Flooding velocity

The packing parameter ( ) (m) reflects the influence of the Hquid flow rate as shown in Figure 20. reflects the influence of the gas flow rate, staying at unity below 50% of the flooding rate but beginning to decrease above this point. At 75% of the flooding velocity, = 0.6. Sc is the Schmidt number of the Hquid. 

The dependencies of all three correlations are remarkably close, as shown by Table 14-13 and the numeric prediction of flooding velocity is also close. 

TABLE 14-13 Dependency of Distillation Flood Velocity on Physical Properties and Tray Open Area... 

These relationships are not restricted to any type of contactor they can be used to predict either the flooding velocity at a given holdup. 

Very low velocities U ill allow particles to drift through the mesh and be carried out with the lea dng vapor. Also, very high velocities will carry liquid to the top of the mesh, establish a flooding condition, and then reentrain the lit]uid from the surface of the mesh. For most situations very good performance can be expected for all velocities from 30% to 100% of the optimum allowable design velocity. The minimum allowable safe design velocity is 10 percent of the value calculated by the equation. The flooding velocity of the mesh is usually about 120 percent to 140 percent of the maximum allowable velocity. 

Ffactor The vapor kinetic-energy parameter, often used as a correlating term for flooding velocity, foam density, etc. 

Estimate the flood point from Figure 8-137, which accounts for liquid flow effects and is a ratio of liq-uid/vapor kinetic effects [79]. Flooding velocity is obtained from... 

In this system, use 85% of flooding condition for design Vapor flooding velocity = Vf = U, flood = Csb/(Pv/(Pi -Pv) ... 

The critical diameter above which the flooding velocity is independent of diameter is given by ... 

Figure 10-79. Effect of entrance tube taper on flooding velocity. (Used by permission Diehl, J. E., and C.R. Kop Company, Chemical Engineering Progress Symposium, Heat Transfer, V. 65, No. 92, 1968. American Institute of Chemical Engineers. All rights reserved.)...

Vf = superficial flooding velocity of the vapor, ft/sec Vf7 = superficial flooding velocity of the vapor when inlet tube taper is 70°, ft/sec... 

The flooding velocity can be estimated from the correlation given by Fair (1961) ... 

No simple correlation is available to predict the flooding velocities in extraction columns, and hence the column diameter needed. The more specialised texts should be consulted to obtain guidance on the appropriate method to use for a particular problem see Treybal (1980), Perry et al. (1997) and Humphrey and Keller (1997). 

The vapor flooding velocity can now be calculated from Equation 8.3 assuming a foaming factor of 0.9 ... 

To obtain the column diameter, an allowance must be made for downcomer area (say 10%), and the vapor velocity should be some fraction (say 80%) of the flooding velocity. 

SAWISTOWSKI, H. Chem. Eng. Sci. 6 (1957) 138. Flooding velocities in packed columns operating at reduced pressures. 

For air and water, in a one inch tube, the superficial gas velocity limits for the film suspension region were about 31 ft./sec. to 41 ft./sec. (see Fig. 10 also). The value of 31 ft./sec. agrees with the value to be expected for the critical gas flooding velocity at zero liquid flow from an extrapolation of the results of Nicklin and Davidson. Hence, the approach of the latter workers in their analysis of unstable slug flow would seem to be valid for net liquid flows down to zero. 

The tests showed the knitted wire mesh has high gas and liquid handling capacities. It was also found to increase in removal efficiency with increased gas velocities when small droplets were involved, until the pad approached a flooding condition. As the flooding velocity-is approached, considerable liquid is held up within the pad between the wire. Since the area left for gas flow is decreased by the presence of the liquid, the gas velocity increases until large droplets are torn away from the back side of the pad and carried from the separator with the gas velocity. 

Figure 14.16. Flooding velocities in liquid-liquid packed towers [J.S. Eckert, Encycl. Chem. Process. Des. 21, 149-165 (1984)]. V = ft/hr (superficial velocity) C = continuous phase D = disperse phase a = sqft area of packing/cuft A = difference s = void fraction in packing p = viscosity centipoise continuous phase p = Ib/cuft o = (dynes/cm) interfacial surface tension F = packing factor.

Spray tower The flooding velocity is found with Eq. (14.33) ... 

Packed tower Flooding velocity is obtained with Figure 14.17. For 1 in. metal pall rings,... 

Good performance can be expected at velocities of 30-100% of those calculated with the given Ks. Flooding velocities are at 120-140% of the design rates. At low velocities the droplets drift through the mesh without coalescing. A popular design velocity is about 75% of the allowable. Some actual data of the harmful effect of low velocities were obtained by Carpenter and Othmer (1955) they found, for example, that 99% of 6 pm droplets were removed at 6.8ft/sec, but 99% of 8 pm at the lower velocity of 3.5 ft/sec. 

A variant on the froth contact is the reverse jet contactor (Example 22), which can be considered as an upside-down distillation tray operated above the flooding velocity in cocurrent flow of gas and liquid. It is limited to one stage. 

Even though gas velocity is well below flooding velocity, the finer droplets of the spray will be entrained. Note the wide spectrum of particle sizes shown by Fig. 14-88. 

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