Phase separation characteristics of the Bunsen reaction when using HIx solution (HI–I2–H2O) in the sulfur–iodine hydrogen production process

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Phase separation characteristics of the Bunsen reaction when using HIx solution (HIeI2eH2O) in the sulfureiodine hydrogen production process Hyo Sub Kim a, Young Ho Kim a,*, Byung Tae Ahn a, Jong Gyu Lee b, Chu Sik Park c, Ki Kwang Bae c a

Department of Fine Chemical Engineering & Applied Chemistry, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea b Energy & Resources Research Department, Research Institute of Industrial Science & Technology, San 32, Hyojadong, Nam-gu, Pohang 790-330, Republic of Korea c Hydrogen Energy Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305343, Republic of Korea

article info

abstract

Article history:

To continuously operate an integrated sulfureiodine (SI) hydrogen production process, the

Received 29 August 2013

HIx solution (HIeI2eH2O) could be recycled from the HI decomposition section as a reactant

Received in revised form

in the Bunsen reaction section. In this study, the temperature, iodine content and water

12 October 2013

content were varied to identify the phase separation characteristics of products from the

Accepted 17 October 2013

Bunsen reaction using the HIx solution with SO2. Increasing the temperature increased the

Available online 22 November 2013

volume of the H2SO4 phase solution and decreased the impurity content in each phase. Increasing the iodine feed concentration somewhat decreased the volume of the H2SO4

Keywords:

phase solution, although the density difference between the phases increased. The amount

Sulfureiodine process

of H2SO4 that separated into the H2SO4 phase was very small under most of these condi-

Hydrogen production

tions, which significantly hindered the continuous operation of the integrated SI process.

Bunsen reaction

The feed of additional water in the separation step was suggested to improve the sepa-

HIx solution

ration performance of the H2SO4 phase solution while minimizing side reactions.

Phase separation

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The current industrial system, which is based mainly on fossil fuels, causes environmental problems such as air pollution and global warming. Therefore, the investigation of alternative energy sources and solutions to environmental problems is inevitable. Hydrogen is an attractive and clean energy carrier that can be produced from fossil fuels, nuclear and

renewable energy sources by various processes, such as water electrolysis, coal gasification, water splitting at high temperatures and natural gas reforming [1]. Among the hydrogen production methods using water, the thermochemical hydrogen production process, which is based on nuclear or solar energy sources, can produce large amounts of hydrogen without emitting CO2 [2,3]. The sulfureiodine hydrogen production process (SI process) that uses the high temperatures

* Corresponding author. Tel.: þ82 42 821 5898; fax: þ82 42 822 6637. E-mail address: [email protected] (Y.H. Kim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.10.098

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produced by a nuclear energy source is a promising thermochemical hydrogen production process [4,5]. The SI process consists of the following three chemical reactions: Bunsen reaction:

5

I 2 /HI molar ratio_feed

4

I2 solidification

20  120  C; exothermic

SO2 þ I2 þ 2H2 O%H2 SO4 þ 2HI 3

(1)

H2SO4 decomposition: H2 SO4 %H2 O þ SO2 þ 0:5O2

2

800  900  C; endothermic

(2)

HI decomposition: 2HI%H2 þ I2

1

30

40

50

60

70

80

90

o

Temperature [ C]

Fig. 1 e The region of phase separation of the products of the Bunsen reaction.

b

a 0.20

7.0 I /HI molar ratio_feed

I /HI molar ratio_feed 1.4

1.8

2.0

H O/H SO molar ratio in H SO phase

HI/H SO molar ratio in H SO phase

1.0 0.15

0.10

0.05

1.0

6.5

1.8

2.0

6.0

5.5

5.0

30

40

50

60

70

80

4.0 20

90

30

40

d

I /HI molar ratio_feed 1.4

80

90

0.5 1.0

2.0

2.0

1.5

1.0

1.4

1.8

2.0

0.4

0.3

0.2

0.1

0.5

0.0 20

70

I /HI molar ratio_feed

1.8

H SO /HI molar ratio in HI phase

2.5

60

Temperature [ C]

3.0

1.0

50

o

o

Temperature [ C]

I /HI molar ratio in HI phase

1.4

4.5

0.00 20

c

(3)

