Incipient fluidization velocity

A definite incipient fluidizing velocity, such that fluidization begins at u = um and the solid particles revert to fixed bed as soon as u < umf. 

The operations listed in Fig. 24 may shift from one mode to the other as the fluid velocity changes. Such a shift depends on the relative magnitudes of the two incipient fluidization velocities un and u[2 and of the two terminal velocities uti and ui2, and on whether the bulk-density-vs.-velocity (as well as the voidage-vs.-velocity) curves of the two kinds of particles are parallel or cross each other. That is, put more succinctly ... 

To test the viability of in quantifying fluidizing characteristics, it is plotted against the ratio of incipient bubbling velocity to incipient fluidization velocity, the latter being calculated after Geldart. Figure 69 shows... 

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. ... 

For practical gas and solids rates, this is possible for very fine powders, for which the terminal velocity ut is small. What happens in self-flow is that the dropping solids fed to the dipleg carry and pump down the surrounding gas, and this compressed gas carries, in turn, the solids up the spout. This is also possible for fine powders, which do not have a sharp incipient fluidization velocity, thus preventing quick defluidization. Self-flow therefore occurs in the band marked in Fig. 13 in the vicinity of e = 1. 

If the critically locked spout is defined by an incipient fluidization velocity uf at the top, then velocity at height z is... 

In fluidized beds, bubbles form at the distribution plate or grid ports where fluidizing gas enters the bed (Figure 169).They form because the gas velocity at the interface to the bed represents an input rate that is larger than what can pass through the interstices with less frictional resistance than the bed weight. Therefore, holes are formed through whose porous surface the gas can enter the bed at the incipient fluidization velocity. This means that bubble formation is a way to increase the active interface between the gas and the particle bed. 

Equation 17 is valid as long as the incipient fluidizing velocity of the falling particle exceeds the bed fluidizing velocity by about 50%. Obviously adequate control over acceptor residence time consistent with the requirements of acceptor kinetics can be effected in a commercial gasifier by appropriately sizing the acceptor particles. 

FIGURE 7.2 Pressure drop versus air velocity to estimate incipient fluidization velocity. 

Another means to estimate the incipient fluidization velocity is to use the pressure drop through the particle bed. This is illustrated in Figure 7.2. Pressure drop increases with air velocity at airflows below the incipient fluidization velocity. Above the incipient fluidization velocity, the pressure drop through the particle bed is relatively independent of airflow. The inflection point at the transition from airflow below the fluidization velocity to above the fluidization velocity in a graph of pressure drop versus airflow or air velocity is the -incipient fluidization point. 

Approximate Incipient Fluidization Velocities for Selected Materials... 

Sketch a plot of pressure drop across a bed of powder versus velocity of the fluid flowing upwards through it. Include packed bed and fluidized bed regions. Mark on the incipient fluidization velocity. 

By applpng a force balance, calculate the incipient fluidizing velocity for a system with particles of particle density 5000 kg/m and mean volume diameter 100 xm and a fluid of density 1.2kg/m and viscosity 1.8x10 Pas. Assume that the voidage at incipient fluidization is 0.5. 

Saxena and Vogel. The measurement of incipient fluidization velocities in a bed of corase dolomite at temperature and pressure. Trans Inst Chem Eng 55 184-189, 1977. 

Figure 11.4 Ratio of terminal velocity to incipient fluidizing velocity with emf = 0.4.

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