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A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process Hyo Sub Kim a, Hyun Kyu Park a, Young Ho Kim a,*, Jong Gyu Lee b, Chu Sik Park c, Ki Kwang Bae c a Department of Chemical Engineering & Applied Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea b Energy & Resources Research Department, Research Institute of Industrial Science & Technology, 67 Cheongam-ro, Nam-gu, Pohang 37673, Republic of Korea c Hydrogen Energy Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
article info
abstract
Article history:
During the integrated operation of the sulfur-iodine process, it is important to conveniently
Received 22 August 2016
and quickly measure the composition of the Bunsen reaction products and to ascertain the
Received in revised form
location of each phase in the liquideliquid phase separator. First, the method to determine
30 October 2016
each composition in HIx phase system which contains four components of HI, I2, H2O, and
Accepted 4 November 2016
H2SO4 was newly proposed using only the data of Hþ and I contents and the density of the
Available online xxx
HIx phase. This method has the advantage to replace the complicated and time-consuming traditional titration step of I2. The calculated I2/HI molar ratios were within an error of ±5%
Keywords:
at all temperature conditions, indicating that this method was suitable for I2 composition
Hydrogen production
analysis. Meanwhile, the use of an electrical conductivity sensor was discussed as a
Sulfur-iodine process
sensing technology during the phase separation of Bunsen reaction products. The electrical
Bunsen reaction
conductivity was measured using different compositions of the Bunsen reaction products.
Phase separation
The conductivity difference between H2SO4 and HIx phase solutions was approximately 478
Electrical conductivity
e822 mS/cm in the main compositional range of the Bunsen reaction products. Therefore, this method can replace the traditional phase separation method using a DP (differential pressure) cell. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Developing alternative energy sources is important because of worsening environmental problems due to the high
consumption of fossil fuels worldwide. Hydrogen produced from water is a promising alternative energy source or energy storage carrier because it produces non-toxic exhaust emissions [1,2]. Splitting water using only heat occurs
* Corresponding author. Fax: þ82 42 822 6637. E-mail address: [email protected] (Y.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2016.11.033 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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spontaneously at approximately 4300 K, indicating that this method is impractical. Thermochemical water splitting methods for hydrogen production have occurred at lower temperatures than direct water decomposition through a combination of chemical reactions [3]. Selecting a reasonable, highly efficient thermochemical cycle remains important. General Atomics (GA) has established a database of thermochemical and hybrid (thermochemical-electrolysis) cycles and has highlighted economically feasible cycles for hydrogen production via water splitting [4]. Among these highlighted cycles, the sulfur-iodine process and the UT-3 cycle have been highly efficient thermochemical cycles. The sulfur-iodine thermochemical hydrogen production process (SI process) is operated using the heat emitted from a very high temperature reactor (VHTR) [5]. The SI process consists of the following three chemical reactions:
SO2 þ I2 þ 2H2O / H2SO4 þ 2HI (293e393 K)
(1)
H2SO4 / SO2 þ H2O þ 0.5O2
(2)
2HI / H2 þ I2
(573e773 K)
(1023e1123 K)
(3)
Sulfuric acid (H2SO4) and hydrogen iodide (HI) are produced as immiscible acids through a reaction between sulfur dioxide (SO2), iodine (I2) and water (H2O) within the Bunsen reaction (Eq. (1)). The Bunsen reaction products spontaneously separate into two acidic phases of different densities when excess iodine is present: a H2SO4 phase (upper phase) and a HIx phase (bottom phase). The H2SO4 decomposes into SO2, H2O and O2 (Eq. (2)), and HI decomposes into H2 and I2 (Eq. (3)). The decomposition products except for H2 and O2 are recycled in a closed cycle while the H2O decomposes into H2 and O2. The Bunsen reaction is a significant section of the SI process because it has linked with the H2SO4 and HI decomposition sections. The major research goals for the operation of Bunsen section can be summarized as the acquisition of composition data, the minimization of impurities and the prevention of side reactions, such as sulfur (S) formation and hydrogen sulfide (H2S) formation. Many studies for the Bunsen reaction have been reported so far [6e19]. In the beginning, results using a quaternary mixture system, including H2SO4, HI, I2 and H2O, were reported to ascertain the overall characteristics of the Bunsen reaction [6e10]. The effects of the operating variables (I2 and H2O concentrations and temperature) on the characteristics of the Bunsen reaction were studied, and an operating window for the Bunsen reaction was proposed. Although the results of a quaternary mixture system were useful to ascertain the characteristics of the Bunsen reaction, these results were insufficient to demonstrate the actual Bunsen reaction, which occurs simultaneously along with side reactions. Therefore, the characteristics of the Bunsen reaction were investigated using SO2, I2 and H2O as reactants [11,12]. The composition of products with different operating parameters, side reactions and the mechanism of the Bunsen reaction were reported.
Additionally, the HIx solution that consisted of HI, I2 and H2O could be recycled as a feed of the Bunsen reaction section from the HI decomposition section in the operation of the integrated SI process. Comprehensive results of the Bunsen reaction using a HIx solution over the wide operating conditions were reported in terms of phase separation and their product composition [13e16]. Recently, a series of experiments using an electrochemical membrane reactor were conducted to reduce the excess I2 in the Bunsen reaction [17e19]. Although the Bunsen reaction products were concentrated at a low I2 concentration, several problems remained such as the corrosion of electrodes and membrane damage. It is important to conveniently and quickly measure the composition of the Bunsen reaction products. Chen et al. determined the composition of a HIx phase solution using density and EED cell voltage instead of traditional titrations [20]. The above HIx phase was only composed of HI, I2 and H2O. Therefore, this method could not be applied directly to determine the composition of the HIx phase including the undesired H2SO4 component as an impurity. In addition, the measurement procedure by traditional titration of I2 in the HIx phase includes a complicated and time-consuming step differed from that of Hþ and I. In this study, therefore, a convenient and quick method to determine the composition of the Bunsen reaction products was investigated to apply the integrated operation of SI process. We analyzed the composition of I2 in the HIx phase, which includes an H2SO4 component, by using the density and the Hþ and I contents. The traditional titration for I2 analysis could be eliminated by this method. A total of 73 data points, which has been reported in previous studies [11,14,21], was applied to improve the accuracy of the composition analysis of HIx phase. On the other hand, the introduction of a DP (differential pressure) cell was proposed to ascertain the locations of the H2SO4 and HIx phases in the liquideliquid phase separator [22]. This method required a dip tube, a differential pressure sensor and a PID controller, and it was difficult to ascertain an exact location of the products. In this study, we proposed measuring the electrical conductivity of the two phases to determine the location of the Bunsen reaction products. The electrical conductivities of the H2SO4 and HIx solutions with different compositions were measured. The phase separation of the Bunsen reaction products measured with an electrical conductivity sensor was also discussed.
