The catalytic oxidative coupling of methane: II. Axial gas sample probing in a bubbling Fluidised-Bed reactor and the effect of side reactions

The atmospheric pressure catalytic oxidative coupling of methane was studied in detail by axial gas concentration probing experiments in a 60 mm OD bubbling fluidised-bed reactor system. Experimental data demonstrated a very fast reaction which occurred within the vicinity of the distributor (normally in the first 5 mm of bed height) under normal reaction conditions of 850°C and 83 vol%/17 vol% CH₄/O₂ feed. The data also showed that hydrocarbon selectivity was deleteriously affected by side reactions which were also promoted by the catalysts even though they were good catalysts for the oxidative coupling reaction. If this reduction in hydrocarbon selectivity is inevitable then there will be optimum operating conditions for each catalyst in the bubbling fluidised bed reactor.     

Performance of an internally circulating fluidized-bed reactor for the catalytic oxidative coupling of methane

The oxidative coupling of methane to C₂-hydrocarbons (OCM) over a La₂O₃/CaO catalyst (27 at.%) was investigated in an internally circulating fluidized-bed (ICFB) reactor (ID_eff = 1.9 cm, H_riser = 20.5 cm). The experiments were performed in the following range of conditions: T = 800?900°C, p_CH4:p_O2p_N2 = 57.1–64:16–22.9:20 kPa. The obtained C₂ selectivities and C₂ yields were compared with the corresponding data from a spouted-fluid-bed reactor (ID = 5 cm) and a bubbling fluidized-bed (FIB) reactor (ID = 5 cm). The maximum C₂ yield in the internally circulating fluidized-bed (ICFB) reactor amounted to 12.2% (T = 860°C, 38.7% C₂ selectivity, 31.5% methane conversion), whereas in the FIB reactor a maximum C₂ yield of 13.8% (T = 840°C, 40.4% C₂ selectivity, 34.2% methane conversion) was obtained. The lowest C₂ yield was achieved in the spouted-bed (SFB) reactor (Y = 11.6%, T = 840°C, 36.2% C₂ selectivity, 32.0% methane conversion). The highest space-time yield of 24.0 mol/kg_cat.h was obtained in the ICFB reactor, whereas in a FIB reactor only a space-time yield of 9.6 mol/kg_catcould be obtained. The performance of the ICFB reactor was strongly influenced by gas-phase reactions. Furthermore, stable reactor operation was possible only over a narrow range of gas velocities.     

Advanced reactor concepts for oxidative coupling of methane

Oxidative coupling of methane (OCM) has been investigated as an interesting way to obtain higher hydrocarbons from natural gas. The aim of this article is to evaluate the reactor concepts for oxidative coupling of methane, from the 1980s through the current state of the art, giving a general insight into the reactor engineering possibilities and perspectives of application of OCM in large scale reactors. The concepts were classified according to the type of reactor bed, the heat management system, the oxygen feeding policy, the degree of integration with separation units, the relative cost, and the current demonstration on industrial scale.     

Catalyst and reactor requirements for the oxidative coupling of methane

Oxidative coupling is not yet competitive for converting methane to liquid fuels. The process is extremely exothermic and is strongly influenced by reaction selectivity. The methane coupling reactor system, which must operate at high temperature, is the primary contributor to the high cost of the oxidative coupling process. This paper investigates two systems—the multitubular reactor and the fluidized bed reactor—for their feasibility and cost effectiveness.

A multitubular reactor of impractical dimensions would be required to control heat transfer and avoid temperature runaway. The fluidized bed reactor, however, could be feasible. Its key advantage is temperature uniformity. However, the catalyst must have stringent mechanical properties, development and scale-up of the catalyst would be complex, and piloting would be expensive. Evaluation of other approaches is needed.     

Catalytic membrane reactor for oxidative coupling of methane. Part II —Catalytic properties of LaOCl membranes

This work presents the potential of a LaOCI porous membrane reactor for the reaction of oxidative coupling of methane. The sol-gel method provided LaOCl membranes supported on alumina tubes which presented mesoporous texture. Actually, the severe operating conditions of this reaction caused textural instability which restricted any transport effects to the macroporous domain. However, despite moderate separation effects between methane and oxygen, beneficial effects of the membrane reactor have been observed, subject to surface composition, structural and feeding configuration requirements.     

Thermodynamics and kinetic modeling of the homogeneous gas phase reactions of the oxidative coupling of methane.

Thermodynamic calculations concerning methane activation have shown that in the equilibrium state hydrocarbons are only present in very small quantities. This is in contrast to experiments carried out under kinetically determined conditions, where considerable amounts of ethane and ethylene are formed via a radical mechanism. A computer model has been developed to simulate and analyze networks of those radical reactions operative in the oxidative coupling of methane.     

