Motion of objects immersed in a bubbling fluidized bed

Fluidized beds are employed in industry because of their excellent properties involving heat and mass transfer, and their capability to establish and promote chemical reactions inside them. A variety of processes can occur inside a fluidized bed, including drying, heat exchange, thermal conversion of solid fuels, and coating of particles. Most of the applications of fluidized beds involve the motion of objects inside the bed. Fuel particles, catalysts, and agglomerates are examples of typical objects found inside a fluidized bed. It is necessary to characterize the motion of these objects within the bed to establish the region for proper performance and to prevent operational problems such as the existence of hot or cold spots in a reactor or the appearance of de-fluidized zones due to the existence of agglomerates. In this work, the motion of large objects immersed in a bubbling fluidized bed was experimentally studied using digital image analysis. The experiments were performed in two facilities designed for such purpose, a 2-D bed and a lab-scale 3-D bed. Different objects were tested, varying density and size. The main characteristics of the object motion were studied on the 2-D bed. By direct visualizations of the object trajectory, the preferential paths and the homogeneity of its spatial distribution were characterized. Then, the cycles described by the object in his way from and towards the surface of the bed were studied in detail. Every cycle consists of processes of descent and processes of ascent. In most cases, a series of movements of ascent and decrease interleaved along the path are observed. From this experimental evidence and considering every cycle independent from the previous history, a simple model was developed to characterize the cycles, based on four fundamental parameters: the average rising and sinking velocities of the object, the maximum depth attained along the cycle and the number of independent rising movements (number of jumps) that take place in each cycle. Concerning the rising and sinking velocities, a methodology was established for the averaging calculations, as the existence of sudden changes of trend and vibratory movements complicates the separation of the processes. The object sinking motion is linked to the dense phase sinking motion and the object rising motion is linked to the evolution of bubbles. The probability of reaching the surface by the action of a single bubble or jump was quantified, along with the existence and relative incidence of cycles with multiple jumps. Finally a simple semi-empirical model was defined to characterize the cyclical motion of the object, using the four fundamental parameters, the number of jumps during the cycle, the maximum attained depth and the average rising and sinking velocities. These latter two parameters were associated to well-known correlations for the average sinking velocity of the dense phase and the average bubble velocity, while the former ones were characterized in relation with the time of circulation of the object and also in his respective distributions of probability. The procedure is tested for a neutrally buoyant object and the results are presented. Then, the procedure is applied to objects with different sizes and densities to study the incidence of buoyant forces. The effect of the gas velocity is also studied. The results show that the semi-empirical model possesses general validity within the range of our experiments. This includes variations of the object density from lightly higher to that of the bed to values lower than half of it, changes of object size around one order of magnitude and even changes of the height of the bed and of the distribution of sizes of the material that conforms the bed. The distribution of probabilities for the number of jumps follows a geometric decay. As a consequence, a value of 45 % is obtained for the average probability by which an object that starts rising by the action of a bubble finishes in the surface of the bed without detaching from it or its trail. This also implies that there is a 55 % probability that the cycle will have a new jump. This average value is kept constant for all the experimental conditions tested. Also a parabolic profile is obtained for the distribution of depths, which can be explained considering the preferential paths of both objects and bubbles. Throughout the study, a negligible effect of buoyant forces is observed during the object rising motion, while it is relatively important and coherent with the above mentioned forces in the sinking path. Finally, in a third part a practical application of object motion in a 3-D bed is presented. The time of circulation of the objects is measured acquiring images of the surface of the bed. A comparison of the distributions of circulation times for a standard bed and for a bed in which an actuator is used to modify and improve the dynamics of the bed. The actuator consists on the low-frequency rotation of the bed distributor. The results show an improvement of the circulation of objects in the whole range being measured. The fundamental aspect consists of the fact that in a standard bed with a perforated plate distributor, the object usually disappears after several cycles, and remains in the dead zones between the holes of the distribution plate. On the other hand the incidence of long periods increases as a consequence of objects passing though the surroundings and being affected proportionally. These effects do not exist (disappearance of the object) or are minimized (long periods) when using the rotating distributor. This result suggests the possibility of increasing the range of bed parameters that assure a proper object circulation, or that to recover objects, by means of the application of the actuator. This work also includes a study of the evolution of the probability distributions of the circulation times depending on the size of the bed, on the speed of the gas and on the density and size of the object.------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Los lechos fluidos se emplean en la industria por sus excelentes propiedades para el transporte de calor y masa y por su capacidad para albergar y promover en su interior reacciones químicas. En el interior de un lecho fluido pueden desarrollarse procesos como el secado de un material granulado, intercambios de calor con una superficie o un tubo, la conversión térmica de combustibles sólidos o el recubrimiento de partículas. Muchas de estas aplicaciones requieren del movimiento de objetos en el interior del lecho. Partículas de combustible, catalizadores y aglomerados son ejemplos típicos de partículas que se pueden encontrar en los lechos fluidos. La caracterización del movimiento de dichos objetos es necesaria para un adecuado funcionamiento y para evitar problemas como por ejemplo la existencia de puntos calientes o fríos en un reactor o la aparición de zonas defluidizadas debido a la aparición de aglomerados. El presente trabajo recoge un estudio experimental, empleando técnicas de análisis digital de imagen, del movimiento de objetos grandes (mayores que el material del lecho) inmersos en un lecho fluido burbujeante. Los experimentos se llevaron a cabo en dos instalaciones, un lecho bidimensional y otro tridimensional y de escala de laboratorio. Se analizaron diferentes objetos, variando su densidad y tamaño. Las características fundamentales del movimiento de los objetos se estudiaron sobre el lecho bidimensional. Siguiendo la trayectoria del objeto, se observaron y caracterizaron los caminos preferentes y la homogeneidad en la distribución. Posteriormente se estudiaron en detalle los ciclos descritos por el objeto en su camino desde y hacia la superficie del lecho. Cada ciclo se compone de procesos de descenso y procesos de ascenso, observándose en muchos casos una serie de movimientos de ascenso y descenso intercalados a lo largo de la trayectoria. A partir de esta evidencia experimental y considerando cada ciclo independiente de la historia previa, se desarrolló una modelización de los ciclos a partir de cuatro parámetros fundamentales: las velocidades medias de ascenso y descenso del objeto, la máxima profundidad alcanzada a lo