Particle segregation is a common problem in many bulk storage systems and its presence creates serious quality control problems. Segregation creates inconsistent batches that can cause dosage variations in pharmaceuticals, significant weight and flavor variations in packaged foods and gas flow problems in chemical reactors.
There are more than a dozen easily identifiable segregation mechanisms that result in out-of-spec product, but the five most common segregation mechanisms -- sifting, angle of repose, fines fluidization, air currents and chute trajectory – are responsible for more than 80 percent of segregation problems in solids handling and storage systems.
Sifting segregation is mostly likely in a mixture containing free-flowing particles of significant size variation, usually three or greater in the mean diameter. As process equipment imposes interparticle motion, usually through vibration, the finer component can sift through coarse components. All bins, batch blenders and chutes have the potential for sifting segregation problems.
Angle of repose segregation occurs whenever solids with greater angles of repose form a steep pile under the deposit point, allowing solids with lesser angles of repose to slide or roll to the head of the pile. A mixture containing components that are cohesive or rough-surfaced are particularly prone to angle-of-repose segregation. Rotating shell-type blenders, stock piles and bins are susceptible to this mechanism.
Fluidization segregation results when a mixture contains a large portion of light or fluffy free-flowing, fine component, enough to form a layer, and smaller portion of a relatively coarse, heavier component. The larger component easily penetrates the fluidized fines, pushing the fines layer to the top of the bin or vessel. Fluidization is especially active in air blenders, high-speed ribbon blenders, bins and piles.
Air current segregation occurs when fine particles become airborne as bimodal material enters a bin. These very fine particles migrate to the vessel walls or toward a dust collection system. If the fines are a minor component of the mixture or are cohesive, migration can be significant.
Chute trajectory segregation results from particles with different friction coefficients. High friction coefficient materials usually contain fine particles and slide more slowly down a chute than low friction materials. This results in different discharge trajectories. Particles with high friction angles dribble off the end close to the chute, while particles with low friction angles accelerate and are thrown further away from the chute.
There are simple, practical steps to reduce these segregation mechanisms.
- DO use a bin with tall cylindrical section that provides flow at the walls.
- DO use a mixing device at the center for charging a multiple-outlet bin. This will create uniform, symmetric segregation.
- DO proportion and mix badly segregated materials just before processing using as little surge capacity as possible.
- DO use a tangential entry for pneumatically conveyed fluidizable solids or install a cyclone at the bin top that uses a deflection plate.
- DO use inclined open chutes to decrease air entrainment in ascending solids. This not only reduces fluidization entrainment, it also reduces dusting.
- DO premix liquid with coarser particles before adding finer components if sifting or repose segregation are present.
- DO use blenders that remix top-to-bottom during hopper discharge or use static stream blenders below belt discharge points to remix segregated solids.
- DON’T split material from a belt conveyor into various bins since the belt may segregate the materials.
- DON’T use a non-symmetrical, multiple-outlet bin.
- DON’T use a uniform velocity mass-flow bin to cure fluidization-type segregation. It will only make it worse.
- DON’T use freefall chutes to transfer materials with different friction angles unless there is a mixing device downstream.
- DON’T charge a mixture of fine fluidizable powder and non-fluidizable coarse particles from a pneumatic conveying line using a vertical downspout.
Segregation problems created by these mechanisms can be solved by modifying equipment or operating procedures, both of which often do not require large capital expenditures. For information on solving segregation problems, contact Diamondback Technology at firstname.lastname@example.org