correlation solely by the readily available specific surface area (Tables 14-13 and 14-14).
Minimum Wetting Rate The minimum wetting rate (MWR) is the lower stability limit of packings. Below this liquid load the liquid film on the packing surfaces breaks up and dewetting occurs. The area available for mass transfer diminishes, and efficiency drops.
Schmidt [IChemE Symp. Ser. 56, 3.1/1 (1979)] described the MWR in terms of a force balance at a dry patch along the path of a falling liquid film. While the gravity and viscous forces resist dewetting, the surface tension and vapor shear forces tend to dewet the falling film. The MWR therefore rises with an increase in surface tension and liquid density, and with a decrease in liquid viscosity. Large packing sizes and poor surface wetting characteristics also contribute to higher MWR.
Schmidt presented a fundamental correlation to predict minimum wetting for Raschig and Pall® rings. More popular have been the Glitsch rules of thumb [Table 14-16; Glitsch, Inc. (now Koch Glitsch), Bulletin 345, Dallas, Tex., 1986] for CMR® random packings with packing surface areas around 200 m2/m3. To extend these rules to other random packings, Kister (Distillation Design, McGraw-Hill, New York, 1992) applied Schmidt's model to give
The Glitsch brochure did not state the surface tension bases of Table 14-16. The author believes that they conservatively apply to organic and hydrocarbon systems (c <25 mN/m). For water (c = 70 mN/m) the author believes that double the values from Table 14-16 is reasonable.
Some surface treatments of the packing (e.g., oxidizing, sandblasting, etching) can substantially reduce the MWR. Chuang and Miller [Can. J. Chem. Eng. 66(6), 377 (1988)] tested a metallic random packing with an aqueous system at low liquid rates (about 0.4 m3/hm2). They used two alternative techniques for oxidizing the packing surfaces. The packings oxidized with the more effective technique gave a column efficiency twice as high as those oxidized by the others.
Superior wetting characteristics characterize structured packings. Satisfactory performance was reported down to 0.25 and 0.12 m3/hm2 in corrugated-sheet and wire-mesh metal structured packings, respectively. FRI's commercial-scale tests (Fitz and Kunesh, Paper presented at the AIChE Annual Meeting, Chicago, Nov. 1996) demonstrated good efficiencies down to 0.5 m3/hm2 (no lower limit reached) with good liquid and vapor distribution.
Two Liquid Phases Two liquid phases often occur in heterogeneous distillation, in steam stripping, and in solvent recovery. Harrison (Chem. Eng. Progr., p. 80, November 1990) and Meier et al. [IChemE Symp. Ser. 152, 267 (2006)] saw no reason to expect unusual capacity or pressure drop limitations due to the presence of the second liquid phase, suggesting that standard correlations for these should apply.
In contrast, both expressed uncertainty in predicting packing efficiency. Harrison presented two case studies: In one, adding water to two water-insoluble organics had no effect on HETP; in another, a key
TABLE 14-16 Glitsch's Rule of Thumb for Minimum Wetting
Minimum wetting rate, m3/(m2'h)
Unglazed ceramic (chemical stoneware) Oxidized metal (carbon steel, copper) Surface-treated metal (etched stainless steel) Glazed ceramic Glass
Bright metal (stainless steel, tantalum, other alloys)
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