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.     

CFD modeling and experimental validation of sulfur trioxide decomposition in bayonet type heat exchanger and chemical decomposer for different packed bed designs

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions has lead to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) Nuclear Hydrogen Initiative was investigating thermochemical cycles for hydrogen production using high-temperature heat exchangers. In this study a three-dimensional computational model of high-temperature heat exchanger and decomposer for decomposition of sulfur trioxide by the sulfur–iodine thermochemical water-splitting cycle with different packed bed designs has been done. The decomposer region of the bayonet heat exchanger also called as silicon carbide integrated decomposer (SID) is designed as the packed bed region. Cylindrical, spherical, cubical and hollow cylindrical pellets have been arranged inside the packed bed. The engineering design of the packed bed was very much influenced by the structure of the packing matrix, which was governed by the shape, dimension and the loading of the constituent particles. Staggered and regular packing methods are used for packing the pellets in the packed bed region. The numerical model is created using GAMBIT and fluid, thermal and chemical analyses were performed using FLUENT. The decomposition percentage of sulfur trioxide is found for the packed bed region with different pellets and the numerical results obtained is compared with the experimental results. A comparison is made for the decomposition percentage of SO₃ for the packed bed approach and the porous media approach.     

Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming

In this theoretical work the performance of different membrane reactor concepts, both fluidized bed and packed bed membrane reactors, has been compared for ultra-pure hydrogen production via methane reforming. Using detailed theoretical models, the required membrane area to reach a given conversion and the prevailing temperature profiles have been compared. The extent of mass and heat transfer limitations in the different reactors has been evaluated, and strategies to decrease (or avoid) these limitations have been proposed for the fluidized bed membrane reactor concept. The results show that the packed bed membrane reactor requires in some conditions double membrane area with respect to the fluidized bed membrane reactor.     

Comparison of experimental and simulation results on catalytic HI decomposition in a silica-based ceramic membrane reactor

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.     

Simulation of bacterial locomotion and attachment in the interspaces of packed bed reactors

Biofilm plays a significant role in biological hydrogen production. Locomotion and attachment of bacteria are the initial processes in the formation of biofilm. In the present study, bacterial locomotion and attachment in the interspace of packed bed reactor is simulated. The model comprehensively combines flagellar propulsion, Brownian motion, running and tumbling. According to the simulation results, compared with other stacking forms, faster bacterial attachment takes place in the tetrahedron stacking, which can lead to faster biofilm formation. Simulation of bacteria distribution in two-dimensional interspace reveals more bacteria accumulate in the central region of the interspace, while less in the corners. Further study shows that smaller packing or bacteria with smaller cell body in the suspension can lead to faster bacterial attachment in the packed bed reactor. In addition, the suspension viscosity has a tiny effect on the attachment of bacteria. The results can be used as a guideline for the design and operation of packed bed reactors with immobilized biofilm.     

HI decomposition over PtNi/C bimetallic catalysts prepared by electroless plating

The catalytic decomposition of hydrogen iodide (HI) has drawn increasing attention because it is the key reaction for hydrogen production in the Iodine–sulfur (IS) thermochemical water splitting cycle, which is considered one of the most promising alternative methods for massive hydrogen production with high efficiency and without CO₂ emissions. Because it is very difficult for HI to decompose without the catalysts even at 500 °C, some catalysts have to be used to catalyze this reaction. In this study, four kinds of PtNi bimetallic catalysts supported on activated carbon (PtNi/C) were prepared by electroless plating and their catalytic activities were compared for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their differences in structures, surface areas, and morphology were characterized by XRD, BET and TEM, respectively. The used catalysts were also analyzed by TEM characterization in order to investigate the stability of catalysts. The results showed that the PtNi/C bimetallic catalysts are promising catalysts for HI decomposition because of their high activity and good stability, especially at high reaction temperature.     

Some issues in modelling methane catalytic decomposition in fluidized bed reactors

This paper reports a model of fluidized bed thermo-catalytic decomposition (TCD) of methane. The novelty of the model consists of taking into account the occurrence of different competitive phenomena: methane catalytic decomposition, catalyst deactivation due to carbon deposition on the catalyst particles and their reactivation by means of carbon attrition. Comparison between theoretical and experimental data shows the capability of the present model to predict methane conversion and deactivation time during the process. The model demonstrates to be also a useful tool to investigate the role played by operative parameters such as fluidizing gas velocity, temperature, size and type of the catalyst. In particular, the model results have been finalized to characterize the attrition phenomena as a novel strategy in catalyst regeneration.     

