The Separation of Hix in the Sulphur-Iodine Thermochemical Cycle for Sustainable Hydrogen Production

One of the main routes currently under development for sustainable hydrogen production is the Sulphur-Iodine thermochemical water splitting cycle based on the Bunsen reaction. A key step in determining the overall efficiency, and indeed eventual feasibility, of the cycle is the separation of a mixture of hydrogen iodide, iodine and water (HIx) into its constituent parts. The hydrogen iodide is broken down to form hydrogen, the product, and iodine which is recycled. Inherent problems are the azeotrope formed by the HIx mixture, the lack of appropriate thermodynamic data and the difficulty in decomposing HI. Early work in the US added H3PO4 or HBr to break the azeotrope, whilst current work in Japan is focussed on using electro-electrodialysis combined with a hydrogen permselective membrane reactor. This paper investigates the scope for improving the process efficiency using membranes and the possible contributions of selective molecular transport both through the membrane and through any associated static gas layer. The choice of membrane is a key issue, both with respect to selectivity but also the relatively extreme operating conditions imposed by the cycle and the chemical and physical properties of the HIx mixture. Simulations have been carried out using ProSimPlus to demonstrate the benefits which could be gained from using a membrane separation in the SI process.