Group B powders

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... 

Bubbles can grow to on the order of a meter in diameter in Group B powders in large beds. The maximum stable bubble size is limited by the size of the vessel or the stabiUty of the bubble itself. In large fluidized beds, the limit to bubble growth occurs when the roof of the bubble becomes unstable and the bubble spHts. EmpidcaHy, it has been found that the maximum stable bubble size may be calculated for Group A particles from... 

Abrahamsen and Geldart (1980) defined Group A powders as those in which UmbIUm > 1, and Group B powders as those where Umb mf = 1 They developed the following equation to predict UmbIUm. ... 

Figure 12. The effect of pressure on the overall heat transfer coefficient for group B powders. (Xavier Davidson.)...

Group B powders can be troublesome (e g., severe pipe vibrations) if high solids/gas loadings are contemplated. 

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... 

Group B powders are characterised by having nmb = umf. Bubbles rise faster than the interstitial gas velocity, coalescence is the dominant phenomenon and there is no evidence of a maximum bubble size, as defined for Group A materials. Bubble size increases with increasing fluidising gas velocity, see Figure 14. The interparticle forces are considered to be negligible for these powders. 

Figure 14 Fluidisation behaviour of a Group B powder effect of gas flow on bubbles in a two-dimensional fluidised bed, (i) lower gas velocity, (ii) higher gas velocity. A max. stable bubble size is never achieved... 

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. ... 

Group B powders have a higher density and/or particle size compared to Group A powders. Sand is a typical example of a Group B powder. 

Behavior of bubbles for a typical fluidized Group B powder (Geldart s classification). 

Strictly, the above equations should be written in terms of M b rather than u i, and Qn,b rather than Q f, so that they are valid for both Group A and Group B powders. Practically, however, it makes little difference, since both M,nb arid u f are usually much smaller than the superficial fluidizing velocity u and, therefore u - u f) u- u ). 

Figure 7.3 Sequence showing bubbles in a two-dimensional fluidized bed of Group B powder. Sketches taken from video...

Botterill (1986) recommends the Zabrodsky (1966) correlation for hmax for Group B powders ... 

What differentiates a Geldart Group A powder from a Geldart Group B powder ... 

Geldart (1972) found that the fluidization behavior of Group B powders was independent both of the mean particle size and of particle size distribution. In particular, the mean bubble size was found to depend only on the type of the distributor, the distance above the distributor plate, and the excess gas velocity above that required at the minimum fluidization condition, U - U t- Mathematically, it can be expressed as... 

The constant 0.027 is a dimensional constant with units of (cm/s) ". Equation (113) gives reasonable agreement with data from fluidized beds using industrial types of orifice distributor plates. Porous distributor plates, as expressed in Eq. (112), behave as though they contained approximately 1 hole per 10 cm of bed area. The principal effect of adding fines to a fluidized bed of group B powders is the reduction of the mean particle size. At equal values of excess (U — U f), this results in increased bed expansion and solid circulation rates but produces no decrease in mean bubble size. 

Baeyens and Geldart (1973) also studied the solid circulation and found that for most Group B powders, the wake fraction, p , had an average of about 0.35... 

A considerable number of studies have been reported on jet penetration, and several correlations have been developed giving the penetration length as a function of the physical properties of gas and particles and of operating conditions (Knowlton and Hirsan, 1980). However, most of these correlations were produced from experimental data obtained at ambient conditions, and they fail when applied at elevated temperatures and pressures. Hirsan et al. (1980) measured jet penetrations in beds of Group B powders at pressures of up to 50 bar and produced a correlation for Ljnax (FiE- 5) in terms of the Froude number and the ratio of fluid to particle density ... 

The deviation from the two-phase theory is thus a function of the ratio of the minimum bubbling velocity to the minimum fluidization velocity, which, as we saw earlier, can be used as a criterion to differentiate group A powders from group B powders. This correlation indicates that there will be almost no deviation from the two-phase theory for group B powders. 

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