largo del ciclo y el número de movimientos ascendentes independientes (número de saltos) que tienen lugar en el ciclo. Se estudiaron las velocidades de ascenso y descenso y se estableció una metodología de cálculo para los promedios, ante la existencia de cambios bruscos de tendencia y movimientos vibratorios que dificultan la tarea de separación de los procesos. El proceso de ascenso del objeto está vinculado al paso de burbujas. Se cuantificó la probabilidad de ascender hasta la superficie mediante la acción de una sola burbuja (un sólo salto) y en función de ella la existencia e importancia relativa de caminos de subida con múltiples saltos, fruto de la acción de una serie de burbujas y separados por periodos de descenso. Finalmente se definió un sencillo modelo semi-empírico para caracterizar el movimiento cíclico del objeto, basado en los cuatro parámetros fundamentales, el número de saltos durante el ciclo, la máxima profundidad alcanzada y las velocidades medias de ascenso y descenso. Estos dos últimos parámetros se relacionaron con correlaciones habituales para la velocidad media de la fase densa y de las burbujas y los dos primeros se modelaron, tanto en cuanto a su relación con el tiempo de circulación del objeto como en sus respectivas distribuciones de probabilidad. Este procedimiento se utilizó inicialmente para caracterizar el movimiento de un objeto cilíndrico grande y de densidad similar a la del lecho y posteriormente se varió su tamaño y densidad para estudiar el efecto de las fuerzas de flotabilidad, teniendo también en cuenta el efecto de la velocidad del gas. Los resultados muestran que el modelo semi-empírico cuenta con validez general dentro del rango de variación de los experimentos. Esto incluye variaciones de la densidad desde ligeramente superiores a la del lecho a valores inferiores a la mitad, cambio de tamaño en el entorno del orden de magnitud e incluso para cambios de altura del lecho y de la distribución de tamaños del material que forma el lecho. La distribución de probabilidades para el número de saltos sigue una proporción geométrica. En función de ello se obtiene un valor de 45% para la probabilidad media de que una burbuja que mueva al objeto acabe depositándolo en la superficie del lecho y supone un 55% de probabilidades de que el ciclo conlleve un nuevo salto. Este valor medio se mantiene constante para todos los experimentos. También se obtiene un perfil parabólico para la distribución de profundidades, que se explica en función de los caminos preferentes de objetos y burbujas. En todos los procesos se observa un efecto despreciable de las fuerzas de flotabilidad en el ascenso de objetos y coherente con dichas fuerzas en los caminos de bajada. Por último, en una tercera parte se estudió una aplicación práctica del movimiento de objetos, utilizando un lecho 3-D de escala de laboratorio. En este lecho se midieron los tiempos de circulación de los objetos mediante la adquisición de imágenes de la superficie del lecho. En el estudio se compararon las distribuciones de tiempos de recirculación para un lecho estándar y para otro en el que se introduce un actuador para modificar y mejorar las características de funcionamiento del lecho. El actuador empleado consiste en la rotación a baja frecuencia del distribuidor. Los resultados muestran la mejora de la circulación de objetos en todo el rango de medida. El aspecto fundamental consiste en que en un lecho estándar con distribuidor de placa perforada, el objeto acaba por desaparecer, quedando inmóvil en los huecos entre agujeros de la placa distribuidora. Por otro lado aumenta la incidencia de periodos de circulación muy largos a causa del paso del objeto por la zona. Estos efectos no existen (desaparición del objeto) o se minimizan (periodos largos) con el distribuidor rotatorio, lo que permite aumentar el rango de acción del lecho o recuperar objetos mediante la aplicación del actuador. Este trabajo incluye asimismo el estudio de la evolución de las distribuciones de probabilidad de los tiempos de circulación en función del tamaño del lecho, de la velocidad del gas y de la densidad y tamaño del objeto

