Dual-membrane reactor for methane oxidative coupling and dry methane reforming: Reactor integration and process intensification

A novel dual-membrane reactor concept was introduced for integrating the oxidative coupling of methane (OCM) and CO₂ methane reforming (dry reforming) reactors. The OCM reactions occur in a conventional porous packed bed membrane reactor structure and a portion of the undesired produced CO₂ and generated heat are transferred through a molten-carbonate perm-selective membrane and consumed in the adjacent dry methane reforming catalytic bed. This integrated reactor provides a very promising thermal performance by controlling the temperature peak to be below 50 °C in reference to the average operating temperature in the OCM section. This was achieved even for the low methane-to-oxygen ratio 2 by introducing 10% CO₂ as the diluent agent and reactant in this integrated reactor structure. This contributed to the improved selective performance of 32% methane conversion and 25% C₂-yield including 21% C₂H₄-yield in the OCM section which also enhances the performance of the downstream units consequently. Around half of the unconverted methane leaving the OCM section was converted to syngas in the DRM section.The dual-membrane reactor alone can utilize a significant amount of the carbon dioxide generated in the OCM catalytic bed. In combination with adsorption unit in the downstream of the integrated process, 90% of the produced CO₂ can be recovered and further converted to valuable syngas products. The experimental data, obtained from a mini-plant scale experimental facility, were exploited to verify the performance of the OCM reactor and the CO₂ separation section.     

Heat Exchangeability of a Catalytic Plate Reactor and Analysis of the Reactivity of Steam and CO2 in Methane Reforming

An assemble‐type plate reactor was developed and its intensified heat transfer compared to that of a conventional tubular reactor in methane reforming was confirmed. This characteristic enables accurate reaction kinetic analysis because of quasi‐isothermal operation with mild pressure loss. Reduced experiment cost is one of the features of the assemble‐type reactor. Simple thermal design equations applicable to plate reactors were also assessed. From experiments and accurate reaction analysis using the plate reactor it is suggested that H₂O and CO₂ have similar reactivity for a commercial Ni/α‐Al₂O₃ catalyst. The partial pressure of the oxidizing agent had much more influence on the reactivity of methane reforming than the species of this agent.     

Highly active and stable Rh/MgOAl2O3 catalysts for methane steam reforming

Highly active and coke-resistant Rh catalysts were developed for methane steam reforming in microchannel chemical reactors. Rh loading was optimized on a stable MgOAl₂O₃ support to improve the volumetric productivity for methane conversion. Catalyst activities were stable over a wide range of steam/carbon ratios. In particular, experimental results demonstrated that Rh/MgOAl₂O₃ catalysts are extremely active for methane steam reforming and are resistant to coke formation at stoichiometric steam/carbon ratio of 1 for over 14 h time-on-stream with no sign of deactivation. Methane steam reforming activities on this catalyst is compared in both a microchannel reactor and a conventional micro-tubular reactor. Significant performance enhancement was observed in microchannel reactors owing to improved heat and mass transfer.     

Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling

A novel metal monolith reactor for coupling methane steam reforming with catalytic combustion is proposed in this work, the metal monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. The transport and reaction performances of the reactor are numerically studied utilizing heterogeneous model based on the whole reactor. The influence of the operating conditions like feed gas velocity, temperature and composition are predicted to be significant and they must be carefully adjusted in order to avoid hot spots or insufficient methane conversion. To improve reactor performance, several different channel arrangements and catalyst distribution modes in the monolith are designed and simulated. It is demonstrated that reasonable reactor configuration, structure parameters and catalyst distribution can considerably enhance heat transfer and increase the methane conversion, resulting in a compact and intensified unit.     

Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors

Microscale autothermal reactors remain one of the most promising technologies for efficient hydrogen generation. The typical reactor design alternates microchannels where reforming and catalytic combustion of methane occur, so that exothermic and endothermic reactions take place in close proximity. The influence of flow arrangement on the autothermal coupling of methane steam reforming and methane catalytic combustion in catalytic plate reactors is investigated. The reactor thermal behavior and performance for cocurrent and countercurrent are simulated and compared. A partial overlapping of the catalyst zones in adjacent exothermic and endothermic channels is shown to avoid both severe temperature excursions and reactor extinction. Using an innovative, optimization‐based approach for determining the catalyst zone overlap, a solution is provided to the problem of determining the maximum reactor conversion within specified temperature bounds, designed to preserve reactor integrity and operational safety. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Preliminary design and simulation of a microstructured reactor for production of synthesis gas by steam methane reforming

The present study considers the potentials of the well-known production of syngas by steam methane reforming (SMR), by operation within microstructured reactors. The model of a microchannel reactor is developed, including very fast kinetic reaction rates on the coated catalytic walls of the reactor module. By varying the characteristic dimensions of the channels, and considering technical constraints on the design and operating conditions, the results demonstrate that the SMR reactor can be drastically miniaturized while maintaining its productivity without any additional pressure drop. Furthermore, by reducing the channel characteristic dimensions, it is possible to suppress heat and mass-transfer limitations enabling SMR reactor operation at thermodynamic equilibrium. A fast method for preliminary design of microstructured heat-exchanger reactors is developed, that enables to identify the optimal channels number and heat power needed to reach process specifications.     

Mathematical Modeling and Optimization of Combined Steam and Dry Reforming of Methane Process in Catalytic Fluidized Bed Membrane Reactor

Mathematical modeling of the methane-combined reforming process (steam methane reforming–dry reforming methane) was performed in a fluidized bed membrane reactor. The model characterizes multiple phases and regions considering low-density phase, high-density phase, membrane, and free board regions that allow study of reactor performance. It is demonstrated that the combined effect of membrane and reaction coupling provides opportunities to overcome equilibrium limits and helps to achieve higher conversion. Additionally, the influence of key parameters on reactor performance including reactor temperature, reactor pressure, steam to methane feed ratio (S/C), and carbon dioxide to methane feed ratio (CO₂/C) were investigated in the multi-objective genetic algorithm to find the optimal operating conditions. Finally, the process of steam reforming was simulated in selected optimal conditions and the results are compared to those of the combined reforming process. Comparison reveals the superiority of the combined reforming process in terms of methane conversion, catalyst activity, and outlet H₂/CO ratio in the syngas product in being close to unity.     

Compact hollow fibre reactors for efficient methane conversion

In this study, a micro-structured catalytic hollow fiber membrane reactor (CHFMR) has been prepared, characterized and evaluated for performing steam methane reforming (SMR) reaction, using Rh/CeO₂ as the catalyst and a palladium membrane for separating hydrogen from the reaction. Preliminary studies on a catalytic hollow fiber (CHF), a porous membrane reactor configuration without the palladium membrane, revealed that stable methane conversions reaching equilibrium values can be achieved, using approximately 36 mg of 2 wt.%Rh/CeO₂ catalyst incorporated inside the micro-channels of alumina hollow fibre substrates (around 7 cm long in the reaction zone). This proves the advantages of efficiently utilizing catalysts in such a way, such as significantly reduced external mass transfer resistance when compared with conventional packed bed reactors. It is interesting to observe catalyst deactivation in CHF when the quantity of catalyst incorporated is less than 36 mg, although the Rh/CeO₂ catalyst supposes to be quite resistant against carbon formation. The “shift” phenomenon expected in CHFMR was not observed by using 100 mg of 2 wt.%Rh/CeO₂ catalyst, mainly due to the less desired catalyst packing at the presence of the dense Pd separating layer. Problems of this type were solved by using 100 mg of 4 wt.% Rh/CeO₂ as the catalyst in CHFMR, resulting in methane conversion surpassing the equilibrium conversions and no detectable deactivation of the catalyst. As a result, the improved methodology of incorporating catalyst into the micro-channels of CHFMR is the key to a more efficient membrane reactor design of this type, for both the SMR in this study and the other catalytic reforming reactions.     

