Civil Engineering Reference
In-Depth Information
Z
NTU
HTU
The NTU term characterizes the difficulty in stripping a compound to a desired
level; it is given by the following equation:
R
(
C
/
C
)(
R
1)
1
NTU
ln
in
out
R
1
R
where
NTU
number of transfer units, dimensionless
R
stripping factor, dimensionless
HG
/
L
H
Henry's constant, atm
G
superficial molar air flow rate, m
3
/m
2
h
atm
L
superficial molar liquid flow rate, m
3
/m
2
h
NTU is dependent on the desired removal efficiency, the air-to-water ratio, and
Henry's constant. Given a specific Henry's constant and a desired removal efficiency,
NTU can be computed for a packed column for a given stripping factor or air-to-water
ratio. Such a relationship is shown in Figure 9-5.
Optimum column designs are typically based on stripping factors between 1.2 and
5. Figure 9-6 shows the effect on NTU of varying the stripping factor for several
removal efficiencies. For 90 percent removal, NTU decreases rapidly for
R
values
slightly greater than 1. Diminishing returns set in as
R
is increased beyond 2. For very
high removals (
90 percent), a stripping factor between 2 and 5 may provide the most
economical design.
2
HTU characterizes the efficiency of mass transfer from water to air and is a function
of the liquid loading rate and
K
L
a,
the overall mass transfer coefficient. It is defined
as follows:
L
HTU
KaC
L
0
where
HTU
height of transfer unit, ft (m)
C
0
molar density of water, lb-mol / ft
3
(kg-mol / L)
L
superficial molar liquid flow rate, lb-mol / (ft
2
-hr) (kg-mol / (m
2
-hr))
K
L
a
overall mass transfer coefficient, hr
1
Computing HTU requires data on the mass transfer coefficients for the system under
consideration. In some cases, packing manufacturers will supply mass transfer data for
air-water systems as a function of temperature and liquid flow rates. It is preferable
for values of
K
L
a
to be determined from pilot studies utilizing the contaminated water.
However, in the absence of such data, mass transfer correlation data from the literature
