Membranes For H2 Separation Research Articles

Hydrogen separation through tailored dual phase membranes with nominal composition BaCe0.8Eu0.2O3-δ:Ce0.8Y0.2O2-δ at intermediate temperatures.

Hydrogen permeation membranes are a key element in improving the energy conversion efficiency and decreasing the greenhouse gas emissions from energy generation. The scientific community faces the challenge of identifying and optimizing stable and effective ceramic materials for H2 separation membranes at elevated temperature (400–800 °C) for industrial separations and intensified catalytic reactors. As such, composite materials with nominal composition BaCe0.8Eu0.2O3-δ:Ce0.8Y0.2O2-δ revealed unprecedented H2 permeation levels of 0.4 to 0.61 mL·min−1·cm−2 at 700 °C measured on 500 μm-thick-specimen. A detailed structural and phase study revealed single phase perovskite and fluorite starting materials synthesized via the conventional ceramic route. Strong tendency of Eu to migrate from the perovskite to the fluorite phase was observed at sintering temperature, leading to significant Eu depletion of the proton conducing BaCe0.8Eu0.2O3-δ phase. Composite microstructure was examined prior and after a variety of functional tests, including electrical conductivity, H2-permeation and stability in CO2 containing atmospheres at elevated temperatures, revealing stable material without morphological and structural changes, with segregation-free interfaces and no further diffusive effects between the constituting phases. In this context, dual phase material based on BaCe0.8Eu0.2O3-δ:Ce0.8Y0.2O2-δ represents a very promising candidate for H2 separating membrane in energy- and environmentally-related applications.

Stability and sintering of MgO as a substrate material for Lanthanum Tungstate membranes

The performance of ceramic gas separation membranes can be increased by reducing their thickness. To maintain the mechanical stability of the membrane, a supporting structure is needed. In this work magnesium oxide (MgO) is identified as support material for H2-separation membranes made of La6-xWO12-δ (LaWO). Therefore MgO is tested under application-related conditions in a water-gas-shift (WGS) catalytic membrane reactor for hydrogen production and shows a good stability. The sequential tape casting serves as the manufacturing method for the preparation of the asymmetric membrane structures. To show the compatibility of the two materials, detailed investigations of the sintering behaviour and stability tests are performed and show the well-matching properties of the two materials. Microstructural analyses reveal the inter-diffusion between LaWO and MgO.

A density functional theory study of hydrogen occupation in VNiTi alloys used for dense metal membranes

Attempting to further the development of non-noble dense metal membranes for H2 separation we conduct a density functional theory study of hydrogen occupancy in V-based alloys with Ni and Ti substitutional solutes. Clusters consisting of 19 quasi-randomly coordinated metal atoms are built to model body-centred cubic VNi and VNiTi alloys with different stoichiometry. The total energy of the target systems is calculated using spatially localised functions. The disposition of a pair of hydrogen atoms within the metal lattice is explored and the binding energy in both tetrahedral and octahedral interstices is evaluated. Large spatial distance between absorbed H atoms is favoured for each of the interstitial sites, rejecting the idea of H clustering in the investigated solid solutions. Moreover, simultaneous occupation of both tetrahedral and octahedral interstices is found to be energetically feasible despite the common believe for solely tetrahedral occupancy in metals with body-centred cubic structure. Nonetheless, the most favourable absorption site depends on the solute concentration in the V-based alloys. Calculations of the binding energy using cluster models with different metal atomic ratio provide information on the hydrogen absorption affinity as a function of alloy composition. Enhancement of the absorption affinity with added Ti until certain limit is found, while Ni solutes influence this property in the opposite direction. The applied methodology can be used further in high-throughput calculations to screen various metal alloys for hydrogen separation membranes.

Chemical Stability and Electrical Conductivity of Ba0.8Ce0.85-xZrxTb0.15O3-δ Proton Conductors with ZnO as Sintering Aid

ABSTRACTThe chemical stability of Ba0.8Ce0.85-xZrxTb0.15O3-δ (0.1≤×≤0.5) has been studied in dry 5% H2/Ar, wet 5% H2/Ar and dry CO2 atmosphere at higher temperature. These materials are promising candidates for mixed ionic electronic conductor based H2 separation membr ane. The materials are prepared by combustion synthesis technique successively followed by calcination at 1100°C and sintering at 1550°C in air. The stabilization mechanism of sintered specimens in various atmospheres is explained with respect to the combined effect of zirconium oxide and zinc oxide as stabilizing agent and sintering aid respectively. Zr4+ introduction into Ce4+ site of doped bar ium cerate perovskite enhances the chemical stability of these materials, particularly for the composition with x>0.3. The stable sintered material is obtained with total electrical conductivity of 0.0189 S.cm−1 for the composition Ba0.8Ce0.35Zr0.5Tb0.15O3-δ in moist reducing atmosphere at 900°C. So, in the present work, we report a composition which shows excellent stability in pure CO2 atmosphere at high temperature without much compromising its ionic conductivity.

