Computational Fluid Dynamics of Catalytic Reactors

Today, the challenge in chemical and material synthesis is not only the development of new catalysts and supports to synthesize a desired product, but also the understanding of the interaction of the catalyst with the surrounding flow field. Computational Fluid Dynamics or CFD is the analysis of fluid flow, heat and mass transfer and chemical reactions by means of computer-based numerical simulations. CFD has matured into a powerful tool with a wide range of applications in industry and academia. From a reaction engineering perspective, main advantages are reduction of time and costs for reactor design and optimization, and the ability to study systems where experiments can hardly be performed, e.g., hazardous conditions or beyond normal operation limits. However, the simulation results will always remain a reflection of the uncertainty in the underlying models and physicochemical parameters so that in general a careful experimental validation is required. This chapter introduces the application of CFD simulations in heterogeneous catalysis. Catalytic reactors can be classified by the geometrical design of the catalyst material (e.g. monoliths, particles, pellets, washcoats). Approaches for modeling and numerical simulation of the various catalyst types are presented. Focus is put on the principal concepts for coupling the physical and chemical processes on different levels of details, and on illustrative applications. Models for surface reaction kinetics and turbulence are described and an overview on available numerical methods and computational tools is provided

Computational design, sensitivity analysis and optimization of fuel reforming catalytic reactor

In this research, the catalytic combustion of methane is numerically investigated using an unstructured, implicit, fully coupled finite volume approach. The nonlinear system of equations is solved by Newton’s method. The catalytic partial oxidation of methane over a rhodium catalyst in one channel of a coated honeycomb reactor is studied three-dimensionally, and eight gas-phase species (CH4, CO2, H2O, N2, O2, CO, OH and H2) are considered for the simulation. Surface chemistry is modeled by detailed reaction mechanisms including 38 heterogeneous reactions with 20 surface-adsorbed species for the Rh catalyst and 24 heterogeneous reactions with 11 surface-adsorbed species for Pt catalyst. The numerical results are compared with experimental data and good agreement is observed. Effects of the design variables, which include the inlet velocity, methane/oxygen ratio, catalytic wall temperature, and catalyst loading on the cost functions representing methane conversion and hydrogen production are numerically investigated. The sensitivity analysis for the reactor is performed using three different approaches: finite difference, direct differentiation and an adjoint method. Two gradient-based design optimization algorithms are utilized to improve the reactor performance. For additional test cases, the performance of two full scale honeycomb-structured reactors with 49 and 261 channels are investigated. The sensitivity analysis of the full reactor is performed using an adjoint method with four design variables consisting of the inlet velocity, inflow methane concentration, inlet oxygen density and thermal conductivity of the monolith

Modélisation des transferts de masse et de chaleur au voisinage de parois réactives : applications à l’oxydation de composés carbonés pour le post-traitement

