Mixing radial-flow impellers

FIG. 6-39 Typical stirred tank configurations, showing time-averaged flow patterns for axial flow and radial flow impellers. From Oldshue, Fluid Mixing Technology, McGraw-Hill, New Yo7 k, 1983.)... 

Impeller Radial Flow. Impeller Mixed Flow. Vertical Regenerative Turbine. 

Figure 5-5X. Type R-500. Very high shear radial flow impeller for particle size reduction and uniform dispersion in liquids. By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of General Signal.

Centrifugal pumps can also be classified by the design of the impeller. Centrifugal pumps may have radial-flow impellers, axial-flow impellers, and mixed-flow impellers (both radial-flow and axial-flow). 

Radial flow impellers Impellers that pump fluid in essentially a radial direction when installed in a baffled mixing tank. 

Using for illustration the radial flow impeller. Fig. 2 shows the velocity profile based on average velocities at a point coming off the blades of an impeller. As shown in the diagram, by taking the slope at any point, we obtain the velocity gradient, which is the shear rate that exists at that point. This shear rate can be calculated at any point in the mixing tank if we will make various velocity measurements and establish the velocity profile at any point. 

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

The solids are kept in suspension if the pumping capacity of the impeller causes strong enough circulation of the liquid. In most processes, complete suspension of the particles is not required. Often, so-called off-bottom suspension is sufficient, which means that all particles are moving above the bottom of the tank with some vertical velocity. Radial flow impellers are usually not very effective in suspending solid particles. Actually, about three times more power is required for a radial turbine to provide the same degree of uniformity compared to an axial turbine. This is because the radial turbines pick up particles from the bottom of the tank by the suction side of the impeller, which is only half of the total flow from the impeller. Due to the appearance of an upper and a lower circulation zone, the contents of the two zones are not sufficiently mixed. Axial impellers are therefore most frequently used for the suspension of solids in stirred tanks [65]. 

The Kiihni column employs radial flow impellers located between perforated plates for compartmentalization. The first Scheibel column used wire mesh zones to promote coalescence and limit backmixing between turbine-agitated mixing zones. A later Scheibel column used a shrouded radial impeller and multiple ring baffles to direct most of the rotor s energy towards dispersion and away from axial mixing. 

For typical reactive mixing systems in a stirred tank, a dual impeller system, Figure 9.23, with a radial-flow impeller at the bottom and an axial flow on the top is recommended [31-33], We recommend the following ratios as initial starting points for a typical batch with liquid height equal to tank diameter ... 

Since radial flow impellers generally produce a higher turbulence zone and axial flow impellers produce more vertical vessel flow, a good combination in applications where two or more impellers are used is to use a radial flow impeller near the bottom where the feed is introduced, and one or more axial flow impellers on the same shaft above it to provide vertical mixing and induce flow through the otherwise partially segregated region under the radial flow impeller. 

As scale increases, a proper mixing state gains in importance that can lead to radial flow impellers, providing very poor mixing conditions in a significant portion of the reactor volume (McFarlane and Nienow, 1995). The solution has been to... 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

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