Hydrogen permeation enhancement in a Pd membrane tube system under various vacuum degrees

Hydrogen purification using palladium (Pd) membrane technology has been seen as a potential solution for producing pure hydrogen form hydrogen-rich gas. Compared to traditional practices of operating the permeate side of the membrane at atmospheric pressure, in this study, a vacuum is applied. The effects of various vacuum degrees applied to the permeate side of the Pd membrane are investigated and compared to the results under normal operation without a vacuum. The feed gas used for experiments consists of a mixture of hydrogen (70 vol%) and nitrogen (30 vol%). Three membrane operating temperatures (320, 350, and 380 °C), four pressure differences (2, 3, 4, and 5 atm) across the membrane, and four vacuum degrees (−15, −30, −45, and −53 kPa) applied to the permeate side are considered. For the three operating temperatures, the best improvements in the performance of hydrogen permeation are at 320 and 350 °C when a −53 kPa vacuum is applied, resulting in 79.4% and 79.1% improvements, respectively, compared to normal operations. Increasing temperatures leads to an increase in H₂ permeation both with and without a vacuum; however, best performances of H₂ permeation are observed in cases without a vacuum.     

Fabrication & performance study of a palladium on alumina supported membrane reactor: Natural gas steam reforming,a case study

In this work, a synthetic mixture of natural gas is considered in a steam reforming process for generating hydrogen by using a membrane reactor housing a composite membrane constituted of a Pd-layer (13 μm) supported on alumina. The Pd/Al₂O₃ membrane separates part of the produced hydrogen through its selective permeation, although it shows a relatively low H₂/N₂ ideal selectivity (>200 at 0.5 bar of trans-membrane pressure and T = 425 °C).The steam reforming reaction is performed at 420 °C, by varying the gas hourly space velocity between 4400 h^?1 and 6900 h^?1 and by using two different mixtures containing some common impurities found within natural gas pipeline. Specifically, the effect of N₂ and CO₂ as impurities in the feed line is analyzed. The reaction pressure and steam-to-carbon ratio (S/C) are kept constant at 3.0 bar (abs.) and 3.5/1, respectively.The best performance of the Pd-based membrane reactor is obtained at 420 °C, 3.0 bar and 100 mL/min of sweep-gas, yielding a methane conversion of 55% and hydrogen recovery >90%.     

Comparison of co–gasification efficiencies of coal,lignocellulosic biomass and biomass hydrolysate for high yield hydrogen production

The diversity in the chemical composition of lignocellulosic feedstocks can affect the conversion technologies employed for hydrogen production. Gasification and co–gasification activities of lignocellulosic biomass, biomass hydrolysate, and coal were evaluated for hydrogen rich gas production. The hydrolysates of biomass materials showed the best performance for gasification. The results indicated that biomass hydrolysates obtained from lignocellulosic biomass were more sensitive to degradation and therefore, produced more hydrogen and gaseous products than that of lignocellulosic biomass. The effects of feed (kenaf and sorghum hydrolysate), flow rate (0.3–2.0 mL/min) and temperature (700–900 °C) on hydrogen production and gasification yields were investigated. It was observed that 0.5 mL/min the optimum feed flow rate for the maximum total gas and hydrogen production. Synergism effects were observed for co–gasification of coal/biomass and coal/biomass hydrolysate. In all co–gasification processes, the main component of the gas mixture was hydrogen (≥70%).     

Application of catalytic membrane reactor for pure hydrogen production by flare gas recovery as a novel approach

In the present study, application of catalytic membrane reactor as a novel approach for the flare gas recovery is proposed. A comprehensive two-dimensional non-isothermal model has been constructed to evaluate the performance of flare gas recovery process in the membrane reactor. The model is developed by taking into accounts the main chemical kinetics, heat and mass transfer phenomena and hydrogen permeation in the radial direction across a Pd–Ag membrane. The model predictions are validated based on different experimental results reported in literature. The impact of reactor operating conditions on the recovery process such as temperature and pressure, feed molar ratio and sweep gas ratio are investigated and discussed. The modeling results confirm that the flare gas conversion and hydrogen recovery improves with increasing the operating temperature, pressure and sweep ratio as a consequence of increasing the driving force for H₂ permeation through membrane. The environmental consideration revealed that by application of catalytic membrane reactor for the flare gas recovery of Asalouyeh gas processing plant (Iran), not only the equivalent mass of greenhouse gases emission reduces from 2179 kg/s to 36 kg/s, but also, 12.7 kg/s pure hydrogen will be produced by flare gas recovery at 750 K, 5 bar, sweep ratio of 5 and feed molar ratio of 4.     

