HYDROGEN PRODUCTION BY THE IODINE-SULPHUR THERMOCHEMICAL CYCLE HIx Vapour Liquid Equilibrium measurements for the sulfur-iodine thermochemical cycle. D. Doizi (ddoizi@cea.fr), B. Larousse, V. Dauvois, J.L. Roujou,, P. Fauvet, P. Carles. Commissariat à l’Energie Atomique, Nuclear Energy Division, Physical Chemistry Department, Saclay, France. J. M. Hartmann, CNRS, LISA, France Outline: Objective and approach. Choice of analytical techniques. Description of the experimental devices. Experimental results. Conclusions. Réunion RCGPH-PROH2 du 12/02/2004

THE IODINE-SULPHUR THERMOCHEMICAL CYCLE H2SO4 Decomposition H2 HI Vaporization O2 Bunsen Reaction SO2 Concentration H20 I2 H2SO4 ↓ H2O + SO2 + ½ O2 [850 °C] 2 HI → H2 + I2 [330 °C] I2 + SO2 + 2 H2O → H2SO4 + 2 HI [120 °C] Réunion RCGPH-PROH2 du 12/02/2004

THE IODINE-SULPHUR THERMOCHEMICAL CYCLE The HIx section: The HIx section is the most critical for the efficiency of the I/S cycle. Three main difficulties must be overcome: the presence of an azeotrope in the mixture HI-H2O, which prevents an efficient distillation of HI, the incomplete and very slow decomposition of HI in H2 and I2, the large heat capacity of the HIx mixture implies very large energy exchanges. Réunion RCGPH-PROH2 du 12/02/2004

Main options for the HIx section H3PO4 I2 HI/(H2O/H3PO4) HI HI/H2/I2 H2O/H2 HI/I2/H2O H2O H2O/H3PO4 Distillate Add third body Recycle HIx mixture from Bunsen section H2 Concentrate HIx Separate H2 Reactive distillation Extractive distillation Electrodialysis Reactive distillation Decompose HI

Objective and approach General objective: The design and optimization of the reactive distillation column of the HIx mixture needs the complete knowledge of the liquid vapour equilibrium (nature and partial pressures of vapour species) of the ternary system HI – I2 – H2O in the temperature range up to 300°C and in the pressure range up to 50 bars. Approach: Progressive methodology to take into account scientific (analytical diagnostics on very concentrated media) and technical difficulties (very corrosive media): two experimental devices. Specifications: Temperature: 20°C < T < 300°C. Total pressure: 0 < P < 50 bars. Liquid phase compositions: 4% < [I2] < 85% (Bunsen exit: 39%), [HI] around the pressure minima:  3% for high [I2];  20% for low [I2]. Réunion RCGPH-PROH2 du 12/02/2004

Initial choice of « online » optical diagnostics: To avoid tedious experiments and prevent any vapour composition changes. HI and H2O: FTIR spectrometry I2: UV-Visible spectrometry HI, I2, H2O and H2: Evaluation of Raman techniques. UV/Visible absorption FTIR spectrometry Raman techniques Réunion RCGPH-PROH2 du 12/02/2004

Experimental devices. The steps leading to the process domain: the low pressure device. Objective: Total and partial pressure measurements around the atmospheric pressure. Experimental device and diagnostics: Glass cell, quartz, teflon in a thermoregulated oven. Total pressure gauge. Reference cells: vacuum/ HI alone / H2O alone or modeling. Free optical pathlengths under argon. FTIR + UV/Visible spectrometries. Operating specifications: up to P = 2 bars; up to T = 130°C (glass). Réunion RCGPH-PROH2 du 12/02/2004

The high pressure device. Experimental devices. The different steps leading to the ternary measurements: The high pressure device. Objective: Total and partial pressure measurements in the process domain. Microautoclave made of Ta equipped with a vapour chamber and placed in a thermoregulated oven. Pressure gauge with a Ta membrane. Specifications: Up to 50 bars, up to 280°C. Use of FTIR and UV Visible spectrometries validated on the low pressure device.

