Thermochemical water splitting is being developed as a potentially energy-efficient hydrogen production process. Of the proposed large-scale hydrogen production processes, the Sulfur-Iodine (SI) thermochemial cycle shows potential to be one of the more cost-effective. In this thermochemical cycle, iodine and sulfur dioxide react with water to form hydrogen iodide and sulfuric acid. The resulting mixture readily splits into two immiscible liquid phases, allowing for effective separation of the acids. Sulfuric acid is then decomposed at high temperatures (~850°C) releasing sulfur dioxide, which is recycled, and oxygen. The hydrogen iodide is similarly decomposed at about 350°C to produce hydrogen and iodine, which is recycled.

The main purpose of this work is to study ways to optimize the third section in this cycle, the concentration and decomposition of hydrogen iodide. This section limits the overall efficiency of the SI thermochemical cycle due to certain conditions present during operation. First, the H₂O-HI system exhibits an azeotrope, which is a bottleneck for their separation. Second, the H₂O-HI-I₂ system has two separate regions of liquid immiscibility, which makes the thermodynamic modeling of this system an intriguing task. Third, the H₂O-I₂ system has a strong immiscibility in the liquid phase and the mechanism of the HI decomposition reaction in the vapor phase is uncertain.

De-bottlenecking of this section is suggested to be achieved through three possible separation options. The first option is using extractive distillation [1] which introduces phosphoric acid to the system to induce the separation of iodine allowing for simple distillation of HI. HI is then decomposed at about 450°C to produce H₂. Separation is then facilitated by membranes to produce pure hydrogen gas. There are distinct advantages to this system, as it simplifies the model being considered, but the disadvantages are severe. By adding phosphoric acid the efficiency of the system drops significantly making the process costly and unattractive. Moreover, the thermodynamic modeling of the multi-electrolyte HI, H₃PO₄ solution is quite complicated. The second separation option being considered is electrodialysis [2,3] which should be able to concentrate the HI_x mixture beyond its azeotropic point more easily allowing for the decomposition and extraction steps. Excess HI is removed by distillation, and the steps for decomposition and extraction are the same as in extractive distillation. This process has several advantages, as it allows for an increase in efficiency of the above process, but the disadvantages are similar to the extractive distillation process. The third separation option being examined is reactive distillation [4], where both HI_x distillation and HI decomposition occur simultaneously at 350°C. A liquid-gas equilibrium occurs in the middle of the column, I₂ is solubilized in the liquid phase, and H₂ is recovered from the top of the column. This process has a distinct disadvantage in that the column has a very large feed stream and, therefore, very high heat demand. Moreover, it requires a complex chemistry-thermodynamic model, a problem augmented by the limited experimental data for the HI_x mixture. Its advantages are that it would provide a cost-effective, efficient method to perform the limiting process in this cycle.

A great deal of problems in the study of the advantages and disadvantages of each process have to do with the high degree of uncertainty regarding the thermodynamics of the HI-H₂O-I₂ mixture and the kinetics of the HI decomposition. A detailed, accurate and consistent thermodynamic model, verified with the majority of experimental data available [5], is required for a reliable study. In the past, problems have been identified with the thermodynamic models being used for the extreme conditions of the SI thermochemical cycle [6]. Using the concepts of hydration and polyiodide ions formation a new thermodynamic model has been developed to address the specific needs of this section. The model was implemented in the aspenONE simulation software and detailed flowsheets for the HI section were examined. Efficiency and feasibility studies were performed on the reactive distillation option, considered the most promising of the separation processes examined.

References

1. Norman JH, Besenbruch GE, Brown LC, O'Keefe DR, Allen CL. Thermochemical water-splitting cycle. Bench-Scale Investigations and Process Engineering. 1982;Final Report - General Atomics.

2. Arifal, Hwang G-J, Onuki K. Electro-electrodialysis of hydriodic acid using the cation exchange membrane cross-linked by accelerated electron radiation. Journal of Membrane Science. 2002;210:39-44.

3. Hwang G-J, Onuki K. Simulation study on the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water-splitting IS process. Journal of Membrane Science. 2001;194:207-215.

4. Roth M, Knoche KF. Thermochemical water splitting through direct Hi-decomposition from H₂O/HI/I₂ solutions. International Journal of Hydrogen Energy. 1989;14:545-549.

5. Doizi D, Dauvois V, Roujou JL, Delanne V, Fauvet P, Larousse B, Hercher O, Carles P, Moulin C, Hartmann JM. Total and partial pressure measurements for the sulphur-iodine thermochemical cycle. International Journal of Hydrogen Energy. 2007;32:1183-1191.

6. Bollas GM, Kazimi MS, Barton PI. Detailed Modeling Of The Thermodynamics Of The Sulfur-Iodine Thermochemical Cycle. AIChE Annual Meeting.2007; Salt Lake City, Utah, USA.