Hydrogen production for PEM fuel cell by gas phase reforming of glycerol as byproduct of bio-diesel. The use of a Pd-Ag membrane reactor at middle reaction temperature

Glycerol as a byproduct of biodiesel production represents a renewable energy source. In particular, glycerol can be used in the field of hydrogen production via gas phase reforming for proton exchange membrane fuel cell (PEMFC) applications. In this work, glycerol steam reforming (GSR) reaction was investigated using a dense palladium-silver membrane reactor (MR) in order to produce pure (or at least CO-free) hydrogen, using 0.5 wt% Ru/Al₂O₃ as reforming catalyst. The experiments are performed at 400 °C, water to glycerol molar feed ratio 6:1, reaction pressure ranging from 1 to 5 bar and weight hourly space velocity (WHSV) from 0.1 to 1.0 h⁻¹. Moreover, a comparative study is given between the Pd-Ag MR and a traditional reactor (TR) working at the same MR operating conditions. The effect of the WHSV and reaction pressure on the performances of both the reactors in terms of glycerol conversion and hydrogen yield is also analyzed. The MR exhibits higher conversion than the TR (∼60% as best value for the MR against ∼40% for the TR, at WHSV = 0.1 h⁻¹ and 5 bar), and high CO-free hydrogen recovery (around 60% at WHSV = 0.1 h⁻¹ and 5 bar). During reaction, carbon coke is formed limiting the performances of the reactors and inhibiting, in particular, the hydrogen permeation through the membrane with a consequent reduction of hydrogen recovery in the permeate side.     

Effect of sweep gas on hydrogen permeation of supported Pd membranes: Experimental and modeling

Membrane reactor processes are being increasingly proposed as an attractive solution for pure hydrogen production due to the possibility to integrate production and separation inside a single reactor vessel. High hydrogen purity can be obtained through dense metallic membranes, especially palladium and its alloys, which are highly selective to hydrogen. The use of thin membranes seems to be a good industrial solution in order to increase the hydrogen flux while reducing the cost of materials. Typically, the diffusion through the membrane layer is the rate-limiting step and the hydrogen permeation through the membrane can be described by the Sieverts’ law but, when the membrane becomes thinner, the diffusion through the membrane bulk becomes less determinant and other mass transfer limitations might limit the permeation rate. Another way to increase the hydrogen flux at a given feed pressure, is to increase the driving force of the process by feeding a sweep gas in the permeate side. This effect can however be significantly reduced if mass transfer limitations in the permeate side exist. The aim of this work is to study the mass transfer limitation that occurs in the permeate side in presence of sweep gas. A complete model for the hydrogen permeation through PdAg membranes has been developed, adding the effects of concentration polarization in retentate and permeate side and the presence of the porous support using the dusty gas model equation, which combines Knudsen diffusion, viscous flow and binary diffusion. By studying the influence of the sweep gas it has been observed that the reduction of the driving force is due to the stagnant sweep gas in the support pores while the concentration polarization in the permeate is negligible.     

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.     

Sustainable hydrogen production options from food wastes

In this study, two thermochemical processes, namely steam gasification and supercritical water gasification (SCWG), were comparatively studied to produce hydrogen from food wastes containing about 90% water. The SCWG experiments were performed at 400 and 450 °C in presence of catalyst (Trona, K₂CO₃ and seaweed ash). The maximum hydrogen yield was obtained at 450 °C in presence of K₂CO₃ catalyst. In second process, hydrothermal carbonization was used to convert food wastes into a high-quality solid fuel (hydrochar) that was further gasified in a dual-bed reactor in presence of steam. The steam gasification of hydrochar was carried out with and without catalysts (iron?ceria catalyst and dolomite). The maximum hydrogen yield obtained from steam gasification process was 28.08 mmol/g dry waste, about 7.7 times of that from SCWG. This study proposed a new concept for hydrogen production from wet biomass, combination of hydrothermal carbonization following steam gasification.     

