Liquid redistribution, in packed

Figure 9-14. Liquid redistribution in packed towers. Used by permission of Norton Chemical Process Products Corp.

Reflux and Intermediate Feed Distribution and Liquid Redistribution in Packed Towers... 

There has been considerable speculation regarding the depth of packing that could be installed before liquid redistribution is required. Silvey and Keller found no loss of efficiency in distillation with 18-ft deep beds of 1 A-in. ceramic... 

Since liquid maldistribution may become a problem unless the flow is periodically redistributed in a tall column, the total packing height must be broken up into a number of individual beds ... 

Liquid Distribution Good initial distribution is not as essential in a sieve tray extractor as it is in a packed extractor, since the trays provide redistribution. While the same distributors used in packed columns are... 

In packed columns, chimney trays are the most effective liquid collection devices for high liquid flow rate services, for interreboilers, and for once-through reboilers. They are also sometimes used for collecting liquid from an upper bed for redistribution (when the redistributor is not self-collecting), and as vapor distributors. 

For fast reactions, change in the flow regime has dramatic effects on performance. Prevent foaming. Liquid distributor design is very important. Carefully plan the liquid redistribution along the walls for columns < 2-3 m diameter. For these small columns place redistributors at distances = 8-10 times the column diameter. The critical surface tension of the solid packing should be greater than the surface tension of the liquid to ensure that the liquid film remains intact in a packed contactor. 

Liquid redistribution was analyzed by Shaw (42) for a two-dimensional model of circular particles. The approach involves a determination of the equilibrium distribution of the liquid in different packing arrangements of particles under the condition that the chemical potential of the liquid in all the pores in a particle array is the same at equilibrium. The chemical potential of an atom under the surface of a liquid-vapor meniscus with an average radius of curvature r is... 

Later experiments in small diameter columns have shown that a bed of random dumped packing develops a definite number of preferred paths of internal liquid flow [8]. If liquid is distributed onto the packed bed with a greater number of streams per square foot than number of preferred liquid paths, the liquid streams within the bed coalesce until the number of preferred liquid paths is reached. If liquid is distributed onto the packed bed with a smaller number of streams per square foot than the number of preferred liquid paths, the packing redistributes the liquid until the number of preferred liquid paths is established. In this latter case, the upper portion of the packed bed has a lower-than-nor-mal separating efficiency. Figure 10-8 shows the performance of such a packed bed in a binary distillation. Curve I represents the separation obtained with a high-performance liquid distribution system. Curve II shows the effect of inadequate liquid distribution to the top surface of the 10-ft deep packed bed. 

A basic requirement for the proper functioning of a packed column is that its diameter should be at least ten times the nominal size of the packing (see Table 5.6) to ensure adequate distribution of the gas and, particularly, the liquid. Redistribution sections, in which the liquid down-flow is collected and redistributed over entire tower cross sections, are often inserted to ensure good performance. On the other hand, the tower cannot be too large for structural and economic reasons. A normal diameter range is 0.5 to 5 m, but diameters as high as 14 m have been reported. 

The hCed water enters the jet distributor of the hollow column 6. After this initial distributor, which di butes the liquid phase in the form of a cone of streams and drops, a sirecial construction of redistributors follows, which redistributes it uniformly over the column cross-section. By the pump 7 the partially purified water is fed in die liquid distributor of the packed bed column 8. The purified hot water by pump 10 is fed in the hC exd ger block to heat the waste water entering through pipe 1. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

For stacked packing the liquid usually has little tendency to cross-distribute, and thus moves down the tower in the cross-sectional area that it enters. In the dumped condition most packings follow a conical distribution down the tower, with the apex of the cone at the liquid impingement point. After about 12 ft vertical height, the liquid flow s vertically downward unless redistributed. For uniform liquid flow and reduced channeling of gas and liquid with as efficient use of the packing bed as pos.sible, the impingement of the liquid onto the bed must be as uniform as possible. 

Other useful distributor types have been referred to and previously illustrated. For redistribution, the v apor risers may be 12 in.-18 in. tall, and with protective hats to prevent liquid dropping from the tray/section above. The space between the cover hat on the riser and the bed above should be 18 in. to 12 in. minimum to allow for proper v apor redistribution entering the packed section above. The importance of a level distributor cannot be overemphasized. 

Redistribute the liquid every 10 ft and allow 2 ft additional for redistribution = 36 ft. Then use 4-10 ft sections of packing. For 20 theoretical plates, total performance packed height 20 (18 in./12 in.) = 30 ft. Allow for loss of equilibrium at (1) reflux entrance = 1 HETP and (2) feed entrance = 2 HETP and (3) redistribution (2) = 2 HETP totals 5 HETP. 

It may be that the random or structured packing efficiency is degraded by the need to limit the bed-height space required for liquid collection redistribution, and by maldistribution and channelling in the packing [33]. 

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