Search

In this study, a corrosion-stable silica membrane was developed to be used in H2 purification during the hydrogen iodide decomposition (2HI → H2 + I2), which is a new application of the silica membranes. From a practical perspective, the membrane separation length was enlarged up to 400 mm and one end of the membrane tubes was closed to avoid any thermal variation along the membrane length and sealing issues. The silica membranes consisted of a three-layer structure comprising a porous α-Al2O3 ceramic support, an intermediate layer, and a top silica layer. The intermediate layer was composed of γ-Al2O3 or silica, and the top silica layer that is H2 selective was prepared via counter-diffusion chemical vapor deposition of a hexyltrimethoxysilane. To the best of our knowledge, this is the first report of 400-mm-long closed-end silica membranes supported on Si-formed α-Al2O3 tubes produced via chemical vapor deposition method. A 400-mm-long closed-end membrane using a Si-formed α-Al2O3 tube exhibited a higher H2/SF6 selectivity of 1240 but lower H2 permeance of 1.4 × 10−7 mol Pa−1 m−2 s−1 with compared with the membrane using a γ-Al2O3-formed α-Al2O3 tube (907 and 5.6 × 10−7 mol Pa−1 m−2 s−1, respectively). The membrane using the Si-formed α-Al2O3 tube was more stable in corrosive HI gas than a membrane with a γ-Al2O3-formed α-Al2O3 tube after 300 h of stability tests. In conclusion, the developed silica membranes using the Si-formed α-Al2O3 tubes seem suitable for membrane reactors that produce H2 on large scale using HI decomposition in the thermochemical iodine–sulfur process.

The thermochemical water-splitting iodine–sulfur process facilitates hydrogen production. This study proposes a new loop operation by subdividing the process configuration into four sections before transferring the continuous operation. The loop operation should define the section affecting the fluctuations to easily stabilize the system. The proposed loop operation was validated by analyzing the material and heat balances using a process simulator. The calculated results showed that the material balances of the respective loop sections were closed without component discharge to outside sections. The loop operation would transfer to the continuous operation by connecting all sections. Considering the switching of operation modes, the material and heat balance showed no or little difference, indicating that two operation modes can only be changed by switching the pipelines. Thus, the loop sections can be operated individually to stabilize the IS process system, and the loop operation can be transferred smoothly to the continuous operation.

The thermochemical water-splitting iodine–sulfur (IS) process requires corrosion-resistant materials owing to usage of corrosive fluids, such as a mixture of HI–I2–H2O. Fluoropolymers, such as PTFE and PFA, are adaptable as lining materials for protecting plant components. However, there has been a concern: PTFE and PFA have the ability to permeate various permeants. From the viewpoint of corrosion of the base material, the permeation characteristics of HI and I2 should be evaluated to improve the integrity of the IS process. In this study, permeation tests on PTFE and PFA membranes were performed to measure the permeated fluxes of HI and I2, and the effects of the operating conditions on them were investigated. The introduction of a permeability parameter could be successful for normalizing the permeated fluxes for a specific membrane thickness and a vapor pressure. Then, the empirical formula of the permeability was given as an Arrhenius-type equation to use as a plant design. Finally, based on the results, the proper conditions for design of a lining material for the inhibition of HI and I2 permeation are summarized.

An electrochemical membrane Bunsen reaction using a cation exchange membrane (CEM) is a key to achieving iodine-sulfur (IS) thermochemical water splitting for the mass-production of hydrogen. In this study, we prepared a radiation-grafted CEM with a high ion exchange capacity (IEC) and a highly-porous Au-electroplated anode, and then used them for the membrane Bunsen reaction to reduce cell overvoltage. The high ionic content of our CEM led to low resistivity for proton transport, while the high porosity of the electrode led to a large effective surface area for anodic SO2 oxidation. The cell overvoltage for the membrane Bunsen reaction was significantly reduced to 0.21 V at 200 mA/cm2, one-third of that achieved using a commercial CEM and non-porous anode. From the analysis of the current-voltage characteristics, the grafted CEM was demonstrated to play a dominant role in the overvoltage reduction compared to the porous Au anode.