Experimental Operating conditions of the Bunsen reaction for composition determination The operating conditions of previous studies [11,14,21] used to detect the composition of Bunsen reaction products were listed in Table 1. Reaction types were classified according to the reactants such as a pure I2, H2O and the HIx solution (HI-I2-H2O mixture). Operating conditions without an occurrence of a side reaction were selected to accurately determine the composition of the products. In each experiment, the density of the resulting HIx phase solution was calculated by measuring its weight and volume. Fig. 1 shows the
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Table 1 e The operating conditions of different reaction types. Reaction type The Bunsen reaction using I2 and H2O Semi batch experiment Continuous experiment
The Bunsen reaction using HIx solution
Operating conditions Temp. [K]
I2/H2O feed molar ratio
298e333 313e333
0.219e0.297 0.250e0.297
Temp. [K]
HI/I2/H2O feed molar ratio
298 313 333
1/2.0/6.0e12.0 1/2.0e2.5/6.0e12.0 1/1.4e2.9/6.0e10.0
representative schematic diagram of the experimental apparatus for the Bunsen reaction using HIx solution.
The electrical conductivity measurement of the Bunsen reaction products A 450 mL glass vessel was used to prepare the H2SO4 and HIx phase solutions. Other parts of the reactor were made of Teflon to prevent corrosion by the product solution. The mixture vessel was installed in a thermostat bath to maintain a constant operating temperature. To ensure sufficient mixing of reactants, a Teflon stirrer was introduced into the mixture vessel. H2SO4 (95 wt%, Junsei), H2O and HI (55e58 wt%, Kanto) were used to prepare the H2SO4 phase solution, and HI, I2 (99%, Junsei), H2O and H2SO4 were used to prepare the HIx phase solution. A selected amount of reactants were added into the mixture vessel, and then the reactants were mixed for 30 min when the desired temperature was achieved. The conductivity of Bunsen reaction products were measured with the conductivity transmitter (CLM253-ID0005, Endress þ Hauser)
Ref.
[21] [11]
[14] [14] [14]
and analog conductivity sensor (CLS50-A4A1, Endress þ Hauser). The HI concentration was measured via the potentiometric titrator (AT-510, KEM) before use. The operating temperature was maintained in the range of 298e363 K. To measure the electrical conductivity of the Bunsen reaction products, different feed composition ranges of the H2SO4 phase and the HIx phase solution were listed in Table 2. The H2SO4/H2O molar ratio was altered in the range of 1/0.6e49.0 to evaluate the effect of H2O concentration on the H2SO4 phase. The H2SO4/H2O/HI molar ratio was altered in the range of 1/5.5/0e0.09 to identify the effect of HI concentration in the H2SO4 phase. The HI/I2/H2O molar ratio was also varied between 1/3.5/5.75e8.00 and 1/0e4.5/5.5 to elucidate the effect of H2O and I2 concentration in the HIx phase. Finally, the HI/I2/ H2O/H2SO4 molar ratio was maintained at 1/3.5/6.5/0e0.09 to identify the effect of H2SO4 concentration as an impurity in the HIx phase.
Measurement method The HI and I2 concentrations were measured by titrating I and I2 with 0.1 N AgNO3 and 0.1 N Na2S2O3 standard solutions (Samchun Chemical), respectively. The H2SO4 concentration was calculated by subtracting the amount of HI from the amount of Hþ titrated with a 0.1 N NaOH standard solution (DC Chemical). The H2O concentration was calculated assuming that only four species (H2SO4, HI, I2 and H2O) constituted each phase. The titrations were performed using the potentiometric titrator and electrodes (acidebase titration electrode: KEM C-171; redox titration electrode: KEM C-272; and precipitation titration electrode: KEM C373). To minimize sampling and analysis errors during processing, three samples for each ion were measured, and the average concentration values were then determined.
Table 2 e Feed composition ranges of H2SO4 and HIx phase solutions.
Fig. 1 e Schematic diagram of the experimental apparatus for the Bunsen reaction using HIx solution.
Bunsen products
Composition range
H2SO4/H2O molar ratio in H2SO4 phase H2SO4/H2O/HI molar ratio in H2SO4 phase HI/I2/H2O molar ratio in HIx phase
1/0.6e49.0 1/5.5/0e0.09 1/3.5/5.75e8.00 1/0e4.5/5.5 1/3.5/6.5/0e0.09
HI/I2/H2O/H2SO4 molar ratio in HIx phase
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Results and discussion Composition determination of the Bunsen reaction products A total of 73 data points obtained from previous works [11,14,21] were used. We calculated the density of HIx solution by substituting the experimental results into Eqs. (4) and (5) to determine the best formula to calculate the density. The assumption that only HI, I2, H2O and H2SO4 constitute the HIx phase was used. n 1 X xi ¼ r r i¼1 i
r¼
n X
xi ri
(4)
(5)
i¼1
where, r was the density of HIx solution, and xi was the weight fraction of component i, and ri indicated the density of component i. The density of H2O and H2SO4 components were found by taking the operating temperature into account. The exact density of HI component was not reported. Therefore, a density of 2.85 g/cm3 HI was applied, which is the density of HI in a liquid state at 226 K. For I2, a solid state density of 4.93 g/ cm3 was used. The experimental density of HIx phase solution was compared with the density calculated by each equation (Eqs. (4) and (5)) (Fig. 2). The error between the experimental density and the density calculated by Eq. (4) was approximately ±5%. There were some results with more than a 5% error at 333 K. This was due to lack of reports of the exact density for the components at 333 K. Meanwhile, the error between the experimental density and the density calculated by Eq. (5) was more than 20%. Therefore, the appropriate calculation equation for HIx phase solution was Eq. (4). Eq. (4) was altered to the form of Eq. (6) to calculate the weight fraction of I2 component (xI2 ; Cal ) in the HIx phase solution. xI2 ;Cal
rH2 O rI2 1 xH2 SO4 xHI 1 xH2 SO4 xHI ¼ r rH2 SO4 rHI rH2 O rI2 rH2 O
(6)
The regression equations were obtained to increase the accuracy of xI2 ; Cal by comparing the experimental weight fractions of I2 component (Fig. 3). We revised the xI2 ; Cal using the regression equation at different temperatures. The calculated I2/HI molar ratios were compared with their experimental counterparts to confirm the accuracy of xI2 ; Cal (Fig. 4). The calculated I2/HI molar ratios were within ±5% error at all temperature conditions, indicating the revised xI2 ; Cal was in good agreement with the experimental values. The procedure for applying this method in the experiment was as follows. The density of HIx phase solution was measured. The Hþ and I concentrations in the HIx phase (mmol/g) were measured using the chemical titration method. These values were converted to weight fractions of H2SO4 and HI via their molecular weights. The weight fractions of I2 (xI2 ; Cal ) and H2SO4 (xH2 SO4 ) were calculated using Eqs. (6) and (7), respectively. The revised xI2 ; Cal was calculated by substituting the obtained xI2 ; Cal into the regression equation at each temperature. Consequently, we can obtain the I2 content by using the I and Hþ concentrations and density of the HIx solution.
