Most processes must produce solids products that are uniformly mixed. Segregation of these particles can occur as they are being handled, and the results can be quite costly. The pharmaceutical industry, among others, is susceptible to problems with particle segregation. If fines and coarse particles segregate, tablet or capsule quality is affected, potentially causing the discarding of valuable drugs. This column discusses two major mechanisms of segregation: sifting and fluidization segregation.
Sifting segregation occurs when small, fine particles move through large, coarser particles. This is the most common means for particles to separate. In order for sifting to occur, the material must: be easy flowing; have a range of particle sizes; have fairly large (> 100 mesh) particles; have some means of interparticle motion (such as forming a pile).
Because of sifting, the particles segregate in a horizontal or side-to-side pattern. As the pile forms, the fines concentrate in the center while the coarse particles roll or slide to the perimeter.
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Fluidization segregation affects fine particles, as they tend to remain airborne longer because they are less permeable to air than coarser particles. As a silo is being filled, say by a pneumatic conveying system, the fine material tends to settle on top, while the coarser or heavier particles fall rapidly, creating a top-to-bottom pattern.
How do sifting and fluidization segregation affect my process operation?
Each of these segregation mechanisms can affect your process in different ways. Typically, as you load your product into its silo, perhaps it segregates by the sifting mechanism. If the silo exhibits a funnel flow pattern (some material moves while most remains stagnant, likely at the walls) at some point in your process, you will discharge all fine particles or all coarse particles, which will affect your process negatively. This occurs because a preferential flow channel forms, usually above the outlet, causing the center (fine particles) to empty first, a mix of coarse and fines follows with large quantities of coarse particles at the end.
Suppose you fill your silo with a pneumatic conveying system, you would likely experience segregation by fluidization. Typically, the pneumatic conveying line enters the silo at the top, center. This method of filling silos causes the coarser, heavier particles to be driven to the bottom, while the finer, lighter particles remain airborne and settle on top. If your silo flows in mass flow (all the material moves whenever any is withdrawn—flow at the walls), and your product segregates top-to-bottom, you may experience segregation. Perhaps you are discharging from your tank to bulk bags or boxes, most of the containers will be filled with coarse particles, while the last few containers will contain all fines. This will obviously affect your product quality and costs. Although in many handling situations, a mass flow pattern is preferred, in this case, mass flow exacerbates segregation.
How do I prevent segregation problems?
A mass flow pattern will minimize sifting segregation effects experienced by funnel flow (ratholing, flushing, etc.). Because of the first-in-first-out type flow sequence, the side-to-side segregated particles will be remixed as they discharge from the outlet.
However, if you experience top-to-bottom (fluidization) segregation, a mass flow pattern will be a detriment; but, there is a way to avert this from occurring. You can address this problem by simply changing the way the product enters the silo at the top, to entering tangentially (on the side) somewhere near the top. As the material enters from the side, it swirls around the tank walls and segregates side-to-side. A side-to-side segregation problem is corrected by using mass flow.
There are other methods to resolving segregation issues not mentioned here. Keep in mind that in order to address a segregation problem, you must know how your material flows. Laboratory tests are available to identify your material’s flow properties such that using this information wisely will result in segregation free flow.
Always remember that the feeder design is as critical as the silo design. They must work in unison!
Joseph Marinelli is a consulting engineer and president of Solids Handling Technologies. He has been providing testing and consulting services since 1972. As a former consultant with Jenike & Johanson Inc., he has years of experience testing powders and designing bins and feeders for reliable flow. He lectures frequently on the topic of powder handling, and has published several papers, including an article in a chemical encyclopedia and two in a food powder book. For more information, contact Joe at 803-802-5527 or firstname.lastname@example.org.