Blenders tumbling

There are currently no mathematical techniques to predict blending behavior of granular components without prior experimental work. Therefore, blending studies start with a small scale, try-it-and-see approach. The first portion of this chapter is concerned with the following typical problem a 5-ft - capacity tumble blender filled to 50% of capacity and run at 15 rpm for 15 minutes produces the desired mixture homogeneity. What conditions... 

Most tumbling blenders are symmetrical in design this symmetry can be the greatest impediment to achieving a homogeneous mixture. The mixing rate often becomes limited by the amount of material that can cross from one side of the symmetry plane to the other (4- 8). Some blender types have been built asymmetrically (e.g., the slant cone, the offset V-blender), and show... 

Our hypothesized set of the variables that are believed to govern particle dynamics in tumbling blenders is shown in Table 1. 

The transition speeds (rotation rates) were determined for the change from the small out pattern to stripes at 50% filling for all the blenders listed in Table 2 (Figure 7 shows results from the 1.9 and 12.9 qt. blenders). As discussed earlier, the most commonly accepted methods for scaling tumbling blenders have used one of two parameters, either the Fr or the tangential speed of the blender. Earlier, we derived V = and showed that... 

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 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. 

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. 

To ensure that specifications established for critical product quality attributes are met in a large-scale operation, the formulation and manufacturing process developed in the laboratory must be transferred to production and validated. It is necessary to start with a small scale in pharmaceutical research and development. Unfortunately, small-scale mixers necessary during the early development phase will not necessarily have the same characteristics as a commercial-scale mixer. Currently no mathematical techniques exist to predict the blending behavior of multicomponent solid mixtures therefore, experimental work to ensure the proper scale-up and transfer to the production facility is required. Consider the following process parameters for a tumbling blender during scale-up trials ... 

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