Redistributor plate

Loss of active surface area due to improper liquid distribution can be reduced by using intermediate redistributor plates, as shown in Fig. 16-14. 

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... 

As indicated above, packed column internals include hqiiid 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. 14-80, based on early 1976 prices, and a Marshall and Swift cost index of 460. 

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).]... 

D. Hold-Down Grid (see Figure 3). According to Strigle, Normally, the upper surface of the packed bed is at least 6 in. below a liquid distributor or redistributor. The bed limiter or hold-down plate is located on top of the packed bed in this space. It is important to provide such a space to permit gas disengagement from the packed bed. Such a space allows the gas to accelerate to the velocity neces-... 

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.

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

Holddown plates and support plates, h) Redistributors. [PikuUk and Diaz, Tower wall thickness, mm... 

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. 

Weir redistributors These are identical to notched-trough distributors. Because weir redistributors cannot collect liquid from the upper section, a liquid collector such as a chimney tray or chevron collector (Chap. 4) or a collecting support plate (e.g., the upper plate in Fig. 3.8c) is usually required above the redistributor. 

---