In the Bunsen reaction (Eq. (1)), sulfur dioxide (SO2) reacts exothermically with iodine (I2) and water (H2O) to produce two immiscible liquid phases: the H2SO4 phase, which consists primarily of sulfuric acid (H2SO4) and water, and the HIx phase, which contains mainly hydrogen iodide (HI), iodine and water. The H2SO4 phase produced by the Bunsen reaction is decomposed into H2O, SO2 and O2 (Eq. (2)), and the HIx phase is decomposed into H2 and I2 (Eq. (3)) via the purification and concentration steps. Subsequently, oxygen and hydrogen are

No separation

0 20

300  500  C; endothermic

30

40

50

60 o

Temperature [ C]

70

80

90

0.0 20

30

40

50

60

70

80

90

o

Temperature [ C]

Fig. 2 e Effects of the temperature on the composition of each phase. (a): HI/H2SO4 in the H2SO4 phase, (b): H2O/H2SO4 in the H2SO4 phase, (c): I2/HI in the HIx phase, and (d): H2SO4/HI in the HIx phase.

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 9 ( 2 0 1 4 ) 6 9 2 e7 0 1

produced separately from the integrated SI process, and the other components are recycled to the Bunsen reaction section. The H2SO4 and HIx phases separate because there is a density difference between the relatively light H2SO4 solution and the heavy HIx solution due to the formation of polyhydriodic acids by the solvation of molecular iodine [6]. Operating the Bunsen reaction section poses the following challenges: (a) the immiscibility of the products should be enhanced (i.e., low cross-contamination between the H2SO4 and the HIx phases), and (b) the Bunsen reaction must occur without undesirable side reactions (the formation of sulfur (S) and hydrogen sulfide (H2S) described in Eqs. (4), (5) and (6)) and the reverse Bunsen reaction (Eq. (7)) [7,8]. Sulfur formation reaction: H2SO4 þ 6HI % S þ 3I2 þ 4H2O

(4)

SO2 þ 4HI % S þ 2I2 þ 2H2O

(5)

Hydrogen sulfide formation reaction: H2SO4 þ 8HI % H2S þ 4I2 þ 4H2O

(6)

Reverse Bunsen reaction: H2SO4 þ 2HI % SO2 þ I2 þ 2H2O

(7)

Sakurai et al. [7,9] investigated the phase separation characteristics of a H2SO4/HI/I2/H2O quaternary mixture system over a temperature range of 0e95  C and the conditions that induce side reactions over a range of 22e95  C. Increasing the temperature under the I2 saturation conditions improved the separation performance, and side reactions (mainly sulfur formation) occurred readily at low iodine concentrations and high temperatures. Giaconia et al. [10] studied the effects of several variables (the temperature and the concentrations of iodine and water) on the phase behavior of products obtained from the Bunsen reaction. Increasing the iodine concentration enhanced the purity of the products, and increasing the water concentration decreased the concentration of the two acids. Kang et al. [11] and Lee et al. [12] investigated the conditions for phase separation and the distribution of water in the phases using a solution that consisted of H2SO4/HI/I2/H2O at molar ratios of 1/2/0.3e13.5/14e30 over a temperature range of 25e120  C. We reported that the impurity content in each phase depends mainly on the temperature and iodine concentration, and the amount of water is slightly higher in the HIx phase than in the H2SO4 phase. The following studies have focused on the optimal operating conditions for the Bunsen reaction and the HI concentration in the resulting HIx phase. Maatouk et al. [13] investigated the effects of the iodine and water contents and the H2SO4/HI molar ratio on the liquid phase separation of products from the Bunsen reaction at 35  C. They reported that the liquideliquid equilibrium improved by increasing the iodine and H2SO4 contents and decreasing the water content. Zhu et al. [14] studied the separation characteristics of the H2SO4/HI/I2/H2O quaternary system to identify the optimal operating conditions. They proposed using I2/H2SO4 molar

