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CEA ASSESSMENT OF THE SULPHUR-IODINE CYCLE FOR HYDROGEN PRODUCTION
It is worth noting that there may remain a degree of freedom for further optimisation of the
section: the amount of heat that is provided to the reactive distillation boiler. For instance, if this heat
is reduced to 181 kJ/mol, the electricity demand increases only to 82 kJ/mol, leading to an overall
improvement in the section efficiency. However, this improvement in energy consumption leads to an
unfavourable decrease in the purity of the iodine which is returned to the Bunsen section. Due to the
lack of adequate thermodynamic models, it is difficult to precisely assess the effect of this reduced
purity, but this option was not selected by CEA for its evaluation.
As mentioned above, these calculations are based on a modified Neumann model, the accuracy
of which accuracy was shown to be not very good at relatively high HI concentrations (Doizi, 2007).
In parallel to the flow sheet evaluation presented above, CEA therefore undertook several actions to
help improve the accuracy of the modelling of the iodine section:
• Liquid-vapour equilibrium properties were studied both around atmospheric pressure and in
close to process conditions (Doizi, 2009).
• The enthalpy of mixing of iodine with HI/H 2 O mixtures was measured (Comte, 2009).
• A new thermodynamic model was built (Hadj-Kali, 2009a).
Future work will have to integrate all these elements into a fully consistent new assessment of
the iodine section, but important, albeit preliminary, tendencies can be extracted from experimental
findings:
• At high temperatures and pressures, the iodine partial pressure in the gas phase above low
iodine content liquid HI/I 2 /H 2 O phases (representative of the top of the reactive distillation
column) is higher than predicted by Neumann’s model. Since this partial pressure limits HI
decomposition, the reactive distillation column will be less effective than expected.
• Experiments show a non-negligible enthalpy of mixing of iodine with HI/H 2 O, possibly
amounting to around 50 kJ/mol. This corresponds to extra heat released in the Bunsen section,
but also extra heat to be provided to the iodine section.
It is therefore to be anticipated that the energetic consumption of the iodine section, when
evaluated with a thermodynamic model taking into account recent experimental information, would
be higher than the figures given above.
On the other hand, at least two new paths for future research are opened by the recently
obtained results:
• The new thermodynamic model, with coefficients fitted to take into account the latest results
from liquid-vapour equilibrium measurements, does, as expected, lead to an increase of the
overall requirements of the iodine section. It also shows that a lower operating pressure
maybe more beneficial than the reference 50 bar, a trend which is not observed with the
Neumann model and should attract further studies.
• In some liquid–vapour equilibrium experiments with low iodine contents (Larousse, 2009), an
unexpected decrease of the pressure with time, especially at elevated temperatures, is observed
(Figure 3). A possible, though not proven, explanation would be HI decomposition with
condensation of the resulting I 2 in the liquid phase, just like what is anticipated to take place
in the reactive distillation column. However, the observed kinetics are about 10 5 faster than
what is expected from gas phase composition, which could be an indication of liquid phase
decomposition.
Overall cycle efficiency
The energy requirements of the three sections (using the modified Neumann model for the iodine
section) are gathered in Table 1. Using a heat to electricity conversion factor of 50%, they correspond
to a cycle thermal efficiency of 39.3%.
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