Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.4 Application of solar photocatalytic oxidation 1559
22.4 APPLICATION OF SOLAR PHOTOCATALYTIC OXIDATION TO
VOC-CONTAINING AIRSTREAMS
K. A. Magrini, A. S. Watt, L. C. Boyd, E. J. Wolfrum, S. A. Larson,
C. Roth, and G. C. Glatzmaier
National Renewable Energy Laboratory, Golden, CO, USA
22.4.1 SOLVENT DEGRADATION BY PHOTOCATALYTIC OXIDATION
Photocatalytic solvent oxidation had been demonstrated at the pilot scale in two recent field
tests located at McClellan Air Force Base (AFB) in Sacramento, California and at the Fort
Carson U. S. Army Installation in Colorado Springs, Colorado (Watt et al., 1999; Magrini et
al., 1998). The objective of the tests was to determine the effectiveness of solar-powered
photocatalytic oxidation (PCO) treatment units for destroying emissions of chlorinated organic
compounds (trichloroethylene and dichloroethylenes) from an air stripper at ambient
temperature and destroying paint solvent emissions (toluene and MEK) from a painting facility
at higher temperatures. Goals for field testing these solar-driven systems were to
gather real-world treatability data and establish that the systems maintained performance
during the duration of the testing.
The photocatalytic oxidation process can effectively destroy hazardous organic pollutants
in air and water streams. Although treatment systems will vary depending on the
type of stream being treated, the basic process remains the same. The key ingredient is the
photoactive catalyst titanium dioxide (TiO2), which is an inexpensive, non-toxic material
commonly used as a paint pigment. When TiO2 is illuminated with lamps or natural sunlight,
powerful oxidizing species called hydroxyl radicals form. These radicals then react
with the organic pollutant to tear it apart and ultimately form carbon dioxide (CO2) and water
(Phillips and Raupp, 1992). When halogenated organics are treated, dilute mineral acids
like HCl form. The process works at both ambient and mildly elevated temperatures
(>200 o C) (Fu et. al., 1995; Falconer and Magrini, 1998).
Researchers throughout the world have been investigating PCO as an advanced oxidation
technology for treating air and water streams contaminated with a variety of organic
and inorganic compounds (Blake, 1996; Cummings et al., 1996). The susceptibility of an
organic species to complete oxidation is typically reported in terms of photoefficiency, defined
as the number of molecules of contaminant oxidized to carbon dioxide, water, and
simple mineral acids divided by the number of photons incident on the catalyst. These values
vary widely, depending on the reactor design, catalyst geometry and the compound of
interest.
Much of the work on photocatalytic oxidation focuses on treating the halogenated
organics trichloroethylene (TCE) and perchloroethylene, contaminants commonly found in
ground water sources. These compounds and other chlorinated ethylenes typically react
rapidly with TiO2 and photons at efficiencies greater than 100%. These rates are likely due
to chain reactions propagated by chlorine radicals (Luo and Ollis, 1996; Nimlos et. al.
1993; Yamazaki-Nishida, 1996). Paint solvent emissions generally consist of toluene,
xylenes, ketones and acetate vapors. Measured photoefficiencies for benzene and other aromatic
compounds like toluene are typically less than 5% (Gratson et. al., 1995; d’Hennezel