Development of a dual functional structured catalyst for partial oxidation of methane to syngas

Rh-LaCoO₃ structured catalysts for the oxidative production of syngas from methane were developed by deposition of the active components on La-γ-Al₂O₃ washcoated honeycomb monoliths. SEM/EDS analysis showed a good adhesion of the washcoat layer and a uniform distribution of La and Co, while Rh was favourably located on the outer shell. Catalytic partial oxidation of methane was tested under both isothermal and pseudo-adiabatic conditions showing that the process can be conducted with high yield and selectivity and stable performance at short contact times over the novel catalysts, characterised by a limited content of noble metal and no need for pre-reduction. Further experiments of CO₂ autothermal reforming indicated the possibility to enhance CO production and to reduce the H₂/CO ratio through secondary endothermic reactions consuming CO₂, which are autothermally self-sustained in a single catalytic reactor operated at short contact time by the heat generated through partial oxidation reactions.     

MODELING AND SIMULATION OF A NONISOTHERMAL CATALYTIC MEMBRANE REACTOR

A two-dimensional nonisothermal mathematical model has been developed to simulate a tube-and-shell configuration, catalytic membrane reactor. The three-layer membrane consists of an inert large-pore support, an o₂ semipermeable dense perovskite layer and a porous catalytic layer. The model is applied to the simulation of the partial oxidation or methane to syngas (oxyreforming). The membrane reactor simultaneously supplies oxygen to the catalytic reaction along the reactor length, and separates oxygen from the air feed, using a dense perovskite layer which is a mixed conductor, thus allowing rapid oxygen permeation without the use of an external circuit. Two configurations of catalytic membrane reactors are simulated, for both bench-scale and industrial-scale conditions. Comparisons are made to the conventional fixed-bed reactor, and to membrane reactors which are isothermal, adiabatic or wall-cooled. The simulation results imply that the temperature rise in exothermic partial oxidation reactions may be mitigated substantially by the use of a dense membrane reactor,     

High yields of synthesis gas by millisecond partial oxidation of higher hydrocarbons

Cyclohexane, n-hexane, and isooctane were reacted with air in a Rh-monolith reactor and converted into synthesis gas (H₂+CO) in yields exceeding 90%, with >95% conversion of fuels and 100% conversion of oxygen. The best catalyst was an 80 ppi washcoated alumina monolith containing 5 wt% Rh. There was a small effect of catalyst contact time on syngas selectivity and conversions for gas space velocities from 3×10⁵ to 3×10⁶ h^–1. Preheating the feed enhances syngas selectivities slightly, but no reactor preheat is necessary provided the fuel remains vaporized. Addition of 25 mol% toluene to isooctane also produces syngas, olefins, and methane with 90% yield, including 70% yield to syngas. Partial oxidation of gasoline–air mixtures was attempted but the catalysts were poisoned after several hours, probably by sulfur and/or metals.     

Catalyst based on BaZrO3 with different elements incorporated in the structure: II. BaZr(1−x)RhxO3 systems for the production of syngas by partial oxidation of methane

New catalytic systems, synthesised by a variant of the citrate route, are proposed for the partial oxidation of methane. They consist of solid solutions – barium, zirconium, rhodium and oxygen – with a perovskite structure of formula BaZr_(1−x)Rh_xO₃. Detailed analysis of the XRD diffractograms and the TGA cycles show that Rh is randomly distributed as Rh^IV among the B sites of the perovskite, together with Zr. The activities of the catalysts have been tested for the catalytic partial oxidation of methane at short contact times to evaluate the potential of materials giving promising results in terms of syngas yield at low Rh loading.     

Syngas formation by direct oxidation of methane: Reaction mechanisms and new reactor concepts

The reaction mechanism of direct catalytic oxidation of methane to syngas over a platinum catalyst under high temperature, short contact time conditions was studied with a detailed reactor and reaction model. Based on a detailed analysis of this mechanism, new integrated reactor concepts were deduced. Two concepts were studied in detail: a fixed bed reactor with integrated recuperative heat exchange, and a catalytic membrane reactor with distributed reactant feed. The reactor concepts are presented, and advantages and problems of the concepts are discussed.     

