Agitation power used

These reactors employ small particles in the range 0.05 - 1.0 mm (0.0020 -0.039 in) with the minimum size being limited by filterability. Small diameters are used to provide as large an interface as possible, since the internal surface of porous pellets is poorly accessible to the liquid phase (Perry and Green, 1999). The catalyst concentration in slurry reactors is limited by the agitation power of the mechanical stirrer or by the gas flow. 

Power Consumption Power consumption sometimes becomes important in industrial bioprocesses, because the power used for aeration and agitation can be highly expensive. The cost of power consumption occupies approximately 15-20% of total production cost in aerobic fermentation processes. 

In recent years attempts have been made to improve the gas-liquid mass transfer by changing the design of the mechanically agitated vessel. Mann et al. (1989) evaluated the use of horizontal baffles mounted near the gas-liquid surface. Horizontal baffles prevent vortex formation, generate less shear than standard baffles, increase gas holdup, and improve gas-liquid mass transfer. The latter two results are due to the rotational flow below the baffles, which causes gas bubbles to move upward in a spiral trajectory and induces surface aeration. For a 12-inch i.d. and 18-inch-tall stirred vessel, they showed kLat to be improved by a factor of 1.6 to 2.3 with 30 to 50% lower agitation power compared to the standard vessel. 

Values for the constant K and the indices b and c for various types of agitator, tank-agitator geometries, and dimensions can be found in the literature see Rushton et al. (1999). A useful review of the published correlations for agitator power consumption... 

A common way to determine the agitator power under gassed conditions is to use the k-factor, sometimes called the relative power demand, defined as the ratio of gassed to ungassed shaft power ... 

Power required for complete suspension of solids in agitated tanks using pitched-blade turbines. 

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