Multi-tube tantalum membrane reactor for HIx processing section of IS thermochemical process

HI_x processing section of Iodine-Sulphur (IS) thermochemical cycle dictates the overall efficiency of the cycle, which poses extremely corrosive HI–H₂O–I₂ environment, coupled with a very low equilibrium conversion (~22%) of HI to hydrogen at 450 °C. Here, we report the fabrication, characterization and operation of a 4-tube packed bed catalytic tantalum (Ta) membrane reactor (MR) for enhanced HI decomposition. Gamma coated clay-alumina tubes were used as supports for fabrication of Ta membranes. Clay-alumina base support was fabricated with 92% alumina (~8 μm particle size) and 8% clay (~10 μm particle size), sintered at a temperature of 1400 °C. An intermediate gamma alumina coating was provided with 4% polyvinyl butyral as binder for a 10% solid loading. Composite alumina tubes were coated with thin films of Ta metal of thickness <1 μm using DC magnetron sputter deposition technique. The 4-tube Ta MR assembly was designed and fabricated with integration of Pt-alumina catalyst for carrying out the HI decomposition studies, which offered >80% single-pass conversion of HI to hydrogen at 450 °C. The hydrogen throughput of the reactor was ~30 LPH at a 2 bar trans-membrane pressure, with >99.95% purity. This is the first time a muti-tube MR is reported for HI_x processing section of IS process.     

Module design of silica membrane reactor for hydrogen production via thermochemical IS process

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.     

DDR zeolite membrane reactor for enhanced HI decomposition in IS thermochemical process

The third section of closed loop Iodine Sulphur (IS) thermochemical cycle, dealing with HI_x processing, suffers from low equilibrium decomposition of HI to hydrogen with a conversion value of only ～22% at 700 K. Here, we report a significant enhancement in conversion of HI into hydrogen (up to ～95%) using a zeolite membrane reactor for the first time. The all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure was synthesized on the seeded clay alumina substrate by sonication mediated hydrothermal process. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX). Corrosion studies were carried out by exposing the membrane samples to simulated HI decomposition reaction environment (at 450 °C) for different durations of time upto 200 h. The FESEM, EDX and XRD analyses indicated that no significant changes occurred in the morphology, composition and structure of the membranes. Iodine adsorption on to the membrane surface was observed which got increased with the exposure duration as confirmed by secondary ion mass spectrometry studies. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of in-house synthesized zeolite membrane and Pt-alumina catalyst for carrying out HI decomposition studies. The tube side was chosen as reaction zone and the shell side as the permeation zone. The HI decomposition experiments were carried out for different values of temperature and feed flow rates. DDR zeolite based PBMR was found to enhance the single-pass conversion of HI up to ～95%. The results indicate that for achieving optimal performance of PBMR, it should be operated with space velocities of 0.2–0.3 s^?1 and temperature in the range of 650 K–700 K with permeate side vacuum of 0.12 kg/cm². It is believed that the in-house developed zeolite PBMR shall be a potential technology augmentation in making the IS thermochemical cycle energy efficient.     

Kinetic and thermodynamic analyses of mid/low-temperature ammonia decomposition in solar-driven hydrogen permeation membrane reactor

It is a promising method for hydrogen generation without carbon emitting by ammonia decomposition in a catalytic palladium membrane reactor driven by solar energy, which could also store and convert solar energy into chemical energy. In this study, kinetic and thermodynamic analyses of mid/low-temperature solar thermochemical ammonia decomposition for hydrogen generation in membrane reactor are conducted. Hydrogen permeation membrane reactor can separate the product and shift the reaction equilibrium forward for high conversion rate in a single step. The variation of conversion rate and thermodynamic efficiency with different characteristic parameters, such as reaction temperature (100–300 °C), tube length, and separation pressure (0.01–0.25 bar), are studied and analyzed. A near-complete conversion of ammonia decomposition is theoretically researched. The first-law thermodynamic efficiency, net solar-to-fuel efficiency, and exergy efficiency can reach as high as 86.86%, 40.08%, and 72.07%, respectively. The results of this study show the feasibility of integrating ammonia decomposition for hydrogen generation with mid/low-temperature solar thermal technologies.     