In this study, the catalytic decomposition of hydrogen iodide was theoretically and experimentally investigated in a silica-based ceramic membrane reactor to assess the reactor's suitability for thermochemical hydrogen production. The silica membranes were fabricated by depositing a thin silica layer onto the surface of porous alumina ceramic support tubes via counter-diffusion chemical vapor deposition of hexyltrimethoxysilane. The performance of the silica-based ceramic membrane reactor was evaluated by exploring important operating parameters such as the flow rates of the hydrogen iodide feed and the nitrogen sweep gas. The influence of the flow rates on the hydrogen iodide decomposition conversion was investigated in the lower range of the investigated feed flow rates and in the higher range of the sweep-gas flow rates. The experimental data agreed with the simulation results reasonably well, and both highlighted the possibility of achieving a conversion greater than 0.70 at decomposition temperature of 400 °C. Therefore, the developed silica-based ceramic membrane reactor could enhance the total thermal efficiency of the thermochemical process.

Thermochemical hydrogen production has attracted considerable interest as a clean energy solution to address the challenges of climate change and environmental sustainability. The thermochemical water-splitting iodine-sulfur (IS) process uses heat from nuclear or solar power and thus is a promising next-generation thermochemical hydrogen production method that is independent of fossil fuels and can provide energy security. This paper presents the current state of research and development (R&D) of the IS process based on membrane techniques using solar energy at a medium temperature of 600 °C. Membrane design strategies have the most potential for making the IS process using solar energy highly efficient and economical and are illustrated here in detail. Three aspects of membrane design proposed herein for the IS process have led to a considerable improvement of the total thermal efficiency of the process: membrane reactors, membranes, and reaction catalysts. Experimental studies in the applications of these membrane design techniques to the Bunsen reaction, sulfuric acid decomposition, and hydrogen iodide decomposition are discussed.

The potential of the silica membrane reactors for use in the decomposition of hydrogen iodide (HI) was investigated by simulation with the aim of producing CO2-free hydrogen via the thermochemical water-splitting iodine-sulfur process. Simulation model validation was done using the data derived from an experimental membrane reactor. The simulated results showed good agreement with the experimental findings. The important process parameters determining the performance of the membrane reactor used for HI decomposition, namely, reaction temperature, total pressures on both the feed side and the permeate side, and HI feed flow rate were investigated theoritically by means of a simulation. It was found that the conversion of HI decomposition can be improved by up to four times (80%) or greater than the equilibrium conversion (20%) at 400 °C by employing a membrane reactor equipped with a tubular silica membrane. The features to design the membrane reactor module for HI decomposition of the thermochemical iodine-sulfur process were discussed under a wide range of operation conditions by evaluating the relationship between HI conversion and number of membrane tubes.

The decomposition of hydrogen iodide in the thermochemical water splitting iodine–sulfur process at an intermediate temperature (400 °C) using a catalytic membrane reactor was reported here, for the first time. The performance of a catalytic membrane reactor based on a hexyltrimethoxysilane-derived silica membranes (H2 permeance of 9.4 × 10−7 mol Pa−1 m−2 s−1 and H2/N2 selectivity of over 80.0.) was evaluated at 400 °C by varying the HI flow rates of 2.6, 4.7, 6.9, 8.4, and 9.7 mL min−1. The silica membranes were prepared by counter-diffusion chemical vapor deposition method on γ-alumina-coated α-alumina tubes. Hydrogen was successfully extracted from the membrane reactor using the silica membrane at 400 °C. A significant increase in HI conversion was achieved. The conversion achieved at an HI flow rate of 2.6 mL min−1 was approximately 0.60, which was greater than the equilibrium conversion in HI decomposition (0.22).

Current R&D on the thermochemical water splitting iodine-sulfur (IS) process in Japan Atomic Energy Agency (JAEA) is summarized. Reactors were fabricated with industrial materials and verified by test operations: a Bunsen reactor, a H2SO4 decomposer, and a HI decomposer. Component materials of the reactors were stable in the operation environment. Small amount of H2SO4 in the anolyte solution in an electro-electrodialysis (EED) cell had no negative impact on cell performance parameters. Relationship between cell solution composition and temperature and cell parameters was formulated by experimental data. Demonstration of the test facility with process design of 100 L/h hydrogen production is performed to verify integrity of process components and stability of hydrogen production. Tests of sections were first conducted individually to show material processing rates were controllable. Based on the result, an 8-h continuous operation of the total IS process was performed in February 2016 with H2 production rate of 10 L/h. Demonstrations are planned for longer operation period and higher H2 production rate after improvement of components to prevent troubles.