Impact of vacuum operation on hydrogen permeation through a palladium membrane tube

Palladium (Pd) membranes are characterized by their high permselectivity to hydrogen and easy operation, and are promising devises for separating hydrogen from hydrogen-rich gases. The membranes are normally operated with atmospheric pressure at the permeate side. Instead of this common operation, hydrogen permeation through a Pd membrane under vacuum operation at the permeate side is investigated and compared with that under normal operation. In this study, two membrane operating temperatures (320 and 380 °C), four H₂ partial pressure differences (2, 3, 4, and 5 atm) across the membrane, and four feed gases are considered. The results suggest that the vacuum operation can efficiently intensify the H₂ permeation rate. The improvement in H₂ permeation rate due to the vacuum operation can be increased up to 136%. The positive effect of the vacuum operation is especially pronounced when the gas mixtures are used as the feed gases, stemming from the effective attenuation of the concentration polarization. An increase in membrane temperature raises the H₂ permeation rate, but its influence in enhancing the H₂ permeation rate with the vacuum operation is not as significant as that without the vacuum one. It is found that the retardation effect of impurities on the mass transfer is always ranked as CO > CO₂ > N₂, regardless of with/without vacuum operation.     

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.     

Methane steam reforming using a membrane reactor equipped with a Pd-based composite membrane for effective hydrogen production

Herein, a methane steam reforming (MSR) reaction was carried out using a Pd composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst under mild operating conditions, to produce hydrogen with CO₂ capture. The Pd composite membrane was fabricated on a tubular stainless steel support by the electroless plating (ELP) method. The membrane exhibited a hydrogen permeance of 2.26 × 10^?3 mol m² s^?1 Pa^?0.5, H₂/N₂ selectivity of 145 at 773 K, and pressure difference of 20.3 kPa. The MSR reaction, which was carried out at steam to carbon ratio (S/C) = 3.0, gas hourly space velocity (GHSV) = 1700 h^?1, and 773 K, showed that methane conversion increased with the pressure difference and reached 79.5% at ΔP = 506 kPa. This value was ～1.9 time higher than the equilibrium value at 773 K and 101 kPa. Comparing with the previous studies which introduced sweeping gas for low hydrogen partial pressure in the permeate stream, very high pressure difference (2500–2900 kPa) for increase of hydrogen recovery and very low GHSV (<150) for increase hydraulic retention time (HRT), our result was worthy of notice. The gas composition monitored during the long-term stability test showed that the permeate side was composed of 97.8 vol% H₂, and the retentate side contained 67.8 vol% CO₂ with 22.2 vol% CH₄. When energy was recovered by CH₄ combustion in the retentate streams, pre-combustion carbon capture was accomplished using the Pd-based composite membrane reactor.     

Kinetic characteristics of in-situ char-steam gasification following pyrolysis of a demineralized coal

This study aims to examine the char-steam reactions in-situ, following the pyrolysis process of a demineralized coal in a micro fluidized bed reactor, with particular focuses on gas release and its kinetics characteristics. The main experimental variables were temperatures (925 °C?1075 °C) and steam concentrations (15%–35% H₂O), and the combination of pyrolysis and subsequent gasification in one experiment was achieved switching the atmosphere from pure argon to steam and argon mixture. The results indicate that when temperature was higher than 975 °C, the absolute carbon conversion rate during the char gasification could easily reach 100%. When temperature was 1025 °C and 1075 °C, the carbon conversion rate changed little with steam concentration increasing from 25% to 35%. The activation energy calculated from shrinking core model and random pore model was all between 186 and 194 kJ/mol, and the fitting accuracy of shrinking core model was higher than that of the random pore model in this study. The char reactivity from demineralized coal pyrolysis gradually worsened with decreasing temperature and steam partial pressure. The range of reaction order of steam gasification was 0.49–0.61. Compared to raw coal, the progress of water gas shift reaction (CO + H₂O ? CO₂ + H₂) was hindered during the steam gasification of char obtained from the demineralized coal pyrolysis. Meanwhile, the gas content from the char gasification after the demineralized coal pyrolysis showed a low sensitivity to the change in temperature.     

A novel tubular membrane reactor for pure hydrogen production in the synthesis of formaldehyde by the silver catalyst process

Formaldehyde-based chemistry plays a significant role in the production of different materials. In this work, attempts have been made to revamp a silver catalyzed formaldehyde plant by applying membrane technology. The conventional silver catalyst packed bed reactor was replaced by a shell and tube membrane reactor. A steady-state one-dimensional model was applied to evaluate the performance of the proposed membrane process. The model was validated with experimental results from the plant.The effects of various parameters including reactor pressure, feed temperature, and membrane thickness on the membrane reactor were investigated. Results showed that the effect of feed temperature on production rates was negligible. The increase in pressure and decrease in membrane thickness, however, leads to increase products. The simultaneous production of 100 tonnes/day of formalin 37% (37 wt% formaldehyde in water) and 500 kg/day pure hydrogen achieved by the proposed process. Furthermore, the exiting reactor temperature can be reduced to 420 °C which is significantly lower than the conventional method (650 °C).     