ratios ranging from 2.45 to 3.99 and temperatures from 72 to 85  C to obtain hyper-azeotropic HI concentrations in the HIx phase. Guo et al. [15] and Lan et al. [16] investigated the effects of the iodine concentration and temperature on the phase separation characteristics of the quaternary mixture and the solubility of iodine. Increasing the iodine concentration improved the phase separation performance, and the region of phase separation was larger at higher temperatures due to the greater solubility of iodine. However, a series of experiments using the H2SO4/HI/I2/ H2O quaternary mixture may be insufficient to explain the characteristics of the actual Bunsen reaction system where the Bunsen reaction, side reactions and phase separation occur simultaneously. Therefore, we comprehensively reported the phase separation characteristics of the products obtained at quasi-equilibrium state from the Bunsen reaction using SO2, I2 and H2O under semi-batch and continuous reaction conditions [17,18]. In a recently reported flow sheet of the integrated SI process, the HIx solution (HIeI2eH2O), which consists of HI, I2 and H2O, could be supplied from the HI decomposition section as a reactant in the Bunsen reaction section [19,20]. The phase separation of products obtained from the Bunsen reaction using the HIx solution has not been widely studied. Parisi et al. [21] performed the Bunsen reaction using the HIx solution and investigated mainly the efficiency of the SO2 conversion and the purification step. The volume of the resultant H2SO4 phase solution was very small, and the composition of products from the Bunsen reaction was not studied in detail. Therefore, in this study, we investigated the effects of various process variables to provide comprehensive data on the Bunsen reaction using the HIx solution during the operation of the integrated SI process. The temperature was varied from 25 to 80  C. The iodine content (I2/HI molar ratio) was varied from 1.0 to 3.8, and the water content (H2O/HI molar ratio) ranged from 6.17 to 12. The volume of the H2SO4 phase solution was very small, indicating that it was very difficult to separate the H2SO4 produced under these conditions.

4.0 I /HI molar ratio_feed 1.0 1.8

3.5 3.0

Density [g/mL]

694

1.4 2.0

HI phase

2.5 2.0 1.5 H SO phase

1.0 0.5 0.0 20

30

40

50

60

70

80

90

o

Temperature [ C]

Fig. 3 e Effects of the temperature on the density of each phase.

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2.

Experimental

The experimental apparatus consisted of a 350 mL jacketed stirred reactor, and the operating temperature was controlled by flowing temperature-controlled water from a thermostatic bath. The vent line was connected to a scrubber, which was filled with an aqueous NaOH solution, to trap unreacted SO2 gas. A series of experiments was performed using a semi-batch procedure. The HI (55e58 wt%, Kanto) concentration was measured using a potentiometric titrator (KEM AT-510, Japan) before the solution was used. A predetermined amount of iodine at an I2/HI molar ratio of 1.0e3.8 and ultra-pure water at

a

I /HI = 1.0

I /HI = 1.4

I /HI = 1.8

HI

15

10

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

I2 /HI molar ratio_feed 35 o

60 C 30

HIx phase

25

I /HI = 2.0

100

80

60

20 15 10

40

5 20

0 0.8

1.2

1.6

0 40

50

60

80

40

60

80

40

60

80

25

40

60

b

c

H2SO4 phase

I /HI = 1.0

I /HI = 1.4

I /HI = 1.8

2.0

2.4

2.8

3.2

I2 /HI molar ratio_feed

80

o

Temperature [ C]

HIx phase

30 o

80 C 25

I /HI = 2.0

100 Concentration [mmol/g]

The distribution ratio of H2 O [mol%]

H2O

20

0 0.8

Concentration [mmol/g]

T h e a m o u n t o f e a c h p h a s e s o lu tio n [v o l% ]

H 2SO4 phase

H2SO4

o

40 C

5

b a

30

25

Concentration [mmol/g]

Additionally, we proposed a method to improve the separation of the H2SO4 phase solution while inhibiting the side reactions based on the comparative study of water addition before and after the reaction.

80

60

20

15

10

40

5 20

0 40

50

60

80

40

60

80

40

60

80

25

40

60

o

Temperature [ C]

Fig. 4 e Effects of the temperature on the amounts of products (a) and the distribution of water between the phases (b).

80

0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

I2 /HI molar ratio_feed Fig. 5 e Effects of the I2/HI feed molar ratio on the composition of the H2SO4 phase at (a): 40  C, (b): 60  C, and (c): 80  C.

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an H2O/HI molar ratio of 6.17e12, based on 0.45 mol HI, were fed into the reactor. After the desired temperature was reached, the HIx solution was homogenized by mixing for 30 min using a mechanical stirrer. Subsequently, the reaction was performed by feeding SO2 gas at 110 mL/min to the solution until the product composition was nearly constant (i.e., a quasi-equilibrium state, which was attained after 340 min). The operating temperature was varied from 25 to 80  C. The products of the Bunsen reaction were transferred to a phase separator, and the weight and volume of each phase were measured. The density of each phase was estimated from its weight and volume. The solid sulfur produced by the side reaction was detected by washing the precipitate with ethanol. The HI and I2 concentrations were measured by titrating I and I2 with standardized 0.1 N AgNO3 and 0.1 N Na2S2O3 solutions (Samchun chemical), respectively. The H2SO4 concentration was calculated by subtracting the amount of HI from the amount of Hþ titrated with a standardized 0.1 N NaOH solution (DC chemical). The H2O content was calculated using a mass balance equation. Titrations were performed using a potentiometric titrator and electrodes (acid-base titration electrode: KEM C-171, redox titration electrode: KEM C-272 and precipitation titration electrode: KEM C-373).

a

3.

