Chemical Processing - September 2007 - (Page 44)

Blender selection guide Type Batch Continuous Blend Time Blender Capacity Scale-up Maintenance Quality, % CoV Blender Attrition Mechanical Doublecone High-rpm paddle Loss-inweight Paddle or ribbon Riffle Screw √ √ √ √ √ ? √ √ √ 1–2 hr. 5–30 sec. 1–15 min. 1–60 min. 3–5 min. small small small small small no moderate easy difficult no easy low moderate low to moderate moderate high high 5–20 <5 1–2 5–10 <1 3–5 moderate but segregates high with agglomeration negligible varies with fines small moderate gravity Flow tube Mass flow Multi-cone √ √ √ √ √ 0–2 turnovers 0–2 turnovers large moderate to high large costly easy easy moderate to high low moderate 10–25 3–5 5–10 negligible negligible negligible Fluid Assist Fluidized bed Jets Lift tube Spouted bed Venturi √ √ √ √ 1–15 min. 15 min. 3 turnovers 1 turnover small to moderate small to moderate large easy easy moderate moderate moderate moderate 1–3 2–5 2–8 small can be high negligible none with big/light particles use only with fines √ moderate moderate low 5–10 √ - - easy high 2–5 Table 1. Using these key criteria can make it much easier to narrow blender choices to the most appropriate options. Barriers to uniformity Several factors can work against a good blend of solids. A key one is the limits of randomness. The best blend would be a random mixture that retains any individual variation (e.g., color, shape) in the particles. It’s important to understand that a random mixture has variations. When it comes to solids, there are two principle measures of blend uniformity: accuracy and precision. To be well blended, the mixture must be both. Accuracy is a measure of the average of multiple tests while precision is the reproducibility of a measurement. These often are expressed as average and standard deviation or some combination, such as a student’s t-value or coefficient of variation (CoV). Precision is limited by randomness for particulate solids. (For more on data analysis, see CP’s ongoing Dr. Gooddata series, Part 1 44 • September 2007 is viewable at www.chemicalprocess ing.com/articles/2005/419.html.) To further complicate the situation, particulate solids have multiple physical properties that limit randomness of a mixture. The most common ones are: Size. As particles become larger, a random mixture has a wider CoV for the same sample size. This can be minimized by taking larger samples for tests or multiple samples and then averaging the results. However, there’s a lower limit to the precision of small samples as the size of the particles in the mixture increases. Shape. Angular particles can interlock with each other to form a pseudo-larger particle. The additional void space isn’t necessarily uniformly distributed and finer particles can become concentrated in different areas of the mixture. Even spherical particles can pack in different ways under pressure and create more void space that can be filled with fine particles. Distribution. Fine particles can coat larger particles and change the frictional characteristics so that flow of the particles over each other is impeded. They can escape the blender’s mixing surfaces (paddles, walls) and accumulate in clumps. The wider the size distribution the more likely the particles will segregate during handling operations. Also, fine particles can increase the ability of the mixture to behave like a fluid. While that would be good for uniformity, it often occurs in only a portion of the mixture, resulting in a poor blend. In addition, electrical and chemical properties of the individual particles can alter their binding characteristics. Consider what happens in a ribbon blender, which is one of the simplest blenders. The solids are added and the unit is run until the desired www.chemicalprocessing.com http://www.chemicalprocessing.com/articles/2005/419.html http://www.chemicalprocessing.com/articles/2005/419.html 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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