Critical Steps to Minimize Segregation in Processes

Dr. Kerry Johanson, chief operations officer, Material Flow Solutions Inc.

Preventing segregation of powders and granular material in process vessels is an important issue. The undertaking can be broken down into three separate tasks. First, measure the segregation profile that is characteristic of your material when it forms a pile, fluidizes, or exits a chute. This can be done in a laboratory setting with limited material, but testing protocol must be representative of what happens in the process. It is important to characterize the segregation pattern, segregation magnitude, and in some cases identify the cause of the segregation.

Second, measure the flow properties of the bulk material under process conditions. This can also be done in a laboratory setting. Again, testing must represent what may happen in the process. This requires that properties are measured at the same temperatures, %RH, and storage conditions expected in the process. It is critical to test the key flow properties at the stress level expected in the process. This means that testing should be done at the high-stress level in typical bins and hoppers (3,000-50,000 Pa). But since segregation happens as a pile is formed, it also means that the properties should be measured at the low-stress conditions comparable to those found at the top of the pile (typically 100-200 Pa). The key flow properties measured should include bulk unconfined yield strength, wall friction angle, bulk density, permeability, and perhaps adhesion to various wall surfaces.

Third, it is important to use the flow property information to compute or infer the velocity profiles in the process equipment. This can be calculated from basic theory of how material flows in process equipment, or it can be inferred by visual inspection of the process vessel. The velocity must be computed in combination with the details of the feed device attached to the bin or hopper. Often the type and placement of the feed device significantly influences the actual velocity profile in the system. It is not enough to know if the material flows in funnel flow or mass flow. Understanding the actual velocity profile is critical to minimizing segregation in a process. The solution to a segregation problem is to match the segregation profile (pattern) with the velocity in the hopper.

For example, if the segregation profile suggests that there may be some significant top-to-bottom segregation in the vessel, then the absolute worst thing to do is to use a perfect mass flow with a uniform velocity across the bin or process to feed this material. In that case, the top-to-bottom segregation profile would remain as the material left the feed system. If on the other hand the segregation pattern is a radial pattern, then using a mass flow bin with a uniform velocity profile will significantly help the segregation problem.

Many other velocity profiles are possible in process equipment, and these should always be considered relative to the segregation pattern. The velocity profile can be fast in the center, fast at the side, or skewed toward one side of the bin. Likewise, the segregation profile may place the key components accumulating in the center of the bin, accumulating at the edge of the bin, or somewhere in between. The question to be asked and answered is: Given the segregation pattern characteristic of my material, what velocity profile in the hopper or bin will induce remixing of the segregated zones? 

Sometimes it is difficult to infer this precisely, but a detailed analysis and calculation of the process velocity profile from bulk solid theory coupled with a detailed description of the segregation pattern can be combined with particle tracking techniques to accurately predict what concentrations of key components leave the feed system over time. This time sequence concentration profile of key components can be used, along with content uniformity limits, to determine how much of the material leaving the process is within specification. More importantly, this approach will allow engineers to determine when in a drawdown cycle the product may become segregated past allowable content uniformity limits. The solution to segregation problems is to match the process velocity profile with the segregation pattern of the material for each unit operation in the system.  

Dr. Kerry Johanson is chief operations officer for Material Flow Solutions Inc. (Gainesville, FL). For more information, visit matflowsol.com.