The effect of fixed-bed reactor configuration on the oxidative coupling of methane over a Li/Pb/Ca catalyst

Introduction of additional O₂ at the midpoint of the catalyst bed of a methane oxidative coupling, fixed bed reactor, increases the C₂ STY more than the CO _x . STY over a Li/Pb/Ca catalyst. This observation is not only a consequence of kinetics but may also be attributed to increased methyl radical generation on the O₂ replenished catalyst surface.     

Numerical simulation of fixed bed reactor for oxidative coupling of methane over monolithic catalyst

A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na₃PO₄-Mn/SiO₂/cordierite monolithic catalyst, and an improved Stansch kinetic model was established to calculate the OCMreactions using the computational fluid dynamicsmethod and Fluent software. The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min^-1 under standard state, the CH₄/O₂ ratio is 3 and the temperature and pressure is 800 ℃ and 1 atm, respectively. The contour of the characteristic parameters in the catalyst bed was analyzed, such as the species mass fractions, temperature, the heat flux on side wall surface, pressure, fluid density and velocity. The results showed that the calculated valuesmatchedwell with the experimental values on the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO,CO₂ and H₂) in the reactor outlet with an error range of ±4%. The mass fractions of CH₄ and O₂ decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and 0.120 at the outlet, where the mass fractions of C₂H₆, C₂H₄, CO and CO₂ were 0.0245, 0.0460, 0.0537 and 0.116, respectively. Due to the existence of laminar boundary layer, the mass fraction contours of each species bent upwards in the vicinity of the boundary layer. The volume of OCM reaction was changing with the proceeding of reaction, and the total moles of products were greater than reactants. The flow field in the catalyst bed maintained constant temperature and pressure. The fluid density decreased gradually from 2.28 kg·m^-3 at the inlet of the catalyst bed to 2.18 kg·m^-3 at the outlet of the catalyst bed, while the average velocity magnitude increased from 0.108 m·s^-1 to 0.120 m·s^-1.     

Reaction engineering studies of methane coupling in fluidised-bed reactors

The oxidative coupling of methane has been conducted in 30 and 60mm dia. fluidised-bed reactors. Methane conversions as high as 40% were achieved at isothermal conditions using methane/oxygen mixtures without diluents. At the same contact time the two reactors had similar selectivities to hydrocarbons. At 850°C the hydrocarbon selectivity decreased dramatically with increasing contact time but this effect was much less severe at lower temperatures. Axial gas concentration profiles through the catalyst bed in the 60mm reactor indicated that at 850°C there was a rapid consumption of oxygen and formation of products in the bottom section of the bed followed by a net loss of hydrocarbon in the oxygen-free zone. This loss was due to carbon formation on the catalyst which was circulated back to the oxygen-containing zone of the bed where the carbon was combusted.     

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.     

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.     

Kinetics and mechanism of oxidative coupling of methane over sodium-manganese oxide catalyst

The kinetics of methane oxidative coupling (OCM) was studied using 1 g of Na-Mn₂O₃ catalyst at 1073 to 1123 K, in an integral flow reactor (I.d. = 10 mm), at atmospheric pressure with methane and oxygen partial pressures of 0.27 and 0.13 bar, respectively, so that the ratio of CH₄ to O₂ was 2. The flow rate range was 50 to 200 ml/min. the kinetic data were analyzed by the Rideal-redox type of rate equation assuming the methyl radical and active surface oxygen to be the steady-state intermediates. Oxidation and reduction rate constants (K_ox, K_red) for methane consumption were calculated from experimental catalysis results by computer simulation using the multiple least squares method. The activation energies at rate constants K_ox and K_red for this type of catalyst were reported as 43.26 and 62.2 kcal/mol, respectively.     

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.     

Transient studies on methane oxidative coupling over alkali-metal promoted titanate catalysts

A transient method to study the role of the different oxygen species involved in oxidative coupling of methane is discussed. The method is applied to two new active catalysts for this process, which are based on alkali metal loaded titanates of lanthanum and nickel and also to a lithium/titania catalyst used in previous works. The results show that the participation of lattice oxygen in the formation of C2 products depends on the Li loading and the type of titanate used.     

Novel vanadium catalyst system with tartaric acid salts for highly selective asymmetric oxidative coupling polymerization

The asymmetric oxidative coupling polymerization (AOCP) of 2,3-dihydroxynaphthalene (DHN) with a novel catalyst system, oxovanadium(IV) stearate in the presence of the sodium or lithium salt of tartaric acid, under an O₂ atmosphere was carried out. For example, the polymerization with a catalytic amount of the d-(−)-tartaric acid disodium salt in THF at room temperature for 48 h followed by acetylation of the hydroxyl groups gave a methanol-insoluble polymer in a 40% yield, which showed the highest specific rotation value ([α]_D) of −223 among the polymers so far obtained by the AOCP of DHN. The enantioselectivity during the polymerization was estimated to be an 88% enantiomeric exess (S).     

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.     

