vvEPA
United States
Environmental Protection
Agency
EPA/540/SR-98/504
March 1999
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Integration of Photocatalytic
Oxidation with Air Stripping of
Contaminated Aquifers
Rajnish Changrani, Gregory B. Raupp, and Craig Turchi
In a recently completed test program,
bench-scale laboratory studies at Ari-
zona State University (ASU) in Tempe,
AZ, and pilot-scale studies in a simu-
lated field-test situation at Zentox Cor-
poration in Ocala, FL, were performed
to evaluate the integration of gas-solid
ultraviolet (UV) photocatalytic oxidation
(PCO) with air stripping of an aquifer
contaminated with chlorinated volatile
organic compounds (VOCs). Chlori-
nated ethylenes such as trichloroethyl-
ene (TCE) can be destroyed in a wide
process window, although chlorinated
ethanes such as trichlorethane (TCA)
are nonreactive. Water vapor signifi-
cantly inhibits the chlorinated ethylene
destruction rate. For this reason, PCO
units should be placed downstream of
a dehumidification unit located between
air strippers and the PCO unit, with
targeted reduction of the relative hu-
midity in the contaminated air stream
to less than 50%. Principal carbon-con-
taining products of PCO identified ex-
perimentally at the bench scale include
carbon dioxide, carbon monoxide, and
phosgene (COCI2). Failure to close car-
bon mass balances under some pro-
cess conditions suggests that not all
byproducts were identified. Further
studies are needed in this area. A panel
bed was identified as the preferred pho-
toreactor configuration. This unit is
characterized by simplicity of construc-
tion, ease of maintenance, and high UV
photon utilization efficiency.
This Emerging Technology Summary
was developed by EPA's National Risk
Management Research Laboratory, Cin-
cinnati, OH, to announce key findings
of the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Contamination of drinking water aqui-
fers with volatile chlorinated organic sol-
vents is a widespread problem across
industrialized areas of the U.S. The. global
objective of this work was to evaluate the
integration of gas-solid ultraviolet (UV) pho-
tocatalytic oxidation (PCO) downstream of
an air stripper unit as a technology for
cost-effectively treating water pumped from
an aquifer contaminated with chlorinated
volatile organic compounds (VOCs). The
photocatalytic oxidation process integrated
with air stripping is shown schematically
in Figure 1. In this configuration, the air
stripper off-gases are fed directly to the
PCO reactor without pretreatment. In the
Printed on Recycled Paper
-------
continuous flow PCO reactor, the con-
taminated air stream contacts the surface
of a near-ultraviolet irradiated titania (TiO2)
catalyst, causing photochemical destruc-
tion of the contaminants at or near room
temperature. Exhaust gas from the PCO
reactor is fed to a dry scrubber for re-
moval of HCI and CI2 generated during
tho oxidation of chlorinated solvents.
The primary objectives of this research
project were as follows:
• Define the optimum gas residence
time, catalyst characteristics, UV light
intensity, and photoreactor
configuration that achieves greater
than 95% destruction of the primary
VOCs (trichloroethylene and
trichtorethane) in air stripper off-gases.
• Characterize the overall performance
of the integrated air stripper/PCO
process by quantifying contaminant
destruction during short-term
transients generated by air stripper
startup, shutdown, and upsets, and
by quantifying destruction perfor-
mance over an extended test period.
The secondary objectives of this project
included the following:
• Quantify catalyst lifetime independent
of factors related to integration of the
PCO unit with an air stripper.
• Identify reaction byproducts and
conditions that Inhibit or enhance their
formation.
Most of these objectives were success-
fully achieved through bench-scale and
pilot-scale controlled testing described in
the body of this report. The first objective
was mot through the following strategy.
Statistically designed experiments enabled
efficient process performance character-
ization as a function of key process vari-
ables. The derived empirical response
surface models allowed prediction of con-
ditions for which high VOC destruction
efficiency could be achieved. These pre-
dictions were subsequently confirmed in a
long-term bench-scale test and in several
short-term pilot-scale tests. Because we
wore unable to obtain actual field test
data, the second primary objective listed
above could not be achieved.
