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.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Mathematical model of a monolith catalytic incinerator

A set ofl-dimensional mathematical models were developed to simulate both the steady state and transient performance of monolithic catalytic incinerators for VOC abatement. In modelling transient performance, quasi-steady state gas phase was assumed since transient response time is determined primarily by the thermal inertia of the monolith. Higher inlet gas temperatures and lower gas velocities were predicted to give higher conversion and faster response times. VOC concentration had little influence on the performance within the concentration ranges used. A catalytic incinerator is shown to operate typically under mass transfer limited conditions, and monolith channel density and shape have significant influence on the conversion and monolith heating time. The metallic monolith was predicted to show superior steady state and transient responses due to its lower thermal inertia generated by higher cell density and thinner wall.     

Modeling for the catalytic coupling reaction of carbon monoxide to diethyl oxalate in fixed-bed reactors: Reactor model and its applications

A two-dimensional (2D) pseudo-homogeneous reactor model was developed to simulate the performance of fixed-bed reactors for catalytic coupling reaction of carbon monoxide to diethyl oxalate. Reactor modeling was performed using a comprehensive numerical model consisting of two-dimensional coupled material and energy balance equations. A power law kinetic model was applied for simulating the catalytic coupling reaction with considering one main-reaction and two side-reactions. The validity of the reactor model was tested against the measured data from different-scale demonstration processes and satisfactory agreements between the model prediction and measured results were obtained. Furthermore, detailed numerical simulations were performed to investigate the effect of major operation parameters on the reactor behavior of fixed bed for catalytic coupling reaction of carbon monoxide to diethyl oxalate, and the result shows that the coolant temperature is the most sensitive parameter.     

Effective approach to cyclic steady state in the catalytic reverse-flow combustion of methane

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air.     

A study of Nusselt and Sherwood numbers in a monolith reactor

A two-dimensional model of a single channel of a monolith reactor is used to evaluate the values of the Nusselt and Sherwood numbers under reaction conditions. The circular channel is assumed to have axisymmetry with a first-order reaction occurring at the wall. The values of the Nusselt and Sherwood numbers do not correlate uniquely with the Graetz number but rather depend on the reaction rate at the wall. Hence they depend on such variables as gas velocity, inlet temperature and reactant concentration.     

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%.     

An investigation of catalytic plate reactors by means of parametric sensitivity analysis

A catalytic plate reactor consisting of closely spaced catalytically coated plates, where endothermic and exothermic reactions take place in alternate channels is studied. The influence of several design parameters on its thermal behaviour and performance is investigated by parametric sensitivity analysis using a two-dimensional reactor model. Parameters considered are: channel size; wall thickness and thermal conductivity; inlet temperature, composition and velocities; and kinetic parameters. It is demonstrated that different catalysts can show similar thermal behaviour and performance but exhibit different sensitivity behaviour. For the system investigated the strongest influence on reactor sensitivity comes from the activation energies of both exothermic and endothermic reactions.     

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.     

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).     

Conceptual design and CFD simulation of a novel metal-based monolith reactor with enhanced mass transfer

This paper introduces a novel structured metallic catalyst that improves mass transfer performance of a monolith reactor for highly exothermic gas–solid reactions. The monolith channels are designed to have metallic substrates that consist of two layers with one of the layers being the metallic support and another layer being a foam metal annular that is tightly deposited onto the support surface by some means. Parametrical studies based on a 2D monolith reactor model showed that the present design yields an enhanced mass transfer between the bulk fluid and the catalyst layer due to a decrease in external film resistance, and an enhanced mass transfer within the solid phase mainly due to the viscous flow effect within the porous catalyst layer.     

Analysis of a two-phase catalytic reaction in a millimeter-scale reactor

The synthesis of a catalyst for a two-phase catalytic reaction, a millimeter-scale reactor experiment, and an analysis model for the prediction of reactor performance are presented in this paper. The catalyst nano-particulate perovskite La_0.8Sr_0.2CoO₃ was prepared by a modified sol-gel method, in which PAA (poly acrylic acid) was added to catalyst precursors. A millimeter-scale reactor experiment with the prepared catalyst was carried. Concentrated hydrogen peroxide was decomposed in the reactor and the characteristics of the reactor were measured in terms of temperature distributions and liquid production rates. The results indicated a flow regime transition, which caused the change of reactor performance. An analysis model for two-phase catalytic reaction based on the lumped flow reactor model and the diagnostic data obtained on the temperature distributions and liquid production rates is proposed. Temperature distributions and heat transfer characteristics of the reactor were predicted by a semi-empirical analysis. In this analysis, the model of the Nusselt number (Nu) was proposed as . This expression of the model reflects the effects of temperature and coordinate location on the heat transfer characteristics of the reactor. From the modeled reactor, characteristics such as the increase of heat transfer in the mid and rear parts of the reactor with the increase of reactant flow rate were obtained. With the obtained results, a tool for the design and analysis of a down-scaled catalytic reaction device was obtained.     