The environmental emergency has led automotive industry to deal with growing constraints as drastic regulations of pollutant emissions are emerging. In order to reduce emissions resulting from the combustion process, one of the solution adopted is to post process pollutants by the means of catalytic after-treatment systems such as three-way converters (TWC) for gasoline applications oroxidation catalysts (DOC) for Diesel applications. These devices present a honeycomb shape which consists in a grid of millimeter-scale narrow channels called monoliths whose interior wall are coated with precious metals presenting catalytic properties.Pollutants are converted through the chemical interaction involving gas-phase molecules and active precious metal sites. Given the laminar flow encountered within these monoliths, weak mixing and molecular diffusion could occur near the catalytic walls. Pollutant conversion rates may therefore prove insufficient for certain operating conditions. In order to promote transfers, obstacles could be introduced by mechanically deforming the channel wall during the manufacturing process. Numerical simulations can contribute to the emergence of innovative technologies based on a profound understanding and mastering of the underlying phenomena that simulation allows. In order to achieve this goal, a first key element was the formulation and integration into the AVBP CFD code of a numerical approach combining specific boundary conditions for reactive walls and ODE solvers for the gas phase and surface chemistry.The approach allowed to account for detailed kinetics and the interplay between the reactive surface and the gas-phase. The resulting tool was first validated using a zero-dimensional heterogeneous reactor computations. The results were shown to perfectly match the ones obtained with the reference kinetic solver SENKIN.Furthermore, the approach was then validated by applying it to the simulation of two planar reactive channel flows, and comparing the predictions with experimental findings of Dogwiler et al.. The developed approach proved to be able of reproducing main features of the catalytic combustion observed for different operating points. Finally, the developed tool was applied to explore the impact of introducing wall obstacles on the conversion rate of catalytic devices. The resulting findings have proved to open very interesting perspectives for contributing to the optimization of the design of catalytic converters using 2D CFD and detailed heterogeneous chemistry. In particular, the study of the impact of wall obstacles indicates the potential for contributing to further increase the efficiency of catalytic converters via the design of monolith geometries that would allow a more efficient and thus less costly usage of Pt-coating as a consequence of optimized interactions between the gas flow, gas phase chemistry and surface chemistry.La crise environnementale a conduit l’industrie automobile à faire face à des contraintes croissantes tandis que les limitations drastiques de polluants entrent en vigueur. Afin de réduire les émissions polluantes issues de la combustion, l’une des solutions adoptées est de post-traiter les fumées à l’aide de systèmes de post-traitement catalytique à l’image du catalyseur 3 voies (TWC) pour les moteurs à essence ou le catalyseur d’oxydation (DOC)pour les moteurs diesel. Ces appareils présentent une structure en nid d’abeille constituée d’un réseau de canaux à l’échelle millimétrique appelés monolithes et dont les parois intérieures sont recouvertes d’une fine couche de métal précieux aux propriétés catalytiques. Les polluants sont transformés via l’interaction entre les molécules présentes dans la phase gaz et les sites actifs du métal précieux. Etant donné les conditions laminaires d’écoulement au sein des monolithes, un mélange faible et une diffusion moléculaire limitée peuvent être rencontrés au voisinage de la paroi réactive. Le taux de conversion des polluants peut être alors insuffisant pour des conditions opératoires données. Dans le but d’optimiser les transferts,des obstacles peuvent être introduits par déformation mécanique des parois du canal catalytique au cours du processus de fabrication.Les simulations numériques peuvent contribuer à l’émergence de solutions innovantes basées sur une compréhension et une maitrise profonde des phénomènes sous-jacents. Afin d’atteindre cet objectif, le premier élément clé a été de formuler et d’intégrer dans le code de dynamique des fluides AVBP une approche numérique combinant d’une part des conditions aux limites dédiées à la prise en compte de parois réactives,et d’autre part, la résolution de la cinétique chimique gaz et surface via un solveur d’EDP.L’approche a permis la prise en compte de la cinétique détaillée et l’interaction entre la phase gaz et les parois réactives. L’outil développé a été validé en premier lieu à l’aide de calculs de réacteurs hétérogènes zéro-dimensionnels. Les résultats ont montré un parfait accord avec le solveur de référence SENKIN. L’approche a été validée ensuite en l’appliquant à la simulation de deux canaux réactifs aux parois planes et en comparant les résultats numériques aux résultats expérimentaux de Dogwiler et al. L’approche développée s’est révélée être capable de reproduire les principales caractéristiques de la combustion catalytique pour différents points de fonctionnement. Enfin, l’outil développé a été appliqué à l’étude de l’impact de l’introduction d’obstacles pariétaux sur les taux de conversion des systèmes catalytiques. Les résultats ont permis d’ouvrir des perspectives très intéressantes quant à la contribution de la CFD2D et de la chimie hétérogène détaillée à l’optimisation du design des systèmes de post traitement catalytique. En particulier, l’étude de l’influence des obstacles pariétaux a montré que le design de la géométrie des monolithes constitue un fort potentiel d’optimisation de l’efficacité des systèmes de conversion catalytique et ce, à moindre coût grâce à une utilisation optimisée du métal précieux rendue possible par une meilleure interaction entre l'écoulement, les réactions chimiques dans la phase gaz et la paroi réactive

Microkinetics modeling of automotive three-way catalysts

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003.Includes bibliographical references (p. 165-169).by Isabelle Pauwels.S.M

A review on exhaust gas after-treatment of lean-burn natural gas engines – From fundamentals to application

Modern lean-operated internal combustion engines running on natural gas, biogas or methane produced from wind or solar energy are highly fuel-efficient and can greatly contribute to securing energy supply, e.g. by mitigating fluctuations in the power grid. Although only comparably low emission levels form during combustion, a highly optimized emission control system is required that converts pollutants over a wide range of operation conditions. In this context, this review article pinpoints the main challenges during methane and formaldehyde oxidation as well as selective catalytic reduction of nitric oxides. The impact of catalyst formulation and operation conditions on catalytic activity and selectivity as well as the combination of several technologies for emission abatement is critically discussed. Additionally, recent experimental and theory-based progress and developments are assessed, allowing coverage of all time and length scales relevant in emission control, i.e. ranging from mechanistic and fundamental insights including atomic-level phenomena to full-scale applications

ONE-DIMENSIONAL PSEUDO-HOMOGENEOUS PACKED BED REACTOR MODELING INCLUDING NO-CO KINETICS

The air pollution generated from mobile sources creates a large impact on the environment and on people's health. In order to meet the stringent emission regulations worldwide, aftertreatment devices are employed to reduce the toxic emissions emanating from the Internal Combustion engines in these mobile sources. In order to continually reduce emissions levels, it is essential to understand and develop more predictive aftertreatment models. Traditional devices are of the monolithic geometry consisting of small channels employing laminar flow. However, often the reaction rate expressions utilized in these models are derived from more conventional packed bed reactor experimental setups. The aim of this thesis is to develop a one-dimensional pseudo-homogeneous packed bed reactor model for this type of reactor setup built in collaboration with the Chemical and Petroleum Engineering Department at the University of Kansas. A brief summary of the pseudo-homogeneous model is presented in order to properly develop the chemical species and energy equations for dynamically incompressible flow in one-dimension. Furthermore, the chemical kinetics on the reduction reaction of nitric oxide by carbon monoxide over rhodium-alumina and platinum-alumina catalysts is investigated in detail. This is accomplished in order to validate the model using fundamentally correct reaction kinetics via a precise global reaction mechanism. Finally, parametric studies including the different model components are presented and the specific choice of model does not largely influence the conversion profiles because of the similar effective transport values. Also, it is found that a careful consideration of source terms are required to model reactions accurately

