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282 Lawrence K. Wang et al.

Each mass transfer problem is unique and deserves individual attention in order that
the most cost-effective and productive solution is obtained for the given set of cir-
cumstances. A responsible environmental engineer should always conduct an
extensive literature study and a pilot-plant study to evaluate and select the most
suitable mass transfer media for the specific scrubbing/stripping applications of
his/her clients.

Example 25
Traditional scrubbing/stripping systems involve distribution of small liquid/slurry droplets
or thin films into the bulk of a flowing airstream (32). Innovative scrubbing/stripping sys-
tems, on the other hand, involve distribution of small air bubbles into a bulk of a flowing
water stream (47–55). Provide a discussion on the following:
1. The flow patterns, advantages, and disadvantages of the innovative scrubbing/strip-
ping systems in comparison with comparable traditional scrubbing/stripping systems.
2. A typical case history of an innovative wet scrubbing system for hydrogen sulfide
reduction from an air emission stream.
Solution
1. The flow patterns, advantages, and disadvantages of the innovative scrubbing/strip-
ping systems in comparison with comparable traditional scrubbing/stripping systems
are discussed here. It has been known that wet scrubbing and gas stripping are both
mass transfer unit operations. Only the direction of the movement of a given pollutant
species is different.
Either the wet scrubbing or gas stripping process will have two streams: a gas steam
and a liquid stream. When the two streams meet in a scrubbing/stripping reactor, the
mass transfer occurs. Because the scrubbing reactor and stripping reactor are similar
to each other, only the wet scrubbers are discussed.

In a traditional wet scrubber, for instance, such as in the previous example of H S
2
reduction (Example 19), the offending specie is H S (or some other malodorous gas)
2
present in an air emission stream or gas phase. The liquid phase is the scrubbing solu-
tion, which is distributed into the wet scrubber as small liquid droplets or thin films.
The traditional wet scrubber is controlled by distributing the liquid phase (i.e., scrub-
bing solution containing the scrubbing chemicals) as liquid droplets or thin films into
a passing bulk gas phase (i.e., air emission stream containing the target pollutant,
H S). The flow pattern in the traditional wet scrubber can be either counterflow or
2
cross-flow. Mass transfer from gas to liquid defines a scrubbing situation.
In an innovative wet scrubber, such as in a new case history for H S reduction to be
2
presented in the second portion of this example, the offending specie is still H S (or
2
some other malodorous gas) present in an air emission stream or gas phase. The liquid
phase is still the scrubbing solution, but it is distributed into the innovative wet scrubber
as a flowing bulk liquid. The innovative wet scrubber is controlled by distributing the
gas phase (i.e., air emission stream containing the target pollutant, H S) as small gas
2
bubbles into a passing bulk liquid phase (i.e., scrubbing solution containing the
scrubbing chemicals). The flow pattern in the innovative wet scrubber can also be
either counterflow or cross-flow. Mass transfer from gas to liquid also defines this
scrubbing situation.
The mass transfer efficiency of a traditional scrubbing/striping process can be
enhanced by packing materials. In comparison with a comparable traditional scrub-