Rhodium supported on thermally enhanced zeolite as catalysts for fuel reformation of jet fuels

Autothermal reformation of military jet fuel (1096 ppmw sulfur) was investigated with rhodium supported on thermally stabilized Y zeolite catalysts. The zeolite catalysts were thermally stabilized by ion exchanging with nitrate solutions of rare-earth metals (La, Ce, Sm, Gd, Dy and Er). Surface area analyses indicated that the exchanged zeolite could maintain its porous structure as high as 950 °C instead of 800 °C for a commercial NaY zeolite. The structure of the exchanged zeolite was characterized by X-ray diffraction (XRD). Rh-SmNaY zeolite reforming catalysts were prepared by incipient wetness and organometallic synthesis. The JP8 reforming experiments were performed in a short contact time adiabatic reactor with a monolithic catalyst with the addition of air and steam at a temperature below 920 °C. The effects of steam and fuel-to-air ratio (C/O ratio) were studied. Hydrogen and carbon monoxide were produced as the main products. Durability tests were performed with Rh/SmNaY-zeolite catalysts. This work shows that zeolite based catalysts can convert transportation fuels such as high sulfur jet fuel (over 1000 ppmw S) to syngas for solid oxide fuel cell applications.     

Optimal Design of A CPO-Reformer of Light Hydrocarbons with Honeycomb Catalyst: Effect of Frontal Heat Dispersions on the Temperature Profiles

This paper extends a previous investigation on the thermal behavior of CH₄-CPO reformers with honeycomb catalysts. Modeling and experimental studies on the short contact time catalytic partial oxidation (CPO) of CH₄ to syngas from our and from other groups have shown that Rh-catalysts rapidly deactivate at the very high temperatures, close to 1000 °C, that establish in the inlet zone of the reactor. We have previously shown that a significant reduction of the surface hot-spot temperature can be obtained by properly designing the catalyst: beneficial effects are observed at increasing opening of the honeycomb channels, which decreases the rate of O₂ inter-phase mass transfer, and at increasing catalyst activity, which promotes the rate of the endothermic reactions. In this work, we explore the effect of the reactor configuration, namely the effect of heat dispersion from the glowing front face of the monolith. Three reactor configurations were compared in CH₄-CPO experiments: (i) a configuration with perfect continuity between the front heat shield (FHS) and the catalytic module, which behaved close to an ideal adiabatic reactor, (ii) a configuration where the FHS was separated from the catalytic monolith and (iii) a configuration where the FHS was at large distance from the catalytic module. State of the art experimental tools, including the spatially resolved measurement of temperature and concentration profiles were used to characterize the thermal behavior of the various configurations. Detailed kinetic modeling supported the analysis of data. The results showed that, at the expense of a small loss of thermal efficiency, a very moderate loss of performance in terms of conversion and selectivity, but, remarkably, an important reduction of the surface inlet temperatures were achieved. Preliminary experiments with propane/air mixtures suggest that the adoption of a moderately dispersive reactor can represent a promising solution for the stable operation of catalytic units treating heavier fuels than methane.     

Temperature profile in a two-stage fixed bed reactor for catalytic partial oxidation of methane to syngas

Detailed axial temperature distribution has been studied in a two-stage process for catalytic partial oxidation of methane to syngas, which consists of two consecutive fixed bed reactors with oxygen or air separately introduced. The first stage of the reactor, packed with a combustion catalyst, is used for catalytic combustion of methane at low initial temperature. While the second stage, filled with a partial oxidation catalyst, is used for the partial oxidation of methane to syngas. A pilot-scale reactor packed with up to 80 g combustion catalyst and 80 g partial oxidation catalyst was employed. The effects of oxygen distribution in the two sections, and gas hourly space velocity (GHSV) on the catalyst bed temperature profile, as well as conversion of methane and selectivities to syngas were investigated under atmospheric pressure. It is found that both oxygen splitting ratio and GHSV have significant influence on the temperature profile in the reactor, which can be explained by the synergetic effects of the fast exothermic oxidation reactions and the slow endothermic (steam and CO₂) reforming reactions. Almost no change in activity and selectivity was observed after a stability experiment for 300 h.     

