Tumbling blenders scaling

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... 

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... 

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. 

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 ... 

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). 

There are three parameters that should be kept similar in scaling tumble blenders. These include geometric, kinematic, and dynamic similarity. This approach is valid for tumble blending without the use of shear (such as an intensifier bar), for low-cohesion materials. Cohesive materials have poor flow and hence blend less efficiently at smaller scales therefore, this technique may overestimate the blending time for scaling up, and underestimate the time when scaling down. 

Tumble blenders also impart shear forces to the powders being blended as slip planes form between the walls of the blender and layers of the blend. The amount of shear force is often low for small-scale blenders, but can increase with increasing scale of the blending container. Because they tend to provide gentler blending and have less of an affect on the particle size of the materials being blended (compared to high shear mixers),... 

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. 

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. 

In this section we outline the commonly accepted scale-up criteria used in industry for agitated mixers and silo blenders. Scale-up of tumbling mixers was addressed in Section 15-5. The complexity of interaction between physical properties of solids, mixer configuration and velocity, and stress profiles within a mixer makes it difficult to formulate generalized scale-up criteria. Various experimental investigations into scale-up, however, do provide useful guidance for scale-up. No concurrence on acceptable approach has been reached, and various manufacturers tend to follow their experience. 

Sometimes, small scale equipment is available in a modular design. For example, to the all-purpose drive stand, shown in Fig. 11.12, a multitude of attachments can be fitted that allow the testing of most tumble/growth agglomeration methods in the laboratory. Fig. 11.13 depicts the most important work modules they are a disc or pan agglomerator (a), a coating pan (b), a bowl blender (c), a planetary bowl blender... 

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