Of the 100+ thermochemical hydrogen cycles that have been proposed, the sulfur-iodine (S-I) cycle is the primary target of international interest for the centralized production of hydrogen from nuclear power. However, this cycle involves complex, highly nonideal phase behavior that is poorly understood, so current performance projections are based on uncertain and incomplete data. Thus, thermodynamic measurements and physical property models for the analysis of the S-I cycle have been identified as a basic research need for the Hydrogen Economy. Only with reliable data, along with carefully constructed property and process models that correctly describe the process, can the true potential of the S-I cycle be assessed.

The current focus of our experimental work is the measurement of phase behavior relevant to the HI decomposition section (i.e., the reactive distillation column) of the S-I cycle, that is, measurements for mixtures of HI, molten iodine, and water at elevated temperatures and pressures. Process simulation of the S-I cycle by our DOE-funded project team has identified the measurement of liquid-liquid equilibrium (LLE) data for the iodine-water system as the highest priority binary for measurement, so initial work is proceeding with this system. Only one study (F. C. Kracek, J. Phys. Chem., 1931, 417-422) has been reported for the iodine-water binary, and those results extend only up to ~200 °C. Our goal is to not only validate this early work, but to extend measurements up to the mixture critical point for the iodine-water system, which is expected to occur in the vicinity of 300 °C.

A continuous-flow apparatus with a windowed equilibrium cell has been designed and constructed to measure equilibrium phase compositions for HI-I₂-H₂O mixtures at temperatures to 350 °C and pressures to 200 bar. Because of the highly corrosive nature of these mixtures, especially at elevated temperatures, the choice of materials for all wetted parts in the continuous-flow setup was an important issue in terms of ensuring both safety and reliability. Details of the construction of this apparatus, materials selection issues, and initial LLE results for the iodine-water binary are to be presented.