Particle pressures generated around bubbles in gas-fluidized beds

2002 ◽  
Vol 455 ◽  
pp. 103-127 ◽  
Author(s):  
KHURRAM RAHMAN ◽  
CHARLES S. CAMPBELL
Keyword(s):  
Fluidized Bed ◽  
Bubble Size ◽  
Fluidized Beds ◽  
Gas Flow ◽  
Solid Particles ◽  
Particle Phase ◽  
Fluid Mixture ◽  
Maximum Value ◽  
Particle Pressure ◽  
Gas Fluidized Beds

The particle pressure is the surface force in a particle/fluid mixture that is exerted solely by the particle phase. This paper presents measurements of the particle pressure on the faces of a two-dimensional gas-fluidized bed and gives insight into the mechanisms by which bubbles generate particle pressure. The particle pressure is measured by a specially designed ‘particle pressure transducer’. The results show that, around single bubbles, the most significant particle pressures are generated below and to the sides of the bubble and that these particle pressures steadily increase and reach a maximum value at bubble eruption. The dominant mechanism appears to be defluidization of material in the particle phase that results from the bubble attracting fluidizing gas away from the surrounding material; the surrounding material is no longer supported by the gas flow and can only be supported across interparticle contacts which results in the observed particle pressures. The contribution of particle motion to particle pressure generation is insignificant.The magnitude of the particle pressure below a single bubble in a gas-fluidized bed depends on the bubble size and the density of the solid particles, as might be expected as the amount of gas attracted by the bubble should increase with bubble size and because the weight of defluidized material depends on the density of the solid material. A simple scaling of these quantities is suggested that is otherwise independent of the bed material.In freely bubbling gas-fluidized beds the particle pressures generated behave differently. Overall they are smaller in magnitude and reach their maximum value soon after the bubble passes instead of at eruption. In this situation, it appears that the bubbles interact with one another in such a way that the de uidization effect below a leading bubble is largely counteracted by refluidizing gas exiting the roof of trailing bubbles.

Particle pressures in gas-fluidized beds

1991 ◽  
Vol 227 ◽  
pp. 495-508 ◽  
Author(s):  
Charles S. Campbell ◽  
David G. Wang
Keyword(s):  
Fluidized Bed ◽  
Bubble Size ◽  
Fluidized Beds ◽  
Particle Density ◽  
Surface Force ◽  
Gas Velocity ◽  
Fluid Forces ◽  
Minimum Fluidization ◽  
Particle Pressure ◽  
Gas Fluidized Beds

The particle pressure is the surface force that is exerted due to the motion of particles and their interactions. This paper describes measurements of the particle pressure exerted on the sidewall of a gas-fluidized bed. As long as the bed remains in a packed state, the particle pressure decreases with increasing gas velocity as progressively more of the bed is supported by fluid forces. It appropriately reaches a minimum fluidization and then begins to rise again when the bed is fluidized, reflecting the agitation of the bed by bubbles. In this fully fluidized region, the particle pressure scales with the particle density and the bubble size.

Expulsion of particles from a buoyant blob in a fluidized bed

1994 ◽  
Vol 278 ◽  
pp. 63-81 ◽  
Author(s):  
G. K. Batchelor ◽  
J. M. Nitsche
Keyword(s):  
Fluidized Bed ◽  
Fluidized Beds ◽  
Particle Concentration ◽  
Small Concentration ◽  
Numerical Calculations ◽  
Significant Feature ◽  
Average Particle ◽  
Remarkable Property ◽  
Gas Fluidized Beds ◽  
Secondary Instabilities

It is a significant feature of most gas-fluidized beds that they contain rising ‘bubbles’ of almost clear gas. The purpose of this paper is to account plausibly for this remarkable property first by supposing that primary and secondary instabilities of the fluidized bed generate compact regions of above-average or below-average particle concentration, and second by invoking a mechanism for the expulsion of particles from a buoyant compact blob of smaller particle concentration. We postulate that the rising of such an incipient bubble generates a toroidal circulation of the gas in the bubble, roughly like that in a drop of liquid rising through a second liquid of larger density, and that particles in the blob carried round by the fluid move on trajectories which ultimately cross the bubble boundary. Numerical calculations of particle trajectories for practical values of the relevant parameters show that a large percentage of particles, of such small concentration that they move independently, are expelled from a bubble in the time taken by it to rise through a distance of several bubble diameters.Similar calculations for a liquid-fluidized bed show that the expulsion mechanism is much weaker, as a consequence of the larger density and viscosity of a liquid, which is consistent with the absence of observations of relatively empty bubbles in liquid-fluidized beds.It is found to be possible, with the help of the Richardson-Zaki correlation, to adjust the results of these calculations so as to allow approximately for the effect of interaction of particles in a bubble in either a gas- or a liquid-fluidized bed. The interaction of particles at volume fractions of 20 or 30 % lengthens the expulsion times, although without changing the qualitative conclusions.

