(182e) Development of Separation Processes Based on Membrane Technology for the S-I Thermochemical Cycle

ENEA (the Italian National Agency for New Technology, Energy and the Environment) is involved in an National R&D 3-years program on thermochemical water-splitting cycles to be coupled with solar-thermal power plants. This project is being carried out in the framework of a integrated national program dealing with the sustainable hydrogen production and storage (TEPSI project). A preliminary screening of the most feasible thermochemical processes for hydrogen generation to be coupled with solar concentration plant has been achieved. On the basis of the literature feasibility information, estimation of the overall theoretical loop efficiency, and the maximum temperature, two thermochemical processes were chosen: sulfur-iodine and mixed ferrites cycles. Both processes are currently under investigation to demonstrate the scientific feasibility at laboratory/bench scale. Intensive theoretical and experimental work on the sulfur-iodine cycle is currently aimed to the assessment of the possible options to carry out the operations within the three main sections of the process loop (Bunsen, HI concentration/decomposition, H2SO4 concentration/decomposition). Hence, membrane technology may be used for the following applications: ion exchange membranes for a Bunsen electrochemical reactor, selective membranes for pervaporation operations, gaseous HI or sulfuric acid decomposition reactors, and, finally, HI concentration by electro-electrodialysis. Here we present our experimental results with assessment of membrane distillation routes to improve HI concentration. This route is simple, with low energetic consumption and does not imply the utilization of special materials thanks to the relatively low operating temperature and pressure. HI concentration from water solutions by membrane distillation was examined with a recirculation batch apparatus. Two commercial hydrophobic membranes have been used with two different configurations: direct contact membrane distillation (DCMD) with a polypropylene (PP) capillary membrane in the first case, and an air-gap membrane distillation (AGMD) with polytetrafluoroethylene (PTFE) flat-sheet membrane in the second case. Operative temperature was 60°C for DCMD and 80°C for AGMD; in both cases the operating pressure was about 1 atm. Results showed that the HI concentration in the feed increased from 0.28 up to 7.0 mol/L for DCMD, and from 0.33 up to 8.0 mol/L for AGMD. This latter value is greater then the azeotropic concentration (7.57 mol/L) of the H2O/HI mixture and, consequently, HI further separation from water can be achieved by means of conventional distillation units. It was determined that the higher the HI concentration in the feed mixture, the higher the HI permeation through the membrane: in fact, from 1 L of feed at 7.2 mol/L HI, 0.7 L feed at 8.0 mol/L HI and 0.3 L of permeate at 5.2 mol/L HI concentration were obtained, with an average flux of 7.81 L/h/m2. In the case of the DCMD process, the maximum feed concentration reached was 7 mol/L, i.e. lower than the azeotropic value, and together with permeate fluxes as low as 0.1 L/h /m2. Durability tests of the membrane were also carried out by soaking the membrane in an aqueous at 57%w/w solution HI at 100 °C. In the first tests only H2O/HI solutions have been examined. In the near future the effect of the presence of residual iodine in the feed mixture (used in excess in the Bunsen section in order to optimize phase separation and minimize side reaction occurrence) will be investigated. New membranes and membrane configurations (e.g. vacuum membrane distillation, VMD) are under investigation as well as the energetic process evaluation even at the higher scales. Results obtained will be discussed and compared with the more conventional methods for concentration and purification of the liquid phases produced in the Bunsen reaction.