Reactions In Membrane Reactors Research Articles

Low-Carbon Hydrogen Production via Ethanol Reforming Reactions in Membrane Reactors: Recent Advances and Future Directions

Low-Carbon Hydrogen Production via Ethanol Reforming Reactions in Membrane Reactors: Recent Advances and Future Directions

Enhancement of oxygen flux through Nb-doped La0.4Sr0.6FeO3-δ ceramic hollow fiber membranes by Fe exsolution

In recent years, numerous studies have focused on the use of mixed ionic-electronic conducting oxides for coupled oxygen separation and catalytic reactions in membrane reactors. A promising strategy for the efficient fabrication of oxygen separation membranes involves modification of the membrane surface via exsolved metal nanoparticles decoration. Here, we present a detailed characterization of the structural and transport properties of a novel membrane material La0.4Sr0.6Fe0.95Nb0.05O3−δ (LSFNb5). The exsolution of Fe nanoparticles was observed after heating of LSFNb5 in a reducing atmosphere (5 % Н2/Ar) and was confirmed by X-ray diffraction analysis, Mössbauer spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The formation of Fe nanoparticles on the surface of LSFNb5 hollow fiber membrane in the reduction process leads to the enhancement of oxygen fluxes and reduces the apparent activation energy. Kinetic parameters for oxygen transport through LSFNb5 hollow fiber membrane estimated using two different models, are in good agreement with the experimental results. Furthermore, LSFNb5 hollow fiber membrane demonstrates stable performance both before and after surface treatment.

Thermodynamic analysis and optimization of solar methane dry reforming enhanced by chemical hydrogen separation

Thermodynamically limited reactions in membrane reactors for hydrogen generation require a high hydrogen concentration gradient for separation, which imposes an energy requirement that reduces the process energy efficiency. To decrease the required separation energy, chemical hydrogen separation by CO2 is proposed by combining solar-driven dry reforming of methane (DRM) in a hydrogen permeable membrane (HPM) reactor with either reverse water gas shift (RWGS) or methanation. This system has the benefit of using solar renewable energy as well as reducing CO2 emissions. The performances of the proposed membrane reactor configurations for in situ hydrogen consumption are compared to DRM and super-dry reforming of methane (SDRM) in a fixed-bed reactor. The thermodynamic, kinetic, and environmental performances are analyzed and compared from 300 °C to 1000 °C. 2.01 mol CO2 can be reduced to CO per mole CH4 consumed at 1000 °C by coupling DRM with RWGS in a membrane reactor, resulting in a CO2 reduction rate of 1064 kg m−2 yr−1. Alternatively, the minimum energy consumption per mole CO2 converted is 0.79 MJ at 860 °C. This study demonstrates the feasibility of DRM enhanced by chemical hydrogen separation in HPM reactors to convert CO2 into fuels and store solar thermal energy as chemical energy.

Progress in Methanol Steam Reforming Modelling via Membrane Reactors Technology.

Hydrogen has attracted growing attention for various uses, and, particularly, for polymer electrolyte membrane fuel cells (PEMFCs) supply. However, PEMFCs need high grade hydrogen, which is difficult in storing and transportation. To solve these issues, hydrogen generation from alcohols and hydrocarbons steam reforming reaction has gained great consideration. Among the various renewable fuels, methanol is an interesting hydrogen source because at room temperature it is liquid, and then, easy to handle and to store. Furthermore, it shows a relatively high H/C ratio and low reforming temperature, ranging from 200 to 300 °C. In the field of hydrogen generation from methanol steam reforming reaction, a consistent literature is noticeable. Despite various reviews that are more devoted to describe from an experimental point of view the state of the art about methanol steam reforming reaction carried in conventional and membrane reactors, this work describes the progress in the last two decades about the modelling studies on the same reaction in membrane reactors.

Thermodynamic analysis on mid/low temperature solar methane steam reforming with hydrogen permeation membrane reactors

Hydrogen production by methane reforming reaction in membrane reactors is a promising means of solar energy harvesting and storage. The integration of endothermic methane reforming with selective removal of H2 through hydrogen permeation membrane theoretically enables a near-complete conversion of methane at the temperature of 400 °C or above (normally 800–900 °C) by shifting the equilibrium of reforming reaction. Operation of methane membrane reforming reactors at lower temperatures offers a special advantage of possible combination with solar trough collector on a large scale. However, such advantages are not immediately apparent due to special characteristics of solar thermal technologies and lower conversion rates of methane, which needs further clarification from a thermodynamic perspective. In this work, a numerical model of solar-driven methane reforming reaction in a palladium (Pd) membrane reactor with vacuum pump and compressor is established. Methane reforming and H2 permeation processes are theoretically investigated in a temperature range of 300 °C to 700 °C. An optimal conversion rate range of methane for efficient solar methane membrane reforming is calculated, and the net solar-to-fuel efficiency can reach as high as 38.25% at 400 °C after taking real vacuum pump and compressor efficiencies into account. The findings of this work show feasibility of integrating methane reforming with mid- or low-temperature solar thermal technologies.

