Phase equilibrium measurements for the I2–H2O and I2–HI–H2O systems of the sulfur–iodine cycle using a continuous-flow apparatus

Liquid–liquid equilibrium (LLE) phase behavior was investigated for the binary iodine–water (I₂–H₂O) and the ternary iodine–hydroiodic acid–water (I₂–HI–H₂O) at the elevated temperatures and pressures of interest for the reactive distillation column of the Sulfur–Iodine Cycle. A continuous-flow apparatus, with wetted parts fabricated from tantalum-tungsten alloys, was designed and constructed for the highly corrosive conditions of this work. A central feature of the apparatus is the equilibrium view cell, which allows for the observation and discernment of vapor-liquid, liquid-liquid, and liquid–liquid–vapor equilibria for HIx systems.     

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H₂O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H₂O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO₂ lowered as the initial HI content and the temperature increased.     

Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H₂O to I₂ in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H₂, with a Section energy requirement of 367 kJ/mol H₂, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.     

SO3 decomposition over CuO–CeO2 based catalysts in the sulfur–iodine cycle for hydrogen production

The effect of several catalyst supports with large specific surface area (such as SiC, Al₂O₃, SiC–Al₂O₃–ball, and SiC–Al₂O₃) on catalytic activity was evaluated in this study. CuO–CeO₂ supported on SiC–Al₂O₃ exhibited high stability and activity, which was considerably close to the thermodynamic equilibrium curve at 625 °C during the stability test for 50 h. The SO₃ decomposition temperature decreased from 750 °C to 625 °C. SiC–Al₂O₃contained numerous micropores and mesopores and had a large specific area, indicating strong adsorption, as determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and nitrogen adsorption measurement. X-ray photoelectron spectroscopy (XPS) revealed that the surface of SiC–Al₂O₃consisted of Al₂O₃, SiC, and SiO₂ and that the cerium oxide surface had the largest number of defects. Temperature-programmed reduction (H₂-TPR) results indicated that the cerium–copper oxides on the surface of powdered SiC–Al₂O₃ had the strongest redox potential and that CuO had the lowest reduction temperature.     

Activity and stability of Pt catalysts supported on carbon nanotubes,active carbon and γ-Al2O3 for HI decomposition in iodine–sulfur thermochemical cycle

Pt catalysts supported on carbon nanotubes (CNT), activated carbon and γ-Al₂O₃ were prepared by the electroless plating method. For comparison, the CNT supported Pt was also prepared by the traditional impregnation–reduction method. The physical properties, structure, morphology and Pt loadings of the different catalysts were characterized by BET, XRD, TEM and ICP, respectively. The catalytic activity for HI decomposition was investigated in a fixed bed reactor under atmospheric pressure. The results of XRD and the activity evaluation indicated that the Pt/CNT prepared by the electroless plating method had better catalytic performance than that prepared by the impregnation–reduction method. Among the three kinds of supported Pt catalysts by the electroless plating method, the CNT supported Pt catalyst not only showed the highest activity for HI decomposition, but also had the best stability in specific surface area, structure and morphology.     

Start-up behaviors of a H2SO4–H2O distillation column for the 50 NL H2/H sulfur–iodine cycle

The sulfur–iodine (SI) cycle to produce hydrogen from water requires a multistage distillation column to concentrate a sulfuric acid solution. To design a concentration process of a sulfuric acid solution that can be applied to the cycle, its static and dynamic simulation is essentially demanded. A 50 NL H₂/h scale SI test facility to be operated under a pressurized environment has been constructed in Korea. This study focuses on the sulfuric acid multi-stage distillation column (SAMDC-50L) for the 50 NL H₂/h SI test facility. The SAMDC-50L was designed and installed in 2012. Based on the design specifications and operation method, a start-up behavior of the SAMDC-50L has been analyzed using the simulation code “KAERI-DySCo”. As a result of the start-up dynamic simulation, it is confirmed that the SAMDC-50L will approach to the steady state value within 30,000 s to fulfill the hydrogen production rate of 50 NL H₂/h. On the other hand, it is expected that the operation time approaching a steady state decreases with an increase in the set point of the condenser temperature until a dew point of the top vapor product and the time required for the transition to the complete steady state is increased with an increasing reflux ratio and reboiler hold-up.     