Hydrogen permeation through Pd-based composite membranes: Effects of porous substrate, diffusion barrier and sweep gas

Gas transport in the diffusion barrier and porous substrate layers is modeled in order to evaluate its impacts on the performance of palladium-based composite membranes for H2 separation. A pseudo-2-dimensional model is created for a cylindrical membrane module with counter-current flow. It is hypothesized that displacement of proton transport paths may occur in the bulk metal membrane due to a non-permeable solid phase in direct contact with it. A flux-weighted effective membrane thickness is proposed to account for this effect, which has not been considered in previous literature. Both the dusty gas model (DGM) and binary fraction model (BFM) are used to simulate gas transport in the porous media. Predictions show that under certain circumstances, a well-designed diffusion barrier may cause a slight increase in H2 flux, contrary to common belief. Two mechanisms are identified which enhance membrane performance by sweeping the permeate side. Though gases with small Schmidt number were found to be more likely to penetrate through the membrane substrate and build significant concentration at the membrane-substrate interface, the choice of sweep gas may depend on other practical considerations, especially gas separation downstream. Combinations of porosity and pore size are recommended for the membrane substrate and diffusion barrier based on modeling results.

Carbon membranes for gas separation processes: recent progress and future perspective

Carbon membrane can be produced using a wide variety of polymer precursor materials via heat treatment process. A general concept route of precursor selection-preparation-modification-performance analysis platform for the carbon membrane has been proposed to promote the development of carbon membrane material for a wide range of application. The current review considers the recent progress of carbon membrane preparation and the potential applications in gas separation and membrane reactor system. In particular, the current carbon membranes for CO2, N2, and H2 separation are reviewed, along with special emphasis to their membrane precursor materials and the technique used to improve the membrane’s performance. Issues affecting membrane performance such as the preparation method to produce supported carbon membrane are explored aligned with the future research to achieve commercially viable processes. For future perspective, carbon membranes hold significant potential and great promise for further investigation, development, and application.

The hydrogen permeability of Pd–Cu based thin film membranes in relation to their structure: A combinatorial approach

Pd–Cu is a well-known alloy for H2 separation membranes. Using a new optical combinatorial method we determined the H2 permeability of Pd–Cu alloys at room temperature in relation to their crystal structure and microstructure.Compositional gradient samples allow us to determine the intrinsic permeability as a function of the alloy composition. From a detailed XRD, TEM and AFM analysis we find that the pure fcc and bcc phases have a very low permeability, whereas the mixed bcc/fcc phases show the highest permeability for thin film alloys prepared at higher temperatures. Therefore interstitial-bulk hydrogen diffusion is most probably not the main transport mechanism. Instead we argue that the hydrogen transport occurs mainly along the heterogeneous grain boundaries. As compared to the bulk phase diagram we find that the mixed bcc/fcc region in our Pd–Cu thin films is larger than reported in literature (ranging from 43 at% – 65 at% Pd at 483 K).

Separation of H<sub>2</sub> and CO<sub>2</sub> Containing Mixtures with Mixed Matrix Membranes Based on Layered Materials

Some membrane separation processes are gradually taking over conventional processes such as distillation, evaporation or crystallization as the technology progresses from bench-scale tests to large-scale prototypes. However, membranes for H2 and CO2 separation constitute a daring technology still under development. This overview focuses on mixed matrix membranes (MMMs), a special type of membranes in which a filler is dispersed in a polymer matrix, as a successful strategy to improve their permeability-selectivity performance while keeping the polymer processability. In particular, among all the possible fillers for MMMs, layered materials (porous zeolites and titanosilicates and graphite derivatives) are discussed in detail due to the several advantages they offer regarding selective microporosity, crystallinity and, what is most important, high specific surface area and aspect ratio. In fact, a selective and as thin as possible, i.e. with high aspect ratio, filler would help to develop high performance MMMs.