Results and discussion

3.1.

The region of phase separation

The execution of the integrated SI process requires the operating conditions that maintain phase separation without iodine precipitation. Fig. 1 shows the region of phase separation for a Bunsen reaction that uses the HIx solution with an H2O/HI molar ratio of 6.17 over a temperature range of 25e80  C. The region of phase separation begins with the onset of phase separation without sulfur formation and terminates when iodine precipitates. As the temperature increased, the I2/HI feed molar ratio for the starting point of the phase separation decreased, while the I2/HI feed molar ratio for iodine saturation increased (Fig. 1). The I2/HI molar ratios for phase separation were 1.9, 1.8, 1.4 and 1.0, and those for iodine saturation were 2.0, 2.3, 2.9 and 3.8 at 25, 40, 60 and 80  C, respectively. The region of the liquideliquid phase separation for a Bunsen reaction using the HIx solution correlated positively with the temperature. This result was similar to those obtained in previous studies [9,11,12,14,16], which observed changes in the temperature and iodine content of a H2SO4/HI/I2/H2O quaternary mixture system. When the I2/

b 10

10 o

HI

25 C

H SO

I

8

Concentration [mmol/g]

8

Concentration [mmol/g]

o

40 C

HO

6

4

6

4

2

2

0 0.8

1.0

1.2

1.4

1.6

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I2/HI molar ratio_feed

c

d 10

o

60 C

1.8

2.0

2.2

2.4

o

80 C

8

Concentration [mmol/g]

8

Concentration [mmol/g]

1.6

I2 /HI molar ratio_feed

10

6

4

6

4

2

2

0 0.8

1.4

1.2

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I2 /HI molar ratio_feed

2.8

3.2

0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

I2 /HI molar ratio_feed

Fig. 6 e Effects of the I2/HI feed molar ratio on the composition of the HIx phase at (a): 25  C, (b): 40  C, (c): 60  C, and (d): 80  C.

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The effects of temperature

The effects of temperature on the HI/H2SO4 and the H2O/ H2SO4 molar ratios in the H2SO4 phase and the I2/HI and the H2SO4/HI molar ratios in the HIx phase are reported in Fig. 2. The HI/H2SO4 molar ratio corresponding to the degree of impurity in the H2SO4 phase decreased as the temperature increased (Fig. 2(a)). The H2O/H2SO4 molar ratio in the H2SO4 phase was maintained between 5.4 and 5.7 over the entire temperature range (Fig. 2(b)). This result indicates that the H2O/H2SO4 molar ratio in the H2SO4 phase was nearly constant when the products were separated. The I2/HI molar ratio in the HIx phase correlated positively with the temperature under these I2 concentration conditions (Fig. 2(c)). The H2SO4/ HI molar ratio corresponding to the degree of impurity in the HIx phase was very high, ca. 0.3 in the reactions at 25  C, but this ratio decreased to ca. 0.1 as the temperature increased to 80  C (Fig. 2(d)). These results were in good agreement with previous studies [9,11,12,14], indicating that increasing the temperature enhanced the immiscibility of each phase. In addition, we considered that the iodine content in the HIx phase increased due to the decrease in the conversion that resulted from the increase in temperature. This effect then improved the phase separation performance by increasing the density difference between the phases. To support this concept, the relationship between the density of each phase and the temperature is plotted in Fig. 3. The density of the H2SO4 phase remained nearly constant at approximately 1.36 g/mL, while that of the HIx phase increased with the temperature. The density difference between the phases increased from 0.97 to 1.58 g/mL, which was consistent with the tendency described by Zhu et al. [14]. The volume fraction of products (vol%) and water distribution in each phase (mol%) at various temperatures are plotted in Fig. 4. Fig. 4(a) shows that the phase separation performance improved due to the increase in the amount of the H2SO4 phase solution at high temperatures. As the temperature increased, the unreacted I2 and H2O concentrations in the HIx phase increased with the decreasing conversion of the Bunsen reaction. As a result, there was an increase in the amount of H2O available to hydrate H2SO4 that existed as an impurity in the HIx phase and subsequently move it to the H2SO4 phase. On the other hand, the phase separation performance deteriorated somewhat as the I2/HI molar ratio increased. This result is because the large amount of H2SO4 formed at high iodine concentrations is present in the HIx phase, and it increases the volume of the HIx phase solution. The distribution ratio of H2O in the H2SO4 phase also increased as the temperature increased (Fig. 4(b)). This finding was in good agreement with the previous study [11] of a quaternary H2SO4/HI/I2/H2O system with a simple mixing experiment.