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.     

Palladium composite membrane fabricated on rough porous alumina tube without intermediate layer for hydrogen separation

A thin palladium composite membrane without any modified layer was successfully obtained on a rough porous alumina substrate. Prior to the fabrication of palladium membrane, a poly(vinyl) alcohol (PVA) layer was first coated onto the porous substrate by dip-coating technique to improve its surface roughness and pore size. After deposition of palladium membrane on the PVA modified substrate, the polymer layer can be completely removed from the composite membrane by heat treatment. The microstructure of the palladium composite membrane was characterized in detail using SEM, EDXS and XRD analysis. Permeation measurements were carried out using H₂ and N₂ at temperatures of 623 K, 673 K, 723 K and 773 K. The results indicated that the hydrogen permeation flux of 0.238 mol m^?2 s^?1 with H₂ separation factor α(H₂/N₂) of 956 for the as-prepared palladium membrane was obtained at 773 K and 100 kPa. Furthermore, the good membrane stability was proven during the total operation time of 160 h at the temperature range of 623 K–773 K and gas exchange cycles of 30 between hydrogen and nitrogen at 723 K.     

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.     

High pressure membrane separator for hydrogen purification of gas from hydrothermal treatment of biomass

For hydrogen purification and green hydrogen production in the context of biomass hydrothermal gasification, a palladium membrane system with microchannels on feed and permeate side was studied. The high pressure in the product gas of the hydrothermal process could potentially be used to generate pressurized pure hydrogen on the permeate side. Stabilizing the membrane by an additional porous metal support, experimental verification of the concept was done at feed pressure up to 50 bar and permeate pressure up to 20 bar. The temperatures were varied between 370 °C and 425 °C. The device was found to be highly selective and efficient for pure hydrogen separation. The membrane was characterized regarding the hydrogen flux and a deviation of the permeation from Sievert's law above 30 bars feed pressure was found. Generally, the microchannels on the feed side minimized concentration polarization effects, leading to high hydrogen fluxes with hydrogen feed mixtures and with real gas samples from hydrothermal gasification.     

Metal coated and nanofiller doped polycarbonate membrane for hydrogen transport

The modification in the polymeric membrane combined with metallic film as well as composites with inorganic nanofiller can be used for separation of various gases, with hydrogen gas as it is widely used as fuel material. The change in free volume due to modification in polymer chains alters the transport mechanism. In this work, the transport behaviour of H₂ across the different metal coated and nanocomposite membrane has been studied and compared with that of standard polycarbonate membrane. Transport properties changes due to the modification in the composition of nanofiller. A PtPd alloy thin film and iridium thin film of around 8–10 nm was coated on polycarbonate substrate using sputtering technique to make layered polymer nanocomposite membrane. Moreover, SiO₂ nanocomposites with 10 wt% and 15 wt % were prepared by solution casting method and the uniform dispersion of silica nanoparticles was obtained by sonication before casting the membrane. Further, the tests were carried at constant upstream pressure 30 psi and at constant temperature 35 °C using constant volume/variable pressure method.     

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.     

Steam reforming of propionic acid: Thermodynamic analysis of a model compound for hydrogen production from bio-oil

The thermodynamic equilibrium of steam reforming of propionic acid (HPAc) as a bio-oil model compound was studied over a wide range of reaction conditions (T = 500–900 °C, P = 1–10 bar and H₂O/HPAc = 0–4 mol/mol) using non-stoichiometric equilibrium models. The effect of operating conditions on equilibrium conversion, product composition and coke formation was studied. The equilibrium calculations indicate nearly complete conversion of propionic acid under these conditions. Additionally, carbon and methane formation are unfavorable at high temperatures and high steam to carbon (S/C) ratios. The hydrogen yield versus S/C ratio passes a maximum, the value and position of which depends on temperature. The thermodynamic equilibrium results for HPAc fit favorably with experimental data for real bio-oil steam reforming under same reaction conditions.     