A comparative computational study of diesel steam reforming in a catalytic plate heat‐exchange reactor

A two‐dimensional steady‐state model of a catalytic plate reactor for diesel steam reforming is developed. Heat is provided indirectly to endothermic reforming sites by flue gas from a SOFC tail‐gas burner. Two experimentally validated kinetic models on diesel reforming on platinum (Pt) catalyst were implemented for a comparative study; the model of Parmar et al., Fuel. 2010;89(6):1212–1220 for a Pt/Al₂O₃ and the model of Shi et al., International Journal of Hydrogen Energy. 2009;34(18):7666–7675 for a Pt/Gd‐CeO₂ (GDC). The kinetic models were compared for: species concentration, approach to equilibrium, gas hourly space velocity and effectiveness factor. Cocurrent flow arrangement between the reforming and the flue gas channels showed better heat transfer compared to counter‐current flow arrangement. The comparison between the two kinetic models showed that different supports play significant role in the final design of a reactor. The study also determined that initial 20% of the plate reactor has high diffusion limitation suggesting to use graded catalyst to optimize the plate reactor performance. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1102–1113, 2017     

Design of mixed conducting ceramic membranes/reactors for the partial oxidation of methane to syngas

The performance of mixed conducting ceramic membrane reactors for the partial oxidation of methane (POM) to syngas has been analyzed through a two‐dimensional mathematical model, in which the material balance, the heat balance and the momentum balance for both the shell and the tube phase are taken into account. The modeling results indicate that the membrane reactors have many advantages over the conventional fixed bed reactors such as the higher CO selectivity and yield, the lower heating point and the lower pressure drop as well. When the methane feed is converted completely into product in the membrane reactors, temperature flying can take place, which may be restrained by increasing the feed flow rate or by lowering the operation temperature. The reaction capacity of the membrane reactor is mainly determined by the oxygen permeation rate rather than by the POM reaction rate on the catalyst. In order to improve the membrane reactor performance, reduction of mass transfer resistance in the catalyst bed is necessary. Using the smaller membrane tubes is an effective way to achieve a higher reaction capacity, but the pressure drop is a severe problem to be faced. The methane feed velocity for the operation of mixed conducting membrane reactors should be carefully regulated so as to obtain the maximum syngas yield, which can be estimated from their oxygen permeability. The mathematical model and the kinetic parameters have been validated by comparing modeling results with the experimental data for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐α (LSCF) membrane reactor. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

基于太阳能蓄热过程的甲烷二氧化碳重整研究进展

热化学储能技术因为其储能密度高、热损小、能长距离运输等优点而成为保证太阳能长久稳定供应的关键技术。本文对基于甲烷二氧化碳重整反应的太阳能热化学储热系统研究现状进行了回顾,重点讨论了甲烷重整催化剂、重整反应器以及储能系统整体的传热特性等3个方向的研究进展。指出新型高效催化剂以及反应器开发和性能测试是目前该领域的主要研究方向。发现辐射热损失、非均匀温度分布特性、辐射热流的时变波动特性,以及由此造成的能量与化学反应的不匹配限制了热化学系统能量储存效率的进一步提高,并提出催化剂的催化特性与物性/结构参数依变关系,反应器辐射吸收特性、传热传质特性和反应特性之间的相互作用机制,以及系统时变动态特性与反应物流/辐射能流的匹配关系是建立甲烷重整热化学储能系统优化设计理论亟待解决的关键问题。     

Syngas formation by direct oxidation of methane: Reaction mechanisms and new reactor concepts

The reaction mechanism of direct catalytic oxidation of methane to syngas over a platinum catalyst under high temperature, short contact time conditions was studied with a detailed reactor and reaction model. Based on a detailed analysis of this mechanism, new integrated reactor concepts were deduced. Two concepts were studied in detail: a fixed bed reactor with integrated recuperative heat exchange, and a catalytic membrane reactor with distributed reactant feed. The reactor concepts are presented, and advantages and problems of the concepts are discussed.     

