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280 Lawrence K. Wang et al.

The success of Q-PAC is a result of the insight of Dr. K. C. Lang of Lantec Products.
His realization was that additional opportunity to force liquid–gas contact existed that
had been ignored in previous packing designs. In addition to having the liquid spread
into a thin film on the solid surface of the packing, if the packing design could be such
that the liquid was forced to pass through the tower as a shower of droplets, each and
every droplet would offer surface for gas–liquid contact through which mass transfer
would occur.
Prior to this innovation, the primary means of creating liquid surfaces was to
spread the liquid over the media, as previously discussed. However, also as previ-
ously discussed, this additional liquid surface was obtained at a price: (1) higher
media costs as the consequence of smaller media size that requires more raw mate-
rial and more pieces per cubic foot; (2) increased operating costs as the conse-
quence of smaller media size causing pressure drop increases; and (3) increased
capital costs as the result of the need to design larger installations to minimize
pressure drop.
In addition to using a specific surface as a comparison parameter, packing suppli-
ers have provided a parameter called void fraction (or free volume) to describe a
given packing. This parameter is expressed as percent (%) of free space. Although
useful, void fraction is nevertheless always subjective and therefore susceptible to
manipulation. This is so because in addition to the free volume of the packing, the
numbers presented to industry also include the percentage of free space within an
absorber tower that results from the “random dump” of the media into the tower.
This tower free volume will obviously depend on the tower diameter, the overall
packing depth, the type of supports used within the tower, and numerous other vari-
ables. A general industry standard has been to use an estimate of 39% tower free
volume, which is used to determine the free volume or void fraction published for
a given media product. This is, as stated, only a general standard; therefore, indi-
vidual suppliers are free to choose their own standard as well as to keep such choice
proprietary.
Industry would be better served if a quantitative parameter free of any possible
manipulation were available for use to evaluate packings. Therefore, it is suggested
here that the absolute void volume (AVV) be introduced and used as the standard param-
eter for the free space of a packing. The absolute void volume is independent of any
subjective interpretation as the result of its definition:
AVV = {1 − (W /W )(SG /SG )} (100 % )
media water water media
where AVV is the absolute void volume (dimensionless), W media is the weight of the
3
3
media (lb/ft ), W is the weight of the water (62.4 lb/ft ), SG is the specif-
water media
ic gravity of the plastic or other material used to produce the media, and SG is
water
the specific gravity of water (= 1). In the case of Q-PAC, W , W , SG ,
media water media
3
3
and SG are 2.1 lb/ft , 62.4 lb/ft , 0.91 (polypropylene), and 1 (for water), respec-
media
tively. The AVV of Q-PAC is calculated to be 96.3%, whereas the AVV of all other
commercial packings using the same plastic material (polypropylene) will be below
95%. As a result of this definition of AVV, it is now possible to evaluate, indepen-
dent of any subjective manipulation, the void volume of a single piece of packing
or 1000 pieces of packings, where the AVV parameter is absolute. Using this new
parameter, an environmental engineer will be able to compare various commercial
packing products.