Page 300 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 279

Solution

1. The past problem is present as follows (36). Packing materials (hereafter referred to
as packing) have been used to enhance gas–liquid contact in chemical engineering
scrubber and stripping unit operation systems (hereafter referred to as tower) as stan-
dard engineering practice for several generations. Many environmental engineers
originally graduated as chemical engineers from their respective university or college
and modern environmental engineering degree programs require several courses be
taken in chemical engineering before graduation. Therefore, the principle that
gas–liquid contact must be maximized for optimum absorber tower performance is
universally understood and accepted.

The packing in an absorption tower is placed there to optimize contact between the
two phases present (liquid and gas) so that a target pollutant in one phase will transfer
into the other phase. As has been previously described in this handbook, the actual
direction of this movement defines a scrubbing (from the gas into the liquid) process
or a stripping (from the liquid into the gas) process. The historic solution to maximiz-
ing gas–liquid contact has been to design the packing with more and more complicated
shapes (38). Famous packing materials (mass transfer media), such as Saddles, Pall
rings, Tellerettes, and Tri-Packs, were patented in 1908, 1925, 1964, and 1973–1978,
respectively. When packed in the absorption tower, such media tend to spread the liquid
into a thin film over the surface of the packing to maximize the liquid–gas contact (38).
A standard measure by which competitive packing products have been historically
compared is as specific surface of media in square feet per cubic foot of the media
2
3
(ft /ft ). With this parameter, the environmental or other design engineer could assess
the area of available packing surface upon which the liquid in the tower could form
a film. A higher specific surface of a media product is equated to larger film sur-
faces. This, in turn, meant that when comparing media products, the media with the
largest specific surface most likely promoted the most efficient mass transfer in an
absorption tower.

Therefore, suppliers of packings responded by inventing products of with increasingly
complicated designs, as well as smaller sizes per individual piece, to increase the spe-
cific surface area of their products. The problem with these early packings is that more
surface and more pieces of media per cubic foot increase costs. This is the result of
increased raw material needed to produce a smaller packing. Operating costs of an
absorption system also increase as a result of an increased pressure drop when a
smaller packing is chosen. Therefore, capital costs increase as towers are sized larger
to minimize pressure drop.
2. The engineering solution is described here (36). Not satisfied with the need to trade
increased costs for improved performance in absorber tower performance, starting in
1987 innovative engineers of many packing manufacturers began introducing new,
high-technology packings. Lantec Products alone patented LANPAC, NUPAC, and
Q-PAC in 1988, 1992, and 1996, respectively. The authors choose one of the best, Q-
PAC, for illustrating how an advanced packing has been conceptually developed, intro-
duced, tested, manufactured, and eventually patented for commercial applications.

The introduction of the latest mass transfer media has revolutionized tower designs.
Previously discussed in this handbook were examples of how towers are significant-
ly smaller when designed with modern media than is possible with any other early
commercial media products.