Page 302 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 281

The design of Q-PAC using rounded surfaces and needles to support droplet forma-
tion was arrived at through extensive trial and error. A liquid stream is forced into a
shower of droplets by the media; in turn, the media’s mass transfer efficiency increases.
It is interesting to note that the new media, in addition to reduced capital costs (the
result of smaller tower diameter) and lower operating costs (a direct result of lower
operating costs due to reduced pressure drop) that have been previously discussed in
this handbook, offer additional savings to industry.
Regardless of the design of a given packing, the cost of that packing will be fixed
based on the amount of material (plastic resin, metal, ceramic, etc.) that is required to
produce the packing. It is important to note that the amount of plastic needed to pro-
duce a cubic foot of Q-PAC as well as the number of pieces of Q-PAC needed to fill
a cubic foot is far less than any other early contemporary packings. This is very sig-
nificant when it is realized that the cost of plastic resin represents approx 40% of the
final cost of a packing when using polyethylene to mold the packing. If a more
expensive plastic resin such as Teflon must be used (because of chemical- or tem-
perature-resistance considerations), the cost of resin can escalate to 95% of the final
cost of the packing.
Also, as plastic media are produced by injection molding, the number of pieces per
cubic foot will directly impact the final cost of a packing. The greater the number of
pieces needed for a cubic foot, the more costly to mold and, hence, the more expen-
sive a given packing will be.
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It should be noted that in order to reduce the cost of injection molding ($/ft ) a greater
number of pieces need to be molded in a single cycle of the injection-molding
machine. However, to accomplish this, a multipiece mold is required for the injection-
molding process. The fabrication cost of this type of mold increases geometrically as
the number of pieces the mold is capable of producing is increased. Therefore,
although the number of pieces being produced in a single cycle of the molder can be
increased, the savings thus realized in reduced labor costs are quickly consumed by
increased capital expenditure and amortization of the mold.

Modern mass transfer media should maximize mass transfer in scrubber towers (gas-
film-limited systems, per previous discussion). The use of modern mass transfer
media provides several advantages:

• Smaller tower diameters: reduced capital and fabrication costs, smaller system
footprint!
• Lower pressure drop: smaller blower motor, lower electrical energy costs, less noise!
• Smaller chemical recirculation pumps: less costly!
• Smaller mist eliminators: less costly!
• Less total packing volume: reduced capital and fabrication costs, smaller system
footprint!
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• Greater mass transfer media: lower cost packing ($/ft )!
• Increase fouling and plugging resistant: reduced maintenance costs!
• Increase capacity of existing towers
Some commercial packings (such as LANPAC), on the other hand, maximize mass
transfer in liquid film-limited systems (per previous discussion), which is commonly
encountered in stripping situations. Other commercial packings (such as NUPAC) are
highly efficient media that have found a niche in keeping tower heights to the absolute
minimum, such as with are indoor tower.