Production of fatty acid methyl esters over a limestone-derived heterogeneous catalyst in a fixed-bed reactor

Production of fatty acid methyl esters (FAME) via the transesterification of different vegetable oils and methanol with a limestone-derived heterogeneous catalyst was investigated in a fixed-bed reactor at 65 °C and ambient pressure. This heterogeneous catalyst, as a 1 or 2 mm cross-sectional diameter extrudate, was prepared via a wet mixing of thermally treated limestone with Mg and Al compounds as binders and with or without hydroxyethyl cellulose (HEC) as a plasticizer, followed by calcination at 800 °C. The physicochemical properties of the prepared catalysts were characterized by various techniques. Palm kernel oil, palm oil, palm olein oil and waste cooking oil could be used as the feedstocks but the FFA and water content must be limited. The extrudate catalyst prepared with the HEC addition exhibited an enhanced formation of FAME due to an increased porosity and basicity of the catalyst. The FAME yield was increased with the methanol/oil molar ratio. The effect of addition of methyl esters as co-solvents on the FAME production was investigated. The structural and compositional change of the catalysts spent in different reaction conditions indicated that deactivation was mainly due to a deposition of glycerol and FFA (if present). The FAME yield of 94.1 wt.% was stably achieved over 1500 min by using the present fixed-bed system.     

The oxidative coupling of methane and the oxidative dehydrogenation of ethane over a niobium promoted lithium doped magnesium oxide catalyst

The promoting effect of niobium in a Li/MgO catalyst for the oxidative coupling of methane (OCM) and for the oxidative dehydrogenation of ethane (ODHE) has been studied in some detail. It has been found that a Li/Nb/MgO catalyst with 16 wt % niobium showed the highest activity for the C₂ production in the OCM reaction; the activity at 600 °C was ten times that of the Li/MgO catalyst at the same temperature. The Li/Nb/MgO catalyst was also slightly more active for the ODHE reaction than was the Li/MgO catalyst. However, the Li/Nb/MgO catalyst produced considerably more carbon dioxide in the both reactions. Structural investigation of the catalyst showed that the addition of niobium to the Li/MgO catalyst increased the surface area and gave an increase in the lithium content of the calcined catalysts. Two niobium phases, LiNbO₃ and Li₃NbO₄, were formed; it is shown that the first of these probably causes the increased activity. Ageing experiments showed that the activity of the catalyst was lost if the catalyst was used above 720 °C, the melting point of the lithium carbonate phase. The catalyst showed a decrease of surface area after ageing and a sharp decrease of the amount of the two niobium phases. The addition of carbon dioxide to the feed could not prevent the deactivation of the Li/Nb/MgO catalyst.     

Cold flow model and computer simulation studies of a circulating fluidized bed reactor for the oxidative coupling of methane

A circulating fluidized bed configuration has been developed for application in the oxidative coupling process. The configuration comprises a bottom turbulent fluidized bed, wherein the oxidative coupling reaction is conducted, followed by a reduced-diameter top fast bed for catalyst entrainment and hydrocarbon cracking. The hydrodynamic characteristics of this configuration have been investigated in a pilot-plant cold flow unit. Detailed experimental results on the turbulent bed flow structure and the gas phase residence time distribution are presented and discussed. The performanceofthe proposed reactor is analyzed by computer simulation studies based on a published oxidative coupling kinetic model. It is shown that improved hydrocarbon yields can be obtained by optimizing the hydrodynamic structure and the mixing characteristics of the turbulent bed.     

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.     

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10^–3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10^–2 molecules/sites, under the same reaction conditions. The initial C₂ formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.     

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.     

Catalytic combustion of methane over a highly active and stable NiO/CeO2 catalyst

In the last decades, many reports dealing with technology for the catalytic combustion of methane (CH₄) have been published. Recently, attention has increasingly focused on the synthesis and catalytic activity of nickel oxides. In this paper, a NiO/CeO₂ catalyst with high catalytic performance in methane combustion was synthesized via a facile impregnation method, and its catalytic activity, stability, and water-resistance during CH₄ combustion were investigated. X-ray diffraction, low-temperature N₂ adsorption, thermogravimetric analysis, Fourier transform infrared spectroscopy, hydrogen temperature programmed reduction, methane temperature programmed surface reaction, Raman spectroscopy, electron paramagnetic resonance, and transmission electron microscope characterization of the catalyst were conducted to determine the origin of its high catalytic activity and stability in detail. The incorporation of NiO was found to enhance the concentration of oxygen vacancies, as well as the activity and amount of surface oxygen. As a result, the mobility of bulk oxygen in CeO₂ was increased. The presence of CeO₂ prevented the aggregation of NiO, enhanced reduction by NiO, and provided more oxygen species for the combustion of CH₄. The results of a kinetics study indicated that the reaction order was about 1.07 for CH₄ and about 0.10 for O₂ over the NiO/CeO₂ catalyst.     

LiCl doped cobalt oxide is an active catalyst for the formation of ethylene in the oxidative coupling of methane

The title compound was found to be an active and selective catalyst for oxidative coupling of methane at temperatures even lower than 900 K, where the selectivity for the formation of ethylene exceeded that of ethane.     

Numerical simulation of particle/monolithic two-stage catalyst bed reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane

A three-dimensional geometry model of the particle/monolithic two-stage reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane (OCM) was set up. The improved Stansch kinetic model adapting different operating temperatures was established to calculate the OCM reactor performance using computational fluid dynamics (CFD) and FLUENT software. The results showed that the calculated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO₂, CO) in the OCM reactor. The distributed dioxygen feeding with the percentage of 5–20% based oxygen flow rate of top inlet promoted the OCM reaction in monolithic catalyst bed and led to the conversion of CH₄ and the selectivity and yield of C₂ (C₂H₆ and C₂H₄) increase obviously. The distributed dioxygen feeding was 15%, the conversion of CH₄, the selectivity and the yield of C₂ reached 34.1%, 68.2% and 23.3%, respectively.