Bunsen Reaction using a HIx Solution ( HII 2H 2 O ) with Countercurrent Flow for Sulfur-Iodine Hydrogen Production Process

In the sulfur-iodine hydrogen production process, the Bunsen reaction is a crucial section because of the linkage with the H2SO4 and HI decomposition sections. The HIx solution (HI-I2-H2O mixture) was fed to the Bunsen reaction section as a reactant from the HI decomposition section. In this study, the Bunsen reaction using the HIx solution with countercurrent flow was performed. The production rate of HIx phase solution increased while that of H2SO4 phase solution was maintained constant when increasing the flow rate of HIx solution. As the SO2 flow rate increased, the production rates of H2SO4 and HIx phase solutions increased. The amount of resultant H2SO4 phase was very lower than that of resultant HIx phase under the conditions examined in this study.


Introduction
The industrial system based on fossil fuels induces various environmental problems.The development of alternative energy is required to resolve the environmental problems.Hydrogen is regarded as an energy carrier that can substitute for fossil fuels.The thermochemical water splitting process produces the hydrogen from water at a lower temperature than operating condition of the direct water splitting process [1].Among the thermochemical water splitting process, the sulfur-iodine hydrogen production process (SI process) can produce a large amount of hydrogen from water [2].This process consisted of the following three reactions: H 2 SO 4 and HI that produced by the Bunsen reaction (Eq.( 1)) were separated spontaneously by a density difference under an excess of iodine.The resultant H 2 SO 4 and HI were decomposed at each decomposition section (Eqs.( 2) and ( 3)).This process was operated in a closed-cycle by recycling the decomposition products except for O 2 and H 2 .Consequently, H 2 O was decomposed into H 2 and O 2 via this process.
Meanwhile, a HI x solution consisting of HI, I 2 and H 2 O was fed to the Bunsen reaction section as a reactant from the HI decomposition section [3].We performed the Bunsen reaction in a semi-batch mode using the HI x solution in a previous study [4].However, these results had limitations to use for the integrated operation of the SI process.Therefore, the Bunsen reaction was conducted in a counter-current mode using the HI x solution to provide the operating data for SI process.The flow rates of HI x solution and SO 2 gas were changed as the operating variables.

Experimental
A 460 mL glass reactor was used for the Bunsen reaction using HI x solution with counter-current flow (Fig. 1).The ceramic packing materials of Raschig ring type were introduced to the reactor to avoid the channeling of SO 2 gas and increase the contact area of reactants.The operating temperature was maintained at 333 K by a thermostatic water bath.
Before the reaction, the H 2 SO 4 phase and HI x phase solutions were prepared for a smooth operation.For the HI x phase solution, I 2 (99 wt%, Junsei) and H 2 O (ultrapure water) to achieve a 1/2/6.17 of HI/I 2 /H 2 O molar ratio based on 1.27 mol of HI were fed to the reactor.The HI x phase solution was prepared by feeding N 2 gas at a constant rate of 20 mL/min for 1 h.Afterwards, the H 2 SO 4 phase solution with the H 2 O/H 2 SO 4 molar ratio of 4.04 based on 0.4 mol of H 2 SO 4 was introduced above the HI x phase solution.I 2 and H 2 O were fed to the HI x solution reservoir to prepare the HI x solution with the HI/I 2 /H 2 O molar ratio of 1/2/6.17based on 1.75 mol of HI.The reaction was performed with feeding of the SO 2 (99.595 vol%), HI x solution and H 2 O. Flow rates of the HI x solution and SO 2 were controlled by using metering pump (QG 50, Fluid Metering, Inc.) and MFC (MKS Instruments Inc.), respectively.The additional H 2 O was fed to be 7.8 of H 2 O/HI molar ratio of the HI x solution to increase the amount of H 2 SO 4 phase.The reaction proceeded by maintaining a constant interface between H 2 SO 4 phase and HI x phase in the reactor.
HI and I 2 concentrations were measured by titrating I -and I 2 with 0.1 N AgNO 3 and 0.1 N Na 2 S 2 O 3 standard solutions (Samchun Chemical), respectively.H 2 SO 4 concentration was calculated by subtracting the amount of HI from the amount of H + titrated with 0.1 N NaOH standard solution (DC Chemical).H 2 O concentration was calculated using a mass balance equation.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 and precipitation titration electrode: KEM C-373).To minimize the error resulting from the sampling and analysis processes, three samples were measured for each ion and the average concentration values were calculated.

