Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production

HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion as well as highly corrosive environment. Corrosion-resistant metal membrane reactor is proposed to be a process intensification tool, which can enable efficient HI decomposition by enhancing the equilibrium conversion value. Here we report corrosion resistance studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. Small pieces of the coated tubes were subject to immersion coupon tests in HI-water environment (57 wt% HI in water) at a temperature of 125–130 °C under reflux environment, and simulated HI decomposition environment at 450 °C. The unexposed and exposed cut pieces were analyzed using scanning electron microscope (SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium.     

Numerical simulations of HI decomposition in packed bed membrane reactors

Numerical simulations to develop an understanding of transport processes inside PBMRs (packed bed membrane reactors) and to evaluate effectiveness of PBMRs in increasing the conversion of HI (hydrogen iodide) decomposition reaction of IS (iodine–sulfur) thermochemical cycle are reported. The computational approach used in the simulations has been validated using the data reported for HI decomposition in a packed bed reactor (PBR). The validated computational approach has been used for parametric studies. Effects of different parameters (temperature, pressure, space velocity, membrane permeability, permselectivity, packed bed porosity and reactor diameter) on HI conversion are reported. The parameters having the maximum impact on the conversion are identified. The findings show that using a PBMR instead of a PBR leads to significant enhancement in conversion and the parameters having high impact on conversion are wall temperature, feed temperature, reactor diameter and packed bed porosity. Based on the findings of parametric studies, ranges of the parameters having maximum impact on conversion are suggested, e.g. the reactor wall temperature is recommended to be in the range of 690–700 K, the bed porosity is recommended to be in the range of 0.2–0.4.     

Synthesis,characterization and corrosion performance evaluation of DDR membrane for H2 separation from HI decomposition reaction

An all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure has been developed for separation of H₂ from HI/I₂ mixture of HI decomposition reaction. In this work, the DDR zeolite membrane was synthesized on the seeded clay-alumina substrate within 5 days. The seeds were synthesized by sonication mediated hydrothermal process within short crystallization time which enhanced the nucleation for the membrane growth. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDAX). The selectivity of hydrogen with respect to CO₂ and Ar was evaluated by single gas permeation studies at room temperature. The tests for corrosion resistance were carried out upto 120 h with both support and DDR membrane at 130 °C which confirmed the stability of membrane under the harsh HI/I₂ environment.     

Sulphonated (PVDF-co-HFP)-graphene oxide composite polymer electrolyte membrane for HI decomposition by electrolysis in thermochemical iodine-sulphur cycle for hydrogen production

In overall iodine-sulphur (I-S) cycle (Bunsen reaction), HI decomposition is a serious challenge for improvement in H₂ production efficiency. Herein, we are reporting an electrochemical process for HI decomposition and simultaneous H₂ and I₂ production. Commercial Nafion 117 membrane has been generally utilized as a separator, which also showed huge water transport (electro-osmosis), and deterioration in conductivity due to dehydration. We report sulphonated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) (SCP) and sulphonated graphene oxide (SGO) composite stable and efficient polymer electrolyte membrane (PEM) for HI electrolysis and H₂ production. Different SCP/SGO composite PEMs were prepared and extensively characterized for water content, ion-exchange capacity (IEC), conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with commercial Nafion117 membrane. Most suitable optimized SCP/SGO-30 composite PEM exhibited 6.78 × 10^?2 S cm^?1 conductivity in comparison with 9.60 × 10^?2 S cm^?1 for Nafion® 117. The electro-osmotic flux ofSCP/SGO-30 composite PEM (2.53 × 10^?4 cm s^?1) was also comparatively lower than Nafion® 117 membrane (2.75 × 10^?4 cm s^?1). For HI electrolysis experiments, SCP/SGO-30 composite PEM showed good performance such as 93.4% current efficiency (η), and 0.043 kWh/mol-H₂ power consumption (Ψ). Further, intelligent architecture of SCP/SGO composite PEM, in which hydrophilic SGO was introduced between fluorinated polymer by strong hydrogen bonding, high efficiency and performance make them suitable candidate for electrochemical HI decomposition, and other diversified electrochemical processes.     