Transient behaviour of catalytic monolith with NOx storage capacity

Transient behaviour of catalytic monolith converter with NO_x storage is studied under conditions typical for automobiles with lean-burn engines (i.e., diesel and advanced gasoline ones). Periodical alternation of inlet concentrations is applied—NO_x are adsorbed on the catalyst surface during a long reductant-lean phase (2–3 min) and then reduced to N₂ within a short reductant-rich phase (2–6 s). Samples of industrial NO_x storage and reduction catalyst of NM/Ba/CeO₂/γ-Al₂O₃ type (NM = noble metal), washcoated on 400 cpsi cordierite substrate, are used in the study. Effects of the rich-phase length and composition on the overall NO_x conversions are examined experimentally. Reduction of NO_x by CO, H₂ and unburned hydrocarbons (represented by C₃H₆) in the presence of CO₂ and H₂O is considered.

Effective, spatially 1D, heterogeneous mathematical model of catalytic monolith with NO_x and oxygen storage capacity is described. The minimum set of experiments needed for the evaluation of relevant reaction kinetic parameters is discussed: (i) CO, H₂ and HC oxidation light-off under both lean and rich conditions, including inhibition effects, (ii) NO/NO₂ transformation, (iii) NO_x storage, including temperature dependence of effective NO_x storage capacity, (iv) water gas shift and steam reforming under rich conditions, i.e., in situ production of hydrogen, (v) oxygen storage and reduction, including temperature dependence of effective oxygen storage capacity, and (vi) NO_x desorption and reduction under rich conditions. The experimental data are compared with the simulation results.     

A fluidized bed membrane reactor for the steam reforming of methane

In the present investigation a realistic two-phase model accounting for the change in the total number of moles accompanying the reaction is utilized to explore a novel reactor configuration suggested for the methane steam reforming process. The suggested design is basically a fluidized bed reactor equipped with a bundle of membrane tubes. These tubes remove the main product, hydrogen, from the reacting gas mixture and drive the reaction beyond its thermodynamic equilibrium. The proposed novel design is also equipped with sodium heat pipes which act as a thermal flux transformer to provide the large amount of heat needed by the endothermic reaction through a relatively small heat transfer surface, assuring better reactor compactness. Two options for fluid routing through the membrane tubes are proposed; each is suitable for a certain industrial application. The performance of this novel configuration is compared with that of an industrial fixed bed steam reformer and the comparison shows the potential advantages of the suggested configuration.     

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.     

CFD modeling of gas flow in porous medium and catalytic coupling reaction from carbon monoxide to diethyl oxalate in fixed-bed reactors

A comprehensive two-dimensional heterogeneous reactor model was developed to simulate the flow behavior and catalytic coupling reaction of carbon monoxide (CO)–diethyl oxalate (DEO) in a fixed-bed reactor. The two-temperature porous medium model, which was revised from a one-temperature porous medium model, as well as one equation turbulent model, and exponent-function kinetic model was constructed for the turbulent velocity scale comparing with laminar flow and simulation of the catalytic coupling reaction. The simulation results were in good agreement with the actual data collected from certain pilot-plant fixed bed reactors in China. Based on the validated approach and models, the distributions of reaction parameters such as temperature and component concentrations in the reactor were analyzed. The simulations were then carried out to understand the effects of operating conditions on the reactor performance which showed that the conduction oil temperature in the reactor jacket and the CO concentration are the key impact factors for the reactor performance.     

MnCu/cordierite monolith used for catalytic combustion of Volatile Organic Compounds

A MnCu‐mixed oxide catalyst supported on a cordierite monolith was synthesized. The catalyst showed very good stability and high homogeneity and presented an excellent catalytic activity in the combustion of ethyl acetate, n‐hexane, and its mixture. The total conversion temperature of the mixture was determined by the temperature at which the most difficult molecule was oxidized.     

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.     

Numerical study of on-board fuel reforming in a catalytic plate reactor for solid-oxide fuel cells

A pseudo-transient numerical model is used for the simulation of a multi-functional catalytic plate reactor (CPR). The work mainly addresses the problems associated with on-board reforming for solid-oxide fuel cells. Heat management is achieved by indirectly coupling partial oxidation with reforming. Water management is achieved by partially recycling the anode stream from a solid-oxide fuel cell. The model uses detailed heterogeneous chemistry for reforming and oxidation reactions occurring on the catalyst beds.