Column liquid redistributors

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

In packed columns, liquid may sometimes be withdrawn from a redistributor. This drawoff method is suitable only when the liquid withdrawn is a small fraction of the column liquid. Compared to the chimney tray, this method consumes far less vertical height and is therefore less expensive. 

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. 

The cause of the poor results in large-diameter columns is apparently poor liquid distribution. The use of liquid redistributor plates every few feet can increase the effectiveness of the units, but they increase the pressure drop. With a number of such liquid distributors, the values... 

Davies et al demonstrated that when two liquid phases are formed in a distillation column, foaming occurs on those plates with a liquid composition near the two-phase transition [37]. This phenomenon may require a reduction in the normal design vapor flow rate to control the foaming action. Liquid redistributors, as well as the feed distributor, require special designs to maintain uniform liquid-phase composition when operating with two liquid phases in the column. 

One of the functions of a liquid redistributor is to remix the liquid phase so as to bring the entire liquid flow onto the next lower bed at a more uniform composition. To perform this function, the liquid redistributor must intercept all the liquid that is flowing down the column, including any liquid on the column walls. If a new feed is to be introduced, a feed sparger or parting box must be used to predistribute this feed onto the redistributor. The redistributor must maintain the uniform vapor distribution that should have been established at the column bottom. To perform these functions, the redistributor must have a large flow area available that is transverse to the gas risers, because only very low gradient... 

In Fig. 52 the liquid redistributor RP 1 [1] is presented. The device consists of distributor troughs, which are open in the direction facing the column wall. The RF 1 lies on a closed supporting ring and is fixed with special clamps. 

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 ... 

FIG. 14-80 Cost of internal devices for columns containing dumped packings, a) Holddown plates and support plates, (h) Redistributors, (c) Liquid distributors. [Pikulik and Diaz, Cbem. Eng., 84(21), 106 (Oct. 10, 1977).]... 

Select and design the column internal features packing support, liquid distributor, redistributors. 

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. 

Figure 1335. Packed column and internals, (a) Example packed column with a variety of internals [Chen, Chem. Eng. 40, (5 Mar. 1984)]. (b) Packing support and redistributor assembly, (c) Trough-type liquid distributor, (d) Perforated pipe distributor, (e) Rosette redistributor for small towers. (0 Hold-down plate, particularly for low density packing.

As indicated above, packed column internals include liquid distributors, packing support plates, redistributors (as needed), and holddown plates (to prevent movement of packing under flow conditions). Costs of these internals for columns with random packing are given in Fig. 14-81, based on early 1976 prices, and a Marshall and Swift cost index of 460. 

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... 

Similar to tray columns, packed columns operated at high gas velocities causes backmixing, and low gas velocities reduce the mass transfer rate. If the gas velocity is too high, the column will flood. In addition, at low liquid flow rates the packing will not wet completely, resulting in a reduction in mass-transfer. Another problem is the tendency for the liquid to channel. To minimize this effect, redistributors have to be installed every 5 to 10 m (16.4 to 30.5 ft) [23] to even out the liquid flow. 

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