Instationäre Reaktionsführung zur Modellierung heterogen katalytischer Reaktionen

Transient processes in modelling heterogeneous catalytic kinetics. It is often difficult to obtain mathematical models to describe the dynamics of heterogeneous catalytic reactors. Kinetics determined under steady-state conditions are normally not valid for the transient behaviour of catalytic reactions. In order to find valid kinetic models, information is needed about the adsorption-desorption processes and the formation of intermediates on the surface. To get single step models, experiments have to be carried out under transient or oscillatory conditions. This is discussed for different examples taken from the literature.     

THREE-WAY CATALYTIC CONVERTER MODELING AND APPLICATIONS

The advent of stricter U.S. and European exhaust emissions regulations has increased the need for reliable 3-way catalytic converter models supporting the design of demanding exhaust systems for low-emitting vehicles. Although a number of tunable models have been presented in the literature, their efficient performance in actual 3-way applicaions requires further development and validation. The major difficulties posed in such modeling efforts arise from the complexities in the reaction schemes and the respective rate expressions for the multitude of currently used catalyst formulations. This paper addresses the details of tuning and real world application of a two-dimensional catalytic converter model, which accounts for the HC (hydrocarbons) and CO oxidation, as well as NO reduction reactions. The model features a number of innovations regarding the catalyst transient behaviour modelling and the reaction kinetics scheme. The advanced oxygen storage submodel presented is capable of accounting for the redox and temperature dependence of the oxygen availability under transient operation. The redox sensitivity of the reaction scheme allows to get clearer insight in the “lambda-window” behavior of 3-way catalysts. It is concluded, that mathematical modelling may successfully describe important aspects of real world three-way catalytic converter operation under dynamic conditions, and thus, is a valid tool in exhaust aftertreatment systems optimization.     

Kinetic experiments and modeling of a three-phase catalytic hydrogenation reaction in supercritical CO2

The three-phase catalytic hydrogenation of an unsaturated ketone using supercritical carbon dioxide as a solvent was studied in order to simulate the performance of a semi-industrial trickle-bed reactor. High pressure kinetic experiments were carried out in a modified internal recycle reactor of Berty type. An industrial Pd on alumina supported catalyst was used, in form of egg-shell pellets. Data were collected over the whole conversion range, allowing for a thorough inspection of the reaction rate composition dependencies. It is shown that supercritical CO₂ strongly increases the reaction rate. Experimental data sets were fit using both simple homogeneous power-law kinetics and complex heterogeneous Langmuir–Hinshelwood models: here, the estimation of liquid concentrations was performed through vapor–liquid equilibrium calculations based on an equation-of-state approach.     

Modelling of kinetics and mass transfer in the hydrogenation of xylose over Raney nickel catalyst

This paper discusses the modelling of xylose hydrogenation kinetics over Raney nickel in aqueous solutions, the determination of the hydrogen solubility in the reaction mixture as well as evaluation of mass transfer effects in the reaction system. The hydrogenation experiments were carried out batchwise in an automatic laboratory‐scale reactor. The reactor system operated at a pressure range of 40–70 bar and at temperatures between 80 and 140 °C. The catalyst‐to‐xylose ratio was approximately 5 wt‐% of the xylose weight normally. The reactor contents were analysed off‐line with a high performance liquid chromatograph. Hydrogen solubility in the reaction medium was determined with a gas‐chromatographic system. The solubility was found to remain fairly constant during the hydrogenation. Only a slight increase in the hydrogen solubility was detected as xylose was hydrogenated to xylitol. The overall hydrogen solubility in the reaction mixture was significantly lower than in pure water, as expected. The main hydrogenation product was xylitol, but small amounts of xylulose and arabinitol were detected as by‐products. A semi‐competitive kinetic model, based on hydrogen and xylose adsorption, was developed. The model accounts for the very different areas covered by a hydrogen atom and an organic species on the catalyst surface. The parameters of the kinetic model were determined with non‐linear regression analysis. It turned out that the kinetic model is able to describe the formation of both xylitol and the by‐products. The mass transfer effects in the batch hydrogenation were evaluated by using measured viscosities and estimated diffusion and mass transfer coefficients. A process simulator, utilizing the kinetic and mass transfer effects, was developed to predict the behaviour of industrial reactors. © 1999 Society of Chemical Industry     

