Hydrogen separation from synthesis gas using silica membrane: CFD simulation

In the present article, an axisymmetric two-dimensional (2D) computational fluid dynamic (CFD) model was adapted to predict the efficiency of the silica membrane for hydrogen (H₂) separation as a renewable energy source. In this model, continuum flows on the shell and tube sides are defined through the Navier-Stokes and transport of chemical species equations. Components transfer through the silica membrane is characterized by introducing source-sink terms based on activating transport mechanisms. To validate the presented model results related to H₂ molar fraction at the retentate and permeate sides were compared with experimental data. The CFD model prognosticates the local information of velocity distribution and the molar fraction of the components. Finally, considering the effects of temperature, pressure difference, gas flow rate, and inner radius of the module on the H₂ molar fraction, silica membrane performance was investigated. Moreover, it has been shown that with increasing working temperature from 323 to 473 K, H₂ molar fraction at the shell side decreases from 59% to 28.4%, and in the tube side, it rises from 78.8% to 82.8%. On the shell side, it could be seen that H₂ permeates better for a low gas flow rate. At the tube side, this parameter has a positive effect on H₂ purification. The result of the impact of pressure differences at shell and tube sides was used to indicate the variation in the H₂ molar fraction. An increase in pressure difference causes a decrease of H₂ molar fraction at the tube side. At the shell side, H₂ molar fraction would be decreased with an addition in pressure difference from 1 to 3 bar. Any further pressure difference rise from 3 to 4 bar, make this trend ascending. Likewise, at the shell and tube sides, by enhancing the inner radius of the module, the molar fraction of H₂ increases.     

Hydrogen production from waste activated sludge by using separation membrane acid fermentation reactor and photosynthetic reactor

The possibility and characteristics of hydrogen production from waste activated sludge were investigated using separation membrane acid fermentation reactor (AR) and photosynthetic reactor (PR). The AR used submerged and external separation membranes and it was followed by the PR. The COD removal efficiencies in the AR with submerged and external separation membrane were about 65% and 40%, respectively. More VFA was produced in the AR with external separation membrane than AR with submerged separation membrane. Hydrogen was produced in the PR but not in the AR and hydrogen productions in the PR connected with submerged membrane AR and external membrane AR were about 50.1 and 160.5 ml _H2/g${\mathrm{H}}_2/\mathrm{g}$ T-VFA, respectively.     

Hydrogen production from ammonia by the plasma membrane reactor

A new plasma membrane reactor (PMR) was developed to efficiently produce hydrogen from NH₃ with the use of atmospheric pressure plasma and a hydrogen separation membrane. The generation of high-purity hydrogen from NH₃ was also examined. First, hydrogen gas flowing into the PMR revealed the effect of the PMR on hydrogen separation. Hydrogen separation depends on the partial pressure of hydrogen gas supplied (P_{in, H2}) and permeated (P_{out, H2}) when P_{in, H2}^0.5 − P_{out, H2}^0.5 > 0. Second, NH₃ gas flowing into the PMR revealed its hydrogen production characteristics: the maximum hydrogen conversion rate of a typical plasma reactor (PR) is 13%, whereas the PMR converted 24.4%. Hydrogen obtained by hydrogen separation was approximately 100% pure. A hydrogen generation rate of 20 mL/min was stably obtained.     

Hydrogen production and carbon dioxide enrichment using a catalytic membrane reactor with Ni metal catalyst and Pd-based membrane

We prepared a catalytic membrane reactor (CMR) by adopting a high-performance metal catalyst and Pd–Au membrane to investigate the possibility of hydrogen production concurrently with carbon dioxide enrichment (up to >80%) in a single-stage reactor from a simulated syngas of a coal gasification, via simultaneous WGS reaction and hydrogen separation process. The CO conversion was above 99% and the H₂ recovery was above 94% at del-P = 30 bar in a CMR. The best result for the concentration of the enriched CO₂ in the retentate side was 85.3% under the conditions of 350 °C, del-P = 30 bar and steam to carbon ratio of 2.0. These results show promise for a feasible simplified process able to achieve CO removal from a high-concentration CO mixture gas coming out of coal gasification via a water-gas shift reaction (WGS), to separate hydrogen and also to enrich CO₂ for pre-combustion capture and storage of CO₂ (CCS) in substitution for the conventional WGS and CO₂ separation stages in integrated gasification and combined cycle process integrated with CCS.     

