Hydrogen separation from mixed gas (H2, N2) using Pd/Al2O3 membrane under forced unsteady state operations

The energy shortage and environmental pollution crises have prompted the investigation of hydrogen based cleaner energy system. Therefore, hydrogen has been considered as a promising energy carrier due to its sustainability and environmentally friendly. This research considered the separation of hydrogen from mixed gas (H₂ and N₂) by using Pd-based membrane. In order to produce extra high purity of hydrogen, the separation of hydrogen using Pd-based membrane under steady state operation suffers from long time lag and membrane deactivation. These two technical problems leading to the decrease of hydrogen permeability were intensively addressed in this work. The separation of hydrogen was conducted by using a Pd/α-Al₂O₃ membrane with aim to improve the performance of separation, indicated by time lag and hydrogen recovery. The novel method of the dynamic membrane operation was applied by performing a composition modulation of the feed gas flow rate. The steady state operation was used as a base case for comparison to dynamic operation. All experiments were carried out at 325 °C, atmospheric pressure, and H₂/N₂ ratio of 1:1, while varying the switching time and concentration amplitude for dynamic operation. The Pd based membrane was prepared, characterized, and it showed no pin hole could be found. The permeability constants for unsteady state condition resulted in higher when compared to steady state condition. The experiment results showed that the recovery of hydrogen under steady state condition was 21%. On the other hands, the recovery of hydrogen under invoked unsteady state operation was significantly improved three times higher than that of the steady state operation. The recovery of hydrogen increased 8–13% when the feed gas amplitude decreased from 1.5 mL/s to 0.5 mL/s. Operations at 300 s switching time and 0.5 mL/s flowrate amplitude reached the hydrogen recovery up to 63%.     

Graphene decorated Pd-Ag nanoparticles for H2 sensing

Hydrogen sensor based on graphene nano-composite with Pd-Ag nanoparticles was fabricated by MEMS process. Structural and morphological properties of the sensing film were studied by an energy dispersive spectroscopy (EDS) and field emission scanning electron microscopy (FESEM), respectively. The H₂ sensing properties of as-formed sensor were investigated by measuring the resistance changes at different H₂ concentrations. The maximum gas response was 16.2% at 1000 ppm of H₂ gas. The gas sensitivity of the as-formed H₂ sensor showed linear behavior with the hydrogen concentration. Experimental results showed that the coupling of graphene with Pd/Ag alloy enhanced significantly hydrogen sensing performance.     

Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

Effect of iron concentration on continuous H2 production using membrane bioreactor

The effect of FeSO₄ on continuous H₂ production in a membrane bioreactor (MBR) was investigated using anaerobic mixed microflora under mesophilic condition. The H₂ production of 41.6 l/day was obtained at 10.9 mg FeSO₄/l, which was 1.59 times higher than that at 2.7 mg FeSO₄/l. Between 2.7 and 13.7 mg FeSO₄/l, the H₂ production rate increased in parallel with the H₂ yield under high-cell-density condition. For the same amounts of FeSO₄, increases in butyric acid together with decreases in lactic acid promoted a reduction of the number of protons and the resultant release of H₂. The hydrogenase activity of 1.08 mg methylene blue (M.B) reduced/min at 10.9 mg FeSO₄/l was about sixfold higher than at 2.7 mg FeSO₄/l. These results suggest that the addition of iron and sulfur to an MBR is an important key factor in the enhancement of H₂ production.     

H2 production via water gas shift in a composite Pd membrane reactor prepared by the pore-plating method

In this work, H₂ production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A completely dense membrane with a Pd thickness of about 10.2 μm over oxidized porous stainless steel support has been prepared. Firstly, permeation measurements with pure gases (H₂ and N₂) and mixtures (H₂ with N_2, CO or CO₂) at four different temperatures (ranging from 350 to 450 °C) and trans-membrane pressure differences up to 2.5 bar have been carried out. The hydrogen permeance when feeding pure hydrogen is within the range 2.68–3.96·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5, while it decreases until 0.66–1.35·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide Fe–Cr catalyst by using a typical methane reformer outlet (dry basis: 70%H₂–18%CO–12%CO₂) and a stoichiometric H₂O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 °C to 400 °C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h⁻¹. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions.     

