以类水滑石为前体的Ni催化剂及其催化甲烷水蒸气重整制氢:催化活性和动力学(英文)

Ni/Mg–Al catalysts derived from hydrotalcite-type precursors were prepared by a co-precipitation technique and applied to steam reforming of methane. By comparison with Ni/γ-Al2O3 and Ni/α-Al2O3 catalysts prepared by in-cipient wetness impregnation, the Ni/Mg–Al catalyst presented much higher activity as a result of higher specific surface area and better Ni dispersion. The Ni/Mg–Al catalyst with a Ni/Mg/Al molar ratio of 0.5:2.5:1 exhibited the highest activity for steam methane reforming and was selected for kinetic investigation. With external and inter-nal diffusion limitations eliminated, kinetic experiments were carried out at atmospheric pressure and over a temperature range of 823–973 K. The results demonstrated that the overal conversion of CH4 and the conversion of CH4 to CO2 were strongly influenced by reaction temperature, residence time of reactants as wel as molar ratio of steam to methane. A classical Langmuir–Hinshelwood kinetic model proposed by Xu and Froment (1989) fitted the experimental data with excellent agreement. The estimated adsorption parameters were consistent thermodynamical y.     

Steam reforming of methane over Ni catalysts prepared from hydrotalcite-type precursors:Catalytic activity and reaction kinetics

Ni/Mg–Al catalysts derived from hydrotalcite-type precursors were prepared by a co-precipitation technique and applied to steam reforming of methane. By comparison with Ni/γ-Al2O3 and Ni/α-Al2O3 cataly...     

Computational analysis of the reacting flow in a microstructured reformer using a multiscale approach

A multiscale methodology is presented to analyze the transport and reaction processes in the catalyst coating of a microstructured reformer and to elucidate the effect of catalyst morphology on transport limitations and the reformer performance. This analysis includes three‐dimensional simulations of methane steam reforming at both reactor level (macroscale) and catalyst microstructure level (microscale). Hypothetical catalyst microstructures are generated using an in‐house particle packing code. Based on the generated structures, the effective transport properties of the porous catalyst and the average reaction rates in the microstructure are determined to be applied in the pseudohomogeneous model used in the macroscale simulation. Parametric study is done to demonstrate the significant effect of the catalyst intraparticle and interparticle porosity as well as the particle size on the reaction effectiveness factor and methane conversion. This study shows that an optimal catalyst coating has a decreasing porosity along the reformer length based on the difference in the degree of diffusion limitation. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2263–2274, 2014     

长链烯烃与苯烷基化外扩散影响的数学模拟

在一定粒度的固体酸催化剂上进行不同反应原料质量流速的烷基化反应实验,通过模型参数估值及模型预测分析,建立评价外扩散阻力影响程度的外扩散有效因子数学模型。在此基础上考察外扩散阻力对烷基化反应的影响规律,确定消除外扩散阻力影响的实验条件。模型预测结果表明,计算值与实验值吻合较好。在一定的反应条件下,随着流体质量流速的增大,外扩散有效因子先快速增大然后缓慢趋于1;随着烷基化反应时间的延长,外扩散有效因子持续增大;随着烷基化反应速率的提高,消除外扩散阻力影响的流体质量流速相应有所增大。当流体质量流速达到4.0g.cm-2.h-1时可消除外扩散阻力影响。     

Novel multi-scale diffusion model for catalytic methane combustion

A multi-scale model of methane catalytic combustion was built by a series of balance equations and diffusion equations, and these equations were solved through the computational fluid dynamics (CFD) software. The difference between this work and previous model is the diffusion process in catalyst coating was considered. By analyzing the methane conversion, temperature distribution and mass fraction contours of every component, the performance of multi-scale model was compared with that of the pure CFD model without diffusion. The effects of diffusion, methane concentration, flow rate on the methane conversion and temperature distribution of monolithic reactor were also evaluated and discussed by the multi-scale model. The multi-scale model showed better accuracy than the pure CFD model without diffusion process. Different methane concentrations and gas flow rates had enormous effects on the methane conversion and temperature. Therefore, it was beneficial to the reaction process to adjust the methane concentration and gas flow rate appropriately.     