Graphite coating on alumina substrate for the fabrication of hydrogen selective membranes

Development of composite membranes is a suitable alternative to improve the hydrogen flux through palladium membranes. The porous substrate should not represent a barrier to gas permeation, but the roughness of its surface should be sufficiently smooth for the deposition of a thin and defect-free metal layer. In this study, the performances of the modification of the outer surface of an asymmetric alumina hollow fibre substrate by the deposition of a graphite layer were evaluated. The roughness of the substrate outer surface was reduced from 120 to 37 nm after graphite coating. After graphite coating, the hydrogen permeance through the composite membrane produced with 2 Pd plating cycles was of 1.02 × 10^?3 mol s^?1 m^?2 kPa^?1 at 450 °C and with infinite H₂/N₂ selectivity. Similar hydrogen permeance was obtained with the composite membrane without graphite coating, also at infinite H₂/N₂ selectivity, but 3 Pd plating cycles were necessary. Thus, graphite coating on asymmetric alumina hollow fibres is a suitable alternative to reduce the required palladium amount to produce hydrogen selective membranes.     

Coupled hydrodynamic and kinetic model of liquid metal bubble reactor for hydrogen production by noncatalytic thermal decomposition of methane

Industrial-scale implementation of liquid metal bubble reactors (LMBRs) to produce hydrogen by methane decomposition will require large gas holdups (e.g., 20–30 vol%) and elevated gas pressures (>20 bar) to allow for practical reactor sizes. A realistic reactor design must account for the coupling between reaction kinetics and hydrodynamic effects. The gas holdup is predicted from the superficial gas velocity with a drift flux model that was experimentally corroborated in gas-molten metal mixtures. Large superficial gas velocities (>0.40 m s⁻¹) are required to achieve gas holdups of about 25 vol% in liquid metal baths (LMBs). A noncatalytic kinetic model is developed to provide thermodynamically consistent decomposition rates at methane conversions approaching equilibrium. The coupled model optimizes the LMB dimensions (diameter and length) and the inlet pressure to minimize the volume of liquid metal when the hydrogen production rate, bath temperature, methane conversion, metal composition, and maximum gas holdup are specified. For example, 200 kt a⁻¹ of hydrogen can be produced in an LMBR containing at least 96.5 m³ of molten tin held at 1100 °C in a bath measuring 3.50 m in diameter and 14.3 m in length, with an inlet methane pressure of 57.8 bar resulting in an average gas holdup of 29.7 vol% and a methane conversion of 65%.     

Multilayer metal membranes for hydrogen separation

A novel multilayer metal membrane has been developed that can be used for the separation of ultra-high purity hydrogen from impure feed streams. The membrane is comprised of very thin layers of dense palladium film deposited on both sides of a thin Group V metal foil, ion-milled prior to the deposition of the palladium. This membrane operates at elevated temperatures on the order of 300 °C and is capable of high rates of hydrogen flow. Flows are dependent on the pressure differential applied to the membrane, but flows of 100 standard cubic centimeters of hydrogen per minute per square centimeter of membrane and higher are regularly observed with differentials of under one atmosphere. Testing of the membrane for a period of 775 h showed stable flows under constant conditions. A membrane system has been successfully applied to a proton exchange membrane fuel cell and was tested using a pseudo-reformate feed stream containing 1% carbon monoxide without any degradation in performance.     

