(148e) Dehydration of Hydriodic Acid and Sulfuric Acid by Pervaporation

The ability to remove water from acidic solutions has numerous potential industrial applications. A particular application of interest to INL has been the development of acid concentration processes to enable the Sulfur-Iodine (S-I) thermochemical cycle for hydrogen production. A thermochemical cycle is simply a chemical process for splitting water in which only oxygen and hydrogen are the only effluents. The S-I cycle is a variant based on concurrent iodine and sulfur oxidation and reduction cycles. Pervaporation is a membrane separation technique that uses solubility selectivity to remove one component and reject another. 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. Common applications for pervaporation are in the removal of water from organic streams, such as ethanol. For the application of acid concentration, other factors must be considered; first and foremost is the chemical stability of the membrane. Thus, durable materials must be selected. Recent efforts at the INL have focused on applying pervaporation of water through Nafion-117 and Nafion-112 membranes for the removal of water from HI/water and HI/Iodine/water, sulfuric acid/water feedstreams. In this work, separations have been performed at temperatures as high as 134 degrees Celsius. Transmembrane fluxes of water are commercially competitive (~5000 g/m2h) and separation factors have been measured as high as 8000, depending on the membrane and the water content. In these 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 with no significant loss in separation factor indicating that the permeability of iodine and HI through Nafion materials can be low depending on experimental conditions. Data for the sulfuric acid concentration suggests performance similar to the HI experiments. All membranes studied for the HI, HI/iodine and sulfuric acid feeds exhibited no degradation in membrane performance during use. Particular attention in the paper will be paid to the performance dependence on the pressure gradient across the membrane.