Redistributor, Redistribution

Packed columns must be provided with good initial distribution of liquid across the column cross section and redistribution of liquid at various height intei vals that decrease with increasing column diameter. A wide variety of distributors and redistributors are available. Packed columns should be considered when ... 

Redistributors are used to collect liquid that has migrated to the column walls and redistribute it evenly over the packing. They will also even out any maldistribution that has occurred within the packing. 

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). 

Liquid collectors are installed when liquid must be collected for redistribution or drawoff (e.g., for external cooling). The common device used is a chimney tray, which is similar to an orifice redistributor, but without perforations. Another common device is the Chevron-type collector, which is a series of Chevron blades, with liquid being collected at the bottom of the blades. 

In large-diameter towers and long beds, a redistributor may help correct a maldistributed composition profile. For instance, remixing stage 4 liquid of the two columns in Example 9.1 would have alleviated the pinch. Good redistribution prectices are discussed elsewhere... 

Comment. The recommended published criteria surveyed by Kister (40) suggest that it is best to redistribute liquid approximately every 20 ft, and to have no more than 10 theoretical stages per bed. Both of these criteria suggest that the bottom section of the tower should have two packed beds with a redistributor in between, while the top section should contain one packed bed. 

Fig. 2.9a shows a Rosette-type, wall-wiper redistributor that must be sealed to the fower wall. Figure 2.9b is a redistributor that achieves total collection of the liquid prior to redistribution. It is designed to be used under a gas-injector support plate. 

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

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... 

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. 

Each tray in a tower is inherently a vapor-liquid redistributor. The holes in the tray decks redistribute the vapor flowing up the tower. 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 height is uneven on a tray, and liquid flow is distorted on that particular tray, the liquid will be properly redistributed on the tray below. 

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. 

The column internals are these important imte of the apparatus idiidi are cxmstructed to ensure i oper conditions fo oi ration of die packiii Ihere are support plates, hoU-4own phrtes, liquid distributors and redistributors, gas (vapour) distributors, gas-liquid phase separators, and liquid collators. As an additional type of column internals, combined devices rinch ensuring redistribution of the phases act also as mass transfer vices. 

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