Packed Columns

At the other end of the spectrum from a ''porous rock'' is the uncon-solidated medium composed of beds of relatively large scale packing elements. These elements may include a variety of shapes, such as rings, saddles, grids, and meshes, which are generally used to provide a large gas/liquid interface for promoting mass transfer in such operations as distillation or absorption or liquid-liquid extraction. A typical application might be the removal of an impurity from a gas stream by selective absorption by a solvent in an absorption column filled with packing. The gas (or lighter liquid, in the case of liquid-liquid extraction) typically enters the bottom of the column, and the heavier liquid enters the top and drains by gravity, the flow being countercurrent as illustrated in Fig. 13-3.

For single-phase flow through packed beds, the pressure drop can generally be predicted adequately by the Ergun equation. However, because the flow in packed columns is normally countercurrent two-phase flow, this situation is more complex. The effect of increasing the liquid mass flow rate (L) on the pressure drop through the column for a given gas mass flow rate (G), starting with dry packing, is illustrated in Fig. 13-4. The pressure drop

Figure 13-3 Schematic of packed column.

Figure 13-3 Schematic of packed column.

for wet drained packing is higher than for dry packing, because the liquid occupies some of the void space between packing elements even in the "drained" condition. As the liquid flow rate increases, the liquid occupies an increasing portion of the void space, so the area available to the gas is reduced and the total pressure drop increases. As the liquid flow rate increases, the curve of AP vs. G becomes increasingly nonlinear. The points labeled "l" in Fig. 13-4 are referred to as the "loading" points and indicate points where there is a marked increase in the interaction between the liquid

and the gas, and this is the desired operation point for the column. The points labeled "f" in Fig. 13-4 are the "flooding" points. At these points, the pressure drop through the column is equal to the static head of liquid in the column. When this occurs, the pressure drop due to the gas flow balances the static head of liquid, so the liquid can no longer drain through the packing by gravity and the column is said to be "flooded." It is obviously undesirable to operate at or near the flooding point, because a slight increase in gas flow at this point will carry the liquid out of the top of the column.

The pressure drop through packed columns, and the flooding conditions, can be estimated from the generalized correlation of Leva (1992), shown in Fig. 13-5. The pressure gradient in millimeters of water per meter of packed height is the parameter on the curves, and interpolation is usually necessary to determine the pressure drop (note that the pressure

0 0

Post a comment