Assessment of Extent of Crystallization of CuCl2 (From Quaternary System of CuCl, CuCl2, HCl, and Water) and Its Limiting Conditions in Crystallization Step of Thermochemical Cu–Cl Cycle

Cu–Cl thermochemical cycle is a promising process for generation of hydrogen without emission of greenhouse gases. It involves four steps viz electrolysis, crystallization, hydrolysis, and thermolysis to achieve overall splitting of water into hydrogen and oxygen in a closed loop. In the electrolysis step of the cycle, electrolyzer produces hydrogen gas and its effluent (spent electrolyzer solution) consists of quaternary system comprising CuCl2, CuCl, HCl, and water. The composition of the electrolyzer effluent may vary based on electrolyzer operating conditions. This effluent is fed to crystallizer to separate CuCl2 crystals which are utilized in hydrolysis step, while the remaining liquid is recycled back as part of the feed to the electrolyzer. Thus, crystallization step is essential for sustained cyclic operation. Owing to narrow range of conditions under which this crystallization occurs, every electrolyzer effluent may not yield crystals on cooling. This makes investigations on the extent of crystallization and its limiting conditions for processing electrolyzer effluent of varying composition important. Experiments are conducted for different electrolytic conversion, acidity, crystallization temperature, and residence time considering 2 M CuCl as the electrolyzer feed. Few experiments have also been done for different feed concentration of CuCl keeping conversion same. The experiments have been conducted with simulated electrolyzer effluent. The crystallization is found to produce needle-shaped bright green colored crystals of different sizes with varying yield based on experimental conditions. XRD analysis of product samples show CuCl2/CuCl2.2H2O crystals. This study throws light on crystallization operation for varying electrolyzer effluent compositions and operating conditions of interest.