Two dimensional modelling of water uptake in proton exchange membrane fuel cell

With the focus on water uptake by proton exchange membrane, a two-phase, non isothermal, transient and two-dimensional model of fuel cell is developed. Further, in order to obtain the equilibrium concentration of water in the membrane, two different approaches of water-uptake by the membrane are considered; though each takes into account the Schroeder's paradox as well as individual contributions of water vapour and liquid water. Furthermore, in both the approaches, rate of water uptake is proportional to the difference between equilibrium concentration and actual concentration of water in membrane. Model results show good agreement with the experimental results. A comparative analysis of the two approaches has been presented for various results, such as liquid saturation, net drag coefficient, temperature, water content in membrane, etc. Obtained results revealed significant difference between predicted current densities, water content of membrane and temperatures for the two approaches. These differences may be reflecting the need to correctly understand water uptake by membrane and its importance for accurate modelling of fuel cell. Response in transient state of fuel cell is also studied when a step change to cell voltage is applied. Likewise, studies on rate of sorption and desorption of water by membrane explain the increase or decrease of the water content of membrane.     

油溶解气体分离影响因素及碳纳米管薄膜应用研究

从膜分离技术原理出发,研究了膜材料、膜结构、气室结构以及温度等对渗透效率和平衡时间的影响,通过仿真和试验测试发现,采用新型碳纳米管膜以及合理控制结构和环境因素可以提高渗透效率和缩短平衡时间,为膜分离技术的有效应用提供参考。     

Supporting the propane dehydrogenation reactors by hydrogen permselective membrane modules to produce ultra-pure hydrogen and increasing propane conversion: Process modeling and optimization

The main aim of this research is supporting the moving bed radial flow reactors in the propane dehydrogenation unit by membrane modules to produce ultra-pure hydrogen and increasing equilibrium conversion. The propane dehydrogenation is a thermodynamically limited and endothermic reaction, which decreasing hydrogen concentration and increasing operating temperature in the system could increase equilibrium conversion. In the first step, the conventional and membrane supported reactors are heterogeneously modeled based on the mass and energy conservation laws considering catalyst decay. To verify the precision of developed model, the results of simulation are compared with the available plant data. Then, the performance of designed membrane configuration is compared with the conventional process at the same feed condition. It appears that the activity of catalyst decreases along the reactors due to coke build up on the catalyst surface, and it results in the lower propane conversion. The results show that supporting the conventional reactors by hydrogen permselective membrane module increases propane conversion up to 3.09%.     

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.     

Production of hydrogen from hydrogen sulfide by means of selective diffusion membranes

The possibilities of using microporous ceramic membranes were examined for the production of hydrogen from hydrogen sulfide. A microporous Vycor-type glass tubing membrane of a mean pore diameter of 45 Å and new microporous alumina tubing membrane of diameter 1020 Å could be used up to 800°C and at higher temperatures, respectively, as a membrane for separating hydrogen and hydrogen sulfide. The microporous alumina tubing membrane has a 30-fold higher permeability than the microporous Vycor glass tubing membrane. When these membranes were applied to the direct decomposition of hydrogen sulfide, the yield of hydrogen increased by about two times the equilibrium yield calculated for the process without hydrogen removal.     

Membrane bubble column reactor model for the production of hydrogen by methane pyrolysis

A membrane reactor model is developed to describe, model, and design molten metal methane pyrolysis bubble column reactors. It is utilized to demonstrate that a membrane reactor allows conversions in excess of the equilibrium conversion implied by the feed and operating conditions. Ultra-high conversion eliminates the need to separate product hydrogen from unreacted methane, thereby eliminating the need to recycle un-reacted methane and reducing the total equipment sizes and energy costs. Furthermore, it is shown that the hydrogen can be completely removed through the membrane reactor walls before the gas bubbles breakthrough the molten metal layer into the reactor headspace. The equations also apply to non-membrane reactors, and are therefore useful for future general conceptual design studies. The general applicability is demonstrated by comparison of the model predictions to published experimental data on methane pyrolysis in a non-membrane bubble column reactor.     