Isobutene hydration over Amberlyst-15 in a slurry reactor

The synthesis of tertiary butyl alcohol (TBA) via isobutene (iB) hydration was studied over Amberlyst-15 sulfonic acid catalyst particles using pure water and aqueous TBA solutions in a bubbling slurry reactor. Preliminary studies to investigate mass transfer effects showed that pore diffusion was present for catalyst particles greater than 165 μm in diameter. Therefore, intrinsic kinetic measurements were made using 90.5 μm catalyst particles and a catalyst loading of 10 kg m⁻³. The kinetic measurements revealed that iB hydration is a pseudo-first-order reaction with an activation energy of 69 kJ mol⁻¹. Isobutene hydration experiments using TBA concentrations in water revealed a hindering effect of TBA, which indicates that separation of TBA formed by iB hydration in three-phase reactors using catalytic distillation is promising from a process design perspective.     

Modeling of three-phase continuously operating open-cell foam catalyst packings: Sugar hydrogenation to sugar alcohols

An advanced comprehensive and transient multiphase model for a trickle bed reactor with solid foam packings was developed. A new simulation model for isothermal three-phase (gas–liquid–solid) catalytic tubular reactor models was presented where axial, radial, and catalyst layer effects were included. The unique feature of this model is that the material balances include most of the individual terms (i.e., internal diffusion, gas–liquid, and liquid solid mass transfer, kinetics) for solid foam packing which is seldom done. Hydrogenation of arabinose and galactose mixture on a ruthenium catalyst supported by carbon-coated aluminum foams was applied as a fundamentally and industrially relevant case study. Parameter estimations allowed to obtain reliable and significant parameters. The effect of the kinetic parameters and the operation conditions on the arabinose and galactose conversions was studied in detail by sensitivity analysis. The model described is applicable for other three-phase continuous catalytic reactors with solid foam packings.     

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part II: Mechanistic aspects and kinetic modeling

In this work, transient and SSITKA experiments with isotopic ¹⁸O₂ were conducted to study the nature of oxygen species participating in the reaction of ethane oxidative dehydrogenation to ethylene and obtain insight in the mechanistic aspects of the ODH reaction over Ni-based catalysts. The study was performed on NiO, a typical total oxidation catalyst, and a bulk Ni–Nb–O mixed-oxide catalyst (Ni_0.85Nb_0.15) developed previously [E. Heracleous, A.A. Lemonidou, J. Catal., in press], a very efficient ethane ODH material (46% ethene yield at 400 °C). The results revealed that over both materials, the reaction proceeds via a Mars–van Krevelen-type mechanism, with participation of lattice oxygen anions. However, the ¹⁸O₂ exchange measurements showed a different distribution of isotopic oxygen species on the two materials. The prevalent formation of cross-labelled oxygen species on NiO indicates that dissociation of oxygen is the fast step of the exchange process, leading to large concentration of intermediate electrophilic oxygen species on the surface, active for the total oxidation of ethane. Larger amounts of doubly exchanged species were observed on the Ni–Nb–O catalyst, indicating that doping with Nb makes diffusion the fast step of the process and suppresses formation of the oxidizing species. Kinetic modeling of ethane ODH over the Ni_0.85Nb_0.15 catalyst by combined genetic algorithm and nonlinear regression techniques confirmed the above, since the superior model is based on a redox parallel-consecutive reaction network with the participation of two types of active sites: type I, responsible for the ethane ODH and ethene overoxidation reaction, and type II, active for the direct oxidation of ethane to CO₂. The kinetic model was able to successfully predict the catalytic performance of the Ni_0.85Nb_0.15 catalyst in considerably different experimental conditions than the kinetic experiments (high temperature and conversion levels).     

