Agitators, power requirements selection

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

Based upon product batch requirements, preliminary vessel dimensions are listed in Table 6. Gas flow rate (ACMM) is determined to obtain the mass transfer required by reaction kinetics. A ring sparger is selected as discussed in conjunction with Fig. 8. Table 7 summarizes the calculation of the required specific power and gas holdup based on mass transfer and the desired oxygen concentration in the liquid. It must be confirmed that vessel dimensions accommodate the aerated liquid height. An appropriate agitation drive system is recommended in Table 8. Fig. 9 is a schematic of the reactor in this design example. 

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