Investigation of 2 liquid phase separation characteristics on the iodine–sulfur thermochemical hydrogen production process

Investigation of 2 liquid phase separation characteristics onthe iodine±sulfur thermochemical hydrogen production

process

M. Sakurai*, H. Nakajima, K. Onuki, S. Shimizu

Nuclear Heat Utilization Engineering Laboratory, Department of Advanced Nuclear Heat Technology, Japan Atomic Energy Research

Institute, 3607, Niibori, Narita-cho, Oarai-machi, Higashi-Ibaraki-gun, Ibaraki 311-1394, Japan

Abstract

Separation characteristics of 2 liquid phase, sulfuric acid phase and poly-hydriodic acid phase, in HI-H2SO4-I2solution of the iodine±sulfur (IS) thermochemical hydrogen production process were measured in the wide operation

temperature range, from 273 to 368 K in order to establish the closed-cycle operation technology and to improvethermal e�ciency of the process. The e�ects of solution temperature and composition of initial solution on theseparation characteristics were investigated. The e�ect of iodine concentration in the solution on the separationcharacteristics was also evaluated. The separation characteristics were found to improve with the increase in iodine

concentration. Iodine concentration at the point where the solution starts to separate and at the point that iodinesaturates, were determined by using measured data. 7 2000 International Association for Hydrogen Energy.Published by Elsevier Science Ltd. All rights reserved.

1. Introduction

Hydrogen is a promising energy carrier for a future

energy system. For example, it is superior to otherenergy carriers for power generation, transportationand storage. Further, hydrogen is a very clean energy

carrier if it is produced from water. Thermochemicalwater decomposition cycle is one of the methods toproduce hydrogen from water, the concept was pro-

posed by Funk et al. [1]. In order to promote thermo-chemical cycle, it is necessary to introduce a primarythermal energy source at around 1273 K. Therefore,

nuclear heat from a high temperature gas-cooled reac-

tor (HTGR) is suitable as thermal energy for a ther-

mochemical cycle. There are some studies on the

thermochemical hydrogen production process by using

HTGR heat source [2,3]. Fig. 1 illustrates the thermo-

chemical hydrogen production system for nuclear heat

utilization. HTGR supplies high temperature thermal

energy in the form of high temperature helium gas to

the thermochemical process through the intermediate

heat exchanger. Then thermochemical process pro-

duces hydrogen by using water as a single source ma-

terial. If the thermochemical process emits waste heat

at a medium temperature of around 673±873 K, other

heat utilization processes can be introduced. This can

improve the overall process e�ciency. The iodine±sul-

fur thermochemical hydrogen production cycle named

the IS process has been studied at the Japan Atomic

International Journal of Hydrogen Energy 25 (2000) 605±611

0360-3199/00/$20.00 7 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

PII: S0360-3199(99 )00078-6

* Corresponding author. Tel.: +81-29-264-8739; fax: +81-

29-264-8741.

E-mail address: [email protected] (M.

Sakurai).

Energy Research Institute (JAERI) [4±6]. This cyclewas originally proposed by General Atomic (GA) Co.[7]. The IS process consists of three chemical reactionsexpressed as follows:

IS PROCESS

xI2 � SO2 � 2H2O � 2HIx� H2SO4 �293±373 K� �1�

2HI � H2 � I2 �473±973 K� �2�

H2SO4 � H2O� SO2 � 0:502 �1073±1173 K�: �3�

The reaction (1) is called Bunsen reaction that pro-

duces two kinds of acid, sulfuric acid (H2SO4) and

hydriodic acid (HI) by using sulfur dioxide (SO2), iod-

ine (I2) and water (H2O) as reactant materials. Reac-

tions (2) and (3) are HI decomposition reaction and

H2SO4 decomposition reaction, respectively. Decompo-

sition products except hydrogen and oxygen are sup-

plied to the Bunsen reaction unit again as reactant

materials. Therefore, by processing these three reac-

tions, water is decomposed to hydrogen and oxygen

and other chemical elements were circulated. The con-

cept of the IS process is described in Fig. 2. The

H2SO4 decomposition reaction, reaction (3), is an

endothermic reaction whose operation temperature is

around 1123 K, therefore, thermal energy from HTGR

Fig. 1. Thermochemical hydrogen production system by using nuclear heat.

Fig. 2. Concept of the IS process.

M. Sakurai et al. / International Journal of Hydrogen Energy 25 (2000) 605±611606

can be brought into this reaction step as a primaryheat source.