Fig. 2 e A calculated density and experimental density of the HIx phase solution with two different formulas for the density; (a) Eq. (4) and (b) Eq. (5).
xH2 SO4 ¼
ðHþ ½mmol=g I ½mmol=gÞ 98:07 2 1000
(7)
The phase separation of the Bunsen reaction products Electrical conductivity of the H2SO4 phase solution To determine the electrical conductivity of H2SO4 phase solution with different concentrations and temperatures, the H2O/H2SO4 molar ratios were altered from 0.6 to 49.0, and the operating temperature was maintained in the range of 298e353 K. The electrical conductivity of H2SO4 phase solution with different H2O/H2SO4 molar ratios was shown in Fig. 5(a). There was a discrepancy between the mixing composition and the measured composition of H2SO4 phase solution due to the interaction among reactants or the evaporation of water. The electrical conductivity of H2SO4 phase solution increased as the operating temperature increased. The electrical conductivity of H2SO4 phase solution also increased up to the H2O/ H2SO4 molar ratio of 12 and then gradually decreased. The electrical conductivity of the H2SO4 phase solution with different H2SO4 concentrations and temperatures was similar to the results of a previous study [23]. The H2SO4 phase solution obtained from the Bunsen reaction included a trace
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
Fig. 3 e The experimental weight fraction of the I2 component (xI2 ) and the calculated weight fraction of I2 (xI2 ; Cal ) at each temperature; The solid lines represent a linear fit to xI2 ; Cal with regression equation: (a) 298 K, y ¼ 0:0130 þ 0:9500x, (b) 313 K, y ¼ 0:0384 þ 1:0813x and (c) 333 K, y ¼ 0:0654 þ 0:9171x.
amount of HI as an impurity. Therefore, the HI solution was fed into the H2SO4 phase solution to confirm the effect of impurity on the electrical conductivity of H2SO4 solution. The electrical conductivity of H2SO4 phase solution decreased from 1182 to 1005 mS/cm when the HI/H2SO4 molar ratio
5
Fig. 4 e Comparisons of the experimental I2/HI molar ratio and the calculated I2/HI molar ratio; (a) 298 K, (b) 313 K and (c) 333 K.
increased from 0 to 0.09 (Fig. 5(b)). This result indicated that the HI component as an impurity affected the electrical conductivity of the H2SO4 phase solution.
Electrical conductivity of the HIx phase solution To ascertain the effects of I2 concentration and temperature on the electrical conductivity of HIx phase solution, the I2/HI molar ratio was varied from 0 to 4.5, and the temperature was
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Fig. 6 e Electrical conductivity of the HIx phase solution with different I2/HI molar ratios and temperatures; H2O/HI molar ratio of 5.5.
ascertain the exact effects of H2SO4 concentration on the electrical conductivity of HIx phase solution.
Electrical conductivity of the Bunsen reaction products
Fig. 5 e Electrical conductivity of the H2SO4 phase solution with different (a) H2O/H2SO4 molar ratios and temperatures and (b) HI/H2SO4 molar ratios at 363 K.
adjusted in the range of 298e363 K. Here, the H2O/HI molar ratio in the HIx solution was measured to be approximately 5.5. The electrical conductivity of HIx phase solution with different temperatures and I2/HI molar ratios was shown in Fig. 6. The range capable of measuring the electrical conductivity of HIx phase solution at each temperature broadened as the temperature increased because the I2 saturation composition increased. The electrical conductivity of HIx phase solution gradually decreased with an increase in the I2/HI molar ratio and further increased with an increase in the temperature. The electrical conductivity of HIx phase solution with different H2O/HI and H2SO4/HI molar ratios was listed in Table 3. The electrical conductivity was nearly constant when the H2O/HI molar ratio was varied from 5.75 to 8 with the I2/HI molar ratio set to 3.5. The electrical conductivity was also steady, in the range of 310e319 mS/cm, regardless of variations in the H2SO4/HI molar ratio. These results indicated that the variation in H2O and H2SO4 concentrations had little effect on the electrical conductivity of HIx phase solution. When the H2SO4 was fed into HIx phase solution, a thin layer of H2SO4 solution formed on the HIx phase solution. Therefore, the measured H2SO4 composition in HIx phase solution was lower than the desired composition. Further study is needed to
We obtained the electrical conductivity of the Bunsen reaction products based on the electrical conductivity data at different compositions of H2SO4 and HIx phase solutions. Compositions of the Bunsen products were determined based on previous studies [10,11,13] that conducted the Bunsen reaction and a separation experiment of quaternary mixture. The electrical conductivity of H2SO4 phase solution was obtained by taking into account the effect of HI and H2O concentrations and temperature. The electrical conductivity of HIx phase solution was obtained by considering the I2 concentration and temperature. The electrical conductivity values with different composition of the Bunsen products between 353 and 363 K were shown in Fig. 7. The electrical conductivity of the H2SO4 phase was obtained with different H2O/H2SO4 molar ratios, which ranged between 4.03 and 6.17. In this range, the electrical conductivity of H2SO4 phase solution was detected to be approximately 907e1182 mS/cm (Fig. 7(a)). Similarly, the electrical conductivity of HIx phase was obtained using different I2/HI molar ratios between 2.30 and 3.26. In this composition range, the electrical conductivity of HIx phase solution was in the range of approximately 360e429 mS/cm (Fig. 7(b)). From these results, the conductivity difference
Table 3 e Electrical conductivity of the HIx phase solution with different H2O/HI and H2SO4/HI molar ratios at 363 K. H2O/HI molar ratio (with I2/ HI ¼ 3.5) 5.75 6 6.5 7 8
Electrical Electrical H2SO4/HI molar conductivity ratio (with HI/ conductivity I2/ [mS/cm] [mS/cm] H2O ¼ 1/3.5/6.5) 345 343 341 338 340
0 0.03 0.06 0.09
302 313 319 310
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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between H2SO4 and HIx phase solutions was determined to be 478e822 mS/cm. It was possible to distinguish between the H2SO4 and HIx phase solutions by the difference in conductivity value. These results indicated that this method can replace the traditional phase separation method using a DP cell. Therefore, we have proposed a schematic diagram of a liquideliquid phase separator including an electrical conductivity sensor (Fig. 8). A sufficient amount of electrical conductivity sensors need to be used to efficiently detect the H2SO4 and HIx phase solutions in a liquideliquid phase separator. The electrical conductivity can be obtained when the Bunsen reaction products have been collected in the liquideliquid phase separator. Once the electrical conductivity value corresponding to the H2SO4 phase solution was obtained near the H2SO4 phase solution outlet, the Bunsen reaction products were discharged to their decomposition sections.