Experimental Methods
The bench-scale PCO system at ASU
Is shown schematically in Figure 2. A CO2-
free synthetic air mixture contaminated to
a predetermined level is prepared by con-
tinuously mixing gases from pressurized
gas cylinders using mass flow controllers
(mfc). Water vapor is added to the feed
flow by a separate nitrogen flow through a
saturator. The humid, contaminated air is
fed to an annular photoreactor incorporat-
VOC-laden
humid air
Air f - T
Stripping T T
Towers *^ *^
VOC-contaminated
groundwater
Surge
Tank
*—fi
Utility
air
Stripped
water
Figure 1. Integrated air stripping with photocatalytic oxidation process.
ing a 1.5" OD 20 W UV black lamp in a
2.25" ID glass tube. The annular space is
filled with titania-coated packing; bed
length is 90 cm. Ultraviolet light fluxes at
the lamp and at the reactor outer wall
were measured with a Minolta integrating
photometer.
The system incorporates three in-line
sensors (Sensl, Sens2 and SensS) and
three discrete sampling locations (Sam1,
Sam2 and SamS). Sam1 and Sam2 are
automatic multiple port GC sampling valves
which send a well-defined volume of gas
to one of two columns in the Varian 3700
GC for VOC and carbon dioxide analysis,
respectively. The third sampling location
(SamS) allows gas samples to be with-
drawn for off-line analysis by gas detec-
tion tubes. Sensl is an in-line relative
humidity sensor (Vaisala). Sens2 and
SensS are in-line electrochemical sensors
for HCI (Detcon) and CO (Sierra) mea-
surement, respectively. Flow, temperature
and pressure measuring devices are also
included as indicated in the figure. Mea-
surements are recorded using automatic
data acquisition driven by a Hewlett-
Packard 486 processor based PC.
Testing Procedures
For a given performance evaluation run,
gas flows are set and stabilized and the
feed gas is analyzed with the sensors and
GCs until a steady reading is achieved. At
this point, the UV lamps are illuminated,
and the reactor outlet is regularly ana-
lyzed. Once steady operating conditions
are achieved (typically several hours with
fresh catalyst beds, several minutes with
conditioned beds), measurements are re-
corded for several more hours. It is these
average steady state values (usually six
to eight points taken over two hours) tnat
are reported. i
Three unique sets of experiments were
run. Set 1 was designed to character12®
the performance of the bench-scale ij"™
under process conditions that are repi'jf"
sentative of those found in the field at jne
site to be tested. Groundwater at the tp~
get industrial field site is currently bejn9
pumped and treated by a 660 gallons |pr
minute remediation plant employing \atr
stripping and carbon bed adsorption
the stripper off-gases. In the planned fi
test, the PCO unit would have been e
ployed to treat a slipstream of the
stripper off-gases. The expected total
concentration ranges from 10 to 18 P
with typically two-thirds of the total attrr"
uted to TCE. An in-line dehumidifier rr"
duces the relative humidity (RH) in tlPe
off-gases to about 50% upstream of tle
carbon beds.
Process characterization was efficiency
achieved by employing statistical desigjn
of experiments (DOE) surface respon6
methodology (SRM). Table 1 summariz
the independent process variables and le
els. The chosen 1/2 fraction of a 25 fact
rial design augmented with axial runs
center point replicates, or "face center
cubic" design, provides a complete r
sponse surface and has a reasonably
stable variance over a large portion of th
design region.
Set 2 is designed to efficiently chara
terize the performance of the bench-scal
unit for conversion of complex contain
nant mixtures. Set 3 experiments wen
designed to test the long-term stability
the photocatalyst in a reactive enviro
ment.
f
-------
GC
Carrier
Gas#1
Sam1 Sam2 Sam3
PCO Reactor Effluent
VOC
in Air
Feed Preparation Section
Sam1 - 3 Sampling locations
Sens'! - 3 Sensing locations
mfc mass flow controller
P Pressure gauge
T Temperature sensor
Figure 2. Bench-scale photocatalytic oxidation apparatus.
ablel. Process Parameters and Levels
actor
Dtal inlet VOC concentration (ppmv)
elative Humidity (%)
V Intensity),^, (mW/cm2)
smperature (°C)
i
?sidence tiine (s)
Code
A
B
C
D
E
Low(-1)
10
20
0.5
40
0.5
Center (0)
20
50
2.0
60
1.0
High (+1)
30
80
3.5
80
1.5
"
'The principal measured responses are
conversion Xvoc of the VOC and selectiv-
ity SC02 of the reaction towards producing
CO2. Conversion is a direct measure of
destruction efficiency, while the selectivity
is a measure of the percentage of the
VOCs destroyed that are converted to the
desired complete combustion product car-
bon dioxide.