Techno-economic analysis: Ethane steam reforming in a membrane reactor with H2 selectivity effect and profitability analysis

With the increased production of shale gas through a new drilling technology of hydraulic fracturing, much attention has been directed to various utilization methods for ethane accounting for about 7% of shale gas. As an efficient utilization method for ethane, ethane steam reforming in a membrane reactor is proposed in this paper to provide improved reactant conversions and product yields thus leading to a reduced operating temperature. To assess techno-economic feasibility of ethane steam reforming in a membrane reactor, parametric studies focusing on a H₂ selectivity and economic analysis predicting profitability from cash flow diagrams based on a purified hydrogen in Korea were performed simultaneously providing very useful design and economic guidelines to implement a membrane reactor for ethane steam reforming.     

A theoretical analysis of methanol synthesis from CO2 and H2 in a ceramic membrane reactor

In this theoretical work the CO₂ conversion into methanol in both a traditional reactor (TR) and a membrane reactor (MR) is considered. The purpose of this study was to investigate the possibility of increasing CO₂ conversion into methanol with respect to a TR. A zeolite MR, able to combine catalytic reactions with separation properties of zeolite membranes, which allows only vapours to permeate, is considered. A mathematical model is used to simulate a traditional chemical reactor: a comparison among the model results and literature experimental data confirmed the validity of the model. Afterwards, the model is used to predict the behaviour of a zeolite MR in terms of both CO₂ conversion and methanol selectivity. The results show that it is possible to obtain both higher CO₂ conversion and methanol selectivity with respect to a TR operating at the same experimental conditions.     

Oxidative steam reforming of ethanol over a Pt/Al2O3 catalyst in a Pd-based membrane reactor

In this study, the ability of a Pd-Ag membrane reactor of producing ultrapure hydrogen via oxidative steam reforming of ethanol has been evaluated. A self supported Pd-Ag tube of wall thickness 60 μm has been filled with a commercial Pt-based catalyst and assembled into a membrane module in a finger-like configuration. In order to evaluate the hydrogen yield behavior under different operating conditions, experimental tests have been performed at temperatures of 400 and 450 °C and pressures of 150 and 200 kPa. The oxidative steam reforming of ethanol has been carried out by feeding the membrane reactor with a gas stream containing a dilute water-ethanol mixture and air. Different water/ethanol feed flow rates (5, 10, 15 g h⁻¹), several water/ethanol (4, 10, 13) and oxygen/ethanol (0.3, 0.5, 0.7) feed molar ratios have been tested. The results pointed out that the highest hydrogen yield (moles of permeated hydrogen per mole of ethanol fed) corresponding to almost 4.1 has been attained at 450 °C and 200 kPa of lumen pressure by using a water/ethanol/oxygen feed molar ratio of 10/1/0.5.The results of these tests have been compared with those reported for the ethanol steam reforming in a Pd-Ag membrane reactor filled with the same Pt-based catalyst. This comparison has shown a positive effect on the hydrogen yield of small oxygen addition in the feed stream.     

Thermodynamic analysis of a membrane-assisted chemical looping reforming reactor concept for combined H2 production and CO2 capture

There is great consensus that hydrogen will become an important energy carrier in the future. Currently, hydrogen is mainly produced by steam reforming of natural gas/methane on large industrial scale or by electrolysis of water when high-purity hydrogen is needed for small-scale hydrogen plants. Although the conventional steam reforming process is currently the most economical process for hydrogen production, the global energy and carbon efficiency of this process is still relatively low and an improvement of the process is key for further implementation of hydrogen as a fuel source. Different approaches for more efficient hydrogen production with integrated CO₂ capture have been discussed in literature: Chemical Looping Combustion (CLC) or Chemical Looping Reforming (CLR) and membrane reactors have been proposed as more efficient alternative reactor concepts relative to the conventional steam reforming process. However, these systems still present some drawbacks. In the present work a novel hybrid reactor concept that combines the CLR technology with a membrane reactor system is presented, discussed and compared with several other novel technologies. Thermodynamic studies for the new reactor concept, referred to as Membrane-Assisted Chemical Looping Reforming (MA-CLR), have been carried out to determine the hydrogen recovery, methane conversion as well as global efficiency under different operating conditions, which is shown to compare quite favorably to other novel technologies for H₂ production with CO₂ capture.     