Effect of the Shape of the Baffles on the Hydrodynamics of a Fluidized Bed

2008 ◽  
Vol 6 (1) ◽  
Author(s):  
Srinivasa Rao Venkata Naga Kaza
Keyword(s):  
Fluidized Bed ◽  
Fluidized Beds ◽  
Gas Flow ◽  
Experimental Studies ◽  
Expansion Ratio ◽  
Blockage Ratio ◽  
Minimum Fluidization Velocity ◽  
Fluidization Velocity ◽  
Minimum Fluidization ◽  
Bubbling Fluidized Bed

Gas flow in a gas–solid fluidized bed is characterized by the predominance of bubbles. When gas flow is more than the minimum fluidization velocity, the top of the fluidized bed may fluctuate vigorously leading to unstable operation. Bed fluctuation and fluidization quality are interrelated. The quality of fluidization can largely be improved by introducing baffles in bubbling and turbulent fluidized beds. In the present work three baffle geometries, i.e., circular, triangular and square are used to determine different hydrodynamic parameters such as minimum fluidization velocity, bed expansion, pressure drop across the bed, fluctuation ratio, expansion ratio, etc. in a bubbling fluidized bed. A new parameter blockage ratio is introduced to analyze the behaviour of baffled fluidized beds. It is found from the current experimental studies that the blockage ratio mainly influences the hydrodynamics of the bed rather than the shape of the baffle.

scholarly journals Bubble size and frequency in gas fluidized beds.

1988 ◽  
Vol 21 (2) ◽  
pp. 171-178 ◽  
Author(s):  
JEONG H. CHOI ◽  
JAE E. SON ◽  
SANG D. KIM
Keyword(s):  
Bubble Size ◽  
Fluidized Beds ◽  
Gas Fluidized Beds

Three-Phase Model of a Fluidized-Bed Catalytic Reactor for Polyethylene Synthesis

2016 ◽  
Vol 14 (1) ◽  
pp. 93-103 ◽  
Author(s):  
R. A. Bortolozzi ◽  
M. G. Chiovetta
Keyword(s):  
Gas Phase ◽  
Fluidized Bed ◽  
Solid Particle ◽  
Active Sites ◽  
Supported Catalysts ◽  
Plug Flow ◽  
Solid Particles ◽  
Particle Phase ◽  
Three Phase ◽  
Bubbling Fluidized Bed

AbstractA mathematical model of a bubbling fluidized-bed reactor for the production of polyolefins is presented. The model is employed to simulate a typical, commercial-scale reactor where the synthesis of polyethylene using supported catalysts is carried out. Results are used to follow the evolution of temperature within the reactor bed to avoid conditions producing polymer degradation. The fluidized bed is modeled as a heterogeneous system with a bubble gas phase and a solid-particle emulsion. The catalyst active sites are considered located within a growing, solid, ever changing particle composed of the support, the catalyst and the polymer being produced. The model sees the reactor as a three phase complex: (a) the bubble phase, transporting most of the gas entering the reactor; (b) the solid-particle phase, where polymerization takes place; and (c) the interstitial-gas phase among solid particles. Both gaseous phases move continuously upward, with different velocities, and are modeled as plug flows. For the solid-particle phase, modeling alternatives are explored, ranging from a descending plug-flow limiting case to the opposite extreme of a perfectly mixed tank related to the particle drag-effect the rising bubble produces in the bed. In the scouting process between these limits instabilities are predicted by the model. The most realistic representation of the bed is that of the two gas phases moving upward in two plug-flow patterns and the solids moving with ascending and descending trajectories due to back-mixing.

