CaBr_2 hydrolysis for HBr production using a direct sparging contactor

摘要

The calcium-bromine cycle being investigated is a novel continuous hybrid cycle for hydrogen production employing both heat and electricity. Calcium bromide (CaBr_2) hydrolysis generates hydrogen bromide (HBr) which is electrolyzed to produce hydrogen. The CaBr_2 hydrolysis at 1050 K (777 ℃) is endothermic with the heat of reaction δG_T = 181.5 KJ/mol (43.38 kcal/mol) and the Gibbs free energy change is positive at 99.6 kJ/ mol (23.81 kcal/mol). What makes this hydrolysis reaction attractive is both its rate and that well over half the thermodynamic requirements for water-splitting heat of reaction of δG_T = 285.8 KJ/mol (68.32 kcal/mol) are supplied at this stage using heat rather than electricity. Molten-phase calcium bromide reactors may overcome the technical barriers associated with earlier hydrolysis approaches using supported solid-phase calcium bromide studied in the Japanese UT-3 cycle. Before constructing the experiment two design concepts were evaluated using COMSOL™ multi-physics models; 1) the first involved sparging steam into a calcium-bromide melt, while 2) the second considered a "spray-dryer" contactor spraying molten calcium bromide counter-currently to upward-flowing steam. A recent paper describes this work [6]. These studies indicated that sparging steam into a calcium-bromide melt is more feasible than spraying molten calcium bromide droplets into steam. Hence, an experimental sparging hydrolysis reactor using a mullite tube (ID 70 mm) was constructed capable of holding 0.3-0.5 kg (1.5-2.5 × 10~(-3) kg mol) CaBr_2 forming a melt with a maximum 0.08 m (8 cm) depth. Sparging steam at a steam rate of 0.02 mol/mol of CaBr_2 per minute (1.2-2.3 × 10~(-5) kg/s), into this molten bath promptly yielded HBr in a stable operation that converted up to 25% of the calcium bromide. The kinetic constant derived from the experimental data was 2.17 × 10~(-12)kmol s~(-1)m~(-2)MPa~(-1) for the hydrolysis reaction. The conversion rate is highly dependent on melt depth and the design for steam sparging. This experimental data provides a basis for designing a larger-scale sparging hydrolysis reactor for the calcium bromide thermochemical cycle where the endothermic heat of reaction can be effectively supplied by heat transfer coils embedded in the melt.