Convection blenders

Carefully identify at least 10 sampling locations in the blender to represent potential areas of poor blending. For example, in tumbling blenders (such as V-blenders, double cones, or drum mixers), samples should be selected from at least two depths along the axis of the blender. For convective blenders (such as a ribbon blender), a special effort should be made to implement uniform volumetric sampling to include the corners and discharge area (at least 20 locations are recommended to adequately validate convective blenders). 

Convection blender snbclasses primarily are distingnished by vessel shape ad impeller geometry ... 

Convection blenders reorient groups of particles in relation to one another as the result of mechanical movement, for example, caused by a paddle or a plow. As a result, circulation patterns result in this type of blenders. Subclasses of convection blenders are typically defined by vessel shape and impeller geometry. Ribbon blenders (Fig. 30), planetary blenders (Figs. 31 and 32), orbiting screw blenders (Fig. 33) are examples of convection blenders. High shear mixers comprise another sub-class of convection blenders that will be discussed separately. 

Convection blenders have some drawbacks associated with their use. Many of the older designs have dead spots due to limited movement of materials in these areas. Problem locations include the comers of the blending vessel and the clearances... 

Applications and Types. The design of convective blenders allows... 

Figure 15-24 Four convective blender impeUCTs (a) double ribbon agitator (i>) center-discharge ribbon agitator (c) paddle agitator (d) sawtooth paddle agitator. (Courtesy of H.C. Davis Sons Manufacturing Co., Inc., Bonna- Springs, KS, printed with permission.)...

In the remainder of this section we discuss recent findings regarding the scale-up of tumbling blenders, which have more easily classified flow fields and mixing mechanisms than convective blenders. As mentioned previously, the description of mixing mechanisms in convective blenders has not been the subject of considerable experimental investigation work, relegating scale-up considerations to trial and error. 

Intershell flow is the slowest step in a V-blender because it is dispersive in nature while intrashell flow is convective. Both processes can be described by similar mathematics, typically using an equation such as... 

Make sure that changes in scale have not changed the dominant mixing mechanism in the blender (i.e., convective to dispersive). This can often happen by introducing asymmetry in the loading conditions. 

Convection mixers use a different principle for blending. These mixers have an impeller. This class includes ribbon blenders, orbiting screw blenders, vertical and horizontal high-intensity mixers, as well as diffusion blenders with an intensifier bar. Scale-up considerations are similar to those for the tumble blenders. 

Convection Mixers Ribbon Blenders Aaron Paul O. Abbe Automatic Industry Machines Azo-Ruberg Custom Metal Craft Jaygo Kemutec Lowe Pneuvac Ross Vrieco-Nauta (Hosokawa)... 

In theory, blend uniformity is achieved by three essentially independent mechanisms (i) convection, which causes the large particles to move in the direction of low owing to blender rotation ... 

Ribbon blenders consist of some moving elements, such as a spiral element, that induce convective motion. They are good for cohesive particulate mixtures, but they require more power than tumbling blenders and are more difficult to clean. In ribbon-type blenders, as well as some other types, PVC dry blend can be prepared by slowly spraying small amounts of liquid additives into the mixture. Such additives may sometimes generate the formation of small, soft balls, which should be avoided if a free-flowing dry blend is desired. Ribbon blenders generate considerable static electricity. 

Fig. 7 Rapid, convective flow seen in particle-dynamic simulation of identical but colored spheres in V-blender. (A) view from front reveals that unlike in some designs, convection in this blender drives grains axially, alternately outward toward the tumbler arms and inward toward its center. This axial flow strongly influences mixing, as described in the section on Mixing Rates. (B) View from side indicates that transport is dominated by a spiraling flow, seen also in drums and other blenders (Fig. 4).

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