considered. They referred that operation of the purification step for the H2SO4 and HIx phases had a possibility of sulfur formation because the operation temperature was higher than 368 K and further investigation should be needed. Bai and Guo et al. studied the purification of H2SO4 and HI phases in both the batch and continuous modes. Their results showed that the higher temperature and introducing the stripping gas N2 were helpful for purification of the above two phases [17,18]. But the influence of the iodine on the purification of HIx phase was not considered in their studies. In 2011, Parisi et al. investigated the Bunsen reaction and hydriodic phase purification by N2 striping at a rate of 40 mL/ min for 30 min. They reported that only when both the temperature and the iodine content are high enough it is possible to quantitatively remove the sulfates from the HIx
2.
Experimental setup and procedures
2.1.
Experimental facility
A schematic diagram of the experimental device is shown in Fig. 1.The experimental setup mainly consists of a reactor, the condenser and the tail gas trap. The reactor temperature was regulated by a thermostatic bath. The gas of N2 was fed at a certain rate into the bottom of the reactor for stripping out the SO2 in the purification process. The flow rate of N2 was controlled by the mass flow controller (MFC). The reactor was connected to a condenser, which was used to cool down the vapor during the purification. The tail gas was analyzed by SO2 analyzer and then introduced into the trap, which was filled with adequate sodium hydroxide solution (0.1 M NaOH) to absorb SO2. The system was continuously stirred at fixed speed by a magnetic stirrer during the purification process.
2.2.
Experimental procedures
The reactor was first filled with pre-determined amount of iodine (Analytical reagent, Rizhao Lideshi Chemical Co., Ltd), hydriodic acid (Analytical reagent, Rizhao Lideshi Chemical Co., Ltd) and water, and then the stirrer and the thermostatic bath were switched on. The mixture solution was thoroughly mixed and heated to the desired operation temperature. When the desired temperature attained, a certain amount of H2SO4 was added into the solution and the time was marked. At the same time, N2 was fed into the reactor at a certain rate. The reaction time was kept at 100e120 min. During the process of purification, 1.0 mL sample was sampled by the micro sampler each 20 min and analyzed. The trap was weighted before and after the experiment. Thus the increasing mass of NaOH solution could be obtained. A series of experiments on HIx phase purification were carried out as follows. (a) Effect of I2 concentration: The HI/ H2O/H2SO4 mole ratio was fixed at 1/6.5/0.078, which came from the experimental results in a lab-scaled closed cycle operation at INET [11]. Effect of I2 concentration was conducted at 80 C by changing the I2/HI mole ratio from 0:1 to
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31
Fig. 1 e Simplified scheme of the reactor setup for purifying HIx.
3.0:1. The flow rate of N2 was kept at 100 mL/min and the reaction time was 100 min. (b) Effect of temperature: The HI/I2/ H2O/H2SO4 mole ratio was fixed at 1/1.5/6.5/0.078. Two kinds of experiments were carried out to investigate the effect of temperature on purification of the above HIx. First, 100 mL HIx phase was heated slowly from room temperature to the boiling temperature at a rate of 10 C per minute. The online analysis of SO2 concentration in the tail gas was conducted by the SO2 analyzer (GXH-1050E, Beijing Junfang Lihua Technology Research Institute). Second, purifying 100 mL HIx phase was carried out for 100 min at 60, 80, 100 and 120 C, respectively. (c) Effect of stripping gas flow rate: The composition of HIx was the same as that of HIx in above (b). A series of purifications were conducted at 80 C for 100 min by varying the stripping gas flow rate from 50 to 200 mL/min.
2.3.
