Chemical Processing - September 2007 - (Page 47)

>> Ribbon blender Vibration Analysis & Balancing Figure 4. The level of fill usually significantly impacts both performance and attrition. of the horizontal ribbon blender. It utilizes a screw along the wall of the cone that rotates around the cone as it pumps solids up the wall. Some cohesiveness helps the performance, especially with the addition of liquids. While blend times can be very low, segregation on discharge and sifting of fine particles can destroy the blend. There’s often an optimum agitator speed that gives a quick blend while minimizing sifting. However, finding it may take an extensive amount of testing. Once the right set of operating conditions have been determined, the screw blender can overcome the segregation effects. In either paddle or ribbon devices, cohesive mixtures don’t do well at the extreme end of the blender, especially if the agitator clearance is large. High rotational speeds may fluidize the solids, resulting in clumps of fine particles that slough off the shell during discharge. Consider paddle blenders for more cohesive solids or when liquids are being added to the solids and ribbons for mixtures containing minor ingredients. Both ribbon and paddle blenders often come with high-speed wall agitators, sometimes called choppers. For very cohesive materials these prevent lumps and can impart enough shear action to the particles that the surface is more active. This highly energetic surface will bind easier to the larger particles and give a quicker end to the blend. While www.chemicalprocessing.com some attrition may occur, the shorter blend time often makes this a wash. Specialized mechanical devices. The ideal mechanical blender would be able to separate the individual particles and put them back together in the mixture based on the overall distribution of the particles. The riffle box separates a mixture into discrete compartments and then recombines the small compartments to give a smaller mass that represents the average of the original mass of material. While this can be repeated to achieve the desired production size or consumer quantity, the method isn’t very practical for very large production rates. However, some proprietary devices such as the Diamondback hopper approach this concept. Another variation on blending is to fluidize the particulate solids as they are loaded from the storage tank. A highspeed ribbon or paddle that doesn’t accumulate solids (no vessel holdup) will provide localized mixing and can improve the filling of tank or rail cars and avoid excessive fine particles on the top of the fill. This is important because many clients open the top hatch of a shipment to take a grab sample for analysis. If the sample doesn’t meet specifications, even though the overall contents of the shipment do, the shipment may be rejected. Flow tube blenders (Figure 5). The riffle concept has been applied with great success on the large scale VIBXPERT® h Watc O IDE e Vn Onli VIBSCANNER® The right tool for every user, job and budget See Us at Booth #1263 Chem Show 305-591-8935 • www.ludeca.com September 2007 • 47 http://www.ludeca.com http://www.ludeca.com 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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