Segregation data has two uses. Engineers may use segregation data to improve product design by developing a product with the least segregation tendency. Engineers may also want to alter the processing technique to diminish the effect of segregation in their handling facility or plant packaging process. In both cases, the segregation mechanism, segregation pattern, and the magnitude of segregation are the main parameters required for product design or process.
Segregation takes place through quite a few mechanisms. The identification of the cause of the main segregation and the segregation pattern created through handling are both important to stop the de-mixing of the final detergent mixture while handling and packaging. Any property variance between materials can result in the separation of crucial material components. However, there are five common causes of segregation issues in standard handling systems.
During handling, fine particles may sift through a matrix of coarse particles. This mechanism necessitates that the void space between the neighboring particles must be sufficiently large to allow fine particles to pass through. Usually, this means there needs to be a particle size difference of about 3:1. Inter-particle motion is also essential to offer a means of revealing empty void spaces to fine particles.
The fine particles should also be sufficiently free flowing to stop arching between neighboring particles and the void spaces must be sufficiently empty to accept fine particles. On the whole, this type of segregation creates a radial pattern as a material develops a pile in process equipment. The fine particles amass near the pile charge point and reduce in concentration toward the pile edge.
Angle of Repose Differences
Two materials may have diverse angles of repose. Therefore, when these two materials flow down a pile they, in essence, form overlapping piles where the material with the steepest repose angle amasses near to the top of the pile, while the material with the flattest repose angle amasses closer to the pile edge.
Usually, there is a spreading of these two materials along the pile’s surface. Repose angle variances of about 2° can cause major segregation. A material of varying particle sizes can contain a sufficient difference in repose angles that results in this type of segregation.
However, particle size variance is not a precondition angle of repose segregation and materials of the same size can separate through this mechanism. Furthermore, the process used must also produce piles during processing or handling to bring about this type of segregation.
The mixture may have fine particles that are quite small to be transported by air currents in the handling system. These fine particles leave the air stream when gas velocities fall below the entrainment velocity. This results in the separation of fine and coarse particles in handling systems. The fine particles mostly deposit near to the container walls.
A source of air currents in process equipment is required for this type of segregation. This source of air can be from a free fall of compressible material. When the falling stream hits the material level, the entrained air is forced out of the interstitial pores and transports the fine particles in the resulting dust cloud. This segregation usually results in a radial pattern during pile formation, but the fine particles are at the bottom of the pile and not the top.
If the mixture is sufficiently fine, then air caught in the interstitial voids can make the material fluidize. As a large particle falls into this fluidized layer, thrust causes the large particles to enter this fluid layer, rendering a top-to-bottom segregation of fine and coarse particles. This mechanism needs a source of air and the capacity of the bulk material to hang onto entrained air for a reasonable amount of time.
A fluidized layer of material can lose its entrained air as it sits idle in a container that has been filled. Percolation forces air up through the bulk material. In general, this process develops fissures in the bulk material where the gas outflows. The local velocity in these fissures is moderately high and can entrain fine particles in the process, resulting in a top-to-bottom segregation. This leads to the size separation of fluidizable material with broad particle size distributions.
It is important to identify the cause of segregation to stop any processing that will trigger the problem. It is also essential to know the pattern of segregation to offer a means of re-mixing material, if necessary. Understanding the mechanism of segregation will also aid in establishing what must be done to the material to develop a product that is less probable to segregate.
Segregation Testing Results: Background
Formerly, scientists have incorporated bulk materials onto a pile, sectioned a conical pile into annular sections, and conducted size or chemical analysis on the material accumulated in each annular section. This is a long and tiresome method and yields just a few measurements along the pile. Plant personnel frequently use thieves to sample piles in process equipment.
This technique also produces only some measurements along the pile and disrupts the surface, therefore changing the segregation pattern. Other scientists measure the segregation pattern by positioning the material in a funnel flow hopper and then discharging the hopper, thereby gathering the output stream and measuring the mixture quality by particle size or chemical composition.
This technique convolutes the segregation with the flow pattern from a specific bin or hopper. Unless the bin design in the tester is exactly the same as the bin design in the process, there is no assurance that the composition during discharge from the tester will be comparable to that which is found in the real process. Furthermore, the cohesive properties of the bulk material can obstruct flow from the segregation tester hopper, despite the fact that hang-ups may not happen in the process with the same material.
One issue with existing segregation testers is that they do not match well to the process. Any two or three materials can segregate if there is a variance in their properties and if the mixture undergoes adequate external stimuli. For instance, incorporating enough fluidization gas will separate materials of varying densities, even though these challenging conditions may never happen in the real process.
The real question that needs an answer is: will the mixture separate when exposed to a feed behavior akin to that which is present in the process? Therefore, any measurement of segregation propensity must have three main elements.
Firstly, the feed should be regulated to permit the measured segregation so that it is scalable to process settings. Secondly, the pattern of segregation should be added as part of the measurement so the tester can predict the anticipated concentration leaving the process equipment.
Lastly, the segregation magnitude must be quantified in order to offer guidance in establishing if the mechanism is truly a potential problem in the process. Preferably, it is beneficial if the concentrations are based on chemical components in the mixture.
Relation of Test Results to Process
Now, the identification of the main attribute must be regulated or measured to make segregation measurements scalable to the process settings. For the sake of simplicity, it will be assumed that the material falls from a mechanical conveyor into the process vessel.
