Numerical simulation of scale-up effects of methanol-to-olefins fluidized bed reactors

Scale-up of fluidized bed reactors has long been regarded as a big challenge in chemical reaction engineering. While traditional scaling theories are mostly based on hydrodynamics similarity, computational fluid dynamics (CFD) aided approach allows direct coupling between hydrodynamics and reaction factors and is expected to speed up the experiment-based scale-up process with lower cost. In this study, we aim to investigate the scale-up effects through simulations of a series of methanol-to-olefins (MTO) reactors of different sizes. The two-fluid model and energy-minimization multi-scale (EMMS)-based drag models, are combined in simulations. The fluidization characteristics in terms of flow structures, velocity distribution, mass fractions of gaseous product and coke distribution are presented against available experimental data for different-sized reactors. It is found that typical hydrodynamic features can be reasonably predicted, while the prediction of reaction behavior shows growing discrepancy with increasing reactor size. Possible reasons are discussed in the last section along with future work presented for scale-up studies. (C) 2017 Elsevier Ltd. All rights reserved

Coarse-Grain DEM Modelling in Fluidized Bed Simulation: A Review

In the last decade, a few of the early attempts to bring CFD-DEM of fluidized beds beyond the limits of small, lab-scale units to larger scale systems have become popular. The simulation capabilities of the Discrete Element Method in multiphase flow and fluidized beds have largely benefitted by the improvements offered by coarse graining approaches. In fact, the number of real particles that can be simulated increases to the point that pilot-scale and some industrially relevant systems become approachable. Methodologically, coarse graining procedures have been introduced by various groups, resting on different physical backgrounds. The present review collects the most relevant contributions, critically proposing them within a unique, consistent framework for the derivations and nomenclature. Scaling for the contact forces, with the linear and Hertz-based approaches, for the hydrodynamic and cohesive forces is illustrated and discussed. The orders of magnitude computational savings are quantified as a function of the coarse graining degree. An overview of the recent applications in bubbling, spouted beds and circulating fluidized bed reactors is presented. Finally, new scaling, recent extensions and promising future directions are discussed in perspective. In addition to providing a compact compendium of the essential aspects, the review aims at stimulating further efforts in this promising field

A Novel Method for Pre-combustion CO2 Capture in Fluidized Bed

La comunidad internacional está realizando enormes esfuerzos para mitigar los efectos de las emisiones de gases de efecto invernadero (GEI) en el cambio climático. Aproximadamente le 25% de las emisiones globales de GEI (fundamentalmente CO2) son generados por la combustión de combustibles fósiles en el sector eléctrico. La captura y almacenamiento de CO2 se ha propuesto como una alternativa para reducir las emisiones de GEI en centrales térmicas. Numerosas tecnologías para la captura de CO2 se han desarrollado en los últimos años, fundamentalmente en tres líneas tecnológicas: postcombustión, oxicombustión y precombustión. Esta tesis presenta un nuevo método para la captura de CO2 en precombustión, produciendo hidrógeno a partir de carbón, sin emisiones de GEI. El objetivo principal de este trabajo ha sido desarrollar un modelo completo, mediante herramientas de fluido dinámica computacional (CFD), del proceso de reformado de un gas de síntesis con alto contenido en metano combinado con la captura de CO2 mediante adsorción con sorbentes sólidos regenerables. Este proceso es conocido como reformado de metano mejorado por adsorción (o SE-SMR, su acrónimo en inglés). SE-SMR representa una novedosa y eficiente energéticamente ruta para la producción de hidrógeno con captura in situ de CO2. Este proceso ha sido estudiado en un lecho fluido burbujeante, usando sorbentes sólidos de óxido de calcio como captores de CO2. Dos sorbentes sólidos han sido estudiados en laboratorio: uno natural (Dolomita) y uno sintético (CaO- Ca12Al14O33). Además, varios tratamientos han sido desarrollados para mejorar la capacidad de captura de estos sorbentes. Un completo modelo CFD del proceso de SE-SMR ha sido desarrollado. Una aproximación Euleriana-Euleriana ha sido combinada con la Teoría Cinética de Flujos Granulares para simular la fluidodinámica del lecho fluido burbujeante. Los reacciones químicas de reformado y carbonatación han sido implementadas en el modelo CFD. Se ha incluido un modelo detallado de captura de CO2 para simular el comportamiento de los diferentes sorbentes sometidos a diferentes pretratamientos para mejorar su rendimiento. Asimismo, un modelo de arrastre de partículas ha sido desarrollado para reducir el coste computacional de las simulaciones a escala semi-industrial. Se ha llevado a cabo una extensa campaña de simulaciones para validar el modelo a escala de laboratorio y semi-industrial. Las simulaciones CFD han sido combinadas con un Diseño de Experimentos Robusto, con el objetivo predecir y evaluar la sensibilidad del proceso SE-SMR a diversos factores operativos