Enhancement of hydrogen production in a novel fluidized-bed membrane reactor for naphtha reforming

In this work, a novel fluidized-bed membrane reactor (FBMR) for naphtha reforming in the presence of catalyst deactivation has been proposed. In this reactor configuration, a fluidized-bed reactor with perm-selective Pd–Ag (23 wt% Ag) wall to hydrogen has been used. The reactants are flowing through the tube side which is a fluidized-bed membrane reactor while hydrogen is flowing through the shell side which contains carrier gas. Hydrogen penetrates from fluidized-bed side into the carrier gas due to the hydrogen partial pressure driving force. Hydrogen permeation through membrane leads to shift the reaction toward the product according to the thermodynamic equilibrium. This membrane-assisted fluidized-bed reactor configuration solves some drawbacks of conventional naphtha reforming reactors such as pressure drop, internal mass transfer limitations and radial gradient of concentration and temperature. In FBMR the hydrogen which is produced in shell side is a valuable gas and can be used for different purposes. The two-phase theory of fluidization is used to model and simulate the FBMR. Industrial packed bed reactor (PBR) for naphtha reforming is used as a basis for comparison. This comparison shows enhancement in the yield of aromatic production in FBMR for naphtha reforming. Although using FBMR reduces hydrogen mole fraction in reaction side and enhances catalyst deactivation due to coking, but this effect can be compensated using advantages of FBMR such as suitable hydrogen to hydrocarbon molar ratio, lowering deactivation rate due to lower temperature, control of permeation rate by adjusting shell side pressure and shifting the equilibrium reactions. The impacts of hydrogen to hydrocarbon molar ratio, pressure, membrane thickness, flow rate and temperature have been investigated in this work.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

Analysis of in situ water transport in Nafion by confocal micro-Raman spectroscopy

A proton exchange membrane fuel cell (PEMFC) is a promising alternative source of clean power for automotive applications, but its acceptance in such applications depends on reducing its costs and increasing its power density to achieve greater compactness. To meet these requirements, further improvements in cell performance are required. In particular, when the fuel cell is operating at high current density, the transport of water through the membrane has considerable impacts on the performance because of the large concentration gradient of water between the cathode and anode. Through-plane water permeation across the membrane is therefore a fundamental process in operational PEMFCs. Recently, resistance to water transport at the membrane-gas interface has been reported, and this is affected by the temperature and relative humidity. We investigated the distribution of water inside a proton exchange membrane during a water permeation test by using confocal micro-Raman spectroscopy with a fine spatial resolution (2-3 μm). In the presence of a water flux, the local water content is not necessarily in equilibrium with the water activity in the gas phase. Interfacial water-transport resistance due to the presence of a non-equilibrium membrane structure at the interface cannot be neglected.     

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.     

Equilibrium potential for the electrochemical Bunsen reaction in the sulfur–iodine cycle

The electrochemical Bunsen reaction was carried out in an electrochemical cell, where the anodic and cathodic compartments were separated by a Nafion 117 membrane. The equilibrium potential of the cell was experimentally measured and theoretically modeled. The effect of electrolyte concentration and temperature was explored. An increase in SO₂ or I₂ concentration reduced the equilibrium potential, whereas increasing H₂SO₄ or HI concentration had a contrary effect. The cell equilibrium potential decreased with increasing temperature. The derived theoretical equilibrium potential model was verified by the experimental data. The regression parameters M and Z in the model were independent of electrolyte concentration, but M decreased and Z kept constant with increasing temperature. An empirical equilibrium potential formula was proposed based on the theoretical and experimental results. The good reproducibility of this formula for measured data indicated its feasibility to estimate the equilibrium potential and also its guidance for optimizing the electrochemical Bunsen reaction.     

Simulation of a porous ceramic membrane reactor for hydrogen production

A systematic simulation study was performed to investigate the performance of a porous ceramic membrane reactor for hydrogen production by means of methane steam reforming. The results show that the methane conversions much higher than the corresponding equilibrium values can be achieved in the membrane reactor due to the selective removal of products from the reaction zone. The comparison of isothermal and non-isothermal model predictions was made. It was found that the isothermal assumption overestimates the reactor performance and the deviation of calculation results between the two models is subject to the operating conditions. The effects of various process parameters such as the reaction temperature, the reaction side pressure, the feed flow rate and the steam to methane molar feed ratio as well as the sweep gas flow rate and the operation modes, on the behavior of membrane reactor were analyzed and discussed.     

Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production

HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion as well as highly corrosive environment. Corrosion-resistant metal membrane reactor is proposed to be a process intensification tool, which can enable efficient HI decomposition by enhancing the equilibrium conversion value. Here we report corrosion resistance studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. Small pieces of the coated tubes were subject to immersion coupon tests in HI-water environment (57 wt% HI in water) at a temperature of 125–130 °C under reflux environment, and simulated HI decomposition environment at 450 °C. The unexposed and exposed cut pieces were analyzed using scanning electron microscope (SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium.     