Results and Discussion
Set 1 Results Under the conditions of
these experiments, TCA conversion is es-
sentially negligible, with measured values
ranging between about 2 and 6%. For
purposes of brevity, subsequent DOE mod-
els for VOC conversion described below
and the accompanying discussion are for
TCE conversion only. Trichloroethylene
conversion data are described by the fol-
lowing quadratic model:
Xvoc =111.02- 0.22 • VOC -2.81 • RH
+ 9.02 • UV - 0.23 • T + 42.0 • T
- 15.11 • i? - 0.07 • RH • UV
+ 0.008 'RH'T
where VOC = VOC concentration (ppm ),
RH = Relative Humidity (%), UV = UV
intensity (mW/cm2), 7= Temperature (K),
and -r = residence time (s). The model
reveals that all five independent factors
are significant. The three-dimensional re-
sponse surfaces are shown in Figures 3
and 4. Increasing residence time and UV
intensity increase VOC conversion,
whereas increasing RH decreases VOC
conversion. Inhibition of the TCE photo-
catalytic oxidation rate by water vapor is a
well-established phenomenon first reported
by our laboratory.
The following linear model adequately
describes the observed selectivity behav-
ior:
SC02 = -23.90 - 1.61 • VOC -0.19' RH
+ 3.93 • UV + 0.43 • T- 12.51 • r
+ 0.0002' VOC'UV'T
where variables are as defined previously.
Increasing VOC concentration decreases
selectivity, while increasing light intensity
(UV) and increasing temperature (T) in-
crease selectivity. In addition, there is an
interaction among VOC concentration, UV
and T. The magnitude of the interaction
-------
VOC conversion
60
VOC concentration (ppm)
1.5
Residence time (s)
Figure 3. Response surface for VOC conversion versus residence time and inlet VOC concentration.
Twenty percent RH, 3.5 mW/cm2UV intensity, 40°C.
60 VOC conversion
20
Ritativ* Humidity (%)
30
1.5
Residence time (s)
Figure 4. Response surface for VOC conversion versus residence time and RH. Ten ppmv VOC
concentration, 3.5 mW/cm2UV intensity, 40°C.
term, while statistically significant, is small
relative to the main factor effects. The
main selectivity responses can be ex-
plained within the framework of a surface-
mediated, free-radical sequential oxidation
mechanism. In this model, hydroxyl radi-
cals created through UV excitation of the
titania photocatalyst initiate oxidative at-
tack on adsorbed VOCs to create VOC
radicals. The radical sites are then at-
tacked by oxygen to create peroxy radi-
cals. These reactive peroxy radicals readily
decompose to produce partial oxidation
products on the catalyst surface. This se-
ries hydroxyl radical attack, oxygen addi-
tion sequence continues until complete
combustion products are produced. We
hypothesize that TCE is first oxidized to
dichloroacetaldehyde, then to dichloroace-
tic acid, and ultimately to chlorinated for-
mic acid intermediates. Under dry
conditions <10% RH), hydroxyl radicals
may not be the primary initiator of oxida-
tion, and an alternative reaction path
through phosgene becomes dominant.
With increasing relative humidity, the pro-
duction of phosgene and other undesir-
able intermediates decreases. For
sufficiently high humidity (>40-50%) and
residence time, essentially complete con-
version to carbon dioxide is observed.
Assuming typical Langmuir-Hinshelwood-
Hougen-Watson (LHHW) reaction kinetics
determine the TCE destruction rate, in-
creasing TCE concentration should in-
crease the average surface coverage by
TCE. For otherwise fixed conditions, the
production rate of reactive hydroxy) radi-
cals will be essentially the same. At steady
state, there will therefore be relatively fewer
hydroxyls to attack adsorbed partial oxi-
dation intermediates, leading to a reduced
integrated selectivity to CO2. A similar ar-
gument can be posed to explain the UV
dependence. With increasing UV inten-
sity, the production rate of hydroxyl radi-
cals increases. A relatively greater number
of hydroxyls will be available to attack
partial oxidation intermediates, leading to
an increased integrated selectivity to CO2.
