Bubble region

A hydrodynamic model of fluidization attempts to account for several essential features of fluidization mixing and distribution of solids and fluid in a so-called emulsion region, the formation and motion of bubbles through the bed (the bubble region ), the nature of the bubbles (including their size) and how they affect particle motion/distribution, and the exchange of material between the bubbles (with little solid content) and the predominantly solid emulsion. Models fall into one of three classes (Yates, 1983, pp. 74-78) ... 

The discussion above suggests a hydrodynamic flow model based on two distinct regions in the fluidized bed a bubble region made up mostly of gas, but also containing solid... 

This model can have as many as six parameters for its characterization Kbe, Pe Pec, and ratios of volumes of regions, of solid in the regions, and of fluid in the regions. The number can be reduced by assumptions such as PF for the bubble region (Pe, - ), all solid in the emulsion, and all fluid entering in the bubble region. Even with the reduction to three parameters, the model remains essentially empirical, and doesn t take more detailed knowledge of fluidized-bed behavior into account. 

In terms of velocity, if we assume, for a vigorously bubbling bed with u. gas flows through the bed only in the bubble region (q = qh, or UjlAc - ft A, is the cross-sectional area of the bed), fl Mm/>that, ubAc, where... 

Note that the continuity equations for product B reflect the fact that B, formed in the cloud + wake and emulsion regions, transfers to the bubble region. This is in contrast to reactant A, which transfers from the bubble region to the other regions. 

The continuity equations 23.4-18 and - 19 are applicable. Thus Continuity equation for A in the bubble region ... 

The flow of gas through the bed (of cross-sectional area A,) is the sum of the flows through the emulsion and bubble regions ... 

Thus, the bubbling region, which is an important feature of beds operating at gas velocities in excess of the minimum fluidising velocity, is usually characterised by two phases — a continuous emulsion phase with a voidage approximately equal to that of a bed at its minimum fluidising velocity, and a discontinous or bubble phase that accounts for most of the excess flow of gas. This is sometimes referred to as the two-phase theory of fluidisation. 

Fig. 60. Regime diagram for A1203. 1. bubbling region 2. turbulent region 3. fast fluidization region 4. pneumatic transport region. [After Chen and Kwauk, 1985.]...

The structure of wakes behind the gas bubbles affects several aspects (such as holdup, gas-liquid mass transfer, etc.) of three-phase fluidized-bed behavior. The magnitude and composition of such wakes are still not known with any certainty. Wake holdups have been estimated from experimental measurements of gas and solid holdups. It is commonly assumed that the bed can be divided into three regions a liquid fluidized region, a gas-bubble region, and a bubble-wake region and that the bubbles and their wakes travel at the same velocity. Different investigators have, however, assumed different values of hws the ratio of solids holdup in the wake to the solids holdup in the liquid fluidized region. Different methods have been used to calculate wake holdups from the experimental... 

The heat transfer coefficient was found to be dependent on the existing flow pattern. In the isolated bubble region, lower q effect in confined bubble region, convection dominant in annular-slug region. 

On the basis of the generalized wake model of Bhatia and Epstein, " a criterion for the bed contraction was developed. In the generalized wake model, the three-phase fluidized bed is assumed fo consist of three regions, the gas bubble region, the wake region, and the liquid-solid fluidized region. Bed contraction will occur when the following criterion is satisfied, ij/ < 0 where... 

Partial Nucleate Boiling (Isolated Bubble) Region. In this region, heat transfer is often considered to be occurring by three separate mechanisms ... 

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