Pt/zirconia catalyst for hydrogen generation from HI decomposition reaction of S–I cycle

Hydrogen energy is considered as one of the ideal solutions for the fulfilment of the ever increasing energy demand. It is mainly due to the following two reasons: firstly, it can be produced from a very abundant source, that is, water; and secondly, it does not leave any harmful effect on the environment. Thermochemical cycles are amongst the most promising ways to generate hydrogen from water in an environment‐friendly manner. Sulfur–iodine cycle is one of the most efficient thermochemical cycles. In this paper, we discuss synthesis of Pt/zirconia catalysts for HI decomposition reaction, which is one of the important steps of S–I thermochemical cycle. The catalysts were characterized by X‐ray diffraction, scanning electron microscopy (SEM), field emission gun‐SEM, transmission electron microscopy, N₂ adsorption and H₂ chemisorption. The catalytic activity and stability of these catalysts, for liquid phase decomposition of hydriodic acid was evaluated. Conversion is found to be dependent on the noble metal loading, with 18.7% conversion for 2% Pt/ZrO₂ catalyst as compared with 2.7% of without catalyst, although the specific activity is highest for 0.5% Pt/ZrO₂ catalyst. The catalyst was found to be stable under liquid phase HI decomposition reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Pt/graphite catalyst for hydrogen generation by HI decomposition reaction in S–I thermochemical cycle

Hydrogen is an attractive energy carrier for future because of various reasons. Therefore its large scale production is the need of the hour. One of the ways to achieve this is sulfur iodine thermochemical cycle and HI decomposition reaction is one of the three reactions constituting the cycle. Pt/graphite catalysts with different loading of platinum were prepared by impregnating colloidal graphite with hexachloroplatinic acid solution followed by reduction under N₂ flow. The catalysts prepared have been characterized by X‐ray diffraction, Raman, scanning electron microscopy, X‐ray photoelectron spectroscopy and Brunauer–Emmett–Teller surface area. These catalysts have been employed for liquid phase HI decomposition under different conditions. To evaluate the stability of this catalyst against noble metal leaching under the reaction conditions, the eluent was analyzed by using ICP‐OES. Platinum loaded catalysts (0.5%, 1% and 2%) show 8.4%, 17.5% and 23.4% conversion respectively. From the present study we conclude that Pt/graphite is a suitable and stable catalyst for liquid phase HI decomposition reaction. Copyright © 2015 John Wiley & Sons, Ltd.     

Tube-wall catalytic membrane reactor for hydrogen production by low-temperature ammonia decomposition

Ammonia has attracted great interest as a chemical hydrogen carrier. However, ammonia decomposition is limited kinetically rather than thermodynamically below 400 °C. We developed a tube-wall catalytic membrane reactor that could decompose ammonia with high conversion even at temperatures below 400 °C. The reactor had excellent heat transfer characteristics, and thus nearly 100% conversion for an NH3 feed of 10 mL/min at 375 °C was achieved with a 2-μm-thick palladium composite membrane, and hydrogen removal from the decomposition side resulted in a large kinetic acceleration.     

Counter-current membrane reactor for WGS process: Membrane design

Water gas shift (WGS) is a thermodynamically limited reaction which has to operate at low temperatures, reducing kinetics rate and increasing the amount of catalyst required to reach valuable CO conversions.     

HIx concentration by electro-electrodialysis using stacked cells for thermochemical water-splitting IS process

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, efficient concentration/separation of HI from HIx solution, a mixture of HI–H₂O–I₂, is very important. In this paper, an experimental study on concentrating HI in HIx using stacked electro-electrodialysis (EED) cells was carried out under the conditions of 1atm, 80 °C and the current density of 0.10 A/cm². The performance of EED stacks including 1, 2 and 4 EED units was evaluated. The results showed that multi-unit EED cells could concentrate HI in catholyte much faster than single-unit cells. The apparent transport number (t₊) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H⁺ (β) changed in a relatively larger range of 1.98–2.89. Although the current efficiency will degrade faster when using a multi-unit stack than a single-unit cell at the late stage of EED process, at high iodine content multi-unit stack could maintain quite high current efficiency.     

Characterization of an ammonia decomposition process by means of a multifunctional catalytic membrane reactor

Ammonia decomposition was studied in a multifunctional catalytic membrane reactor filled with Ruthenium catalyst and equipped with palladium-coated membranes. To characterize the system we measured NH₃ conversion, H₂ yield and its partial pressure, the internal and external temperatures of the reactor shell and the electric consumption under several NH₃ flow and pressure conditions. Experimental results showed that the combined effect of Ruthenium catalyst and palladium membranes allowed the reaction to reach the equilibrium in all the conditions we tested. At 450 °C the ammonia conversion resulted the most stationary, while at 7 bar the hydrogen yield was almost independent of both the ammonia flow and temperature. In addition, the experimental system used in this work showed high values of NH₃ conversion and H₂ permeation also without heating the ammonia tank and therefore renouncing to control the feeding gas pressure. When ultra-pure hydrogen is needed at a distal site, a reactor like this can be considered for in situ hydrogen production.     

