Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.4 Application of solar photocatalytic oxidation 1565
Figure 22.4.5. Solar insolation (left) and fractional TCE conversion (right) versus illumination time. Flow rate and
relative humidity of the incoming air stream was 20 SCFM at 20% RH. Data taken on April 10, 14, and 15, 1997.
Figure 22.4.4 shows the effect of clouds on solar insolation (left) and TCE conversion
(right) as a function of illumination time at 10 SCFM for two separate days. Again TCE conversions
greater than 95% were obtained when the insolation levels were greater than 1.5
mW/cm 2 . In general, clear skies had UV intensities of 2 mW/cm 2 or better. Haze and high
clouds reduced the intensity to 1.5 mW/cm 2 , and thicker clouds reduced available intensity
to about 1 mW/cm 2 . Conversions greater than 90% were still achievable during periods of
significant cloud cover due to the photoreactor’s ability to use global horizontal (diffuse)
UV illumination.
TCE conversion versus solar insolation for three days of operation at 20 SCFM is
shown in Figure 22.4.5. Doubling the flow rate halves the residence time of the gas in the
photoreactor. Conversions greater than 95% are achieved with UV intensities greater than
2.0 mW/cm 2 . The reproducibility of TCE conversion indicates that the little or no catalyst
deactivation occurred during the three weeks of testing.
Pooling all of the data taken at the two flow rates provides a relation between TCE
conversion and UV intensity (Figure 22.4.6). Significant scatter, which exists for both flow
rates, is likely due to the rapidly varying UV intensity levels during cloud events. The variable
illumination makes it difficult
to establish a precise correlation
between an isolated UV measurement
and the UV exposure received
by the gas as it flows through and
reacts with the photocatalyst bed.
The 10 SCFM data suggests a
square root relationship between
UV intensity and conversion. The
square root dependence was observed
in a continuous flow reactor
when TCE concentrations were
less than 60 ppmv (Nimlos et. al.,
1993). Higher concentrations
Figure 22.4.6. Pooled UV intensity versus TCE conversion data for
all 10 SCFM and 20 SCFM runs.
yielded an approximately linear dependence.