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 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.     

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.     

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.     

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.     

Effect of reactor internal properties on the performance of a flow reversal catalytic reactor for methane combustion

This paper describes the use of a flow reversal reactor for the destruction of lean emissions of methane with ambient temperature feed. The reactor consisted of two parallel sections, each containing a packed bed reactor and inert sections to act as heat traps. In this paper, the effect on reactor performance of the inert properties is illustrated. Three different inert types are described. These are a ceramic monolith of 100 cells per square inch cell density, a metal monolith and a packed bed of Denstone balls. Use of monolith inserts reduces the reactor pressure drop. Inert sections with lower thermal mass give rise to greater movement in temperature fronts, requiring the use of shorter cycles.     

The effects of kinetics, hydrodynamics and feed conditions on methane coupling using fluidized bed reactor

A previously developed model describing bubbling fluidized bed reactors is used in this investigation to study the effect of various important hydrodynamic, operating and design parameters on the performance of a large scale fluidized bed reactor used in oxidative coupling of methane. Three kinetic schemes obtained from the literature have been used in this study. The model predicted fairly well the experimental results reported recently under different reaction conditions. The simulation results revealed that increasing the ratio of methane to oxygen in the feed leads to lower methane conversion but higher C₂ selectivity. As the ratio is decreased the system loses its fixed-point stability to a periodic stability. Higher methane conversion and product selectivity are obtained upon decreasing the feed flow rate and particle diameter.     

Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor

Methane decomposition over a Ni/Cu/Al₂O₃ catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures.     

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.     

Effect of washcoat modification with metal oxides on the activity of a monolithic Pd-based catalyst for methane combustion

The deposition of Ni, Co, Ce or Fe oxides onto the washcoat surface in the 0.5%Pd/Al₂O₃ catalyst enhances conversion of CH₄. Catalytic activity of the Pd-catalysts containing cobalt oxide depends on the incorporated amount of cobalt oxide and the method of incorporation. The highest activities were those of the 0.5%Pd/0.3%Co/Al₂O₃ and 1%Pd/0.3%Co/Al₂O₃ catalysts (cobalt oxide deposited onto the surface of Al₂O₃) and the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst (mixed washcoat). Total SSA, Pd dispersion and Pd crystallite size in the x%Pd/y%Co/Al₂O₃ catalysts depend on the incorporated amount of PdO and cobalt oxide. Pd dispersion in the 1%Pd/Al₂O₃ catalyst increases from 4% to 20% upon deposition of 14 wt.% Co₃O₄ (by mass Al₂O₃) onto the Al₂O₃ surface (1%Pd/0.3%Co/Al₂O₃). This increase in Pd dispersion influence the increase in the activity of the 1%Pd/Al₂O₃ catalyst. On the surface of the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst Pd occurs mainly in the form of PdO and displays considerable mobility under conditions of temperature variations—cyclically undergoing reduction and oxidation. At 500 °C, in vacuo, the reduction was irreversible and parallelled by the agglomeration of metallic Pd crystallites. At room temperature, cobalt occurred on the catalyst surface in the form of Co⁺² ions (CoAl₂O₄) and was reduced to Co⁰ at 500 °C (in vacuo). Up to 500 °C, the reduction of Co was reversible.     

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.     

A novel structured bioreactor: Development of a monolithic stirrer reactor with immobilized lipase

Cordierite monoliths were functionalized with polyethylenimine (PEI) and with different types of carbon, consisting of carbonized sucrose, carbonized ployfurfurryl alcohol, or carbon nanofibers, in order to create adsorption sites for a lipase from Candida antarctica. The prepared supports were compared in terms of immobilization capacity, activity, and stability. The supports with a carbon nanofiber coating displayed the highest enzyme adsorption capacity. The biocatalysts were assayed in the acylation of 1-butanol with vinyl acetate in toluene, yielding butanyl acetate and acetaldehyde. For catalyst performance testing a novel reactor type was employed, the monolithic stirrer reactor, in which monolithic structures are applied as stirrer blades. No profound effect of stirrer rate on the reaction rate was observed, implicating the absence of external mass transfer limitations. For comparison, free enzyme and a commercial (particulate) immobilized lipase were also included in the study. Compared to the free enzyme, the immobilized lipase shows a significantly lower activity. Increased stability, easy catalyst separation and the possibility to reuse the enzyme in immobilized form can overcome this difference. The commercial immobilized lipase initially has a significantly higher activity than the monolithic biocatalysts, but deactivates relatively fast. For the monolithic biocatalysts, no deactivation was observed; the prepared catalysts were stable for several weeks.     

