HTGR Hydrogen and Heat Application Research

6-4 Investigation of the Process Conditions of Inhibition Reactions for Thermal-Efficiency Improvement
ᴪ Effects of Impurity Contamination in an HI Concentrator ᴪ

H2SO4 decomp.

Bunsen18 m

HI decomp.

H2

H2O

O2Decomposer
Decomposer

Reactor

220 mmI (Iodine)
Cell stack ofelectoro-electoro
dialysis

Distillation
columnS (Sulfur)
Separator

EED

H2 Production Test Facility

W18.5 m × D5 m × H8.1 m

Purifiers

Hydrogen iodine(HI) Bunsen
decomposition
reactionsectionsectionpurification process by heating solution should be
required to remove H2SO4 prior to EED

Fig.6-7 External view of the hydrogenproduction test facility
We constructed a hydrogen-production test
facility with all process components composedof industrial structural material in order toverify the integrity of the components anddemonstrate continuous hydrogen-production
ability. An electoro-electoro dialysis device(EED) was employed to concentrate the HIsolution.

H2SO4
decomposition
section

Effect of sulfuric acid on electro-electro dialysis of Hlx solution
Exp. conditions

CathodeBunsen
reactorPurifiers

Cation-exchange
membrane

ConcentratedHI solution

EED
AnodeSolution returns to
Bunsen reactor

Bunsen reactor
produces HIsolution which
includes small
amount of H 2SO4

HI in HI-I2-H 2O solution is concentratedusing selective H+ permeation through aion exchange membrane by oxidationreduction reactions

Fig.6-8 Production and concentration of HI Solution
The HI solution produced by the Bunsen reaction is purified toprevent harmful side reactions in HI concentration using theEED device. The purification process for the one-sided EEDchannel can be eliminated by adapting the proposed method.

We have pursued R&D on a thermochemical hydrogenproduction iodine–sulfur process as a heat-utilization application
for the high-temperature gas-cooled reactor. This chemical
process uses chemical compounds of iodine and sulfur tosplit water to produce hydrogen by combining three chemical
reactions. This process, which can harness heat for nuclear orrenewable energy, is a promising next-generation hydrogenproduction method that is independent of fossil fuels and thatcan provide energy security. One important task is to improve
the thermal efﬁciency of hydrogen production.
Reduction of the heat for driving processes is an effective
method of improving thermal efficiency. We focused on apurification operation for processing the Bunsen-reaction
solution. Bunsen reaction, which is core chemical reaction inthe iodine–sulfur process, produces two types of acids in HIand H2SO4-rich solutions from H2O, I2, and SO2; the two acids
separate into upper and lower phases with a clear boundary set
by the liquid–liquid phase-separation phenomenon. The lower
phase, which is rich in HI and I2, includes an impurity of H2SO4;
this contamination probably causes HI concentration (an EEDwith a cation-exchange membrane, Fig.6-7) and HI distillation
to occur as a harmful side reaction with the production of solid
sulfur. Because of mutual solubility between the two phases,a certain amount of H2SO4 is solved into the lower phase; aCase A(Purificationof both sides)

Catholyte: no H2SO4Anolyte:

no H2SO4

Case B(Purificationcatholyte)

Catholyte: no H2SO4

Case C(No purification)

Catholyte: with H2SO4Anolyte:

Anolyte:with H2SO4

with H2SO4Exp. results: transportnumber ofprotons

Visual inspection

1.0(No change)

1.0(No change)

0.7

Sulfurdeposition

Fig.6-9 View of cation-exchange membrane after HIconcentration experiments using the EED device
No contamination by H2SO4 in the bath sides (Case A); onesided contamination by H2SO4 in anolyte (Case B); no sulfur
is evidenced. Contamination by H2SO4 in catholyte; sulfur isproduced (Case C).

purification process by heating of the solution is necessary toremove H2SO4. This puriﬁcation causes an increase of heat input
because of the heating requirement for solution vaporization.
While optimizations of the operating temperature for thepurification process have been studied, we proposed a flowrate-reduction method of the processed solution itself to reduce
the required heat (Fig.6-8), based on the fact that H2SO4 turns
into sulfur in the reduction environment. The solution after the
puriﬁcation should be concentrated by EED; the solution is fed
to both channels of the EED device in the previous ﬂowsheet;
H2SO4 in the anolyte of the oxidation environment is therefore
probably stable. If the method is workable, the required heatcan be reduced by reducing the processing solution for the
puriﬁcation; moreover, the ﬂowsheet can be simpliﬁed.
To validate the proposed method, we conducted concentration
experiments of the HI solution including H 2SO 4 using anEED device. Results met our expectations; the sulfur was notproduced in the anolyte; moreover, the contamination in theanolyte was not affected by the cell voltage or the membraneselectivity (Fig.6-9). We revealed that our proposal can savethe input heat required for purification. An increase of thehydrogen-production thermal efﬁciency is approximately 10%of that of the previous method.

ReferenceTanaka, N., Kubo, S. et al., Effect of Sulfuric Acid on Electro-Electrodialysis of HIx Solution, International Journal of Hydrogen Energy, vol.39, issue 1,
2014, p.86-89.

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JAEA R&D Review 2016-17