MULTIPLE ORIFICE BUBBLE GENERATION IN GAS-SOLID FLUIDIZED BEDS: THE ACTIVATION REGION APPROACH

This work addresses the bubble generation mechanism at multi-orifice distributors in gas-solid fluidized beds (FB). Different measurements techniques such as high speed video camera and Kistler pressure transducers were applied to obtain information from both local, and global bed dynamics. Pressure fluctuation time series are used for dynamic diagnosis of the 2-D facility used during the study. The bed was operated with different distributor plates at several bubbling conditions leading to different bubble flow patterns characterized by digital image analysis of both the dense and the bubble phases. In order to explain the bubble pattern developed within the bed and the measured bubble dynamics, a phenomenological discrete bubble model is used. This model proposes an activation region (AR) mechanism for multi-orifice bubble generation. The underlying hypothesis is that the bubble formation can be placed in a region above the distributor plate where the initial bubble size is the result of the dynamical interaction of neighbour orifices. From the analysis of the experimental results, it is observed how for two different uniform gas distribution across the distributor plate, bubble dynamics interactions play the main role as the driver of the resulting bubble flow pattern developed within the bed. Moreover, when the activation region hypothesis is used as a bubble generation mechanism in a phenomenological discrete bubble model, it is seen that the proposed activation region mechanism, explains the observed bubble generation phenomena at multi-orifice distributors, and leads to a substantial decrease of the computational cost to simulate bubbling FB dynamics

Simulation of fuel particles motion in a 2D fluidized bed using a hybrid-model considering wall friction

The mixing of fuel particles is a key issue on the performance of fluidized bed reactors. In this work, the motion of a non-reactive fuel particle in a 2D bubbling fluidized bed at ambient conditions is simulated employing a hybrid-model. The hybrid-model, implemented in the code MFIX, simulates the dense and gas phases using a Two-Fluid Model (TFM) while the fuel particles are modeled using a Discrete Element Method (DEM). The importance of the present hybrid-model is that the interaction of the continuum phases with the fuel particles behavior is fully coupled. In a previous study, Hernández-Jiménez et al. (1) compared the fuel particles motion obtained from the simulation with experimental results measured in a cold 2D fluidized bed by Soria-Verdugo et al. (2, 3). The simulation results related to the location of the fuel particle in the bed were similar to the experimental data (Figure-1). Nevertheless, some discrepancies were found in important parameters such as the circulation time of the fuel particles. These discrepancies were associated to the overprediction of the simulated solids velocity. In the present work, in order to improve the accuracy of the simulated fuel particle motion in a bubbling fluidized bed, a friction term accounting for the effect of the walls of the bed on the continuum solid phase is introduced in the hybrid-model, as proposed by Hernández-Jiménez et al. (4). According to the results, prediction of the fuel circulation time is clearly improved when the friction term is included in the simulation (Figure-2). REFERENCES Hernández-Jiménez F. , Garcia-Gutierrez L.M., Soria-Verdugo A., Acosta-Iborra A. 2015. Fully coupled TFM-DEM simulations to study the motion of fuel particles in a fluidized bed, Chem. Eng. Sci.,134, 29, 57-66. Soria-Verdugo, A., Garcia-Gutierrez, L.M., Sánchez-Delgado, S., Ruiz-Rivas,U., 2011a. Circulation of an object immersed in a bubbling fluidized bed. Chem. Eng. Sci. 66, 78–87. Soria-Verdugo, A., Garcia-Gutierrez, L.M., García-Hernando, N., Ruiz-Rivas, U., 2011b. Buoyancy effects on objects moving in a bubbling fluidized bed. Chem. Eng. Sci.66, 2833–2841. Hernández-Jiménez, F., Cano-Pleite, E., Sánchez-Prieto, J., Garcia-Gutierrez, L.M., Acosta-Iborra, A. Development of an empirical wall-friction model for 2D simulations of pseudo-2D fluidized beds. Submitted for publication. Please click Additional Files below to see the full abstract

Critical Evaluation of Euler-Euler and Euler-Lagrangian Modelling Strategies in a 2-D Gas Fluidized Bed

