932 PCBs AND ASSOCIATED AROMATICS
migrates to the surface. The sodium carbonate is converted to
sodium chloride. PCB destruction in a 6-inch depth of molten
salt was determined in the Rockwell Energy Systems Group
process as a function of the pool temperature using 53–65%
excess air and the destruction efficiencies found to vary in the
range 99.99917% at 745°C to 99.99971% at 912°C. As the
depth of the molten salt pool is increased so the destruction
efficiency increases because of the longer residence time and
Rockwell points out that the deeper salt beds of commercial
units would probably achieve higher destruction efficiencies
than was observed with the laboratory scale system.
The product gases were analyzed for CO, CO 2 , HCl,
Cl 2 and COCl 2. The sodium carbonate tends to react with
acid gases and reduces the need for downstream scrubbing
systems. It was found that less than 2 ppm HCl was formed
provided that 1–2%, Ca 2 CO 3 remained in the melt. Less than
0.25 ppm of phosgene was detected and about 0.5 ppm of
Cl 2 (or perhaps NO x ).
Inorganic, high melting, products such as sodium
chloride build up in the melt until it eventually solidifies
and the destruction efficiency is reduced. The continuous
process provides for this by adding more sodium carbon-
ate to maintain the fluidity of the pool. However, even
when the pool had been almost completely converted to
sodium chloride PCB destruction continued. Under these
circumstances the process can no longer be classified as
a high temperature organic/inorganic reaction but rather is
a pyrolysis. The EPA requires a 1200°–1600°C temperature
for incineration coupled with a sufficient residence time at
the chosen temperature to avoid inadvertent generation of
products of incomplete combustion (PlCs) such as PCDFs
and PCDDs.
Mobile molten salt reactors have been constructed but are
costly. A system with a capacity of 225 lb/h was estimated
by Rockwell to cost approximately $1.5 million. The solids
handling equipment increases the cost by a further $250,000.
The operational costs are not high once the capital cost is
overcome because the reaction exotherm provides heat and
the only reagent expended is sodium carbonate.DIESEL DISPOSALD. and D. Disposals Ltd., based in Smithville, Ontario have
evaluated the use of diesel engines to incinerate PCBs. Three
six cylinder units were mounted onto a flatbed truck to give a
throughout of about 8 gal. of PCB per hour. The cost estimate
for each engine was about $300,000. The Ontario Research
Foundation has undertaken measurements of PCB destruc-
tion efficiencies and found that 99.998% can be achieved
when an 80/20 blend of diesel fuel/PCBs is burned at about
590°C in the cylinder head.OZONOLYSISThe Royal Military College in Kingston, Ontario and the
University of California, Riverside, have both investigatedthe use of ozone to oxidize PCBS. A six- to eightfold excess
of ozone over the amount required for stoichiometric reac-
tion was found to produce a 90–95% conversion of PCBs.
The extent of formation of PCDFs and PCDDs from askarel
fluids can be expected to depend upon the particular reaction
conditions so that the potential problems which the opera-
tion of the method may have are intended to be alleviated
by critical control. About 100 lb of PCBs require approxi-
mately 100 lb of ozone. About 1000 kWh of electricity are
used to produce this quantity of reagent and therefore the
method is not likely to be as cost competitive as some incin-
eration processes although it may be proven useful for the
destruction of PCBs diluted into trace quantities in the envi-
ronment. The application of the ozonolysis reaction to PCB
contaminated waters is discussed below in the discussion on
radiation methods.
Palauschek and Scholz^94 have applied ozone destruction
methods to PCDFs and PCDDs in water. They found that
degradation takes place only under alkaline conditions of pH
10 as a result of the generation of hydroxyl radicals by reac-
tion between ozone and water. Ozone does not directly attack
PCDDs/PCDFs in aqueous solution. The method is therefore
not likely to be useful for the treatment of oily wastes in
which the chlorinated aromatics are confined to the oil.CHLOROLYSISHydrocarbon wastes have been reacted into useful products
by adding rather than removing chlorine. The chlorolysis
reaction has been used on a large scale by Hoechst A.G.
(Jackson^95 ). The process involves the reaction of chlorine
gas at high temperature (about 640°C) and pressure (about
25 MPa) with the partly chlorinated waste stream to yield
carbon tetrachloride and hydrogen chloride. The generalized
reaction isC x H y Cl z + (4x + y − z)/2Cl 2 → X CCl 4 + y HClThe method has been used primarily to treat aliphatic
wastes rather than PCBs and is limited to feedstocks which
contain approximately 5% aromatics unless considerable
modifications are undertaken. The reaction conditions nec-
essary to treat askarels may involve an increase in the pro-
cess residence time or the use of a catalyst to promote the
reaction. The reaction products are easily separated from
the process by distillation and the mixture which remains is
then recycled to the reactor.
Carbon tetrachloride and hydrogen chloride are market-
able reaction products. However, the health effects associ-
ated with carbon tetrachloride have tended to curtail its use.
The reaction is exothermic, which helps to keep operating
costs low, but the disadvantage of the process lies in the need
to use high purity nickel construction at a high capital cost
in order to overcome the extremely corrosive nature of the
chemicals involved.C016_003_r03.indd 932C016_003_r03.indd 932 11/18/2005 1:12:43 PM11/18/2005 1:12:43 PM