3.3.

The effects of the I2/HI feed molar ratio

To determine the effects of the iodine feed concentration on the separation performance of the Bunsen reaction product, we

a

3.5

3.0 I 2 /HI molar ratio in HI x phase

3.2.

performed a series of experiments by varying the I2/HI feed molar ratio from 1.0 to the I2 saturation composition at each temperature (i.e., 2.0, 2.3, 2.9 and 3.8 I2/HI molar ratios at 25, 40, 60 and 80  C, respectively). The H2SO4 content in the H2SO4 phase increased slightly, and the H2O content decreased slightly, as the I2/HI molar ratio in the feed increased (Fig. 5). Iodine in the H2SO4 phase could hardly be detected under these operating conditions. The concentration of HI (an impurity in the H2SO4 phase) decreased to ca. 57% as the molar ratio of I2/HI in the feed increased from 1.0 to the I2 saturation composition. As the feed molar ratio of I2/HI increased, the concentrations of HI, H2O and H2SO4 (impurities in the HIx phase) decreased almost linearly, while that of the I2 increased in the HIx phase (Fig. 6). The observed decrease in the impurity content in each phase as the I2/HI molar ratio increased is consistent with previous studies [11,17]. Therefore, increasing the iodine concentration in the feed enhanced the immiscibility of the products. The I2/HI molar ratio in the HIx phase and the phase densities are shown in Fig. 7. The I2/HI molar ratio in the HIx phase

2.5

2.0

1.5

1.0

25 40 60 80

0.5

0.0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

C C C C

3.6

4.0

I2 /HI molar ratio_feed

b

3.5

3.0 HI phase

2.5 Density [g/mL]

HI molar ratio exceeded the iodine saturation point, the transfer tube was clogged with precipitated iodine, which significantly hindered the continuous operation of the integrated SI process.

2.0

1.5

H SO phase

1.0

25 C 40 C 60 C 80 C

0.5

0.0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

I2/HI molar ratio_feed

Fig. 7 e Effects of the I2/HI feed molar ratio on the I2/HI molar ratio in the HIx phase (a) and the density of each phase (b).

4.0

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increased linearly with the I2/HI molar ratio in the feed. As the feed concentration of iodine increased, the density of the HIx phase increased from 2.26 to 3.16 g/mL, while that of the H2SO4 phase decreased slightly from 1.40 to 1.10 g/mL. These trends were in good agreement with previous studies [13,14,22]. The density difference between the phases increased as the I2/HI feed molar ratio increased, which indicates that higher concentrations of iodine improved the phase separation performance. On the basis of the theoretical study, Calabrese et al. [23] reported that I2xHþ complexes such as I2Hþ, I4Hþ, I6Hþ and I8Hþ could exist at high iodine concentrations. They also reported that the concentrations of I2xHþ complexes with high values of x increased as the iodine concentration increased. Therefore, the increase in density of the HIx phase implies the existence of high concentrations of I2xHþ complexes in the HIx phase. The volume fraction of each phase solution (vol%) and the distribution ratios of H2SO4 and H2O in each phase (mol%) are listed in Table 1. The amount of the H2SO4 phase solution decreased gradually, while that of the HIx phase solution increased as the iodine concentration increased. In particular, the H2SO4 phase solution obtained from the I2 saturation composition at each temperature was approximately 2.8 vol%. Therefore, it was very difficult to continuously separate the H2SO4 and the HIx phases. Similarly, the amounts of H2SO4 and H2O that moved to the H2SO4 phase decreased, while those that transferred to the HIx phase increased linearly by increasing the iodine concentration. A comparable relative decrease in the amount of water that moved to the H2SO4 phase as the iodine concentration increased was observed also in previous studies of a H2SO4/HI/ I2/H2O quaternary mixture system [11,12]. Therefore, the decreased volume of the H2SO4 phase solution was attributed

to the decrease in the H2SO4 and H2O contents that moved into the H2SO4 phase. It was considered that the amount of water required for hydration of HIx complexes in the HIx phase was somewhat greater under conditions with higher iodine concentrations. Consequently, phase separation was difficult due to the low volume of the H2SO4 phase solution in the Bunsen reaction using the HIx solution. The amount of water that existed in the Bunsen reaction product was insufficient for phase separation.

