Tumbling blenders rates

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... 

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... 

Rotation speed impacts shear rate and therefore blending efficiency, especially when cohesive materials are being blended. However, the relationship between rotation speed and uniformity is affected by the complex nature of the flow of cohesive materials, and the number and size of avalanches per revolution. Blending performance in bench scale tumble blenders demonstrated that the rotation speed did not significantly impact the blending of free-flowing materials (5,20). 

The ensuing discussion will revolve around experiments run in 14, 56, and 300 L tote blenders using two mixtures a free-flowing binary 50 50 w/w% mixture of 400 ttm sand particles and a cohesive mixtme of 3% micronized acetaminophen ( 30 ttm) in a 50 50 w/w% matrix of PH102 Avicel and Fast-Flo lactose. All experiments were run at 60% of blender capacity at a rotation rate of 10 rpm. In raw form the acetaminophen was extremely cohesive and agglomerates (up to 0(1 cm) in diameter) formed in the bulk mixtme. The effect of blender scale on the breakup of these agglomerates is an important consideration for scale-up of tumbling blenders. 

Probably the most important rule governing the basic dynamics of mixing in tumbling blenders is that the loading protocol has the most direct impact on the mixing rate. When the blender is symmetrically loaded in a top/bottom fashion. 

Fig. 21 Mixing rates in a V-blender tumbled at 24 rpm with and without shear induced by a high speed intensifier bar.

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