Wetting rates, packing

However, in certain cases, the impingement plate design is preferred over packed-tower columns when either internal cooling is desired, or where low liquid flow rates would inadequately wet the packing. 

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. 

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... 

Packing Wetting Rates Related to Pacldng Material Surface... 

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. 

Lmin = Minimum liquid wetting rate in packed tower, ft /(hr) (ft cross-section)... 

If very low liquid rates have to be used, outside the range of FLV given in Figure 11.44, the packing wetting rate should be checked to make sure it is above the minimum recommended by the packing manufacturer. 

Chlorine is to be removed from a vent stream by scrubbing with a 5 per cent w/w aqueous solution of sodium hydroxide. The vent stream is essential nitrogen, with a maximum concentration of 5.5 per cent w/w chlorine. The concentration of chlorine leaving the scrubber must be less than 50 ppm by weight. The maximum flow-rate of the vent stream to the scrubber will be 4500 kg/h. Design a suitable packed column for this duty. The column will operate at 1.1 bar and ambient temperature. If necessary, the aqueous stream may be recirculated to maintain a suitable wetting rate. 

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. 

Figure 1337. Corn-elation of flow rates, typical pressure drop behavior, and packing factors of random packed beds. [Eckert, Foote, and Walter, Chem. Eng. Prog. 62(1), 59 (1966) Eckert, Chem. Eng. 14 Apr. 1975)]. (a) Correlation of flow rate and pressure drop in packed towers, (b) Typical pressure drop data 2 in. porcelain intalox saddles, with F = 40, in a bed 30 in. dia by 10 ft high, (c) Packing factors, F, of wet random packings-...

Zhavoronkov et al. (Z2), 1951 Mass transfer studies (CO into water film) in two diameters of wetted-wall column and on wetted-plate packing (liquid mixed at intervals). Gas velocity had little effect on transfer rates. 

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. 

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... 

Under trickle-flow conditions, determinations of the reactor conditions (i.e., gas and liquid flow rates, packing size, liquid properties, etc.) when all catalyst particles are effectively wetted and the catalyst surface is 100 percent utilized. 

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