Fig. 3 shows the ¯ow sheet of the IS process. Theprocess can be divided into three sections, Bunsenreaction section, H2SO4 decomposition section and HI

decomposition section, respectively. On the conditionof excess iodine concentration, the product of a Bun-sen reaction can be separated into an H2SO4 phase,

the lighter phase, and a poly-hydriodic acid (HIx )phase, the heavier phase, in the form of a liquid phasein the liquid phase separator. Each of the phases,

H2SO4 and HIx, are introduced into the H2SO4 de-composition and HI decomposition sections, respect-ively. The decomposition reaction proceeds after apuri®cation step at each decomposition section. Then,

hydrogen is produced from the HI decomposition reac-tor and oxygen is produced from the H2SO4 decompo-sition reactor, respectively. The rest of the product

from each decomposition section except H2 and O2 arerecycled to the Bunsen reaction section as reactant ma-terials. Therefore, in order to conduct closed-cycle op-

eration of the IS process on the condition of steadystate, the composition of the solution from the liquidphase separator to each decomposition section should

be maintained to be constant. This means that thecomposition of solution in the liquid phase separatoris a very important factor.For industrialization of the IS process, there are

some problems to be solved. Those are the establish-ment of closed-cycle operation technology, the

improvement of process e�ciency and the developmentof materials for the construction of process units. Con-cerning the establishment of closed-cycle operation

technology, it is important to maintain the compo-sition and the ¯ow rate of the process solution to beconstant for the steady state operation. In order to

keep the constant composition and ¯ow rate of theprocess solution, the separation characteristics of sol-ution in the liquid phase separator are necessary to be

clari®ed.Regarding the improvement of the process e�-

ciency, one of the promising methods is operatingthe Bunsen reaction and the 2 liquid phase separ-

ation at high iodine concentration. The separationcharacteristics are supposed to be improved byincreasing the iodine concentration. Since the solubi-

lity of iodine in the solution increases with the tem-perature increase, in order to increase iodineconcentration, the operation temperature of both

Bunsen reaction and the 2 liquid phase separationhas to be increased. Therefore, it is important toknow the separation characteristics in the liquid

phase separator on the condition of high tempera-ture and high iodine concentration.In this paper, an experimental study of measure-

ments of the 2 liquid phase separation characteristics

Fig. 3. Simpli®ed ¯ow sheet of the IS process.

M. Sakurai et al. / International Journal of Hydrogen Energy 25 (2000) 605±611 607

that can be useful for the establishment of closed-cycleoperation technology and the improvement of process

e�ciency of the IS process was reported.

2. Experimental

Fig. 4 shows the set-up for the measurements of 2liquid phase separation characteristics. The solution of

a mixture of HI, H2SO4, I2 and H2O was prepared ina glass vessel with a jacket. The volume of the vesselwas about 100 ml. This prepared solution was stirred

by using magnetic stirrer. The temperature of the sol-ution was maintained to be constant by ¯owing tem-perature-controlled water through the jacket. Afterenough time for mixing the solution passed, the stir-

ring was stopped. Then, the solution was separatedinto 2 liquid phase, as H2SO4 phase (upper phase) andHIx phase (lower phase). Samples were taken for both

phases, respectively. The amount of proton ion (H+),iodine ion (Iÿ) and molecular iodine (I2) in bothsamples was measured by the method of chemical ti-

tration. By using these results of analysis, the concen-tration of H2SO4, HI and I2 in the solution in eachH2SO4 phase and HIx phase were determined.

The following are the experimental conditions.The range of solution temperature: 273, 301, 313,

333, 353 and 368 K. The initial molar ratio of HI,H2SO4 and H2O of the solution: 0.070/0.048/0.882, for

273, 301, 313, 333, 353 and 368 K; 0.085/0.058/0.857,0.049/0.069/0.882, for 313 K.In this study, the ratio of HI to H2SO4 di�ers from

2, the stoichiometric value of a Bunsen reaction pro-duct. In the actual operation condition, the compo-sition of solution in the liquid phase separator is also

not a stoichiometric value [6].All the experimental conditions in this work were

carried out in the range that includes an excess amountof H2O. Therefore, the results of measurements were

expressed in the form of the molar ratios of three com-

ponent elements except H2O, that is, the molar ratios

of HI, H2SO4 and I2.

3. Results and discussion

3.1. Separation characteristics of 2 liquid phase

Figs. 5 and 6 describe the results of measurementsof the separation characteristics on various solution

temperature conditions. The vertical axis and the hori-zontal axis in these ®gures show the molar fraction ofI2 and HI to three components, HI, H2SO4 and I2 in

Fig. 5. E�ect of solution temperature on the separation

characteristics (273, 301 and 313 K).


characteristics (333, 353 and 368 K).

