Choosing the right blender for your material can be a difficult and frustrating job. Vendors claim their blenders work efficiently and their claims are generally true, provided their blenders operate with the right material.
Three conditions must exist for a blender to operate efficiently. First, a blender must have no stagnant regions. Second, a blender must promote differences in retention times in various sections of the blender. Third, blender operation must not separate mixture components.
Stagnant regions exist in the freeboard area and between agitator blades and blender walls. Limited flow channels during operation also produce stagnant regions. The effect of these stagnant regions depends on mixture and component flow properties. For example, using a tube blender to mix a cohesive material (Rathole Index, RI > tube diameter) results in stable rathole formation around each tube inlet and destroys blender effectiveness.
An air blender, vertical shaft blender, horizontal shaft blender, or even a ribbon blender operating at a large rpm can blow fine particles into the air that adhere to the freeboard surfaces if the fine material is adhesive (Chute Index, CI = 90 degrees). Air blenders may require vibrators or special coatings and liners to prevent material accumulation in these regions.
J.R. Johanson's Diamondback hopper, a mainstay of the firm's blending expertise,
eliminates the need for noisy vibrators or air cannon
Pile formation blenders rely on avalanche flow in a small region on top of the pile to mix material. An excessively cohesive material will create thick avalanche layers with little interparticle motion. The result is decreased blender effectiveness. However, completely free-flowing material can have very thin avalanching zones that also have lower-than-optimal interparticle motion. This, too, produces blender inefficiencies. Actual blender performance depends heavily on mixture cohesion and can be cor-related with the Arching Index (AI).
Some blenders produce differences in retention time because of inefficient material transport during operation. For example, a ribbon blender lifts and translates only a small quantity of material during one revolution. In addition, atypical ribbon blender tends to lift material more efficiently than it transports material from side to side. This action produces differences in retention time and can also cause poor blender operation. Some ribbon blender users have discovered that sometimes optimal blending is possible only by layering mixture components in the blender.
Other blenders produce dif-ferences in retention time by promoting velocity gradients in the bulk material during opera-tion. The cone-in-cone blender promotes a faster Velocity in the center of the bin than on the side. This blending velocity profile extends up from the cone--in-cone hopper about one hopper diameter high. This results in typical short, squatly blenders. However, using proprietary cylinder-in-cylinders above the cone-in-cone hopper section ex-tends the blending profile far up into the vertical section above the hopper. It is not unusual to maintain a 5:1 height-to--diameter velocity profile ratio, thereby allowing larger blender volumes.
Sometimes blender operation segregates individual components during operation and discharge. For example, V-blenders rely on continual pile formation to blend material. Segregation may occur if the material is susceptible to repose angle differences of the individual components or if sifting takes place. This produces non-uniform mixtures. Selecting another blender style may help decrease segregation, as can retrofitting a V-blender. For example, placing a Diamond-back Hopper at the V-blender outlet causes flow across the entire width of the blender, which can reduce segregation to acceptable levels (see figure).
Air currents within the blender can also segregate mature com-ponents. For example, a ribbon blender typically has several feeder ports on the top panel. Connecting one of the ports to a dust collection system tends to accumulate fines below the collection port. Since a-ribbon blender mixes poorly from one end to the other, operating a dust collection system during blending may reduce blender effectiveness and can increase blending times.
Material properties and segregation tendencies are at the heart of intelligent blender selec-tion. J.R. Johanson routinely measures these material characteristics and can provide blender evaluations and help in blender selection. This will help reduce time-consuming blender trials with multiple blenders.
For more information: Kerry Johanson, chief technology officer, J.R. Johanson, Inc., 1237 Archer St., San Luis Obispo, CA 93401. Telephone: 805-544-3775. Fax: 805-549-8282.