Analysis of attainable reactor performance for the oxidative methane coupling process

In this work, a comparative analysis of attainable performance is presented for three different reactor structures including a fixed-bed reactor, and two different feeding structures of packed bed membrane reactors. For this purpose, three types of kinetic models, namely La₂O₃/CaO, Mn/Na₂WO₄/SiO₂, and PbO/Al₂O₃ have been used under a wide range of operating conditions. Thus, the effect of several variables such as operating temperature, membrane thickness, methane-to-oxygen ratio, feed flow rate, gas streams composition, and reactor length are investigated.Moreover, kinetic-analysis based on a proposed graphical approach enables determination of the suitable operational condition range and allows analysis of the feasible and optimal performance that corresponds to the effect of several dependent operating variables. The results show that tracking the optimum area of operation has a monotonic direction under some range of operating conditions, while it reflects qualitative trade-offs under some other ranges of operating conditions. For all investigated reactor concepts and catalysts, optimal operating conditions and the best corresponding performance are presented.     

The oxidative coupling of methane in a fluidised-bed reactor

A small fluidised-bed reactor has been used by the CSIRO Division of Coal Technology to study the oxidative coupling of methane to higher hydrocarbons. Methane conversions of 9.6 to 13.5% were obtained in preliminary experiments using a lithium-promoted magnesium oxide catalyst at 850°C and with feed gases containing 5.6 to 10.7% v/v oxygen. Total hydrocarbon selectivity declined from 82 to 72% with increasing methane conversion. When operating with ethane in the feed at concentrations found in natural ethylene, the fluidised-bed reactor converted the ethane with good selectivity to ethylene, a key result in the context of using oxidative coupling for natural gas conversion. In view of these promising results, current work is directed towards increasing methane conversion and hydrocarbon selectivity in fluidised-bed reactors by development of more active and selective catalysts.     

A novel particle/monolithic two-stage catalyst bed reactor and their catalytic performance for oxidative coupling of methane

A novel two-stage catalyst bed reactor was constructed comprising of the 5%Na₂WO₄-2%Mn/SiO₂ particle catalyst and the 5%Na₃PO₄-2%Mn/SiO₂/cordierite monolithic catalyst. The reaction performance of the oxidative coupling of methane (OCM) in the two-stage bed reactor system was evaluated. The effects of the bed height and operation mode, as well as the reaction parameters such as reaction temperature, CH₄/O₂ ratio and flowrate of feed gas on the catalytic performance were investigated. The results indicated that the two-stage bed reactor system exhibited a good performance for the OCM reaction when the feed gases were firstly passed through the particle catalyst bed and then to the monolithic catalyst bed. The CH₄ conversion of 32.6% and C₂ selectivity of 67.5% could be obtained with a particle catalyst bed height of 10 mm and a monolithic catalyst bed height of 50 mm in the two-stage bed reactor. Both of the CH₄ conversion and C₂ selectivity have been increased by 4.8% and 2.5%, respectively, as compared with the 5%Na₂WO₄-2%Mn/SiO₂ particle catalyst in a single-bed reactor and by 7.7% and 16.1%, respectively, as compared with the 5%Na₃PO₄-2%Mn/SiO₂/cordierite monolithic catalyst in a single-bed reactor. The catalytic performance of the OCM in the two-stage bed reactor system has been remarkably improved. The TPR results indicate the high temperature reduction oxygen species in the monolithic catalyst might be favorable to the formation of C₂ products.     

Catalytic membrane reactor for oxidative coupling of methane. Part 1: preparation and characterisation of LaOC1 membranes

The sol-gel process was investigated in order to prepare LaOC1 inorganic membranes on commercial alumina tubular supports which were catalytically active for the oxidative coupling of methane. The preparation of LaOCI meso and microporous membranes is described on the basis of the fundamental phenomena occurring during the sol preparation and thermal treatment. Membranes were synthesised from LaC1₃ precursor in aqueous media. Important parameters, mastering the sol formation and consequently the final material textural characteristics, were the molar ratio [aceta]/La],[NH₃/La] and the total concentration of lanthanum in the sol. Without any acetate addition, turbid sols were obtained leading at 800°C to mesoporous membranes. The effect of acetate ions is shown to be of prime importance in order to limit the polycondensation reactions in the sol and to prepare microporous materials. Under these conditions, quite dense membranes were obtained at 200°C due to lanthanum acetate polymerisation. The formation of carbonates and their decomposition at 600°C explain the maximum micro-porosity observed at this temperature. When these membranes were treated at 800°C, the microporous volume decreased.     

