Liquid-liquid equilibrium for iodine-hydroiodic acid-water mixtures at elevated temperatures and pressures for the sulfur-iodine cycle

An alternative scenario to the overdependence on fossil fuels is the use of thermochemical cycles to split water into hydrogen and oxygen. The Sulfur-Iodine (SI) cycle, developed at General Atomics in the mid 1970s, is a leading candidate of international interest for the centralized production of hydrogen from nuclear and/or solar power. For a comprehensive assessment of the SI cycle, thermodynamic data for I2-HI-H2O mixtures at elevated temperatures and pressures have been identified as a basic research need.;The focus of this study was liquid-liquid equilibrium (LLE) measurements for the above system. To carry out the measurements, a continuous-flow apparatus (CFA) with corrosion-resistant wetted surfaces rated for 350°C and 150 bar was designed and constructed. In the course of the development of this apparatus, tantalum and its tungsten alloys were found to be capable of withstanding the aggressive chemicals and conditions.;Using our CFA, the first observations for the LLE for the binary I 2-H2O above 225°C are presented, with measurements being made to 300°C and 70 bar. Based on these results, we estimate the upper critical solution temperature for this binary to be 310-315°C. For the ternary I2-HI-H2O, phase boundaries for the LLE in the HI-lean region of the ternary diagram were mapped at 160 and 200°C. In addition, equilibrium-tie lines were determined at 160°C. Phase compositions for the water-rich side were accurately determined by titration, but for the I2-rich side, they were estimated from the overall mass balance. Our results indicate that current models are still inadequate for prediction of the LLE phase behavior for this highly nonideal system.