Experimental study of Ni/CeO2 catalytic properties and performance for hydrogen production in sulfur–iodine cycle

The Ni/CeO₂ catalysts with different calcination temperatures have been tested for hydrogen production in sulfur–iodine (SI or IS) cycle. TG-FTIR, BET, XRD, HRTEM and TPR were performed for catalyst characterization. It was found that the Ni²⁺ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO₂ and the generation of oxygen vacancies. Perfect crystallites were formed in the catalysts. It was evident that there was a change in particle size and morphology as the calcination temperature increased from 300 to 900 °C. The Ni/CeO₂ catalysts with different calcination temperatures showed better catalytic activity by comparison with blank yield, especially Ni/Ce700. A hypothetic mechanism of HI catalytic decomposition on Ni/CeO₂ has been constructed. The two important reactive sites were assumed for HI catalytic decomposition.     

Effects of the second metals on the active carbon supported Pt catalysts for HI decomposition in the iodine–sulfur cycle

The active carbon supported monometallic Pt and Pt-based bimetallic catalysts (including Pt–Ni, Pt–Pd and Pt–Rh) were prepared by the impregnation-reduction method. Their catalytic activities were compared for HI decomposition in a fixed bed reactor. The fresh and used monometallic Pt and Pt-based bimetallic catalysts were characterized by BET, XRD and TEM in order to investigate their changes in surface area, structure, and morphology, respectively. The results showed that the Pt-based bimetallic catalysts had better activity and higher stability than the monometallic Pt catalyst in HI decomposition.     

Preparation and characterization of bimetallic Ni–Ir/C catalysts for HI decomposition in the thermochemical water-splitting iodine–sulfur process for hydrogen production

A series of bimetallic 10%Ni-xIr/C (x = 0.5, 1.0, 1.5, 2.0 wt%) and monometallic 10%Ni/C and 2%Ir/C catalysts were prepared through the impregnation–reduction method modified by adding the ionic surfactant hexadecyltrimethylammonium bromide (CTAB) as the stabilizing agent during the impregnation. Their catalytic performance was tested by HI decomposition under atmospheric pressure at 400 °C and 500 °C. X-ray diffraction, Brunauer–Emmett–Teller surface area, transmission electron microscopy, and X-ray photoelectron spectroscopy were adopted to characterize the structure, specific surface area, morphology, and surface chemical state, respectively. Results showed that the addition of Ir metal and the use of CTAB played important roles in enhancing the activity and stability of the Ni-based bimetallic catalysts. Among all the catalysts tested, the bimetallic 10%Ni-1.5%Ir/C catalyst presented excellent activity and stability for HI decomposition.     

Process model-free analysis for thermodynamic efficiencies of sulfur–iodine processes for thermochemical water decomposition

Material, energy, and entropy balances, which depend only on stream conditions and flows entering and leaving a system, have been used to evaluate different scenarios for thermochemical decomposition of water to manufacture hydrogen using the Sulfur–Iodine cycle. Energy efficiencies have been found for idealized systems with variable stream amounts, as well as for a common flowsheet, to locate the greatest effects on energy requirements and inefficiencies. Aspen Plus^®, OLI Engine, and ProSimPlus property models have been used on the sections of a General Atomics process to reveal the effects of differences in computed energies and entropy generation. While the calculated efficiencies are generally consistent with those of the literature, differences in stream properties and phase behaviors suggest that optimal process configurations from simulations may have significant uncertainties.     

Effects of the composition on the active carbon supported Pd–Pt bimetallic catalysts for HI decomposition in the iodine–sulfur cycle

A series of binary Pd–Pt catalysts supported on active carbon were prepared by the co-impregnation and reduction method. For comparison, active carbon supported monometallic Pt and Pd catalysts were also prepared by the impregnation–reduction method. Their structure, morphology and surface area were investigated by means of X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface area, respectively. Their catalytic activities were evaluated for the decomposition of hydrogen iodide (HI). Furthermore, their thermal stabilities were also investigated. The results of activity tests showed that the composition of Pd–Pt binary catalysts played the important role in dictating the catalyst activity. Among the Pt, Pd and binary Pd–Pt catalysts, the 2.5%Pd–2.5%Pt/C showed the best catalytic performance for the decomposition of HI. The results of thermal stability tests showed that the binary Pd–Pt catalyst had the higher stability than the monometallic Pt and Pd catalysts.     

