A computational framework for the simulation of a gas solid catalytic reactor based on a multiregion approach (original) (raw)
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Chemical Engineering Research and Design, 2007
A method is described by which a CFD code known as FLUENT, may be adapted to model the reactions that take place inside catalyst structures. To test the CFD code, simulations of gas flow in a circular tube are performed and compared with analytical solutions. Then the coupled processes of diffusion and chemical reaction, combined with heat and mass transfer effects are modelled in a catalyst pellet. To illustrate the technique, the catalytic combustion of propane is simulated in spherical and cylindrical shaped pellets, at gas temperatures from 500 to 700 K and at atmospheric pressure. To help validate the approach adopted, the results of the CFD simulations are compared with solutions obtained from a one-dimensional model using MATLAB. It is shown that the CFD simulations provide comparable results with MATLAB, and that the CFD code can provide valuable additional information about temperature and concentration gradients in and around the catalyst pellet-this is not available in a simple one-dimensional approach. It is discussed how the technique could be extended to model reactions in a packed bed which would be a valuable design tool.
Towards a particle based approach for multiscale modeling of heterogeneous catalytic reactors
Chemical Engineering Science, 2018
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Kinetic Modelling at the Basis of Process Simulation for Heterogeneous Catalytic Process Design
2017
Process simulation represents an important tool for plant design and optimisation, either applied to well established or to newly developed processes. Suitable thermodynamic packages should be selected in order to properly describe the behaviour of reactors and unit operations and to precisely define phase equilibria. Moreover, a detailed and representative kinetic scheme should be available to predict correctly the dependence of the process on its main variables. This review points out some models and methods for kinetic analysis specifically applied to the simulation of catalytic processes, as a basis for process design and optimisation. Attention is paid also to microkinetic modelling and to the methods based on first principles, to elucidate mechanisms and calculate thermodynamic and kinetic parameters. Different case histories support the discussion. At first, we have selected two basic examples from the industrial chemistry practice, e.g. ammonia and methanol synthesis, which ...
Bulletin of Chemical Reaction Engineering & Catalysis, 2018
In this work, dynamic simulation at particle scale is carried out to predict the kinetics of solid catalyzed esterification reaction between acetic acid and methanol to produce methyl acetate and water. The reaction kinetic data utilized for modeling and validation is with solid catalyst as Indion 180. It was observed that the reaction rate and kinetics inside the pores of the catalyst is of higher magnitude as compared to bulk liquid. Each solid catalyst particle is surrounded by reactant solution of equal volume. A dynamic simulation is carried out using COMSOL Multiphysics which has solver for diffusion-reaction equation for both in liquid phase and inside porous catalyst particle. The intrinsic reaction rate constants for bulk liquid phase and inside the particle are obtained by solving the full diffusion-reaction equation and optimization method. Three different models (model 1,2,3) were proposed for evaluating the rate constants from the experimental kinetic data. The three mo...
Multiscale Modelling in Computational Heterogeneous Catalysis
The goal of multiscale modelling of heterogeneous catalytic reactors is the prediction of all steps, starting from the reaction mechanism at the active centre, the rates of reaction, adsorption and diffusion processes inside the porous system of the catalyst support, based on first principles, quantum chemistry, force field simulations and macroscopic differential equations. The progress in these fields of research will be presented, including linking models between the various levels of description. Alkylation of benzene will be used as an example to demonstrate the various approaches from the active centre to the reactor.
Chemical Engineering Journal, 2006
This study is devoted to a chemical reaction, carried out in a gaseous phase under isothermal conditions and without internal convective flow. Traditionally, in the mass-balance equations, the transport of reactants and products in a porous catalyst is introduced by the diffusion phenomenon in gaseous phase. The reaction rate then is written as a function of the reactant's and product's concentrations in gaseous phase [G.F. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design, Wiley, New York, 1979]. However, in stationary conditions and with the hypothesis of the thermodynamic equilibrium between the gaseous and the adsorbed phase, the existence of a concentration gradient in the gaseous phase brings about a concentration gradient in the adsorbed phase. The diffusion process occurring in the adsorbed phase is usually neglected in the traditional modeling. The present work aims to investigate the influence of the considered diffusion processes on the evaluation of the performance of the catalyzed chemical reaction. The analysis is based on the study of a simple chemical reaction and consecutive reactions. The study shows that the flux of consumed reactant, evaluated by a traditional way could be different from the flux obtained with the diffusion in the adsorbed phase. Also, in these conditions the kinetic parameters obtained by the traditional modeling of the experimental results do not represent then the real kinetic parameters.
