Optimum operation of oxidative coupling of methane in porous ceramic membrane reactors

This paper presents analysis of oxidative coupling of methane on Li/MgO packed porous membrane reactor (PMR) by the fixed-bed reactor (FBR) model with reliable reaction kinetic equations. PMR can improve the selectivity and yield by controlling the oxygen feed to the catalyst bed through manipulating the feed pressure. At a fixed methane feed rate there is an optimal oxygen feed pressure that will achieve the highest yield. With a commercial ultrafiltration ceramic membrane, theoretical analysis shows that PMR can achieve, by operating with both side pressures at 1 bar at 750 °C, a maximal 30% yield at 53% selectivity. The maximal yield achieved in the FBR of identical dimension and temperature is 20.7% at 52.5% selectivity. Parametric study shows that lowering the membrane permeability improves the performance. Higher oxygen feed pressure will reduce the yield as well as the selectivity. Homogeneous reactions at high shell-side pressure can have adverse effect on the performance due to the fact that homogeneous reaction rates are strongly pressure dependent. The shell (oxygen feed) side volume must be minimized to reduce the homogeneous reactions. The results of PMR model calculation fit the published experimental result unexpectedly well.     

Fischer–Tropsch synthesis: Overview of reactor development and future potentialities

An overview of the reactors utilized for the Fischer–Tropsch synthesis is considered during three time periods: discovery to 1945, 1945–1970, and 1970 to date. A brief outline of the scientific and engineering developments related to chemical reactor design for the same three periods is also presented. In general, the reactor developments outpaced the ability to utilize the scientific and engineering advances on the academic level. However, today it appears that the academic and industrial developments closely match each other in content and interests. Even so, the availability of reliable data from large-scale pilot plants and/or commercial operations remain available only to the organization developing the data.     

费托浆态床反应器蜡分离系统的设置和优化

在16万t/a煤间接液化项目运行参数的基础上,对费托浆态床反应器蜡分离系统内置、外置过滤器精度、滤芯选择与安装布置、配套设施及控制操作进行了探讨;并结合实际运行经验,在操作温度选择、恒速过滤、错流比控制等8个方面提出优化,为百万吨级规模放大提供参考依据。     

Multi-objective optimization of simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane

A comprehensive optimization study on a simulated countercurrent moving bed chromatographic reactor (SCMCR) is reported in this article for oxidative coupling of methane (OCM) reaction. The selection of the operating parameters such as the switching time, make-up feed rate, methane to oxygen ratio in feed, length of columns and flow rates in different sections are not straightforward in an SCMCR. In most cases, conflicting requirements and constraints govern the optimal choice of the decision (operating or design) variables. An experimentally verified mathematical model was selected to optimize the performance of the SCMCR for OCM. A few multi-objective optimization problems were solved for both existing setup and at design stage. The optimization was performed using a state-of-the-art AI-based non-traditional optimization technique, non-dominated sorting genetic algorithm with jumping genes (NSGA-II-JG), which resulted in Pareto optimal solutions. It was found that the performance of the SCMCR could be improved significantly under optimal operating conditions.     

Modeling and simulation of simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane

Oxidative coupling of methane (OCM) is a reaction of industrial importance but per pass equilibrium conversion and product yield in a single reaction column is severely low. The simulated countercurrent moving bed chromatographic reactor (SCMCR) has been reported to significantly improve the methane conversion and C₂-product yield. This paper addresses the mathematical modeling of a five section SCMCR for OCM, which is particularly important for understanding the operation of this SCMCR system. In order to obtain the various process parameters, a realistic and rigorous kinetics was adopted in reactors for OCM and subsequently a kinetic model was developed which can best describe the associated kinetics of OCM in SCMCR. Adsorption isotherm parameters were then derived based on the experimental breakthrough curves acquired using single adsorption column. The proposed mathematical model demonstrated extremely good predictions of the experimental results. Finally, effects of operating parameters, such as switching time, methane/oxygen feed ratio, raffinate flow rate, eluent flow rate, etc., on the behavior of the SCMCR were studied systematically.     

Catalyst poisoning and fixed bed reactor dynamics—II: Adiabatic reactors

Prior work on experimental and modeling studies of nonisothermal nonadiabatic reactor dynamics induced by catalyst poisoning is extended in this paper to adiabatic reactors. Thiophene poisoning of the nickel catalysed hydrogenation of benzene is used as the experimental system.A pseudohomogeneous one dimensional dispersion model was used to model both steady state and transient behavior of the reactor on introduction of poison into the feed. Poisoning kinetics were interpreted via a shell-progressive mechanism which appears to provide a plausible and simpler alternative to the two site mechanism previously postulated [1]. In general a reasonable agreement between experiment and model was obtained; however the accumulation of errors involved in separate determination of the reactor/reaction parameters ultimately limits the degree of precision which such simulations can attain.     

