Electro-electrodialysis for concentration of hydroiodic acid

Aqueous solution of iodine and hydrogen iodide (HIx) 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 I2 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 H2 and was found to change by about 11.6% with change in temperature; it was lowest (210.4 kJ/mol-H2) at 293 K and highest (235.0 kJ/mol-H2) 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.