Problem

The oxygen transfer rate OTR and power demand P/V of a Simplex aerator (Type A) with a diameter of d = 0.206 m (Fig. 5.6) was measured for different speeds. The efficiency E was calculated and the results plotted in Fig. 5.7. Following this, the dimensionless numbers Ne, Fr and Y were calculated using Eqs. (5.83), (5.87) and (5.90). First, Ne was plotted versus Fr in Fig. 5.14. In addition, Fig. 5.15 shows the plot of Y as a function of Fr. A large aerobic tank with a volume of V = 250 m3 and with only one Simplex aerator is to be constructed with an oxygen transfer rate of OTR = 100 g O2 (m3 h)-1. An aerator with a diameter of 2.0 m is available (Zlokarnik 1979).

1. What is the speed n needed for the demanded oxygen supply?

2. What is the efficiency E of the aeration system?

Constant parameters:

v = 10-6 m2 s-1, g = 9.81 m s-1, c* = 8 mg L-1, c' = 1 mg L-1 Solution

1. Calculation of the required speed n: (a) KLa = ?

9.812/3

0.89

116 I 5 Gas/Liquid Oxygen Transfer and Stripping (c) The speed n follows from:

p d5 n3

we obtain the specific power consumption: P = Ne p d5 n3

m s3 m3

(b) With the given OTRv = 100 g (m3 h) 1 O2 the efficiency follows:

P/V 18 h W m3 k W h The experimental results in Figs. 5.13 and 5.14 are valid for: H/d = 1 and D/d = 4.37

For d = 2 m, we obtain H = 2 m and D = 8.74 m for a cylindrical tank with:

In a tank with V = 250 m3, H and/or D must differ from those values. For D = 12 m, H follows: H = 4 V/nD2 = 2.21m

References

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