Conclusions The methods for the composition determination and phase separation of the Bunsen reaction products were investigated and summarized the results as follows:
Fig. 7 e Electrical conductivity with different compositions of the Bunsen reaction products in the temperature range of 353e363 K; (a) H2SO4 phase and (b) HIx phase.
Fig. 8 e The schematic diagram of a liquideliquid phase separator fixed with electrical conductivity sensor in the Bunsen section.
(1) The method was proposed using density and Hþ and I concentrations to conveniently and quickly analyze the I2 composition in the HIx phase solution, including an H2SO4 component as an impurity. The accuracy of composition determination in the HIx phase was improved by applying data reported in literature. The formula to calculate the density of the HIx phase solution was decided, and an equation to determine I2 content was derived. The accuracy of the I2 composition analysis was improved via the revised equation at different temperatures (error within ±5%). The I2 content could be obtained by using the Hþ and I concentrations, as well as the density. This method can replace the completed and time-consuming traditional titration step of I2. (2) The use of the electrical conductivity sensor was proposed to ascertain the location of Bunsen reaction products in the liquideliquid phase separator. The electrical conductivity of H2SO4 and HIx phase solutions was measured with different compositions. The electrical conductivity of H2SO4 phase solution was increased with increasing temperature. It was also increased up to the H2O/H2SO4 molar ratio of 12 and then gradually decreased. The electrical conductivity of H2SO4 phase solution was decreased with increasing the HI content as an impurity. The electrical conductivity of HIx phase solution was decreased with an increase of I2 concentration and increased with an increase of temperature. Variations of H2O and H2SO4 concentrations had little effect on the electrical conductivity of HIx phase solution. The conductivity difference between H2SO4 and HIx phase solutions was approximately 478e822 mS/cm. Therefore, this method can replace the phase separation method using a DP cell. The liquideliquid phase separator fitted with electrical conductivity sensors was proposed to discern
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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the phase separation boundary of the Bunsen reaction products.
Acknowledgment This research was performed for the Nuclear Hydrogen Technology Development and was funded by the Ministry of Science, ICT and Future Planning (NRF-2014M2A8A2048725), Korea.
Nomenclature xi xHI xH2 SO4 xI2 ;Cal r ri rH2 SO4 rHI rI2 rH2 O
weight fraction of component i in the product, g/g weight fraction of HI component in the product, g/g weight fraction of H2SO4 component in the product, g/g calculated weight fraction of I2 in the product, g/g density of HIx solution, g/mL density of component i, g/mL density of H2SO4 component, g/mL density of HI component, g/mL density of I2 component, g/mL density of H2O component, g/mL
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[9] Maatouk SC, Mokrani NB, Tabarant M, Fleche JL, Carles P. Study of the miscibility gap in H2SO4/HI/I2/H2O mixtures produced by the Bunsen reaction e part I: preliminary results at 308 K. Int J Hydrogen Energy 2009;34:7155e61. [10] Zhu Q, Zhang Y, Zhou C, Wang Z, Zhou J, Cen K. Optimization of liquideliquid phase separation characteristics in the Bunsen section of the sulfureiodine hydrogen production process. Int J Hydrogen Energy 2012;37:6407e14. [11] Kim HS, Kim YH, Han SJ, Park CS, Bae KK, Lee JG. Continuous Bunsen reaction and simultaneous separation using a counter-current flow reactor for the sulfureiodine hydrogen production process. Int J Hydrogen Energy 2013;38:6190e6. [12] Zhang Y, Peng P, Ying Z, Zhu Q, Zhou J, Wang Z, et al. Experimental investigation on multiphase Bunsen reaction in the thermochemical sulfureiodine cycle. Ind Eng Chem Res 2014;53:3021e8. [13] Ying Z, Zhang Y, Zhu Q, Liu J, Zhou J, Wang Z, et al. Influence of the initial HI on the multiphase Bunsen reaction in the sulfureiodine thermochemical cycle. Int J Hydrogen Energy 2013;38:15946e53. [14] Kim HS, Kim YH, Ahn BT, Lee JG, Park CS, Bae KK. Phase separation characteristics of the Bunsen reaction when using HIx solution (HIeI2eH2O) in the sulfureiodine hydrogen production process. Int J Hydrogen Energy 2014;39:692e701. [15] Zhang P, Xue L, Lan S, Guo H, Chen S, Wang L, et al. Reaction parameters of the Bunsen reaction under simulated recycling conditions. Int J Hydrogen Energy 2014;39:14565e73. [16] Kim HS, Keum YS, Kim YH, Lee JG, Park CS, Bae KK. Characteristics of Bunsen reaction using HIx solution (HIeI2eH2O) in a co-current flow mode for the sulfur-iodine hydrogen production process. Int J Hydrogen Energy 2016;41:10530e7. [17] Zhang Y, Ying Z, Zhou J, Liu J, Wang Z, Cen K. Electrolysis of the Bunsen reaction and properties of the membrane in the sulfureiodine thermochemical cycle. Ind Eng Chem Res 2014;53:13581e8. [18] Ying Z, Zhang Y, Xu S, Zhou J, Liu J, Wang Z, et al. Equilibrium potential for the electrochemical Bunsen reaction in the sulfureiodine cycle. Int J Hydrogen Energy 2014;39:18727e33. [19] Zhang Y, Ying Z, Peng P, Zhou J, Liu J, Wang Z, et al. Performance of the electrochemical Bunsen reaction using two different proton exchange membranes in the sulfureiodine cycle. Ind Eng Chem Res 2014;53:4966e74. [20] Chen S, Zhang P, Wang L, Xu J. A detection method for HIx composition in thermochemical water splitting iodineesulfur cycle. Int J Hydrogen Energy 2016;41:2247e52. [21] Lee KJ, Kim YH, Park CS, Bae KK. Phase separation characteristics via Bunsen reaction in sulfur-iodine thermochemical hydrogen production process. Trans Korean Hydrogen New Energy Soc 2008;19:386e93. [22] Shin YJ, Lee KY, Chang JH, Lee WJ. Bunsen reactor comprising automatic separator for producing nuclear hydrogen and method of separating Sulfuric acid -Iodinic acid automatically using thereof. 2008. [23] Darling HE. Conductivity of sulfuric acid solutions. J Chem Eng Data 1964;9:421e6.
