Resumen de Experimental study and Monte Carlo modeling of object motion in a bubbling fluidized bed

Luis Miguel García Gutiérrez

• Fluidized beds are employed for a wide variety of applications such as drying, coating of particles, catalytic reactions, or thermal conversion processes. In a number of these applications, objects differing in density and/or size from the dense phase material are found in the bed. These objects can be agglomerates, catalysts or reactants. In this PhD thesis, a fundamental study of the motion of objects is presented, but considering also the main characteristics of the thermal conversion processes for these objects. Fluidized beds are used for the thermal conversion of fuels with low heating value and/or large humidity content, applications in which the high heat and mass transfer exchange provided by fluidized beds becomes relevant. In general, fuel particles of such characteristics are much larger in size than the dense phase material, and have a density that can range between the density of the dense bed to rather smaller values. In all cases, a good mixing of the fuel particles throughout the bed involves a higher efficiency in the thermal conversion process. In fluidized beds, the mixing rate in the vertical direction is higher than that in the lateral direction, as a result of the bubble motion. A proper distribution of the fuel particles in the whole bed is fundamental for an adequate development of the chemical reaction, and to avoid the formation of cold or hot spots. Therefore, the lateral mixing becomes a relevant parameter. The lateral and vertical displacement of the fuel particle and the dimensions of the reactor have to be taken into account together with the fuel particle residence time in the bed for a proper characterization. The residence time of a fuel particle during its thermal conversion can be represented either by the devolatilization time or the char conversion time. For the purposes of this thesis, the first one will be used, as it is the limiting factor in time. Also, a comparison between the time that the fuel particle remains in the freeboard, the time it remains immersed in the bed, and the devolatilization time is relevant. Finally, a significant design parameter of reactors, the location and number of feeding ports for a proper distribution of the fuel throughout the bed, depends on the capacity of the fuel particle to move laterally. In this dissertation, the main parameters that characterize the object motion in a bubbling fluidized bed are obtained experimentally and related to bed variables such as the dimensionless gas velocity. These main parameters include the time that an object spends during its motion throughout the bed, either immersed in the dense bed or in the freeboard, and the vertical and lateral displacements. The experimental characterization, analyzing the dynamics of an object in a fluidized bed, provides the information to develop a model for the object motion. The model is divided in several sub-models, taking into account the different dynamics to which the object is subjected throughout the bed. In each case, the relevant parameters of the object motion, displacement and time, are found to relate to more elemental factors with definite statistical distributions, which are obtained experimentally. The different sub-models are based on the simulations of the relations between the statistical distributions of such factors using a Monte Carlo method. The object behavior in a bubbling fluidized bed can be divided in two parts: when it is in the freeboard and when it is immersed in the dense bed. In the freeboard, a large object is only affected by the gravitational force; the drag force and the interactions with dense phase particles being negligible. Therefore, its motion can be characterized as a ballistic motion. This motion can be described by the object velocity at the instant of its ejection by the bubbles to the freeboard. Such a velocity can be characterized with statistical information of the modulus and angle, and modeled as a function of the bed operational conditions. On the other hand, the behavior of the object when it is immersed in the bed can still be divided in two different processes: a sinking process and a rising process. The sinking process is governed by the dense phase and thus the object moves according to its motion. This process can be defined by statistical parameters such as the probability of the object to attain a maximum depth or the probability of the object to start a rising process at each position during its sinking motion. In the rising process, the object is mainly affected by the bubbles, and the capacity of reaching the bed surface directly depends on its attachment to the bubbles. This behavior can be characterized by a parameter that represents the probability of an object to reach the surface directly when it starts a rising path and its opposite, the probability of detaching from the bubble and restart a sinking path before reaching the bed surface. As a result, the motion of the object either when it is in the freeboard or immersed in the dense bed can be described by the statistical parameters obtained experimentally, and models based on Monte Carlo simulations of such parameters can be derived. Finally, the combination of the different sub-models of the object motion throughout the bed permits to describe the global behavior of an object in a large-scale bubbling fluidized bed. A global model based on Monte Carlo simulations of the elemental parameters obtained experimentally is developed, based on the elemental statistical parameters of the object motion. The relevant parameters that describe the behavior of a fuel particle, the lateral and vertical displacement or the time spent during its motion throughout the bed, are determined using the model, and validated with experiments reported in the literature. ---------------------------------------------------------------------------------