Fig. 4. Experimental set-up for separation characteristics

measurements.


the solution, respectively. Fig. 5 shows the temperaturerange from 273 to 313 K and Fig. 6 shows that from

333 to 368 K. The raw material solutions, shown asthe plots on the straight line, separate into H2SO4

phases, shown as the plots on the curve on the lower

side of the graph, and HIx phase, shown as the plotson the curve on the upper side of the graph. Thegraphs indicate that the increase in molar fraction of

I2 in the raw material solution causes a higher fractionof I2 and a lower one of HI and H2SO4 in the HIxphase at all temperature conditions. It also causes

lower concentrations of HI and I2 in the H2SO4 phaseat all temperature conditions. In other words, as the I2concentration in the raw material solution increases inthe region of the 2 liquid phase separation state, the

separation characteristic of the 2 liquid phaseimproves. Each curving line, on which the compositionof each phase follows, is almost the same in spite of

the di�erence of solution temperatures. The fact thatthe iodine concentration of the raw material solutionat the point that the solution starts to separate into 2

liquid phase increased a little bit with increase in sol-ution temperature is discussed later. Fig. 7 shows thee�ect of the molar ratios of HI, H2SO4 and H2O of

the raw material solution on the separation character-istics. The composition of each phase, H2SO4 phaseand HIx phase, also follows almost same curving lineon the graph from the di�erent initial solution compo-

sitions. From these results, the shape of the curvinglines of both the H2SO4 and HIx phases are found notto be a�ected by the solution temperature and the

composition of the initial solution. This suggests thatit can be possible to establish a simple operation tocontrol each solution composition in the H2SO4 and

HIx phases to be stable against the variation of overall

composition or temperature of the solution coming tothe separator.

3.2. Iodine fraction to maintain 2 liquid phase separation

The iodine fraction of the initial solution at thepoint that 2 liquid phase separation starts wasmeasured experimentally at each solution temperature

by adding iodine to the solution of the state of 1 liquidphase. The I2 fraction of the initial solution for thecondition of I2 saturation at each solution temperature

was also determined by the following graphical con-sideration. In the region of the state of 2 liquid phaseseparation, the three points of each composition, the

raw material solution, the H2SO4 and HIx phase sol-utions are on the straight line, that is the tie-line. How-ever, when iodine concentration in the raw material

solution exceeds a certain value, the compositions ofseparated H2SO4 and HIx phases stop changingbecause this region exceeds the iodine saturation pointof the solution. The excess iodine exists in the solution

as the state of solid phase, therefore, this solid iodineis supposed not to a�ect the separation characteristics.According to this consideration, the point of iodine

saturation can be determined from the point of inter-section of the line that connects the best separatedpoints of each H2SO4 and HIx phase compositions

and the line which the compositions of the initial sol-ution follow. Fig. 8 shows the e�ect of solution tem-perature on the I2 fraction for the point thatseparation starts (fI2-sep.) and that I2 saturates (fI2-

sat.) determined by using the procedure previouslymentioned. It was found that both fI2-sep. and fI2-sat.increase with the increase in the solution temperature.

In the case of this solution mixture system, the mutualsolubility of the H2SO4 and HIx phase solutions issupposed to increase with the increase in the solution

Fig. 8. E�ect of solution temperature on the I2 molar fraction

at the points where 2 phase separation starts (fI2-sep.) and I2saturates (fI2-sat.).

Fig. 7. E�ect of initial solution composition on the separation

characteristics at 313 K.


temperature. Therefore, the necessary amount of iodineincluded in the solution for 2 liquid phase separation is

considered to increase with the increase in solutiontemperature. For this reason, the value of fI2-sep.becomes higher with the temperature increase. In the

case of fI2-sat., the solubility of iodine in the solutionincreases also with the increase in the solution tem-perature, therefore, fI2-sat. increases with the tempera-

ture increase. The relationship between fI2-sep. and fI2-sat. values and the solution temperature, T, wereexpressed by Eqs. (4) and (5), respectively. For fI2-sep.,

the values were approximated by the linear equation asshown in Eq. (4). For fI2-sat., the values were approxi-mated by the quadratic equation as shown in Eq. (5).

