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HP/1000 GALS. GASSED

Figure 32. Effect of horsepower and impeller diameter on mass transfer coefficient at 0.08 ft/sec gas velocity..

Figure 32. Effect of horsepower and impeller diameter on mass transfer coefficient at 0.08 ft/sec gas velocity..

HP/1000 GALS. GASSED

Figure 33. Effect of horsepower and impeller diameter on mass transfer coefficient for gas velocity of 0.13 ft/sec.

At the far right of Fig. 29 is shown high mixer power levels relative to the gas rate, and it can be seen that D/T makes no difference to the mass transfer. This occurs in some types of hydrogenation, carbonation, and chlorination. In those cases, the power level is so high relative to the amount of gas added to the tank that flow to shear ratio is of no importance.

In Figs. 30 through 33, the gas rate is successively increased in each of the four figures. At the low gas rate, the 4-inch impeller is more effective than the 6 or 8-inch impeller under all power levels. At the higher gas rates, the larger impellers become more effective at the lower gas rates, while the smaller impellers are more effective at the higher power levels, fitting generally into the scheme shown in Fig. 29.

A sparge ring about 80% of the impeller diameter is more effective than an open pipe beneath the impeller or sparge rings larger than the impeller. Figure 34 shows this effect and indicates that the desired entry point for the gas is where it can pass initially through the high shear zone around the impeller.

This has led to the common practice today of using the distribution of power in a three-impeller system, for example, 40% to the lower impeller and 30% to each of the two upper impellers, Fig. 35.

KW/ cu METER GASSED

T - 460 mm ~L - 460 mm D -150 mm -6FBT 150mm F = ,007m/sec.

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