Predicting, diagnosing, and solving mixture segregation problems Herman Purutyan and John W. Carson Jenike & Johanson, Inc. Keeping a dry bulk material mixture together from the blender to the final package isn t always easy. This article covers segregation fundamentals, then explains how to predict a new mixture s segregation potential, how to diagnose mixture segregation problems in an existing handling system, and how to remedy the problems. Problems stemming from particle segregation in dry bulk material mixtures during handling can be extremely costly and in some cases can even pose serious safety risks. Customer complaints in reaction to taste variations in a powder drink mix, too much or too little bleach in a box of powder detergent, or an unpredictable amount of leavening agent in a cake mix can kill a product. Tablet-to-tablet variation in the amount of the active ingredient in a pharmaceutical drug batch can mean scrapping the batch or, worse, delaying a new drug s launch, costing millions of dollars. Even more serious, a runaway reaction caused by chemistry variations in the feed to a reactor can create hazardous conditions in your plant. In fact, as particle size decreases, dry bulk products generally become more difficult to handle. For instance, while the coarse portion may be free-flowing, the fines can be cohesive, with a tendency to arch or rathole in a storage bin or stockpile, as shown in Figure 1. Higher concentrations of fines can cause particles to adhere together and promote caking during handling and storage. Processes that are designed for a feed with a certain particle size can be adversely affected if the feed s particle size distribution varies. And conveying, feeding, and bagging and packaging systems designed to handle a mixture with a certain particle size distribution or bulk density can have problems handling the mixture once it segregates. Before discussing how to avoid or remedy these costly problems, let s explore how segregation occurs. Figure 1 Rathole in a stockpile Not only mixtures of discrete components, but products that contain just one chemically homogeneous material can have segregation problems. If such a product consists of particles of varying size, segregation can create regions of fine and coarse particles, and each can behave very differently.
Understanding segregation mechanisms Segregation can occur by several different mechanisms, depending on the particles physical characteristics and the handling method. The most common segregation mechanisms are sifting, fluidization, and dusting. One or more of them can be behind mixture segregation problems in a given handling system. Sifting. Sifting, perhaps the most common segregation mechanism, can be described simply as the movement of smaller particles through a matrix of larger ones. This mechanism is vividly illustrated in Figure 2, which shows a formed stockpile of a mixture containing corn fines (white), roasted shells (brown), and ceramic beads (gray). The fines are concentrated in the pile s center under the incoming stream. But as the pile is formed, the slope stability causes layers of larger particles to move intermittently from the central feedpoint, carrying some of the fines with them. This results in the striations visible in the photograph. Experiments 1 and field observations have shown that all four of the following conditions must be present for sifting to occur: 2. A large enough mean particle size: If a mixture consists of 1-micron fumed silica and 10-micron microspheres of another material, the mixture won t be as easy to segregate as a mixture of golf balls and ball bearings. Experiments 2 on binary mixtures have shown that below about 500 microns (number 35 US mesh) the tendency to segregate by sifting drops substantially, as illustrated in Figure 3b. More than likely, this is because the forces acting between finer particles tend to make them less mobile. However, some sifting can occur down to and below a mean particle diameter of 200 microns (number 70 US mesh) when the particle diameter ratio is as low as 2-to-1. Figure 3 Results of sifting segregation experiments a. Variation of segregation coefficient in binary mixtures with various particle diameter ratios 2 100 80 1. Particle size differences between individual components: If all the particles in a mixture are the same size, no movement can occur through the void space between particles. Experiments 2 on binary mixtures of spherical particles have indicated that sifting can occur with a particle diameter ratio as low as 1.3-to-1, as shown in Figure 3a. In general, the larger the ratio of particle sizes, the greater the tendency for particles to segregate by sifting. To illustrate, imagine mixing a bag of white golf balls with a bag of yellow golf balls. Once mixed, the balls won t segregate by color because the balls are all the same size. But if you mix golf balls with small ball bearings, the ball bearings will sift fairly easily through the matrix of golf balls. Segregation coefficient 60 40 20 0 1 2 3 4 5 6 Particle diameter ratio Figure 2 Sifting segregation in a stockpile Segregation coefficient 100 90 80 70 60 50 40 30 20 10 b. Variation of segregation coefficient in binary mixtures with various particle diameter ratios and mean particle diameters 2 Ratio = 2.84 0 0 100 200 300 400 500 600 700 800 900 1,000 Mean particle diameter (microns) Ratio = 2.38 Ratio = 1.42
3. Free-flowing material: For sifting to occur in a mixture, no agglomerates can form, and no significant particle-toparticle bonding can occur between particles of the same size or different sizes. This generally requires the mixture to have a low moisture content and few or no fines. 