Segregation data has two uses. Segregation data may be used by engineers to enhance product design by making a product with the least segregation tendency. In addition, engineers may wish to alter the processing to decrease the effect of segregation in their handling facility or plant packaging process.
In both cases, the important parameters essential for process or product design are the segregation mechanism, segregation pattern, and magnitude of segregation.
Segregation takes place through a number of mechanisms. Identifying the cause of primary segregation and the segregation pattern generated by handling is crucial to avoid the de-mixing of the final detergent mixture at the time of handling and packaging. Any difference in property between materials can lead to the separation of key material components. However, segregation issues in typical handling systems happen due to five common causes.
While handling, fine particles may sift through a matrix of coarse particles. For this mechanism to occur, the void space between the adjacent particles should be large enough to allow the fine particle to traverse. Normally, this needs a particle size difference of around 3:1. Inter-particle motion is also necessary to offer a way to reveal empty void spaces to fine particles.
In addition, the fine particles must be free flowing enough to avoid arching between neighboring particles and the void spaces must be vacant enough to allow fine particles to reach them. Overall, this type of segregation generates a radial pattern as material creates a pile in process equipment. The fine particles collect near the pile charge point and reduce in concentration toward the pile edge.
The Angle of Repose Differences
Two materials may not have the same angles of repose. Therefore, as these two materials flow down a pile, they fundamentally form overlapping piles where the material having the steepest repose angle builds up close to the top of the pile, while the material which has the flattest repose angle collects near the edge of the pile.
Normally, these two materials are distributed along the surface of the pile. Repose angle differences of approximately 2 degrees can cause considerable segregation. A material of different particle sizes can have enough difference in repose angles to create this type of segregation.
Yet, the particle size difference is not a precondition for the angle of repose segregation and materials with the same size can separate through this mechanism. Moreover, the process must form piles during handling or processing to result in this kind of segregation.
The mixture may consist of fine particles that are so small that the air currents in the handling system can carry them. When gas velocities drop below the entrainment velocity, these fine particles fall from the air stream leading to the separation of coarse and fine particles in handling systems. The fine particles normally pile up close to the container walls.
This type of segregation needs a source of air current in the process equipment. Such a source of air can be obtained from the free fall of a compressible material. When the falling stream bumps the material level, the entrained air is expelled out of the interstitial pores and the fine particles are carried in the resulting dust cloud. This segregation usually creates a radial pattern during the formation of the pile; however, the fines are at the bottom of the pile and not at the top.
If the mixture is sufficiently fine, then the material can be fluidized by the air trapped in the interstitial voids. When a large particle falls into this fluidized layer, momentum makes the large particles enter this fluid layer, causing a top-to-bottom segregation of coarse and fine particles. This mechanism needs a source of air and also the ability of the bulk material to hold onto the entrained air for an average amount of time.
If a fluidized layer of material remains still in a container that was just filled, it can lose its entrained air. Percolation drives air up through the bulk material. Typically, this process causes fissures in the bulk material through which the gas escapes. The local velocity in these fissures is comparatively high and can trap fine particles in the process, resulting in top-to-bottom segregation. This leads to the size separation of fluidizable material with broad particle size distributions.
The cause of segregation needs to be identified in order to prevent processing that will give rise to the issue. Furthermore, the pattern of segregation must also be known in order to offer a way for re-mixing material, if necessary. Gaining insights into the segregation mechanism will also help in determining what exactly should be done to the material to develop a product that is less likely to segregate.
Segregation Testing Results: Background
Earlier, investigators have fed bulk materials onto a pile, partitioned a conical pile into annular sections, and carried out chemical or size analysis on the material obtained in each annular section. This is a tiresome and lengthy procedure which enables only a few measurements to be made along the pile.
Most often, plant personnel prefer thieves to sample piles in process equipment. This technique also results in making only a few measurements along the pile and disturbs the surface, thus altering the segregation pattern. Other investigators quantify the segregation pattern by putting the material in a funnel flow hopper and subsequently discharging the hopper, eventually collecting the output stream and quantifying the quality of the mixture 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 and the bin design in the process are exactly the same, there is no assurance that the composition during discharge from the tester will be analogous to that which is found in the real process.
Furthermore, the bulk material’s cohesive properties can hinder flow from the segregation tester hopper, although hang-ups may not happen in the process with the same material.
One drawback with present segregation testers is that they do not associate well with the process. Any two or three materials can segregate if there is a dissimilarity in their properties and if the mixture undergoes sufficient external stimuli. For instance, combining an adequate amount of fluidization gas will segregate materials of various densities, although these adverse conditions may never happen in the real process.
The definite question to be answered is whether the mixture will segregate on subjecting it to a feed behavior similar to that which is present in the process. Thus, any measurement of segregation tendency must have three vital aspects. Firstly, the feed should be controlled in order to enable the measured segregation to be scalable to process conditions.
Secondly, the segregation pattern must be incorporated as part of the measurement to estimate the expected concentration leaving the process equipment. Finally, the extent of the segregation should be measured to offer guidance to establish whether the mechanism is actually a potential challenge in the process. Ideally, the concentrations based on chemical components in the mixture are preferred.
Relation of Test Results to Process
In order to make segregation measurements scalable to process conditions, it is essential to identify the vital attribute that must be controlled or measured. For the sake of simplicity, consider that material just falls from a mechanical conveyor into the process vessel.