Parameter optimization for the oxidative coupling of methane in a fixed bed reactor by combination of response surface methodology and computational fluid dynamics

A resource and time saving method is introduced for optimizing fixed bed reactors by the combination of response surface methodology (RSM) and CFD simulation. This is demonstrated for the oxidative coupling of methane (OCM) based on the reaction kinetics by Stansch et al. (1997). Firstly, a parameter screening is performed to identify the power factors modified reaction time, heating temperature and CH₄ to O₂ ratio. Secondly, utilizing Central Composite Design, meta models for the most interesting responses are developed, i.e. C₂ selectivity and yield as well as CH₄ conversion. The statistical models describe the characteristics of the responses over a wide range. Thirdly, an optimization of C₂ yield is carried out using RSM. The maximum is detected to be approx. 14% and validated by three dimensional CFD simulations. In the investigated parameter space the optimized parameter conditions are found for a feed composition of 20% nitrogen, 26.7% oxygen, 53.3% methane (CH₄ to O₂ ratio of 2), a modified reaction time of 61 kg s/m³ and a heating temperature of 801.5 °C.     

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.     

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.     

Oxidative coupling of methane in a bubbling fluidized bed reactor

The oxidative coupling of methane to higher hydrocarbons (C₂₊) was studied in a bubbling fluidized bed reactor between 700°C and 820°C, and with partial pressures of methane from 40 to 70 kPa and of oxygen from 2 to 20 kPa; the total pressure was ca 100 kPa. CaO, Na₂CO₃/CaO and PbO/γ-Al₂O₃ were used as catalytic materials. C₂₊ selectivity depends markedly on temperature and oxygen partial pressure. The optimum temperature for maximizing C₂₊ selectivity varies between 720 and 800°C depending on the catalyst. Maximum C₂₊ selectivities were achieved at low oxygen and high methane partial pressures and amounted to 46% for CaO (T = 780°C; P_CH4 = 70 kPa; P_O2 = 5 kPa), 53% for Na₂CO₃/CaO (T = 760°C; P_CH4 = 60 kPa; P_O2 = 6 kPa) and 70% for PbO/γ-Al₂O₃ (T = 720°C; P_CH4 = 60 kPa; P_O2 = 5 kPa). Maximum yields were obtained at low methane-to-oxygen ratios; they amounted to 4.5% for CaO (T = 800°C; P_CH4 = 70 kPa; P_O2 = 12 kPa), 8.8% for Na₂CO₃/CaO (T = 820°C; P_CH4 = 60 kPa; P_O2 = 20 kPa) and 11.3% for PbO/γ-Al₂O₃ (T 2= 800°C; P_CH4 = 60 kPa; P_O2 = 20 kPa).     

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.     

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.     

Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor

Ionic-electronic mixed-conducting perovskite-type oxide La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the p_O2-range of 10⁻³−1 bar at 1073–1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m⁻² s⁻¹ at 1173 K with a corresponding activation energy of 130–140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1–3% at 1073–1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30–35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized.     

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.     

Comparison of diffusion models in the modeling of a catalytic membrane fixed bed reactor coupling dehydrogenation of ethylbenzene with hydrogenation of nitrobenzene

Coupling of dehydrogenation of ethylbezene with hydrogenation of nitrobenzene in a catalytic membrane reactor can lead to a significant improvement in the conversion of ethylbenzene and production of styrene. In this work, the homogeneous reactor model for a cocurrent flow configuration is compared to two heterogeneous models based on the Fickian diffusion model and the dusty gas model for both isothermal and non-isothermal pellets. It is observed that both heterogeneous models predict a significant drop in yield and conversion compared to the homogeneous model, indicating the importance of heterogeneity. This drop is generally less severe for the dusty gas model than for the Fickian diffusion model. The assumption of isothermality causes larger deviations than the assumption of Fickian diffusion. The deviations in the predictions of the homogenous model and the heterogeneous models from those of the dusty gas model for non-isothermal pellets are ∼6% and ∼11%, respectively.     

Influence of oxygen supply rates on performances of catalytic membrane reactors: Application to the oxidative coupling of methane

The impact of oxygen permeability using an ionic oxygen conducting membrane reactor with surface catalyst was investigated for the oxidative coupling of methane to higher hydrocarbons. Dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), Ba_0.5Sr_0.5Mn_0.8Fe_0.2O_3−δ (BSMFO) and BaBi_0.4Fe_0.6O₃ (BBFO) membrane disks with Pt/MgO catalysts were prepared by sol–gel deposition or wash-coating. It is demonstrated that the oxygen supply by permeation needs to fit to the consumption during the coupling reaction. In case of insufficient oxygen supply comparably poor conversions are observed while higher oxygen fluxes lead to increased methane conversions, especially in the presence of an efficient catalyst. Generally, increasing catalytic activity leads to lower C₂ selectivity, especially for low oxygen permeation fluxes. The concept of a reactor employing dense catalytic membranes is viable, but the present study identifies further potential when the activity of the catalyst for the oxidative coupling is improved, leading to an overall enhanced performance of the membrane reactor.     

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.     

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.     

Cooling exothermic catalytic fixed bed reactors: Co-current versus countercurrent operation in a methanol conversion reactor

A comparison between two possible designs of exothermic catalytic fixed bed reactors, fully co-current and fully countercurrent flow, was performed for the process of methanol transformation into hydrocarbons. The fully co-current operation proved more appropriate, yielding, under low parametric sensitivity, higher productivities and allowing higher inlet methanol partial pressures. The co-current design showed, in a wide range of operating conditions, the Pseudoadiabatic Regime, a useful mode of operation. The countercurrent design did not allow Pseudoadiabatic Operation and had drawbacks such as multiple steady states and reactor ignition.     

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%.