Results and Discussion
It was found that the feed pipe for the HI x solution was clogged with solid I 2 when the HI x solution was fed to the reactor that filled the H 2 SO 4 phase and HI x phase solutions.This result can be explained by the occurrence of the reverse Bunsen reaction (Eq.( 4)) due to a direct contact between the HI x feed solution and the H 2 SO 4 phase solution.Therefore, the HI x feed solution should be introduced to the HI x phase in the reactor to avoid the solidification of I 2 .Flow rate of the HI x solution was changed to confirm the effect of the HI x solution flow rate on the composition of Bunsen products.Flow rate of SO 2 was controlled at 110 mL/min.HI/H 2 SO 4 /I 2 /H 2 O molar ratio was 1/0.14/1.61/4.28 at 180 min.For the H 2 SO 4 phase, the H 2 SO 4 /HI/H 2 O molar ratio was 1/0.09/5.55.The production rates of H 2 SO 4 and HI x phase solution were 0.26 and 3.09 g/min, respectively.Fig. 2(b) shows the composition change in HI x phase at 4.9 g/min of HI x solution flow rate.The composition of HI x phase was maintained constant after 60 min and the HI/H 2 SO 4 /I 2 /H 2 O molar ratio was 1/0.13/1.67/3.39 at 180 min.The H 2 SO 4 /HI/H 2 O molar ratio in H 2 SO 4 phase was 1/0.09/5.55 at 180 min.The production rates of H 2 SO 4 and HI x phase solution were 0.16 and 5.32 g/min, respectively.The composition change in HI x phase at 7.7 g/min of HI x solution flow rate was shown in Fig. 2(c).The composition of HI and H 2 SO 4 was maintained constant after 60 min while the composition of I 2 and H 2 O were changed significantly.The HI/H 2 SO 4 /I 2 /H 2 O molar ratio in HI x phase was 1/0.13/1.51/4.50 and the H 2 SO 4 /HI/H 2 O molar ratio in H 2 SO 4 phase was 1/0.07/6.19 at 180 min.The production rates of H 2 SO 4 and HI x phase solution were 0.22 and 8.55 g/min, respectively.As the flow rate of HI x solution increased, the production rate of HI x phase solution increased while that of H 2 SO 4 phase solution did not changed significantly.In addition, the HI content in H 2 SO 4 phase and the I 2 and HI contents in HI x phase were increased by increasing the flow rate of HI x solution.This result was because the amounts of HI, I 2 and H 2 O in feed increased relatively compared with the supplied amount of SO 2 .
Flow rate of SO 2 gas was controlled to identify the effect of SO 2 flow rate on the characteristics of Bunsen reaction in a counter-current flow mode.The HI x solution was fed at approximately 7.7 g/min.The composition and production rates of HI x phase and H 2 SO 4 phase at 180 min with different SO 2 flow rate were listed in Table 1.Regardless of the variation in the SO 2 flow rate, the compositions of HI and H 2 SO 4 in HI x phase were maintained constant after 60 min.The amount of H 2 SO 4 corresponding to an impurity in HI x phase increased by increasing the SO 2 flow rate.In addition, the production rates of H 2 SO 4 phase and HI x phase solutions increased when increasing the SO 2 flow rate.This result can be explained by an increase in the amount of SO 2 used as a reactant.On the other hand, the composition of products obtained in the counter-current flow mode was similar to the results obtained in a semi-batch mode [4].The amount of H 2 SO 4 phase was lower than that of HI x phase, which was in accordance with the results obtained in a semi-batch mode [4].A method to increase the amount of H 2 SO 4 phase solution for the continuous operation of Bunsen reaction with countercurrent flow should be further investigated.

Conclusions
In this study, the effects of the flow rates of HI x solution and SO 2 on the Bunsen reaction using the HI x solution with counter-current flow were investigated.Major results were summarized as follows.
1) When the HI x solution was fed to the H 2 SO 4 phase solution in the reactor, the solidification of I 2 occurred by the reverse Bunsen reaction.The HI x solution should be introduced to the HI x phase in the reactor for a continuous operation of Bunsen reaction.
2) The composition of HI x phase was maintained constant after 60 min regardless of the variation in the HI x solution flow rate.The production rate of HI x phase solution increased while that of H 2 SO 4 phase solution did not changed significantly as the flow rate of HI x solution increased.
3) The compositions of HI and H 2 SO 4 were maintained constant after 60 min regardless of the variation in the SO 2 flow rate.The impurity content in HI x phase was increased by increasing the SO 2 flow rate.The production rates of H 2 SO 4 phase and HI x phase solutions also increased as the SO 2 flow rate increased.The amount of resultant H 2 SO 4 phase was very lower than that of HI x phase under all experimental conditions.

Fig. 1 .
Fig. 1.Schematic diagram of experimental apparatus for the Bunsen ration using the HI x solution with countercurrent flow.

Fig. 2 .
Fig. 2. The composition change in HI x phase with different the HI x solution flow rate; (a) 2.8 g/min, (b) 4.9 g/min and (c) 7.7 g/min.

Fig. 2 (
a) shows the composition change in HI x phase at 2.8 g/min of HI x solution flow rate.The composition of HI x phase was maintained constant after 60 min and the DOI Bunsen reaction: SO 2 + I 2 + 2H 2 O ֖ H 2 SO 4 + 2HI (293 ~ 393 K)

Table 1 .
The Composition And Production Rate Of Hi x Phase And H 2 so 4 Phase At 180 Min With Different The So 2