Pt‐Rh/TiO2/activated carbon as highly active and stable HI decomposition catalyst for hydrogen production in sulfur‐iodine (SI) process

Pt‐TiO₂ loaded on activated carbon was studied as an active and stable catalyst to HI decomposition for H₂ formation in the sulfur‐iodine process. Although the activity of TiO₂‐loaded catalyst was slightly lower HI conversion than that of CeO₂ loaded one, the higher stability against HI decomposition reaction was achieved and almost equilibrium conversion was sustained over ~65 h examined. Moreover, effects of Rh or Ir addition on HI conversion were studied and it was found that Pt‐Rh bimetallic system was highly active and stable to HI decomposition. Scanning transmission electron micrograph observation suggested that the increased HI decomposition activity was assigned to the increased dispersion of Pt particles. High dispersion state of Pt was sustained after HI decomposition at 773 K by addition of Rh. Since the formation of PtI₄ was suggested by X‐ray photoelectron spectroscopy measurement during HI decomposition, increased stability by addition of Rh seems to be assigned to the high chemical stability of Rh against iodine. Almost the equilibrium HI conversion on Pt‐Rh‐TiO₂/M563 was sustained over 300 hours at 673 K.     

Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes

The possibilities of using microporous ceramic membranes were examined for the production of hydrogen from hydrogen sulfide. A microporous Vycor-type glass tubing membrane of a mean pore diameter of 45 Å and new microporous alumina tubing membrane of diameter 1020 Å could be used up to 800°C and at higher temperatures, respectively, as a membrane for separating hydrogen and hydrogen sulfide. The microporous alumina tubing membrane has a 30-fold higher permeability than the microporous Vycor glass tubing membrane. When these membranes were applied to the direct decomposition of hydrogen sulfide, the yield of hydrogen increased by about two times the equilibrium yield calculated for the process without hydrogen removal.     

Enhanced catalytic decomposition of HI by using a microporous membrane

The attractive method in order to upset the equilibrium condition, i.e. product gases are continuously separated by using a selective membrane under reaction, is analyzed quantitatively. From the computer simulation of the catalytic decomposition of HI by use of a microporous membrane, the possibility is shown that the conversion rises more than the equilibrium value.     

Evaluation of Bunsen reaction at elevated temperature and high pressure in continuous co-current reactor in iodine-sulfur thermochemical process

Bunsen reaction is one of the three reaction steps of iodine-sulfur process. In present study, Bunsen reaction is carried out in co-current reactor to identify effect of different operating conditions on concentrations of Bunsen reaction product mixture. Bunsen reaction studies have been done in tubular reactor, which is made of tantalum tube and stainless steel jacket, in 50–80 °C temperature range, 2–6 bar (g) pressure range. Feed flow rates are varied for HIx (mixture of hydroiodic acid, water and iodine) 1.2 l/h - 3 l/h, SO₂ 0.02 g/s – 0.24 g/s and O₂ 0.008 g/s ?0.016 g/s. It has been observed that, increasing SO₂ feed flow rate and pressure results in increased mole fraction of HI in HIx phase and H₂SO₄ in sulfuric acid phase. Increase in temperature increased the mole fraction of HI in HIx phase but decreased the mole fraction of H₂SO₄ in sulfuric acid phase. Increase in feed I₂/H₂O ratio and HIx feed flow rate, decreased the mole fraction of HI in HIx phase. Higher pressure improved the conversion of Bunsen reactants to products.     

Concept design of a novel reformer producing hydrogen for internal combustion engines using fuel decomposition method: Performance evaluation of coated monolith suitable for on-board applications

Hydrogen addition effectively reduces the fuel consumption of spark ignition engines. We propose a new on-board reformer that produces hydrogen at high concentrations and enables multi-mode operations. For the proposed reformer, we employ a catalytic fuel decomposition reaction via a commercial NiO–CaAl₂O₄ catalyst. We explore the physical and chemical aspects of the reforming process using a fixed bed micro-reactor operating at temperatures of 550–700 °C. During reduction, methane is decomposed to form hydrogen and carbon. Carbon formation is critical to hydrogen production, and free space for carbon growth is essential at low temperatures (≤600 °C). We define a new accumulated conversion ratio that quantitatively measures highly transient catalytic decomposition. The free space of the coated monolith clearly aided low-temperature decomposition with negligible pressure drop. The coated substrate is therefore suitable for on-board applications considering that our reformer concept also utilizes the catalytic fuel decomposition reaction.