Environmental Engineering Reference
In-Depth Information
option than collecting them once they get mixed with high volumes of air. Moving large
amount of air requires high power motors that eventually consume massive amounts of energy.
Reducing emissions from processes
VOCs
As in the case of particles, when processes produce VOCs, the best approach is to capture
these streams at the source before they get released into the plant and become fugitive VOCs.
Ideally, totally closed equipment is the most efficient way to capture VOCs. When not pos-
sible, recapture in the vicinity of the equipment using hoods or baffles is a second less desirable
option. In this case, powerful vacuum systems are necessary to suck high volumes of air that
contain the VOCs. A third option is to enclose the source in a room and capture the VOCs with
an air-handling system. Again, the more concentrated the captured VOCs are, the less energy is
necessary to mobilize air and in downstream operations to remove the VOCs.
Several technologies are available for the removal of VOCs from airstreams: thermal
oxidizers, catalytic oxidizers, carbon adsorption, and gas scrubbers. Thermal oxidation
consists of burning the VOCs at very high temperatures (760 to 871°C) thus transforming the
VOCs into carbon dioxide and water. Regenerative thermal oxidation is a fuel-efficient version
that captures heat generated during the combustion and uses it to preheat incoming air.
Catalytic oxidizers are thermal oxidizers, with regeneration or not, that use a catalyzer to
decrease the burning temperatures to a range between 315 and 538°C (Sims, 2009). Both
systems require the input of auxiliary fuel (generally natural gas) to develop the temperatures
needed for an efficient destruction of the offensive compounds.
Carbon adsorption is suited for airstreams with low concentrations of VOCs. The air con-
taining VOCs is passed through an activated carbon bed where the organic compounds are
adsorbed and a clean airstream exists in the adsorber. These systems are normally semicon-
tinuous in nature because they require regeneration of the carbon bed once it gets saturated.
Scrubbers are probably the most versatile equipment to clean VOCs from airstreams. With
the right scrubbing liquid, practically any compound can be removed. Depending on the
volume of air to treat, scrubbers can be small compact units sitting in a corner of a plant or
large devices with hundreds of horse power motors that run the fan to mobilize the air through
the equipment. Basically, a scrubber is a device that put gas (carrying contaminants) in contact
with a liquid (the absorbent) for a period of time long enough for the contaminants to diffuse
from the gas to the liquid phase. The absorbent is a water-based additive that either chemically
reacts or has a high affinity for the contaminants. During operation, the adsorbent is recircu-
lated by means of a pump until it gets saturated with the contaminants and needs to either be
discharged into the wastewater treatment system or regenerated.
Scrubbers and activated carbon filters are typically used not only for removal of VOCs but
also for capturing odors. They have both advantages and disadvantages. Activated carbon
filters are more suited to handle smaller volumes. Scrubbers, on the other hand, can be scaled
up to handle large volumes of air but with the penalty of high-energy consumption to run fans
and pumps.
Waste and waste treatment
These topics are covered extensively in Chapters 9 and 10; therefore, this section presents only
a short description. Solid and liquid wastes are important sources of emissions, particularly
liquids, because they are prone to fermentation with the resulting emission of GHGs.