Bench-scale WGS membrane reactor for CO2 capture with co-production of H2

This study investigated the water-gas shift reaction in a bench-scale membrane reactor (M-WGS), where three supported Pd membranes of 44 cm in length and ca. 6 μm in thickness were used, reaching a total membrane surface area of 580.6 cm². The WGS reaction was studied with the syngas mixture: 4.0% CO, 19.2% CO₂, 15.4% H₂O, 1.2% CH₄ and 60.1% H₂, under high temperature/pressure conditions: T = 673 K, p_feed = 20–35 bar(a), p_perm = 15 bar(a), mimicking CO₂ capture with co-production of H₂ in a natural gas fired power plant. High reaction pressure and high permeation of Pd membranes allowed for near complete CO conversion and H₂ recovery. Both the membranes and the membrane reactor demonstrated a long-term stability under the investigated conditions, indicating the potential of M-WGS to substitute conventional systems.     

Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

A sorption rate hypothesis for the increase in H2 permeability of palladium-silver (Pd-Ag) membranes caused by air oxidation

Hydrogen permeation measurements were performed at 300 °C for 25-μm cold-rolled Pd-Ag 25 wt% membranes before and after air oxidation at the same temperature as permeation. The air oxidation resulted in enhanced H₂ permeation through the membrane, as well as a roughening of the surface with the formation of surface grains and defects. The protruding grains can be leveled off by exposure to H₂ but the surface defects cannot. These microstructure changes are only on the membrane surfaces and do not create transmembrane defects that would allow permeation for gas species other than H₂. The H₂ permeability of the oxidized membrane increased by 25-90% compared to that of the as-received film at the same permeation condition, and the membranes retained perfect H₂ selectivity over N₂. The percent improvement of H₂ permeability decreases with increasing H₂ feed pressure. A new sorption kinetics hypothesis is proposed to elucidate the increase in H₂ permeability of Pd-Ag membranes caused by oxidation. H₂ solubility and sorption rate results were presented to test the new hypothesis. It is found that air oxidation does not change the H₂ solubility in Pd-Ag membranes, but enhances the H₂ sorption kinetics significantly. The extent of kinetics enhancement also decreases with increasing H₂ pressures. The much faster sorption equilibrium implies higher effective H₂ diffusivity at the Pd-Ag membrane surface for the oxidized sample and a higher transfer rate of atomic hydrogen from surface/sub-surface to the membrane bulk that contributes to the increase of H₂ permeability observed in experiments.     

H2 production in silica membrane reactor via methanol steam reforming: Modeling and HAZOP analysis

The main aim of this work is the presentation of both qualitative safety and quantitative operating analyses of silica membrane reactor (MR) for carrying out methanol steam reforming (MSR) reaction to produce hydrogen. To perform the safety analysis, HAZOP method is used. Before the HAZOP analysis, a comprehensive investigation of most important operating parameters effects on silica MR performance is required. Therefore, for a quantitative analysis, a 1-dimensional and isothermal model is developed for evaluating the reaction temperature, reaction pressure, feed molar ratio (steam/methanol) and feed flow rate effects on silica MR performance in terms of methanol conversion and hydrogen recovery. The model validation results show good agreement with experimental data from literature. As a consequence, simulation results indicate that the reaction pressure and feed molar ratio have dual effect on silica MR performance. In particular, it is found that methanol conversion is decreased by increasing the reaction pressure from 1.5 to 4.0 bar, whereas over 4.0 bar, it is improved. Moreover, the hydrogen recovery is decreased by increasing the feed molar ratio from 1 to 5, while over 5, it was approximately constant. After the evaluation of modeling results, the HAZOP analysis for silica MR is carried out during MSR reaction. The analysed operating parameters in the modeling study have been considered as key parameters in the HAZOP analysis. The safety assessment results are presented in tables as check list. By considering the HAZOP results, safety pretreatment works are recommended before or during the experimental tests of MSR reaction in silica MR. According to different parameters consequences, reaction temperature is the most critical parameter in MSR reaction for the silica MR studied in this work. In particular, to avoid the consequences of temperature deviation, it is recommended to use a PID temperature controller in the silica MR for MSR reaction.     

