Page 296 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 275

odor control (Example 19), the offending species is H S (or some other malodorous
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gas) and it is controlled by being absorbed into a passing liquid phase. Mass transfer
from gas to liquid defines a scrubbing situation.
On the other hand, in certain situations the desire is to remove a given species from a
liquid into a passing gas phase. This is the definition of air stripping. Using the exam-
ple of H S gas, when H S is dissolved in groundwater in small amounts, the offending
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odor of H S may prevent the use of an otherwise potable water source. Therefore, a
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stripping tower is one plausible technique to use to remove the H S from the water.
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Other possible solutions are aeration or tray tower technologies (47,50–55). The actu-
al choice of removal technology will depend on the space available for the equipment
used to treat the water. An aeration basin will require a large available area and will
lose significant amounts of water to evaporation, as well as have high power (and
hence operating) costs. A tray tower will be less costly to fabricate than a stripper sys-
tem. However, if the water flow being treated is large, the large pressure loss in a tray
tower and subsequent cost of operation will make a stripper tower the logical choice
to treat the water.
Briefly, in mass transfer, a species must leave one phase and enter another phase. This
movement of a molecule from one phase to another is treated extensively in standard
academic texts by McCabe et al. (38). The two-film theory presented by McCabe et al.
(38) is widely accepted as the model to explain how mass transfer occurs in both a
scrubber and a stripper tower.
A simple, graphical explanation of the two-film theory of mass transfer is presented by
Heumann (56). The concentrations of the species being scrubbed/stripped at the film
interface will be less than the bulk concentrations of the species in the bulk phases as
the specie transfers from one phase to the other. The difference in concentration
between the bulk phases, and actually between the two-film interface, is the driving
force to mass transfer. If the concentrations of the species at the film interface are equal
to the bulk concentrations of the same species in the bulk phases, no mass transfer will
occur.
In actual practice, the specie being treated in the system will have limited solubility
in one of the two phases. In a scrubbing situation, the specie being scrubbed must
cross the barrier of the gas film in order to pass into the liquid film. This resistance
of passage of the molecule out of the gas film is the limiting factor to mass transfer
in a scrubber system. So, with exceptions noted below, scrubbing is said to be gas
film controlled.
The exceptions referred to are CO , NO , phosgene, or similar scrubbing situations.
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Although these gases have high solubility in water and one would think that as such
gas film resistance would limit their mass transfer in a scrubber, in reality these and
similar compounds are liquid film controlled in a scrubber system. This is so
because, although readily absorbed into water, the subsequent chemical reactions of
these compounds in water are relatively slow, therefore, the liquid film resistance is
the controlling factor when scrubbing these compounds from an air emission stream.
In a stripping situation, the specie of concern is moving in the other direction, out of
the liquid film into the gas film. Thus, in a stripping situation, the limiting factor to
mass transfer is the ability of the molecule in question to break out of the liquid film
to enter the gas phase. Thus, with very few exceptions, stripping is said to be liquid
film controlled.