No VOCs other than TCE and TCA
were observed in the GC chromatograms.
Carbon monoxide levels were between
0.5 to 1.5 ppmv for all experimental runs.
Molecular chlorine (Cl) and phosgene
(COCI2) levels were observed at levels
<0.2 ppmv; no significant hydrogen chlo-
ride was detected. The calculated carbon
mass balances for the Set 1 experimental
runs range from ca. 60 to 100%. In geh-
eral, highest percent closures were ob-
tained for low inlet VOC concentration.
Low inlet TCE concentrations also favpr
higher selectivity. Lack of closure could
not be attributed to inaccuracy in C02
measurements but is instead most likely
due to non-detect of volatile partial oxida-
tion products including, but not necessar-
ily limited to, dichloroacetyl chloride. A Cl
atom balance is not reported since, ex-
cept for unconverted TCE, only very small
amounts of chlorine-containing compounds
were detected in the reactor outlet for all
runs! It is likely that chlorine buildup on
the catalyst surface is responsible for the
low levels of chlorine-containing com-
pounds detected in the reactor effluent.
-------
Sef 2 Results At the target Superfund
site, co-contaminant or secondary VOCs
present along with the primary VOCs
are, 1,1-dichloroethylene, cis 1,2-
dichloroethylene, vinyl chloride, tetra-
chloroethylene (also known as
perchloroethylene), and 1,2-dichloroben-
zene. Each of these co-contaminants is
present in varying amounts depending
on air stripper operation, but under no
circumstance have levels higher than 1
ppmv been detected. A mixture experi-
mental design in which the chlorinated
ethylenes were lumped together to create
a "secondary chlorinated ethylenes"
pseudocomponent was employed. This
strategy yields the following three-compo-
nent mixture: trichloroethylene (mole frac-
tion x,), secondary chlorinated ethylenes
(mole fraction x2), and 1,2-dichlorobenzene
| (mole fraction x3). The initial concentra-
tions for each of the chlorinated VOCs in
the lumped secondary chlorinated ethyl-
enes category were held in the same ratio
for all experiments, at levels designed to
, approximate the average ratios found at
the Superfund site.
A 32 factorial mixture experiment was
designed to capture the maximum pseudo-
component ratios R, = x, / x2 and R2 = x, /
x3 expected at the Superfund site. Table 2
summarizes these ratios and the corre-
spibnding pseudo mole fractions of the
thr,ee pseudocomponents for the nine ex-
peHmental runs performed in this set.
Otner process conditions were fixed at
representative values as follows: TCE in-
let, concentration = 10 ppmv, RH = 20%,
UV Intensity = 3.5 mW/cm2, Temperature
= 40°C., and residence time = 0.5 s. These
process values were chosen to yield a
TCE conversion that was reasonably high
. (order of 70%) but not so high (i.e., >95%)
thp conversion sensitivity to the presence
of| co-contaminants would be masked by
pqor measurement statistics.
:A statistical analysis of the experimen-
tal data revealed that nearly the entire
Table 2. Mixture Experimental Design
Run
ID
1
2
3
4
5
6
Ri
2
4
6
2
4
6
2
4
R2
6
6
6
11
11
11
16
16
x,
0.60
0.70
0.75
0.63
0.74
0.80
0.64
0.76
X2
0.30
0.18
0.13
0.31
0.19
0.13
0.32
0.19
X3
0.10
0.12
0.12
0.06
0.07
0.07
0.04
0.05
sum of squares is explained by the sum
of squares for the mean, I.e., that the
effect of the co-contaminants on TCE con-
version is not statistically significant. Se-
lectivity depends on the pseudocomponent
ratios according to the following quadratic
model:
Sco2 = ~39-19 +10.69 • R, + 9.23 • R2
+ 0.71-Rf -0.26• R22 -0.525•R1-R2
Figure 5 shows this response visually.