Dynamic modelling of catalytic liquid-phase reactions in fixed beds—Kinetics and catalyst deactivation in the recovery of anthraquinones

Dynamic modelling of catalytic fixed-bed reactors with liquid-phase feed is of crucial importance, since catalyst deactivation often plays a central role in reaction engineering. General dynamic modelling of liquid-phase fixed beds was considered, including complex reaction kinetics and catalyst deactivation. The modelling concept was applied on a catalytic liquid-phase conversion reaction. The model was tested with pilot-plant data and showed a good predictivibility. The model can be used to optimize the production life cycles of fixed beds with catalyst deactivation.     

松节油催化异构-分子间氢转移反应集总动力学

以H2SO4-Pd/C为复合催化剂、N2为保护气研究了松节油直接异构-分子间氢转移反应的集总动力学。在消除内、外扩散影响的条件下,在线跟踪反应产物并用气相色谱法测定反应体系组成随时间的变化关系,借鉴集总思想和方法,建立了单萜烯、对孟烯结构的单环单萜烯、异构单萜烯、对伞花烃和氢化单萜烷复杂反应体系的集总动力学模型;采用Levenberg-Marquart法,以Matlab编程和SPSS软件对实验数据进行回归估算了模型参数,得到该复杂反应体系的活化能分别为77.86,80.18,71.33 kJ/mol,所建动力学模型与实验数据吻合良好。     

“State defining” experiment in chemical kinetics—primary characterization of catalyst activity in a TAP experiment

This paper presents a new strategy, “state-by-state transient screening”, for kinetic characterization of states of a multicomponent catalyst as applied to TAP pulse-response experiments. The key idea is to perform an insignificant chemical perturbation of the catalytic system so that the known essential characteristics of the catalyst (e.g. oxidation degree) do not change during the experiment. Two types of catalytic substances can be distinguished: catalyst state substances, which determine the catalyst state, and catalyst dynamic substances, which are created by the perturbation. The general methodological and theoretical framework for multi-pulse TAP experiments is developed, and the general model for a one-pulse TAP experiment is solved. The primary kinetic characteristics, basic kinetic coefficients, are extracted from diffusion-reaction data and calculated as functions of experimentally measured exit-flow moments without assumptions regarding the detailed kinetic mechanism. The new strategy presented in this paper provides essential information, which can be a basis for developing a detailed reaction mechanism. The theoretical results are illustrated using furan oxidation over a VPO catalyst.     

Dynamic operation of a three-phase upflow reactor for the hydrogenation of phenylacetylene

The selective catalytic hydrogenation of phenylacetylene to styrene presents an attractive and interesting process both from scientific and industrial viewpoints. The process mechanism consists of two consecutive reaction steps in which a high selectivity towards the intermediate product is desired.

There have been many papers published so far, that report an increase in the selectivity of adsorption-desorption-based gas-phase processes performed on solid catalysts, due to a periodic (pulsing) operation of the reactor system. Pulsing the gas feed is believed to create more preferential adsorption conditions for the required reactant (transient conditions).

This paper presents the results of a theoretical and experimental study concerning the influence of forced periodic oscillations of the gas-phase component on the performance of a three-phase reactor. The theoretical results are based on a heterogeneous mathematical reactor model considering both the transient mass balance over the reactor length as well as accumulation inside the catalyst pellets.

Numerical dynamic reactor simulations with a simplified model neglecting accumulation of components within the catalyst pellet showed little improvement in conversion and selectivity when compared to results of the steady-state model. These results were supported by dynamic experiments in a bench scale reactor. It is shown that due to mass transfer limitations, the original square wave shape of the oscillation in the gas-phase concentration was almost completely faded into a smooth and rather flat oscillation at the catalyst surface. Simulations with a model in which accumulation of the relevant components inside the particles was considered showed no further improvement in performance at transient conditions, but demonstrated the impact of accumulation capacity of the pellets on the shape of oscillation present at the catalyst surface.

