Fluidization bubble velocity

Bubbles and Fluidized Beds. Bubbles, or gas voids, exist in most fluidized beds and their role can be important because of the impact on the rate of exchange of mass or energy between the gas and soflds in the bed. Bubbles are formed in fluidized beds from the inherent instabiUty of two-phase systems. They are formed for Group A powders when the gas velocity is sufficient to start breaking iaterparticle forces at For Group B powders, where iaterparticle forces are usually negligible, and bubbles form immediately upon fluidization. Bubbles, which are inherently... 

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. 

Figure 30. Particle size effects on minimum fluidization and bubbling velocities.

Cocurrent three-phase fluidization is commonly referred to as gas-liquid fluidization. Bubble flow, whether coeurrent or countereurrent, is eonveniently subdivided into two modes mainly liquid-supported solids, in which the liquid exeeeds the minimum liquid-fluidization veloeity, and bubble-supported solids, in whieh the liquid is below its minimum fluidization velocity or even stationary and serves mainly to transmit to the solids the momentum and potential energy of the gas bubbles, thus suspending the solids. 

The flow pattern of gas within the emulsion phase surrounding a bubble depends on whether the bubble velocity Ug is less than or greater than minimum fluidization velocity U . For Ubflow lines. For Ub> U , the much different case of Figure 4(B) results. Here a gas element which leaves the bubble eap rises much more slowly than the bubble, and as the bubble passes, it remms to the base of the bubble. Thus, a cloud of captive gas surrounds a bubble as it rises. The ratio of eloud diameter to bubble diameter may be written... 

Measurements in large fluidized beds of fine particles indicate that bubble coalescence often ceases within a short distance above the gas distributor plate. Indications from density measurements or single bubble velocities are that bubble velocity Ug and diameter often reach maximum stable values, which are invariant with height or fluidizing gas velocity. 

Ratio of Minimum Bubbling Velocity to Minimum Fluidization Velocity (Umb/Umf). This ratio can be calculated as follows ... 

Cracking catalysts are members of a broad class characterized by diameters of 30-150 im, density of 1.5 g/mL or so, appreciable expansion of the bed before fluidization sets In, minimum bubbling velocity greater than minimum fluidizing velocity, and rapid disengagement of bubbles. 

Rough correlations have been made of minimum fluidization velocity, minimum bubbling velocity, bed expansion, bed level fluctuation, and disengaging height. Experts recommend, however, that any real design be based on pilot plant work. 

Fig. 6 shows the FFT spectrum for calculated bed pressure drop fluctuations at various centrifugal accelerations. The excess gas velocity, defined by (Uo-U ,, was set at 0.5 m/s. Here, 1 G means numerical result of particle fluidization behavior in a conventional fluidized bed. In Fig. 6, the power spectrum density function has typical peak in each centrifugal acceleration. However, as centrifugal acceleration increased, typical peak shifted to high frequency region. Therefore, it is considered that periods of bubble generation and eruption are shorter, and bubble velocity is faster at hi er centrifugal acceleration. 

Glicksman and McAndrews (1985) determined the effect of bed width on the hydrodynamics of large particle bubbling beds. Sand particles with a mean diameter of 1 mm were fluidized by air at ambient conditions. The bed width ranged from 7.6 cm to 122 cm while the other cross sectional dimension remained constant at 122 cm. Most experiments were carried out with an open bed. The bubble rise velocity increased with the bed width, in the representation of bubble velocity as... 

Group A powders show a limited tendency to form bubbles and generally exhibit considerable bed expansion between the minimum fluidization velocity Vmp and the minimum bubbling velocity Vmb. These powders also retain aeration and the fluidized bed collapses very slowly when the gas is turned off. 

Group B materials fluidize readily and tend to form bubbles, which grow rapidly by coalescence. However, bed expansion is small. That is, the minimum bubbling velocity, Vmb, usually is approximately equal to (or only slightly greater than) the minimum fluidizing velocity Vmj, The fluidized bed does not retain its aeration and collapses quickly when the gas supply is removed. 

A one-parameter model, termed the bubbling-bed model, is described by Kunii and Levenspiel (1991, pp. 144-149,156-159). The one parameter is the size of bubbles. This model endeavors to account for different bubble velocities and the different flow patterns of fluid and solid that result. Compared with the two-region model, the Kunii-Levenspiel (KL) model introduces two additional regions. The model establishes expressions for the distribution of the fluidized bed and of the solid particles in the various regions. These, together with expressions for coefficients for the exchange of gas between pairs of regions, form the hydrodynamic + mass transfer basis for a reactor model. 

Two-Phase Theory of Fluidization The two-phase theory of fluidization assumes that all gas in excess of the minimum bubbling velocity passes through the bed as bubbles [Toomey and Johnstone, Chem. Eng. Prog. 48 220 (1952)]. In this view of the fluidized bed, the gas flowing through the emulsion phase in the bed is at the minimum bubbling velocity, while the gas flow above U j, is in the bubble phase. This view of the bed is an approximation, but it is a helpful way... 

Particulate Fluidization Fluid beds of Geldart group A powders that are operated at gas velocities above the minimum fluidizing velocity (U ) but below the minimum bubbling velocity (U ) are said to be particulately fluidized. As the gas velocity is increased above Umf, the bed further expands. Decreasing (ps - py), dp and/or increasing if increases the spread between Umf and U. Richardson and Zaki [Trans. Inst. Chem. Eng., 32, 35 (1954)] showed that U/Ut = n, where n is a function of system properties, 8 = void fraction, U = superficial fluid velocity, and Ut = theoretical superficial velocity from the Richardson and Zaki plot when 8 = 1. 

Minimum bubbling velocity timb is defined as the gas velocity at which bubbles first appear in aggregative fluidization. For coarse uniformly-sized particles, for example those in Geldart group B, it is usually the case that M i, = u /- However, very fine non-uniformly sized particles such as those in group A exhibit smooth bed expansion and no bubbling until a gas velocity considerably in excess of the minimum... 

Group B powders They give only bubbling fluidization. Bubbles are formed as soon as the gas velocity exceeds the minimum fluidization velocity ( bm = u(n). Most particles... 

The minimum bubbling velocity for Group A particles (or more generally, for Type A fluidization) and gas-solid systems is (Abrahamsen and Geldart, 1980 Ye et al., 2005)... 

Using the Bayens equation (3.460) and eq. (3.481), the ratio of minimum bubbling velocity to minimum fluidization velocity is... 

Figure 3.54 Ratio of minimum bubbling velocity to minimum fluidization velocity. For dp > 0.045 mm, X4S = 0 and for df < 0.045 mm, X45=l is assumed.

Bubble velocity and diameter In the context of the bubbling bed theory, the rise velocity of a single bubble in a fluidized bed is given by (Wen, 1984)... 

This equation has been deduced from studies conducted with bed diameters of 7.6-130 cm, minimum fluidization velocities of 0.5-20 cm/s, solid particle sizes of 0.006-0.045 cm, and us - wfm< 48 cm/s. To calculate an average value of the bubble velocity, an average bubble diameter should be used. This diameter can be taken to be equal to the bubble diameter at z = Hfl2. Thus, to calculate the bubble diameter and thus the bubble velocity, the fluidized bed height should be known. To solve the problem, an iteration method should be used (Figure 3.60). 

Daiton el al. (1977) and Werther (1983) presented different relationships for bubble diameter and bubble velocity for Group A and Group B particles (for bubbling fluidization). The mean rise velocity of a bubble in the bed ( bljb) can also be evaluated using the following equations, which include a wall effect collection (Darton et al., 1977 Werher, 1983 Wen, 1984). 

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