3.4.

The effects of H2O content

The first attempt to improve the phase separation performance of the products was to increase the H2O/HI feed molar ratio. A series of experiments was performed with high iodine concentrations. Water with molar ratios of 8e12 for H2O/HI based on the 0.45 mol HI was fed into the reactor. Table 2 shows the amount, density and composition of each phase and the DHI/DH2SO4 molar ratio in solution. As shown in Table 2, the volumes of the H2SO4 and HIx phase solutions increased as the H2O/HI feed molar ratio increased. The amounts of H2SO4 and HI generated in the total solution after the Bunsen reaction increased. Parisi et al. [21], who performed the Bunsen reaction using the HIx solution, reported that it was very difficult to observe the H2SO4 phase solution and that additional water was required to separate the H2SO4 phase solution. It means that the Bunsen reaction proceeds easily under conditions with excess water and the extra of water is required to hydrate H2SO4 and HIx complexes in each phase during the phase separation process. The density of the H2SO4 phase and the H2O content in the H2SO4 phase remained stable as the H2O/HI feed molar ratio

Table 1 e Effects of the I2/HI feed molar ratio on the amounts of products and the distribution ratios of H2SO4 and H2O. Temp. ( C)

25 40

60

80

I2/HI molar ratio

2.0 1.0 1.4 1.8 2.0 2.2 2.3 1.0 1.4 1.8 2.0 2.2 2.4 2.6 2.8 2.9 1.0 1.4 1.8 2.0 2.9 3.8

H2SO4 phase

HIx phase

The amount of H2SO4 H2SO4 H2O distribution The amount of H2O distribution product (vol%) distribution ratio ratio (mol%) product (vol%) distribution ratio ratio (mol%) (mol%) (mol%) 0.32 13.44 6.94 3.14 3.66 2.23 2.22 27.62 19.47 11.68 10.92 7.45 5.47 5.44 4.75 3.70 34.58 23.85 16.51 12.32 4.10 2.38

0.57 33.98 19.74 9.24 12.62 10.50 7.16 75.34 69.06 56.60 54.16 54.64 43.06 46.06 33.97 26.72 95.03 87.36 81.16 75.05 53.09 35.60

0.52 20.24 12.66 2.93 8.12 4.53 3.41 41.59 32.71 28.23 20.80 18.00 15.91 17.43 10.11 7.54 46.79 36.15 25.77 21.16 7.38 4.05

99.68 86.56 93.06 96.86 96.34 97.77 97.78 72.38 80.53 88.32 89.08 92.55 94.53 94.56 95.25 96.30 65.42 76.15 83.49 87.68 95.90 97.62

99.43 66.02 80.26 90.76 87.38 89.50 92.84 24.66 30.94 43.40 45.84 45.36 56.94 53.94 66.03 73.28 4.97 12.64 18.84 24.95 46.91 64.40

99.48 79.76 87.34 97.07 91.88 95.47 96.59 58.41 67.29 71.77 79.20 82.00 84.09 82.57 89.89 92.46 53.21 63.85 74.23 78.84 92.62 95.95

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Table 2 e Effects of the H2O/HI molar ratio on the amount, density, composition and HI/H2SO4 molar ratio of the products. Temp. ( C)

I2/HI molar ratio

H2O/HI molar ratio

Volume (mL)

Density (g/mL)

