Page 342 - Air Pollution Control Engineering

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Fig. 7. Schematic of an air stripping system. (From US EPA.)

When VOC concentrations are too low for direct condensation, gas-phase carbon
adsorption can provide proper initial treatment. Activated carbon will adsorb the VOC
until the carbon particles are saturated, reaching capacity. The carbon then undergoes
regeneration (heating) to desorb the VOC vapors at a higher concentration, which may
be condensed for removal/recovery. In this case, the control device shown in Fig. 7
will be gas-phase carbon adsorption followed by condensation. The condenser in this
case is used for recovery of VOC from the adsorber during its desorption stage for
adsorbent regeneration (17–18).
A freeze-condensation vacuum system (19) has been developed for both chemical
and environmental engineering applications. The freeze-condenser operates with heat
transfer surfaces below the freezing point of the vapors. VOC or steam solidify on the
heat transfer surfaces through condensation followed by freezing, by direct deposition.
Placing a freeze-condenser upstream of an ejector system traps unwanted vapors before
they enter the ejector system. The ejector system becomes less expensive and requires
less utility consumption. Furthermore, the production of waste and vent streams is
lessened, reducing environmental impacts.
Condensers are sized and their costs estimated by environmental engineers. Once an
air pollution control system involving the use of one or more condensers is properly
designed, individual condensers can be purchases commercially (20).
An effective training program can be provided to a condenser operator, in turn, to
operate the condenser efficiently (22–25). According to Buecker (21), a monitoring pro-
gram can improve operations and yield substantial energy savings for a condenser.

6. DESIGN EXAMPLES
Example 1
Perform the following design steps for a condensing system to remove VOCs from an air
emission stream. Stream characteristics appear in Table 7.
1. Gather important air emission stream characteristics data.
2. Confirm the required VOC removal efficiency (RE).