Exergy optimization in a steady moving bed heat exchanger

Proceedings of: Interdisciplinary Transport Phenomena V: Fluid, Thermal, Biological, Materials and Space Sciences (ITP 2007), 14-19 of October, 2007, Bansko, Bulgaria (Oral paper nº 70)This work provides an exergy analysis of a moving bed heat exchanger to obtain for a range of incoming fluid flow rates the operational optimum and the incidence on it of the relevant parameters such as the dimensions of the exchanger, the particle diameter and the flow rate of the fluid. The MBHE proposed can be analyzed as a cross flow heat exchanger where one of the phases is a moving granular medium. In the present work the exergy analysis of the MBHE is carried out over operation data of the exchanger obtained in two ways: a numerical simulation of the steady state problem and the analytical solution of the simplified (avoiding conduction terms) equations. The numerical simulation is carried over the two steady energy equations (fluid and solid), involving for the solid the convection heat transfer to the fluid and the diffusion term in both directions, and for the fluid only the convection heat transfer to the solid. The analytical solution is the wellknown solution of the simplified problem neglecting conduction effects.Publicad

Thermal analysis and optimization of a heat regenerator composed of two coupled moving bed heat exchangers

This work presents a study to optimize the performance of a heat regenerator composed by two coupled moving bed heat exchangers (MBHE). A MBHE is used to recover heat, from a hot gas stream, and the other one is used to preheat an air stream. A direct application might be a gasifier. The heat exchangers performance was studied in two cases, considering or not the conduction heat transfer in the solid phase. When the solid conduction is taken into account, a numerical solution is obtained, while an analytical solution is possible when the conduction terms are neglected. In both cases, the optimum values of bed length (in the air flow direction) and particle diameter were obtained from an exergy point of view. Finally, an energy optimization of the heat regenerator was carried out, obtaining the optimal heat regenerator dimensions as a function of gas velocity and gas flow rate.Publicad

Coherent structures and bubble-particle velocity in 2-D fluidized beds

This work presents an experimental study to characterize ascending bubbles and granular velocity in the dense phase of a 2-D fluidized bed. Three different non-intrusive techniques based on images obtained with a high speed camera are developed, and applied to the images. First the bubble paths are characterized with time-average concentration maps and the bubble velocities are obtained, using a tracking algorithm over the mass centers of the bubbles. Finally, a PIV (particle image velocimetry) method is used to characterize the particle velocity vectors. This procedure is repeated for different bed aspect ratios, and different superficial gas velocities. This study analyzes the superficial gas velocity influence on the bed behavior, and how the bubble path configuration depends on the bed aspect ratio. The PIV measurements give us information on the location of the recirculation regions and the influence of the superficial gas velocity.Publicad

The role of fuel mixing on char conversion in a dual fluidized bed gasifier

Operational conditions, such as the fluidization velocity or the solids cross-flow, have been found to affect both lateral and axial mixing of fuel particles in a fluidized bed (1, 2). The fuel axial mixing has been shown to influence the heat and mass transfer between the bed and the fuel particles, which in turn affect the char conversion rate (3). Furthermore, the fuel lateral mixing affects the residence time of the fuel inside the fluidized bed reactor. The aim of this work is to investigate the effect of the operational conditions on the resulting char conversion in the gasification chamber of an indirect gasifier, while accounting for and assessing the role played by lateral and axial fuel mixing in the residence time and conversion kinetics, respectively. A 1-dimensional model for indirect gasification (4) was used in the present study. The model includes empirical inputs regarding the influence of the operational parameters on the fuel mixing in the lateral and axial directions. The axial mixing of fuel undergoing pyrolysis is described by results from previous large-scale experiments (2), while that of char particles is characterized in laboratory-scale experiments under cold conditions carried out in this work. Regarding the kinetics, previous findings on the char gasification rate resulting from fuel conversion at different axial locations in the bed (3) were used as empirical input data to the modeling. The experiments conducted in this work showed that the fluidization velocity significantly influences the axial mixing of char. Furthermore, results from the modeling showed that the fluidization velocity has a strong effect on the degree of char conversion in the gasification chamber: increasing the excess velocity from 0.05 m/s to 0.20 m/s resulted in a 10-fold decrease in the degree of char conversion. This is due to the decrease in the fuel residence time caused by the enhanced fuel lateral mixing. In comparison to this, the fuel axial mixing and its effect on the char conversion kinetics played a smaller role: disregarding the axial mixing of fuel resulted in char conversion degrees up to 1.2 times higher than those obtained when accounting for it. REFERENCES E. Sette, D. Pallarès and F. Johnsson. Influence of bulk solids cross-flow on lateral mixing of fuel in dual fluidized beds. Fuel Process. Technol., 140:245-251, 2015. E. Sette, T. Berdugo Vilches, D. Pallarès and F. Johnsson. Measuring fuel mixing under industrial fluidized bed conditions – a camera-probe based fuel tracking system. To be submitted, 2015. L. Lundberg, P.A. Tchoffor, D. Pallarès, R. Johansson, H. Thunman and K. Davidsson. Influence of Conversion Conditions on the Gasification Rate of Biomass Char in a Fluidised Bed. Submitted to Fuel Process. Technol., 2015. L. Lundberg, D. Pallarès, R. Johansson and H. Thunman. A 1-dimensional model of indirect biomass gasification in a dual fluidised bed system. 11th International Conference on Fluidized Bed Technology, CFB 2014. Beijing: Chemical Industry Press, 607-612, 2014