Fabrication,permeation, and corrosion stability measurements of silica membranes for HI decomposition in the thermochemical iodine–sulfur process

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.     

Preliminary process analysis for the closed cycle operation of the iodine–sulfur thermochemical hydrogen production process

In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H₂SO₄ phase and HI_x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I₂ molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I₂ solution from the HI_x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I₂ solution from the distillation column decreased the flow rate of I₂ solution fed to the distillation column from the separator. The variation of I₂ molar fraction in the H₂SO₄ phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I₂ solubility, 2-liquid phase disappearance and SO₂ reaction amount.     

Activated carbon catalysts for the production of hydrogen via the sulfur–iodine thermochemical water splitting cycle

Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydrogen iodide (HI) to produce hydrogen, a key reaction in the sulfur–iodine (S–I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. Within the group of lignocellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a lignocellulosic chemically activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites, as determined by Boehm's titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.     

Survey of Bunsen reaction routes to improve the sulfur–iodine thermochemical water-splitting cycle

A large excess of water and iodine is typically employed in the Bunsen reaction step of the sulfur–iodine thermochemical cycle in order to induce liquid–liquid phase separation of the two acid products. This paper presents an overview of some alternative routes for carrying out the Bunsen reaction. The use of a reaction solvent other than water is first discussed, and experimental results obtained with tributylphosphate are presented. Another approach is separation of the product acids by selective precipitation of insoluble salts, and the addition of lead sulfate as the precipitating agent is discussed in detail. Finally, the electrochemical Bunsen reaction route is investigated. All of these methods have the potential to reduce the iodine and/or water requirement of the sulfur–iodine cycle.     

Sulfur trioxide electrolysis studies: Implications for the sulfur–iodine thermochemical cycle for hydrogen production

In this paper we describe our efforts to develop a sulfur trioxide (SO₃) electrolyzer that could lower the temperature of the SO₃ decomposition step in the sulfur–iodine and hybrid sulfur thermochemical cycles. The objective is to develop an alternative to the standard process of converting SO₃ to SO₂, which is thermal decomposition at 830 °C and above. Thermodynamic calculations show that high SO₃ conversions can be obtained at 590 °C if oxygen is removed during the SO₃ decomposition stage. One way of achieving oxygen removal during SO₃ decomposition is electrolysis, if suitable electrode and electrolyte materials can be found. Active oxygen electrode materials are already developed and we have demonstrated suitability of a thin doped-zirconia electrolyte in this study. The main difficulty came in the development of an active and stable SO₃ electrode. Using Ga–V–O/NbB₂/Au electrodes we demonstrated high catalytic activity, but could not achieve acceptable electrochemical performance.     

Experimental vapour–liquid equilibrium data of HI–H2O–I2 mixtures for hydrogen production by Sulphur–Iodine thermochemical cycle

The Sulphur–Iodine thermochemical cycle for hydrogen production has been investigated by ENEA in the framework of the Italian TEPSI Project whose main objective is the realization of an integrated loop plant at a laboratory scale. For the design of the separation–purification equipments, the study of vapour–liquid equilibrium characterization of the ternary HI–H₂O–I₂ system is considered a key factor. The aim of the present work is to provide new experimental isobaric vapour–liquid equilibrium data for this system by ebulliometry varying both temperature and solution composition. The temperature range has been extended up to about 144 °C, the iodine concentration range from 0.2%w/w to 90%w/w while HI weight fraction varies from 4%w/w to 67%w/w in the liquid phase. Most of the data obtained in this work are in good agreement with other experimental data retrieved from literature, which have been recorded in similar operative conditions but acquired by different procedures.