Segregation data has two applications. Engineers can use the segregation data to improve product design, creating a product with the least segregation tendency. Engineers may also want to modify processing 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 factors required for product design or process.
Segregation takes place because of many mechanisms. The identification of the key segregation cause and the segregation pattern created through handling is vital to stop the de-mixing of the final detergent mixture during handling and packaging. Any property variance between materials can lead to the separation of crucial material components. However, there are five general causes of segregation issues in basic handling systems.
During handling, fine particles may move through a matrix of coarse particles. This mechanism necessitates that the void space between neighboring particles is quite large in order to allow the fine particles to pass through. Usually, this necessitates 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. The void spaces must also be sufficiently empty in order to accept fine particles. On the whole, this type of segregation creates a radial pattern as material develops a pile in the process equipment. The fines accumulate near the pile charge point and reduce in concentration toward the edge of the pile.
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 collects near the top of the pile, while the material with the flattest repose angle collects closer to the pile edge. Usually, there is a spreading of these two materials along the pile’s surface.
Repose angle variances of approximately 2° can cause substantial segregation. A material containing various particle sizes can contain an adequate difference in repose angles to result in this type of segregation. However, particle size variance is not a precondition in the angle of repose segregation and materials of the same size can also separate through this mechanism. Moreover, the process used must also make piles during processing or handling to facilitate this type of segregation.
The mixture may have fine particles that are too small to be pushed by air currents in the handling system. These fine particles leave the air stream when gas velocities diminish below the entrainment velocity. This results in the separation of fine and coarse particles in handling systems. The fine particles typically deposit near to the container walls.
A source of air currents in process equipment is needed for this type of segregation. This source of air can be from the free fall of a 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 suitably fine, then air caught in the interstitial voids can make the material fluidize. As a large particle drops into this fluidized layer, the momentum caused makes the large particles enter this fluid layer, rendering a top-to-bottom segregation of coarse and fine 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 shiftless in a container that was just filled. Percolation moves air up through the bulk material. In general, this process causes fissures in the bulk material where the gas outflows. The local velocity in these fissures is quite 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 extensive particle size distributions.
It is essential to find the cause of segregation to stop processing that will trigger the problem. It is also important 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 exactly should be done to the material to develop a product that is not very likely to separate.
Segregation Testing Results: Background
In the past, experts have incorporated bulk materials onto a pile, sectioned a conical pile into annular sections, and conducted a size or chemical analysis on the material accumulated in each annular section. This is a long and laborious method which leads to just a few measurements being made along the pile. Plant personnel frequently use thieves to sample piles in process equipment.
This technique also produces only a few measurements along the pile and disrupts the surface, thereby changing the segregation pattern. Other scientists measure the segregation pattern by putting material in a funnel flow hopper and then discharging the hopper, therefore 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 match that found in the real process. Moreover, 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 downside of the existing segregation testers is that they are not compatible with the process. Any two or three materials can segregate if there is a variance in properties and 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 has to be answered is: will the mixture separate when exposed to a feed behavior akin to that 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 to be scalable to process settings.
Secondly, the segregation pattern 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 to offer guidance in establishing whether the mechanism is truly a potential problem in the process. Preferably, it is better if the concentrations are based on the chemical constituents in the mixture.
Relation of Test Results to Process
At present, the identification of the chief attribute must be measured or regulated to make segregation measurements scalable to process settings. For ease, 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 fall height at some process velocity.
It is vital to consider, in this case, the dynamic effects brought on by particle rebound or sliding down the pile scale linearly with the geometry of the process equipment (shown 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 formulated.
For the purpose of this article, and because the segregation data is for 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 set to 0.57 times the diameter or width of the receiving bin. It must be noted 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 fall of about 0.85 m.
Figure 1. Typical rebound of free fall particles down the pile surface.
The segregation pattern necessitates that segregation is measured by examining spatial concentration data. This spatial examination always has a sample size or 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 takes up just one or two particles, then any segregation measurements are destined to fail 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 into the process vessel. The correct segregation volume should be selected someplace in between.
Clearly, these spatial segregation measurements need access to some spatial view of the material after 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 the material and note the segregation pattern using the side of the slice model and optical methods (see 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 (as seen in the 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. Therefore, care must be taken when loading the tester to guarantee that an illustrative sample is visible through 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 in 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 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 (see 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 length of the pile 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 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. Thus, an automatic instrument was built to control the feed, acquire the pure component spectra, and measure spatial concentration profiles for spectral measurements (as illustrated in Figure 4).
Figure 4. SPECTester used in segregation analysis.
Results of SPECTester Analysis
The segregation pattern and data is illustrated in Figures 5, 6, and 7. The concentrations are plotted as a function of the dimensionless radius. A radius of 0 is the top of the pile and a radius of 1.0 is the bottom of the pile. This profile reveals substantial segregation of the yellow and white sands, as well as some segregation of the blue sand, as specified by the segregation intensity values seen in Figure 7.
Furthermore, blue sand collects at the bottom of the pile, while white sand collects towards the top of the pile. Yellow sand shows a propensity to amass towards the center of the pile with a smaller amount of accumulation towards the top of the pile as seen in Figure 5.
This pattern of segregation is mainly suggestive of the angle of repose segregation, integrated with some sifting segregation. These sands are used to color the cement or grout for specialty pool decks, decorative slabs, and so on. If a uniform mixture of these three elements were needed to realize the anticipated effect, this pattern of segregation would indicate that the end user would need to use dedicated mixing equipment and bin designs to maintain steadiness of color.
Figure 5. Radial segregation profile for tri-color sand mixture.
Figure 6. Cumulative radial segregation profile for tri-color sand mixture.
Figure 7. Segregation intensity for tri-color sand mixture.
Occasionally, the segregation trend is clearer to see when observed from a cumulative concentration viewpoint. It is possible to sum, or integrate, the fraction of any component along the pile and to standardize it relative to the real average concentration of that component along the pile as shown in Figure 6.
If segregation is not present, then this process would reveal a straight line passing through the point (0, 0) and (1, 1) when plotted against dimensionless radius. A positive deviation of this line shows buildup near the top of the pile. A negative deviation of this line specifies build-up towards the bottom of the pile. An S-shaped curve indicates accumulation at both the top and bottom of the pile.
The deviation magnitude of this line shows the percent deviation from the mean concentration for any one component. The reason for plotting segregation in this way is to connect the segregation profile to the deviation from the mean or average concentration. The plot offers a fast way to view how bad the segregation is from a mean concentration viewpoint while giving an idea of the type of segregation that is occurring.
This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
For more information on this source, please visit Particulate Systems.