Chemical Engineering July 2023 - 26

pounds with good to moderate water
solubility are found in the waste gas,
oxidation chemicals, such as hypochlorite
or peroxide, are required.
Air stripper systems. Air stripper
units are designed to remove hydrocarbons
or chlorinated compounds
from water. Recovered groundwater
is pumped into the top of the system.
As contaminated groundwater enters
through the top of the air stripper,
millions of air bubbles are forced
by the blower to pressure up through
the perforated trays, vigorously aerating
the water to a froth and removing
VOCs as gravity pulls the
water down through each tray in the
stripping column.
Oil-water separator systems. Oilwater
separators are designed to accelerate
natural separation between
water and hydrocarbons. Recovered
groundwater is pumped into the first
chamber of the system, where small
oil droplets contained in the mixture
coalesce and form larger droplets
that rise to the surface. The large
oil droplets are then collected for
proper disposal.
Carbon-bed systems. Carbon
beds work by physically adsorbing
pollutants from the air. Adsorption
is a process in which pollutant
molecules adhere to the surface of
the activated carbon. Although it is
evaluated first as a Best Available
Control Technology (BACT), carbonbed
technology may not always be
the best longterm solution, because
carbon replacement and usage can
be expensive. There are also safety
concerns associated with carbonbed
fires if VOCs cause an exothermic
reaction. To reduce such risks,
proper air circulation should be included
in the design of the bed. Depending
on the VOCs, other types of
concentrators can be used.
Fabric filters. Also known as baghouses,
fabric filters are used for control
of particulate matter. These filters
consist of numerous fabric bags or
tubes through which the gas stream
passes. The fabric material captures
and retains the particulate matter, allowing
the clean gas to pass through.
Periodically, the collected particulate
matter is removed from the bags using
mechanical shaking or pulsing methods.
Baghouses are widely employed
26
Ship & Shore Environmental
FIGURE 2. Regenerative thermal oxidizers (RTOs) enable a very high destruction efficiency for pollutants.
There are a variety of RTO configurations that provide different benefits in different applications
in industries where high-efficiency
dust control is required, including cement
manufacturing, metal processing
and coal-fired power plants.
Combustion technologies
The majority
of combustion-based
pollution-control methods fall into the
thermal oxidizer category. There are
several varieties of thermal oxidizers,
but they all are designed to sustain
the optimal conditions for oxidation of
combustible components of the gas
stream. This is done by a careful control
of the operating temperature, so
that it is sufficiently above the autoignition
level to provide enough time
and excess oxygen to complete the
necessary reactions.
Regenerative thermal oxidizer
(RTO) systems. RTOs can handle
dilute waste gases and achieve a
destruction efficiency of 95-99% at
operating temperatures of 1,400-
1,500°F (Figure 2). Through use of
a ceramic heat exchanger, a thermal
efficiency of up to 97% can be
achieved. Depending on the VOC
loading, a specially designed RTO
can be used to accommodate higher
solvent loading, with a slightly lower
thermal efficiency, and use of a
hot bypass.
A special poppet-valve design can
achieve 99% destruction without use
of a puff chamber. The " puff " refers to
the small volume of unprocessed air
that may remain after RTO treatment.
In some applications demanding an
extremely high destruction efficiency
(above 99%), the " puff " may require
re-treatment in a dedicated chamber.
On the same production line, there
may be varying exhaust-air volumes,
as well as varying solvent mixtures,
resulting in high flows and volatile
organic compound (VOC) concentrations
of up to 25% of the lower
explosive level (LEL).
Direct-fired thermal oxidizer
(DFTO) systems. DFTOs (Figure 3)
are ideal for very high solvent emissions
(self-sustaining, with VOC concentrations
of up to 50% LEL). They
can achieve a destruction efficiency
of more than 99%, but normally do
not provide any energy recovery. Depending
on the solvent loading, this
system could be a high energy consumer.
However, if periodic vent gas
can be optimized and controlled as
a continuous flow, thermal oxidation
equipment also has heat-recovery
options. If vent waste-gas streams
have a high heating value to sustain
combustion, then thermal oxidation
technologies would prove to be a
better fit, since the gas streams can
be repurposed as fuel gas.
Multi-stage thermal oxidizers.
Multi-stage thermal oxidation systems
operate more like a standard
thermal oxidizer, with sufficient excess
oxygen and temperature to destroy
the combustibles from the initial
stage, while keeping oxygen and
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