Optical diagnostics on the high pressure device FTIR spectrometry for the measurement of HI and H2O concentrations. 42 mm pathlength, Bruker Tensor 27 Error <15% UV-Visible spectrometry for the measurement of I2 concentration. 1 mm pathlength, Varian Cary 300 Error <15%

Compositions tested in the ternary mixture HI-I2-H2O. Experimental results and comparison with the models: the low pressure device Compositions tested in the ternary mixture HI-I2-H2O. (Total pressures and for most of them, partial pressures). Top of the column Entrance of the column Bottom of the column Réunion RCGPH-PROH2 du 12/02/2004

Experimental results and comparison with the literature: the low pressure device / Ternary system HI – I2 - H2O For various iodine concentrations (4%, 12%, 24%, 39%, 65%, 85%), the total and partial pressures have been measured around 100°C up to 130°C for different HI concentrations. A minimum of four different HI concentrations are prepared for a fixed iodine concentration. This enables us to describe the total pressure curve and find the pressure minima. Total pressures: Comparison with the results of the literature (Knoche, Neumann). Pressure minima position versus [HI] in good agreement with the values of the literature. Réunion RCGPH-PROH2 du 12/02/2004

Total pressures: Partial pressures: Experimental results and comparison with the models: the low pressure device / Ternary system HI – I2 - H2O Total pressures: Good agreement on the left of the azeotrope, At the right of the azeotrope, the pressures measured experimentally are higher than the one calculated with the model. Ex: Comparison with Prophy (Prosim) model for [I2] ~ 39 %. Partial pressures: Good agreement on the left of the azeotrope, At the right of the azeotrope, the [HI] concentration measured experimentally is higher than the one calculated with the model. Réunion RCGPH-PROH2 du 12/02/2004

4 % 12% 39% 65% 14 experiments realized. Experimental results and comparison with the literature: the high pressure device 14 experiments realized. 65% iodine: 3 39% iodine: 5 12% iodine: 3 4% iodine: 3 4 % 12% 39% 65%

I2 HI H2O Good agreement Prophy model vs experience for H2O. Experimental results and comparison with the models: the high pressure device / Ternary system HI – I2 - H2O Results: 63% iodine [HI] concentrations: 8.2 %, 7.2 %, 6.3 % Bottom of the column. I2 HI H2O Good agreement Prophy model vs experience for H2O. Prophy model underestimates HI and I2 concentrations in the vapour phase.

HI H2O I2 Good agreement Prophy model vs experience for HI and H2O. Experimental results and comparison with the models: the high pressure device / Ternary system HI – I2 - H2O Results: 39 % iodine [HI] concentrations: 13.5 %, 11.5 %, 10 % Distillation column entrance. HI H2O I2 Good agreement Prophy model vs experience for HI and H2O. Prophy model underestimates I2 concentration in the vapour phase.

HI H2O I2 Good agreement Prophy model vs experience for HI and H2O. Experimental results and comparison with the models: the high pressure device / Ternary system HI – I2 - H2O Results: 12 % iodine. [HI] concentrations: 19.5 %, 16.1 %, 13.8 % Top of the column. HI H2O I2 Good agreement Prophy model vs experience for HI and H2O. Prophy model underestimates I2 concentration in the vapour phase. H2 formation for high [HI] concentrations.

High [HI] concentration Experimental results and comparison with the models: the high pressure device / Ternary system HI – I2 - H2O High [HI] concentration 215°C plateau Results: 4% iodine Low [HI] concentration Good agreement for H2O and HI. Discrepancy with I2 (overestimated at low temperature and underestimated at high temperature) [HI] concentration decreases vs time. H2 formation by HI dissociation. Decomposition rate: 5 orders of magnitude lower than Pascal literature values.

Comparison of Prophy model with experimental results. Iodine Composition 63% 39% 12% 4% HI underestimated #OK H2O I2 Overestimated low temperature Underestimated high temperature Remarks H2 formation if [HI] high

Conclusions VLE measurements were performed on a large domain of ternary HI-I2-H2O compositions representative of the compositions found in a reactive distillation column. Optical diagnostics enable the determination of the gaseous species concentrations without modifying the speciation up to the process domain. Comparison of experimental results with Prophy model show discrepancies especially at high [HI] concentration for low fixed iodine concentration due to hydrogen formation in the vapour phase.. For iodine concentrations close to the total pressure minimum, a good agreement between experimental results and Prophy model is observed. Iodine concentration are generally underestimated. These new VLE data are used to build a new thermodynamic model

Conclusions Publications: Speciation of the gaseous phase of the HI section of the iodine sulphur thermochemical cycle by modeling and inversion of FTIR spectra, J. M. Hartmann and al., Int J Hydrogen Energy (2009);34:162-168.  Experimental study of the vapour-liquid equilibria of HI-I2-H2O ternary mixtures.Part 1: Experimental results around the atmospheric pressure, D. Doizi and al., IJHE (2009, under press). Experimental study of the vapour-liquid equilibria of HI-I2-H2O ternary mixtures. Part 2: Experimental results at high temperature and pressure. B. Larousse and al., IJHE(2009, under press).