Thermal stability and effect of typical water gas shift reactant composition on H2 permeability through a Pd-YSZ-PSS composite membrane

This work comprises a study of hydrogen separation with a composite Pd-YSZ-PSS membrane from mixtures of H₂, N₂, CO and CO₂, typical of a water gas shift reactor. The Pd layer is extended over a tubular porous stainless steel support (PSS) with an intermediate layer of yttria-stabilized-zirconia (YSZ). YSZ and Pd layers were incorporated over the PSS using Atmospheric Plasma Spraying and Electroless Plating techniques, respectively. The Pd and YSZ thickness values are 13.8 and 100 μm, respectively, and the Pd layer is fully dense. Permeation measurements with pure, binary and ternary gases at different temperatures (350–450 °C), trans-membrane pressures (0–2.5 bar) and gas composition have been carried out. Moreover, thermal stability of the membrane was also checked by repeating permeation measurements after several cycles of heating and cooling the system. Membrane hydrogen permeances were calculated using Sieverts' law, obtaining values in the range of 4·10⁻⁵–4·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5. The activation energy of the permeance was also calculated using Arrhenius' equation, obtaining a value of 16.4 kJ/mol. In spite of hydrogen selectivity being 100% for all experiments, the hydrogen permeability was affected by the composition of feed gas. Thus, a significant depletion in H₂ permeate flux was observed when other gases were in the mixture, especially CO, being also more or less significant depending on gas composition.     

Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures

Using a high-pressure shock tube facility, the ignition delay times of stoichiometric C₂H₆/H₂/O₂ diluted in argon were obtained behind reflected shock wave at elevated pressures (p = 1.2, 4.0 and 16.0 atm) with ethane blending ratios from 0 to 100%. The measured ignition delay times were compared to the previous correlations, and the results show that the ignition delay times of ethane from different studies exhibit an obvious difference. Meanwhile, numerical studies were conducted with three generally accepted kinetic mechanisms and the results show that only NUIG Aramco Mech 1.3 agrees well with the measurements under all test conditions. Sensitivity analysis was made to interpret the poor prediction of the other two mechanisms. Furthermore, the effect of ethane blending ratio on the ignition delay times of the mixtures was analyzed and the results show that ethane blending ratio gives a non-linear effect on the auto-ignition of hydrogen. Finally, chemical interpretations on this non-linear effect were made from the reaction pathway analysis and normalized H radical consumption analysis.     

Quantification of “delivered” H2 by a volumetric method to test H2 storage materials

We set up and validated a volumetric method to quantify the amount of hydrogen “delivered” after saturation of a solid material as adsorber at different pressures (up to 100 kg_f/cm²) and temperatures (down to 77 K). This is the practically most relevant datum to quantify the effectiveness of an adsorbent for the present application. A complementary dynamic method has been also developed to take into account the reversibility of adsorption and to assess in at least a semi-quantitative way the strength of interaction between H₂ and the adsorbent. The method has been applied to compare the hydrogen storage capacity of some significant different carbon-based materials (two active carbons and one graphite), as supplied or after thermal treatments under oxidising or reducing conditions. The best results, ca. 7 wt% H₂ “delivered”, were achieved after saturation at 77 K, 20 kg_f/cm² with an active carbon with ca. 3000 m²/g of apparent specific surface area. The thermal treatments, almost always inducing a drop in surface area, showed effective only for saturation at 273 K, in particular the oxidising procedure. This was correlated to the formation of surface oxidised species, likely carboxylic groups, which improved the interaction strength between H₂ and the adsorbent.     

