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CEA ASSESSMENT OF THE SULPHUR-IODINE CYCLE FOR HYDROGEN PRODUCTION
Figure 1: Counter-current Bunsen reactor schematics
I 2
H 2 SO 4
H 2 SO 4
(H 2 SO 4 + 4 H 2 O)
9 I 2 + SO 2 + 16 H 2 O → +
(2 HI + 10 H 2 O + 8 I 2 )
HI
SO 2
H 2 O
HI
can therefore be seen as a reactive liquid-liquid extractor in which water is the solvent of H 2 SO 4 and
iodine the solvent of HI. Provided pure enough iodine is returned from the iodine section, it can
be used to counter-currently purify the sulphuric acid phase by extraction of HI. Furthermore, in
counter-current streams, circulating phases are never at chemical equilibrium, theoretically resulting
in a possible increase in reaction rate by modifying the reactants amounts (particularly SO 2 ).
CEA’s flow sheet calculations (Leybros, 2009) indicate no heat requirements in Bunsen section
(the Bunsen reaction is actually quite exothermal), and a small electricity requirement of 4 kJ/mol for
SO 2 /O 2 separation through compression. Due to the lack of adequate thermodynamic models, the effect
of the presence of impurities in the acid phases is not taken into account. However, CEA, together
with the University of Toulouse, has undertaken to build a model capable of describing hydroiodic and
sulphuric acid mixtures (Hadj-Kali, 2009b), which will serve as the basis for future evaluations.
Sulphur section
As stated above, it is anticipated that sulphuric acid enters the sulphur section along with four water
molecules. In order to avoid heating these gpit water molecules up to the very high temperatures
(around 850°C) required to decompose SO 3 and release oxygen, the section is split into several
subsections:
• a concentration section, which uses temperature- and pressure-staged flashes to concentrate
sulphuric acid up to a roughly equimolar H 2 SO 4 /H 2 O composition;
• a decomposition section where this mixture is brought to around 850°C in the presence of a
catalyst, leading to SO 2 regeneration and O 2 release;
• a coupling component which recovers undecomposed SO 3 to form H 2 SO 4 and send it back to
the decomposition section.
Heat integration is of course essential in this section, and CEA’s flow sheet (Leybros, 2009) limits
V/HTR secondary helium uses to powering the high temperature part of the section, namely the H 2 SO 4
dehydration and SO 3 decomposition reactors. Low temperature components in charge of sulphuric
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