Numerical Study of Ozone Decomposition Reaction Behaviours in Gas-Solids Circulating Fluidized Bed Reactors

This study numerically investigated the reaction behaviours of catalytic ozone decomposition reaction in a 10.2-meter-tall gas-solids circulating fluidized bed (CFB) reactor. A pseudo-homogeneous reactive transport model for ozone decomposition, integrated with the two-fluid model, was developed and validated using experimental data. Based on the model, the impacts of turbulence models, specularity coefficients, and simulation methods on reaction behaviours in the CFB riser reactor were explored. These three factors were found to significantly affect the hydrodynamic characteristics and the reaction behaviours in the riser. A comparative study of CFB riser and downer reactors was conducted. Operations in the direction of and against gravity resulted in drastically different flow fields and particle clustering in the two reactors. More uniform flow and reaction fields make the downer have better gas-solids contact efficiency than the riser. Flow structure and residence time distributions of gas and solids in the riser and downer were characterized by tracing the gas and particles. The results showed that flow in the downer reactor resembles plug flow, while significant axial backmixing occurs in the riser. An internal circulation mechanism is proposed to explain the backmixing. A sub-grid reactive transport model was developed using a filtering method and an artificial neural network (ANN) to explore the impact of particle clustering on reaction behaviours. In the development of the filtered model, employing gradient features as inputs enhanced regression accuracy. Additionally, ANN demonstrated superior performance over traditional fitting methods. Consequently, the filtered reactive transport model showed improvements in predicting the reaction behaviours in a CFB riser. In summary, the hydrodynamic characteristics within CFB predominantly influence reaction behaviours. High-resolution simulations combined with machine learning techniques effectively aid in understanding mechanisms in fluidization system and developing new models, which are crucial for designing and optimizing large-scale reactors

Principes fondamentaux de la RTD en phase gazeuse dans les réacteurs à lit fluidisé