Influences of cation and anion substitutions on oxidative coupling of methane over hydroxyapatite catalysts

Lead substituted hydroxyapatite (Pb-HAP) has been an active catalyst for oxidative coupling of methane (OCM) reactions. CO32− substituted HAP (HAP-CO3) has showed enhanced oxide ion conductivity than bare HAP in high temperature solid oxide fuel cells. Substitutions for both cations and anions in HAP structure (Pb-HAP-CO3) are promising to integrate the catalytic property of Pb-HAP and oxide ion conductive property of HAP-CO3 into one apatite-based ceramic material that can be manufactured into membrane reactors for possessing CH4 activation and O2 permeation capabilities for efficient OCM reactions. In this work, the effects of substitutions for both cation (Pb2+) and anion (CO32−) in HAP structure on OCM reactions were studied. The composition and physicochemical properties of HAP catalysts were changed by the cation and anion substitutions, respectively, and as consequences, they influenced the catalytic performances of HAP structure in OCM reactions. The selectivity to C2 (ethylene and ethane) products increased in the order of HAP-CO3<HAP<Pb-HAP-CO3<Pb-HAP, while Pb-HAP-CO3 showed the best stability and comparable C2 yield (under optimized reaction conditions) to Pb-HAP catalyst. Under different reaction temperature and/or CH4/O2 ratio in the OCM reactions, the CH4 conversion and C2 or COx (CO and CO2) selectivity showed a strong dependence on the composition of HAP-based catalysts. The present study forms a basis for understanding of the correlations between the composition, structure, and catalytic performance of HAP and other apatite structured catalysts, which are potential membrane materials for OCM reactions in membrane reactors.

Water gas shift reaction in membrane reactors: Theoretical investigation by artificial neural networks model and experimental validation

In this work, a theoretical approach via artificial neural networks model has been followed for studying the water gas shift reaction in hydrogen selective membrane reactors, based on an experimental campaign useful for training the aforementioned model. In particular, such parameters as the reaction pressure (from 150 to 300 kPa), reaction temperature (from 300 to 360 °C), gas hourly space velocity (GHSV) between 2000 and 6000 h−1, sweep gas flow rate (between 35.75 and 130.42 mL/min of N2), H2O/CO feed molar ratio (from 1/1to 4.5/1) and feed configuration (co–or counter-current mode with respect to the sweep gas) have been considered from both a modeling and an experimental point of view in order to analyze their influence on the water gas shift performance (in terms of CO conversion, hydrogen recovery, hydrogen permeate purity) in two membrane reactors, allocating dense Pd–Ag membranes, having different active membrane surface areas. As best experimental results, by using a Cu–Zn based catalyst, at GHSV = 3340 h−1, T = 350 °C, H2O/CO feed molar ratio = 2/1 and co-current configuration of sweep gas, CO conversion around 100% and H2 recovery of about 70% were reached. Meanwhile, the artificial neural networks model has been validated by using part of the experimental tests as training values and, then, it was used for optimizing the system to achieve as much as possible high hydrogen recovery. The model predicted the experimental performance of the water gas shift membrane reactors with an error on CO conversion lower than 0.5% and around 10% for the H2 recovery over the experimental tests not used during the model training.

Mixed ionic-electronic conducting (MIEC) membranes for hydrogen production from water splitting

Hydrogen as a clean energy carrier for power generation through fuel cells has attracted much attention. Water is a suitable source for hydrogen production by the splitting reaction. Significant amount of hydrogen can be produced at moderate temperature if a mixed ionic-electronic conducting (MIEC) membrane is used to remove the produced oxygen from water splitting reaction, although the equilibrium constant for water splitting is small. In this review, the principle of hydrogen production via water splitting in MIEC membrane reactor is illustrated, and the factors affecting the hydrogen production rate (such as membrane materials, thickness, modification of membrane surface) and the stabilities of membranes under high oxygen chemical potential gradient are discussed. Furthermore, following the concept of process intensification, the researches on water splitting coupling with other catalytic reactions in membrane reactors are summarized.

Model biogas steam reforming in a thin Pd-supported membrane reactor to generate clean hydrogen for fuel cells

Steam reforming of a model biogas mixture is studied for generating clean hydrogen by using an inorganic membrane reactor, in which a composite Pd/Al2O3 membrane separates part of the produced hydrogen through its selective permeation. The characteristics of H2 perm-selectivity of the fresh membrane is expressed in terms of H2/N2 ideal selectivity, in this case equal to 4300. Concerning biogas steam reforming reaction, at 380 °C, 2.0 bar H2O:CH4 = 3:1, GHSV = 9000 h−1 the permeate purity of the recovered hydrogen is around 96%, although the conversion (15%) and hydrogen recovery (>20%) are relatively low; on the contrary, at 450 °C, 3.5 bar H2O:CH4 = 4:1, GHSV = 11000 h−1 the conversion is increased up to more than 30% and the recovery of hydrogen to about 70%. This novel work constitutes a reference study for new developments on biogas steam reforming reaction in membrane reactors.

