Systematic selection of off-gas treatment at the Savannah River Site Page: 4 of 11
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-► CLEAN AIR
SOLUTION TO OUTFALL
VOC-LADEN | HEAT !
Affi * ‘exchanger!-
Figure 1. Catalytic Oxidation Process Diagram
cost and should be avoided if permit limits
allow other alternatives. Maintenance costs
are relatively low, and a thermal incineration
system should be one of the least demanding
systems to operate and maintain.
A heat exchanger is a highly recommended
alternative to reduce lifetime operating costs.
The cost of the heat exchanger can usually be
recovered in one to two years. Typical
conventional heat exchangers can obtain 40 to
70 percent heat recovery. This energy savings
will greatly lower the life cycle cost of a
thermal incineration system. Regenerative
thermal incineration systems use a heat
exchanger system that is claimed to recover up
to 95% of the combustion energy. Some
manufacturers offer a system that contains
several beds of ceramic material that act as
heat sinks. The process is operated by
alternating treatment between the beds. When
the beds are switched, the VOC stream is
preheated by passing it through the recently
heated bed before it is combusted. This
method of recovering heat could result in the
lowest cost system depending on the VOC
concentration and the amount of air to be
Catalytic oxidation is a method that lowers the
high energy costs of thermal incineration by
lowering the reaction temperature. A
schematic diagram of the process is shown in
Figure 1. Normally, a gas fueled preheater
provides heat and combustion air to the
system. The hot gases arc then mixed with the
VOC laden air stream to the required
temperature, and the heated stream is fed to
the catalyst bed where the oxidation reactions
occur. Certain noble metals (platinum,
palladium, etc.), typically supported by a
ceramic matrix, act as catalysts to enhance the
reaction rate and lower the reaction
temperature (600 - 800 0 F). These two factors
reduce the energy requirements for catalytic
oxidation considerably below thermal
incineration. The overall operating costs,
however, are not reduced by the full amount of
this energy savings. The catalyst bed must be
replaced, on average, every three years. One
potential problem with the technology is the
fact that many noble metals are poisoned or
masked by chlorides, sulfur, lead, silicones, or
phosphorous containing compounds. The
presence of these substances could greatly
shorten the life of the catalyst used. A limited
number of manufacturers now make catalysts
which are resistant to chloride poisioning
problems. A heat exchanger is also
recommended for use with a catalytic oxidation
system. While the cost savings are not as
dramatic as those for a thermal system, the
cost of a heat exchanger can still be recovered
in approximately two years.
Carbon adsorption typically uses a granular
activated carbon (GAC) bed to adsorb the VOC
onto the carbon. A schematic of the process is
shown in Figure 2. This method of VOC removal
has been used for years in a number of
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McKillip, S.T. & Rehder, T.E. Systematic selection of off-gas treatment at the Savannah River Site, article, January 1, 1992; Aiken, South Carolina. (digital.library.unt.edu/ark:/67531/metadc1072116/m1/4/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.