Catalysts
Process simulation represents an important tool for plant design and optimization, either applied to well established or to newly developed processes. Suitable thermodynamic packages should be selected in order to properly describe the behavior of reactors and unit operations and to precisely define phase equilibria. Moreover, a detailed and representative kinetic scheme should be available to predict correctly the dependence of the process on its main variables. This review points out some models and methods for kinetic analysis specifically applied to the simulation of catalytic processes, as a basis for process design and optimization. Attention is paid also to microkinetic modelling and to the methods based on first principles, to elucidate mechanisms and independently calculate thermodynamic and kinetic parameters. Different case studies support the discussion. At first, we have selected two basic examples from the industrial chemistry practice, e.g., ammonia and methanol synthesis, which may be described through a relatively simple reaction pathway and the relative available kinetic scheme. Then, a more complex reaction network is deeply discussed to define the conversion of bioethanol into syngas/hydrogen or into building blocks, such as ethylene. In this case, lumped kinetic schemes completely fail the description of process behavior. Thus, in this case, more detailed-e.g., microkinetic-schemes should be available to implement into the simulator. However, the correct definition of all the kinetic data when complex microkinetic mechanisms are used, often leads to unreliable, highly correlated parameters. In such cases, greater effort to independently estimate some relevant kinetic/thermodynamic data through Density Functional Theory (DFT)/ab initio methods may be helpful to improve process description.
Summary The application of a newly developed computational tool, DETCHEMMONOLITH, for the transient two- and three- dimensional simulations of catalytic combustion monoliths is presented. The simulation is based on the coupling of a transient 2D / 3D heat balance of the solid monolith structure with steady-state calculations of the reactive flow in a representative number of channels. The two-dimensional single-channel model uses a boundary-layer approximation including detailed models for heterogeneous and homogeneous reactions as well as transport phe- nomena. As an example, the computational tool is applied to study the catalytic partial oxidation of methane. In the present work, we focus on those spatially structured monolithic catalysts, where the time scales of varia- tions in the gas phase are much smaller than those of the thermal changes in the monolithic structure. Then, the flow through the single monolith channels, which are assumed to have an cylindrical shape, is model...
1991
It is shown that the linear operator, representing the heterogeneous model of a packed-bed reactor in which convective mixing occurs in the fluid phase, and diffusive transport occurs in every catalyst particle, has, among other eigenvalues found by Ramkrishna and Arce (1989). also as eigenvalues those of the particle operator if allowance is made for multiple particles in a given fluid environment. The physica consequence of this is that the reactor can display spatial patterns when single particles possess multiple steady states leading to very diverse variations in reactor performance. Other spatial patterns are also envisaged in which the catalyst phase concentration profile may possess a fractal character. A discussion is also included of how heterogeneous reactor models may be analyzed for the appearance of such patterns.
Mathematical analysis of TAP models for porous catalysts
Chemical Engineering Journal, 2005
The mathematical models for porous catalysts involving interparticle and intraparticle Knudsen diffusion with and without a first order irreversible reaction in a TAP reactor during a single pulse experiment were analyzed. If the ratio of the interparticle to the intraparticle transport characteristic times (γ) is sufficiently large, the intraparticle concentration distribution follows an intraparticle pseudo-steady state (IPSS) condition. For a threeequal-zone reactor, the IPSS assumption is valid when γ ≥ 12.5, corresponding to a macro-porous domain. For γ < 12.5, the validity of the IPSS assumption depends on the magnitude of the effectiveness factor. The expressions for the valid domain are proposed. The validity of the IPSS assumption for a thin-zone reactor is also discussed. Moment analysis shows that analytical expressions for the gas conversion are the same for the cases with and without application of the IPSS assumption. The conversion expressions for different shapes of catalyst pellets and different reactor configurations are reported.