The study is a result of direct cooperation between Politecnico di Torino, where the modelling and calculations were performed, and DSM Research. Its aim was to offer an attractive alternative to the conventional approach.     

Dynamic modelling of a three-phase catalytic slurry reactor

Two dynamic models for a three-phase catalytic slurry reactor with appropriate solution procedures were developed in this work. The models consist of mass and heat balance equations for the catalyst particles, for the gas and liquid bulk phases as well as for the heat exchange through the jacket of the reactor. The models of the tubular reactor were applied to describe the dynamic behaviour of the reactor during the hydrogenation of o-cresol on Ni/SiO₂ catalyst. These models differ in solid phase modelling, which allows to evaluate the reactor dynamic behaviour prediction capacity. The models successfully reproduce the main characteristics of the reactor dynamic behaviour.     

Comparison of the dynamics of the high-temperature water-gas shift reaction on oxide catalysts

The dynamics of the high-temperature water-gas shift reaction on iron oxide and Co-Mo-oxide catalysts was studied with transient response experiments performed at 563–673 K at atmospheric pressure in a gradientless spinning basket reactor. At the reaction start-up the step response of CO₂ on the iron oxide catalyst was always faster than the response of H₂, whereas the response of H₂ on the Co-Mo catalyst was faster than the response of CO₂. Water pretreatment retarded the response of H₂ on the iron oxide catalyst, whereas a similar pretreatment accelerated the response of H₂ on the Co-Mo catalyst. Based on the results of the transient response experiments reaction mechanisms were proposed for the water-gas shift reaction on both catalysts. The rate determining steps on the iron oxide catalyst were assumed to be the CO₂ desorption and surface hydroxyl decomposition steps. The rate determining steps on the Co-Mo catalyst were assumed to be the surface reaction and CO₂ desorption steps. The transient responses were modelled with non-steady-state rate equations based on the mechanisms, and the kinetic constants were determined by regression analysis. The kinetic models were able to describe the transient behaviours of the oxide catalysts.     

Steam gasification of German hard coal using alkaline catalysts: Effects of carbon burn-off and ash content

Laboratory-scale experiments were performed on chars from German hard coals with potassium carbonate addition. The steam gasification rate at 4 MPa and 700 °C as a function of the amount of catalyst added is described for a low-and high-ash char. From the burn-off behaviour the reaction order relative to carbon was determined. For the low-ash char a uniform reaction order was found but the high-ash char indicated a complex interaction of catalytic gasification, catalyst deactivation, and the development of the reacting surface.     

Building and characterization of catalysts on single crystal surfaces

The components of complex catalyst systems include transition metals, oxides, alkali metal or halogen additives and strongly chemisorbed sulfur or carbonaceous species. Modern surface science techniques identified the chemical and structural roles of these ingredients when used in combination with catalytic reaction rate studies. Using metal single crystal substrates, catalyst components have been deposited from the vapor phase and the complex catalyst system could be built this way. The Pt-Re-S, Rh-TiO₂, and Mo-Co-S systems are reviewed and the requirements for the proper surface structure and composition to obtain high reaction rates and selectivities are discussed.     

Oxidation of Aqueous Sulphur Dioxide Catalysed by Poly-4-vinylpyridine–Cu(II) Complex. Part 2: Combined Homogeneous and Heterogeneous Phase Oxidation