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process Hyo Sub Kim a, Hyun Kyu Park a, Young Ho Kim a,*, Jong Gyu Lee b, Chu Sik Park c, Ki Kwang Bae c a Department of Chemical Engineering & Applied Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea b Energy & Resources Research Department, Research Institute of Industrial Science & Technology, 67 Cheongam-ro, Nam-gu, Pohang 37673, Republic of Korea c Hydrogen Energy Research Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
article info
abstract
Article history:
During the integrated operation of the sulfur-iodine process, it is important to conveniently
Received 22 August 2016
and quickly measure the composition of the Bunsen reaction products and to ascertain the
Received in revised form
location of each phase in the liquideliquid phase separator. First, the method to determine
30 October 2016
each composition in HIx phase system which contains four components of HI, I2, H2O, and
Accepted 4 November 2016
H2SO4 was newly proposed using only the data of Hþ and I contents and the density of the
Available online xxx
HIx phase. This method has the advantage to replace the complicated and time-consuming traditional titration step of I2. The calculated I2/HI molar ratios were within an error of ±5%
Keywords:
at all temperature conditions, indicating that this method was suitable for I2 composition
Hydrogen production
analysis. Meanwhile, the use of an electrical conductivity sensor was discussed as a
Sulfur-iodine process
sensing technology during the phase separation of Bunsen reaction products. The electrical
Bunsen reaction
conductivity was measured using different compositions of the Bunsen reaction products.
Phase separation
The conductivity difference between H2SO4 and HIx phase solutions was approximately 478
Electrical conductivity
e822 mS/cm in the main compositional range of the Bunsen reaction products. Therefore, this method can replace the traditional phase separation method using a DP (differential pressure) cell. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Developing alternative energy sources is important because of worsening environmental problems due to the high
consumption of fossil fuels worldwide. Hydrogen produced from water is a promising alternative energy source or energy storage carrier because it produces non-toxic exhaust emissions [1,2]. Splitting water using only heat occurs
* Corresponding author. Fax: þ82 42 822 6637. E-mail address: [email protected] (Y.H. Kim). http://dx.doi.org/10.1016/j.ijhydene.2016.11.033 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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spontaneously at approximately 4300 K, indicating that this method is impractical. Thermochemical water splitting methods for hydrogen production have occurred at lower temperatures than direct water decomposition through a combination of chemical reactions [3]. Selecting a reasonable, highly efficient thermochemical cycle remains important. General Atomics (GA) has established a database of thermochemical and hybrid (thermochemical-electrolysis) cycles and has highlighted economically feasible cycles for hydrogen production via water splitting [4]. Among these highlighted cycles, the sulfur-iodine process and the UT-3 cycle have been highly efficient thermochemical cycles. The sulfur-iodine thermochemical hydrogen production process (SI process) is operated using the heat emitted from a very high temperature reactor (VHTR) [5]. The SI process consists of the following three chemical reactions:
SO2 þ I2 þ 2H2O / H2SO4 þ 2HI (293e393 K)
(1)
H2SO4 / SO2 þ H2O þ 0.5O2
(2)
2HI / H2 þ I2
(573e773 K)
(1023e1123 K)
(3)
Sulfuric acid (H2SO4) and hydrogen iodide (HI) are produced as immiscible acids through a reaction between sulfur dioxide (SO2), iodine (I2) and water (H2O) within the Bunsen reaction (Eq. (1)). The Bunsen reaction products spontaneously separate into two acidic phases of different densities when excess iodine is present: a H2SO4 phase (upper phase) and a HIx phase (bottom phase). The H2SO4 decomposes into SO2, H2O and O2 (Eq. (2)), and HI decomposes into H2 and I2 (Eq. (3)). The decomposition products except for H2 and O2 are recycled in a closed cycle while the H2O decomposes into H2 and O2. The Bunsen reaction is a significant section of the SI process because it has linked with the H2SO4 and HI decomposition sections. The major research goals for the operation of Bunsen section can be summarized as the acquisition of composition data, the minimization of impurities and the prevention of side reactions, such as sulfur (S) formation and hydrogen sulfide (H2S) formation. Many studies for the Bunsen reaction have been reported so far [6e19]. In the beginning, results using a quaternary mixture system, including H2SO4, HI, I2 and H2O, were reported to ascertain the overall characteristics of the Bunsen reaction [6e10]. The effects of the operating variables (I2 and H2O concentrations and temperature) on the characteristics of the Bunsen reaction were studied, and an operating window for the Bunsen reaction was proposed. Although the results of a quaternary mixture system were useful to ascertain the characteristics of the Bunsen reaction, these results were insufficient to demonstrate the actual Bunsen reaction, which occurs simultaneously along with side reactions. Therefore, the characteristics of the Bunsen reaction were investigated using SO2, I2 and H2O as reactants [11,12]. The composition of products with different operating parameters, side reactions and the mechanism of the Bunsen reaction were reported.
