Handbook of Solvents - George Wypych - ChemTech - Ventech!

1560 K. A. Magrini, et al.
and Ollis, 1997). Besides exhibiting low photoefficiencies, aromatic species tend to form
less reactive, nonvolatile intermediates during gas phase PCO. These intermediates build up
on the catalyst surface and block or inhibit the active catalytic sites for further reaction
(Larson and Falconer, 1997). The addition of heat and small amounts of platinum to the
TiO2 catalyst overcome these problems (Falconer and Magrini, 1998; Fu et. al., 1996). Oxygenated
organics such as ethanol and acetone have photoefficiencies typically around
1%-10% (Peral and Ollis, 1992).
Several field demonstrations of PCO using sunlight to treat groundwater contaminated
with TCE have been reported (Mehos and Turchi, 1993; Goswami et. al., 1993).
These field tests found that nontoxic constituents in the water can non-productively react
with or “scavenge” the photogenerated hydroxyl radicals and reduce the rate of the desired
reaction. Common scavengers such as humic substances and bicarbonate ions increase
treatment costs for the technology (Bekbolet and Balcioglu, 1996). Turchi and co-workers,
(1994) found that by air stripping the volatile contaminants from the water stream, the regulated
compounds at many contaminated sites could be transferred to the air, leaving the radical
scavengers behind. The water can then be safely discharged and the air effectively
treated with PCO. The improved photoefficiency reduces treatment costs and more than
offsets the added cost of air stripping these contaminants from water. Read et. al., (1996)
successfully field tested a lamp-driven, PCO system on chloroethylene vapors from a soil
vapor extraction well located at DOE’s Savannah River Site. Magrini et al., 1996, and
Kittrell et al., 1996, both used modified TiO2 catalysts and lamp-driven reactors to treat
VOCs representative of semiconductor manufacturing and contact lens production, respectively.
1,2-dichloroethane, stripped from contaminated groundwater, was successfully
treated in a pilot scale PCO demonstration at Dover AFB (Rosansky et al., 1998).
The second field test assessed PCO to treat paint solvent vapors. Painting operations
for military and civilian vehicles are conducted in ventilated enclosures called paint booths.
Filters in the exhaust ducts trap paint droplets from the paint overspray while the
VOC-laden air is typically exhausted through roof vents. The vent emissions can contain
several hundred parts per million (ppm) of the paint solvents, which continue to evaporate
from the vehicle after painting is complete. Most types of paint generally contain significant
amounts of VOCs such as toluene, a suspected carcinogen, as well as other hazardous solvents
such as methyl ethyl ketone, methyl isobutyl ketone, hexanes, xylenes, n-butyl acetate,
and other components in lesser amounts.
Current technologies for treating these emissions include catalytic combustion of the
vapors over supported Cu and Cr-oxides at temperatures of 350 o C (Estropov et. al, 1989);
air-flow reduction and recirculation strategies (Ayer and Darvin, 1995); and regenerative
thermal oxidation at near incineration temperatures (Mueller, 1988). The use of platinized
TiO2 and temperatures of 180-200 o C provide significant energy savings in treating these
emissions. Our goals for testing the paint booth emissions was to gather real-world
treatability data and establish that the system maintained performance during the duration
of the testing.
22.4.2 PCO PILOT SCALE SYSTEMS
22.4.2.1 Air stripper application
Figure 1 shows a schematic of the pilot-scale system used at McClellan AFB. This system,
scaled from an optimized laboratory reactor, was fabricated by Industrial Solar Technology