Incipient bubbling velocity

To test the viability of in quantifying fluidizing characteristics, it is plotted against the ratio of incipient bubbling velocity to incipient fluidization velocity, the latter being calculated after Geldart. Figure 69 shows... 

Since rising gas bubbles are replaced with particulate solids from the bed, the superficial gas velocity minus the incipient bubbling velocity approximates the volumetric bulk solids movement of any unit bed cross section per unit time. This amounts to a substantial mass movement. Therefore, it is not surprising that a fluidized bed exhibits a rather uniform particle size distribution and bed temperature throughout. 

Figure 172. Influence of particle size on incipient buoyancy and incipient bubbling velocities ...

Fluidization may be described as incipient buoyancy because the particles are still so close as to have essentially no mobility, whereas the usual desire in fluidization is to create bed homogeneity. Such homogeneity can be achieved only by violent mixing. This is brought about by increasing the fluid velocity to the point of blowing "bubbles" or voids into the bed, which mix the bed as they rise. The increased fluid velocity at which bubbles form first is referred to as the incipient (or minimum) bubbling velocity. 

Group A represents the best powders. They are fine and aeratable, such as cracking catalyst. They fluidize nicely, and expand particulately after reaching the point of incipient fluidization mf, until the first bubbles appear at a higher velocity, the minimum bubbling velocity umb. It is thus evident that the ratio of umJum is always greater than unity, and the greater is this value, the better the powder performs in fluidization. 

Group B powders are of intermediate particle size, such as sand. They do not fluidize so smoothly as Group A, for bubbles form as soon as the incipient fluidization velocity is reached. It is thus evident that the ratio umJum is equal to unity. When the fluidizing gas is turned off suddenly, Group B powders would collapse immediately. In the fluidized state, the rising bubbles travel upward faster than the interstitial gas flow rate, and are therefore designated as fast bubbles. ... 

In fluidized beds, bubbles form at the distribution plate or grid ports where fluidizing gas enters the bed (Figure 169).They form because the gas velocity at the interface to the bed represents an input rate that is larger than what can pass through the interstices with less frictional resistance than the bed weight. Therefore, holes are formed through whose porous surface the gas can enter the bed at the incipient fluidization velocity. This means that bubble formation is a way to increase the active interface between the gas and the particle bed. 

Particulate solid matter can exist over a wide range of bulk densities and, therefore, exhibits substantial differences between incipient buoyancy and incipient bubbling. This is illustrated qualitatively in Figure 172 the curves for Dpi and Dp2 represent the most typical shape of fluidization curves (bed expansion versus gas flow rate). Because there is no way to predict precisely a powder s range of bulk densities and the optimum operating gas velocity does not have narrow limits, it is always safe to select a higher gas velocity if in... 

Observations of bubbles emerging through the bed surface show that bubble shape is markedly dependent on liquid velocity. This indicates the existence of a relationship between bed viscosity and liquid velocity. A bed near incipient fluidization is characterized by a high viscosity, and an emerging bubble is of nearly spherical shape, whereas a fluidized bed of high porosity is characterized by a viscosity not very much higher than that of water, so that an emerging bubble is of spherical cap shape. 

The same conclusion is evident from results obtained by Hino and Ueda (1975) and presented above in Fig. 6.4. The conclusion that A7s is almost unaffected by inlet flow velocity as at 7) -C 1 as at Z) < 1 was established from experiments carried out in the channels of diameters about d = 1—10 mm. What has been commonly observed at incipient boiling for subcooled flow in channels of this size is that small bubbles nucleate, grow and collapse while still attached to the wall, as a thin bubble layer formed along the channel wall. 

The velocity at which gas flows through the dense phase corresponds approximately to the velocity that produces incipient fluidization. The bubbles rise, however, at a rate that is nearly an order of magnitude greater than the minimum fluidization velocity. In effect, then, as a consequence of the movement of solids within the bed and the interchange of fluid between the bubbles and the dense regions of the bed, there are wide disparities in the residence times of various fluid elements within the reactor and in... 

For G/S systems, on the other hand, especially when relatively coarse solid particles with close size distributions are used, fluid velocity increment beyond incipient fluidization is accompanied by the formation of bubbles, or rising cavities with hardly any solid particles in them, as shown in the top row of Fig. 3. In general, gas flow beyond incipient fluidization mostly reports to bubble flow, thus implying that gas velocity through the surrounding dense... 

At incipient fluidization, e = 0 and the volume fraction for bubbles (e — eQ) diminishes to zero, and so does the correction factor at the particle terminal velocity, = 1, and the volume fraction of clusters (1 — e) diminishes to zero, and so does the correction factor. 

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