Hydrogen production system combined with a catalytic reactor and a plasma membrane reactor from ammonia

Ammonia is a 1promising raw material for hydrogen production because it may solve several problems related to hydrogen transport and storage. Hydrogen can be effectively produced from ammonia via catalytic thermal decomposition; however, the resulting residual ammonia negatively influences the fuel cells. Therefore, a high-purity hydrogen production system comprising a catalytic decomposition reactor and a plasma membrane reactor (PMR) has been developed in this work. Most of the ammonia is converted to hydrogen and nitrogen by the catalytic reactor. After the product gas containing unreacted ammonia is introduced to the PMR, unreacted ammonia is decomposed and hydrogen is separated in the PMR. Based on these processes, hydrogen with a purity of 99.99% is obtained at the output of the PMR. Optimal operation conditions maximizing the hydrogen production flow rate were investigated. The gap length of the PMR and the gas differential pressure and applied voltage of the plasma influence the flow rate. A pure hydrogen flow rate of ∼120 L/h was achieved using the current operating conditions. The maximum energy efficiency of the developed hydrogen production system is 28.5%.     

Hydrogen separation from blended natural gas and hydrogen by Pd-based membranes

Hydrogen separation membranes based on a heated metal foil of a palladium alloy, offer excellent permeability for hydrogen as a result of the solution-diffusion mechanism. Here, the possibility to separate hydrogen from the mixture of Natural Gas (NG) and hydrogen (NG+H₂) with various NG concentrations using Pd, PdCu₅₃ and PdAg₂₄ hydrogen purification membranes is demonstrated. Hydrogen concentrations above ∼25% (for Pd and PdCu₅₃) and ∼15% (for PdAg₂₄) were required for the hydrogen separation to proceed at 400 °C and 5 bar pressure differential. Hydrogen permeability of the studied alloys could be almost fully recovered after switching the feed gas to pure hydrogen, indicating no significant interaction between the natural gas components and the membranes surface at the current experimental condition. Hydrogen flux of the membranes at various pressure differential was measured and no changes in the hydrogen permeation mechanism could be noticed under (NG 50%+H₂) mixture. The hydrogen separation capability of the membranes is suggested to be mainly controlled by the operating temperature and the hydrogen partial pressure.     

Hydrogen production from water gas shift reactions in association with separation using a palladium membrane tube

Hydrogen production from water gas shift reactions (WGSRs) of synthesis gas (syngas) followed by separation via a Pd membrane was studied experimentally. In the reactions, a variety of combinations of a high‐temperature shift reaction (HTSR), a low‐temperature shift reaction (LTSR) and a palladium (Pd) membrane tube were considered. The results indicated that the CO conversion from the LTSR was close to that of the HTSR and LTSR in series; however, the latter with the Pd membrane could provide a much low CO concentration at the permeate side. On the other hand, while the produced hydrogen diffused through the membrane, methane was also found at the both sides of the membrane due to the methanation reaction activated by the Pd membrane. In the present system, increasing the steam/CO ratio enhanced the forward reaction of the WGSRs and elongated the residence time of the reactants in the catalyst beds, resulting in the increases of CO conversion and hydrogen recovery. As a whole, the concentration of CO in the separated hydrogen was lower than 50 ppm from the combination of the HTSR and the LTSR with the membrane, whereby the produced hydrogen could be applied in proton exchange membrane fuel cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen recovery from ammonia purge gas by a membrane separator: A simulation study

