Advances in acid concentration membrane technology for the sulfur-iodine thermochemical cycle

One of the most promising cycles for the thermochemical generation of hydrogen is the Sulfur-Iodine (S-I) process, where aqueous HI is thermochemically decomposed into H₂ and I₂ at approximately 350 °C. Regeneration of HI is accomplished by the Bunsen reaction (reaction of SO₂, water, and iodine to generate H ₂SO₄ and HI). Furthermore, SO₂ is regenerated from the decomposition of H₂SO₄ at 850 °C yielding the SO₂ as well as O₂. Thus, the cycle actually consists of two concurrent oxidation-reduction loops. As HI is regenerated, co-produced H₂SO₄ must be separated so that each may be decomposed. Current flowsheets employ a large amount (-83 mol% of the entire mixture) of elemental I₂ to cause the HI and the H₂SO₄ to separate into two phases. To aid in the isolation of HI, which is directly decomposed into hydrogen, water and iodine must be removed. Separation of iodine is facilitated by removal of water. Recent efforts at the INL have concentrated on applying pervaporation through Nafion-117, Nafion-112, and sulfonated poly(ether ether ketone) (SPEEK) membranes for the removal of water from HI/water and HI/Iodine/water feedstreams. In pervaporation, a feed is circulated at low pressure across the upstream side of the membrane, while a vacuum is applied downstream. Selected permeants sorb into the membrane, transport through it, and are vaporized from the backside. Thus, a concentration gradient is established, which provides the driving force for transport. In this work, membrane separations have been performed at temperatures as high as 134 °C. Transmembrane fluxes of water are commercially competitive (-5000 g/m ²h) and separation factors have been measured as high as 8000, depending on the membrane and the water content. For the Nafion-117 experiments, the common trade off in membrane performance is observed in that as flux is increased, separation factor decreases. Nafion-112, a thinner membrane, exhibited much higher fluxes than the Nafion-117 without loss in separation factor. All membranes exhibited no degradation in membrane performance during use. Pervaporation was also applied with similar results for the concentration of sulfuric acid. Decomposition of sulfuric acid is equilibrium limited requiring a recycle stream. Concentration by distillation involves serious corrosion issues at the vapor-liquid interface. In this work, Nafion-112® was used successfully as a membrane to remove water from sulfuric acid using pervaporation.