(258f) Use of Membranes and Reactive Distillation for the Separation of Hix in the Sulphur-Iodine Cycle

One of the most promising routes currently under development for sustainable hydrogen production is the Sulphur-Iodine thermochemical water splitting cycle. Iodine, sulphur dioxide and water react according to the Bunsen reaction forming a two-phase solution; aqueous sulphuric acid (the light phase) and HIx, a mixture of HI, water and iodine (the heavy phase). A key step in determining the overall efficiency, and indeed eventual feasibility, of the cycle is the separation of HIx into its constituent parts. The hydrogen iodide can then be broken down into hydrogen, the product, and iodine, which is recycled. Inherent problems are the azeotrope formed by HIx solution, 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 work in Japan has focussed on using electro-electrodialysis combined with a hydrogen permselective membrane reactor. This paper investigates the scope for improving the global process efficiency by the application of membrane separations to the HIx processing section. As part of the EC funded HYTHEC project, simulations have been carried out using ProSim Plus. These model the reactive distillation column in combination with a membrane separation unit. Heat integration is studied with and without a heat pump. Several flowsheet configurations are investigated; with a membrane separation unit on the column feed, on a side stage from the column or at the column reboiler. Pervaporation and membrane distillation are considered as methods of dewatering the HIx solution, along with the possible contributions of selective molecular transport both through the membrane and through any associated 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. The benefits to be gained from using a membrane separation unit are clearly demonstrated by these simulations. The work has been funded under Framework VI of the European Communities.