In this study, a two‐dimensional mathematical model is proposed for modeling the hollow fiber membrane (HFM) separators for hydrogen (H₂) recovery unit implemented in the Razi Petrochemical Company (Imam Khomeini Port, Iran) to capture hydrogen from ammonia (NH₃) purge gas. In this regard, computational fluid dynamics is applied to solve the equations of momentum and mass transfer in the laminar flow conditions. Axial and radial diffusion for mass transfer inside the membrane fibers and axial diffusion within the shell side of separator were considered. The distributions of concentration, velocity, and mass transfer fluxes were achieved by the model. As the new insights, the effects of feed flow rate and feed gas concentration on mass transfer of H₂ were investigated. Moreover, fluid velocity profile and H₂ fluxes in the tube (fiber), membrane, and shell sides of the HFM separator were studied. The results of simulation were compared with the industrial data and showed that the present developed model has excellent agreement with the experimental data with a low mean deviation value of 3.5%.     

Hydrogen recovery from methylcyclohexane as a chemical hydrogen carrier using a palladium membrane reactor

A Pd membrane has been prepared by electroless plating on the surface of a porous TiO₂ tube in this work. The mean thickness of the resulting Pd membrane on the modified tube was 13 µm. The hydrogen permeation flux was as high as 0.16 mol m^?2 s^?1 with a pressure difference of 108.75 Pa at 648 K. H₂ permeances were 1.6 × 10^?7–6.4 × 10^?7/molm^?2 s^?1 Pa^?1/at 580–650 K. The separation factor of H₂/N₂ was over 1,000. Measurements of the temperature coefficient for hydrogen permeation through the membrane gave a value of 9.49 kJ mol^?1 in good agreement with previous reports. The results showed that the prepared membranes can be used in membrane reactors for H₂ permeation in the dehydrogenation of methylcyclohexane.     

Production of synthesis gas by co-gasifying coke and natural gas in a fixed bed reactor

The production of synthesis gas has gained increasing importance because of its use as raw material for various industrial syntheses. In this paper synthesis gas generation during the reaction of a coal/methane with steam and oxygen, which is called the co-gasification of coal and natural gas, was investigated using a laboratory scale fixed bed reactor. It is found that about 95% methane conversion and 80% steam decomposition have been achieved when the space velocity of input gas (oxygen and methane) is less than 200 h⁻¹ and reaction temperature about 1000 °C. The product gas contains about 95% carbon monoxide and hydrogen. The reaction system is near the equilibrium when leaving the reactor.     

Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: Proof-of-Concept

This paper presents a new membrane reactor concept for ultra-pure hydrogen production with integrated CO₂ capture: the membrane-assisted gas switching reforming (MA-GSR). This concept integrates alternating exothermic and endothermic redox reaction stages in a single fluidized bed consisting of catalytically active oxygen-carrier particles, by switching the feed between air and methane/steam, where the produced hydrogen is selectively removed via Pd-based membranes. This concept results in overall autothermal conditions and allows easier operation at high pressure compared to alternative novel technologies. In this work, the MA-GSR concept is demonstrated at lab scale using four metallic supported membranes (Pd–Ag based) immersed into a fluidized bed consisting of a Ni-based oxygen carrier. The performance of the reactor has been tested under different experimental operating conditions and high methane conversions (>50%) have been obtained, well above the thermodynamic equilibrium conversion of a conventional fluidized bed as a result of the selective H₂ extraction, with (ultra-pure) H₂ recoveries above 20% at relatively low temperatures (<550 °C). These results could be further improved by working at elevated pressures or by integrating more membranes. Even though the concept has been successfully demonstrated, further research is required to develop suitable membranes since post-mortem membrane characterization has revealed defects in the membrane selective layer as a consequence of the frequent exposure to thermal cycles with alternating oxidative and reducing atmospheres.     

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.     