Experimental quantification of the particle-wall frictional forces in pseudo-2D gas fluidised beds

In this work a novel measurement technique for pseudo-2D fluidised beds is developed. The objective is to give an estimation of the overall frictional force between the solids and the front and rear walls of the bed. For doing this, the measured pressure signal in the bed is processed in combination with the solids distribution (i.e. centre of mass position, velocity and acceleration) obtained from digital image analysis of the optically accessible front view of the bed. This is performed by acquiring the pressure signal in the bed simultaneously to the digital images. Both the pressure and the digital images are connected through a simple force balance in the bed, and a particle-wall interaction coefficient is obtained assuming that the overall frictional force is proportional to the centre of mass velocity. The particle-wall interaction coefficient found using this technique is of the order of 40-120 kg/m(2) s in the bed tested, and the standard deviation of the frictional forces reaches more than 70% of the weight of the bed. Therefore, the results indicate that the contribution of the particle-to-wall friction on the fluctuation of the pressure drop in a pseudo-2D bed is not negligible.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad

Combining the lumped capacitance method and the simplified distributed activation energy model to describe the pyrolysis of thermally small biomass particles

The pyrolysis process of thermally small biomass particles was modeled combining the Lumped Capacitance Method (LCM) to describe the transient heat transfer and the Distributed Activation Energy Model (DAEM) to account for the chemical kinetics. The inverse exponential temperature increase predicted by the LCM was considered in the mathematical derivation of the DAEM, resulting in an Arrhenius equation valid to describe the evolution of the pyrolysis process under inverse exponential temperature profiles. The Arrhenius equation on which the simple LCM-DAEM model proposed is based was derived for a wide range of pyrolysis reactor temperatures, considering the chemical kinetics data of four lignocellulosic biomass species: pine wood, olive kernel, thistle flower, and corncob. The LCM-DAEM model proposed was validated by comparison to the experimental results of the pyrolysis conversion evolution of biomass samples subjected to various inverse exponential temperature increases in a TGA. To extend the validation, additional biomass samples of Chlorella Vulgaris and sewage sludge were selected due to the different composition of microalgae and sludge compared to lignocellulosic biomass. The deviations obtained between the experimental measurements in TGA and the LCM-DAEM predictions for the evolution of the pyrolysis conversion, regarding the root mean square error of temperature, are below 5 degrees C in all cases. Therefore, the simple LCM-DAEM model proposed can describe-accurately the pyrolysis-process of a thermally small biomass particle, accounting for both the transient heat transfer and the chemical kinetics by solving a simple Arrhenius equation.The authors express their gratitude to the BIOLAB experimental facility and to the “Programa de movilidad de investigadores en centros de investigación extranjeros (Modalidad A)” from the Carlos III University of Madrid (Spain) for the financial support conceded to Antonio Soria-Verdugo for a research stay at the German Aerospace Center DLR (Stuttgart, Germany) during the summer of 2018. Funding by Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR), the German Aerospace Center, is also gratefully acknowledged

Analyzing the pyrolysis kinetics of several microalgae species by various differential and integral isoconversional kinetic methods and the Distributed Activation Energy Model

