Membrane processes for the sulfur-iodine thermochemical cycle

Removal of water from aqueous hydriodic acid (HI) by pervaporation using Nafion-117^® membranes was studied. Specifically, Nafion membranes were used to separate water from HI at concentrations from 18% to 70% and at temperatures ranging from 22 to 109 ^{{ring operator}} C. Initial fluxes of the more dilute acid solutions were as high as 1500 g / m² h with a separation factor (α) of 139. Furthermore, separation factors as high as 500-700 were obtained for more concentrated samples. In general, increased temperatures yielded higher fluxes with lower separation factors and higher feed concentrations gave lower fluxes with higher separation factors. Activation energies of transport (E_J) values were calculated for lower and higher concentration HI feeds and compared to pure water pervaporation. These data suggest that the degree of protonation and mole fraction water in the feed influence transport kinetics and that boundary layer issues become more prevalent at higher HI concentrations.