Dow Offers Tower Packing Material The low temperature, thermoplastic packing will be marketed through Saran Lined Pipe Co. Dow Chemical will market a line of low-temperature, thermoplastic tower packing through the Saran Lined Pipe Co., of Ferndale, Mich. The packing, Dow claims, will confer lower pressure drops to high volume liquid-gas contractors than other types of packing. This, coupled with light weight of the plastic and high contacting efficiency because of the packing shape, leads Dow to credit the line with advantages ranging from lighter tower structures to a minimum of liquid channeling. The packing, which is tradenamed Maspac, has evolved over a seven-year period of test and internal
use in Dow's plants. Its shape is difficult to describe. Dow calls it a perforated saddle with fingers above and below. The packing has been put to such test uses as stripping volatile gases, degasifying water, and scrubbing atmospheric pollutants from waste gases. Maspac comes in a nominal 3 3 / 4 -in. size, called FN-90, and a nominal 2-in. size, called FN-200. FN-90 comes in general purpose polystyrene, low and high density polyethylene, and polypropylene. The 2-in., FN-200 Maspac comes in high density polyethylene and polypropylene. These pack into a tower with a low bulk den-
TEST CELL. A worker checks the efficiency of Dow's Maspac—a thermoplastic tower packing material. Windows in the test cell allow a visual check on the packing, which, Dow says, can mean lighter tower structures 58
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sity. Highest is 5.8 lb. per cu. ft. for the FN-200 high density polyethylene to 4.3 lb. per cu. ft. for the 3 3 / 4 -in. polypropylene. In all cases, this leaves more than 90% voidage in a packed tower, while providing from 25 sq. ft. per cu. ft. of packing volume in the 3 3 / 4 -in. size to 43 sq. ft. per cu. ft. of packing volume for the 2-in. size. When these factors are totaled, they spell out a number of advantages. Take light weight, for example. Ceramic and porcelain Raschig rings (2-in. size) weigh between 35 and 40 lb. per cu. ft. Carbon rings and ceramic saddles are about the same. This dictates a stronger tower structure to support the packing weight than that needed for Maspac, Dow says. Strength is another factor. Heavier ceramics are sometimes stacked by hand or dumped into a water-filled tower to prevent breakage. Maspac can be added in almost any convenient way. The high void volume helps keep pressure drop low, while the large surface area keeps contact high. These are also helped by the shape of the Maspac. When the packing is dumped into a tower, it presents such a tortuous profile that channeling is kept to a bare minimum, and water redistribution is excellent, Dow says. If the packing is removed for cleaning it separates easily and is easily cleaned. Dow has some fundamental performance data to bear out its claims. Packing factors, dimensionless numbers which indicate relative pressure drops, run 20 for the 3 3 / 4 -in. Maspac, 30 for the 2-in. Maspac. Ceramic Raschig rings (2-in. size) are typically around 75. With water mass velocities at 10,000 lb. per hr. per sq. ft. and air at 1000 lb. per hr. per sq. ft., pressure drop is 0.2 in. of water per foot of packed height using FN-200. With liquid mass velocity at 2000, gas at 1000, height of a gas phase transfer unit runs about 2.3 ft. for adiabatic transfer of water to air in a tower using FN-90. Temperature limits these thermoplastics. General purpose polystyrene, for example, has an ASTM deflection temperature of 185° F. This is under high stress, though, and the packing can be used at various higher temperatures, depending on application. Dow will make recommendations covering operating temperatures and chemical resistance for any proposed application of the packing.