The response shows a complex depen-
dence on the pseudocomponent ratios.
As the ratio (R,) of TCE to secondary
chlorinated ethylenes increases, selectiv-
ity increases regardless of the chloroben-
zene level. The increase is most dramatic
at the lowest ratio of TCE to chloroben-
zene. The Selectivity tends to go through
a broad maximum as the ratio (R2) of TCE
to chlorobenzene increases, with the lo-
cation of the maximum shifting to lower R2
ratios as the ratio of TCE to secondary
chlorinated ethylenes increases.
Carbon mass balance closures range
from ca. 60 to near 100% (basically the
same range observed for the factorial ex-
periments in Set 1). As for Set 1, a chlo-
rine atom balance is not reported since
essentially no chlorine-containing com-
pounds other than those found in uncon-
verted VOCs were detected in the reactor
outlet for all runs.
Set 3 Results These experiments were
designed to test the long-term stability of
the photocatalyst in a reactive environ-
ment. Process conditions were chosen so
that high TCE conversion (>95%) and se-
lectivity (100%) were obtained during the
initial stages of the run. To simplify gas
analysis, TCE was the only VOC in the
contaminated air stream. A freshly-pre-
pared catalyst bed was employed.
Over the course of the first six days of
operation, TCE conversion held steady at
greater than 95%, while Selectivity was
constant at approximately 100%. The C
mass balance was essentially closed in
all measurements (=100%). The Cl atom
mass balance was only about 3-4%. After
six days on stream, a dramatic change in
performance was observed. The Cl atom
balance closure jumped to the 70-80%'
closure range. This improved closure is
associated with the appearance of sub-
stantial concentrations of HCI and CI2, as
well as lesser amounts of COCI2. Selectiv-
ity fell below 100% at this point as a
portion of the inlet C appeared in the
COCI2 byproduct. Over the remainder of
the run, the TCE conversion gradually fell,
although it remained above 90% over the
entire duration. The selectivity also gradu-
ally fell, although it remained above 95%.
The findings in this experiment are con-
sistent with a model in which the fresh
14
12
10
R2 (TCE: Aromatic)
Figure 5. Influence of pseudocomponent ratios R, and R2 on selectivity.
5
R1 (TCE: Cl-
ethylenes)
-------
surface becomes chlorinated as TCE is
dochlorinated and oxidized in the PCO
process. Initially this deposited Cl is non-
volatile and an inventory of adsorbed Cl
builds up on the surface as TCE-contain-
ing air Is treated. At the point of Cl atom
saturation on the surface, TCE conver-
sion continues but chlorine produced sub-
sequently Is volatile, appearing in the gas
phase as HCI, CI2, and COCI2.
Pilot-Scale Experiments
A commercial scale reactor should be
designed to achieve high integrated pho-
ton utilization efficiencies, while providing
low pressure drop, high throughput opera-
tion in a reasonably compact physical con-
figuration. The preferred reactor
configuration is a key technical issue ad-
dressed in this research. Figure 6 com-
pares the two principal reactor
configurations tested in this research
project. Configuration 1 is the packed bed
design, in which a cylindrical photoreactor
containing a number of equally-spaced par-
allel UV lamps is packed with a catalyst-
coated support material. The air flows
parallel to the lamps that irradiate the sup-
ported catalyst. Configuration 2 is panel
bed design, in which a rectangular duct-
shaped photoreactor contains a number
of alternating UV lamp banks and a re-
movable panel-type catalyst support ma-
terial. In this design, the air flows
perpendicular to the lamps that irradiate
the panels.
Set 1: Reactor Configuration 1 Table
3 summarizes TCE conversion perfor-
mance for a set of controlled pilot-scale
runs at Zentox's production facility with
Reactor Configuration 1. Note that the tem-
perature reported refers to the ambient air
temperature, not the temperature of the
reactor, which should be substantially
higher (20 to 40°C.) due to thermal en-
ergy dissipation of the UV lamps. Inhibi-
tion of the VOC conversion rate by the
presence of water vapor is substantial at
the pilot-scale as evidenced by a com-
parison of Run 1 with Run 2, and by a
comparison of Run 7 with Run 8. The
inhibition effect appears to be more se-
vere at the lower TCE concentrations
(Runs 1 and 2). Second, higher destruc-
tion efficiencies are achieved for lower air
flow rates (compare Runs 3 and 4). This
behavior is essentially a residence time
effect expected for rate-based processes.
