Electro-electrodialysis for concentration of hydroiodic acid

Aqueous solution of iodine and hydrogen iodide (HI) was successfully concentrated beyond its azeotropic composition using electro-electrodialysis (EED). This process can be used as an alternative to azeotropic distillation for concentration of HIx solution in the HI-decomposition section of thermochemical Iodine-Sulfur (IS) process for production of hydrogen. EED was carried out in a two-compartment cell with graphite electrodes and nafion-117 membrane was used as a separator between the two compartments. Experiments were performed at three different temperature values of 293 K, 313 K and 333 K. For each temperature, variation of the cell voltage and concentrations of I-2 and HI in the two compartments with time was measured. HI concentration in catholyte increased to a value just above the azeotropic value while that in anolyte was found to decrease. Current efficiency of the cell decreased from 85.1% at 293 K to 82.5% at 333 K. Energy required for EED operation was calculated in terms of heat equivalent of electric energy consumed per mole of H-2 and was found to change by about 11.6% with change in temperature; it was lowest (210.4 kJ/mol-H-2) at 293 K and highest (235.0 kJ/mol-H-2) at 333 K. Iodine was also found to deposit on the membrane resulting in change of color of the membrane to reddish brown; however the deposition was reversible and iodine could be easily removed from the membrane. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.