Aqueous phase oxidation of sulphur dioxide at low concentrations catalysed by a PVP–Cu complex in the solid phase and dissolved Cu(II) in the liquid phase is studied in a rotating catalyst basket reactor (RCBR). The equilibrium adsorption of Cu(II) and S(VI) on PVP particles is found to be of the Langmuir-type. The diffusional effects of S(IV) species in PVP–Cu resin are found to be insignificant whereas that of product S(VI) are found to be significant. The intraparticle diffusivity of S(VI) is obtained from independent tracer experiments. In the oxidation reaction HSO₃⁻ is the reactive species. Both the S(IV) species in the solution, namely SO₂(aq) and HSO₃⁻, get adsorbed onto the active PVP–Cu sites of the catalyst, but only HSO₃⁻ undergoes oxidation. A kinetic mechanism is proposed based on this feature which shows that SO₂(aq) has a deactivating effect on the catalyst. A rate model is developed for the three-phase reaction system incorporating these factors along with the effect of concentration of H₂SO₄ on the solubility of SO₂ in the dilute aqueous solutions of Cu(II). Transient oxidation experiments are conducted at different conditions of concentration of SO₂ and O₂ in the gas phase and catalyst concentration, and the rate parameters are estimated from the data. The observed and calculated profiles are in very good agreement. This confirms the deactivating effect of non-reactive SO₂(aq) on the heterogeneous catalysis. © 1997 SCI.     

Kinetic and catalytic aspects in melt transesterification of dimethyl terephthalate with ethylene glycol

The kinetics of melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of zinc acetate as catalyst has been studied in semibatch conditions. We observed that this reaction occurs with the formation of many oligomers characterized from the terminal groups of the chains that can be hydroxyl–hydroxyl, methyl–hydroxyl or methyl–methyl. Experimental runs have been performed at different temperatures, initial reagents ratios, and catalyst concentrations, following the amount of methanol released during the time as well as the concentration of any kind of oligomer. All the oligomers have been identified and determined by HPLC analysis. A classic Kinetic model based on a complex reaction scheme containing four or five reaction sequences has been developed. The scheme with four sequences foresees 24 oligomeric species involved in 58 different reactions, while the scheme with five sequences has 48 oligomeric species involved in 228 reactions. Despite the large number of oligomers and occurring reactions, only two kinetic parameters and two equilibrium constants are necessary to simulate the kinetic behaviour of all the oligomers. A kinetic constant is related to the reaction of a methyl group with a hydroxyl of ethylene glycol, while the other corresponds to the reaction of a methyl group with a hydroxyl in a chain. Both kinetic constants show an activation energy of about 15 kcal/mol. We observed a nonlinear correlation between activity and catalyst concentration and interpreted this fact by assuming two different catalytic activity levels for a dissociated and an undissociated zinc ionic couple, respectively. © 1994 John Wiley & Sons, Inc.     

Some kinetic aspects of unsteady-state partial oxidation reactions. Dynamic processes on metal oxide surfaces

It is shown that steady-state kinetic data do not allow the discrimination between the redox and associated mechanisms of the partial oxidation reactions. A discrimination between these mechanisms was performed using transient experiments. The obtained rate expressions are in agreement with experimental kinetic data for catalytic partial oxidation of o-xylene.

An influence of the conjugate oxidation of a catalyst surface on dynamics and kinetics of the heterogeneous catalytic oxidative reactions is considered. Computing simulation of methane oxidative coupling of methane reaction at lowered temperature and elevated pressure has been performed. It showed that the reaction order with respect to oxygen exceeding unity is consistent with the chain branching mechanism of the reaction in the presence of TiSi₂ and TiB₂ and showed the important role of the branching chain cycles in the low-temperature OCM reaction at elevated pressure.     

Investigation of travelling waves on catalytic wires

Equations governing the transient behaviour of a catalytic reaction occurring on a catalyst wire are formulated. A simple transformation of variables if proposed which makes it possible to develop in analytical form an expression for the velocity of a travelling wave. This expression allows also to determine the direction of the wave propagation. A detailed discussion of the underlying assumptions is performed. It is shown that the existence of a travelling wave is associated with the occurrence of multiple steady states. To verify the theoretical results the catalytic oxidation of ammonium on a Pt wire was investigated. Two types of travelling waves were experimentally observed (i) travelling wave which results in a transition of kinetic regime into a diffusion regime (ii) travelling wave which results in a transition of diffusion regime into a kinetic regime. For the conditions of zero velocity of wave propagation a great number of steady states were experimentally observed. It is shown that the experimental observations can be explained by the theory suggested.