Two-phase granular systems are commonly encountered in industry, and fluidized beds are particularly important due to their excellent heat and mass transfer characteristics. Here, we critically evaluate the differences between two modelling strategies, Euler-Euler and Euler-Lagrangian models. Euler-Euler simulations were performed using MFIX and an in-house code was used for Euler-Lagrangian simulations. A 2D bed of width, height and transverse thickness of respectively, 0.2 m, 0.5 m and 0.01 m, served as a test case. The settled bed height was H0 = 0.2 m. Particles of density ρ = 1000 kg/m³ and diameter dp = 1.2 mm were fluidized with air. The drag-law proposed by Benyahia et al. (10) was used in both models. Comparison between the simulation results was based on both instantaneous and time-averaged properties. A particular focus of this study was the influence of the coefficients of restitution and friction on the simulation results

Comparison between two-fluid model simulations and particle image analysis & velocimetry (PIV) results for a two-dimensional gas-solid fluidized bed

This work compares simulation and experimental results of the hydrodynamics of a two-dimensional, bubbling air-fluidized bed. The simulation in this study has been conducted using an Eulerian–Eulerian two-fluid approach based on two different and well-known closure models for the gas–particle interaction: the drag models due to Gidaspow and Syamlal & O'Brien. The experimental results have been obtained by means of Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV) techniques applied on a real bubbling fluidized bed of 0.005 m thickness to ensure its two-dimensional behaviour. Several results have been obtained in this work from both simulation and experiments and mutually compared. Previous studies in literature devoted to the comparison between two-fluid models and experiments are usually focused on bubble behaviour (i.e. bubble velocity and diameter) and dense-phase distribution. However, the present work examines and compares not only the bubble hydrodynamics and dense-phase probability within the bed, but also the time-averaged vertical and horizontal component of the dense-phase velocity, the air throughflow and the instantaneous interaction between bubbles and dense-phase. Besides, quantitative comparison of the time-averaged dense-phase probability as well as the velocity profiles at various distances from the distributor has been undertaken in this study by means of the definition of a discrepancy factor, which accounts for the quadratic difference between simulation and experiments The resulting comparison shows and acceptable resemblance between simulation and experiments for dense-phase probability, and good agreement for bubble diameter and velocity in two-dimensional beds, which is in harmony with other previous studies. However, regarding the time-averaged velocity of the dense-phase, the present study clearly reveals that simulation and experiments only agree qualitatively in the two-dimensional bed tested, the vertical component of the simulated dense-phase velocity being nearly an order of magnitude larger than the one obtained from the PIV experiments. This discrepancy increases with the height above the distributor of the two-dimensional bed, and it is even larger for the horizontal component of the time-averaged dense-phase velocity. In other words, the results presented in this work indicate that the fine agreement commonly encountered between simulated and real beds on bubble hydrodynamics is not a sufficient condition to ensure that the dense-phase velocity obtained with two-fluid models is similar to that from experimental measurements on two-dimensional bedsThis work has been partially funded by the Spanish Government (ProjectDPI2009-10518) and the Autonomous Community of Madrid (ProjectS2009/ENE-1660). Their supports are greatly appreciatedPublicad

Reversal of gulf stream circulation in a vertically vibrated triangular fluidized bed

