Contemporary scenarios for future energy supply include using enormous amounts of hydrogen as an energy carrier for fuel cells and as a reagent for upgrading heavy oils and converting coal to liquid fuels. Many processes have been proposed to decompose water to hydrogen and oxygen via a series of chemical reactions. The Sulfur-Iodine cycle is a continuous, all fluid-phase process that may be the most energy efficient among the many proposed. However, current studies using only process simulation suggest that the energy requirements are unacceptably high. Unfortunately, great uncertainties in the modeling exist, particularly in the section where H₂ is formed from HI and separated from the recycled I₂, due to a lack of reliable data and the difficulties of modeling these complicated solutions.

In aqueous solution, HI, H₂O and I₂ have a number of complexing and ionization reactions. The result is binary and ternary phase behavior where both aqueous binaries show partial immiscibility, but a single liquid phase is found in the ternary. While the pressure is fixed over the LL region, when the single liquid appears with sufficient HI, the pressure first decreases and then increases dramatically over a small range of added HI, with large amounts of HI going to the vapor.

Literature models for this system do not fully capture all of these variations. In particular, the rapid rise in HI partial pressures is not obtained even if the miscibility is described. This paper will describe a new properties model for the HI-I₂-H₂O system using alternative speciation reactions that are consistent with molecular calculations and spectroscopy. This leads to new parameters for the Electrolyte-NRTL model and more reliable energy estimates for the HI decomposition section.