4. Interparticle motion: If particles are stationary or moving with a uniform velocity, they re essentially locked together, almost eliminating the tendency to segregate, even for highly segregating mixtures. For sifting to occur, particles in the mixture must flow at different velocities. If any one of these four conditions is not present, the mixture won t segregate by sifting. Fluidization. If a column of particles of various sizes and densities is entrained in gas (that is, the particles are fluidized or aerated) and then allowed to deaerate, the larger or denser particles (or both) will settle to the bottom. The fines will accumulate near the top because their interparticle voids retain air, resulting in a vertical segregation pattern. mixture of coarse and fine particles is loaded into a bin, a vertical segregation pattern commonly develops because the coarse particles are driven into the material bed as the bin fills and the fines remain fluidized near the top surface. Dusting. Since ancient times, segregation by dusting (also called particle entrainment) has been used to separate chaff from wheat. In the presence of air currents, lighter particles that are suspended in air can separate from the bulk of heavier particles that settle. For instance, when filling a bin, fine particles remain suspended in air and often are carried by secondary air currents away from the fill point into the bin s outer areas, scattering the particles in a way that bears no resemblance to their calculated trajectory. This dusting effect becomes stronger with smaller particle sizes of about 50 microns and is very common with particles smaller than 10 microns. For sifting to occur, particles in the mixture must flow at different velocities. Fluidization (also called gas entrainment) can also occur as particles free-fall, such as during bin filling. When a Figure 4 Fluidization segregation test equipment Filter Column Filled collection cups Collection cups Fine Medium Coarse Gas-permeable membrane Control panel Gas supply
Predicting a new mixture s segregation potential Before you design a handling system for a new mixture, it s important to anticipate the mixture s segregation potential so that you can select equipment and configure the system to prevent or minimize segregation problems. While no comprehensive models exist for precisely predicting a mixture s blend composition at all points in a handling system, it s possible to anticipate segregation problems, including segregation type and severity, based on understanding the mixture s physical characteristics. By characterizing the particle size distribution, bulk cohesive strength, and particle densities of each component in the mixture, you can obtain some information about whether the mixture may segregate by sifting, fluidization, or dusting. However, you can obtain much more information about your new mixture s segregation potential by conducting segregation tests. You can conduct these tests in your plant s in-house lab if it has segregation test equipment; otherwise, you can contract an independent test lab to perform the tests. Sifting and fluidization segregation tests 3, 4 provide information about whether your mixture is likely to segregate by these mechanisms, and the test results also provide a basis for deducing the mixture s dusting segregation potential. The apparatus for a fluidization segregation test is shown in Figure 4. In this test, a mixture sample is fluidized in a column of gas, then allowed to settle. The collection cups at the column s left then pass through the column to collect material from the top, middle, and bottom. If fluidization segregation has separated the mixture (causing vertical segregation), each cup will contain a different size fraction fine, medium, or coarse particles as shown at the top right. If your particle characterization and segregation test results indicate that your mixture is likely to segregate, you now have a much better idea of how to design a successful handling system for it. Find advice on how to do this in the final section, Solving segregation problems. Also expect to work closely with your equipment supplier or bulk solids handling consultant for help designing a system that can prevent your new mixture from segregating. Diagnosing mixture segregation problems in an existing system If you have a segregation problem in an existing handling system, you need to determine where segregation is occurring in the system before you can reduce or eliminate the problem. This requires taking representative mixture samples from meaningful locations in the system, then using sample analysis to identify the segregation s source. [Editor s note: Providing extensive details about available sampling equipment and methods is beyond this article s scope; for more information, see the later section For further reading. ] Choosing the sampling locations. It s not always apparent at what point in a handling system a segregation problem starts, so it s important to take samples from several system locations and particularly at transfer points to determine where your mixture has acceptable composition and the first point at which it begins to segregate. For instance, to find the source of segregation in a bagged product, you may take a sample inside the bin above the bagging operation and another sample from the bin s discharge stream. If the bin sample is well-mixed and the discharge sample is segregated, you know that the mixture doesn t start to segregate until it discharges from the bin. When you take a mixture sample from a flowing stream, such as from a fill chute, making sure that the sample is representative is especially important. Getting a representative sample. Obtaining a representative sample of a mixture isn t always easy. Analyzing blend quality for instance, for particle size, chemical makeup, or color often requires relatively small amounts of material. Yet samples collected from a process are almost always larger than such analyses require. As a result, a subsample must be taken from the collected sample for analysis. But taking a random subsample doesn t ensure that the subsample will be representative of the collected sample. For this reason, it s best to use a rotary riffler (also called a spinning riffler) or similar device to divide the sample into identical subsamples. A typical rotary riffler, as shown in Figure 5, has a hopper that discharges onto a rotating wheel. The wheel is divided into individual sampling cups, and when material is discharged from the hopper, it flows into each sampling cup as each passes in turn under the discharge. When you take a mixture sample from a flowing stream, such as from a fill chute, making sure that the sample is representative is especially important. In this case your sample must capture the stream s entire cross section, because segregation can occur anywhere within the stream. For instance, just inserting a sample cup into the stream will capture only a portion of the stream, and the sample s analysis results can be quite different from those of a sample taken from another portion of the stream. To get an accurate sample from a flowing stream, you must collect a cross section of the entire stream, then divide that sample into subsamples for analysis. Identifying the segregation problem s source. Once you ve analyzed the samples from your handling system, you can identify the process or handling step where segregation occurs. From this information you can determine how to change the system to eliminate the problem.