Analytical method
The concentration of Hþ and I2 was titrated individually in the automatic titrator (Metrohm 905 Titrando) by using 0.1 mol/L NaOH and 0.2 mol/L Na2S2O3 standard solutions, respectively. The sulfate (SO2 4 ) and iodide (I ) contents were measured by ion chromatography (ICS2000, Dionex Corporation). The conversion of H2SO4 (100%) is calculated by the following formula: CSA ¼ nH2 SO4;0 nH2 SO4 ;t nH2 SO4;0 100%
ratio of the change in the amount of HI to that of H2SO4, DnHI =DnH2 SO4 ¼ ðnHI;o nHI;t Þ=ðnH2 SO4 ;0 nH2 SO4 ;t Þ in the solution during purification decreases from around 6 to 2 as the mole ratio value of I2/HI increases from 0 to 1.5. Considering that the stoichiometric values of HI to H2SO4 in the reactions of (4), (5) and (6) are 6, 8 and 2, respectively, it could be deduced that the iodine concentration plays the important role in dictating the type of reaction during the purification. When the I2/HI ratio value is 0 or 0.5, the value of the ratio DnHI =DnH2 SO4 is almost 6 which is equal to the stoichiometric value of HI to H2SO4 in the reaction of sulfur formation as shown in reaction (4). This means that in these cases, the predominant reaction is the sulfur formation. The experimental phenomena also confirmed the conclusion. The solid sulfur was obviously obtained after the purification, although the smell of rotten eggs was smelt and trace amount of SO2 was detected (The highest concentration of SO2 in the tail gas was 0.38 v/v%.). When the mole ratio value of I2/HI is 0.88, the value of the ratio DnHI =DnH2 SO4 is about 4 which is the intermediate value
where CSA is signified as the conversion of H2SO4; nH2 SO4 ;t is donated as the amount of H2SO4 after purification; nH2 SO4 ;0 means the amount of H2SO4 in the initial solution.
3.
Experimental results and discussion
3.1. The effect of iodine concentration on the purification process Fig. 2 shows the effect of iodine concentration on the reactions occurred during the purification at 80 C. In this figure, the
Fig. 2 e The effect of iodine concentration on the reactions during purification.
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between 6 (the stoichiometric value of HI to H2SO4 in the sulfur formation reaction) and 2 (the stoichiometric value of HI to H2SO4 in the reverse Bunsen reaction), which indicates that both the sulfur formation reaction and the reverse Bunsen reaction occur at same time. When the mole ratio values of I2/HI are in the range of 1e1.5, the values of the ratio DnHI =DnH2 SO4 are close to 2, which show that the predominant reaction changes into the reverse Bunsen reaction. However, when the mole ratio value of I2/HI was above 1.5, the mixed solution would be stratified to two phases so that the components were not evenly distributed, which affected the accuracy of the analysis results. That is the reason why the ratio value of HI to H2SO4 becomes disorder when the ratio of I2/HI is above 1.5. The different effects of the iodine on the three reactions (4e6) can be explained from the reaction thermodynamic equilibrium. The stoichiometric coefficients of I2 in the H2S formation, S formation and the reverse Bunsen reaction are 4, 3 and 1, respectively. So increase of the concentration of I2 will limit the occurrence of the former two reactions more seriously than that of the reverse Bunsen reaction. The above results about the role of iodine in dictating the type of reaction are consistent with those obtained by Sakurai et al. [16]. Sakurai et al. reported that the sulfur formation side reaction predominated at low I2 concentrations (x of HIx was 1.32, equivalent to 0.16 of the I2/HI ratio value). And at high I2 concentrations (x of HIx was 3.56, equivalent to 1.28 of the I2/ HI ratio value), both the S formation and the reverse Bunsen reaction proceeded. In order to avoid the side reaction occurrence at 368 K, Sakurai et al. proposed that the x of HIx should be increased to at least 4.41 which corresponded approximately to 1.7 of the I2/HI ratio value. The differences between our studies and the results obtained by Sakurai et al. could be attributed to the different experimental condition, such as different reaction temperature and the compositions of examined solution. Furthermore, striping gas was used in the present study. Fig. 3 shows that the effect of iodine concentration on the conversion of H2SO4. It is found that conversion of H2SO4 rises evidently with the reaction time regardless of the iodine content. CSA decreases from 72.1% to 34.3% when the mole
Fig. 3 e The effect of iodine concentration on the conversion of H2SO4.
ratio of I2/HI increases from 0 to 3.0. The iodine is the product of the three reactions (Equations (4)e(6)) between HI and H2SO4. According to the equilibrium theory of the chemical reaction, the increase of the products will shift the reaction equilibrium toward the reactants’ direction. So the conversion of H2SO4 would be decreased by increasing the iodine concentration. Fig. 4 shows the effect of the iodine concentration on the increasing mass of alkaline solution after the purification. Compared with the sulfur formation reaction, the hydrogen sulfide formation reaction is subsidiary [9]. In addition, SO2 is very easy to react with hydrogen sulfide to produce sulfur in the solution. Thus, the increasing mass of NaOH solution could indicate the amount of the produced SO2. The mass of NaOH solution increases distinctly with enhancing the iodine content, which also confirms the iodine’s function to promote the reverse Bunsen reaction. Based on the above analysis of Figs. 2e4, it is deduced that the side reactions could be effective depressed through controlling the concentration of iodine in the initial HIx phase. The advantage of increasing the iodine concentration is to promote the occurrence of the reverse Bunsen reaction, but the disadvantage is to decrease the conversion of H2SO4. Purification of HIx phase should be carried out in such a manner that almost all the minor component of H2SO4 is removed in the form of SO2 and recycled in the IS process. So, temperature, another important factor, should be given more expectancy in order to promote the conversion of H2SO4.
