Minimum wetting

Resin type and nominal percent cross-linkage Minimum wet capacity, mequiv mL Density (nominal), g mL Comments... 

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. 

Wetting For operation at low liquid loads, the onset of minimum wetting can adversely affect scale-up, particularly with random packings and aqueous systems. Scale-up reliability at low liquid loads can be improved by pilot-testing at the composition range expected in the prototype, and by using identical packing materials and surface treatment in the pilot tests and in the prototype. 

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. 

TABLE 14-16 Glitsch s Rule of Thumb for Minimum Wetting... 

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. 

Schmidt (102) observed that flooding and minimum wetting are caused by two different mechanisms. A low-liquid-rate column may therefore flood even when it operates below the MWR. This is most likely to occur in vacuum systems (where liquid rates are low and vapor velocities are high) and in high-surface-tension, low-viscosity systems. 

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

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. 

---