Kinetic and thermodynamic analyses of mid/low-temperature ammonia decomposition in solar-driven hydrogen permeation membrane reactor

It is a promising method for hydrogen generation without carbon emitting by ammonia decomposition in a catalytic palladium membrane reactor driven by solar energy, which could also store and convert solar energy into chemical energy. In this study, kinetic and thermodynamic analyses of mid/low-temperature solar thermochemical ammonia decomposition for hydrogen generation in membrane reactor are conducted. Hydrogen permeation membrane reactor can separate the product and shift the reaction equilibrium forward for high conversion rate in a single step. The variation of conversion rate and thermodynamic efficiency with different characteristic parameters, such as reaction temperature (100–300 °C), tube length, and separation pressure (0.01–0.25 bar), are studied and analyzed. A near-complete conversion of ammonia decomposition is theoretically researched. The first-law thermodynamic efficiency, net solar-to-fuel efficiency, and exergy efficiency can reach as high as 86.86%, 40.08%, and 72.07%, respectively. The results of this study show the feasibility of integrating ammonia decomposition for hydrogen generation with mid/low-temperature solar thermal technologies.     

Radiation-induced mass transfer through membranes

The influence of excitation of molecules on mass transfer in membranes is investigated theoretically. It is shown that excitation of the molecules in the gas phase at one side of the membrane can lead to the occurrence of the resulting mass flux in an initially equilibrium system.     

Methane membrane steam reforming: Heat duty assessment

Membrane reactors are an innovative technology with huge application potentialities for equilibrium limited endothermic reactions. Assembling a membrane selective to a reaction product avoids the equilibrium conditions to be achieved, supporting the reactions at lower operating temperatures. Taking as an example the natural gas steam reforming, a methane conversion around 98% can be reached imposing an operating temperature of 823 K, much lower than that of the traditional process. In the present paper, a stringent analysis of heat power requirement needed to carry out the natural gas steam reforming process by applying a membrane reactor is made. The simulations allows to understand how the main operating parameters (inlet temperature, inlet methane flow-rate, steam to carbon ratio, ratio between sweeping steam and inlet methane, operating reaction pressure) influence the total heat power required by the process, divided among power contributions for the reaction heat duty, reactant steam and permeation steam generation and preheating. Moreover, the specific thermal energy per mole of pure H₂ is computed and assessed. Optimizing the operating conditions set, a specific thermal energy per mole of pure hydrogen of 92.3 kWh kmol⁻¹ is obtained corresponding to a total thermal power of 687.4 kW required to convert, in a single membrane reactor, a methane flow-rate of 2 kmol h⁻¹ (GHSV = 9.590 h⁻¹) with a conversion around 98%.     

A comparative study on a novel combination of spherical and membrane tubular reactors of the catalytic naphtha reforming process

Refineries have been looking for ways of improving the performance of the reformer by enhancing the octane number of the product via increasing the aromatics content. To reach this goal, more improved configurations should be investigated. The aromatics production rate could be enhanced by shifting the reactions to the production side by using hydrogen perm-selective membranes. In the present study, we have investigated theoretically the best combination of membrane tubular reactors and spherical radial-flow reactors for the conventional naphtha reforming unit consist of three fixed-bed reactors. Hydrogen permeation through the membrane shifts the reaction to the product side (aromatics and hydrogen) according to the thermodynamic equilibrium. Spherical reactors reduce the pressure drop in the catalytic naphtha reforming units and consequently increase the efficiency. The results show higher aromatics production in the new configurations compared with the membrane tubular and conventional reactors despite using lower membrane surface area.     

A theoretical model of the membrane electrode assembly of liquid feed direct methanol fuel cell with consideration of water and methanol crossover

An algebraic model of the membrane electrode assembly of the direct methanol fuel cell is developed, which considers the simultaneous liquid water and methanol crossover effects, and the associated electrochemical reactions. The respective anodic and cathodic polarization curves can be predicted using this model. Methanol concentration profile and flux are correlated explicitly with the operating conditions and water transport rate. The cathode mixed potential effect induced by the methanol crossover is included and the subsequent cell voltage loss is identified. Water crossover is influenced by the capillary pressure equilibrium and hydrophobic property within the cathode gas diffusion layer. The model can be used to evaluate the cell performance at various working parameters such as membrane thickness, methanol feed concentration, and hydrophobicity of the cathode gas diffuser.     

Study of current distribution and oxygen diffusion in the fuel cell cathode catalyst layer through electrochemical impedance spectroscopy

In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.     

A simulation study of the steam reforming of methane in a dense tubular membrane reactor

The methane steam reforming reaction has been investigated from a modelling viewpoint, considering the effect of different parameters on methane conversion. For example, considering the influence of the lumen pressure on methane conversion at constant temperature, it has been found that increasing this parameter, equilibrium methane conversion increases for the membrane reactor, while it decreases for the traditional one. Moreover, in a realistic membrane reactor (i.e. considering a simulation performed using kinetic expressions), the behaviour of methane conversion versus lumen pressure at various temperatures shows a minimum value, depending on the membrane thickness, on the reactor length and on the temperature.