A high-performance, H2-permselective silica membrane derived from hexyltrimethoxysilane (HTMOS) was developed for application in the thermochemical water-splitting iodine–sulfur process. Silica membranes, referred to here as HTMOS membranes, were prepared via counter-diffusion chemical vapor deposition on γ-alumina-coated α-alumina support tubes with outer diameters of 10 mm. Special attention was devoted to obtain high H2/HI selectivity, high H2 permeance, and good stability in the presence of corrosive HI gas. The effects of the deposition conditions, temperature, and period were investigated. The HTMOS membrane prepared at 450 °C for 5 min exhibited high H2/HI selectivity (>175) with H2 permeance on the order of 10−7 mol Pa−1 m−2 s−1. On the basis of stability experiments, it was found that the HTMOS membrane was stable upon HI exposure at a temperature of 400 °C for 11 h.

The effect of sulfuric acid on the concentration of HIx solution by electro-electrodialysis (EED) was examined for the thermochemical water-splitting iodine–sulfur process. Presence of sulfuric acid in the anolyte HIx solution did not affect the concentration behavior. However, sulfuric acid in the catholyte solution caused side reaction(s) producing whitish precipitates, which indicates that the sulfur compound should be removed prior to the EED operation.

The separation of HI from HI–I2–H2O mixture is an essential subsection of the Iodine–Sulfur (IS) process for thermochemical hydrogen production. The energy requirement of the separation determines, to a large extent, the hydrogen production efficiency of the IS process. In order to examine duty of the separation using electro-electrodialysis (EED) and distillation, a process simulation study was carried out using an analytical model of EED based on ideal membrane properties and properties of the reported EED experiments using a Nafion® membrane and graphite electrodes. For both of the ideal-membrane case and Nafion-membrane case, effects of the operating parameters on heat duty were estimated, which comprised column pressure, HI molality in the column feed, and the flow rate ratio of the input from Bunsen section to distillate rate. Low column pressure, and high HI molality in the column feed were preferable for the ideal-membrane case; column pressure of 1.0 MPa and optimized HI molality in the column feed were desired for the Nafion-membrane case. The flow rate ratio had little effect on the minimum heat duty in the ideal-membrane case; a value in the vicinity of the lower limit of the flow rate ratio was optimal for the Nafion-membrane case. The difference of the inclination of parameters resulted from the fractional vaporization of the column feed in the ideal-membrane case and weight of the EED cell duty on the total duty due to the membrane voltage drop. The optimization of these parameters was also carried out. The minimum total heat duty of the Nafion-membrane case was 3.07 × 105 J/mol-HI, and that of the ideal-membrane case was 12.5% of this value.

Among many thermochemical cycle hydrogen production methods, the iodine sulfur (IS) cycle, originally proposed by General Atomics is expected to be a clean, economical, and sustainable method of large-scale hydrogen production using nuclear heat at high temperature. However, there still exist some technical problems to be solved before realization of industrial hydrogen production using the IS process. One such problem is purification of Bunsen reaction products. The Bunsen reaction products in the IS cycle, H 2 SO 4 and HIx phases, containing minor acids of HI and H 2 SO 4 , respectively, should be purified in order to avoid occurrence of side reactions. In this paper, simulation study on the purification of H 2 SO 4 and HIx phases were carried out at pressure both higher and lower than 1 atm by a chemical process simulator ESP with the thermodynamic database based on the Mixed Solvent Electrolyte model. The results showed that purification of H 2 SO 4 under subatmospheric pressure could increase the conversion of HI, improve the selectivity of SO 2 , and promote I 2 and SO 2 conversion products distributing in the vapor phase. Although purification of HIx phase under subatmospheric pressure could also increase the conversion of H 2 SO 4 , it did not facilitate the occurrence of the reverse Bunsen reaction.

An experimental study of applying electro-electrodialysis (EED) for improved HI concentration in the HIx solution, a mixture of HI–I2–H2O of approximately quasi-azeotropic compositions has been carried out in the conditions of around 90 °C and using Nafion 117 and graphite electrodes. A range of 25–80% increase in initial current efficiency of HI molality in catholyte is measured with the use of EED. In general, the efficiency increases with increasing iodine molality and weight ratio of anolyte solution to catholyte solution. The EED performance degrades in time. In some cases, the HI concentration limits are observed. Electric conductivity of the HIx solution, overvoltage of electrode reaction, and the membrane voltage drop is measured in a temperature range of 20–120 °C. It is found that the EED cell voltage, which is an important cell performance parameter, is governed by the membrane voltage drop.