Hydrogen permeation studies of composite supported alumina-carbon molecular sieves membranes: Separation of diluted hydrogen from mixtures with methane

One alternative for the storage and transport of hydrogen is blending a low amount of hydrogen (up to 15 or 20%) into existing natural gas grids. When demanded, hydrogen can be then separated, close to the end users using membranes. In this work, composite alumina carbon molecular sieves membranes (Al-CMSM) supported on tubular porous alumina have been prepared and characterized. Single gas permeation studies showed that the H₂/CH₄ separation properties at 30 °C are well above the Robeson limit of polymeric membranes. H₂ permeation studies of the H₂–CH₄ mixture gases, containing 5–20% of H₂ show that the H₂ purity depends on the H₂ content in the feed and the operating temperature. In the best scenario investigated in this work, for samples containing 10% of H₂ with an inlet pressure of 7.5 bar and permeated pressure of 0.01 bar at 30 °C, the H₂ purity obtained was 99.4%.     

Enhanced hydrogen permeability and reverse water–gas shift reaction activity via magneli Ti4O7 doping into SrCe0.9Y0.1O3−δ hollow fiber membrane

Hydrogen proton conducting perovskite-based hollow fiber membrane is an attractive hydrogen separation technology that shows higher stability relative to Pd-based membranes above 800 °C. One of the challenges towards high hydrogen (H₂) permeability on such proton conducting membrane is enabling simultaneously high proton and electronic conductivities to be achieved in single phase membrane. This has been addressed by developing dual-phase membrane. Here, we showed another promising approach, i.e., exploitation of beneficial phase reactions to create new conductive phases along the grain boundaries. By doping up to 8 wt. % magneli Ti₄O₇ into SrCe_0.9Y_0.1O_3?δ (SCY), Ce-doped SrTiO₃ and Y-doped CeO₂ were created in-between SCY grains. Electrical conductivity tests confirmed higher conductivities for 5 and 8 wt. % Ti₄O₇-doped SCY relative to SCY between 750 and 950 °C. These higher conductivities manifested into higher H₂ permeation fluxes for the doped SCY membranes. The highest flux of 0.17 mL min^?1 cm^?2 was observed for 5 wt. % Ti₄O₇-doped SCY at 900 °C when 50 vol. % H₂/He and 100 vol. % N₂ were used in the feed side and the permeate side, respectively. This is much higher than the flux of 0.05 mL min^?1 cm^?2 obtained from SrCe_0.9Y_0.1O₃ membrane at identical condition. More essential is the fact that the doped SCY membranes displayed catalytic activity for the reverse water-gas shift (RWGS) reaction which consumed H₂ in the permeate side; increasing the H₂ flux up to 0.57 mL min^?1 cm^?2 at 900 °C. The 5 wt. % Ti₄O₇-doped SCY furthermore showed stable flux for more than 140 h at 850 °C despite the formation of minor amount of SrCO₃ in H₂-CO₂-containing atmosphere; highlighting its potential application as membrane reactor for RWGS or dehydrogenation reaction.     

Conversion of biodiesel synthesis waste to hydrogen in membrane reactor: Theoretical study of glycerol steam reforming

Hydrogen production from waste glycerol, mainly producible as a by-product of biodiesel synthesis, is investigated as an attractive opportunity for exploiting renewable energy sources for further applications. Glycerol steam reforming using membrane technology was modeled by taking into accounts the maim transport phenomena, thermodynamic criteria and chemical process kinetics. A sensitivity analysis of operating conditions was made for key performance metrics such as glycerol conversion, hydrogen yield and hydrogen recovery. Glycerol conversion intensifies with enhancement of operating pressure and temperature, whereas high feed molar ratio and sweep ratio have limiting effect. Hydrogen permeation and subsequently, hydrogen recovery facilitates with increasing sweep gas ratio and sweep gas temperature. Hydrogen recovery enhances from 70% to 99% with increasing temperature from 350 to 500 °C at feed molar ratio of 3. Also, hydrogen recovery improves from 50% to 71% with increasing sweep ratio from 0 to 20 at 350 °C and 1 bar.     

