Helical ribbon agitator

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

Carreav. P, J.. Patterson, I., and Yap, C. Y. Can J. Chem. Eng. 54 (1976) 135. Mixing of viscoelastic fluids with helical ribbon agitators 1. Mixing time and flow pattern. 

This patent then calls for two or more reflux cooled conical CSTR s with helical ribbon agitators operating in series in an "intermediate conversion zone" ranging from 65 to 85% conversion. The conversion in succeeding reactors in this zone should show a relative difference of 15-25%. As with the earlier patent,... 

Paddle, anchor and helical ribbon agitators (Figures 10.56a, b, c), and other special shapes, are used for more viscous fluids. 

Figure 10.56. Low-speed agitators (a) Paddle (b) Anchor (c) Helical ribbon...

Gray (1963) investigated the mixing times of helical ribbon agitators and found the following equation to hold ... 

PVC bulk polymerizations are two-slage processes in which a very porous PVC seed particle is produced in a vessel provided with very high speed agitation. The wet polymer powder from this stage is fed to horizontal autoclaves in which the polymerization is finished. Reaction heal is removed by cooling the helical ribbon blender-type agitators as well as the vessel jacket. 

This is still a low power level of 6.2/1.74 = 3.55 HP/1,000 gal. With this agitator, a reasonable upper hmit for agitator speed would be 100 rpm, for which the impeller power would be 22 HP with a specific power input of 13HP/l,000gal and Npe = 9. This change would move up into the Reynolds number near the lower limit recommended by Hemrajani and Tatterson (in Paul (2004), 345). This example illustrates the great impact of fluid viscosity on (1) the power requirement of a 6BD and (2) the choice of an impeller style between a turbine and a helical ribbon impeller. 

Carreau P., Patterson I., Yap C.Y., Mixing of ciscolelastic fluids with helical-ribbon agitators. Can. J. Chem. Engng. 54 (1976) 3, p. 135-142... 

Kuriyama M., Ohta M., Yanagawa K., Arai K., Saito S., Heat Tranter and Temperature Distribution in an Agitated Tank equipped with Helical Ribbon Impeller, J. Chem. Eng. Japan 14 (1981) 4, p. 321-330... 

Savinsky )., Havas G., Deak A., Power requirement of anchor and helical ribbon impellers for the case of agitating Newtonian and pseudo-elastic liquids, Chem. Eng. Sci. 31 (1976), p. 507-509... 

The effect of fluid rheology on the power consumption of helical ribbon agitators has also been evaluated [54] and power consumption as a function of generalized Reynolds number for shear thinning but inelastic fluids defined. When shear thinning effects are small, and elasticity is negligible, deviations from the Newtonian... 

Proximity and nonproximity impellers are the two major designs used in mixing applications. Proximity relates to distance from the vessel wall. Figure 9.50(a) shows a nonproximity impeller typically used for turbulent conditions. Its blades are not close to the vessel wall. Close proximity agitators like anchors and helical ribbons, illustrated in Figures 9.50(b, c), are typically used for high-viscosity applications. 

A distinction is usually made between close clearance/large diameter and small diameter/high speed impellers. The first class is used for viscous reaction mixtures. It includes anchor stirrers and helical ribbons. The diameter of these agitators typically is between 70 and 100 % of the tank diameter. 

Other types of impellers may be preferred for mixing certain liquids. A helical ribbon agitator gives much shorter mixing times for the same power input with very viscous liquids but is slower than the turbine with thin... 

The speed of such mixers is between 1800 and 400 rpm, higher for smaller volume, and lower for larger volumes. Baffles can be used to reduce the vertex formation that diminishes the active volume. For low viscous liquids (10 to 1 Ns/m ) turbines, impellers and propellers are suitable. For viscous liquids, above 10 Ns/m, anchor and helical ribbon agitators are recommended. 

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