Vibrated fluidized beds are a process intensification technique consisting in introducing vibratory kinetic energy in a fluidized bed (1). In this work we assess experimentally the effect of vibration on the gulf-stream circulation pattern of particles in a fluidized bed that is of triangular shape. The bed has 0.206 m span and 0.01 m thickness. The base of the bed is composed of two inclined walls, each one forming an angle of 45º with the horizontal. Air was injected through the inclined bed walls to fluidize the bed (see Figure 1a). This gas injection, together with vibration, can make the dynamics of this bed different to that found in a spouted fluidized bed (2). The bed is filled with ballotini particles with a mean diameter of 1.15 mm up to the top of the inclined walls. The bed vessel is made of antistatic PMMA to allow optical access with a high-speed camera. The bed was mounted on an electrodynamic shaker which produces the vibration. A high speed camera is used to record the motion of particles. The particle velocity was obtained via Particle Image Velocimetry (PIV). As a function of vibration amplitude and frequency, we observe several circulation patterns when the fluidization velocity is just below and above the minimum fluidization velocity. Noticeably, for zero gas velocity, particles ascend close to the side walls descend in the center of the bed. By injecting fluidization gas, the circulation pattern of the bed could be reversed (i.e. particles descending near the side walls ascend in the center of the bed) for certain conditions. For example, reversal of the gulf stream circulation of particles appeared in the triangular bed for gas superficial velocities higher than the minimum fluidization velocity and sufficiently high values of the vibration strength. This phenomenon is illustrated in Figure 1b in which, for the same vibrating conditions, the injection of gas superficial velocity through the walls reverses the gulf stream motion of particles in the bed. REFERENCES R. Gupta, A.S. Mujumdar, Hydrodynamic of vibrated fluidized bed, Can. J. Chem. Eng., 58:332-338, 1980. Vinayak S. Sutkar, Niels G. Deen, J.A.M. Kuipers, Spout fluidized beds: Recent advances in experimental and numerical studies, Chem. Eng. Sci., 86:124:136, 2013. Please click Additional Files below to see the full abstract

Gas interchange between bubble and emulsion phases in a 2D fluidized bed as revealed by two-fluid model simulations

Using two-fluid model simulations, the present work aims at characterizing the interchange due to gas advection between the emulsion phase and bubbles in fully bubbling beds of Geldart group B particles that are fluidized with air. In the studied beds the bubbles are slow, which means that the advection transport of gas through the bubble boundary is the main mechanism of gas interchange. In an initial verification step, the pressure distribution and the gas interchange coefficient for isolated bubbles obtained in the two-fluid simulation are compared with the classical potential flow theory of fluidized beds, providing concordant results. In a second step, the work analyzes the gas interchange in fully bubbling beds and the effects of the superficial velocity, bed height, and particle diameter on the interchange coefficient and the crossflow ratio. The results indicate that both the interchange coefficient and the crossflow ratio in bubbling beds are about two times those predicted by the potential theory of isolated bubbles. A corrected model for the gas interchange is proposed based on the introduction of the gas throughflow into the classical potential flow theory. As a consequence, the gas interchange coefficient in the corrected model is a function of the superficial gas velocity instead of the minimum fluidization velocity.This work has been partially funded by the Spanish Government (Project DPI2009-10518) and the Autonomous Community of Madrid (Project S2009/ENE-1660).Publicad

Statistical accuracy of scattered points filters and application to the dynamics of bubbles in gas-fluidized beds

A novel analytical equation for the assessment of the accuracy of filters used for the interpolation and differentiation of scattered experimental data is presented. The equation takes into account the statistical nature of the filter output resulting from both the arbitrary positions of the data points and the randomness and noise present in the experimental data. Numerical estimation of the accuracy of the filter, using a Monte Carlo procedure, shows good agreement with the deduced analytical equation. This numerical procedure was also used to determine the accuracy of variance filters aimed at calculating the mean-square fluctuation of experimental data. The combination of the numerical results and analytical equations reveals the exact sources of inaccuracy arising in scattered point filters, namely: (i) the spectral inaccuracy of the weighting function; (ii) the noise or stochastic signal amplification; and (iii) the error arising from the random collocation of points within the filter window. The results also demonstrate that the use of the local mean in the calculation of the quadratic fluctuation leads to smaller estimation errors than the central mean. Finally, all these filters are used and critically evaluated in the framework of the stochastic position, diameter, and velocity of bubbles in a gas-fluidized bed. It is shown that the empirical coefficient of bubble coalescence in the two-dimensional bed tested, $\bar {\lambda }$ , is in the range 2.0-2.4 when incorporating only the visible flow of bubbles. Here, the vertical distance over which a bubble survives without coalescing is $\bar {\lambda } {L}_{c}$ , where ${L}_{c}$ is the characteristic separation between neighbouring bubbles in the horizontal direction prior to coalescence. It was also seen that the relative mean-square-root fluctuation of both bubble diameter and velocity is more than 50 % at the centre of the bed and remains nearly constant along the height of the be