Minimum bubbling velocity

A simple but subjective way of determining the minimum bubbling velocity is by visual observation. By this method, the velocities are noted at which the first distinct bubbles appear when the gas flow is gradually increased and at which bubbling ceases when the gas flow is decreased. The average of the two values may be taken or, alternatively, the values obtained during decreasing flow may be more appropriate. 

Make sure not to mistake the tiny fissureis of volcanoes which appear on the surface just below uMB for genuine bubbles bubbles generally pop in several places on the surface and are about 1 cm in diameter whereas the volcanoes usually stay in one place (at a given superficial gas velocity) and can be seen as semi-permanent channels, a few millimeters in diameter. 

The minimum bubbling velocity can be found less subjectively, according to Geldart66, when using bed depths greater than about 

The intersection of the curves below and above MB, as shown in Fig. 37, give MB reproducibly and usually coincides with visual observations. At low bed heights, however, the value determined visually is less than MB found from the graph. 

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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. 

Minimum Bubbling Velocity (Umb). The velocity at which discrete bubbles begin to form. Typical minimum bubbling velocity for an FCC catalyst is... 

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. 

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. 

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. 

The upper limit of gas velocity for particulate expansion is termed the minimum bubbling velocity, umb. Determining this can present difficulties as its value may depend on the nature of the distributor, on the presence of even tiny obstructions in the bed, and even on the immediate pre-history of the bed. The ratio umb/umf, which gives a measure of the degree of expansion which may be effected, usually has a high value for fine light particles and a low value for large dense particles. 

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... 

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.

At this point, it is important to know if the operating gas superficial velocities, which are from 2.5-15 cm/s, are higher than the minimum bubbling velocity and lower than the minimum slugging velocity, respectively. 

Since, the particle size used is greater than 45 pm, the value of (X45) is zero. The resulting minimum bubbling velocity is less than the operating velocity (1 cm/s). Thus, the operating regime is purely bubbling fluidization. 

Ordinarily under practical conditions the flow rate is at most a few multiples of the minimum fluidizing velocity so the local maximum bed level at the minimum bubbling velocity is the one that determines the required vessel size. The simplest adequate equation that has been proposed for the ratio of voidages at minimum bubbling and fluidization is... 

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