硫碘制氢中碘化氢分解的化学模拟及实验研究

对热化学硫碘制氢中的碘化氢分解反应进行了化学热力学、动力学模拟以及实验研究,同时利用热力学的方法研究了氢气选择性膜对碘化氢分解率的影响.化学热力学模拟中,500℃下,HI的分解率只达到22.8%.但利用膜分离只移走氢气和同时移走氢气和碘蒸气的情况下,分解率分别增长了30.3%和54.8%.化学动力学模拟中,随温度的升高,Ⅷ浓度降低曲线呈现一定的线形规律,该分解反应对温度的敏感性较高.通过实验结果和动力学模拟结果的对比,该动力学模型能较好地描述HI分解的化学反应历程.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

Preparation of a novel bimodal catalytic membrane reactor and its application to ammonia decomposition for COx-free hydrogen production

A novel bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a silica separation layer was proposed. The catalytic activity of the support was successfully improved due to enhanced Ru dispersion by the increased specific surface area for the γ-Al₂O₃/α-Al₂O₃ bimodal structure. The silica separation layer was prepared via a sol–gel process, showing a H₂ permeance of 2.6 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹, with H₂/NH₃ and H₂/N₂ permeance ratios of 120 and 180 at 500 °C. The BCMR was applied to NH₃ decomposition for CO_x-free hydrogen production. When the reaction was carried out with a NH₃ feed flow rate of 40 ml min⁻¹ at 450 °C and the reaction pressure was increased from 0.1 to 0.3 MPa, NH₃ conversion decreased from 50.8 to 35.5% without H₂ extraction mainly due to the increased H₂ inhibition effect. With H₂ extraction, however, NH₃ conversion increased from 68.8 to 74.4% due to the enhanced driving force for H₂ permeation through the membrane.     

Equilibrium shift of methylcyclohexane dehydrogenation in a thermally stable organosilica membrane reactor for high-purity hydrogen production

A high-performance organosilica membrane was prepared via sol–gel processing for use in methylcyclohexane (MCH) dehydrogenation to produce high-purity hydrogen. The membrane showed a high H₂ permeance of 1.29 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, with extremely high H₂/C₃H₈ and H₂/SF₆ selectivities of 6680 and 48,900, respectively, at 200 °C. The extraction of hydrogen from the membrane reactor led to the MCH conversion higher than the thermodynamic equilibrium, with almost pure hydrogen obtained in the permeate stream without considering the effect of carrier gas and sweep gas in the membrane reactor, and the organosilica membrane reactor was very stable under the reaction conditions employed.     

Membrane distillation of and mixtures for the sulfur–iodine thermochemical process

The concentration of hydriodic and sulfuric acid aqueous solutions by membrane distillation (MD) was experimentally investigated. Two commercial hydrophobic membranes, with two different recirculation batch configurations, were tested: direct contact membrane distillation (DCMD) with a polypropylene (PP) capillary membrane, and air-gap membrane distillation (AGMD) with polytetrafluoroethylene (PTFE) flat-sheet membrane. Feed temperatures were 58 °C for DCMD and 80 °C for AGMD, with cooling water at 15 °C and 1 atm operating pressure in both cases. H₂SO₄ concentration in the feed solution increased from 1.1 up to 7.0 mol/L with the DCMD and from 0.9 up to 10.1 mol/L with the AGMD. HI concentration increased from 0.3 up to 7.0 mol/L with the DCMD and from 0.3 up to 8.0 mol/L with the AGMD. The latter value is higher than the azeotropic concentration of the HI/H₂O (7.57 mol/L) mixture and, hence, HI further separation from water can be easily achieved with conventional distillation units. Durability of PTFE membrane in acid solutions was assessed too.     

