Minimum gas velocity

Minimum Fluidization Velocity (Umf). The lowest velocity at which the full weight of catalyst is supported by the fluidization gas. It is the minimum gas velocity at which a packed bed of solid particles will begin to expand and behave as a fluid. For an FCC catalyst, the minimum fluidization velocity is about 0.02 ft/sec. 

J Annular to slug or churn Minimum gas velocity to lift largest drop... 

The main significance of the terminal velocity lies in the fact that it defines the minimum gas velocity requested. The particle enters the system at Point 1 at zero velocity. If a gas velocity is employed so that u, < uh the particle cannot get into but drops out of the system if the gas velocity makes the relative velocity equal the terminal velocity, i.e., u, = it, the particle will move with a constant velocity of zero with respect to the ground, and so will remain at Point 1 while if the gas velocity makes the relative velocity greater than the terminal velocity, i.e., u,> it, the particle will be accelerated by the gas flow to the terminal velocity and will then move towards Point 2 at that velocity. Therefore, the terminal velocity is necessary for the determination of the operational range of the gas flow velocity for a specific vertical gas-solid impinging stream system. 

Hydrodynamics of slurry reactors include the minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

Combining eqs 1-3 results in a minimum gas velocity necessary to suspend the particles uq (min, theor). However, comparison with the experimental values taught that a correction factor has to be introduced for the particle concentration and vessel diameter, dc ... 

Chum flow or slug flow-annular (dispersed) flow transition. The flow becomes annular when the gas flow rates are enhanced to a certain point. The high gas flow rate causes a wavy interface of the liquid film. As a consequence, parts of the waves will enter the gas core as entrained drops. This results in an upward flow direction of the liquid, due both to interfacial shear and to drag on the waves and drag on the droplets. Annular flow can therefore exist only when the gas velocity is sufficient to lift the droplets in the gas core. The minimum gas velocity required to suspend a drop is determined from the balance between the gravity and drag forces on the drop. 

The transport velocity is an important design parameter since it determines the minimum gas velocity required to fully develop a stable fast fluidization mode in the riser. Smolders et al. [4] gathered literature data on the transport velocity of different particles (dp=24 - 1000 im, = 600 -3000 kg/m ) in CFBs of different size (D = 0.08 - 1 m) They compared these experimental data with the predictions by 8 correlations of several authors. They conclude that the transport velocity can best be calculated by the correlation of Ad ez [S] for small particles ... 

In practice the minimum gas flow velocity is used, which corresponds to the critical pressure drop required for fluidisation. If the particle size is larger than 100 pm, then the minimum gas velocity (umf) is calculated by the following equation [65] ... 

If the size of particles is smaller than 100 pm, then the minimum gas velocity is... 

Gas Velocity and Pressure Drop. The minimum gas velocity in feet per second required to fluidize a fixed bed of solid particles has been correlated by Leva et al. ... 

Thus, the minimum gas velocity in the flow ducts and the minimum particle diameter are respectively given by... 

Another transition velocity, Uj,, from fast fluidization to pneumatic transport is defined as the minimum gas velocity required to fully suspend a given flux of solid particles over the entire length without solids downflow along the wall. In pneumatic transport, gas also is the continuous phase, and solids hold-up is very low (typically, less than 1%). 

Fig. 2 Experimental technique to find the minimum gas velocity for particle suspension in gas-liguid-solids sparged columns.

Narayanan et al. [2S] used a visual observation technique and have given relations for the minimum gas velocity to suspend the particles. To obtain a theoretical basis, they compared a pick-up velocity previously derived [2S] on the basis of a force balance ... 

Thomas (1961, 1962) has developed a correlation base on the friction velocity principle. The technique for determining the minimum gas velocity for saltation involves various implicit expressions. The gas velocity is given as... 

As noted earlier, at the onset of fluidization the pressure drop across the bed multiplied by its cross-sectional area equals the weight of the bed this relationship may then be used in conjunction with the Ergun equation to calculate the minimum fluidization velocity, i.e., the minimum gas velocity at which fluidization is initiated. 

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