Liquid Redistributors

Liquid redistributors are used wherever an intermediate liquid feed is introduced into a packed column, and between packed sections, wherever liquid redistribution is required. 

Apart from distributing an incoming feed, evenly irrigating the bed below, and providing a uniform vapor flow to the bed above, liquid re- 

Since the main function of a redistributor is to counteract the effects of large-scale maldistribution, less frequent redistribution is required when large-scale maldistribution is absent or minimized. For instance, less frequent redistribution is needed when the ratio of column to packing diameter is small (443), because large-scale maldistribution is well-counteracted by lateral mixing in the packing (Sec. 3.1). 

The importance of adequate redistribution is illustrated by a field experience reported for two identical refinery debutanizers operating in parallel and in identical service (237). Both were packed with 3-and 4-in packings the only difference between the two was the presence of redistributors. The tower containing four 19-ft beds with redistributors achieved an HETP of 39 in the other contained two 38-ft beds and achieved an HETP of 72 in. The author also experienced a case of a poorly performing packed bed which was about 35 ft deep and 

Orifice redistributors (Fig. 3.8a) These are identical to orifice distributors, either the pan or the trough type, except that hats, caps, or strips are usually installed above the risers to prevent liquid from the packed hed above from entering the vapor risers, and also to promote lateral mixing of vapor. 

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Liquid redistributors are needed every 5-10 tower diameters with pall rings but at least every 20 ft. The number of liquid streams should be 3-5/sqft in towers larger than 3 ft dia (some experts say 9-12/sqft), and more numerous in smaller towers. 

Each tray in a tower is inherently a vapor-liquid redistributor. The outlet weir, or more exactly the crest height of the liquid overflowing the weir forces the liquid to flow evenly across the tray. Even if the weir... 

Depth of packing without intermediate supports is limited by its deformability metal construction is limited to depths of 20-25 ft, and plastic to 10-15 ft. Intermediate supports and liquid redistributors are supplied for deeper beds and at sidestream withdrawal or feed points. Liquid redistributors usually are needed every 2 -3 tower diameters for Raschig rings and every 5-10 diameters for pall rings, but at least every 20 ft. 

Liquid redistributors are installed at frequent intervals in a packed column to remix the liquid, thus counteracting the propagation of maldistribution effects and the natural tendency of liquid to migrate toward the wall. A common design practice is to redistribute the liquid every 20 ft (6-7 m). 

Figure 10. Orifice deck liquid redistributor. (Courtesy of Norton Chemical Process Products Corporation.)...

Liquid redistributors collect downcoming liquid and distribute it uniformly to the bed below. Initially, after entering the tower the liquid tends to flow out to the wall, the redistributor makes that portion of the liquid more available again to the gas flow. It also breaks up the coalescence of the downcoming liquid, and it will eliminate factors that cause a loss of efficiency in the tower and reestablish a uniform pattern of liquid... 

Liquid collectors provide for withdrawal of products, mixing of new feeds with the internal liquid traffic, and remixing to avoid composition gradients after a certain height of packed bed has been traversed. Liquid collectors are often designed as one item to be integral with vapor distributors and liquid redistributors. 

Figure 3.8 Liquid redistributors, (a) Orifice redistributor (W "rosette wall wiper (c) support mixer redistributor. (Part a Gilbert K. Chen, excerpted by special permission from Chemical Engineering, March 5,1984, copyright by McGraw-Hill, Inc., New York, NY 10020 part b reprinted courtesy of Norton Company part c reprinted courtesy of Koch Engineering Company, Inc.)...

Liquid distributor for filling material and packings (Table 2-33) and liquid redistributors (Fig. 2-82)... 

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