Three-dimensional numerical modeling on high pressure membrane reactors for high temperature water-gas shift reaction

This study presents a three-dimensional numerical model that simulates the H2 production from coal-derived syngas via a water-gas shift reaction in membrane reactors. The reactor was operated at a temperature of 900 °C, the typical syngas temperature at gasifier exit. The effects of membrane permeance, syngas composition, reactant residence time, sweep gas flow rate and steam-to-carbon (S/C) ratio on reactor performance were examined. Using CO conversion and H2 recovery to characterize the reactor performance, it was found that the reactor performance can be enhanced using higher sweep gas flow rate, membrane permeance and S/C ratio. However, CO conversion and H2 recovery limiting values were found when these parameters were further increased. The numerical results also indicated that the reactor performance degraded with increasing CO2 content in the syngas composition.

Compatibility of Transport and Reaction in Membrane Reactors Used for the Oxidative Dehydrogenation of Short-Chain Hydrocarbons

The possibility of process intensification by enhancing selectivity and yield in networks of parallel and series reactions was investigated applying asymmetric multilayer ceramic and sintered metal membranes in a dead-end configuration for a controlled distributed reactant feeding. The oxidative dehydrogenation of ethane to ethylene was selected as a model reaction applying three different doped and/or active VOx/?-Al2O3 catalysts. Experimental investigations were performed in a pilot scale in order to evaluate the potential of a distributed dosing via membranes with respect to operation conditions and compatibility of reaction and membrane properties. It was demonstrated that the rates of reaction and trans-membrane mass transfer have to be compatible for an optimal membrane reactor operation avoiding back diffusion of reactants out of the catalytic zone as well as achieving safety aspects. Therefore, a detailed modeling of the trans-membrane mass transfer under reaction conditions was carried out. As a main result, it was found metal membranes possess a favorable mechanical stability, relatively low costs for production and the possibility to control mass transfer if the rate of reaction and mass transfer in the membrane is compatible which can adjusted by the trans-membrane pressure and the catalyst activity, respectively.

Compatibility of Transport and Reaction in Membrane Reactors Used for the Oxidative Dehydrogenation of Short-Chain Hydrocarbons

Compatibility of Transport and Reaction in Membrane Reactors Used for the Oxidative Dehydrogenation of Short-Chain Hydrocarbons

Hydrolase-catalyzed reactions in membrane reactors at atmospheric and high pressure

Membrane reactors could serve as a tool for enzyme-catalyzed reactions and they enable the performance of catalytic reaction, enzyme recovery and product isolation as a one-step process. In the presented work, the use of continuous flat-shape and continuous tubular membrane enzymatic reactors for different enzymatic systems at atmospheric pressure and at supercritical conditions are described. In a high-pressure continuous enzymatic flatshape membrane reactor, where supercritical carbon dioxide (SC-CO 2) was used as a solvent, the high-pressure enzyme-catalyzed hydrolysis of sunflower oil was performed. The influence of enzyme concentration and temperature on the production of total free fatty acids and especially oleic acid in biphasic (SC-CO 2/H 2O) medium were investigated. In the same type of reactor hydrolysis of soybean oil at atmospheric pressure was also performed. The application of tubular ceramic membranes in the high-pressure reaction system was studied on hydrolysis of carboxy-methyl cellulose (CMC) at atmospheric pressure and in biphasic (SC-CO 2/H 2O) medium. The reaction was catalyzed with covalent linked cellulase from Humicola insolens on the surface of ceramic membrane. The physical characteristics for tubular ceramic membranes before and after immobilization of enzyme onto membrane were defined, as well.

An analysis of the performance of membrane reactors for the water–gas shift reaction using gas feed mixtures

The water–gas shift (WGS) reaction in membrane reactors has been widely studied by several authors. From these works, the increase of the CO conversion above the equilibrium values appears to be possible when hydrogen is removed through the membrane. However, to date, this feasibility has been verified mostly when feeding pure reagents to the reactor, although in an industrial context the feed normally contains several other compounds. The objective of this work has been to analyse the effect of the feed composition on the membrane reactor efficiency in order to determine the best conditions in terms of CO conversion. At this purpose, experimental tests with mixtures of different compositions have been carried out in three different systems of reaction: (1) traditional fixed-bed reactor; (2) membrane reactor with mesoporous ceramic membrane; (3) membrane reactor with palladium membrane. The experiments included permeation (for the membrane reactors) and reaction tests. The experimental results obtained with the various systems of reaction have been compared. A mathematical model has been also formulated for the different type of reactors used in order to verify the experimental results obtained. From the work carried out it can be concluded that by using the palladium membrane reactor it is possible to overcome the equilibrium conversion. Moreover, a complete conversion has been achieved for one of the mixtures fed to the reactor.