Material going through the process usually free-falls from a certain fall height at some process velocity. In this case, it is necessary to note that the dynamic effects are due to particle rebound or due to sliding down the pile scale linearly with the process equipment’s geometry (see Figure 1).
Figure 1. Typical rebound of free fall particles down a pile surface.
This is not the same in the case where gas is carried with the input stream. Conversely, similar models can be produced for this case and a scale law can be developed. For the sake of this article, and since the segregation data is for easy comparison, little to no gas effects are assumed during the filling of a standard pharmaceutical process.
The small-scale segregation tester can subsequently be operated at a steady fall height and drop rate. The material was dumped in a slice model, creating half a pile, at a rate of around 1 liter per minute. The free-fall dimension was configured to be 0.57 times the width or diameter of the receiving bin.
It must be noted that the complete diameter of the receiving bin in the process would correspond to two times the width of the slice mode bin in the tester as the slice model was filled on one side. For a standard 1.5 m wide bin, this would correspond to a drop of roughly 0.85 m.
The segregation pattern demands the measurement of segregation by analyzing spatial concentration data. This spatial analysis always includes a sample size or viewport associated with it. It is essential to decide in advance the volume of material which would represent a reasonable segregation basis. The viewport should include adequate particles in order to be statistically pertinent.
In case the volume takes up just one or two particles, then any segregation measurements would lead to failure because an average sample cannot be represented by the segregation measurement volume. If that sample volume and the bin size are same, then every segregation measurement will represent the overall average concentration placed in the process vessel. The proper segregation volume should be selected somewhere in between.
Certainly, these spatial segregation measurements need access to some spatial view of the material once it has been separated through initial process filling. One method to attain access to the cross-section of a pile is by filling a slice model with material and looking at the segregation pattern through the side of the slice model using 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 (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 using the sides of a slice model will vary according to the optical pattern existing at the wall of the segregation tester. Therefore, one must be careful when loading the tester in order to ensure that a representative sample is seen through the side of the tester. The material will be distributed to the tester wall by filling the tester across the width of the slice model, thereby forming an even material across the tester.
Reducing the thickness of the slice model will also facilitate the distribution of representative material to the tester wall. Conversely, one should know that very thin slice models are put through banding because of wall effects that would not exist in broader slice models. In the case of typical materials, this reduces the thickness of the slice model to a minimum of about 25 mm.
Reflectance spectroscopic techniques can be used to measure slight variations in color. Since this article considers a discrete particle system, multiple measurements are needed to measure the average concentration within a particular viewport. Segregation measurement also represents a scale issue.
The size of the selected viewport must be large enough to include a representative number of particles, while, at the same time, it should be small enough 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 around 30 points along the pile length at a position that was approximately 1.27 cm below the top surface of the pile as illustrated in the schematic in Figure 2.
Figure 3. Measurement zone along the pile top surface.
If the spectra of the pure components and the spectra of the mixture in different viewport boxes along the pile are known, 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 to collect accurate data, there needs to be several spectral measurements collected at multiple points along the pile.
The manual collection of this data is tiresome and lengthy, and hence an automatic instrument was developed to control the feed, achieve the pure component spectra, and quantify spatial concentration profiles for spectral measurements (see Figure 4).
Figure 4. SPECTester used in segregation analysis.
Results of SPECTester Analysis
The segregation pattern and data are represented in Figures 5, 6, and 7. The concentrations are mapped as a function of the dimensionless radius. Radii of 0 and 1.0 are at the top of the pile and the bottom of the pile respectively. This profile demonstrates that there is a considerable segregation of surfactant, as denoted by the segregation intensity (see Figure 7). Additionally, surfactant has a tendency to segregate heavily toward the bottom of the pile (see Figure 5).
Figure 5. Radial segregation profile for household powdered cleanser.
Figure 6. Cumulative radial segregation profile for household powdered cleanser.
Figure 7. Segregation intensity for household powdered cleanser.
It is fascinating to note that the key active ingredient does not exhibit much propensity to segregate. By contrast, the surfactant exhibits a major propensity to segregate. The pattern of segregation indicates that both sifting and angle of repose are apparent causes of particle segregation with this mixture.
The key active ingredient may be filling the spaces between the other particles in the mixture, producing a well-graded mixture that is not sensitive to segregation. On the other hand, the surfactant exhibits a large propensity to collect at the bottom of the pile. This is a case of complex segregation patterns when handling mixtures of over two elements.
Although inert A material is interacting predominantly with the surfactant to promote segregation, this sub-mixture (the combination of surfactant and inert A material) does not exhibit much segregation potential with the key active ingredient. There, a formulator would only have to optimize the segregation with the sub-mixture to decrease the overall segregation of this blend.
At times, the segregation trend can be easily viewed if observed from a cumulative concentration viewpoint. The fraction of any component along the pile can be integrated or summed and can be normalized in relation to the actual average concentration of that component along the pile (see Figure 6).
When there is no segregation, this procedure would represent a straight line passing through the point (0,0) and (1,1) when plotted against dimensionless radius. The build up near the top of the pile is denoted by a positive deviation outside of this line. The accumulation towards the bottom of the pile is denoted by a negative deviation off this line.
The accumulation at both bottom and top are represented by an S-shaped curve. The percentage deviation from the mean concentration for any one component is indicated by the magnitude of the deviation off this line.
The segregation is plotted in this way to relate the segregation profile to the deviation from the average or mean concentration. These plots offer a rapid method to see how bad the segregation is from a mean concentration perspective while providing a feel of the type of segregation that is taking place.
This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
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