Fluidization wake region

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... 

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... 

Here, n is the Richardson-Zaki index, k is the ratio of wake size to bubble size, and x is the ratio of solid concentration in the wake region to that in the liquid-solid fluidized region. 

Figure 1. Slugging fluidized bed. (A) well-mixed wake region (B) piston-flow region.

The fluidizing (reactant) gas is in convective flow through the bed only via the bubble-gas region (with associated clouds and wakes) that is, there is no convective flow of gas through the emulsion region. 

Figure 9.8. (a) An NO2 bubble rising in a two-dimensional ballotini bed showing the cloud region of the bubble (b) A two-dimensional bubble rising through a layer of black particles showing the bubble wake in a gas-solid fluidized bed (from Rowe, 1971 reproduced with permission). 

As noted by the correlation of Kim et al.,58 any quantitative evaluation of the holdup characteristics of a three-phase fluidized bed must consider the phenomenon of bed contraction.137 The wakes of the bubbles rising in a three-phase fluidized bed consist of a particle-free liquid immediately below the bubble (commonly known as liquid wake ) and a lower region of particles and liquid which apparently moves with liquid (known as particulate region ). Particulate elutrition is caused by this particulate region, whereas bed contraction is caused by the liquid wakes. 

Since the liquid divides itself between the solids-free wake and the particulate region of the liquid-fluidized sohds. 

The bubble phase formed by gas in excess of that required for the onset of fluidization. The bubble is usually surrounded by a cloud of gas-soM mixture and is characterized by an indentation caused by suction due to the upward movement of the bubble. The solids that flU up this region are called the wake. The bubbles are usually large and move faster than the surrounding emulsion gas flowing at u p thus giving rise to the cloud. This behavior is usually characteristic of Geldart A and B particles (see Section 11.4.2.2). 

The foregoing observations can be interpreted in terms of the bubble behavior in fluidized beds. Werther and Molerus (1973) reported that very close to the distributor region, intensified bubble activity exists in an annular region near the wall. As bubbles detach and rise, they tend to move toward the center. If the bed is sufficiently deep, they will eventually merge at the center. Because the solids are carried upward in the wake of the bubbles, they basically move along the bubble tracks. Therefore, the solids would... 

Eq. (12) assumes that the liquid divides itself between the solids-free wakes and the particulate region of the fluidized bed. On the other hand, Eq. (13) considers that the interstitial velocity of the liquid can be represented by an equation similar to the one proposed by Richardson and Zaki [14] ... 

The New Model, Mechanistic Cyclic Form In this form we follow each slug, the wake of the slug, and the piston-flow region from formation at the bottom of the fluidized bed to the top of the bed where the slug bursts. Thus the steady-state is approached from an initial state in which the bed is fluidized with non-reacting gas and then reacting gas is admitted at zero time. Only the final steady-state solutions are presented here for comparison with the time-averaged version, the two-phase model of Hovmand and Davidson and the experimental data. 

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