Tttt

Collecting pan (f)

Inlet conduit

Outlet

Inlet conduit

Outlet conduit

Outlet

Outlet conduit

FIGURE 9.2 Nozzles for sprays and units for aeration or stripping: (a-d) nozzle types; (e) inclined apron that may be studded with riffle plates; (f) perforated plates; (g) spray tower; and (h) cascade.

Surface Aerator Figures

FIGURE 9.3 Aeration units: (a) turbine aerator with an air sparger; (b) porous ceramic diffuser; and (c) surface aerator.

FIGURE 9.3 Aeration units: (a) turbine aerator with an air sparger; (b) porous ceramic diffuser; and (c) surface aerator.

FIGURE 9.4 Dome-type diffusers. (Courtesy of Aerocor Co.)
FIGURE 9.5 An activated sludge aeration tank at Back River wastewater treatment plant, Baltimore, MD.

aluminum oxide. This diffuser produces very tiny bubbles. As indicated, the diffusers are mounted on rows of pipes. Figure 9.5 shows an actual aeration in action. This happens to be one of the activated sludge process tanks at the Back River Wastewater Treatment Plant, Baltimore, MD.

A common device used in gas absorption and stripping is the packed tower, the elevational section of which is shown in Figure 9.6i. The device consists of a column or tower equipped with a gas inlet and distributor at the bottom and a liquid inlet and distributor at the top. It also consists of a liquid outlet at the bottom and a gas outlet at the top and a supported mass of solid shapes called tower packing or filling.

The liquid trickles down through the packing while the gas goes up the packing. The packing causes a thin film of liquid to be created on the surfaces which are contacted by the gases flowing by. Two phases and an interface between liquid and gas are therefore created inducing mass transfer.

Figures 9.6a to Figure 9.6h show the various shapes of packings used in practice. Packings are either dumped randomly into the tower or are stacked manually. Dumped packings consists of units that are 0.6 cm to 5 cm in major dimensions; they are mostly utilized in small towers. Stacked packings are 5 cm to 20 cm in major dimensions and are used in large towers. The spiral partition rings single, double, and triple are stacked. The Berl and Intalox saddles, the Raschig, Lessing, and cross-partition rings are normally dumped packings. Large Raschig rings 5 to 7 cm in diameter are often stacked.

9.2 INTERFACE FOR MASS TRANSFER, AND GAS AND LIQUID BOUNDARY LAYERS

Figure 9.7a shows the formation of boundary layers in absorption operations, and Figure 9.7b shows the formation of boundary layers in stripping operations. The right-hand side in each of these figures represents the liquid phase as in the liquid phase of a droplet and the left-hand side represents the gas phase as in the gas phase of the air.

Consider the absorption operation. Imagine the two phases being far apart initially. As the phases approach each other, a point of "touching" will eventually be reached. This point then determines a surface; being a surface, its thickness is equal to zero. This surface is identified as the interface in the figure. This figure shows the section cut across of the interface surface. The line representing the interface must have a zero thickness.

From fluid mechanics, when a fluid flows parallel to a plate, a boundary layer is formed closed to the plate surface. At the surface itself, the velocity is zero relative to the plate, because of the no-slip condition. Considering the two phases mentioned previously, either the liquid or the gas may be considered as analogous to the plate. Taking the liquid phase as analogous to the plate, the gas phase would be the fluid. The interface would then represent the surface of the plate. Because of the no-slip condition, the relative velocity of the phases parallel to the interface at the interface is equal to zero. In other words, the two phases are "glued" together at the interface and they do not move relative to each other.

As in the case of the fluid flow over a plate, a boundary layer is also formed. With the liquid phase considered as analogous to the plate, a gas phase boundary layer is formed; with the gas phase considered as analogous to the plate, the liquid phase boundary layer is formed (see figures). These boundary layers are also called films.

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