Comparisons of Pt catalysts supported on active carbon,carbon molecular sieve,carbon nanotubes and graphite for HI decomposition at different temperature

A series of Pt catalysts supported on activated carbon (AC), carbon molecular sieve (CMS), carbon nanotubes (CNT) and graphite (GR) were prepared by the impregnation method. Their catalytic performances in HI decomposition were evaluated in a fixed bed reactor at temperatures ranging from 400 to 550 °C under atmospheric pressure. The different Pt catalysts before and after HI decomposition at different temperature were characterized by BET, XRD and TEM, respectively. The results of the activity evaluation indicated that the activity order of different Pt catalysts changed significantly with the variation of reaction temperature. At 400 °C, different supported Pt catalysts activities decreased in order of Pt/CMS > Pt/AC > Pt/CNT > Pt/GR. At 450 °C, the activities of different Pt catalysts followed the order of Pt/AC ≈ Pt/CNT > Pt/CMS > Pt/GR. At 500 and 550 °C, the Pt/CNT showed the optimum activity and stability during HI decomposition, which could be attributed to the high dispersion of Pt particles and the special microstructure of CNT. The XRD and TEM results illustrated that the Pt particle size or Pt dispersion in different supported Pt catalysts showed different sensitivity to the reaction temperature.     

Reaction/separation coupled equilibrium modeling of steam methane reforming in fluidized bed membrane reactors

An equilibrium model of steam methane reforming coupled with in-situ membrane separation for hydrogen production was developed. The model employed Sievert’s Law for membrane separation and minimum Gibbs energy model for reactions. The reforming and separation processes were coupled by the mass balance. The model assumed a continuously stirred tank reactor for the fluidized bed hydrodynamics. The model predictions for a typical case were compared with those from the model of Ye et al. [15] which assumed a plug flow for bed hydrodynamics. The model predictions show satisfactory agreement with experimental data in the literatures. The influences of reactor pressure, temperature, steam to carbon ratio, and permeate side hydrogen partial pressure on solid carbon, NH_x and NO_x formation were studied using the model.     

Experimental investigation on hydrogen production by methanol steam reforming in a novel multichannel micro packed bed reformer

A novel multichannel micro packed bed reactor with bifurcation inlet manifold and rectangular outlet manifold was developed to improve the methanol steam reforming performance in this study. The commercial CuO/ZnO/Al₂O₃ catalyst particles were directly packed in the reactor. The flow distribution uniformity in the reactor was optimized numerically. Experiments were conducted to study the influences of steam to carbon molar ratio (S/C), weight hourly space velocity (WHSV), reactor operating temperature (T) and catalyst particle size on the methanol conversion rate, H₂ production rate, CO concentration in the reformate, and CO₂ selectivity. The results show that increase of the S/C and T, as well as decrease of the WHSV and catalyst particle size, both enhance the methanol conversion. The CO concentration decreases as the S/C and WHSV increase as well as the T and catalyst particle size decrease. Moreover, T plays a more important role on the methanol steam reforming performance than WHSV and S/C. The impacts on CO concentration become insignificant when the S/C is higher than 1.3, WHSV is larger than 1.34 h⁻¹ and T is lower than 275 °C. A long term stability test of this reactor was also performed for 36 h and achieved high methanol conversion rate above 94.04% and low CO concentration less than 1.05% under specific operating conditions.     

Fixed bed membrane reactors for WGSR-based hydrogen production: Optimisation of modelling approaches and reactor performance

The production of high-purity hydrogen using the water–gas-shift reaction in both conventional fixed bed reactor and hydrogen perm-selective membrane reactor at low to medium scale is studied in this work by developing and comparing models with different complexity levels. A two-dimensional rigorous reactor model considering radial and axial variations of properties (including bed porosity), setting mass, energy and momentum differential balances, and nesting a rigorous model for mass transfer within the porous catalyst was considered as reference for comparison. Different simplifications of this model for taking into account mass-transfer effects within the catalyst pellet (Thiele modulus, evaluation of apparent kinetic constants, empirical correlation for effectiveness factors or just neglecting these effects) were tested, being observed that these effects are not negligible and that the first two approaches are accurate enough for taking into account mass transfer within catalyst pellets. Regarding to the reactor model, it was observed that one-dimensional models are not adequate, especially for the membrane reactor. Analogously, neglecting the momentum balances in the reactor (as made is most simulations reported in the literature) leads to important misspredictions in the behaviour of the membrane reactor performance. Finally, the influence of the main operation parameters (inlet temperature, pressure, space velocity, etc.) was studied using the detailed reactor model, concluding that space velocity and pressure are the most important parameters affecting reactor performance for membrane reactors.     

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%.     

Steam reforming of heptane in a fluidized bed membrane reactor

n-Heptane served as a model compound to study steam reforming of naphtha as an alternative feedstock to natural gas for production of pure hydrogen in a fluidized bed membrane reactor. Selective removal of hydrogen using Pd₇₇Ag₂₃ membrane panels shifted the equilibrium-limited reactions to greater conversion of the hydrocarbons and lower yields of methane, an intermediate product. Experiments were conducted with no membranes, with one membrane panel, and with six panels along the height of the reactor to understand the performance improvement due to hydrogen removal in a reactor where catalyst particles were fluidized. Results indicate that a fluidized bed membrane reactor (FBMR) can provide a compact reformer for pure hydrogen production from a liquid hydrocarbon feedstock at moderate temperatures (475-550 °C). Under the experimental conditions investigated, the maximum achieved yield of pure hydrogen was 14.7 moles of pure hydrogen per mole of heptane fed.     

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.     

