PCBs AND ASSOCIATED AROMATICS 931
A secondary reaction chamber maintains gases and sus-
pended particulates at about 1100°C for 2 seconds in order
to ensure that the required DRE is obtained. A wet scrubber
is used to neutralize the acidic off-gas.
The Penberthy glass furnace for hazardous waste
was developed by Penberthy Electromelt Inc. in Seattle,
Washington. The Unit is a tunnel incinerator with a pool of
molten glass maintained electrically at 1200°C. The high
thermal capacity bed acts as the heat transfer medium.
The unit is insensitive to the form of the waste introduced
and the ash is periodically removed with the molten glass.
The product is a non-leachable solid matrix which can be
landfilled.
Offgas scrubbing is accomplished using either calcium
carbonate towers, which react to produce calcium chloride,
or wet scrubbing towers using alkaline water followed by
an electrostatic precipitator. Particulates are removed with
a ceramic filter element. The capacity of the unit can be
provided from 100 lb/h to 25,000 lb/h. Its application to the
incineration of PCBs has not yet been documented and its
usefulness will probably depend on the relative cost of alter-
native energy sources.
Shirco Infrared Systems have applied a large scale com-
bustion system in which the waste was treated by infrared
energy developed by electrically powdered silicon carbide
rods in a primary chamber at 1000°C. The residence time
of material could be adjusted between 10 and 90 minutes.
Volatile materials were given a 2 second residence time in a
secondary chamber at 1260°C.
Each mobile unit could be erected on-site in about a
week and be capable of processing between 10 and 45 Kg
per hour.SUPERCRITICAL WATER OXIDATIONThe oxidation of organics by supercritical water oxidation
is being developed by Modar Inc. of Natick, Massachusetts.
A mobile pilot plant is being developed and has shown
destruction efficiencies for chlorinated organics of 99.99%.
The equipment is mounted on a flatbed truck with a capac-
ity of about 0.5 gal. of organics/min. A fullscale unit with a
capacity of 25,000 gallons per day is being designed.
The operating principle utilizes the properties of water
above 375°C and 3,200 psi where the liquid and vapor
phases have the same density (about 0.05 to 0.30 g/mL).
Under these conditions hydrogen bonding is much reduced
and the dielectric constant is diminished to between 3 to 10
with the result that water becomes a much better solvent for
organics and salts are only partially ionized.
High pressure air or oxygen injected into the system
causes rapid oxidation of organic material to carbon
dioxide and water with chlorine eliminated as chloride.
Inorganic salts are precipitated and the supercritical water
used for power generation or high pressure steam. The
waste stream is slurried with water to give a mixture of
5 to 10% organics and is heated and pressurized to super-
critical conditions without charring. The exothermic heatof reaction is able to sustain the process at about 550°C.
At the present time the application of this technology is
assessed on a site by site basis. Supercritical oxidation of
2,3,7,8-TCDD wastes has been shown to take place with a
DRE of 99.9999%.WET AIR OXIDATIONWet air oxidation of PCBs and aqueous wastes has been
developed by Zimpro Inc., Rothschild, Wisconsin. Zimpro is
a subsidiary of ENSCO. The operating principle of the pro-
cess is similar to that of supercritical water oxidation. The
pressure is adjusted to the autogenous pressure of water at the
operating temperature, that is, in the range 300 to 3,000 psi
between 175°C and 320°C. The process is primarily aimed at
the treatment of aqueous wastes which are either too dilute to
incinerate or which are toxic to biological treatment systems.
If the chemical oxygen demand (COD) is greater than 10 to
15 g/L the exotherm of the oxidation reactions is sufficient
to sustain the reaction, whereas incineration does not usu-
ally achieve energy sufficiency until the COD of the waste
reaches 300 to 500 g/L, depending upon the temperature.
Chlorinated aromatics containing non-halogen func-
tional groups such as phenols and anilines are relatively
easily oxidized by the process compared with chloroben-
zenes or PCBs. Zimpro is therefore evaluating the use of
homogeneous catalysts to increase the destruction efficiency
of refractory organics.
The Wetox process is similar to Zimpro’s technology.
It has been developed by the Ontario Research Foundation
and is being marketed by Wet Com Engineering Ltd.,
Toronto, Ontario. A 25 gpm full scale unit has been put
into successful operation at a chemical plant.
The turbulence produced by mixers in the reactor vessel
creates eddies and induces the formation of bubbles. The air
or oxygen in the system can then react at a faster rate and
this, in turn, allows operation of the system at temperatures
which are considerably less than in the wet oxidation pro-
cess (about 225°C/580 psi compared with 175°–320°C/300–
3,000 psi). The Wetox process typically reduces the COD
of the waste feed by 75 to 95% and produces low molecular
weight species in addition to carbon dioxide. Its usefulness
for the oxidation of PCBs has not been reported.MOLTEN SALTThe molten salt process is an old technology which has been
considered during the past 25 years as a method for coal
degassification. It can, however, be applied (Barclay, 1980^93 )
to the destruction of organic and inorganic hazardous wastes,
including PCBS. Rockwell Energy Systems Group has devel-
oped a system to treat both liquid and solid PCB waste. In
1986 Hydro Quebec built a $4 million salt system housed in
several forty foot trailers. The PCB material is introduced at
the bottom of a pool of molten sodium carbonate which is
maintained at a temperature of about 900°C and reacts as itC016_003_r03.indd 931C016_003_r03.indd 931 11/18/2005 1:12:43 PM11/18/2005 1:12:43 PM