Concentration (mmol/g) I2

25

2

6.17

8

10

12

40

2.3

6.17

8

10

12

60

2.9

6.17

8

10

12

a

H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase

H2SO4 HI

DHI/DH2SO4 in solutiona

H2O

e

0.000 4.906 0.290 26.785

2.09

133.89 5.30

2.69 1.37

1.401 1.023 3.393 6.126 0.000 4.829 0.374 26.605

2.02

147.15 11.12

2.54 1.36

1.121 1.133 3.655 7.607 0.000 4.649 0.543 26.383

1.94

158.91 16.57

2.34 1.40

0.915 1.244 3.884 8.280 0.000 4.457 0.727 26.122

1.79

170.29 3.06

2.24 1.28

0.701 1.390 4.041 9.394 0.000 5.103 0.234 26.107

1.72

134.61 14.33

2.78 1.16

2.214 0.536 2.171 5.998 0.000 4.897 0.345 26.440

1.66

140.60 24.88

2.77 1.28

1.937 0.539 2.384 8.373 0.000 4.759 0.439 26.524

1.60

150.24 35.68

2.65 1.40

1.828 0.578 2.628 7.958 0.000 4.633 0.517 26.660

1.34

155.54 5.56

2.56 1.23

1.739 0.644 2.656 8.662 0.000 5.173 0.176 26.141

1.26

144.70 18.34

3.02 1.28

2.790 0.222 1.406 5.017 0.000 4.957 0.265 26.687

1.10

149.00 30.60

2.81 1.27

2.648 0.135 1.434 7.298 0.000 4.874 0.332 26.659

0.86

156.00 47.01

2.73 1.29

2.554 0.174 1.511 7.858 0.000 4.773 0.373 26.917

0.70

160.15

2.64

2.497 0.195 1.557 8.229

0.43

DHI/DH2SO4 is the ratio of the changes in the moles of HI and H2SO4 in the solution before and after the reaction.

increased. Interestingly, the H2O/H2SO4 molar ratio in the H2SO4 phase, which corresponds to the extent of hydration of H2SO4 in the H2SO4 phase solution, was maintained between approximately 5 and 6. Thus, each mole of H2SO4 in the HIx phase should be contact with ca. 5e6 mol of water to move into the H2SO4 phase. On the other hand, the formation of solid yellow sulfur, which indicates the occurrence of a side reaction, was observed under the conditions with excess water (a H2O/HI molar in the ratio range of 10e12). The degree of the side reaction was evaluated from the ratio of the changes in the moles of HI and H2SO4 in the solution before and after the reaction (DHI/DH2SO4). When the Bunsen reaction occurs without side reactions, the DHI/ DH2SO4 molar ratio in the solution is expected to be 2 on the basis of Eq. (1). However, the DHI/DH2SO4 molar ratio in the solution is less than 2 due to the greater decrease in the HI content when side reactions occur such as Eqs. (4), (5) and (6). As shown in Table 2, the molar ratio of DHI/DH2SO4 inversely correlated with the increased water content in the feed: the DHI/DH2SO4 molar ratios were 1.79, 1.34 and 0.70 at 25, 40 and 60  C,

respectively, when the H2O/HI molar ratio in the feed was 12. The extent of sulfur formation in the reaction system visible to the naked eye also increased as the DHI/DH2SO4 molar ratio decreased. Therefore, increasing the H2O/HI feed molar ratio for the Bunsen reaction enhanced the occurrence of side reactions (mainly sulfur formation). These side reactions are a serious problem for the continuous operation of the integrated SI process. Thus, a method was required to increase the volume of the H2SO4 phase solution while minimizing the side reactions. Extra water was added to the product obtained from the Bunsen reaction at 40  C and an HI/I2/H2O molar ratio of 1/2.3/6.17. The solution was stirred for 30 min to allow sufficient mixing after the addition of water. The amount, density and composition of each phase as well as the DHI/DH2SO4 molar ratio in the solution obtained from each of these experiments are listed in Table 3. As shown in Table 3, the amount of the H2SO4 phase solution increased as the H2O/HI molar ratio increased. Sulfur was hardly observed under these experimental conditions, and the DHI/DH2SO4 molar ratios in solution were very similar

700

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 9 ( 2 0 1 4 ) 6 9 2 e7 0 1

Table 3 e The amount, density, composition and HI/H2SO4 molar ratio of the products after the addition of water (40  C and the I2/HI molar ratio of 2.3). H2O/HI molar ratio

8 10 12

a

Volume (mL)

H2SO4 phase HIx phase H2SO4 phase HIx phase H2SO4 phase HIx phase

3.06 134.61 7.76 143.44 10.53 155.77

Density (g/mL)

