Making a selection
All this raises questions: What are some blender choices? Which characteristics of products cause the most concern? Can any single type of blender successfully handle a diverse variety of products?
Table 1 summarizes key criteria for some of the more common devices — broken down into three general classes: mechanical, gravity and fluid assist — and so should help in selecting the most appropriate unit.
Click image to enlarge.
However, the table requires a little explanation. Manufacturers offer many mechanical or physical variations of each of the blender types. Most devices can be adapted to either batch or continuous operation but the checked items represent my opinion of the best choice. Note that there’s a question mark for the high-rpm paddle. Although that unit can be an effective way to ensure that a loading operation (e.g., for a tank or rail car) is uniform, the particle strength and electrical characteristics may make it unacceptable. The blend times assume free-flowing materials and reflect relative, not actual, performance.
The comments on capacity, scale-up and maintenance provide a guide to the factors that need to be included and how much material testing may be necessary to properly design a device. When scale-up is difficult or costly, much more effort can be justified for material testing. The term “% COV” points out how much variation generally may be possible — under ideal conditions (free-flowing, narrow particle size or shape variation, and uniform composition) these numbers can be much better. The last column indicates how much attrition is common for the device.
Another way to look at this table is by the extent of blending that can be achieved with the most difficult materials. For example, an easy blend would be one where the particles are of uniform size (e.g., nylon pellets) but have small chemical or color differences that may not matter much because the customer melts and mixes the product during manufacture. A single-pass flow tube or multi-pass gravity blender may suffice depending upon the desired COV. A more difficult blend would involve small amounts of very fine particles where the small particles may clump, electrically bind or stick to the larger mass of large particles. Flow tube blenders may not be appropriate in this case. Even ribbon or paddle blenders may not work due to the clumping of fines between the clearances of the ribbon or paddle and wall.
In the following section, I’ll discuss some of the ways to achieve the more difficult blends along with a more general classification of the blender types and their advantages and disadvantages. Fluidization effects, stickiness and particle size distribution can complicate the blender selection process, Often it’s better to look back at the objective of the blender and not focus on the mechanical features of the equipment.
We’ll also delve into some of the limitations in achieving a blend as well as discharge considerations to retain the blend.
Let’s look at some of the commonly used blending devices.
Loss-in-weight feeders (Figure 1). When you are producing a 10-gram tablet that has 10 mg of active ingredient, it’s very important to have a uniform feed. Starting with a 10-kg batch of the mixture is unlikely to give the final desired composition due to segregation or attrition in the feeder. Many times the best blending is no blending.
Figure 1. When a uniform feed is essential such feeders often are better than blenders.
For example, feeding an extruder with a wide range of particle sizes and chemical components is best done by setting the appropriate rates of each component over the feed chute. This can be an expensive proposition when many components are involved but provides very high precision and accuracy. Equipment reliability is the major drawback. A feeder failure can upset the process. Surge bins usually aren’t a good solution to feeder failures both due to product loss and potential segregation of the mixture. Online sensors that monitor triboelectricity can give a quick response to the failure, which will minimize product upsets.
Tumble blenders. These are some of the most common devices and include double-cone (Figure 2), V-cone and bin agitators. They can have internal baffles or even added sloping walls to offset the tumbling action of the device. The units tend to be inexpensive and easy to clean. For the bin agitator, the shipping container is the blender, so the customer can be certain of the overall contents. Discharge from these devices must be done carefully because segregation can be extensive. In one double-cone-blender test of a pellet product of about 6 mm, the product came out in precise order from 5.9 mm to 6.1 mm — this was a disaster because the individual packages were used to fill a catalytic reactor and the particle size variation made the flow different in each tube, which affected the reaction time.
Figure 2. Such units generally are inexpensive and suit moderately cohesive materials.
The shape of the device provides a riffle effect with cross mixing between the chambers. The V-cone has the least effective mechanism of cross mixing, followed by square bins and then the double-cone blender. Adding baffles can improve the blend for uniform-particle-size materials but can make emptying more difficult. Increasing the speed of rotation and filling the vessel to a lower level can improve the blend quality but at the price of attrition and throughput.