Although this reactor configuration
yielded acceptable performance charac-
teristics with respect to throughput and
light utilization, the packed bed design
used a support material that was difficult
to coat with catalyst, and the loosely
packed bed was cumbersome to exchange
if a catalyst replacement was necessary
or desired. Moreover, because the sup-
port was organic polymer based, it tended
to degrade under UV irradiation. An im-
proved reactor configuration was there-
fore designed, constructed and tested
based on experience gained in the labo-
ratory and in these pilot-scale tests.
Set 2: Reactor Configuration 2 Table
4 summarizes TCE conversion perfor-
mance for a set of short-term, controlled
pilot-scale runs at Zentox's production fa-
cility with a layered, perpendicular flow
PCO reactor (Reactor Configuration 2).
This design employed consecutive, alter-
nating banks of lamps and catalyst-coated
panels.
In a general qualitative sense, this re-
actor configuration exhibits the same per-
Airin
Catalyst-coated packing
UV lamps
UV lamp bank
Catalyst panels
Air in
Air out
Figure 6. Photocatalytte oxidation reactor configurations. Configuration 1 (top) is a packed bed design, and Configuration 2 (bottom) is a panel bed desig
6
-------
Table 3. TCE Conversion Results for Reactor Configuration 1
Run UV Air Flow Temp. TCE RH
No. Lamps
(scfm)
(ppmv)
Conversion Photon
(%) Efficiency (%)
1
2
3
4
5
6
7
8
22
,22
22
22
22
0
22
22
28
28
28
20
38
39
39
39
21
33
24
24
24
22
22
32
22
19
21
31
15
67
69
64
76
23
45
45
44
68
68
25
55
90
90
96
90
5
77
90
1.2
1.7
1.9
2.1
1.9
—
7.4
8.0
Table 4. TCE Conversion Results for Reactor Configuration 2
Run
No.
10
11
12
13
14
15
16
17
18
)9
20
UV
Lamps
18
18
18
18
18
18
18
46
46
50
50
Air Flow
(scfm)
28
43
27
43
22
41
25
50
50
75
75
Inlet
Temp.
(°c.)
28
29
23
24
24
26
26
29
32
27
41
TCE
Inlet
(Ppmv)
85
55
81
29
29
53
76
45
46
52
49 "
RH
(%)
24
24
47
45
43
60
45
40
20
70
21
TCE
Conversion
(%)
90
81
85
77
89
62
87
91
99
36
92
Photon
Efficiency
(%)
9.4
8.4
8.0
4.3
2.5
6.0
7.1
3.5
3.9
2.2
5.3
i 10
_ 8
0 Configuration 2
© Configuration 1
6..
4. .
o
£ 2
0 10 20 30 40 50 60 70 80 90
Inlet VOC Concentration (ppm)
.Figure7. Integrated photon efficiency versus inlet TCE concentration for the two pilot reactor
configurations.
formance behavior as the first reactor con-
figuration. A direct performance compari-
son of the two reactor configurations is
not possible since the reactors were not
tested under identical conditions of flow
rate, VOC concentration, relative humid-
ity, and temperature. Apparent photon ef-
ficiency is a performance metric adopted
by PCO equipment suppliers to allow com-
parison of the inherent energy efficiencies
of different photoreactor designs. Figure 7
is a visual comparison of the effective
photon utilization in each configuration ver-
sus inlet VOC concentration. Although
there are significant run-to-run variations
in process parameters, the figure shows
that the two designs behave roughly the
same at equivalent inlet VOC concentra-
tions in terms of effective photon utiliza-
tion. The lower fabrication cost and simpler
maintenance make the second configura-
tion a superior design.
Conclusions
Controlled testing of two pilot-scale re-
actor systems revealed that they exhibit
qualitatively similar performance charac-
teristics; Integrated photon utilization
efficiencies depend on inlet TCE con-
centration, with higher efficiencies ob-
served for higher inlet concentration.