Multistage two-membrane ATR reactors to improve pure hydrogen production

A plate-type auto-thermal reforming (ATR) reactor with both hydrogen permeable and oxygen permeable membranes has unique characteristics of mass transfer and heat transfer, which have a big effect on the hydrogen production and hydrogen recovery. In order to study the special mass and heat transfer inside the two-membrane ATR reactor, a 2-D reactor model was developed, and a reactor simulation was carried out. In the single-stage two-membrane ATR reactor, the large gradients of temperature and hydrogen concentration indicate the limitation of mass transfer and heat transfer. To improve the mass and heat transfer, multistage reactors are suggested. The simulation results show that the multistage reactors have better mass and heat transfer, a lower rate of oxygen consumption, higher operating temperatures of the H₂ membrane, and a larger driving force for hydrogen permeation, and hence can produce more than three times the amount of pure hydrogen than the single-stage reactor.     

A mathematical model for the dehydrogenation of methylcyclohexane in a packed bed reactor

A model for the dehydrogenation of methylcyclohexane in a tubular reactor over an industrial catalyst Pt-Sn/Al₂O₃ has been established. This model takes into account the axial dispersion at the inlet of the catalytic bed reactor as well as the heat transfer at the wall of the reactor. The heat transfer at the wall is satisfactorily represented by using a heat transfer coefficient correlation for which the parameters are obtained by fitting to the experimental data. The model provides a good representation of the radial and axial temperature profiles in the packed bed and can be also used to calculate the conversion.     

Coupling of highly exothermic and endothermic reactions in a metallic monolith catalyst reactor: A preliminary experimental study

An LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl metallic monolith catalyst for the exothermic catalytic combustion of methane and an Ni/SBA-15/Al₂O₃/FeCrAl metallic monolith catalyst for the endothermic reforming of methane with CO₂ have been prepared. A laboratory-scale tubular jacket reactor with the Ni/SBA-15/Al₂O₃/FeCrAl catalyst packed into its outer jacket and the LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl catalyst packed into its inner tube was devised and constructed. The reactor allows a coupling of the exothermic and endothermic reactions by virtue of their thermal matching. An experimental study in which the temperature difference between the chamber of the external electric furnace and the metallic monolith catalyst bed in the jacket was kept very small, by adjusting the power supply to the furnace, confirmed that the heat absorbed in the reforming reaction does indeed partly come from that evolved in the catalytic combustion of methane, and that the direct thermal coupling of the two reactions in the reactor can be realized in practice. When the temperature of the electric furnace chamber was 1088 K, and the gas hourly space velocities (GHSVs) of the reactant mixtures passed through the inner tube and the jacket were 382 h⁻¹ and 40 h⁻¹, respectively, the conversions of methane and CO₂ in the reforming reaction were 93.6% and 91.7%, respectively, and the heat efficiency reached 81.9%. Stability tests showed that neither catalyst underwent deactivation during 150 h on stream.     

An Ru-based catalytic membrane reactor for dry reforming of methane—its catalytic performance compared with tubular packed bed reactors

The methane reforming with CO₂ seems to be a promising reaction system useful to reduce the greenhouse contribution of both gases into the atmosphere. On this basis, and considering the potentiality of this reaction system, the dry reforming reaction has been carried out in an Ru-based ceramic tubular membrane reactor, in which two Ru depositions have been performed using the co-condensation technique. Experimental results in terms of CH₄ and CO₂ conversion versus temperature during time are presented, as well as product selectivity and carbon deposition. These experiments have also been carried out using a traditional reactor. A comparison with literature data regarding dry reforming reaction is also provided. Experimental evidence points out a good catalyst activity for the methane dry reforming reaction, confirming the potentiality of a catalytic membrane applied to the reaction system.     

