In process design, the segregation problem can be overcome by two possible solutions. One is to modify the process to incorporate segregation patterns produced by the different mechanisms, and the other is to make changes to the process to minimize the cause of segregation. In both solutions, gaining insights into segregation mechanisms is vital when devising powerful processes to deal with segregating materials.
Usual Material Segregation Patterns
There could be many reasons for the segregation of materials upon being handled. A number of solid flow practitioners can quickly determine the potential for fine materials to sift through the coarse particle matrix when the material slides down a pile.
In fact, sifting segregation is a major reason for separation while handling particles of different sizes. Often, this mechanism leads to a radial segregation pattern in which fine particulates accumulate close to the center of a pile while the coarse material accumulates principally at the edge of the pile (illustrated in Figure 1).
Figure 1. Typical sifting segregation profile on pile.
Causes of Sifting Segregation Problems
However, intense sifting segregation can lead to a top-to-bottom segregation pattern in which the fine particulates are beneath the coarse particles. This especially true if some external effect such as vibration induces inter-particle motion inside of the material. In general, particle size differences of more than three to one are sufficient to create substantial sifting segregation problems.
Particulate material separation during handling is not just caused by the sifting segregation mechanism. The differences in inter-particle friction in certain particles leads to the formation of piles with distinct repose angles. The formation of piles inside of process equipment results in the sliding of the less frictional particles further down the pile, accumulating at the edge of the pile.
This mechanism leads to a radial segregation pattern. Materials with more than 2° difference in angle of repose can exhibit considerable repose angle segregation. Figure 2 depicts the segregation of coarse and fine fractions of roofing granules caused by the angle of repose. Here, the difference in particle size between the fine and coarse fractions is just ∼30%, making sifting impossible. The repose angles differ by just 2° and lead to considerable segregation.
Figure 2. Typical repose angle segregation.
Fine material could be carried by air currents that are formed during filling, to regions in which the air currents adequately decrease to deposit the fine material. A radial pattern or side-to-side pattern could be produced by this air entrainment segregation based on the geometry of the vessel and the position of the inlet.
Most often, fine particulates pile up close to the process vessel walls due to this segregation. A typical profile for air entrainment segregation is illustrated in Figure 3, where the fine particulates pile up near to the wall.
However, it is vital to draw attention to the fact that this figure also demonstrates the outcome of sifting segregation where the fine particulates pile up close to the drop point. Figure 3 further shows that a number of segregation mechanisms can take place simultaneously, thereby creating a complex overall segregation pattern.
Figure 3. Typical air entrainment segregation.
In case the bulk material is compressible and very fine, then it could be fluidized when a process vessel is filled. Fluidization such as this is not stable since it would occur in a fluid bed that includes an external source of air.
The material starts losing its entrained air immediately once the filling process is completed. Although, materials such as these retain their fluid-like behavior for several minutes or even hours. It is possible for coarse particles that enter the bin at this time to have an effect on this fluidized material and penetrate the fluidized solid’s top layer before coming to rest underneath the top surface.
This leads to a top-to-bottom particle separation in the bulk mixture, resulting in the formation of fine and coarse material layers (shown in Figure 4).
Figure 4. Typical impact fluidization segregation.
A number of solid flow practitioners consider mass flow to be the solution to the segregation problem, which is an imprudent thought. It is essential for the flow pattern inside a specific piece of process equipment to be matched with the segregation profile to realize a process that will reduce segregation while handling.
For instance, in case the material segregated by impact fluidization (see Figure 5) forms layers upon being placed in a hopper or a bin, the placement of a usual steep mass flow hopper on this bin would not help the segregation. By contrast, it would considerably improve the separation of bulk materials. A typical, well-designed mass flow bin induces uniform velocity, which would lead the coarse particulates to exit, followed by the fine particulates, rendering the segregation problem even worse.
Figure 5. Mass flow velocity profile with impact fluidization segregation profile.
By contrast, the conversion of the bin to mass flow will help a radial segregation pattern to form. Material will exit the bin in the same way as it entered the hopper. Although there will be a segregation profile throughout the outlet, the material at each cross-section will be at the optimum consistency. In order to achieve better mixing than this, it is essential to add additional in-line blenders to the process to realize optimal blend consistency.
Figure 6. Mass flow profile with radial segregation pattern.
To summarize, it is vital to know the segregation mechanism and the flow profiles in the process equipment to overcome possible segregation problems. The type and magnitude of segregation that occurs in a user’s systems can be measured through simple tests.
It is also possible to measure flow properties to identify the flow patterns in the process equipment. This information can be used to design a reliable solution to complex segregation problems and put the equipment back on track to ensure quality of production.
This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
For more information on this source, please visit Particulate Systems.