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PREDICTING THE ENERGY EFFICIENCY OF A RECUPERATIVE BAYONET DECOMPOSITION REACTOR
Predicting the energy efficiency of a recuperative bayonet decomposition
reactor for sulphur-based thermochemical hydrogen production*
Maximilian B. Gorensek, Tommy B. Edwards
Computational Sciences Directorate, Savannah River National Laboratory
Aiken, SC, USA
Abstract
High-temperature decomposition of sulphuric acid is a major step for sulphur-based thermochemical
cycles such as hybrid sulphur and sulphur-iodine. It is generally also the most energy-intensive, so
that the overall heat requirement for this step can determine whether a particular decomposition
reactor or flow sheet design is practical.
A recuperative bayonet sulphuric acid decomposition reactor has been designed by researchers at
Sandia National Laboratories (SNL) that features all-silicon carbide (SiC) construction for the heated
parts, can be made from readily available SiC shapes, makes the most use of heat recuperation, and
has all of its fluid connections at sufficiently low temperatures that conventional seal materials can be
used. Bench-scale experiments using electric resistance heaters as the energy source have verified that
the design functions as intended.
The purpose of this work was to apply a pinch analysis to the SNL concept, using a statistically
designed set of computer model experiments to establish the limiting performance (heating target) of
the reactor as a function of operating conditions. Mapping out the limiting performance helped ascertain
the practical operating range, allowing the reactor to be integrated into conceptual flow sheets. Seven
input variables were considered: temperature (48 to 150°C), pressure (10 to 90 bar), and acid
concentration (30 to 90 wt.%) of the feed; process fluid temperatures at the inlet (600 to 740°C) and the
outlet (750 to 900°C) of the catalyst bed; temperature approach to equilibrium of the decomposition
reaction in the catalyst bed (-25 to 0°C); and minimum temperature difference for recuperative heat
transfer (10 to 100°C). Of particular interest was the effect of temperature, which will be limited by
the operating temperature of the nuclear reactor heat source.
A space-filling Latin Hypercube Design was used to generate model control variable input sets that
were spread throughout factor space. Statistical tools were used to develop models from the results of
the simulation “experiments” and to suggest additional simulation points to explore regions of interest.
Over 130 different sets of control variables were eventually simulated.
Results indicate that practical operation (high temperature heat target < 400 kJ/mol SO 2 ) requires acid
feed concentrations in excess of 65 wt.% H 2 SO 4 . The optimum (lowest achievable high temperature
heat target of 321 kJ/mol SO 2 ) occurs at roughly 80 wt.% H 2 SO 4 at the highest pressure (90 bar) and
highest peak process temperature (900°C). As the peak process temperature is decreased from 900°C,
the heating target increases. Below peak process temperatures of about 700°C, heating targets well in
excess of 400 kJ/mol SO 2 are predicted. This means that HTGR operating below 800°C reactor outlet
temperature will not allow efficient bayonet operation. Feed and catalyst bed inlet temperatures had
no effect on the heat target. Not surprisingly, a zero value of the decomposition reaction temperature
approach to equilibrium and 10°C minimum temperature difference for recuperative heat transfer was
found to be best.
* The full paper being unavailable at the time of publication, only the abstract is included.
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