Characterization and testing of sol-gel catalysts prepared as thin layers in a plate reactor

Catalytic plate reactors offer many advantages over conventional reactors, including a major reduction in size and much better temperature control. This study examines the characteristics of thin catalyst coats prepared by the sol-gel method and calcined at different conditions. Employing the catalyst as a thin layer (< 100 μm) on the surface of plate reactors reduces mass and heat transfer limitations compared with pellet catalysts and can improve the effectiveness factor. A sol-gel of Ni/Al₂O₃ catalyst, with good rheological properties and good adherence onto stainless steel substrate, was prepared and characterized. The effects of calcination temperature, nickel content and calcination environment on the catalyst properties were investigated. The results revealed that the highest catalyst surface area was obtained at 400 °C for all calcined coatings. The presence of nitrogen gas during drying and calcining seemed to increase the catalyst surface areas and improve its adherence properties. Rheological evaluation of the prepared coats proved to be an effective tool in characterizing the thin coatings. The Ni/Al₂O₃ catalyst exhibited high activity and achieved more than 80% conversion for steam reforming of methane. The reactions were not diffusion limited based on the values of activation energy.     

Pure hydrogen generation in a fluidized bed membrane reactor: Application of the generalized comprehensive reactor model

A generalized comprehensive model was developed to simulate a wide variety of fluidized-bed catalytic reactors. The model characterizes multiple phases and regions (low-density phase, high-density phase, staged membranes, freeboard region) and allows for a seamless introduction of features and/or simplifications depending on the system of interest. The model is implemented here for a fluidized-bed membrane reactor generating hydrogen. A concomitant experimental program was performed to collect detailed experimental data in a pilot scale prototype reactor operated under steam methane reforming (SMR) and auto-thermal reforming (ATR) conditions, without and with membranes of different areas under diverse operating conditions. The results of this program were published in Mahecha-Botero et al. [2008a. Pure hydrogen generation in a fluidized bed membrane reactor: experimental findings. Chem. Eng. Sci. 63(10), pp. 2752-2762]. The reactor model is tested in this second paper of the series by comparing its simulation predictions against axially distributed concentration in the pilot reactor. This leads to a better understanding of phenomena along the reactor including: mass transfer, distributed selective removal of species, interphase cross-flow, flow regime variations, changes in volumetric flow, feed distribution, and fluidization hydrodynamics. The model does not use any adjustable parameters giving reasonably good predictions for the system of study.     

Ceramic microchannel reactors with channel sizes less than 100 μm prepared by a mesh-assisted phase-inversion process

Microchannel reactors show fast mass transfer and heat transfer while limited active surface area, which can be greatly increased by reducing channel sizes. However, conventional extrusion makes honeycomb ceramics with channel sizes above 100 μm. This study has prepared ceramic microchannel reactors with channel sizes in the range of 1–100 μm by a one-step phase-inversion process, and channels are formed through the convection between coagulant and solvent. The effect of channel size on reaction performance was investigated by comparing two reactors with different channel sizes in dry reforming of methane. In addition, the microstructure of the reactors can be further tuned via sintering temperature to achieve high catalytic performance owing to the balanced active surface and mass transfer. Therefore, the microchannel reactors developed in this study represent a diagram-shift in the preparation of microchannel reactors by making channels with sizes less than 100 μm, which has potential applications in many catalytic reactions.     

Tube-wall reactor modelling for multiple reaction networks with non-linear kinetics and volume change

Tube-wall reactors are gaining importance for highly exothermic and fast chemical reactions because of their simple construction and improved temperature control. A generalized mathematical model of the non-isothermal annular tube-wall reactors for simultaneous catalytic reactions with mixed type non-linear reaction kinetics and volume change is developed. Numerical solutions of two-dimensional component transfer equations, heat transfer equation and fluid flow equation may be obtained with appropriate boundary conditions using a recently developed orthogonal collocation method for annular geometries. The model has been successfully applied to the tube-wall reactors for the Fischer-Tropsch Synthesis. Comparison between model predictions and previous experimental results is good.