Performance of silicon‐carbide foams as supports for Pd‐based methane combustion catalysts

BACKGROUND: Cellular foam materials are a new type of catalyst support that provides improved mass and heat transport characteristics and similar pressure drops to other well‐established structured supports such as monoliths. RESULTS: A Pd‐based catalyst has been prepared using a moderate surface area (25 m² g^?1) β‐SiC foam support without further washcoating. The stability of this catalyst has been tested for methane combustion at lean conditions, showing a small decrease of activity during the first 10 h followed by stable performance. Characterization of fresh and aged catalyst shows no significant changes. The influence of the most important reaction conditions, such as reactor loading (0.25–1 g), temperature (300–550 °C) and inlet methane concentration (833 and 1724 ppm), was studied in a fixed‐bed reactor. The results were fitted to three kinetic models: Mars‐van Krevelen; Langmuir‐Hinselwood; power‐law kinetics. CONCLUSIONS: The Pd/β‐SiC foam catalyst, prepared without the previous addition of a washcoating has been demonstrated to be stable for the combustion of methane‐air lean mixtures. A Mars‐van Krevelen kinetic model provides the best fit to the results obtained. Copyright © 2011 Society of Chemical Industry     

The influence of various parameters on the oxidative coupling of methane reaction over lithium doped lanthanum oxide

The influence of reaction temperature, space velocity and methane and oxygen partial pressures were studied for the oxidative coupling of methane reaction (OCM) over a lithium doped lanthanum oxide catalyst. The kinetic data obtained with this catalyst, at temperatures between 650 and 750°C, indicate that oxygen is adsorbed non-dissociatively and non-competitively. Product selectivities extrapolated to zero percent methane conversion are similar to those obtained with Li/MgO, suggesting that both rare earth and alkaline earth based catalysts involve similar mechanisms. Carbon monoxide and ethylene were found to be secondary products exclusively.     

High yield synthesis of propanal from methane and air

High yield synthesis of propanal from methane and air can be obtained in a single pass at atmospheric pressure. Three catalytic processes are combined to give 13% yield of propanal based on total methane input. Ethene is made from the oxidative coupling reaction and carbon monoxide and hydrogen is generated from partial oxygenation of methane. These gases are combined and passed to a hydroformylation catalyst to give liquid propanal. The unreacted methane is inert in the hydroformylation stage, while oxygen deactivates the catalyst readily. The results imply that propanal can be obtained, in good yield, from methane and air provided that total oxygen conversion is achieved. The yield of propanal from the three combined processes can be substantially higher than that of ethene from the oxidative coupling reaction. Thus, higher yields of a condensible and oxygenated product are obtained.     

Catalytic Conversion of Bio-ethanol to Ethylene over La-Modified HZSM-5 Catalysts in a Bioreactor

Ethylene production from petroleum or natural gas is an energy intensive process. Bio-ethanol catalytic dehydration to ethylene is an attractive alternative for oil based ethylene. Catalytic dehydration conversion of bio-ethanol to ethylene using HZSM-5 modified by 3 wt% rare earth metal (lanthanum) was carried out in a laboratory bioreactor. The physicochemical properties of the catalyst were characterized. The stability test showed that ethanol conversion and selectivity over this catalyst could be maintained above 98% for more than 950 h. The regenerated catalyst also displayed high reactivity and stability of up to 830 h can be obtained. The effects of temperature, liquid hourly space velocity, particle size of catalyst, and bio-ethanol partial pressure on products formation rate were investigated. The external and internal diffusion resistances were eliminated and the kinetic control range was identified. An apparent kinetics model was used to describe the dehydration reaction of ethanol over 3 wt% La-HZSM-5 catalyst, and the kinetic parameters were determined.     