Outstanding hydrogen permeation through CO2-stable dual-phase ceramic membranes

Mixed electronic- and protonic-conducting composites made up of physical mixtures of La5.5WO11.25−δ–La0.87Sr0.13CrO3−δ (LWO–LSC) have been evaluated as H2 separation membranes for operation at temperatures greater than 550 °C. The mixture of these two ion-conducting phases led to non-linear synergetic effects; i.e. unexpected enhancement of the total conductivity and well-balanced ambipolar conductivity, resulting in appealing H2 permeation fluxes through robust ceramic membranes. The preparation, primary characterization, H2 permeation and stability studies of various composites is presented. Mixing LWO and LSC phases makes it possible (1) to improve the LSC sintering behavior and to achieve very high membrane densities and (2) to obtain compounds with high total conductivity, higher than that shown for LWO and LSC, separately. The highest permeation rate is achieved for the 50 vol%-LWO–LSC membrane, though other composite compositions showed higher total conductivity. Moreover, the influence on the H2 permeation of the composite composition, the humidification of gas streams, temperature and the use of various catalytic coatings on the membrane surface is evaluated. The nature of the transport mechanism is investigated by the permeation studies using deuterium tracers. The H2 permeation rates reported in this investigation for a 370 μm thick 50 vol%-LWO–LSC membrane, e.g. 0.15 mL min−1 cm−2 at 700 °C, are the highest reported values, up to date, for any bulk mixed protonic-electronic membranes. The H2 permeation magnitude achieved at moderate temperatures along with the proven stability in CO2-rich atmospheres are firm steps towards the future application of this type of membrane for industrial processes.

Integrated Approach to CO2 Capture: Fuel Gas Decarbonisation

In this presentation, the opportunities for pre-combustion capture of fuel gas at a world-scale manufacturing complex will be evaluated. The work focusses on decarbonising fuel gas at a central location in a manufacturing complex to create a hydrogen rich fuel gas pool ready for use in existing or slightly modified heating equipment and a high pressure CO2 stream ready for storage. Evaluations are based on current reforming and gasification technologies in combination with available gas treating line-ups, as well as new line-ups with emerging gas separation technologies, like high temperature membranes. Dependent on the local process boundary conditions (sweet or sour fuel gas) one or more technologies can be suitable. Current conclusion is that the concept of pre- combustion capture at a world scale manufacturing location is feasible with available technologies. It is advised to demonstrate at a scale which is sufficiently large to evaluate the performance and reliability of the individual technologies as well as the complexity of integration with the manufacturing. This presentation will reveal some of the technology aspects regarding the minimum scale for a demo regarding membrane technology. Several membrane based process schemes have been short listed for these fuel decarbonisation demonstration projects. Recovery of CO2 at high pressure and high purity (>90 mol%) as well as carbon conversions in excess of equilibrium limits may be achieved using various process schemes patented by Shell [1], [2].Successful evaluations of CRI/Criterion H2 separation membranes in membrane steam reformers and as high temperature membrane separators have been performed by CRI/Criterion [3]. These membranes can be produced with a hydrogen permeance in the range of 50-70 [Nm3/m2.h.bar0.5]. Both the hydrogen flux and the separation selectivity are stable at temperatures of 300 - 500 oC and large differential pressures (tested up to 68 bar). H2 purity of >99% has been demonstrated for long periods (over 1 year) in high temperature gas separations.

GHGT-12 Performance of the IGCC with distributed feeding of H2 in the gas turbine burner

Abstract A main hurdle for realizing the IGCC with CO 2 capture is the lack of gas turbines capable of burning hydrogen in an environmental-friendly, yet energy-efficient manner. With currently proposed combustion schemes an efficiency penalty is caused by the preparation (separation and compression) of the inert diluent, i.e. nitrogen. Distributed Fuel Injection (DFI) , where hydrogen is provided to the combustion air through a H2-separating membrane or a porous wall, could be a means of avoiding both concentrated fuel point sources and N2 dilution of the fuel. The paper presents two potential schemes for DFI: Scheme 1 with upstream H2 separation and scheme 2 with integrated H2 separation from the shifted syngas. First process simulation results for an IGCC with a reheat gas turbine yielded an IGCC efficiency increase of 0.7%-points with Scheme 1 and of 1.3%-points with scheme 2, compared to a reference case with nitrogen fuel dilution. This is mainly due to savings in N2 compression work but for scheme 2 also because the partial pressure difference of H2 over the Pd membrane can be employed as driving force. The results should however be regarded as indicative only one of the reasons being that fuel injection pressure drop for DFI is yet unknown, and is presumably a trade-off between membrane/porous wall size.

Hydrogen-induced high-temperature segregation in palladium silver membranes.