Hydrogen production from propane in Rh-impregnated metallic microchannel reactors and alumina foams

Rh-impregnated alumina foams and metallic microchannel reactors have been studied for production of hydrogen-rich syngas through short contact time catalytic partial oxidation (POX) and oxidative steam reforming (OSR) of propane. Effects of temperature and residence time have been compared for the two catalytic systems. Temperature profiles obtained along the central axis were valuable in understanding the different behaviour of the reactor systems. Gas phase ignition occurs in front of the metallic monolith at furnace temperatures above 700 °C, leading to lower hydrogen selectivity. Lowering the residence time below 10 ms for the microchannel monolith increases the syngas selectivity. This probably due to quenching of the gas phase reactions at high linear gas velocity, and suggests that microchannel reactors have potential for isolating kinetic effects and minimising gas phase contributions. The Rh/Al₂O₃ foam systems show higher initial syngas selectivity than the Rh-impregnated microchannel reactors, but deactivate rapidly upon temperature cycling, especially when steam is added as a reactant.     

Partial oxidation of CH4 and C2H6 over noble metalcoated monoliths

The direct partial oxidation of hydrocarbons offers promising alternatives to chemical synthesis. By replacing endothermic processes such as steam reforming and steam cracking, fast and exothermic oxidation reactions should require much smaller and simpler reactors. However, direct oxidation reactions are much more difficult to manage because of potential heat release in total oxidation and hazardous because of the possibility of homogeneous reactions which are nonselective and can produce flames and explosions. We describe experiments in which monolith catalysts are used for partial oxidation of CH₄ and C₂H₆ to produce synthesis gas or alkenes by direct oxidation at or above atmospheric pressure in pure O₂ in nearly adiabatic reactors operating at 1000°C with very high flowrates (space velocities of 10⁶h⁻¹ and residence times of 10⁻³ s). With methane oxidation we obtain over 90% selectivities to synthesis gas (a 2:1 H₂:CO mixture) with> 90% conversion of the methane and complete conversion of O₂ on Rh coated ceramic monoliths with contact times of 10⁻³ s. With Pt catalysts under the same conditions, the H₂ selectivity drops to 70%; while with Pd, the catalyst rapidly forms carbon. This process appears to be primarily a surface reaction in which CH4 pyrolyzes on the hot Rh surface and the H atoms dimerize and the carbon is oxidized to CO. A model has been constructed which accurately predicts the conversions and selectivities and the variations between Rh and Pt. With higher alkanes, synthesis gas is produced on Rh with comparable selectivities and conversions on metal-coated monoliths. However, with Pt we observe up to 70% selectivity to alkenes with 80% conversion of alkanes at adiabatic temperatures near 1000°C with approximately 5 ms contact times. These results can be explained as occurring by predominantly surface reactions in which the alkane adsorbs to form the alkyl by H abstraction with adsorbed O atoms. Then the adsorbed alkyls undergo primarily β-elimination reactions on Pt to produce alkenes. These products are therefore far from thermodynamic equilibrium at these very short contact times, even though the temperatures are very high. The use of very short contact times and high temperatures promises to provide new routes to production of partial oxidation products with very small adiabatic reactors and thus opens up new types of reactions and reactors for chemical synthesis.     

Tuning millisecond chemical reactors for the catalytic partial oxidation of cyclohexane