3.2.
The effect of temperature on the purification process
Fig. 5 shows a temperature profile of the SO2 evolution during heating the HIx phase (HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078) slowly from room temperature to the boiling temperature. As shown in the figure, when temperature is in the range of 25e57 C, the SO2 was not detected in the tail gas. Trace of SO2 in tail gas (0.01 v/v%) is first detected at 58 C. The concentration of SO2 increases slightly from 0.01% to about 1% when the temperature changes from 58 to 95 C. When the temperature is above 95 C, the concentration of SO2 increases sharply and reaches its peak at about 109 C. When the
Fig. 4 e The effect of iodine concentration on the increasing mass of 0.1 M NaOH (temperature: 80 C, N2 flow rate: 100 mL/min, and reaction time: 100 min).
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Fig. 5 e The effect of temperature on the concentration of SO2 in the tail gas.
temperature increases from 109 to 120 C, the concentration of SO2 decreases quickly from the maximum of 31.91 v/v% to 0.18 v/v%, respectively. The changing curve of SO2 evolution with the temperature shows that purification at above 95 C will be very helpful for the evolution of SO2. According to the changing curve of SO2 evolution with the temperature, four kinds of purifying HIx phase were carried out at 60, 80, 100 and 120 C, respectively. Fig. 6 shows the effect of temperature on the reactions occurred during the purification. When the temperature increases from 60 to 120 C, the ratio value of DnHI =DnH2 SO4 decreases from 3.6 to 2.3. It indicates that both sulfur formation reaction and reverse Bunsen reaction proceed at low temperature. And the reverse Bunsen reaction becomes predominant at high temperature. This result is in accord with those reported by Parisi et al. [13]. Parisi et al. reported that it was possible to satisfactorily reduce the impurities content without the occurrence of secondary reactions only when high temperature and high iodine content were applied. Fig. 7 indicates that the effect temperature on the conversion of H2SO4. In the temperature range examined, the conversion of H2SO4 increases obviously with the increase of temperature. That is
Fig. 6 e The effect of temperature on the reactions during purification.
33
Fig. 7 e The effect of temperature on the conversion of H2SO4.
because all the three kinds of reactions between H2SO4 and HI are endothermic. In addition, the conversion of H2SO4 increases with the reaction time more sharply at 100 and 120 C than at 60 and 80 C. The reasons are that increasing temperature promotes the occurring of the reverse Bunsen reaction more significantly than that of the sulfur formation, and operation at elevated temperatures facilitates the vaporization of SO2 from the aqueous phase to the gas phase, which is verified by both the changing curve of SO2 evolution with the temperature (shown in Fig. 5) and the increasing mass of alkaline solution with the temperature (as shown in Fig. 8).
3.3. The effect of stripping gas flow rate on the purification process The composition of feed solution is HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078 (molar ratio). Stripping gas flow rate varies from 50 to 200 mL/min under a fixed temperature of 80 C. The results are shown in Figs. 9,10. Fig. 9 shows the effect of stripping gas flow rate on the ratio of the change in the concentration of HI to that of H2SO4 in the solution. When the
Fig. 8 e The effect of temperature on the increasing mass of 0.1 M NaOH (N2 flow rate: 100 mL/min, and reaction time: 100 min).
