Fast bubbles

Additionally, macroscopic flow structure of 3-D bubble columns were studied [10]. The results reported can be resumed as follows (a) In disperse regime, the bubbles rise linearly and the liquid flow falls downward between the bubble stream, (b) If gas velocity increases, the gas-liquid flow presents a vortical-spiral flow regime. Then, cluster of bubbles (coalesced bubbles) forms the central bubble stream moving in a spiral manner and 4-flow region can be identified (descending, vortical-spiral, fast bubble and central flow region). Figure 10 shows an illustrative schemes of the results found in [10]. 

Pass an excess of gas upward through a bed of fine particles. With a large enough bed diameter we get a freely bubbling bed of fast bubbles. As simplifications, assume the following ... 

We ignore the flow of gas through the cloud since the cloud volume is very small for fast bubbles. 

In effect, we consider the emulsion gas as stagnant. Of course more general expressions can be developed for beds where bubbles have thick clouds (not too large and fast bubbles), or where flow through the emulsion is significant (wq close to thus where = 1-2 u f). However, for fast bubble, vigorously bubbling fine particle beds the above assumptions are reasonable. 

Now see whether the fast bubble model of this chapter applies. 

Check the fast bubble assumption Take the velocity ratio... 

Since the bubble rises 25 times as fast as the emulsion gas we have a fast bubble with thin cloud—less than 1 cm thick. 

Most bubbles in gas-solid fluidized beds are of spherical cap or ellipsoidal cap shape. Configurations of two basic types of bubbles, fast bubble (clouded bubble) and slow bubble (cloudless bubble), are schematically depicted in Fig. 9.7. The cloud is the region established... 

Figure 9.7. Bubble configurations and gas flow patterns around a bubble in gas-solid fluidized beds (a) Fast bubble (clouded bubble) Ub > /mf/ mf (b) Slow bubble (cloudless bubble)...

Davidson and Harrison (1963) expressed the total interchange coefficient for mass transfer from the bubble to the emulsion, K, by using Eq. (12.84), which is reasonable for very fast bubbles with negligible cloud. For bubbles with a large cloud, the cloud-emulsion mass transfer coefficient, Kce, should also be considered, as indicated in Eq. (12.77). 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

The gas flow through the bubbles depends on whether the bubble rise velocity, 0.7l/g d (29) is slow or fast relative to the gas velocity in the emulsion phase (umf/emf). When the bubble rise velocity exceeds the emulsion velocity, the bubbles are said to be fast or clouded and the gas circulates through a cloud surrounding the bubble as shown schematically in Figure 5 (adapted from Ref. 32). When the emulsion velocity is fast relative to the bubble rise velocity the bubbles are said to be slow or cloudless and the emulsion gas uses the bubble as a by-pass (see Figure 5). The transition from slow to fast bubble depends upon the... 

Figure 5. The dependence of the transition from the slow to fast bubble regime on bubble diameter dg and minimum fluidization velocity umf.

ASTM D3825-90(2000). Standard test method for dynamic surface tension by the fast-bubble technique. 

B 100-800 intermediate size e.g., sand bubble at incipient fluidization viz., umb/umf = 1 collapse immediately upon shutoff of gas flow bubbles have clouds fast bubbles ... 

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. ... 

The celebrated Kunii-Levenspiel [78, 79, 81, 82] (p 289) reactor model presented in this section was designed for the particular case of fast bubbles in vigorously bubbling beds which is relevant for industrial applications with Geldart A and AB solids. In such beds there are definite gross mixing patterns for the solid, downward near the wall and upward in the central core. This has a marked effect on the gas flow in the emulsion phase, which is also forced downward near the wall. However, based on experimental data analysis, Kunii... 

The volume fraction of the bed consisting of bubbles fb is determined by the prevailing flow regime. For fast bubbles in vigorously bubbling beds, where Ub > Uf /Emf and the clouds are thin and one may use the... 

The time for bubble growth to a hemisphere t, is the difference between the total bubble time Tb and the dead time x, p and L are pressure and gas flow rate and p and are the respective values at a critical point. From considerations discussed above about adsorption processes at the surface of growing drops it is concluded that the situation with the growing bubble is comparable, at least until the state of the hemisphere. Then, the process runs without specific control and leads to an almost bare residual bubble after detachment due to the very fast bubble growth. 

Let us first check where we are with respect to the Geldhart classification of Figure 8.4. The values of dp and (p, — pfj given as data land us well into the A region of Geldhart A particles, so the parameter correlations presented above should be valid. Further mq = 16M ,y, and 90 u f (shown subsequently), so that we are dealing with a fine-particle bed, fast bubbles, and a thin cloud phase. From equation (8-52),... 

Fast bubble Thin cloud Bubble-cloud isolated from emulsion... 

Features Fast bubble-break does not separate or settle easy to incorp. Regulatory AlCS listed... 

Uses Defoamer for high-gloss paints/coatings (acrylic, styrene acrylic, vinyl acrylic latex), wh. bases and pastes Features Fast bubble-break does not separate or settle easy to incorp. ... 

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