It is demonstrated that millisecond partial oxidation of cyclohexane can be tuned by varying the catalyst and operating conditions to generate product distributions that favor (1) oxygenates, (2) olefins, or (3) syngas (H₂ and CO). High selectivities to parent oxygenates require low conversions using low-temperature catalysts, such as Ag or Co. Olefins are favored by Pt or Pt-Sn and H₂ addition eliminates the production of CO and CO₂, thereby increasing olefin selectivities. For syngas, Rh is the catalyst of choice. Finally, a Pt-10% Rh single gauze gives high selectivities to both oxygenates and olefins.Conventional methods for the partial oxidation of cyclohexane are liquid-phase processes that are plagued by poor conversions, high recycle costs, long residence times (minutes to hours), and expensive catalysts. In contrast, with a cyclohexane–oxygen feed at C₆H₁₂/O₂=2, a Pt-10% Rh single gauze catalyst can give total selectivities exceeding 80% to oxygenates and olefins at 25% cyclohexane conversion and complete oxygen conversion. The products consist of nearly 60% selectivity to the C₆ products, cyclohexene and 5-hexenal. The temperature profile attained in the single-gauze reactor allows the preservation of these highly non-equilibrium products.Alternative catalysts for cyclohexane oxidation to oxygenates and olefins include α-alumina monoliths coated with Pt, Rh, Pt-Rh, Pt-Sn, Co, Mo or Ag. The Co, Mo and Ag catalysts give very high selectivities to C₆ oxygenates but are hindered by poor conversions (<5%) of both cyclohexane and oxygen at these millisecond contact times. H₂ addition to cyclohexane oxidation feed mixtures over Pt and Pt-Sn is shown to significantly increase the selectivities to C₆ olefins while reducing the formation of CO and CO₂.Cyclohexane oxidation in air over Rh monoliths enables the production of high yields (>95%) of syngas. This process could find applications in the automotive industry as the production of hydrogen from liquid fuels becomes important.     

Experimental and modeling analysis of the effect of catalyst aging on the performance of a short contact time adiabatic CH4-CPO reactor

Catalytic partial oxidation of methane at short contact time was studied in a lab-scale packed bed reactor over a 0.5 wt% Rh/A₂O₃ catalyst. Experiments were focused on the investigation of catalyst stability and durability upon repeated start-up/shut-down tests at different inlet temperatures and flow rates. Measurements of the axial temperature profiles evidenced a high sensitivity of the steady state thermal behavior of the reactor on catalyst activity: a decrease of the intrinsic catalytic activity was interpreted as the cause of a progressive over-heating of the bed which, in turn, moderated the loss of methane conversion and syngas productivity. At sufficiently high flow rate the observed temperature rise spread along the whole catalytic bed. Under such conditions both steady state and dynamic reactor performances were affected by the progressive decay of catalyst activity. A rationalization of the observed results was pursued by applying a one dimensional (1D) heterogeneous model of the reactor to the quantitative analysis of experimental results. Model predictions revealed the occurrence of operating surface temperatures up to 1100 °C and allowed to quantify the progressive worsening of reactor performances in terms of a loss of reforming activity localized in correspondence of the catalyst hot spot.     

整体型催化剂在天然气部分氧化中的研究进展

由于热量散失和均相反应使直接部分氧化甲烷很难控制,本文介绍了整体型催化剂在天然气部分氧化中的应用,使用整体型催化剂反应可在大空速下自热进行,基本实现干氧化。     

Modeling, simulation and control of dimethyl ether synthesis in an industrial fixed-bed reactor

Dimethyl ether (DME) as a clean fuel has attracted the interest of many researchers from both industrial communities and academia. The commercially proven process for large scale production of dimethyl ether consists of catalytic dehydration of methanol in an adiabatic fixed-bed reactor. In this study, the industrial reactor of DME synthesis with the accompanying feed preheater has been simulated and controlled in dynamic conditions. The proposed model, consisting of a set of algebraic and partial differential equations, is based on a heterogeneous one-dimensional unsteady state formulation. To verify the proposed model, the simulation results have been compared to available data from an industrial reactor at steady state conditions. A good agreement has been found between the simulation and plant data. A sensitivity analysis has been carried out to evaluate the influence of different possible disturbances on the process. Also, the controllability of the process has been investigated through dynamic simulation of the process under a conventional feedback PID controller. The responses of the system to disturbance and setpoint changes have shown that the control structure can maintain the process at the desired conditions with an appropriate dynamic behavior.     

Performance of Ba0.5Sr0.5Co0.8Fe0.2O3 + δ membrane after laser ablation for methane conversion

A cross-shaped pattern was formed on the surface of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ oxygen permeation membrane by laser ablation. A membrane reactor made from this membrane was operated for partial oxidation of methane to syngas in the presence of Ni/ZrO₂ catalyst. The CH₄ conversion and CO selectivity of the membrane reactor were 98.8% and 91.5%, respectively, and the oxygen permeation flux through the membrane was 11.0 ml/cm² min at 850 °C. The effects of space velocity (SV) on CH₄ conversion and CO selectivity in such reactor were discussed. The mechanism of POM in such membrane reactor may follow the combustion and reforming mechanism.     

