Packed towers flooding velocity

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

Packed Tower Flooding velocity is obtained using Equations 14.42-14.43. For one inch metal Pall rings,... 

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.

Therefore, the gas mass velocity is 1372 lb/hr ft for 60% flooding condition with a gas mass flow of 1.28 x 10 lb/hr. It is determined that 3 towers are required each one 17.6 ft in diameter. The height of a transfer unit is a 11.4 ft and the total height of a packed tower is 22.8 ft. Pressure drop in the tower is 0.41 lb/in2. A summary of the composition of various streams is given in Table IV and stripper characteristics are given in Table III. 

Early RPB researchers discovered that this flooding correlation for packed towers applied equally well to RPBs when the gravity term (g) was replaced by centrifugal acceleration (ra>2). As acceleration increases, the gas flooding velocity (Uq) increases in order to maintain the same value of the first term. Since the ratio of liquid (L) to gas (G) flow remains constant, liquid flow increases commensur-ately with gas flow. Most researchers observed higher gas velocities before the onset of flooding than predicted by the Sherwood correlation (17,26,27). 

Flood point Packed towers are usually designed to 70 to 80 percent of the Good point velocity (17,55,56,96). This practice provides sufficient margin to allow for uncertainties associated with the flood-point concept (Sec. 8.2.3) and prediction (Sec. 8.2.6) and to keep the design point away from the region at which efficiency rapidly diminishes (just below the flood point). 

For additional information and methods for estimating pressure drop and flooding velocities in packed towers, see R. H. Perry and D. Green, Chemical Engineers Handbook, 6th ed., Sect. 18, McGraw-Hill Book Company, New York, 1984. 

Simplified Approximate Method for Estimating Flooding Velocities in Packed Distillation Towers ... 

At the flooding velocity in a packed tower, liquid starts to build up over the packing. It can be assumed, therefore, that flooding occurs when the downward pressure of the descending films or droplets equal the upward velocity pressure of the rising vapor. With this assumption and considering the gas density as negligible in comparison to the liquid density, the pressure-balance equation can be written as... 

Packed towers may be operated above 90% of the flooding velocity when pressure controls are provided to maintain a fixed maximum pressure drop not to exceed the desired flooding velocity. Strigle suggests... 

Packed towers are typically operated at a gas velocity that corresponds to about 50-80% of flooding condition. This usually results in a AP of 0.5-1.0 in. HjO/ft of packing. Alternatively, a large decrease in flow... 

The curves have been fitted to a series of equations developed by Blackwell [13]. In addition, the program computes the vapor rate and vapor velocity at 70% of flooding condition. Table 7-21 illustrates the input data and computer output of the packed tower. 

Figure 53 Flooding velocities in packed extraction towers [1]. L. McCabe, Unit Operations of Chemical Engineering. Reproduced with permission of McGraw-HiU companies.

Flooding velocities in packed towers. If the flow rate of either the dispersed phase or the continuous phase is held constant and that of the other phase gradually increased, a point is reached where the dispersed phase coalesces, the holdup of that phase increases, and finally both phases leave together through the continuous-phase outlet. The effect, like the corresponding action in an absorption column,... 

Example 22.1. A tower packed with 1-in. 25.4-mm) ceramic Intalox saddles is to be built to treat 25,000 ft (708 m ) of entering gas per hour. The ammonia content of the entering gas is 2 percent by volume. Ammonia-free water is used as absorbent The temperature is 68°F (20°C), and the pressure is 1 atm. The ratio of gas flow to liquid flow is I lb of gas per pound of liquid, (a) If the gas velocity is to be one-half the flooding velocity, what should be the diameter of the tower b) What is the pressure drop if the packed section is 20 ft (6.1 m) high ... 

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