Teklay Weldeabzgi Asegehegn

Hydrodynamic Simulation of Gas-Solid Bubbling Fluidized Bed Containing Horizontal Tubes

Hydrodynamic simulation of 2-D gas-solid bubbling fluidized bed containing staggered horizontal tubes was performed. The bubble hydrodynamics, bubble diameter and bubble rise velocity, were investigated and compared with experimental results elsewhere in the literature (Hull et al., 1999, Influence of Horizontal tube Banks on the Bubbling and Solids Mixing Behavior of Fluidized Beds. 15th Int. FBC Conference). The Eulerian-Eulerian Two Fluid Model (TFM) implemented in Fluent, version 6.3, was used for the governing equations with closure equations based on the Kinetic Theory of Granular Flow (KTGF). The numerical simulation showed that the horizontal tubes were the main source of bubble break up where bubbles break when they interact with the tubes and grew by coalescence until they reach the next row of tubes. Quantitative investigation of the bubble hydrodynamics also revealed that the predicted average bubble diameter and bubble rise velocity were in good agreement with the experimental results reported in the literature. It was observed that there were small bubbles formed on the lower-half part of the tubes which were usually interacted with an incoming bubble from below and left the tube after coalescence. As a result, the numerical simulation predicted a lower average bubble diameter and bubble rise velocity at the bottom of the tube banks than that reported in the literature.

Numerical Study of Bubbling Gas-Solid Fluidized Beds Hydrodynamics: Influence of Immersed Horizontal Tubes and Data Analysis

Numerical simulations of two dimensional gas-solid bubbling fluidized beds with and without immersed horizontal tubes were performed using Eulerian - Eulerian Two Fluid Model (TFM). The influences of immersed horizontal tubes and different data analyses techniques on the bed and bubble hydrodynamics were investigated. The results were compared with experimental data and correlations available in the literature.Different ways for extracting and defining hydrodynamic properties, such as bed expansion ratio and bubble properties, were found to influence the simulation results. Furthermore, the time-averaged values showed greater sensitivity to the length of averaging time in the first few seconds. With regard to tube influence, immersed tubes were found to be the main source of bubble breakup. Thus, the calculated mean bubble diameters and rise velocities were found to be lower with tubes than without for the same bed geometry and superficial velocity. The bubble shapes were observed to elongate in the vertical direction in the tube bank region compared to the bed region below and above the tube bank. In addition, the TFM was found to successfully predict the overall time-averaged solid motion and distribution. For beds with immersed tubes, defluidized regions were observed at the upper part of the tubes where solid particles rested without moving. On the other hand, the lower parts of the tubes were usually covered with gas pockets. These effects were seen to reduce with increasing superficial velocity.

Numerical Simulation of Dense Gas-Solid Multiphase Flows Using Eulerian-Eulerian Two-Fluid Model

Gas-solid fluidized beds are widely applied in chemical processes such as drying, combustion, synthesis of fuels, granulation, polymerization etc. They have several advantageous properties including; excellent heat and mass transfer, nearly isothermal conditions due to intense gas-solid mixing, large gas-solid surface area, smooth transport of solids, uniform solid product in batch processes, and possibility of continuous and largescale operations. On the other hand, these reactors have several drawbacks that provide a strong motivation for further studies and developments. These include; difficulty in scalingup and design, erosion of vessel and internals, formation of agglomerates, non-uniform products due to non-uniform solids residence time during continuous operation, and high particle entrainment. These drawbacks are bottlenecks for practitioners to reliably design and scale-up commercial fluidized bed reactors. The main reason for this is that the gassolid multiphase flow dynamics coupled with heat and mass transfer and chemical reactions that occur in these systems are very complex and not yet fully understood. In bubbling gas-solid fluidized beds, bubble characteristics such as size, shape, velocity, distribution have a vital influence on the hydrodynamics of bed and hence on its performance as a chemical reactor and/or a heat exchange unit. The extent of gas-solid mixing and segregation, heat and mass transfer as well as reaction conversion are governed by the number, size and motion of bubbles passing through the bed (Kunii & Levenspiel, 1991). Therefore, fundamental understanding of the hydrodynamics of fluidized beds thereafter their heat and mass transfer as well as chemical conversion come only after a sound understanding of bubbling behaviour is achieved. However, prediction of bubble characteristics is extremely complex as bubbles can grow, coalesce, split or even disappear as they move from the distributor where they are formed to the top of the bed where they finally erupt. Moreover, bubble characteristics vary with geometric construction of the bed and operating conditions. In many applications, heat exchanger tubes are inserted to enhance the rate of heat and mass transfer and chemical conversion, control the operating temperature, promote good mixing and reduce gulf circulation of solids. In these systems, the bubbling behaviour is also strongly influenced by the geometry and arrangement of the internals (Yates et al., 1990; Hull et al., 1999; Asegehegn et al., 2011a). Therefore,