Additionally, the HIx solution that consisted of HI, I2 and H2O could be recycled as a feed of the Bunsen reaction section from the HI decomposition section in the operation of the integrated SI process. Comprehensive results of the Bunsen reaction using a HIx solution over the wide operating conditions were reported in terms of phase separation and their product composition [13e16]. Recently, a series of experiments using an electrochemical membrane reactor were conducted to reduce the excess I2 in the Bunsen reaction [17e19]. Although the Bunsen reaction products were concentrated at a low I2 concentration, several problems remained such as the corrosion of electrodes and membrane damage. It is important to conveniently and quickly measure the composition of the Bunsen reaction products. Chen et al. determined the composition of a HIx phase solution using density and EED cell voltage instead of traditional titrations [20]. The above HIx phase was only composed of HI, I2 and H2O. Therefore, this method could not be applied directly to determine the composition of the HIx phase including the undesired H2SO4 component as an impurity. In addition, the measurement procedure by traditional titration of I2 in the HIx phase includes a complicated and time-consuming step differed from that of Hþ and I. In this study, therefore, a convenient and quick method to determine the composition of the Bunsen reaction products was investigated to apply the integrated operation of SI process. We analyzed the composition of I2 in the HIx phase, which includes an H2SO4 component, by using the density and the Hþ and I contents. The traditional titration for I2 analysis could be eliminated by this method. A total of 73 data points, which has been reported in previous studies [11,14,21], was applied to improve the accuracy of the composition analysis of HIx phase. On the other hand, the introduction of a DP (differential pressure) cell was proposed to ascertain the locations of the H2SO4 and HIx phases in the liquideliquid phase separator [22]. This method required a dip tube, a differential pressure sensor and a PID controller, and it was difficult to ascertain an exact location of the products. In this study, we proposed measuring the electrical conductivity of the two phases to determine the location of the Bunsen reaction products. The electrical conductivities of the H2SO4 and HIx solutions with different compositions were measured. The phase separation of the Bunsen reaction products measured with an electrical conductivity sensor was also discussed.
Experimental Operating conditions of the Bunsen reaction for composition determination The operating conditions of previous studies [11,14,21] used to detect the composition of Bunsen reaction products were listed in Table 1. Reaction types were classified according to the reactants such as a pure I2, H2O and the HIx solution (HI-I2-H2O mixture). Operating conditions without an occurrence of a side reaction were selected to accurately determine the composition of the products. In each experiment, the density of the resulting HIx phase solution was calculated by measuring its weight and volume. Fig. 1 shows the
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Table 1 e The operating conditions of different reaction types. Reaction type The Bunsen reaction using I2 and H2O Semi batch experiment Continuous experiment
The Bunsen reaction using HIx solution
Operating conditions Temp. [K]
I2/H2O feed molar ratio
298e333 313e333
0.219e0.297 0.250e0.297
Temp. [K]
HI/I2/H2O feed molar ratio
298 313 333
1/2.0/6.0e12.0 1/2.0e2.5/6.0e12.0 1/1.4e2.9/6.0e10.0
representative schematic diagram of the experimental apparatus for the Bunsen reaction using HIx solution.
The electrical conductivity measurement of the Bunsen reaction products A 450 mL glass vessel was used to prepare the H2SO4 and HIx phase solutions. Other parts of the reactor were made of Teflon to prevent corrosion by the product solution. The mixture vessel was installed in a thermostat bath to maintain a constant operating temperature. To ensure sufficient mixing of reactants, a Teflon stirrer was introduced into the mixture vessel. H2SO4 (95 wt%, Junsei), H2O and HI (55e58 wt%, Kanto) were used to prepare the H2SO4 phase solution, and HI, I2 (99%, Junsei), H2O and H2SO4 were used to prepare the HIx phase solution. A selected amount of reactants were added into the mixture vessel, and then the reactants were mixed for 30 min when the desired temperature was achieved. The conductivity of Bunsen reaction products were measured with the conductivity transmitter (CLM253-ID0005, Endress þ Hauser)
Ref.
[21] [11]
[14] [14] [14]
and analog conductivity sensor (CLS50-A4A1, Endress þ Hauser). The HI concentration was measured via the potentiometric titrator (AT-510, KEM) before use. The operating temperature was maintained in the range of 298e363 K. To measure the electrical conductivity of the Bunsen reaction products, different feed composition ranges of the H2SO4 phase and the HIx phase solution were listed in Table 2. The H2SO4/H2O molar ratio was altered in the range of 1/0.6e49.0 to evaluate the effect of H2O concentration on the H2SO4 phase. The H2SO4/H2O/HI molar ratio was altered in the range of 1/5.5/0e0.09 to identify the effect of HI concentration in the H2SO4 phase. The HI/I2/H2O molar ratio was also varied between 1/3.5/5.75e8.00 and 1/0e4.5/5.5 to elucidate the effect of H2O and I2 concentration in the HIx phase. Finally, the HI/I2/ H2O/H2SO4 molar ratio was maintained at 1/3.5/6.5/0e0.09 to identify the effect of H2SO4 concentration as an impurity in the HIx phase.
Measurement method The HI and I2 concentrations were measured by titrating I and I2 with 0.1 N AgNO3 and 0.1 N Na2S2O3 standard solutions (Samchun Chemical), respectively. The H2SO4 concentration was calculated by subtracting the amount of HI from the amount of Hþ titrated with a 0.1 N NaOH standard solution (DC Chemical). The H2O concentration was calculated assuming that only four species (H2SO4, HI, I2 and H2O) constituted each phase. The titrations were performed using the potentiometric titrator and electrodes (acidebase titration electrode: KEM C-171; redox titration electrode: KEM C-272; and precipitation titration electrode: KEM C373). To minimize sampling and analysis errors during processing, three samples for each ion were measured, and the average concentration values were then determined.
Table 2 e Feed composition ranges of H2SO4 and HIx phase solutions.
Fig. 1 e Schematic diagram of the experimental apparatus for the Bunsen reaction using HIx solution.
