Incipient buoyancy

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. 

Particulate solid matter can exist over a wide range of bulk densities and, therefore, exhibits substantial differences between incipient buoyancy and incipient bubbling. This is illustrated qualitatively in Figure 172 the curves for Dpi and Dp2 represent the most typical shape of fluidization curves (bed expansion versus gas flow rate). Because there is no way to predict precisely a powder s range of bulk densities and the optimum operating gas velocity does not have narrow limits, it is always safe to select a higher gas velocity if in... 

Figure 172. Influence of particle size on incipient buoyancy and incipient bubbling velocities ...

Fluidization begins when the upward flow of gas through the voids between the particles in the bed attains a frictional resistance equal to the weight of the bed. This condition is called incipient buoyancy. However, at that flow rate, the particles are still so closely together that they do not have any appreciable mobility. The desired bed homogeneity and particle mobility is achieved if the gas velocity is increased... 

Frequently, both kinds of adiabats must be employed to follow the behavior of a parcel of air. A parcel of dry air may rise by its own buoyancy, or may be pushed up by orographic lifting, which occurs when winds meet mountains and are deflected upward. The temperature of the air parcel follows the dry adiabat until water vapor condensation is incipient with further cooling, condensation can occur. If it is assumed that supercooling (a nonequilibrium situation in which air cools below the dew point without condensation) does not occur, the parcel of air then moves upward following the corresponding wet adiabat. Conditional stability refers to conditions under which dry,... 

With increase in both Op and cIb, a bubble attached to the poorly wetted plate starts to slide along the plate due to the buoyancy force acting on the bubble. The point of incipient slide is represented in Fig. 4.48 by the solid and broken lines. The method used to determine these lines are described below. 

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