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CaBr 2 HYDROLYSIS FOR HBr PRODUCTION USING A DIRECT SPARGING CONTACTOR
Introduction
The objective of this study was to demonstrate experimentally that a sparging contactor can be used
for CaBr 2 hydrolysis as the first stage for hydrogen production. The calcium-bromine (Ca-Br) cycle
investigations combine both modelling and experimental studies of a novel continuous “hybrid” cycle
employing both heat and electricity for hydrogen production (Yang, 2008). The interest in engineering
the calcium-bromine cycle for continuous operation is that it permits the use of components and
materials that will operate in a consistent, non-cycling chemical and thermal environment. The
simplified hybrid Ca-Br cycle has two reaction stages and one electrochemical stage, compared with
the four chemical stages in the Japanese UT-3 cycle. At these temperatures, theoretical considerations
would suggest that an integrated water-splitting cycle would require three to four theoretical stages
(Funk, 1966).
“CaBr 2 hydrolysis with HBr formation” (1 020-1 050 K) CaBr 2 + H 2 O → CaO + 2HBr (1)
This reaction is highly endothermic. At a design temperature of 1 050 K, the heat of reaction is
181.5 kJ/mol, and the free energy change is positive at 99.6 kJ/mol. However, analysis of these
experiments provides evidence for a second order hydrolysis reaction in CaBr 2 with the formation of a
complex involving CaBr 2 and CaO. A more accurate representation of the system appears to be:
3CaBr 2 + H 2 O → (CaBr 2 ) 2·CaO + 2HBr (2)
A moisture level of ~1.4 moles H 2 O:CaBr 2 is needed to close and reconcile the material balance
where the calcium bromide initial charge to the melt bath is employed since the final complex is still
uncertain.
“CaBr 2 regeneration with oxygen formation” (850-1 050 K) CaO + Br 2 → CaBr 2 + ½O 2 (3)
“Bromine regeneration – PEM electrochemical” (333 K) 2HBr → Br 2 + H 2 (4)
The equilibrium constant K, based on experimentally measured partial pressure or gas
concentration of H 2 O and HBr (Marchetti, 1973) is two to four orders of magnitude greater than the
thermodynamically calculated equilibrium constant based on the US National Institute of Standards
and Technology (NIST) data. The explanation of this is that the basic chemistry is not adequately
expressed in Eq. (1), but rather is linked to the complex formation shown in Eq. (2). While the
equilibrium constant suggests the difficulty of getting higher conversion, a reactor can be designed to
overcome the kinetic limitations. Therefore, it is important to obtain experimental kinetic data using a
prototype reactor contacting heterogeneous reactant and products.
A classic study of the hydrolysis of calcium, strontium, barium and their common halide salts
(Robinson, 1926) concluded that “bromides in general are more easily hydrolysed than chlorides”, and
it found that running steam over solid CaBr 2 in a ceramic boat for 30 minutes at 963 K (690°C) caused
63% of the calcium bromide (CaBr 2 ) to react to CaO while hydrolysing the CaBr 2 to hydrogen bromide
(HBr.) HBr was recovered in a sodium hydroxide bath and determined by wet-chemistry titration. This
work was later pursued by Euratom (EUR, 1972, 1973, 1974) as the initial stage of the Mark-1 and then
in Japan as the UT-3 cycles. The Euratom report indicated that a significantly greater rate of reaction
is expected for the molten phase reaction, which is the focus of this investigation.
Hydrolysis modelling
Before constructing and experiment, two options were considered for bringing the reactants together
continuously (Lottes, 2008). In the first option, molten CaBr 2 is sprayed with or into a high temperature
steam environment. The CaBr 2 droplets act as heat carriers for the reaction as well as one of the
reactants. In the second option, steam bubbles are sparged through a pool of molten CaBr 2 . Both of
these candidate systems consist of a continuous fluid medium as one reactant and a dispersed fluid
in the form of droplets or bubbles as the other reactant. Modelling of these systems is, therefore,
similar with the roles of the gas and liquid phases reversed.
Given the much greater heat carrying capacity of a pool of molten CaBr 2 , sparging steam into
molten CaBr 2 appeared to be the best candidate for a calcium bromide hydrolysis reactor in a hybrid
thermochemical water-splitting cycle. For a single 2 mm diameter bubble, a negligible amount of CaO
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