Chemical Processing - September 2007 - (Page 45)

blend is reached. While this can be successful in a wide variety of cases, there’re numerous examples where the solids never reached an acceptable blend or the quality of the blend was variable. In addition, segregation can occur on discharge of the batch. Material testing is often cited as a way to get uniform results but variations in product temperature, moisture or even surface roughness can make this an expensive and time-consuming task. A systematic approach for choosing a blender that starts with measurement of material flow properties has been known to work very well (see “Selecting an effective blender,” CP, Oct. 2001, p. 65). However, even with the material properties well-defined by flow testing, solids can be over-blended. For instance, it’s not unusual for materials in a ribbon blender to reach a “random” state after a few minutes and then slowly drift away from the perfect blend. This can be due to triboelectric effects (charging of the particle surface due to the impact of flow), attrition, fluidization or simply coating of large particles with smaller particles due to temperature gradients (thermophoresis). So, even if you use a materials testing approach for design, you still need to understand these additional factors during operation. 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. However, the table requires a little explanation. Manufacturers offer many mechanical or physical variations of each of the blender types. www.chemicalprocessing.com 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 highrpm 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 dis- >> Loss-in-weight feeders Figure 1. When a uniform feed is essential such feeders often are better than blenders. cuss 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. The options 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. 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 September 2007 • 45 http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - September 2007

Contents From the Editor Field Notes In Process Energy Saver Compliance Advisor Succeed at Simulation Rethink Your Approach to Process Safety Avoid Blending Blunders Get the Right Cartridge or Bag Filter Wireless Proponents Take HART Membrane Boasts Material Benefits Process Puzzler Plant InSites Chem Show Product Preview ISA Product Preview Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - September 2007

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