Prediction of the Flotsam Component in a Gas-Fluidized Bed of Two Dissimilar Solids

2012 ◽  
Vol 10 (1) ◽  
Author(s):  
Alberto Di Renzo ◽  
Francesco P. Di Maio ◽  
Vincenzino Vivacqua
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Computational Fluid Dynamics ◽  
Simple Model ◽  
Discrete Element Method ◽  
Fluidized Bed ◽  
Fluidized Beds ◽  
Computational Simulations ◽  
The Difference ◽  
Gas Fluidized Beds

In the present paper the segregating behaviour of solids of different size and density in gas-fluidized beds is studied. In particular, the attention is focussed on pairs composed of a bigger/less dense species and a smaller/denser species. Typical industrial examples of such combinations are encountered in fluidized beds of biomass/sand mixtures. Their behaviour is not easily predictable, as the segregation tendency promoted by the difference in density is counteracted by the difference in size. While typically the denser component is expected to appear predominantly at the bottom of the fluidized bed, experiments on mixtures exhibiting the reverse behaviour have been reported (e.g. Chiba et al., 1980).A simple model to predict the segregation direction of the components, i.e. which species will segregate to the top of the bed (the flotsam), depending upon their difference in properties (size, density) and the mixture composition, is discussed. The predicted behaviour is compared with experimental data available in the literature and agreement is found for the majority of them. For one mixture, experiments are conducted as well as computational simulations based on the combined Discrete Element Method and Computational Fluid Dynamics (DEM-CFD) approach. This allows investigating how an initially mixed bed upon suspension evolves as a result of the segregation prevalence in the bed.

Instabilities and the formation of bubbles in fluidized beds

1995 ◽  
Vol 303 ◽  
pp. 327-366 ◽  
Author(s):  
K Anderson ◽  
S. Sundaresan ◽  
R. Jackson
Keyword(s):  
Fluidized Beds ◽  
Equations Of Motion ◽  
Glass Beads ◽  
Solid Particles ◽  
The Other ◽  
Ambient Conditions ◽  
Small Perturbations ◽  
Gas Fluidized Beds ◽  
Direct Numerical Integration ◽  
Bed Expansion

As is well known, most gas-fluidized beds of solid particles bubble; that is, they are traversed by rising regions containing few particles. Most liquid-fluidized beds, on the other hand, do not. The aim of the present paper is to investigate whether this distinction can be accounted for by certain equations of motion which have commonly been used to describe both types of bed. For the particular case of a bed of 200 μm diameter glass beads fluidized by air at ambient conditions it is demonstrated, by direct numerical integration, that small perturbations of the uniform bed grow into structures resembling the bubbles observed in practice. When analogous computations are performed for a water-fluidized bed of 1 mm diameter glass beads, using the same equations, with parameters modified only to account for the greater density and viscosity of water and to secure the same bed expansion at minimum fluidization, it is found that bubble-like structures cannot be grown. The reasons for this difference in behaviour are discussed.

2D and 3D CFD Simulations of Bubbling Fluidized Beds Using Eulerian-Eulerian Models

2003 ◽  
Vol 1 (1) ◽  
Author(s):  
Luca Cammarata ◽  
Paola Lettieri ◽  
Giorgio D. M. Micale ◽  
Derek Colman
Keyword(s):  
Kinetic Model ◽  
Bubble Size ◽  
Fluidized Beds ◽  
Numerical Procedure ◽  
Cfd Simulations ◽  
3D Simulations ◽  
Gas Fluidized Beds ◽  
Group B ◽  
Particle Bed ◽  
2D And 3D

This paper reports on CFD simulations of freely bubbling gas fluidized beds using CFX-4, a commercial code developed by CFX Ltd. (formerly AEA Technology). Two Eulerian-Eulerian modelling approaches, the granular kinetic model and the particle-bed model (Gibilaro, 2001), have been investigated. The particle bed model has been recently implemented in CFX-4 for 2D simulations and a numerical procedure was developed to allow for a tight control of the fluid-bed voidage at maximum packing during the simulations, see Lettieri et al. (2003). The work has now been extended to 3D simulations and qualitative and quantitative results are presented in this paper for both the 2D and 3D simulations of the bubbling fluidization of a Geldart Group B material. Results on bed expansion, bubble size and bubble hold-up are reported. In particular, simulated bubble size is compared with predictions given by the Darton et al. (1977) equation at different bed heights. The paper shows that the bubble size predicted by both the granular kinetic model and the particle-bed model is in good agreement with the Darton's equation.

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