1.34 2.59 1.35 2.58 1.35 2.36

Concentration (mmol/g) I2

H2SO4

HI

H2O

0.000 2.209 0.000 2.240 0.000 2.092

4.858 0.493 4.485 0.442 4.108 0.447

0.397 2.238 0.583 2.083 0.739 2.090

26.287 5.823 26.997 6.757 27.939 8.771

DHI/DH2SO4 in solutiona

1.78 1.64 1.70

DHI/DH2SO4 is the ratio of the changes in the moles of HI and H2SO4 in the solution before and after the reaction.

before and after the addition of extra water. We concluded that this method of adding water to the products of the Bunsen reaction favors the separation of products because the volume of the H2SO4 phase solution increases while the side reactions are minimized. In general, it is necessary to add excess water to facilitate phase separation by moving the large amount of H2SO4 in the HIx phase to the H2SO4 phase. The H2O/H2SO4 molar ratio that implies the extent of hydration of H2SO4 in the H2SO4 phase solution was between approximately 5 and 6. On the other hand, it was difficult to determine the exact extent of hydration of HIx complexes in the HIx phase (H2O/HI molar ratio) due to the large amount of H2SO4 that remained in the HIx phase. Thus, the phase separation data obtained from the simple mixing experiment of a quaternary H2SO4/HI/I2/H2O system was reconsidered to deduce the extent of hydration of HIx complexes. It was confirmed that the H2O/HI molar ratio for the hydration of HIx complexes was approximately 5 in the HIx phase with little or no H2SO4 from the previous studies [11,12]. Maatouk et al. [13] and Spadoni et al. [24] reported that the HI and HI3* (HþI 3 ion pair) compounds predominate when the I2/HI molar ratio is less than 1 in the HIx phase while the HI3* and HI5* (HþI 5 ion pair) coexist when the I2/HI molar ratio is greater than 1 in the HIx phase. On the basis of these results, a

conceptual illustration of the separation behavior of the products by the addition of water is depicted in Fig. 8. Fig. 8(a) describes the products obtained from the Bunsen reaction that uses the HIx solution. The H2SO4 phase consists of H2SO4 and H2O, while the HIx phase consists of HIx, H2SO4 and H2O. The H2SO4 in the HIx phase could not move to the H2SO4 phase due to the water deficiency when most of the water combined with the HIx complexes in the HIx phase (Fig. 8(a)). When a sufficient amount of water was added, the H2SO4 component could transfer to the H2SO4 phase due to the decrease in density (approximately 1.20 g/mL) by combining with the additional supplied water (Fig. 8(b)). Moving the considerable amounts of the H2SO4 and water into the H2SO4 phase after the mixing process increased the volume of that phase (Fig. 8(c)). Therefore, we concluded that the use of excess water is inevitable in terms of the phase separation of products in the Bunsen reaction section, although the use of excess water imposed a high thermal burden on the SI process.

4.

Conclusions

The effects of the temperature and the iodine and water contents on the phase separation characteristics of the

Fig. 8 e Conceptual illustration of the phase separation behavior of the products obtained from the Bunsen reaction.

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 9 ( 2 0 1 4 ) 6 9 2 e7 0 1

Bunsen reaction using the HIx solution were experimentally investigated. As the temperature increased, the region of phase separation for the Bunsen reaction broadened, and the phase separation performance improved with the increasing volume of the H2SO4 phase solution. Meanwhile, increasing the iodine feed concentration somewhat decreased the volume of the H2SO4 phase solution. When excess water was present in the reaction system, the Bunsen reaction occurred easily, and extra water was required to separate the products. However, sulfur formation was observed when the H2O/HI feed molar ratios greatly increased above 8, indicating the occurrence of side reactions. When extra water was added to the products of the Bunsen reaction, the side reactions were greatly inhibited, while the volume of the H2SO4 phase solution increased. The amount of water required to move H2SO4 in HIx phase into the H2SO4 phase was discussed relative to the extent of hydration of the H2SO4 and HIx complexes. The H2O/H2SO4 molar ratio for hydration of H2SO4 in the H2SO4 phase solution was between approximately 5 and 6 on the basis of experimental results. The H2O/HI molar ratio for the extent of hydration of HIx complexes was deduced to be approximately 5. Therefore, we conclude that the additional supply of water in the product separation step is necessary to improve the phase separation performance in the Bunsen reaction section using the HIx solution.

[9]

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Acknowledgments This research was performed for the Nuclear Hydrogen Technology Development Project and funded by POSCO through the Ministry of Education, Science and Technology of Korea.

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