Curing kinetics of epoxy‐urethane copolymers

The cure of the epoxy resin diglycidyl ether of bisphenol A (Araldyt GY9527) with a mixture of cycloaliphatic amines (Distraltec) was studied, and the focus was on the effect of the copolymerization with a commercial polyurethane (PU) elastomer (Desmocap 12). A simplified phenomenological model was proposed to represent the copolymerization reaction. It considered the effect of the temperature and the concentration of the elastomer on the reaction rate, and it was simple enough to be included in models of processing conditions. A nonlinear regression analysis of the experimental conversion data obtained from differential scanning calorimetry was utilized to find the best fitting parameters to Kamal's equation for the chemically controlled part of the reaction (short times) under isothermal and constant heating‐rate conditions. The Rabinowitch approach together with the Addam–Gibbs theory was utilized to introduce the effect of diffusion control at the end of the reaction on the overall constant for the reaction rate. The Di Benedetto equation was used to predict the conversion at which vitrification takes place for each run. Experimental results for conversions higher than this critical conversion were utilized to obtain information about the diffusion kinetic constant using a nonlinear regression analysis as previously. The overall model obtained was used to calculate a calorimetric conversion and reaction rate as functions of time, which was in excellent agreement with the experimental results. The addition of PU elastomers affected the values of the activation energies of the chemically and diffusion controlled parts of the reaction, as well as the final conversion reached by the epoxy–amine system. The proposed model allowed prediction of all the observed features using parameters that were independent of the temperature of the curing reaction. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1771–1779, 2001     

Kinetic study of methane reforming with carbon dioxide over NiCeMgAl bimodal pore catalyst

The kinetic behavior of NiCeMgAl bimodal pore catalyst for methane reforming with CO₂ was investigated after the elimination of external and internal diffusion effects in a fixed‐bed reactor as a function of temperature and partial pressures of reactants and products. Increase in CO₂ partial pressure favors the consumptions of CH₄ and CO₂ but inhibits the formation of H₂ due to the existence of reverse water gas shift (RWGS) reaction. The reforming rate increased first and then reached a horizontal stage with the rise of CH₄ partial pressure. A Langmuir–Hinshelwood model was developed assuming that the carbon deposition is ignorable but the RWGS reaction is non‐ignorable and the removal of adsorbed carbon intermediate is the rate‐determining step. A nonlinear least‐square method was applied to solve the kinetic parameters. The derived kinetic expression fits the experimental data very well with a R² above 0.97, and also predicts the products flow rate satisfactorily. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2019–2029, 2017     

Light-off curve of catalytic reaction and kinetics

The light-off curve is the conversion-temperature plot of a catalytic reaction; it is usually used in catalyst development. The possibility to deduce kinetic information such as the apparent reaction order or mass transfer limitation from the shape of the curve is examined here. The light-off plots obtained by numerical simulation with several different rate equations are compared between themselves and with experimental curves. This is illustrated with the oxidation of carbon monoxide and methane on noble metal catalysts with different degree of mass transfer resistance. The comparison is complemented with the Arrhenius plots, i.e. the logarithm of the apparent rate constant versus the reverse of temperature.From the shape of the curves, the three following situations can be discriminated: first-order kinetics, negative first-order kinetics in the absence or in the presence of external mass transfer limitation. The range of diffusional limitation is well defined for negative-order reaction. However, the mass transfer resistance is difficult to evidence for first-order reaction. It is observed that the Langmuir-Hinshelwood reaction kinetic model is not convenient to represent the effect of strong external transport limitation on a reaction in the regime of reactant inhibition (negative apparent reaction order).     

Desulfurization of coal by air + steam at 400°C in a fixed bed

A high-volatile coal was treated by mixtures of air and steam at 400°C. The coal particle size, air/steam ratio and treatment time were varied. The degree of conversion of total sulfur depended on the particle size (but not proportionally) and the air/steam ratio (proportionally). The kinetics of the heterogeneous desulfurization by oxidation of FeS/FeS₂ was investigated. The kinetic data were well described by the model of an unreacted shrinking core in a coal particle of unchanging size. The coal particle size affected the external and internal diffusion of O₂ molecules only; the air/steam ratio affected the internal diffusion of O₂ molecules only. On average, the process was governed 14% by external diffusion, 76% by internal diffusion and 10% by chemical reaction. It was possible to elucidate the desulfurization mechanism merely from a knowledge of the experimental relation between total sulfur conversion degree and treatment time.     