Bunsen products
Composition range
H2SO4/H2O molar ratio in H2SO4 phase H2SO4/H2O/HI molar ratio in H2SO4 phase HI/I2/H2O molar ratio in HIx phase
1/0.6e49.0 1/5.5/0e0.09 1/3.5/5.75e8.00 1/0e4.5/5.5 1/3.5/6.5/0e0.09
HI/I2/H2O/H2SO4 molar ratio in HIx phase
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Results and discussion Composition determination of the Bunsen reaction products A total of 73 data points obtained from previous works [11,14,21] were used. We calculated the density of HIx solution by substituting the experimental results into Eqs. (4) and (5) to determine the best formula to calculate the density. The assumption that only HI, I2, H2O and H2SO4 constitute the HIx phase was used. n 1 X xi ¼ r r i¼1 i
r¼
n X
xi ri
(4)
(5)
i¼1
where, r was the density of HIx solution, and xi was the weight fraction of component i, and ri indicated the density of component i. The density of H2O and H2SO4 components were found by taking the operating temperature into account. The exact density of HI component was not reported. Therefore, a density of 2.85 g/cm3 HI was applied, which is the density of HI in a liquid state at 226 K. For I2, a solid state density of 4.93 g/ cm3 was used. The experimental density of HIx phase solution was compared with the density calculated by each equation (Eqs. (4) and (5)) (Fig. 2). The error between the experimental density and the density calculated by Eq. (4) was approximately ±5%. There were some results with more than a 5% error at 333 K. This was due to lack of reports of the exact density for the components at 333 K. Meanwhile, the error between the experimental density and the density calculated by Eq. (5) was more than 20%. Therefore, the appropriate calculation equation for HIx phase solution was Eq. (4). Eq. (4) was altered to the form of Eq. (6) to calculate the weight fraction of I2 component (xI2 ; Cal ) in the HIx phase solution. xI2 ;Cal
rH2 O rI2 1 xH2 SO4 xHI 1 xH2 SO4 xHI ¼ r rH2 SO4 rHI rH2 O rI2 rH2 O
(6)
The regression equations were obtained to increase the accuracy of xI2 ; Cal by comparing the experimental weight fractions of I2 component (Fig. 3). We revised the xI2 ; Cal using the regression equation at different temperatures. The calculated I2/HI molar ratios were compared with their experimental counterparts to confirm the accuracy of xI2 ; Cal (Fig. 4). The calculated I2/HI molar ratios were within ±5% error at all temperature conditions, indicating the revised xI2 ; Cal was in good agreement with the experimental values. The procedure for applying this method in the experiment was as follows. The density of HIx phase solution was measured. The Hþ and I concentrations in the HIx phase (mmol/g) were measured using the chemical titration method. These values were converted to weight fractions of H2SO4 and HI via their molecular weights. The weight fractions of I2 (xI2 ; Cal ) and H2SO4 (xH2 SO4 ) were calculated using Eqs. (6) and (7), respectively. The revised xI2 ; Cal was calculated by substituting the obtained xI2 ; Cal into the regression equation at each temperature. Consequently, we can obtain the I2 content by using the I and Hþ concentrations and density of the HIx solution.
Fig. 2 e A calculated density and experimental density of the HIx phase solution with two different formulas for the density; (a) Eq. (4) and (b) Eq. (5).
xH2 SO4 ¼
ðHþ ½mmol=g I ½mmol=gÞ 98:07 2 1000
(7)
The phase separation of the Bunsen reaction products Electrical conductivity of the H2SO4 phase solution To determine the electrical conductivity of H2SO4 phase solution with different concentrations and temperatures, the H2O/H2SO4 molar ratios were altered from 0.6 to 49.0, and the operating temperature was maintained in the range of 298e353 K. The electrical conductivity of H2SO4 phase solution with different H2O/H2SO4 molar ratios was shown in Fig. 5(a). There was a discrepancy between the mixing composition and the measured composition of H2SO4 phase solution due to the interaction among reactants or the evaporation of water. The electrical conductivity of H2SO4 phase solution increased as the operating temperature increased. The electrical conductivity of H2SO4 phase solution also increased up to the H2O/ H2SO4 molar ratio of 12 and then gradually decreased. The electrical conductivity of the H2SO4 phase solution with different H2SO4 concentrations and temperatures was similar to the results of a previous study [23]. The H2SO4 phase solution obtained from the Bunsen reaction included a trace
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
Fig. 3 e The experimental weight fraction of the I2 component (xI2 ) and the calculated weight fraction of I2 (xI2 ; Cal ) at each temperature; The solid lines represent a linear fit to xI2 ; Cal with regression equation: (a) 298 K, y ¼ 0:0130 þ 0:9500x, (b) 313 K, y ¼ 0:0384 þ 1:0813x and (c) 333 K, y ¼ 0:0654 þ 0:9171x.
amount of HI as an impurity. Therefore, the HI solution was fed into the H2SO4 phase solution to confirm the effect of impurity on the electrical conductivity of H2SO4 solution. The electrical conductivity of H2SO4 phase solution decreased from 1182 to 1005 mS/cm when the HI/H2SO4 molar ratio
5
Fig. 4 e Comparisons of the experimental I2/HI molar ratio and the calculated I2/HI molar ratio; (a) 298 K, (b) 313 K and (c) 333 K.
increased from 0 to 0.09 (Fig. 5(b)). This result indicated that the HI component as an impurity affected the electrical conductivity of the H2SO4 phase solution.
Electrical conductivity of the HIx phase solution To ascertain the effects of I2 concentration and temperature on the electrical conductivity of HIx phase solution, the I2/HI molar ratio was varied from 0 to 4.5, and the temperature was
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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Fig. 6 e Electrical conductivity of the HIx phase solution with different I2/HI molar ratios and temperatures; H2O/HI molar ratio of 5.5.
ascertain the exact effects of H2SO4 concentration on the electrical conductivity of HIx phase solution.
