Hydrolysis of CuCl2 in the Cu–Cl thermochemical cycle for hydrogen production: Experimental studies using a spray reactor with an ultrasonic atomizer

The Cu–Cl thermochemical cycle is being developed as a hydrogen production method. Prior proof-of-concept experimental work has shown that the chemistry is viable while preliminary modeling has shown that the efficiency and cost of hydrogen production have the potential to meet DOE's targets. However, the mechanisms of CuCl2 hydrolysis, an important step in the Cu–Cl cycle, are not fully understood. Although the stoichiometry of the hydrolysis reaction, 2CuCl2 + H2O ↔ Cu2OCl2 + 2HCl, indicates a necessary steam-to-CuCl2 molar ratio of 0.5, a ratio as high as 23 has been typically required to obtain near 100% conversion of the CuCl2 to the desired products at atmospheric pressure. It is highly desirable to conduct this reaction with less excess steam to improve the process efficiency. Per Le Chatelier's Principle and according to the available equilibrium-based model, the needed amount of steam can be decreased by conducting the hydrolysis reaction at a reduced pressure. In the present work, the experimental setup was modified to allow CuCl2 hydrolysis in the pressure range of 0.4–1 atm. Chemical and XRD analyses of the product compositions revealed the optimal steam-to-CuCl2 molar ratio to be 20–23 at 1 atm pressure. The experiments at 0.4 atm and 0.7 atm showed that it is possible to lower the steam-to-CuCl2 molar ratio to 15, while still obtaining good yields of the desired products. An important effect of running the reaction at reduced pressure is the significant decrease of CuCl concentration in the solid products, which was not predicted by prior modeling. Possible explanations based on kinetics and residence times are suggested.