The pyrolysis kinetics of the microalgae Chlorella vulgaris (CV), Isochrysis galbana (IG), Nannochloropsis gaditana (NG), Nannochloropsis limnetica (NL), Phaeodactylum tricornutum (PT), and Spirulina platensis (SP) were studied by non-isothermal thermogravimetric analysis conducted at nine different constant heating rates. The kinetic parameters of each microalgae species were calculated using several kinetic methods, such as those of Kissinger, Friedman, Ozawa-Flynn-Wall (OFW), Kissinger-Akahira-Sunose (KAS), Vyazovkin, and the simplified Distributed Activation Energy Model (DAEM). The results show that the kinetic parameters calculated from the integral isoconversional methods OFW, KAS and Vyazovkin are similar to those determined by applying the simplified DAEM. In contrast, application of the differential isoconversional method of Friedman led to moderate deviations in the activation energies and pre-exponential factors computed, whereas the unique values of the kinetic parameters determined by the Kissinger method resulted in the highest deviations.The authors express their gratitude to the BIOLAB experimental facility. Funding by Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR), the German Aerospace Center, is gratefully acknowledged as well as funding by the DLR international collaboration project “Accurate Kinetic Data of Biomass Pyrolysis”.Publicad

On the characteristic heating and pyrolysis time of thermally small biomass particles in a bubbling fluidized bed reactor

Pyrolysis of crushed olive stone particles in a lab scale Bubbling Fluidized Bed (BFB) reactor wasinvestigated. The time evolution of the pyrolysis conversion degree of the olive stone particles, while moving freely in the BFB, was determined from the evolution of the mass of olive stones remaining in thebed, measured by a precision scale holding the whole reactor installation. The experimental measurements of the pyrolysis conversion degree were employed to validate a simple model combining heattransfer and chemical kinetics, which is valid for thermally small particles. The model combines the Lumped Capacitance Method (LCM) and the simplified Distributed Activation Energy Model (DAEM) toaccount for heat transfer and pyrolysis chemical kinetics, respectively. The estimations of the combined LCM-DAEM model for the pyrolysis conversion degree were found to be in good agreement with the experimental measurements for the pyrolysis of olive kernels in a BFB operated at various bed temperatures,fluidizing gas velocities, and biomass particle size ranges. From the combined LCM-DAEM model, the characteristic heating time and the pyrolysis time of the olive stone particles were derived, obtaining a direct relation between these two parameters for constant values of the bed temperature.The authors express their gratitude to the BIOLAB experimentalfacility and to the program "Research Stays for University Academics and Scientists" from the German Academic Exchange Service (DAAD) for the financial support conceded to Antonio Soria-Verdugo for a research stay at the German Aerospace Center(DLR) (Stuttgart, Germany) during the summer of 2019. Funding by Deutsches Zentrum für Luft-und Raumfahrt e. V. (DLR), the German Aerospace Center, and the Helmholtz Association in the research fields energy, fuels and gasification, especially in the Program "Energy Efficiency, Materials and Resources" Topic 4 "Efficient Use of Fuel Resources" is also gratefully acknowledged

Fully coupled TFM-DEM simulations to study the motion of fuel particles in a fluidized bed

In the present work, novel numerical simulations using a hybrid model are carried out to study the motion of objects, representing fuel particles, in a pseudo-2D fluidized bed. The hybrid model combines the continuum treatment of the gas phase with the possibility to treat different solid phases either as continuum, or discrete. In the present case, both the gas and the dense phase of the bed are modelled as continuum phases, as typically done in two-fluid model simulations, whereas fuel particles are simulated as discrete entities whose movement affects, and can be affected by the dense phase motion (i.e. fully coupled TFM-DEM simulations). The results obtained from the model are qualitatively and quantitatively compared with reported experimental findings available in the literature. Firstly, the motion of the fuel particle with regards to the bubble phase and dense phase is proved to be realistic. Secondly, the location probability of the particle in the simulated bed is calculated and compared with the experimental data. Then, the ballistic path followed by the particle in the freeboard is also compared with experimental measurements. These results show good agreement between experiments and simulation. The numerical results reflect the same behaviour during the ascending and descending motion of the fuel particles as that observed in the experiments. The results also show that the most probable locations of the particles predicted by the simulations are consistent with the experimental findings, both inside the fluidized bed and in the freeboard. Overall, the hybrid model tested shows quite promising results, which indicates the potential usability of the model.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad