where:

depth of packing, m = diameter of the tower, m = liquid loading rate, m3/m2/sec K1a = overall liquid mass-transfer coefficient, sec"1 R = stripping factor, dimensionless Ci = influent concentration, mg/l Ce = effluent concentration, mg/l Q = flow rate m3/sec

The key variables to define in the preceding equations are the overall mass-transfer coefficient K1a and the stripping factor R. The mass-transfer coefficient is a function of the type of packing, the liquid and gas flow rates, and the viscosity and density of the water. Therefore, the masstransfer coefficient is usually determined from a pilot test on actual field data. When pilot testing is not feasible, theoretical correlations, such as those developed by Onda, Takeuchi, and Okumoto (1968), can be used.

The stripping factor R is related to the air-water ratio as follows (Noonan and Curtis 1990):

where:

(G/L)min = minimum air-water ratio, dimensionless (G/L)actual = actual air-water ratio, dimensionless G = gas (air) loading rate, m3/m2/sec

L = liquid loading rate, m3/m2/sec

H = Henry's constant, dimensionless

The actual air-water ratio, however, is related to the gas pressure drop through the column as shown in Figure 9.17.15 (brand-specific pressure drop curves are available from packing vendors). Therefore, engineers should examine several combinations of air-water ratio and pressure drop to determine the most cost-effective design. A high pressure drop reduces the size of the tower and capital costs; however, it increases the size of the blower and operation costs. Studies have shown that the most cost-effective stripping factor R usually falls between 3 and 5 (Hand et al. 1986).

After a stripping factor is selected, the actual air-water ratio can be calculated with Equation 9.17(12), and the gas (air) loading rate can be obtained from Figure 9.17.15 for a given pressure drop. Then the tower height and diameter can be calculated with Equations 9.17(10) and 9.17(11), respectively. This procedure should be repeated for several combinations of stripping factor and pressure drop until the most cost-effective design is obtained. Several computer cost models can be used in this process (Nirmalakhandan, Lee, and Speece 1987; Cummins and Westrick 1983; Clark, Eilers, and Goodrick 1984).

FIG. 9.17.15 Generalized pressure drop curves. (Reprinted from R.E. Treybal, 1980, Mass transfer operations, 3d ed., New York: McGraw-Hill.)

FIG. 9.17.15 Generalized pressure drop curves. (Reprinted from R.E. Treybal, 1980, Mass transfer operations, 3d ed., New York: McGraw-Hill.)

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