For fI2-sep.:

fI2-sep: � 6:448� 10ÿ4 � T� 0:149 �4�For fI2-sat.:

fI2-sat: � 2:528� 10ÿ5 � T 2 ÿ 0:0128� T� 2:048 �5�In Fig. 8, in the lower region below the fI2-sep. line,the solution does not separate into 2 liquid phase, andin the upper region of the fI2-sat. line, iodine solidi®es

in the solution. Therefore, the continuous closed-cycleoperation can not be carried out within these tworegions. The composition of the solution must be con-

trolled so as not to get into these two regions. Byusing Eqs. (4) and (5), the iodine concentration forcontinuous closed-cycle operation at a certain solutiontemperature of T, fI2-op. (T ), is expressed as follows in

Eq. (6):

2:528� 10ÿ5 � T 2 ÿ 0:0128� T� 2:048

> fI2-op:�T � > 6:448� 10ÿ4 � T� 0:149: �6�

3.3. The separation characteristics at the I2 saturationpoint

The separation characteristics at the point of I2 sat-uration were described for each solution temperature

in Fig. 9. As shown in this ®gure, it was found thatthe separation characteristics became better with theincrease in the solution temperature. In other words,

an increase in the I2 saturation concentration com-bined with an increase in the solution temperature canimprove separation characteristics. In order to clarify

this point more quantitatively, the amount of impurityin each of the phases of HI in H2SO4 and H2SO4 inHIx on the I2 saturation condition for each tempera-ture was calculated. Fig. 10 shows the relationship

between the amount of impurity in both the H2SO4

and HIx phases and the I2 concentration in the sol-ution under the I2 saturation condition. It was found

that the amount of impurity decreased drastically withthe increase in solution temperature because theamount of dissolved I2 increased. For example, the

molar ratio of H2SO4 to HI in HIx phase decreasedfrom 0.103 at 273 K to 0.025 at 368 K. In the case ofthe H2SO4 phase, the ratio of HI to H2SO4 decreased

from 0.097 at 273 K to 0.018 at 368 K. From thisevaluation, the possible improvement of separationcharacteristics with an increase in I2 concentration isclari®ed quantitatively. Namely, comparing the impuri-

ties at 273 K, and 368 K was reduced to about one-fourth for the HIx phase and about one-®fth forH2SO4 phase, respectively. In the continuous operation

of the IS process, it is necessary for the puri®cation

Fig. 10. E�ect of I2 concentration in the initial solution on

the amount of impurities in the HIx and H2SO4 phases under

I2 saturation condition.


characteristics under I2 saturation condition.


step to be installed before the decomposition steps ofH2SO4 and HI because impurities in H2SO4 and HI

cause side reactions and corrosion. If the amount ofimpurity can be reduced by increasing the temperatureand the iodine concentration of the product solution

of the Bunsen reaction, this has a great advantage inreducing many of the puri®cation steps for the H2SO4

and HI decomposition sections. In future, the operat-

ing condition of the Bunsen reaction section isassumed to be brought close to that which GA Co.proposed [7]. That is the condition where the solution

mixture includes higher concentrations of both H2SO4

and HI acids. Therefore, at the next step, 2 liquidphase separation characteristics of the aqueous sol-ution mixtures of HI, H2SO4 and I2 that include higher

concentration of H2SO4 and HI acid solutions have tobe measured and investigated. As for the operation ofthe IS process on the condition of high temperature

and high iodine concentration in the Bunsen reactionsection, special attention should be paid to prevent theiodine from solidifying in the solution. Although the

iodine solidi®cation is a serious problem for continu-ous transportation of the process solution, the resultsof this study suggest that increasing iodine concen-

tration in the 2 liquid phase separator by increasingsolution temperature is one of the e�ective methodsfor closed-cycle operation at high process e�ciency.

4. Conclusions

In conclusion, the separation characteristics of the 2

liquid, H2SO4 and HIx phases, from the mixture ofHI, H2SO4 and I2 aqueous solution are summarized asfollows.

1. The separation characteristics are improved with theincrease in iodine concentration of the initial sol-ution in the temperature range of all experimental

conditions.2. Each composition of the H2SO4 and HIx phases is

not a�ected by solution temperature and compo-sition of the raw material solution, that is, each

almost follows the curving line for the H2SO4 andHIx phases, respectively, even if it is separated at a

di�erent solution temperature and from a di�erentraw material composition of the solution.

3. The I2 concentration at both points where 2 liquidphase separation starts and I2 saturates increasewith the increase in solution temperature.

4. On the condition of I2 saturation, separationcharacteristics can be improved by increasing thesolution temperature, because solubility of iodine

increases with the increase in the solution tempera-ture. The amount of impurity at 368 K could bereduced to 25% of that at 273 K for the HIx phase

and 20% of that at 273 K for H2SO4 phase.

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Documents

Investigation of 2 liquid phase separation characteristics on the iodine–sulfur thermochemical hydrogen production process