Mathematical modelling of a low-temperature hydrogen production process with In Situ CO2 capture

Imperial Users onl

Development of a computationally efficient monolith reactor simulator: CFD-hybrid model analysis of methane oxidation monolith catalysed systems.

Doctoral Degrees. University of KwaZulu-Natal, Durban.The optimisation of complex geometries such as that of monolith reactors can be supported by computation and simulation. However, complex boundaries such as those found in multi-channel monoliths render such simulations of extremely high computational expense. Adding to the computational expense is the strong coupling among reaction kinetics, heat and mass transfer limitations in these channels. This severely limits the possibilities for geometric optimisation. In the first step toward developing a fast-solving hybrid simulation, a detailed CFD simulation was used to obtain the unsteady state, spatial temperature and concentration (and hence reaction rate) profiles for a range of input conditions. The results of the CFD simulation were then accepted as the benchmark to which faster-solving models were measured against to be considered as viable descriptions. A modified plug flow with effectiveness factor correction for wall mass-transfer was developed and evaluated as the first step towards the development of a multi-channel model. However, the modified plug model is only applicable to single channel monoliths and cannot account for heat transfer across high-density multi-channel beds. For multichannel simulations, the modified plug flow model is embedded into a hybrid-model framework. The hybrid model is based on the principle that, due to the high density of channels in a monolith, there will exist an equivalent homogeneous cylindrical model that approximates the behaviour of a bundle of channels acting as axial heat sources. This model entails the coupling of analytical solutions to single channel mass and momentum transfer with heat transfer across the single-shell extra-multi-channel space. Due to the application of effectiveness-factor type approaches, it is shown that the model can be represented by algebraic models that accurately represent the partial differential equations (PDEs) that describe monolith reactors. A close agreement between both temperature and species mole fraction profiles predicted from the modified plug flow model and a detailed CFD model was found with R2 values of 0.994 for temperature. The time needed to find a converged solution for plug flow model on an Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz workstation was found to be 53 seconds in comparison to 1.3 hours taken by a CFD model. The hybrid model was itself validated against the CFD multichannel model. The hybrid model axial temperature and species concentration profiles at various radial positions were found to be in a close agreement with CFD simulations, with relative error found to be in the 0.35 % range. The clock time on an Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz workstation was found to be 38 hours for a CFD multi-channel simulation which when compared with the 53 seconds clock time of the hybrid model implies the suitability of hybridisation for the application to geometric optimisation in the design of monolith reactors. The hybrid-model is developed to facilitate geometric optimization with the view of reducing hot spot formation, pressure drop and manufacturing costs. This is because monolith reactors applied in catalytic partial oxidation of methane are coated with precious metal catalysts, significantly contributing to capital costs. By isolating regions of high catalytic activity, it becomes possible to reduce the amount of precious metal coating required to achieve high conversion. The fast-solving hybrid model was used in the economic analysis of the catalytic partial oxidation of methane to syngas. Due to the low computational expense of the hybrid model, it was possible to investigate a wide range of design geometry and operating condition .It is shown that, for methane oxidation over a Platinum gauze catalyst, the channel diameter could be optimised to the 0.8 mm level resulting in the highest syngas revenue (R 65754.14 /day). The distribution of the catalytic material on the monolithic walls was found to influence the reactor performance hence the process profitability. The non-uniform distribution was found to significantly reduce the cost of fabrication while maintaining a high syngas productivity. In general, a method is proposed to optimise design and operation of catalytic monolith reactors through the application of fast-solving models.Author's Keywords : Hybrid model, catalytic partial oxidation, modified plug flow model, CF

NASA SBIR abstracts of 1991 phase 1 projects

The objectives of 301 projects placed under contract by the Small Business Innovation Research (SBIR) program of the National Aeronautics and Space Administration (NASA) are described. These projects were selected competitively from among proposals submitted to NASA in response to the 1991 SBIR Program Solicitation. The basic document consists of edited, non-proprietary abstracts of the winning proposals submitted by small businesses. The abstracts are presented under the 15 technical topics within which Phase 1 proposals were solicited. Each project was assigned a sequential identifying number from 001 to 301, in order of its appearance in the body of the report. Appendixes to provide additional information about the SBIR program and permit cross-reference of the 1991 Phase 1 projects by company name, location by state, principal investigator, NASA Field Center responsible for management of each project, and NASA contract number are included