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

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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

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          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

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                                                              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.

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    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)

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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

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 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

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   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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