Minimum wetting rate

The minimum wetting rate (MWS) is the lower stability limit of packings. It is the liquid load below which the falling liquid film breaks up, and the liquid shmtage causes dewettii of the padiii surface. The area available for mass transfer diminishes, and efficiency drops (Sec. 8.2.2 point A on F%. 8.16a). 

MacDougall (58) added the surface tension gradient to the list of relevant factors. A system is surface tension positive (o+) when siirface 

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Sdimidt (102) observed that flooding and minimum w ting are caused two diffoent mechamsms. A low-liquid rate column may therrfore flood evmi when it (iterates below the MWR. This is most likely to occur in imcuum systems (udimre liquid rates are low and vapor velocities are hi ) and in hi surface-tension, low-viscosity sy8  

Prediction by correlation. Schmidt (102) developed the following correlation fiom his fundamental model which describes minimum wetting for random packings. 

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Minimum Wetting Rate The minimum liquid rate required for complete wetting of a vertical surface is about 0.03 to 0.3 kg/m s for water at room temperature. The minimum rate depends on the geom-etiy and nature of the vertical surface, liquid surface tension, and mass transfer between surrounding gas and the liquid. See Ponter, et al. Int. J. Heat Mass Tran.fer 10, 349-359 [1967] Trans. Inst. Chem. Eng. [London], 45, 345—352 [1967]), Stainthorp and Allen Trans. Inst. Chem. Eng. [London], 43, 85-91 [1967]) and Watanabe, et al. ]. Chem. Eng. [Japan], 8[1], 75 [1975]). 

To feed enough liquid into the tower to effectively place a wet film of liquid over all the packing, a minimum wetting rate (MWR) has been evaluated for guidance in operation and design. Morris and Jackson [52] recommended the MWR shown in Table 9-24. 

MWR = value of minimum wetting rate from Table 9-24... 

The minimum wetting rate is a function of the packing material surface (Table 9-25) and the physical properties of the liquid involved, particularly the viscosity and the... 

Kister [90] has evaluated Schmidt s [92] somewhat complicated equation for minimum wetting rate and proposes ... 

Note that when packing is changed from one material of construction to another, it is important to recognize the effect on minimum wetting rate for the new condition. 

Vacuum service requires recognition of minimum liquid flow refer to section on minimum wetting rate. Pressure drops are designed to be low, but normally not lower than 0.10 in. water/ft. 

MWRg = Minimum wetting rate, gpm/ft, (Eq. 9-14) MWRj = Minimum wetting rate, gpm/ft, constant,... 

Similar effects occur in a packed column, although the flow patterns and arrangement of the surfaces are then obviously much more complex. Morris and Jackson(57) have recommended minimum wetting rates of 2 x 10-5 m3/s m for rings 25-75 mm in diameter and grids of pitch less than 50 mm, and 3.3 x 10-5 m3/s m for larger packings. 

Generally, the minimum wetting rate is at 0-5 to 2 gpm/ft2 for random packings, and 0.1 to 0.2 gpm/ft2 for structured packings (Sec. 8.2.15). It follows that point A is usually a distributor turndown limit. Regardless of which limit point A represents, it is extremely sensitive to maldistribution (Fig. 8.16b). When liquid distribution is poor, it will take more liquid to wet the entire bed, and point A will shift to the right. If distribution is very poor, point A may never be observed, and the curve will have no flat region at all. A V-shaped curve is not uncommon, and is indicative of poor distribution. 

Because of minimum wetting rate considerations, Ludwig (83) recommends against designing for pressure drops lower then 0.1 in woter/ft Billet (66) also warns against designing packed vacuum towers with too low a pressure drop. 

Prediction by rule o> thumb. Popular rules of thumb for minimum wetting rates in random packings are... 

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. 

Qmw Minimum wetting rate, gpm/ft2 of tower cross-section area... 

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