Screening of bifunctional water-gas shift catalysts

A large number of different formulations have been recently proposed in the literature as new catalysts for the water-gas shift (WGS) reaction. These formulations typically consist of a metal deposited on a reducible support. As these catalysts have been synthesized and tested by different groups in different operating conditions, a true comparison of their activities is not really possible. The aim of this study is to screen these samples under identical conditions using a model reformate as reaction mixture. A commercial parallel reactor has been used for this task. Comparison of the data obtained for the Pt catalysts from the parallel reactor with those obtained from a single fixed bed reactor showed deviations of 20–30% in the kinetic parameters. Rh and Ru based catalysts produced significant amounts of methane. Pt/CeO₂/Al₂O₃ and Pt/TiO₂ were found to be the most active catalysts for the high temperature water-gas shift while gold and copper catalysts showed promising results for low temperature applications, but they require testing at lower CO partial pressures.     

Fuel processor based on syngas production via short contact time catalytic partial oxidation reactors

Short contact time catalytic partial oxidation (SCT-CPO) of natural gas is a promising technology for syngas production, representing an appealing alternative to existing processes. The high conversion and selectivity observed since the earlier works in this field can make this process attractive. Moreover, the SCT-CPO reactors can be autothermally operated and the possibility to use air as oxidant appears a feasible route to reduce syngas production costs: these two issues make possible the use of a SCT-CPO reactor as the reformer of a fuel processor for H₂ production for fuel cells.

The present work refers to an experimental study of syngas production from CH₄ and O₂ via a SCT-CPO reactor made of a fixed bed of Rh/-Al₂O₃ spheres. The main obtained results are: (i) an increase in GHSV produces an enhancement of transport rates and this in turn determines an improvement in CH₄ conversion, despite the reduction in residence time; (ii) the catalyst pellets get hotter than the gas phase thus favouring the H₂ and CO production; syngas formation is in fact both thermodynamically and kinetically promoted at high temperatures; (iii) a similar improvement of conversion was obtained with a reduction of the catalyst particle size, thanks once again to an increase in the heat transport and a higher geometrical surface area of the catalyst itself. By a slight increase of the O₂ fed to the reactor, H₂ and CO yields can be maximised and a complete CH₄ conversion achieved.     

Oxidative dimerisation of methane on a supported Pd-Ag catalyst using a diffusion controlled ceramic wall reactor

Supported palladium-silver oxides were used as catalysts for the partial oxidation of methane by molecular oxygen in a tubular reactor with ceramic wall separation. The ceramic wall controls the O₂ supply in the catalyst bed. The results indicate that the reactor configuration can play an important role in methane oxidation. C₂H₆, C₂H₄, CO₂ and H₂O were obtained at temperatures less than 300 °C. At this temperature any contribution from homogeneous gas phase reaction can be ruled out.     

Fuel-rich methane combustion: Role of the Pt dispersion and oxygen mobility in a fluorite-like complex oxide support

For catalysts comprised of Pt supported onto dispersed complex fluorite-like oxides (ceria doped by Pr, Gd, Sm, or CeO₂–ZrO₂ doped by La, Gd or Pr), the effects of the oxygen mobility in supports and Pt dispersion on the performance in methane selective oxidation into syngas at short contact times were elucidated using combination of kinetic and spectroscopic methods. While in general any simple universal relation between the oxygen mobility, Pt dispersion and the rate of methane transformation into syngas was not found, for some series, a good correlation was observed agreeing with the bifunctional scheme of the methane selective oxidation into syngas.     

甲烷常压非催化部分氧化制合成气研究

考察了常压下甲烷非催化部分氧化制合成气过程中,甲烷转化率、气体产物产率以及积炭率随反应器温度、进气配比、原料气流量变化的影响。通过考察反应器温度对制备合成气过程的影响结果,分析了甲烷非催化部分氧化制合成气过程的反应机理。