Kinetic study of the effect of catalysts on the curing of biphenyl epoxy resin

The investigation of cure kinetics of biphenyl epoxy (4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl)dicyclopentadiene type phenolic resin system with different kinds of catalysts was performed by a differential scanning calorimeter using an isothermal approach. All kinetic parameters of the curing reaction including the reaction order, activation energy, and rate constant were calculated and reported. The results indicate that the curing reaction of the formulations using triphenylphosphine (TPP), 1-benzyl-2-methylimidazole (1B2MI), and tris(4-methoxyphenyl)phosphine (TPAP) as a catalyst proceeds through an nth-order kinetic mechanism, whereas thatof the formulations using diazabicycloundecene (DBU) and tetraphenyl phosphonium tetraphenyl borate (TPP–TPB) proceeds by an autocatalytic kinetic mechanism. To describe the cure reaction in the latter stage, we have used semiempirical relationship proposed by Chern and Poehlein. By combining an nth-order kinetic model or an auto-catalytic model with a diffusion factor, it is possible to predict the cure kinetics of each catalytic system over the whole range of conversion. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1125–1137, 1998     

Catalytic upgrading of tarry fuel gases: A kinetic study with model components

As a contribution to the development of a process for catalytic upgrading of tarry fuel gases, e.g. coke-oven gas, the conversion of naphthalene, benzene and methane on a nickel catalyst in the presence of H₂ and H₂O was studied. The experiments were performed in a tubular flow reactor (total pressure: 1.6 bar; residence time with respect to the empty reactor: 0.3 s; temperature: 400-950°C; and particle diameter of catalyst: 1.5 mm). The kinetic data were obtained by systematic variation of the reaction conditions.

At temperatures of more than 800°C, each hydrocarbon is cracked and converted with H₂O to CO and H₂. Soot formation does not occur at any temperature. In case of simultaneous conversion of all three hydrocarbons, competitive reactions have to be considered.

The rate of chemical reaction on the catalyst is substantially decreased in the presence of H₂S. Nevertheless, in a reactor of industrial scale, H₂S has only slight influence. The catalyst would be applied with a particle diameter of 19 mm (experiments: 1.5 mm), and the overall reaction rate of hydrocarbon conversion is significantly affected by gas film diffusion.     

The combustion of methane on platinum—alumina fibre catalysts—II design and testing of a convective-diffusive type catalytic combustor.

A convective—diffusive catalytic combustor has been used to study the total oxidation of methane over a platinum on alumina fibre catalyst. Temperature profiles across and through the catalyst pad have been measured as a function of methane feed rate, as has the radiant energy and the gas composition near the pad. Carbon dioxide and water were the only detectable products of reaction.The results have been compared with the predictions of a one dimensional—one phase model. Good agreement was obtained with experimental results, except near to the edge of the combustor: this is probably due to small inaccuracies in the thermal conductivity values used.The model has been used to predict the behaviour of a combustor as a function of variables in the system. The combustor performance is found to depend upon a balance between different effects. Heat generation depends on the rate of chemical reaction, and an increased feed rate of methane may (a) increase the rate by increasing the concentration of methane (b) decrease the rate by limiting the diffusion of air from the other side of the pad or (c) result in increased methane slippage by reducing the contact time in the pad. Varying packing density has a similar effect, with the added problem that radiation (heat recovery) is also affected by the density of fibres. It is shown that only ca. 30% of the fibre pad radiates significantly. As a result, although the thickness of the bed increases the internal bed temperature, it has little effect on heat recovery.     

