United States
Environmental Protection
Agency
EPA/540/F-95/505
May 1995
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology Bulletin
Development of a Photothermal Detoxification Unit
Environmental Science and Engineering Group
University of Dayton Research Institute
Technology Description: The University of Dayton Research
Institute has developed a novel photochemical process embodied
in a device called a Photothermal Detoxification Unit (PDU) which
offers an efficient means of destroying hazardous organic wastes.
The PDU, which overcomes the problems of slow reaction rates
and incomplete destruction of hazardous materials often associ-
ated with photochemical waste reduction, is a relatively simple
device. It consists of a thermally insulated vessel (Figure 1)
Side View
Top View
General Layout of a Four Chamber PDU
10 cm
Side View Top View
Chamber Details
Figure 1. UDRI Photothermal Detoxification Unit (PDU)
enclosing a set of large, medium pressure mercury vapor lamps
which provide an efficient source of near-UV radiation as well as
heat for the process. The PDU uses the radiation from the lamps
to induce destructive photochemical reactions at moderate tem-
peratures (200-600°C) so they proceed to completion quickly and
efficiently. Since the process requires both light and heat, it is
referred to as a photothermal detoxification process.
The process is capable of destroying organic materials at tem-
peratures much lower than thermal processing alone and at
temperatures easily achievable through non-combustion means.
The specific exposure time, temperature and radiant intensity will
be largely dependent on the materials of interest and the required
level of destruction. In general, these aspects of the PDU design
and operation are brought together through a reactor perfor-
mance model such as:
f =exp[-(kgn
gnd
kab0)t]
where fr is the fraction remaining in the process stream exiting
the PDU, K nd is the rate of thermal reactions, Kab is the rate of
light absorption, 0 is the efficiency of the photothermal reactions
(quantum yield), and t is the mean exposure time.
To predict the performance of the PDU, it is necessary to have
knowledge of the rates of thermal reactions (Kgnd), the photothermal
quantum yields (0), and the UV absorption rates (Kab) of the
system. Since none of the required high temperature spectro-
scopic and photochemical data is available from the literature,
researchers at UDRI designed and built a special high tempera-
ture spectrophotometer and a bench-scale photothermal detoxifi-
cation unit for the basic thermal and photothermal information.
Waste Applicability: Organic compounds which efficiently ab-
sorb near-UV radiation are relatively easily destroyed by the
photothermal process. Toxic organic compounds whose molecu-
lar structure includes alkene or aromatic structures (i.e., chlori-
nated alkenes, chlorinated aromatics, chlorinated dibenzo-
p-dioxins, etc.) are likely to absorb the near-UV radiation which is
necessary for the photothermal detoxification process. Molecules
which only weakly absorb near-UV radiation (i.e. alkanes and
chloroalkanes) may require deep UV sources such as low pres-
sure mercury lamps.
Laboratory and Bench-scale Test Results: Photothermal
detoxification at elevated temperatures improves the overall effi-
ciency of the process in three important areas: the spectroscopy,
Printed on Recycled Paper
-------
the rate of destruction, and the completeness of the destruction.
The most important aspect of the process is whether the light
absorbed by the waste feed results in the destruction of the
waste feed. The Laboratory Scale-Photothermal Detoxification
Unit (LS-PDU) data for trichloroethylene (TCE) exposed to 18.1
W/cm2 of xenon arc radiation for 10 sec in air showed that the
process is capable of destroying a significant portion of the TCE
where no thermal destruction is occurring. For example, at 500°C
the thermal decomposition has not yet begun, while the
photothermal process has destroyed approximately 60% of the
sample.
The last important aspect of the photothermal process is its
ability to completely mineralize the waste feed. The data for
1,2,3,4-tetrachiorodibenzo-p-dioxin (TCDD) demonstrated that the
process can easily destroy this type of hazardous material which
has traditionally challenged conventional waste disposal tech-
niques. For example, GC/FID chromatograms from TCDD ex-
posed to at 600°C photothermal (17.6W/cm2) for 10 sec in air
show that not only is the parent TCDD destroyed, but nearly all
the associated products of incomplete conversion (PICs) as well.
Under the same thermal conditions 35% of the TCDD remained
with numerous organic compounds.
A large-scale PDU should include at least four cylindrical reactor
chambers operating in series, enclosing lamps mounted near the
chamber centerline, and at a relatively high temperature (500-
600°C). Linear, medium pressure mercury lamps are the most
suitable for a large-scale PDU because of their high near-UV
output, long service life, and geometry. The capacity of the
system can be adjusted by selecting appropriate operating condi-
tions, (number of lamps, operating temperature, etc.), operating
chambers in series to increase efficiency and capacity, or sets of
chambers in parallel.
A project summary and complete report have been submitted
and will be available in the near future.
For Further Information:
EPA Project Manager:
Chien T. Chen
U.S. EPA Risk Reduction Engineering Laboratory
2890 Woodbridge Avenue
Edison, NJ 08837-3679
(908) 906-6985
Technology Developer:
Barry Dellinger or John Graham
University of Dayton Research Institute
300 College Park
Dayton, OH 45469-0132
(513) 229-2846
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/540/F-95/505
------- |