Jl y

(uLapr

ReL in Eq. (8.32) is obtained from Eq. (8.8). € and Op are obtained from Table 8.2. Eq. (8.31) is only valid in the preloading regime (i.e.t at less than 65 percent of the flood vapor velocity). The range of application of the correlation is

m and

17 < ap < 130 ft!/ft8 0.57 < e < 0.99 58 < pL < 70 lb/ft3 0.6 < < 14 cP

The effect of liquid and vapor rates on the operating holdup is shown in Fig. 8.21. In the preloading regime holdup is essentially independent of vapor flow (100,101), but is a strong function of liquid flow rate and packing size. Smaller-size packings and high liquid rates tend to have more holdup.

8.2.15 Minimum wetting rate

The minimum wetting rate (MWR) is the lower stability limit of packings. It is the liquid load below which the falling liquid film breaks up, and the liquid shortage causes dewetting of the packing surface. The area available for mass transfer diminishes, and efficient drops (Sec. 8,2.2; point A on Fig. 8.16a).

Schmidt (102) described the MWR in terms of a force balance between wetting and dewetting forces 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. The effect of vapor rate on the MWR becomes important near the loading point, when the shear force becomes significant, and higher vapor rates also increase the MWR.

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

25nwn HSW-typ C.Hostic. System Air/Woter.lbor, 293K

0 0

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