MoO3/MgO as a catalyst in the oxidative dehydrogenation of n-butane in a two-zone fluidized bed reactor

Butadiene has been produced from butane by oxidative dehydrogenation on a MoO₃/MgO catalyst using a two-zone fluidized bed reactor (TZFBR). The effect of the main TZFBR operating variables was studied, and its performance was compared with that of conventional fluidized beds and fixed bed reactors loaded with the same catalyst. These results have been compared with those obtained on a selective V₂O₅/MgO catalyst.     

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.     

Numerical simulation and optimisation of unconventional three‐section simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane reaction

The simulated countercurrent moving‐bed chromatographic reactor (SCMCR) has been reported to significantly enhance methane conversion and C₂ product yield for oxidative coupling of methane (OCM) reaction, which is otherwise a low per pass conversion reaction. A mathematical model of an unconventional three‐section SCMCR for OCM was first developed and solved using numerically tuned kinetic and adsorption parameters. The model predictions showed good agreement with available experimental results of SCMCR for OCM. Effects of several process parameters on the performance of SCMCR were investigated. A multi‐objective optimisation problem was solved at the operating stage using state‐of‐the‐art AI‐based non‐dominated sorting genetic algorithm with jumping genes adaptations (NSGA‐II‐JG), which resulted in Pareto Optimal solutions. It was found that the performance of the SCMCR could be significantly improved under optimal operating conditions. © 2011 Canadian Society for Chemical Engineering     

Hydrogen production via catalytic steam reforming of fast pyrolysis bio-oil in a two-stage fixed bed reactor system

Hydrogen production was prepared via catalytic steam reforming of fast pyrolysis bio-oil in a two-stage fixed bed reactor system. Low-cost catalyst dolomite was chosen for the primary steam reforming of bio-oil in consideration of the unavoidable deactivation caused by direct contact of metal catalyst and bio-oil itself. Nickel-based catalyst Ni/MgO was used in the second stage to increase the purity and the yield of desirable gas product further. Influential parameters such as temperature, steam to carbon ratio (S/C, S/CH₄), and material space velocity (W_BHSV, GHSV) both for the first and the second reaction stages on gas product yield, carbon selectivity of gas product, CH₄ conversion as well as purity of desirable gas product were investigated. High temperature (> 850 °C) and high S/C (> 12) are necessary for efficient conversion of bio-oil to desirable gas product in the first steam reforming stage. Low W_BHSV favors the increase of any gas product yield at any selected temperature and the overall conversion of bio-oil to gas product increases accordingly. Nickel-based catalyst Ni/MgO is effective in purification stage and 100% conversion of CH₄ can be obtained under the conditions of S/CH₄ no less than 2 and temperature no less than 800 °C. Low GHSV favors the CH₄ conversion and the maximum CH₄ conversion 100%, desirable gas product purity 100%, and potential hydrogen yield 81.1% can be obtained at 800 °C provided that GHSV is no more than 3600 h^− 1. Carbon deposition behaviors in one-stage reactor prove that the steam reforming of crude bio-oil in a two-stage fixed bed reaction system is necessary and significant.     

Simulation of a spouted bed reactor for solid catalyst alkylation

A process simulator was developed to afford a better understanding of the performance of a butene-isobutane alkylation plant that uses a three-phase spouted bed reactor and a super acid solid catalyst. The mass and energy balances were numerically solved using lump of reactions and kinetic expressions previously developed. The results indicate the strong influence of the gas linear velocity and temperature in the riser and downcomer behavior that affects the activity, selectivity, and stability of the catalytic systems that cannot be predicted otherwise. The optimal operating conditions were determined for a particular catalyst and set of costs. The impacts of the recycling alkylate and isobutane to the reactor as well as the presence of hydrogen to control the coke formation are analyzed.     

Effects of various barium precursors and promoters on catalytic activity of Ba-Ti perovskite catalysts for oxidative coupling of methane

The influences of barium precursor and promoter type on the catalytic performance of perovskite catalysts in OCM reaction were studied. Catalysts (BaTiPO₃, P: promoter) were prepared by carbonate, hydroxide and propionate precursors of barium and SnCl₂ and CeO₂ as promoters by sol-gel method, tested in a fixed-bed microreactor and characterized by XRD, BET, CO₂-TPD, FT-IR and UV-Visible analysis. The experiment results showed that based on the extent of effect upon catalyst efficiency, the barium anions can be ranked as; propionate > carbonate > hydroxide, and the CeO₂ promoted catalysts were more active than the SnCl₂ promoted ones. The characterization results showed that the substitution of metal precursors caused formation of different phases with different particle sizes, influenced the basicity of the catalysts, resulted in the appearance of the peaks corresponding to different groups in IR spectroscopy, and shifted the absorption peaks in UV-Visible spectra. These results suggested that OCM reaction over perovskite catalysts is structure sensitive and depended on the type of used precursor and promoter.