Electrical conductivity of the Bunsen reaction products
Fig. 5 e Electrical conductivity of the H2SO4 phase solution with different (a) H2O/H2SO4 molar ratios and temperatures and (b) HI/H2SO4 molar ratios at 363 K.
adjusted in the range of 298e363 K. Here, the H2O/HI molar ratio in the HIx solution was measured to be approximately 5.5. The electrical conductivity of HIx phase solution with different temperatures and I2/HI molar ratios was shown in Fig. 6. The range capable of measuring the electrical conductivity of HIx phase solution at each temperature broadened as the temperature increased because the I2 saturation composition increased. The electrical conductivity of HIx phase solution gradually decreased with an increase in the I2/HI molar ratio and further increased with an increase in the temperature. The electrical conductivity of HIx phase solution with different H2O/HI and H2SO4/HI molar ratios was listed in Table 3. The electrical conductivity was nearly constant when the H2O/HI molar ratio was varied from 5.75 to 8 with the I2/HI molar ratio set to 3.5. The electrical conductivity was also steady, in the range of 310e319 mS/cm, regardless of variations in the H2SO4/HI molar ratio. These results indicated that the variation in H2O and H2SO4 concentrations had little effect on the electrical conductivity of HIx phase solution. When the H2SO4 was fed into HIx phase solution, a thin layer of H2SO4 solution formed on the HIx phase solution. Therefore, the measured H2SO4 composition in HIx phase solution was lower than the desired composition. Further study is needed to
We obtained the electrical conductivity of the Bunsen reaction products based on the electrical conductivity data at different compositions of H2SO4 and HIx phase solutions. Compositions of the Bunsen products were determined based on previous studies [10,11,13] that conducted the Bunsen reaction and a separation experiment of quaternary mixture. The electrical conductivity of H2SO4 phase solution was obtained by taking into account the effect of HI and H2O concentrations and temperature. The electrical conductivity of HIx phase solution was obtained by considering the I2 concentration and temperature. The electrical conductivity values with different composition of the Bunsen products between 353 and 363 K were shown in Fig. 7. The electrical conductivity of the H2SO4 phase was obtained with different H2O/H2SO4 molar ratios, which ranged between 4.03 and 6.17. In this range, the electrical conductivity of H2SO4 phase solution was detected to be approximately 907e1182 mS/cm (Fig. 7(a)). Similarly, the electrical conductivity of HIx phase was obtained using different I2/HI molar ratios between 2.30 and 3.26. In this composition range, the electrical conductivity of HIx phase solution was in the range of approximately 360e429 mS/cm (Fig. 7(b)). From these results, the conductivity difference
Table 3 e Electrical conductivity of the HIx phase solution with different H2O/HI and H2SO4/HI molar ratios at 363 K. H2O/HI molar ratio (with I2/ HI ¼ 3.5) 5.75 6 6.5 7 8
Electrical Electrical H2SO4/HI molar conductivity ratio (with HI/ conductivity I2/ [mS/cm] [mS/cm] H2O ¼ 1/3.5/6.5) 345 343 341 338 340
0 0.03 0.06 0.09
302 313 319 310
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
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between H2SO4 and HIx phase solutions was determined to be 478e822 mS/cm. It was possible to distinguish between the H2SO4 and HIx phase solutions by the difference in conductivity value. These results indicated that this method can replace the traditional phase separation method using a DP cell. Therefore, we have proposed a schematic diagram of a liquideliquid phase separator including an electrical conductivity sensor (Fig. 8). A sufficient amount of electrical conductivity sensors need to be used to efficiently detect the H2SO4 and HIx phase solutions in a liquideliquid phase separator. The electrical conductivity can be obtained when the Bunsen reaction products have been collected in the liquideliquid phase separator. Once the electrical conductivity value corresponding to the H2SO4 phase solution was obtained near the H2SO4 phase solution outlet, the Bunsen reaction products were discharged to their decomposition sections.
Conclusions The methods for the composition determination and phase separation of the Bunsen reaction products were investigated and summarized the results as follows:
Fig. 7 e Electrical conductivity with different compositions of the Bunsen reaction products in the temperature range of 353e363 K; (a) H2SO4 phase and (b) HIx phase.
Fig. 8 e The schematic diagram of a liquideliquid phase separator fixed with electrical conductivity sensor in the Bunsen section.
(1) The method was proposed using density and Hþ and I concentrations to conveniently and quickly analyze the I2 composition in the HIx phase solution, including an H2SO4 component as an impurity. The accuracy of composition determination in the HIx phase was improved by applying data reported in literature. The formula to calculate the density of the HIx phase solution was decided, and an equation to determine I2 content was derived. The accuracy of the I2 composition analysis was improved via the revised equation at different temperatures (error within ±5%). The I2 content could be obtained by using the Hþ and I concentrations, as well as the density. This method can replace the completed and time-consuming traditional titration step of I2. (2) The use of the electrical conductivity sensor was proposed to ascertain the location of Bunsen reaction products in the liquideliquid phase separator. The electrical conductivity of H2SO4 and HIx phase solutions was measured with different compositions. The electrical conductivity of H2SO4 phase solution was increased with increasing temperature. It was also increased up to the H2O/H2SO4 molar ratio of 12 and then gradually decreased. The electrical conductivity of H2SO4 phase solution was decreased with increasing the HI content as an impurity. The electrical conductivity of HIx phase solution was decreased with an increase of I2 concentration and increased with an increase of temperature. Variations of H2O and H2SO4 concentrations had little effect on the electrical conductivity of HIx phase solution. The conductivity difference between H2SO4 and HIx phase solutions was approximately 478e822 mS/cm. Therefore, this method can replace the phase separation method using a DP cell. The liquideliquid phase separator fitted with electrical conductivity sensors was proposed to discern
Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
the phase separation boundary of the Bunsen reaction products.
Acknowledgment This research was performed for the Nuclear Hydrogen Technology Development and was funded by the Ministry of Science, ICT and Future Planning (NRF-2014M2A8A2048725), Korea.
Nomenclature xi xHI xH2 SO4 xI2 ;Cal r ri rH2 SO4 rHI rI2 rH2 O
weight fraction of component i in the product, g/g weight fraction of HI component in the product, g/g weight fraction of H2SO4 component in the product, g/g calculated weight fraction of I2 in the product, g/g density of HIx solution, g/mL density of component i, g/mL density of H2SO4 component, g/mL density of HI component, g/mL density of I2 component, g/mL density of H2O component, g/mL
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Please cite this article in press as: Kim HS, et al., A convenient method for phase separation and composition determination of the Bunsen reaction products in sulfur-iodine hydrogen production process, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.11.033