Steam reforming of propane in a fluidized bed membrane reactor for hydrogen production

Steam reforming of propane was carried out in a fluidized bed membrane reactor to investigate a feedstock other than natural gas for production of pure hydrogen. Close to equilibrium conditions were achieved inside the reactor with fluidized catalyst due to the very fast steam reforming reactions. Use of hydrogen permselective Pd₇₇Ag₂₃ membrane panels to extract pure hydrogen shifted the reaction towards complete conversion of the hydrocarbons, including methane, the key intermediate product. Irreversible propane steam reforming is limited by the reversibility of the steam reforming of this methane. To assess the performance improvement due to pure hydrogen withdrawal, experiments were conducted with one and six membrane panels installed along the height of the reactor. The results indicate that a compact reformer can be achieved for pure hydrogen production for a light hydrocarbon feedstock like propane, at moderate operating temperatures of 475–550 °C, with increased hydrogen yield.     

Pure hydrogen production via autothermal reforming of ethanol in a fluidized bed membrane reactor: A simulation study

In this paper the production of ultra-pure hydrogen via autothermal reforming of ethanol in a fluidized bed membrane reactor has been studied. The heat needed for the steam reforming of ethanol is obtained by burning part of the hydrogen recovered via the hydrogen perm-selective membrane thereby integrating CO₂ capture. Simulation results based on a phenomenological model show that it is possible to obtain overall autothermal reforming of ethanol while 100% of hydrogen can in principle be recovered at relatively high temperatures and at high reaction pressures. At the same operating conditions, ethanol is completely converted, while the methane produced by the reaction is completely reformed to CO, CO₂ and H₂.     

Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus

Hydrogen production via steam methane reforming with in situ hydrogen separation in fluidized bed membrane reactors was simulated with Aspen Plus. The fluidized bed membrane reactor was divided into several successive steam methane sub-reformers and membrane sub-separators. The Gibbs minimum free energy sub-model in Aspen Plus was employed to simulate the steam methane reforming process in the sub-reformers. A FORTRAN sub-routine was integrated into Aspen Plus to simulate hydrogen permeation through membranes in the sub-separator based on Sieverts' law. Model predictions show satisfactory agreement with experimental data in the literature. The influences of reactor pressure, temperature, steam-to-carbon ratio, and permeate side hydrogen partial pressure on reactor performances were investigated with the model. Extracting hydrogen in situ is shown to shift the equilibrium of steam methane reactions forward, removing the thermodynamic bottleneck, and improving hydrogen yield while neutralizing, or even reversing, the adverse effect of pressure.     

Stability of pore-plated membranes for hydrogen production in fluidized-bed membrane reactors

Pd-based membranes prepared by pore-plating technique have been investigated for the first time under fluidization conditions. A palladium thickness around 20 μm was achieved onto an oxidized porous stainless steel support. The stability of the membranes has been assessed for more than 1300 h in gas separation mode (no catalyst) and other additional 200 h to continuous fluidization conditions. Permeances in the order of 5·10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ have been obtained for temperatures in a range between 375 and 500 °C. During fluidization, a small decrease in permeance is observed, as consequence of the increased external (bed-to-wall) mass transfer resistances. Moreover, water gas shift (WGS) reaction cases have been carried out in a fluidized bed membrane reactor. It has been confirmed that the selective H₂ separation through the membranes resulted in CO conversions beyond the thermodynamic equilibrium (of conventional systems), showing the benefits of membrane reactors in chemical conversions.     

Hydrogen production in an upflow anaerobic packed bed reactor used to treat cheese whey

In this work, an upflow anaerobic packed bed reactor configuration to produce hydrogen using cheese whey as the substrate was tested. The microbiological composition was linked to the reactor operation.     

H2SO4 poisoning of Ru-based and Ni-based catalysts for HI decomposition in SulfurIodine cycle for hydrogen production

The sulfur–iodine (SI) cycle is deemed to be one of the most promising alternative methods for large-scale hydrogen production by water splitting, free of CO₂ emissions. Decomposition of hydrogen iodide is a pivotal reaction that produces hydrogen. The homogeneous conversion of hydrogen iodide is only 2.2% even at 773 K [1]. A suitable catalyst should be selected to reduce the decomposition temperature of HI and attain reaction yields approaching to the thermodynamic equilibrium conversion. However, residual H₂SO₄ could not be avoided in the SI cycle because of incomplete purification. The H₂SO₄ present in the HI feeding stream may lead to the poisoning of HI decomposition catalysts. In this study, the activity and sulfur poisoning of Ru and Ni catalysts loaded on carbon and alumina, respectively, were investigated at 773 K. HI conversion efficiency markedly decreased from 21% to 10% with H₂SO₄ (3000 ppm) present, which was reversible when H₂SO₄ was withdrawn in the case of Ru/C. In the case of Ru/C and Ni/Al₂O₃, catalyst deactivation depends on the concentration of H₂SO₄; the higher the concentration of H₂SO_4, the greater the severity of deactivation. Catalysts before and after sulfur poisoning were characterized by transmission electron microscopy (TEM), energy-dispersive X-Ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Experimental results and characterization of poisoned and fresh catalysts indicate that the catalyst deactivation could be ascribed to the competitive adsorption of sulfur species and change in its surface properties.