Hydride phase formation in LaMg2Ni during H2 absorption

Hydrogen absorption and desorption properties in nanocrystalline LaMg₂Ni are presented. Nanostructured phases have been obtained by milling grain coarse ingot and by mechanically alloying the parent elements. The structural and hydriding properties were examined by X-ray diffraction, thermal analysis and thermal desorption measurements. Ball milling and mechanical alloying give a significant refinement of the microstructure. Reactive milling has been used for hydrogen absorption experiments. Hydrogenation by means of reactive milling at 300 K under a pressure of 0.4 MPa leads to the formation of a stable La-hydride phase together with an amorphous phase. Thermal desorption up to 983 K of hydrogenated samples leads again to parent LaMg₂Ni phase.     

Optimal Ag concentration for H2 production via Ag:TiO2 nanocomposite thin film photoanode

TiO₂ thin films containing different concentrations of Ag nanoparticles have been synthesized by sol-gel method. According to UV–visible spectra, presence of an intense surface plasmon resonance peak at 490 nm of wavelength indicated formation of silver nanoparticles in the TiO₂ films. Based on atomic force microscopy (AFM) analysis, the surface roughness and the effective surface ratio increased by increasing the Ag mol%. Moreover, scanning electron microscopy (SEM) images showed formation of Ag nanoparticles on the surface for the samples containing high Ag concentration. X-ray diffraction (XRD) patterns revealed that the size of Ag nanocrystals increased by increasing the Ag content in the films while the nanocrystalline size of TiO₂ reduced in the presence of silver nanoparticles. Based on x-ray photoelectron spectroscopy (XPS) data, a stoichiometric chemical composition was detected for TiO₂ while, Ag presented in a combination a metal/oxide states on the surface. Studying photoresponse of the samples showed that the highest photocurrent was obtained for the sample containing 1 mol% Ag. By measuring the photovoltage versus time, it was found that addition of silver nanoparticles to the TiO₂ layer resulted in reduction of the transient time of the photogenerated carriers in the samples. Impedance spectroscopy determined a slight decrease in charge transfer resistance by addition of Ag to the films. Moreover, measuring the amount of hydrogen produced during water splitting reactions verified that the highest quantum yield of 9.6% was obtained for the sample with 1 mol% Ag.     

La1−xSrxMO3 (M = Mn, Fe) perovskites as materials for thermochemical hydrogen production in conventional and membrane reactors

La_1−xSr_xMO₃ (M = Mn, Fe) perovskites are investigated as potential redox materials for the thermochemical production of hydrogen. Thermogravimetric oxidation/reduction experiments indicated that the materials are able to lose and uptake oxygen reversibly from their lattice up to 5.5 wt.% for La_1−xSr_xMnO₃ with x = 1 and up to 1.7 wt.% for La_1−xSr_xFeO₃ with x = 0. Pulse reaction experiments indicated that the materials can be used as redox catalysts in a redox process where water is dissociated giving rise to the production of pure hydrogen during the oxidation step. The oxidation and reduction steps can be combined in a membrane reactor constructed from dense perovskite membranes towards a continuous and isothermal operation. The system is also able to operate on partial pressure-based desorption without the need of a carbon-containing reductant, so that a process towards hydrogen production, based only on renewable hydrogen source such as water, can be established. At steady state and 900 ^°C, 25 ± 7 cm³ (STP) H₂ m⁻² min⁻¹ is produced in purified state.     

Effect of ZrO2 addition on Ni/Al2O3 catalyst to produce H2 from glycerol

In this work, ZrO₂ was employed as support and as Al₂O₃ modifier of Ni based catalysts due to its special interesting characteristics. The catalytic activity of these systems was studied in steam reforming of glycerol to produce H₂. As the activity results at 773 K and 873 K showed, the NiZ catalyst allowed low glycerol conversion and H₂ production when compared to the NiγA catalyst. Moreover, the NiZ catalyst was not able to reform intermediate liquid products into gaseous products.