Flooding conditions, packed towers

The only available test data [44] indicate that the plain flat plate (26-45% free area) has a decided detrimental effect on the allowable flooding conditions of the tower whereas the wire screen, weir-type, and fused Raschig ring designs have very little effect when using Intalox saddles for packing. 

The drop in pressure for the flow of gas and liquid over packings is discussed in Chapter 4. It is important to note that, during operation, the tower does not reach flooding conditions. In addition, every effort should be made to have as high a liquid rate as possible, in order to attain satisfactory wetting of the packing. 

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. 

You will note that the lowest DP value curve shown in Fig. 3.9 is 0.05 in/ft. This is because lesser DP values are subject to severe liquid channeling. This DP value of 0.05 is 40 to 50% of flood value for the packed tower. A lesser value is most surely a liquid channeling condition, meaning the liquid will channel to one side of the packing or will skim only the tower s inside surface. In both of these conditions the needed contact between gas and liquid is severely missing. To avoid this loss of contact, keep your packing Dp at an absolute minimum of 0.05. Some vacuum systems however, are reported to function reasonably well at a DP lower limit of 0.02. 

The possibility of causing flooding in the tower at the redistribution point must not be overlooked, as too much restriction by a wall wiper, or by packing on a plate can be the focal point for poor tower performance. The velocity conditions should be checked for the smallest crosfrsection. 

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. 

As one increases the gas flow rate or liquid flow rate further, one encounters what is generally known as flooding of the column. Flooding describes a variety of conditions the appearance of a layer of liquid at the top of the packings through which gas appears as bubbles liquid fills the tower completely and gas appears dispersed as bubbles slugs of foam rise through the column (Treybal, 1980). The gas pressure drop is increased considerably. It is not advisable to run the packed tower under such conditions. 

Liquid hold up in a tower represents the liquid held in the void spaces of the packing during operating conditions. At flooding, essentially all of the voids are filled with liquid. 

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