RÉSUMÉ: La distribution du temps de résidence (RTD) est une technique de diagnostic appliquée par les chercheurs pour évaluer l’hydrodynamique dans les réservoirs, les tuyaux, les réacteurs et les systèmes à composants multiples. Il détecte les écarts par rapport aux modèles d’écoulement idéaux tels que les zones mortes, les canaux et la dispersion. Les expériences RTD consistent à introduire un traceur détectable soit un gaz inerte, un solide ou un liquide, puis à détecter sa concentration à un certain point du système, généralement à la sortie. Nous avons développé une méthodologie RTD précise et reproductible pour mesurer l’hydrodynamique en phase gazeuse et l’avons appliquée à un micro-réacteur à lit fluidisé (8mm de diamètre). Nous avons comparé une injection en continu (heaviside step) à une injection d’un volume précis (bolus). Dans le premier cas, nous substituons instantanément la composition du débit de gaz et examinons l’évolution de la concentration avec le temps à l’aide d’un spectromètre de masse à une fréquence de 2 Hz. Dans le second cas, nous pivotons une vanne multiport qui pousse un volume du traceur gazeux à travers une bobine vers le réacteur. Pour comprendre l’effet de la diffusivité, de la dispersion et de l’adsorption, nous avons comparé 7 gaz avec des coefficients de diffusivité distincts - Kr, O2, CO2, CO, CH4, He et H2. Nous avons étudié l’hydrodynamique de ces gaz dans le réacteur à lit fluidisé avec des catalyseurs du groupe Geldart A - l’hémihydrate d’hydrogénophosphate de vanadyle (VPOP), le pyrophosphate de vanadyle calciné (VPPC) et équilibré (VPPE), le catalyseur pour le craquage catalytique en lit fluidisé (FCC) ainsi qu’avec une poudre du groupe Geldart B — le sable. H2 et He, avec leurs coefficients de diffusivité élevés, sortent toujours du réacteur à l’avance autant avec un catalyseur poreux et que non poreux ainsi qu’à température ambiante ou à 300 �C. Nous avons ajusté toutes les courbes de réponse RTD avec un modèle de dispersion axiale avec des conditions frontière fermées ouvertes. Lorsqu’un gaz de synthèse simulé — CO2, CO, CH4 et H2 — est injecté sous forme de pulse dans le micro-réacteur chargé de catalyseur FCC, le CO2 est adsorbé à température ambiante. Ceci est représenté sur la courbe de réponse par une queue étendue et un grand écart par rapport au modèle de dispersion axiale. De plus, le CO2 prend deux fois plus de temps pour sortir du réacteur que H2. Ce phénomène chromatographique était inattendu et pourrait être appliqué pour éliminer le CO2 de l’air ou des courants industriels.----------ABSTRACT: Residence time distribution (RTD) is a diagnostic technique researcher apply to evaluate hydrodynamics in vessels, pipes, reservoirs, and multi-component systems. It detects deviations from ideal flow patterns non-homogeneity like dead zones, channelling, and dispersion. RTD experiments consist of introducing an identifiable, inert gas, solid or liquid tracer and then detecting it at some point in the system, usually the exit. We developed a precise, reproducible RTD methodology to measure gas phase hydrodynamics and applied it to a micro-fluidized bed reactor (8mm diameter). We compared a Heaviside step function injection against a bolus injection. In the former, we instantaneously switch the composition of the gas flow and monitor the change in concentration with time with a mass spectrometer at a frequency of 2 Hz. In the second case, we switch a multiport valve that pushes a volume of tracer gas from a coil into the reactor. To understand the diffusivity, dispersion, and adsorption effect, we compared 7 gases with distinct diffusivity coefficients – Kr, O2, CO2, CO, CH4, He and H2. We examined the hydrodynamics of these gases in the fluidized bed reactor for Geldart group A catalyst – vanadyl hydrogen phosphate hemihydrate (VPOP), vanadyl pyrophosphate calcined (VPPC) and equilibrated (VPPE), fluid catalytic cracking (FCC), and Geldart group B powder – sand. H2 and He, with high diffusivity coefficients, always egress from the reactor in advance with porous and non-porous catalyst as well as ambient temperature and 300 �C. We fitted all the response curves with an axial dispersion model with closed-open boundary conditions. When a simulated syngas – CO2, CO, CH4, and H2 – is injected as a pulse in the micro reactor loaded with FCC catalyst, CO2 adsorbed at ambient temperature. This is represented on the response curve by an extended tail and a large deviation to the axial dispersion model. Also, CO2 takes double the time to egress the reactor compared to H2. This chromatographic phenomenon was unexpected and might be applied to remove CO2 from the air or industrial streams

CFD-DEM Modeling of Spouted Beds With Internal Devices Using PTV

195 p.Esta tesis se centra en la extracción de perfiles de velocidad de sólidos, tanto esféricos como irregulares, en un spouted bed y el análisis de estos valores bajo la influencia de diferentes dispositivos internos en el contactor y caudales. El análisis se ha centrado en un contactor cónico mientras que un contactor de perfil prismático también ha sido utilizado para analizar el efecto de esta geometría en la dinámica del sistema. Estos valores experimentales de sólidos regulares e irregulares han sido modelados y simulados a través de un modelo CFD-DEM en el que la fase continua y discreta se han acoplado, a fin de garantizar simulaciones realistas y capaces de predecir parámetros difíciles de obtener de manera experimental y cruciales para el diseño y escalado de estos tipos de lechos; como son los tiempos de ciclo de los sólidos y la distribución de tiempos de residencia del gas bajo diferentes condiciones. Estos parámetros determinan la capacidad de un sistema y la eficacia a la hora de utilizar el volumen del reactor