of the ShallowTray aerator at these flow rates is
rium to the mass of contaminant in the liquid
so high that >99.99% reductions of VOC concen-
phase at equilibrium. By assuming that 100% of
trations were achieved independent of liquid
the liquid phase contaminants removed were
temperature and influent concentrations as high
transferred to the gas phase (a good assumption
as 46.2 ppm TCE and 10.8 ppm PCE. All factor
based on the near 100% mass balance from off-
level combinations produced an effluent water
gas hydrocarbon measurements), the mass of con-
with TCE and PCE concentrations below the EPA
taminant in the gas phase is therefore known and
maximum contaminant levels (MCLs) of 5 ppb.
the mass of contaminant in the liquid phase at
equilibrium can be estimated as
Estimation of VOC liquid
CG CL = HC .
*
*
(4)
mass transfer coefficients
Contaminant property effects on ShallowTray
*
Assuming CG = CG and rewriting, yields
stripping efficiency were determined by compar-
ing overall liquid mass transfer coefficients (KLa)
CG HC = CL .
*
(5)
derived empirically from this research to pub-
lished values for other types of air stripping sys-
Since all the VOCs are transferred to the air, M =
tems. The rate of transfer of a volatile compound
CG and eq 3 can be rewritten to solve for KLa in
from water to air is generally proportional to the
this manner:
difference between the existing concentration and
the equilibrium concentration of the compound
CG
KL a =
(CL - C0 )
(6)
in solution. The relationship is expressed as a
*
modification of Fick's law:
()
data from this research for TCE and PCE are sum-
M = KL a CL - C0
*
(3)
marized in Table 5. Because Henry's law constant
varies with liquid temperature, three KLa values
where M = mass of substance transferred per
were determined (one at each temperature) for
unit time and volume (mg/min L)
each VOC. As expected the KLa values at both
KL = overall liquid mass transfer coeffi-
flow rates for a given temperature were also cal-
cient (m/min)
culated and each pair were found to be equal.
a = effective mass transfer area (m2/m3)
Values derived from this research show good
*
agreement with those published by McCarty
with the gas phase conc. (mg/L)
(1983). Mass transfer of both contaminants, in fact,
C0 = bulk phase (existing) liquid concen-
indicate that the ShallowTray strippers are very
tration (mg/L)
competitive with packed towers in removal effi-
The mass transfer coefficient KL is a function of
ciency per unit volume.
the properties of the compound being stripped
(solubility, partial pressure and diffusivity), the
TPH removal
physical characteristics of the air stripping
A graphic summary of all TPH removal data is
equipment, and the temperature and flow rate
shown in Figure 5. Trial data are plotted in the
of the liquid (Hess et al. 1983). The effective
same sequence in which trials were conducted,
area a represents the total air/water
Table 5. Estimate of mass transfer coefficients for ShallowTray
interface area created in the stripper
stripping of TCE and PCE at three liquid temperatures.
and is a function of the air stripping
equipment. The conventional approach
Temperature
TCE
PCE
McCarty (1983)
assumes that the effective area is too
(C)
a ( sec1)
a (sec1)
†
(sec1)
HC*
KL
HC
KL
difficult to estimate by itself and is
22.2
0.375
0.01
0.586
0.024
0.025**
evaluated with KL as a single constant,
0.0009††
15.56
0.384
0.0104
0.599
0.025
8.89
0.393
0.0108
0.613
0.026
0.007***
*
centration CL is determined by apply-
* source--U.S. EPA 1990
ing Henry's law constant. The dimen-
† source--Munz and Roberts 1979
sionless Henry's law constant (Hc) is
** countercurrent tower (VOC stripping at 20C, Q = 6600 L/min)
essentially the ratio of mass of con-
†† cross flow tower (VOC stripping, Q = 19,800 L/min)
taminant in the gas phase at equilib-
*** mechanical aeration basin (VOC stripping, Q = 2640 L/min)
9
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