The primary advantages of the second
configuration lie in ease of fabrication and
maintenance. In particular, the second or
panel configuration utilizes a catalyst sup-
port that is simple to produce, and that
allows simple, rapid installation or ex-
change of the active catalyst loaded pan-
els.
Based on the results of our bench-scale
studies, we can make the following con-
clusions on PCO process performance for
the target application:
• Although a large process conditions
window exists for which TCE can be
destroyed, frichlorethane (TCA) is
nonreactive. Thus, PCO as presently
employed is not a viable technology
for treating TCA-contaminated air
streams.
• Increasing incident UV light intensity
and mean gas residence time
increase TCE conversion. The
quantitative operating window
(combination of process conditions)
required to yield 95% TCE destruction
can be predicted through an empirical
response surface model. In general,
UV intensities greater than 3.5 mW/
cm2 and residence times greater than
2.5 s will yield greater than 95% TCE
destruction.
• Water vapor significantly inhibits the .
VOC destruction rate for chlorinated
-------
ethylenes such as TCE. For this
reason, PCO units should be placed
downstream of a dehumidification unit
located between air strippers and the
PCO unit. For high VOC conversion
operation in the PCO unit, these
dehurnldiflers should reduce the RH
to less than 50%.
Principal carbon-containing products
of POO identified experimentally at
the bench scale include CO2, CO,
and phosgene (COCL). Failure to
close carbon mass balances under
some conditions suggests that not all
byproducts were identified.
The presence of secondary
chlorinated ethylenes and chloro-
benzene did not significantly affect
the conversion of the primary VOC
TCE. However, selectivity to the
desired complete combustion product
carbon dioxide decreased with
increasing co-contaminant concen-
tration, suggesting that the presence
of the co-contaminants enhanced the
production of undesirable partial
oxidation products.
TCE destruction activity could be
maintained for up to thirty days on
stream. Results suggested that
chlorine atoms build up on a fresh
catalyst surface during an initial period
of operation until the surface is
saturated. During this time, only small
amounts of chlorine-containing
products are detected in the reactor
effluent. After this initial period of
operation, Cl-containing compounds,
including HCI, CI2 and COCI2 are
evolved from the surface.
Recommendations
Based on the research described in this
report, the following recommendations are
made for further investigation:
• The hypothesis that the lack of carbon
mass balance closure under some
conditions was due to undetected/
unidentified volatile partial oxidation
intermediates should be tested.
• The preferred PCO reactor
Configuration 2 should be tested at
an acceptable field site so that the
objectives related to practical
Rajnlsh Changrani and Gregory B. Raupp are with Arizona State University,
Tempo, AZ85287-6006. Craig Turchiis with NEPCCO Environmental Systems,
Oca/a, FL 34470.
Norms M. Lewis Is the EPA Project Officer (see below).
The complete report, entitled "Integration of Photocatalytic Oxidation with Air
Stripping of Contaminated Aquifers," (Order No. PB99-127920; Cost: $25.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-605-6000
Tha EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U,S. Environmental Protection Agency
Cincinnati, OH 45268
integration of PCO with air stripping,
and perhaps vacuum soil vapor
extraction can be achieved.
• First principles modeling of the
preferred PCO reactor configuration
should be undertaken to allow reactor
optimization to be performed in a
rapid, efficient and scientifically sound
manner. The modeling effort should
include submodels for the UV radiation
field and intrinsic VOC destruction rate
expressions, as well as the reactor-
scale advection-diffusion-reaction
model.
• Testing efforts should be focused on
sites that are contaminated with
chlorinated ethyienes. Avoid sites
contaminated with such recalcitrant
compounds as aromatics 'and
chlorinated ethanes.
• Equipment development efforts should
continue to focus on advanced reactor
designs. Based on previous
experience, it is likely that a large
margin for process performance
improvement exists through this
avenue.
The full report was submitted in fulfill-
ment of contract number CR 821100-01-0
by Arizona State University under the
sponsorship of the United States Environ-
mental Protection Agency. The report cov-
ers a period from October 1, 1993, to
June 30, 1997, and work was completed
as of June 30, 1997. !
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United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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detach or copy, and return to the address in the upper
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upper left-hand corner.
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EPA/540/SR-98/504
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