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Table 1 e The compositions of the initial and purified HIx phases. Components C0 (mol/kg) n0 (mol) Ct (mol/kg) HI H2SO4 I2
1.578 0.123 2.367
0.357 2.781*102 0.535
1.552 1.380*102 2.345
nt (mol) 0.304 2.707*103 0.460
3.4. HIx phase purification under the optimized condition
Fig. 9 e The effect of sweeping gas flow rate on the reactions during purification.
flow rate of N2 is 50 mL/min, the ratio value of DnHI =DnH2 SO4 is about 4, which indicates that not only the sulfur formation reaction but also the reverse Bunsen reaction occurs at the low flow rate of stripping gas. As the flow rate increases above 100 mL/min, the ratio value is very close to 2, which shows that the reverse Bunsen reaction predominates during the purification at the high flow rate of stripping gas. Fig. 10 shows the effect of stripping gas flow rate on the time profile of the SO2 evolution. It is clearly found that both the peak time and peak areas are different for different flow rate of N2. The maximum of SO2 evolution appeared at 13, 11, 9 and 6 min when the stripping gas flow rate was 50, 100, 150 and 200 mL/ min, respectively, which indicated that increasing the flow rate of N2 accelerated the rate of the reverse Bunsen reaction. The peak area of the SO2 evolution increased evidently with the increase of the stripping gas flow rate, which showed that increasing the flow rate of N2 favored the conversion of H2SO4 into SO2. Increasing the flow rate of N2 will help SO2 shift from the liquid phase to the vapor phase, thereby enhancing the reverse Bunsen reaction. These results were in good agreement with the results obtained by Guo and Wang et al. [17e19].
Based on the above analysis of purification parameters, an experiment of HIx purification was carried out at the optimized condition. Purification of HIx phase (molar ratio of HI:I2:H2O:H2SO4 ¼ 1:1.5:6.5:0.078) was operated at 120 C and the flow rate of N2 was 200 mL/min. The compositions of the initial and purified HIx phases are listed in Table 1. In terms of the material balance in mass, the mass of the initial HIx solution (226.059 g) was compared with that of obtained products (224.988 g) which consisted of 196.182 g purified HIx solution, 27.910 g solid product (which was mainly iodine) attached in the inner surface of the condenser and 0.896 g SO2 adsorbed in 0.1 M NaOH solution. The mass loss and the mass loss ratio were 1.071 g and 0.47%, respectively. It was easy to obtain that the ratio value of DnHI =DnH2 SO4 was 2.1:1, which indicated that the predominant reaction during purification was the reverse Bunsen reaction. However, the conversion of H2SO4 was 90.3%. Although the reaction time reached 120 min and almost no evolution of SO2 was detected by the SO2 analyzer, a small amount of H2SO4 remained in the purified HIx phase. Simulation results for HIx purification studied by Wang et al. also showed that it was very difficult to realize the desired purification of HIx phase [19,20]. The thermodynamic equilibrium restriction may be the main reason that H2SO4 is very difficult to be removed completely. In addition, some stable intermediates may be formed between HI, I2 and the small amount of H2SO4, which needs to be verified. Additional experiments should be further carried out for effective removal of H2SO4 from HIx phase by some new ways, such as using special reactors and purifying under subatmospheric pressure.
4.
Conclusion
A series of experimental studies were conducted to investigate effects of the iodine concentration, reaction temperature and striping gas flow rate on the purification of HIx phase. The main results are summarized as follows.
Fig. 10 e The effect of sweeping gas flow rate on the time profiles of the SO2 evolution.
(1) The iodine concentration played the significant role in dictating the reactions during the purification of HIx phase. When the mole ratios of I2/HI were less than abut 0.88, the predominant reaction is the sulfur formation. When the mole ratios of I2/HI were in the range of 1e1.5, the predominant reaction changed into the reverse Bunsen reaction. However, increasing the iodine concentration decreased the conversion of H2SO4.
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(2) Temperature was also an important factor to influence the purification. At low temperature, both sulfur formation and reverse Bunsen reactions occurred. The reverse Bunsen reaction became predominant at high temperature. In addition, increasing reaction temperature favored the conversion of H2SO4. (3) In the examined N2 flow rate of 50e200 mL/min, increasing the stripping gas flow rate could not only increase the conversion of H2SO4 but also enhance the occurrence of the reverse Bunsen reaction. (4) In terms of the reaction type and the conversion of H2SO4, suitable operation conditions were recommended as follows. The mole ratio of I2/HI in the initial HIx phase should be controlled at about 1:1e1.5:1. Purification had better be carried out at above 95 C and the striping gas flow rate is above 100 mL/min. (5) The experiment of HIx purification at the optimized condition (at molar ratio of I2:HI ¼ 1.5:1, 120 C and 200 mL/ min of the N2 flow rate) showed that above 90% H2SO4 in the HIx phase could be removed in the style of SO2 by the reverse Bunsen reaction.
Acknowledgments This work was supported by Chinese National S&T Major Projects (2010zx06901).
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