Kinetics of selective hydrogenation of pyrolysis gasoline over an egg-shell catalyst

The kinetics of liquid-phase selective hydrogenation of pyrolysis gasoline over a commercial egg-shell catalyst was investigated in a stirred semi-batch reactor in the absence of external transport limitations. Experimental observations showed that competitive hydrogenation between monoolefins and diolefins was noticeable, and that the reaction rates of diolefins were much faster than those of monoolefins. A Langmuir–Hinshelwood-type reaction mechanism was proposed for the reaction system, and then a rigorous diffusion-reaction mathematical model was developed. For the egg-shell catalyst particle, the diffusion and reaction of species were described separately in two distinct regions, i.e., the outer active region, where both diffusion and reaction occurred, and the inner inertia region, where only diffusion happened. The kinetic and adsorption parameters were estimated from the diffusion-reaction model with the experimental data. The model was able to describe the experimental observations very well. The simulation results showed that significant concentration gradients existed inside the egg-shell catalyst, which revealed that the influence of the internal diffusion resistances on the reaction kinetics was considerable.     

Auto-cyclic reactor: Design and evaluation for the removal of unburned methane from emissions of natural gas engines

A non-adiabatic fixed bed auto-cyclic reactor (ACR) consisting of two counter-current concentric compartments was designed and built for removing low concentrations of methane from exhaust gases from natural gas engines. The length was based on simulations by a simple heterogeneous one dimensional model using literature parameters and kinetic data, while the diameter was selected to assure a linear fluid velocity between 0.5 and 2 m/s. Its innovative design consists of a judicious combination of 14 longitudinal fins welded to the outlet part of inner reactor compartment to maximize the heat transfer to the inlet section and highly active pellet type catalyst filling the space between fins to lower the ignition temperature.The experimental ACR pilot unit was loaded by a combination of highly active laboratory prepared catalysts: palladium/alumina pellets and palladium/alumina coated cordierite monoliths. The efficiency of methane removal from air and from synthetic exhaust gas containing 7 vol% CO₂ and 14 vol% H₂O was evaluated under a wide range of operating conditions: temperature from 290 to 500 °C, methane concentration between 500 and 3800 ppm. The reactor performance was monitored in terms of axial temperature profiles and methane conversion both in transient and steady state conditions.Reproducible performance of the ACR was observed even after 1200 h of cumulative operation and complete methane removal was obtained at relatively low temperatures.To simulate the obtained experimental data, a heterogeneous one-dimensional model was developed to suit the final reactor configuration using actual laboratory determined kinetic data. The model described adequately the experimental temperature profiles and methane conversion when heat transfer between the reactor compartments and heat loss were taken into account.     

Kinetics of polyvinyl butyral (PVB) synthesis reaction catalyzed by deep eutectic solvent

In this work, the kinetics of the polyvinyl butyral (PVB) synthesis reaction catalyzed by the deep eutectic solvents (DESs) at the low-temperature stage was studied to control the particle size of PVB resin and optimize its synthesis process. The effects of stirring speed, the concentration of polyvinyl alcohol (PVA), catalyst dosage, and temperature on the synthesis reaction of PVB were investigated. The data obtained from kinetic experiments at 283.15–298.15 K were fitted with the shrinking core models, and the results showed that the model controlled by internal diffusion fitted well with the experimental data. The internal diffusion coefficient, D_A, of n-butanal in the product layer was further calculated by the obtained model parameters, and it was applied to predict the synthesis reaction of PVB at different concentrations of n-butanal. The results indicated that the shrinking core model controlled by internal diffusion is suitable to describe the kinetics of the PVB synthesis reaction. DES played a dual role in catalysis and dispersion in the synthesis of PVB, and it was a green catalyst with good potential for PVB industrial applications.     

Effect of transport limitations on C2+ selectivity in the oxidative methane coupling reaction using a NaOH/CaO catalyst

The dependence of C₂₊ selectivity on oxygen conversion (20 to 95%) and on particle size (0.2 mm, 1.2 mm, 4×4 mm) of the catalyst, i.e. under conditions of mass transport limitation during the oxidative coupling of methane on a NaOH/CaO catalyst was experimentally studied at three temperatures (953, 983, 1013 K) and at a partial pressure of oxygen of 0.075 bar and of methane of 0.7 bar. A kinetic reaction scheme was derived for which the overall kinetic parameters were reported. Reaction engineering calculations with respect to the particle size effect, taking into account transport phenomena, are presented and compared with experimental evidence.