3D simulation of hydrogen production by ammonia decomposition in a catalytic membrane reactor

Ammonia decomposition in an integrated Catalytic Membrane Reactor for hydrogen production was studied by numerical simulation. The process is based on anhydrous NH₃ thermal dissociation inside a small size reactor (30 cm³), filled by a Ni/Al₂O₃ catalyst. The reaction is promoted by the presence of seven Pd coated tubular membranes about 203 mm long, with an outer diameter of 1.98 mm, which shift the NH₃ decomposition towards the products by removing hydrogen from the reaction area. The system fluid-dynamics was implemented into a 2D and 3D geometrical model. Ammonia cracking reaction over the Ni/Al₂O₃ catalyst was simulated using the Temkin-Pyzhev equation.Introductory 2D simulations were first carried out for a hypothetic system without membranes. Because of reactor axial symmetry, different operative pressures, temperatures and input flows were evaluated. These introductory results showed an excellent ammonia conversion at 550 °C and 0.2 MPa for an input flow of 1.1 mg/s, with a residual NH₃ of only a few ppm. 3D simulations were then carried out for the system with membranes. Hydrogen adsorption throughout the membranes has been modeled using the Sievert’s law for the dissociative hydrogen flux. Several runs have been carried out at 1 MPa changing the temperature between 500 °C and 600 °C to point out the conditions for which the permeated hydrogen flux is the highest. With temperatures higher than 550 °C we obtained an almost complete ammonia conversion already before the membrane area. The working temperature of 550 °C resulted to be the most suitable for the reactor geometry. A good matching between membrane permeation and ammonia decomposition was obtained for an NH₃ input flow rate of 2.8 mg/s. Ammonia reaction shift due to the presence of H₂ permeable membranes in the reactor significantly fostered the dissociation: for the 550 °C case we obtained a conversion rate improvement of almost 18%.     

Mathematical modeling of hydrogen production using methanol steam reforming in the coupled membrane reactor when the output materials of the reformer section is used as feed for the combustion section

Hydrogen is one of the most abundant elements on Earth's surface. It is not in nature in its pure form, but it can produce by various methods such as methanol steam reforming, partial oxidation, electrolysis, etc. In the present study, using the mass and energy conservation law, hydrogen production in coupled membrane reactors has been modeled using the methanol steam reforming process using two different methods in the coupled membrane reactor. A separate (fresh) methanol is used as feed for the combustion section in the first method. While in the second method, the reformer reactor's output material is used as feed for the combustion section. After simplifying using the required assumptions, the governing equations solved using the ode45 (shooting method) numerical method using MATLAB software. The results of this study show that the conversion of methanol in the coupled membrane reactor when separate methanol is used as feed in the combustion reactor, while in the same reactor, the output material of the reformer section used as feed in the combustion section, is 92% and 88.5% respectively. In this study, the effect of different parameters on the methanol conversion rate is investigated. Finally, it found that with increasing temperature and pressure and decreasing membrane thickness in coupled membrane reactors, methanol conversion rate increases. The percentage of hydrogen recovery in the two coupled membrane reactors is almost equal to 92%.     

Development and test of a solar reactor for decomposition of sulphuric acid in thermochemical hydrogen production

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for hydrogen production by thermal splitting of water. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: firstly, sulphuric acid is decomposed by high temperature heat of 800–1200 °C forming sulphur dioxide, which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS aims at a technical realisation of the HyS. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore, a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb up to 950 °C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in order to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe₂O₃) and mixed oxides (i.e. CuFe₂O₄) were used a conversion of SO₃ to SO₂ of more than 80% at a thermal efficiency of over 25% could be reached.     

Detailed kinetic modeling of homogeneous HI decomposition for hydrogen production—Part II: Effect of I2 on HI decomposition

The kinetic modeling of homogeneous decomposition of hydrogen iodide (HI) and HI/H₂O vapors with the addition of diatomic iodine (I₂) using the mechanism proposed in the companion work (part I) in the sulfur–iodine cycle was investigated in this paper. Thermodynamic results calculated by FactSage and the kinetic experiment verified the applicability of the mechanism. The effect of temperature, residence time, pressure, HI/H₂O/I₂ molar ratio, HI/I₂ molar ratio, and sensitivity analysis on the HI conversion was observed in the modeling process. The addition of small amount of diatomic iodine greatly decreases the HI conversion, and the overall pressure could promote the HI decomposition rate in the kinetic process. Sensitivity analysis shows that hydrogen yield was most sensitive to reactions (4) HI + H = H₂ + I, (1) HI + HI = H₂ + I₂, (5) HI + I = I₂ + H, and (8) HI + OH = H₂O + I. The existence of diatomic iodine increases the reverse reaction of (1) and (5).