Higher operation temperatures benefit H2 permeability and selectivity of metal membranes and they are interesting for e.g. water gas shift and steam reforming in membrane reactors. Hence the behaviour of PdAg-ceramic composite membranes has been investigated between 823 K and 923 K. The H2 flux of membranes with less than 10 μm thick alloy layers decreased continuously with time during operation under H2 at 873 K and above. This was accompanied by a steady increase of the activation energy for H2 permeation and the growth of Ag-depleted crystallites on the membrane surface. All phenomena could be reversed through annealing under N2 at 923 K. The textural and permeability changes are consistent with a segregation mechanism starting with metal sublimation from hydrogenated PdAg layers and subsequent metal resublimation. This implies an enhancement of the yet unknown metal activities in PdAg hydride phases over metallic PdAg alloys. Ramifications for application of thin-layered, supported PdAg membranes for H2 separation above 823 K are discussed.

Sol–gel synthesis of thin alumina layers on porous stainless steel supports for high temperature palladium membranes

The synthesis of thin, defect-free and long lifetime Pd-based membranes for H2 separation is still a challenging task. A porous support is necessary in order to give mechanical stability to very thin layers. Sintered porous metal supports are very promising candidates having numerous advantages over the widely studied ceramic supports. The deposition of a non-metallic barrier between the stainless steel support and the Pd dense layer can prevent the intermetallic diffusion of elements from the support to the Pd-based layer and improve the characteristics (e.g. pore size, topography) of the support surface which actually limits the minimum Pd thickness required in order to obtain high selectivity membranes.In the present paper, the deposition of intermetallic barrier layers made of alumina obtained via sol gel technique was investigated using commercial porous stainless steel supports, both with their original roughness and after a mechanical smoothing treatment. The quality of the deposited alumina layer was related to the surface characteristics of the supports, sol composition (aluminium and additive content) and viscosity. Pd dense layers were grown via electroless plating technique on the alumina barrier by activation with an additional Pd doped thin alumina layer.

Solid State Transport and Hydrogen Permeation in the System Nd5.5W1–xRexO11.25−δ

Nd5.5WO11.25−δ is a mixed proton–electron conducting oxide, which shows an important mixed conductivity and stability in moist CO2 environments. However, the H2 fluxes obtained with this material are not high enough in order to apply them as H2 separation membranes in industrial applications. Re6+ cation presents similar ionic radii than W6+ and Re6+ can be reduced to different oxidation states under the operating conditions typical for hydrogen membrane separation. This fact leads to the improvement of the electronic conductivity and produce the generation of oxygen vacancies with the subsequent increase in the ionic conductivity. This work presents the synthesis as nanosized powders as well as the structural and electrochemical characterization of mixed conducting materials based on the system Nd5.5W1–xRexO11.25−δ where x = 0, 0.1, 0.5 and 1. The evolution of the crystalline structure and the shrinkage behavior are studied as a function of the sintering temperature. Total conductivity in reducing and ox...

Limiting and achievable efficiencies for solar thermal hydrogen production

For reversible conversion, we derive theoretical Sun-to-H2 (STH2) efficiencies for water-splitting processes harnessing solar energy as heat predominantly. At solar concentration ratios (C) of 2,000–10,000, the derived STH2 efficiency limits are 72.4–80.1%. For real processes with irreversibilities, we conceptualize direct and two-stage thermal water-splitting processes to estimate the achievable STH2 efficiency, the favorable operating conditions and design challenges that must be overcome. For direct thermal water-splitting, achievable STH2 efficiencies between 35 and 50% are possible at reaction temperatures of 1300–2000 K, and C = 2,000–10,000. This STH2 efficiency range is greater than the estimates of achievable values available for low and high temperature water electrolysis or single bandgap methods for generating H2. The direct process requires efficient heat integration, and high temperature membranes for H2(g) and O2(g) separation to surpass reaction equilibrium limitations. Alternatively, for two-stage water-splitting using Fe3O4/FeO with solar heat recovered at 1600–2300 K, the calculated estimates for the achievable STH2 efficiency are 38–54%.

Dense Membranes for Oxygen and Hydrogen Separation (DEMOYS): Project Overview and First Results

This paper provides an overview of objectives, structure and first results of the DEMOYS project, financially supported by the European Commission in the frame of the 7th FP Energy. The project started on May 1, 2010 and brings together fifteen Partners, including three Universities, five Research Organizations and seven Industries. The objective of DEMOYS is the development of thin mixed conducting membranes for O2 and H2 separation by using a “Plasma Spraying – Thin Film” PS-TF) in combination with nano-porous, highly catalytic layers.

