Agitation circulation rate

Pitched-blade turbines (Fig, 18-3) are used on top-entering agitator shafts instead of propellers when a high axial circulation rate is desired and the power consumption is more than 2,2 kW (3 hp), A pitched-blade turbine near the upper surface of liquid in a essel is effecth e for rapid submergence of floating particulate solids,... 

Equation 7-93 was established for turbine agitators with flat blades and Z/Da = 0.5. The criteria for Equation 7-93 relate to a specific type of suspension. The distribution of the solid as a function of the height in the liquid is not uniform in every case. Therefore, the uniformity can only be approximated by obtaining a circulation rate Q as high as possible. Nienow [42] found that H/E = 7 in tanks for which H/Dx =1.0. 

A single, isothermal reactor design more reliably yields gypsum crystals than multiple-reactor designs with poorer temperature control [39]. This benefit has also been claimed for a two-vessel loop system. The effect of surfactants on crystallization of phosphogypsum has been examined [40]. Higher reactor circulation rates and increased agitation coupled with other refinements also improve acid recovery [41]. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

Another empirical rule based on considerable practical experience (Bennett, Fiedehnan and Randolph, 1973) suggests that for scale-up at constant magma density and residence time, the quantity R /T should be kept constant, R = agitator tip speed and T = turnover time, i.e. crystallizer volume/volumetric liquor circulation rate. 

An agitator acts like a centrifugal ptimp impeller without a casing and gives a flow at a certain pressure head. This circulation rate Q in m /s from the edge of the impeller is the flow rate perpendicular to the impeller discharge area. Fluid velocities have been measured in mixers and have been used to calculate the circulation rates. Data for baffled vessels have been correlated using the dimensionless flow number Nq (Ul). 

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