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Process Flowsheet Analysis of Hydrogen Iodide Decomposition in the Sulfur-Iodine Cycle

Robert T. Buckingham, Lloyd Brown, Benjamin E. Russ, Gottfried Besenbruch, and Wendi Sweet. General Atomics, 3550 General Atomics Ct., San Diego, CA 92121

As oil prices rise, the concept of economical, large-scale production of hydrogen from nuclear energy becomes more attractive. The Sulfur-Iodine (S-I) process for thermochemical water decomposition is one of the most promising methods currently being developed. The S-I cycle consists of three key reactions, one of which is the decomposition of HI into H2 and I2. Reactive distillation is one potential method for effective decomposition of HI. A feed solution consisting of I2, HI, and water is fed to a distillation column, where the I2 is separated from the HI and water. The upper section of the column consists of a packed bed of activated carbon catalyst, and the HI decomposes here in the presence of water into the hydrogen product and I2. The hydrogen is purified and unreacted HI and the iodine are recycled back into the process. A second method, extractive distillation, separates the I2 from HI and H2O by contacting it with concentrated H3PO4. The mix of H3PO4, HI, and water is fed to a distillation column. The H3PO4 breaks an azeotrope between HI and H2O, thus pure HI can be distilled out of the H2O/H3PO4 solution. The HI is then decomposed at approximately 450oC in the presence of a carbon catalyst. The H3PO4 is reconcentrated in a boiler and recycled to the extraction column. Flowsheets for each HI decomposition process were developed using Aspen Plus. Heat exchanger networks were developed for both alternatives and the resultant for thermal efficiencies are compared.