A membrane-assisted hydrogen and carbon oxides separation from flare gas and recovery to a commercial methanol reactor

The push to control the greenhouse gas emissions is motivated by environmental regulations. For the aim to be achieved, the suggestion of eliminating or at least reducing gas flaring is currently taken under environment. In this regard, a new configuration for flare gas treatment is proposed in this study. This configuration is aimed to collect H₂ and CO₂ from flare gas, simultaneously. The collected components would be sent to the methanol synthesis reactor in the upstream section. The proposed configuration is made up of a multi-step membrane-assisted separation unit. In order to clarify what lies behind the idea, we proposed a mathematical formulation which is composed of conservation equations and kinetic rate equations is developed. H₂ and CO₂ elimination in the first step followed by a membrane-assisted water gas shift reactor for catalytic CO conversion and H₂ recovery in tandem, and removing the remaining CO₂ in the supplementary step is investigated numerically. The collected H₂/CO₂ mixture is aimed to recover into the upstream methanol synthesis reactor. The obtained results reveal that by utilizing such a strategy, about 2500 kmol/day CO₂ (almost 98% of total input) is eliminated from the flare gas stream. Moreover, by considering the converted CO, about 4050 kmol/day CO₂ is recovered to the methanol reactor. As a whole, 0.68% enhancement in the methanol generation and the reduction of about 4050 kmol/day flare gas pollutants are achieved in tandem when 98% N₂ and 92.9% CH₄ is separated the from purge gas.     

Hydrogen gas production from sago industry wastewater using electrochemical reactor: Simulation and validation

This study describes the production of hydrogen gas from sago industry wastewater under varying operating conditions, such as time of electrolysis, electrode surface area, and current intensity. The process was investigated by employing a four-factor three-level Box–Behnken statistical design (BBD). The results were examined using analysis of variance (ANOVA). This study developed a second-order polynomial model and utilized three-dimensional (3D) response graphs to study the interactive effect of process variables on the production of hydrogen gas. The optimum conditions for maximum hydrogen gas production were determined and found to be 45 cm² for electrode surface area, 22 min for time of electrolysis, and 13 A for current intensity. The predicted hydrogen gas output in optimum conditions was 1.12 mL/L. These results show that it is possible to achieve efficient hydrogen gas production, by electrochemically processing in sago industry wastewater.     

Hydrogen enrichment effects on the second law analysis of a lean burn natural gas engine

This paper presents a computational work aimed at investigating the effects of hydrogen addition on the exergy (or availability) balance in a lean burn natural gas spark ignition (SI) engine. A thermodynamic engine cycle simulation was extended to perform the exergy analysis. A zero dimensional, two-zone computational model of the engine operation was used for the closed part of the cycle. The results of the model were compared with experimental data to demonstrate the validation of the model. Exergetic terms, such as exergy transfer with heat, exergy transfer with work, irreversibilities, fuel chemical exergy, and total exergy, were computed based on principles of the second law. The exergetic (the second law) efficiency was also calculated. The results of exergy analysis show that increasing hydrogen content and lean burn have considerably affected the exergy transfers, irreversibilities and second law efficiency. With increasing hydrogen content, the irreversibility produced during combustion decreases, and the second-law efficiency sharply increases at near the lean limit.     

Hydrogen production by silica membrane reactor during dehydrogenation of methylcyclohexane: CFD analysis

A comprehensive computational fluid dynamic model has been developed using COMSOL Multiphysics 5.4 software to predict the behavior of a membrane reactor in dehydrogenation of methylcyclohexane for hydrogen production. A reliable reaction kinetic of dehydrogenation reaction and a permeation mechanism of hydrogen through silica membrane have been used in computational fluid dynamic modeling. For performance comparison, an equivalent traditional fixed bed reactor without hydrogen removal has been also modeled. After model validation, it has been used to evaluate the operating parameters effect on the performance of both the silica membrane reactor and the equivalent traditional reactor as well. The operating temperature ranged between 473 and 553 K, pressure between 1 and 2.5 bar, sweep factor from ?6.22 to 25 and feed flow rate from 1 to 5 × 10^?6 mol/s. The membrane reactor performed better than the equivalent traditional reactor, achieving as best result complete methylcyclohexane conversion and 96% hydrogen recovery.     

