Underwetting

Underwetting is a packing surface phenomenon, which breeiks up liquid film. The tendency of the liquid film to break (the degree of wet- 

If the Koshy and Rukovena theory is correct, then only hig -relative-volatility systems are likely to be affected by X, because M 

Rgura 8.22 Contact anglos. (a) Acute, good wetting ( ) obtuse, poor wetting. 

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Underweter Mines. See under Mines (Military) in Vol 8, M131-R to M133-L... 

The above rules of thumb apply to organic and hydrocarbon systems, whose surface tensions are relatively low (a < 25 mN/m). For higher surface tensions, the liquid does not adhere well to the packing surfaces (underwetting), causing higher HETPs. In a water-rich system (a = 70 mN/m or so) HETPs obtained from Eqs. (14-156), (14-158), and (14-159) need to be doubled. For intermediate surface tension systems (some amines and glycols, whose surface tension at column conditions is 40 to 50 mN/m), HETPs obtained from Eqs. (14-156), (14-158), and (14-159) need to be multiplied by 1.5. 

Underwetting is most significant in aqueous-organic systems, and tends to occur at the high-surface-tension (aqueous) end of the composition range. Liquid viscosity may also have an effect. 

Underwetting may be alleviated by changing the material and surface roughness of the packing. 

In systems susceptible to underwetting, column efficiency can sometimes (but not always) be improved by the addition of small amounts of surfactants. 

This rule is conservative a more realistic range for minimum wetting rates is 0.5 to 2 gpm/ft2. In some nonaqueouB services, liquid rates sometimes as low as 0.1 to 0.25 gpm/ft2 are successfully handled. This rule applies only where underwetting (Sec. 8.2.16) is nota problem. 

The underwetting theory appears to be supported by more evidence compared to the X theory. The author analyzed Koshy and Rukovena s data (110,111) and believes that all their observations can be explained in terms of the underwetting theory. Further, the analysis showed that Koshy and Rukovena s results were in striking agreement with those of Ross, Ponter et al. (36,104), and even the critical methanol concentration appsared the same. 

Some tests by Koshy and Rukovena (110,111) with aqueous, high-relative-volatility systems (a >2) gave much lower random packing efficiencies at high and low L/V than close to total reflux. They interpreted the results as an LfV ratio effect however, underwetting (below) can also explain these data. 

Underwetting (Sec. 8.2.16). With aqueous-organic systems, HETP tends to increase at the aquedus end of the column, both with random and structured packings. 

Summary. In the preloading regime, packing size, type, and distribution affect HETP. With aqueous-organic systems, HETP may be sensitive to underwetting and composition. HETP of structured packings may also be affected by pressure (at high pressure), and vapor and liquid loads. 

In high-vacuum columns (<2 psia), and where underwetting is a problem, these rules may be optimistic. Further discussion is in Sec. 9.1.3. 

Be on the watch for wetting and underwetting effects. When these are likely, scaleup can be particularly unreliable. 

When wetting or underwetting effects are likely, pilot-test at the composition range expected in the prototype. Also, use identical materials of construction and surface treatment in the pilot tests and the prototype. 

Examine Sec. 9.3.3, and use its guidelines to scale up the HETP from the test data to your column. Pay attention to the effects of diameter, height, and underwetting. Judgment is required here. It pays to look at the original reference from which the data was extracted in order to check whether distribution, data scatter, or test procedure could have influenced the data. 

Ponchon-Savarit diagram. 26 Ponter underwetting theory, 516, 517 Porter rivulet model. 542 Porter and Jenkins packing HETP, 532-534 regime transition. 332 Poynting factor. 7 Prado and Fair tray efficiancy, 375 PRO/II. 169, 170,180... 

Comperison of Underweter Effectiveness of Explosives with their Meet of Detonetion... 

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