Question 5.24: Discuss the past problem and the recent developments in pack...

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 standard 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 maximizing 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– 978, 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 (ft²/ft³). 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 surfaces. 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 specific 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, QPAC, for illustrating how an advanced packing has been conceptually developed, introduced, 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 significantly smaller when designed with modern media than is possible with any other early commercial media products.

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 previously 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 material and more pieces per cubic foot; (2) increased operating costs as the consequence 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 suppliers 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 variables. 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, individual 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 parameter for the free space of a packing. The absolute void volume is independent of any subjective interpretation as the result of its definition:

AVV = \left\{1  −  (W_{media}/W_{water})(SG_{water}/SG_{media})\right\}  (100 \% )

where AVV is the absolute void volume (dimensionless), W_{media} is the weight of the media (lb/ft³), W_{water} is the weight of the water (62.4 lb/ft³), SG_{media} is the specific gravity of the plastic or other material used to produce the media, and SG_{water} is the specific gravity of water (= 1). In the case of Q-PAC, W_{media}, W_{water}, SG_{media}, and SG_{media} are 2.1 lb/ft³, 62.4 lb/ft³, 0.91 (polypropylene), and 1 (for water), respectively. 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, independent 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.

The design of Q-PAC using rounded surfaces and needles to support droplet formation 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 produce 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 significant 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 temperature- 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 expensive a given packing will be.

It should be noted that in order to reduce the cost of injection molding ($/ft³) 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 injectionmolding 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 (gasfilm- 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!
• Greater mass transfer media: lower cost packing ($/ft³)!
• 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.

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 circumstances. 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.