Enhancement of pure hydrogen production through the use of a membrane reactor

Pure hydrogen production is of great interest as it is an energy carrier which can be used in PEM fuel cells for power production. Methane Steam Reforming (MSR) is commonly used for hydrogen production although the produced hydrogen is not free of other components. Membrane Reactors (MR) enable a pure hydrogen product stream and allows the reaction to take place at significantly lower temperatures (lower than 550 °C) than in conventional reactors (greater than 800 °C) with comparable methane conversion. This is achieved by hydrogen removal through a permselective Pd–Ag based membrane that cause a favorable shift in chemical equilibrium towards hydrogen production. In the present study, a two-dimensional, nonlinear, and pseudo-homogeneous mathematical model of a catalytic fixed-bed membrane reactor for methane steam reforming over a nickel-based foam supported catalyst is presented. Simulated results referring to the distribution of species, methane conversion, temperature and hydrogen flowrate along the reactor for different radial positions are obtained and analyzed. The performance of structured catalyst and catalyst supported on foam configurations under the same operating conditions is also studied. Experimental results for the membrane facilitate the identification of suitable operating conditions.     

Hydrogen networks synthesis considering separation performance of purifiers

In hydrogen networks, purifiers are quite often used to reduce operating costs. They should be properly integrated with the whole network in order to maximize the benefit. In this paper, a graphical method is proposed for targeting the minimum fresh resource consumption of hydrogen networks considering separation performance of purifiers. The material balance of the whole hydrogen network shows that the extent of fresh hydrogen reduction is subject to the maximum hydrogen surplus. Based on such observation, the mass transfer triangle is developed to describe the hydrogen transformation from maximum hydrogen surplus to fresh hydrogen. With both the purity and the flow rate of purification streams optimized, the minimum fresh hydrogen consumption can be determined through the proposed graphical method. Two cases are studied to illustrate the proposed methodology.     

Hydrogen production by steam methane reforming in a membrane reactor equipped with a Pd composite membrane deposited on a porous stainless steel

With the aim of producing hydrogen at low cost and with a high conversion efficiency, steam methane reforming (SMR) was carried out under moderate operating conditions in a Pd-based composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst. A Pd-based composite membrane with a thickness of 4–5 μm was prepared on a tubular stainless steel support (diameter of 12.7 mm, length of 450 mm) using electroless plating (ELP). The Pd-based composite membrane had a hydrogen permeance of 2.4 × 10^?3 mol m^?1 s^?1 Pa^?0.5 and an H₂/N₂ selectivity of 618 at a temperature of 823 K and a pressure difference of 10.1 kPa. The SMR test was conducted at 823 K with a steam-to-carbon ratio of 3.0 and gas hourly space velocity of 1000 h^?1; increasing the pressure difference resulted in enhanced methane conversion, which reached 82% at a pressure difference of 912 kPa. To propose a guideline for membrane design, a process simulation was conducted for conversion enhancement as a function of pressure difference using Aspen HYSYS^®. A stability test for SMR was conducted for ～120 h; the methane conversion, hydrogen production rate, and gas composition were monitored. During the SMR test, the carbon monoxide concentration in the total reformed stream was <1%, indicating that a series of water gas shift reactors was not needed in our membrane reactor system.     

Hydrogen production by glycerol reforming in a two-fixed-bed reactor

A two-stage fixed bed system was used in the hydrogen production from glycerol reforming. The calcined dolomite catalyst was used in the first fixed bed, and the Nickel-based catalyst was used in the second fixed bed to produce hydrogen from the glycerol steam reforming. The results showed that the hydrogen yield and carbon conversion gradually increased with the temperature increasing. When the temperature exceeded 800 °C, the growth rate of hydrogen yield and carbon conversion decreased. As the space velocity increased, the hydrogen yield and carbon conversion gradually decreased. When the space velocity was greater than 2 h^?1, the decline rate of hydrogen yield and carbon conversion decreased rapidly. As the water-to-carbon ratio (S/C) increased, the hydrogen yield and carbon conversion gradually increased. The growth rate of hydrogen yield and carbon conversion became smaller when the S/C was more than 5. Compared with the single-stage fixed-bed reactor, the utilization of two-stage fixed-bed catalytic reaction system can not only increase the hydrogen yield and carbon conversion, but extend the life of the Nickel-based catalyst. Under the optimal reaction conditions, the hydrogen yield is as high as 84.3%, and the carbon conversion is as high as 88.23%.