Chemical Processing - September 2007 - (Page 49)

>> Jets in cone Figure 7. Blasting solids from several points with a gas fosters fast localized mixing. Quality controls may ensure that the ingredients, including their size and color, are correct but segregation during filling, transport and discharge can give the impression that the product has not been blended if the customer simply takes grab samples. The particle size and distribution may provide clues that this may happen. Solids flow testing can identify mixtures that are more likely to behave badly than other materials. You must do more than make the customer aware of the steps you’re taking to ensure a good blend, it’s important to pass on tips on how to avoid segregation of the product and how to obtain representative samples. 2. Educate your operators. A good first impression is important when delivering a product. If you’re filling a drum, don’t shake out the chute onto the top of the drum. If you’re loading a cargo ship, don’t dump the baghouse catch on top of the fill. If you do, send a note to the customer explaining why it looks so bad. Sometimes a simple demonstration of sifting and segregation principles to the operators can go a long way to improve product quality. Once I was having trouble explaining why we wanted to www.chemicalprocessing.com avoid using an intermediate hopper with a slowly deaerating material. So I took a coffee can, put some material in it and shook it up. In the first minute, I tossed in a quarter. It disappeared. Three minutes later, the same thing happened. After five minutes, the quarter landed on top and did not sink. The other two quarters were at or near the bottom of the can. It convinced the operators they had to bag as they made the material. 3. Don’t over-blend. Determine the correct blending time, speed and fill for each mixture and ensure that they’re used. Over-blending leads to attrition, heating and change in surface characteristics of the solids. If you must take samples during the blend, shut off the blender while the sample is analyzed. This is especially true if the sample size must be large to be representative. Return the sample to the blender before proceeding, if possible. 4. Use statistical methods to find hiddenvariables.Subtle changes in humidity, temperature and even the operator can influence blending. Emphasize using sixsigma methodology when developing the correct blending parameters (time, speed, etc.) for each type of blender. 5. Select the correct blender. While it may be easier, simpler and keep capital costs down to use one blender, as this article should’ve made clear, a single type of blender can’t handle all materials. Use Table 1 to find the most likely candidate based on the type of particulate solids being blended. However, keep in mind that this table only gives general patterns of behavior. It may be necessary to do flow testing to determine if the sol- ids fluidize or have a strong tendency to segregate. If you don’t have the correct device, rely on a toll producer until you can afford one. 6. Consider clean-out. It’s not economical to have a blender for every product. So determine what cleaning between products will entail. Plows and ribbons are very difficult to clean and fluidized blenders may build up coating in hard-to-reach places. The more internals in the blender, the harder it will be to clean. 7. Test, test, test! This isn’t al ways the most practical suggestion due to the cost. Flow testing is an expensive proposition but can improve both the selection and operation of a blender. However, reserve it for high value or high volume products, especially when the blender choice is in doubt. A less expensive version of the testing scenario is to match the sample size to the packaging size and use statistical techniques to reproduce the correct operating conditions. Make a solid start To select the best blender follow a systematic approach that considers where the blender fits in the process and the difficulty of the material. The amount of testing required can be a confusing issue. You can base some selections on similar materials being handled but other materials will require extensive and more expensive procedures. However, often there’s no single correct answer. Temper any expert opinions with an evaluation of overall process needs and potential problems. Ill-founded certainty is far worse than a realistic appreciation of the confusing issues. CP Tom Blackwood is director of technology for Healthsite Associates, St. Louis, Mo. E-mail himattrblac@att.net September 2007 • 49 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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.