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CaBr 2 HYDROLYSIS FOR HBr PRODUCTION USING A DIRECT SPARGING CONTACTOR
CaBr 2 hydrolysis for HBr production using a direct sparging contactor
Richard D. Doctor, Jianhong Yang, Chandrakant B. Panchal,
Steven A. Lottes, Robert W. Lyczkowski
Argonne National Laboratory, USA
Abstract
We investigated a novel, continuous hybrid cycle for hydrogen production employing both heat and
electricity. Calcium bromide (CaBr 2 ) hydrolysis, which is endothermic, generates hydrogen bromide
(HBr), and this is electrolysed to produce hydrogen. CaBr 2 hydrolysis at 1 050 K is endothermic with a
181.5 KJ/mol heat of reaction and the free energy change is positive at 99.6 kJ/mol. What makes this
hydrolysis reaction attractive is both its rate and the fact that well over half the thermodynamic
requirements for water-splitting free energy of ΔG T = 285.8 KJ/mol are supplied at this stage using
heat rather than electricity. These experiments provide support for a second order hydrolysis reaction
in CaBr 2 forming a complex involving CaBr 2 and CaO and the system appears to be: 3CaBr 2 + H 2 O →
(CaBr 2 ) 2·CaO + 2HBr. This reaction is highly endothermic and the complex also includes some water
of hydration. COMSOL TM multi-physics modelling of sparging steam into a calcium bromide melt
guided the design of an experiment using a mullite tube (ID 70 mm) capable of holding 0.3-0.5 kg
(1.5-2.5 10 –3 kmol) CaBr 2 forming a melt with a maximum 0.08 m depth. Half of the experiments
employed packings. Sparging steam at a steam rate of 0.02-0.04 mol/mol of CaBr 2 per minute into this
molten bath promptly yielded HBr in a stable operation that converted up to 19 mol% of the calcium
bromide. The kinetic constant derived from the experimental data was kinetic constant was
2.17 10 –12 kmol s –1 m –2 MPa -1 for the hydrolysis reaction.
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