Civil Engineering Reference
In-Depth Information
More recent research has applied these procedures for the removal of radon.
13,14
This
section discusses considerations associated with packed towers. For a more detailed,
step-by-step discussion of design procedures, the reader is referred to the literature just
cited.
A process schematic for a countercurrent tower with cross-sectional area
A
and
depth of packing
Z
is shown in Figure 9-4. Water containing a high level of contam-
inant (concentration
C
in
) enters the top of the tower and flows downward at a superficial
velocity
V
L
Q
L
/
A
(where
Q
L
volumetric liquid flow rate), exiting at the bottom
with a low concentration (
C
out
). Correspondingly, forced air containing little or no
contaminant (partial pressure
P
in
) enters the bottom of the tower and travels upward
at a superficial velocity
V
G
Q
G
/
A
(where
Q
G
volumetric gas flow rate), exiting
the top of the tower with a higher level of contaminant or partial pressure (
P
out
).
The steady-state mass transfer equation can be solved for the case of dilute solutions
(for which Henry's law is valid). This solution leads to a packed tower design relation
stating that the depth of packing,
Z
, required to achieve a desired removal performance
is the product of the number of transfer units (NTUs) and the height of a transfer unit
(HTU).
2
The HTU reflects the rate of mass transfer for a particular packing material
and contaminant, whereas the NTU is a measure of the overall mass transfer driving
force.
Q
L
,
C
in
Q
G
,
P
out
Cross-Sectional Area
A
dZ
Z
Q
L
,
C
out
Q
G
,
P
in
Fig. 9-4.
Process schematic for countercurrent packed tower (From Culp, Gordon, and Williams,
Robert,
Handbook of Public Water Systems.
Copyright
1986 by John Wiley & Sons, Inc.
Reprinted by permission of John Wiley & Sons, Inc.)
