Chemical Processing - September 2007 - (Page 46)

>> Double cone tumble blender Figure 2. Such units generally are inexpensive and suit moderately cohesive materials. 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), Vcone 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, 46 • September 2007 which affected the reaction time. 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. Tumble devices are well suited to moderately cohesive materials that do not sift or segregate. Wide variations (say, more than 3:1) in particle size of free flowing materials can create blend variations or even discharge segregation. Blend time isn’t as critical as with many other devices but the process is long. Paddle or ribbon blenders. There are many variations, with the differences primarily in the shape of the agitator. The paddles (Figure 3) can be flat blades at an angle on the shaft or large plows at right angles to the shaft. The plow has long been thought of as very effective for cohesive materials but the mixing relies on a tumbling action as the plows or paddles lift material. The biggest problem with a plow is the limited lateral mixing along the shaft. This can be overcome by feeding at one end and emptying at the other but segregation can occur as the solids go over the discharge weir. Fine solids can accumulate in the clearance between the wall and paddle so that dead areas develop where the solids don’t move at the same speed as the bulk contents. This can give wide variations in blend quality even with a continuous blender. When evaluating a paddle blender, conduct long term tests to determine the extent of this segregation. High rpm paddle blenders are a specialized version of this device, but are best used in very light loadings and short residence times. Attrition can be severe. Ribbon blenders (Figure 4) well suit batch mixes as the ribbons can move material in opposite directions, which adds tumbling to a back-andforth flow. The most effective blenders for wide particle-size differences use ribbons of different pitch and diameters. Overfilling and underfilling negatively affects performance. The optimum, which varies with the type of solids, generally is around twothirds full — but attrition is highest at that level and needs to be considered. If a dedusting occurs after the blender, this may improve product quality by eliminating fine particles that would be generated in subsequent packaging or handling. Ribbons can achieve a quicker blend, and better blend in the vertical plane, but still are slower in the lateral plane (along the shaft). The screw blender is a variation >> Paddle blender Figure 3. These devices often get the nod for more cohesive solids or liquid addition. www.chemicalprocessing.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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