A material getting added into the process usually free-falls from a certain height at some process velocity. In this case, it is vital to consider that the dynamic effects brought on by particle rebound or sliding down the pile scale linearly with the geometry of the process equipment (as seen in Figure 1).
The same cannot be said of the case where gas is transported with the input stream, but analogous models can be made for this case and a scale law can be formulated. For the purpose of this article, and because the segregation data is for a basic comparison, it will be assumed that there is little to no gas effects during the filling of a standard pharmaceutical process.
The small scale segregation tester can then be worked at a constant drop rate and fall height. The material was deposited in a slice model, forming half a pile, at a rate of about 1 liter per minute.
The free-fall dimension was fixed at 0.57 times the diameter or width of the receiving bin. It is crucial to note that the total diameter of the receiving bin in the process would match twice the width of the slice mode bin in the tester since the slice model was filled on one side. For a usual 1.5 meter wide bin, this would match a drop of about 0.85 m.
Figure 1. Typical rebound of free fall particles down a pile surface.
The segregation pattern necessitates that segregation is measured by examining spatial concentration data. This spatial examination always has a sample size or a viewport connected with it. It must be planned earlier what volume of material signifies a practical segregation basis. The viewport must hold sufficient particles to be statistically applicable.
If that volume inhabits just one or two particles, then any segregation measurements are fated to failure since the segregation measurement volume cannot signify an average sample. If that sample volume is the same as the bin size, then all segregation measurements will signify the universal average concentration put in the process vessel. The correct segregation volume should be selected someplace in between.
Evidently, these spatial segregation measurements need access to some spatial view of the material once it has segregated because of early process filling. One way to get access to the cross-section of a pile is to fill a slice model with material and note the segregation pattern by the side of the slice model using optical methods (as shown in Figure 2).
Figure 2. Schematic of a segregation tester.
Dump material into a box and observe the change in color intensity along the pile as measured just below the top surface of the pile (rectangle section).
These changes in color intensity are an indication of differences in either chemical composition or in particle size and can be used to estimate the segregation of key components in the system.
The observation of segregation through the sides of a slice model will be prejudiced to the optical pattern that is present at the wall of the segregation tester. Thus, care must be taken when loading the tester to guarantee that an illustrative sample is visible using the side of the tester.
Filling the tester across the width of the slice model will spread the material to the tester wall, forming an even material across the tester. Restricting the thickness of the slice model will also help with spreading the representative material to the tester wall. However, one should be conscious that very thin slice models are subject to banding because of wall effects that would not exist in broader slice models. For standard materials, this restricts the slice model to a minimum of approximately 25 mm in thickness.
Reflectance spectroscopic techniques can be used to measure the subtle variances in color. Since a discrete particle system is dealt with here, several measurements are needed to establish the average concentration within a predetermined viewport. Segregation measurement is also a scale issue.
The size of the chosen viewport should be large enough to contain an illustrative number of particles, yet adequately small so that differences in local compositions are not lost in the averaging scheme (as shown in Figure 3).
The tester viewport area was defined as 1.27 cm², and 36 sample readings were averaged within the viewport area. The viewport was moved to collect data at about 30 points along the pile’s length at a position that was about 1.27 cm below the top surface of the pile as specified in the schematic illustration in Figure 2.
Figure 3. Measurement zone along the pile top surface.
If the spectra of the pure components are established, and the spectra of the mixture in various viewport boxes along the pile are established, then the pure component spectra can be used to measure the concentration of the components along the length of the pile.
One disadvantage of this method is that good data needs a number of spectral measurements gathered at many points along the pile. Manually gathering this data is laborious and time-consuming. Therefore, an automatic instrument was built to control the feed, acquire the pure component spectra, and measure spatial concentration profiles for spectral measurements (shown in Figure 4).
Figure 4. SPECTester used in Segregation Analysis.
Results of SPECTester Analysis
Figures 5, 6, and 7 illustrate the segregation pattern and data in detail. The concentrations are plotted as a function of the dimensionless radius. Radii of 0 and 1.0 are at the top and the bottom of the pile respectively. This profile shows substantial segregation of both blue dots and brightener, as specified by the segregation intensity (illustrated in Figure 7).
Figure 5. Radial segregation profile for laundry detergent with brightener.
Figure 6. Cumulative radial segregation profile for laundry detergent with brightener.
Figure 7. Segregation Intensity for laundry detergent with brightener.
The detergent base tends to collect at the top of the pile while blue dots and brightener collect at the bottom of the pile (displayed in Figure 5). The measurements show that the blue dots are the worst acting element for segregation. This profile is suggestive of the angle of repose segregation, indicating that the formation of piles during filling of any equipment downstream of the mixing unit and prior to the packing step should be diminished.
Occasionally, the segregation trend is clearer to see when observed from a cumulative concentration point of view. It is possible to sum, or integrate, the fraction of any constituent along the pile and normalize it in relation to the real average concentration of that component along the pile (see Figure 6). If there is no segregation, then this procedure will reveal a straight line passing by the point (0,0) and (1,1) when mapped against dimensionless radius.
A positive deviation off this line specifies build up near the top of the pile. A negative deviation off this line specifies build up towards the bottom of the pile. An S-shaped curve specifies build up at both the top and bottom of the pile. The magnitude of the deviation off this line specifies the percent deviation from the mean concentration for any one constituent.
The reason for plotting segregation in this way is to associate the segregation profile to the deviation from the mean or average concentration. These plots provide a fast way to see how extensive the segregation is from a mean concentration standpoint while offering a feel of the type of segregation taking place.
This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
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