Enhancement of membrane hydrogen separation on glycerol steam reforming in a fluidized bed reactor

Membrane hydrogen separation can effectively promote fuel conversion and hydrogen yield by means of altering chemical equilibrium of reforming reactions. In this work, the enhancing process of glycerol steam reforming via a fluidized bed membrane reactor is numerically investigated. Under the framework of the Euler-Euler method, chemical kinetic model is implemented and the reforming performance with and without membrane separation is compared. The effect of densified zones caused by membrane separation is examined. Meanwhile, the impacts of operating parameters including hydrogen partial pressure on the permeate side and fuel gas velocity on densified zones and hydrogen yield are evaluated. The results demonstrate that the excessive reduction of hydrogen partial pressure on the permeate side and the increase of feed gas velocity are detrimental to fuel conversion and hydrogen yield.     

A solid state thermogalvanic cell harvesting low-grade thermal energy

A high performance polymer electrolyte thermogalvanic cell, which converts thermal energy to electrical energy directly, is transformed from a proton exchange membrane fuel cell. The transform is realized by connecting the anode and cathode chamber with a gas tube and filling hydrogen to both chambers. Provided a heat flux through the cell, hydrogen is consumed in the cold side and regenerated in the hot side while circulating in two chambers during operation. The Seebeck coefficient is 0.531 mV K^?1 at a cold side temperature of 60.0 °C and the maximum power density could reach up to 20 μW cm^?2 with a temperature difference of 15.3 °C between two electrodes.     

Membrane reformer module with Ni-foam catalyst for pure hydrogen production from methane: Experimental demonstration and modeling

Results of experiments and modeling of a compact (800 cm³) membrane reformer module for the production of 0.25–0.30 Nm³/h hydrogen by methane steam reforming are reported. The module consists of a two-sided composite membrane disc with a 50 μm PdAg layer and two adjacent 4 mm thick Ni foam discs (60 ppi). A nickel catalyst and a porous support were deposited on the foam discs to give the final composition of 10%Ni/10%MgO/Ni-foam. Membrane permeability by pure hydrogen was investigated, and coefficients of transverse hydrogen transport across the Ni foam to the membrane in the case of inlet binary N₂H₂ mixture were refined in order to account for concentration polarization effect into the model. Activity of the catalytic discs was measured in a differential laboratory scale reactor at a pressure of 1 bar and temperature of 400–600 °C. Modules were tested at a 8–13 bar pressure of the mixture in the reforming zone and at 1 bar of pure hydrogen under the membrane, H₂O/C = 2.5–3 and a module temperature of 550–680 °C (with and without hydrogen removal). Two modifications of the module were tested: consecutive (I-type) and parallel (II-type) flow of the reaction mixture around two sides of the membrane disc. In order to optimize construction of the module, calculations were made for revealing the effect of thickness of the PdAg membrane layer (5–50 μm), thickness of the Ni foam discs (0.5–8 mm) and temperature (600–700 °C) on the hydrogen output of the module. A comparison of the values obtained in our experiments (>1 MW/m³ and >0.7 kg(H₂)/h/m²) with the literature data reported by other authors showed that the developed modules are promising for practical application as components of a fuel processor section for mobile applications.     

Characteristics and prediction model of hydrogen production of oily sludge by supercritical water gasification

Harmless treatment and resource utilization of oily sludge are urgent and related to the sustainable green, and low-carbon development of the petroleum industry. Aiming to the supercritical water gasification (SCWG) of oily sludge for hydrogen production, this paper investigated the effects of critical factors, including reaction temperature, initial pressure, retention time, and feed concentration, on the mole fraction, the gas yield, the gasification efficiency, and the hydrogen yield potential. The interaction mechanisms among these four factors were discussed and revealed with a reasonable prediction model of hydrogen production. Results showed that the longer retention time, higher temperature, and lower feed concentration could accelerate hydrogen production from oily sludge by SCWG. The synthetic promotion of the hydrogen yield exists between the temperature and the retention time, while the temperature predominates. A 2.63-fold increase in the H₂ yield was obtained when the condition changed from 135 min to 380 °C to 10 min and 555 °C. The hydrogen production of oily sludge by SCWG, at lower temperature and higher pressure was worse than that at higher temperature and lower pressure.