Compensation Effect in H2 Permeation Kinetics of PdAg Membranes

Knowledge about the (inter)dependence of permeation kinetic parameters on the stoichiometry of H2-selective alloys is still rudimentary, although uncovering the underlying systematic correlations will greatly facilitate current efforts into the design of novel high-performance H2 separation membranes. Permeation measurements with carefully engineered, 2–7 μm thick supported Pd100–xAgx membranes reveal that the activation energy and pre-exponential factor of H2 permeation laws vary systematically with alloy composition, and both kinetic parameters are strongly correlated for x ≤ 50. We show that this permeation kinetic compensation effect corresponds well with similar correlations in the hydrogen solution thermodynamics and diffusion kinetics of PdAg alloys that govern H2 permeation rates. This effect enables the consistent description of permeation characteristics over wide temperature and alloy stoichiometry ranges, whereas hydrogen solution thermodynamics may play a role, too, as a yet unrecognized source of kinetic compensation in, for example, H2-involving reactions over metal catalysts or hydrogenation/dehydrogenation of hydrogen storage materials.

High-flux H2 separation membranes from (Pd/Au)n nanolayers

A novel strategy for the preparation of supported PdAu alloy layers allows the facile and fast fabrication of highly permeable and selective H2 separation membranes from refractory metals via electroless plating and low-temperature alloying. Homogenous alloying of multiple, separately deposited Pd and Au layers with thickness in the nm range required less than one week at 773 K under atmospheric H2 as evidenced by X-ray diffraction and H2 permeation measurements. The H2 permeation rate JH2 became stable within a day even, reaching 0.62 mol m−2 s−1 at 773 K and ΔPH2 = 100 kPa. The corresponding N2 leak rate remained constant during a 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400 and demonstrating that such membranes tolerate extended operation at that temperature well.

Isothermal solid‐state transformation kinetics applied to Pd/Cu alloy membrane fabrication

AbstractIn this work, time‐resolved, in situ high‐temperature X‐ray diffraction was used to study the solid‐state transformation kinetics of the formation of the fcc Pd/Cu alloy from Pd/Cu bilayers for the purpose of fabricating sulfur‐tolerant Pd/Cu membranes for H2 separation. Thin layers of Pd and Cu (total ∼15 wt % Cu) were deposited on porous stainless steel with the electroless deposition method and annealed in H2 at 500, 550, and 600°C. The kinetics of the annealing process was successfully described by the Avrami nucleation and growth model, showing that the annealing process was diffusion controlled and one dimensional. The activation energy for the solid‐state transformation was 175 kJ/mol, which was similar to the activation energy of Pd‐Cu bulk interdiffusion. Furthermore, the Avrami model was able to successfully describe the changes in permeance and activation energy observed in Pd/Cu alloy membranes during characterization as they were annealed at high temperatures. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Carbon-Free Hydrogen and Electricity From Coal: Options for Syngas Cooling in Systems Using a Hydrogen Separation Membrane Reactor

Conversion of coal to carbon-free energy carriers, H2 and electricity, with CO2 capture and storage may have the potential to satisfy at a comparatively low cost much of the energy requirements in a carbon-constrained world. In a set of recent studies, we have assessed the thermodynamic and economic performance of numerous coal-to-H2 plants that employ O2-blown, entrained-flow gasification and sour water-gas shift (WGS) reactors, examining the effects of system pressure, syngas cooling via quench versus heat exchangers, “conventional” H2 separation via pressure swing adsorption versus novel membrane-based approaches, and various gas turbine technologies for generating coproduct electricity. This study focuses on the synergy between H2 separation membrane reactors (HSMRs) and syngas cooling with radiant and convective heat exchangers; such “syngas coolers” invariably boost system efficiency over that obtained with quench-cooled gasification. Conventional H2 separation requires a relatively high steam-to-carbon ratio (S/C) to achieve a high level of H2 production, and thus is well matched to relatively inefficient quench cooling. In contrast, HSMRs shift the WGS equilibrium by continuously extracting reaction product H2, thereby allowing a much lower S/C ratio and consequently a higher degree of heat recovery and (potentially) system efficiency. We first present a parametric analysis illuminating the interaction between the syngas coolers, high temperature WGS reactor, and HSMR. We then compare the performance and cost of six different plant configurations, highlighting (1) the relative merits of the two syngas cooling methods in membrane-based systems, and (2) the comparative performance of conventional versus HSMR-based H2 separation in plants with syngas coolers.