Comparison of the surface and catalytic properties of rare earth‐promoted CaO catalysts in the oxidative coupling of methane

Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650–800 °C, CH₄/O₂ ratios, 2.0–8.0 and space velocity, 51 360 cm³ g^?1 h^?1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm‐promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below ?700 °C, whereas at higher temperature no further effect is seen. The La₂O₃? CaO, Nd₂O₃? CaO and Yb₂O₃? CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd‐promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH₄ conversion with 70.8% C₂₊ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) and the C₂₊ selectivity and/or C₂₊ yield has been observed. Copyright © 2005 Society of Chemical Industry     

The catalytic oxidative coupling of methane: I. Comparison of experimental performance data from various types of reactor

The methane oxidative coupling performance of a fixed-bed reactor was successfully translated to a bubbling fluidised-bed reactor and this reaction mode was superior to spouted-bed, inclined and mechanically-agitated fluidised-bed units. Also, a two-stage bubbling fluidised-bed reactor with inter-stage addition of oxygen had the same performance as the single-stage unit, for the same total oxygen input, over a wide range of operating conditions. Overall the bubbling fluidised-bed is preferred, catalytic reactions dominate over non-catalytic gas phase reactions in determining the reactor performance, the gas phase exhibited plug-flow behaviour and the performance was independent of the gas phase oxygen partial pressure for a given oxygen input. The best hydrocarbon yield achieved in this study was 19.4%.     

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.     

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.     

Design and demonstration of an experimental membrane reactor set-up for oxidative coupling of methane

In this experimental research, the performance of the oxidative coupling of methane (OCM) reactions in a porous packed bed membrane reactor was investigated. A commercially available porous alpha-alumina membrane was modified to obtain the characteristics needed for a stable and catalytically inert OCM membrane reactor. The silica-sol impregnation–calcination method and a new silicon oxycarbide (SiOC) coating-calcination approach were applied to modify the membrane. The characteristics of the resulted membrane and its typical performance as OCM membrane reactor are reported.     

Structure sensitivity of the catalytic oxidative coupling of methane on lanthanum oxide

The oxidative coupling of methane (OCM) has been found to be structure sensitive on La₂O₃ catalysts exhibiting different crystallite morphologies. Thin plates obtained by thermal decomposition of lanthanum nitrate at 650 °C are more selective on OCM reaction performed at 750 °C than the particles obtained by decomposition of the nitrate at 800 °C. It is assumed that the oxycarbonate observed is formed from the methane deep oxidation on the catalyst surface. This compound appears to act as an intermediate in the production of CO₂ and is thus important hi the resulting selectivity.     

Redox properties and catalytic performance of complex oxides in oxidative coupling of methane

The redox properties of model Mn-containing mixed oxide are studied. It is shown that water evolution can determine the rate of oxide reduction. The kinetic model of the redox process taking into account surface interactions and diffusion of oxygen, hydrogen and hydroxy ions in the oxide lattice can describe successfully the experimental data. The influence of slow water evolution on catalytic performance of the oxides in oxidative coupling of methane is discussed.     

Positioning of the reaction zone for gas–liquid reactions in catalytic membrane reactor by coupling results of mass transport and chemical reaction study

The location of the reaction zone in the catalytic membrane contactor was determined for two-phase reaction system. As a model reaction oxidation of formic acid by Pt catalyst deposited in the ceramic membrane filtration layer was used. It was determined from gas–liquid mass transport experiments that at gas side overpressures from 0.5 to 0.9 bar the liquid was displaced from membrane support layer and that the tortuosity factor of membrane filtration layer is 3.1. It was estimated on the basis of CO₂ fluxes that the reaction zone is close to gas–liquid interface and that the locus of the reaction zone matches with O₂ penetration depth calculated on the basis of formic acid disappearance rate. It was also shown that partition coefficient of formic acid in the membrane filtration layer is higher than 1, otherwise formic acid could not migrate to the reaction zone. Only a small fraction of the catalyst was involved in the reaction. The apparent activation energy was estimated to be around 44 kJ/mol.     

Characterization of porous ceramic membranes for their use in catalytic reactors for methane oxidative coupling

Catalytic inorganic membranes were prepared by depositing active materials over porous alumina tubes, using sol-gel and impregnation techniques. The concentration profiles of the different species in the membrane were obtained using XPS. Also, the stability of the membranes was tested by subjecting the samples to different thermal treatments. Different preparation methods are compared with the aim of attaining controlled distributions of the active phases in the membrane. Finally, some reaction results are also presented.