For instance, consider how you would track a segregation problem in the final packaged product from a system in which material is mixed in a blender, then pneumatically conveyed to a bin before flowing to the bagging operation. You would start by taking samples from inside the blender, 5 from the blender s discharge stream, from inside the bin, and from the bin s discharge stream. Next, you would analyze each sample s particle size distribution (or chemical composition or other attribute) to determine at what point segregation occurs. If the results show that the sample from inside the blender is well-mixed but the sample from inside the bin is segregated, you can eliminate the blending step as the segregation s source and instead concentrate on the blender-discharging or bin-filling steps to determine how the mixture is segregating. Solving segregation problems To reduce or eliminate mixture segregation problems in your existing handling system, you have three avenues for solving the problem: changing the material, changing the process, or changing the equipment. You can also use Figure 5 Rotary riffler for subdividing samples many of the principles discussed here to design a handling system for a new mixture that s likely to segregate. Changing the material. Most highly segregating materials are free-flowing, which means that the particles easily separate from each other. So one obvious change you can make to reduce a mixture s segregation tendency is to increase its cohesiveness, such as by adding water, oil, or another liquid binder to the mixture s major component prior to blending or adding the binder during blending. However, be careful not to overdo this, since increasing the mixture s cohesiveness too much can lead to other flow problems, such as arching or ratholing, that cause even greater problems downstream. Another option is to change the particle size distribution of your mixture s components. For instance, if the mixture is segregating by sifting, you can reduce or eliminate the sifting by reducing the particle diameter ratio between components below 1.3-to-1 or by reducing each component s mean particle diameter below 100 microns. These changes will also minimize segregation that can occur because of particle velocity differences between components. If each component has nearly uniform particle size and density, reducing its mean particle diameter will also minimize its fluidization segregation tendencies. However, controlling the particle size of your mixture components comes at a substantial cost because it typically requires further size reduction or a screening step. Particle size changes may also unacceptably change your final product s performance, such as its suitability for a required chemical reaction. If your handling system includes a bin with multiple outlets, the outlets and their hopper sections should be located symmetrically. Changing the process. You can change your process in any of several ways to minimize a highly segregating mixture s tendency to segregate during handling. If your mixture consists of several components and each is more or less uniform in itself but varies distinctly from the other components, it s always a good idea to handle each component separately up to the final handling step and then proportion and mix the components just before this step. You should also avoid using a free-fall chute to transfer a mixture that segregates easily unless your handling system includes a mixing device downstream from the chute. When pneumatically conveying a fine fluidizable mixture into a bin, you can prevent segregation by using a tangential entry into the bin sidewall rather than a 90-degree entry at the sidewall or top. With a 90-degree entry, the coarse
particles flow through the conveying line directly into the material bed s center and have more momentum than the fine particles, which can be entrained by air currents inside the bin. The tangential entry, which enters the bin sidewall at a much shallower angle, keeps the coarse and fine particles in the incoming stream flowing at the same velocity, as in a cyclone, preventing vertical segregation. Another method for preventing bin-entry segregation in a pneumatically filled bin is to carry the conveying line into the bin s center and then direct the line upward so that the material stream flows up toward a deflector plate. When the material stream hits the deflector plate, the stream s velocity drops, reducing fluidization. Changing the equipment design. Some bin designs can facilitate segregation. A funnel-flow bin, which has a relatively shallow hopper section and no material flow along the hopper walls, has a first-in last-out flow sequence. A mass-flow bin has a taller hopper section with a steeper wall angle to cause flow along the walls and a first-in firstout flow sequence. When materials segregate from side-toside during filling that is, when fine particles collect at the center while coarse particles flow to the sides the funnel-flow bin tends to make segregation worse when the bin discharges. But the mass-flow bin tends to minimize such segregation at discharge. In fact, increasing the cylinder section s height-to-diameter ratio above 1-to-1 in a mass-flow bin usually produces a uniform velocity pattern across the material s top surface in the bin, further reducing the material s tendency to segregate compared with using a mass-flow bin with a shorter cylinder section or none at all. If your handling system includes a bin with multiple outlets, the outlets and their hopper sections should be located symmetrically that is, at the same distance and angle from the bin s fill point. This will avoid segregating the fines, which can concentrate in one hopper section and plug that outlet or create downstream quality control problems. Examples of improper and proper designs for center-filled bins with multiple outlets are shown in Figure 6. In Figure 6a, six of the seven outlets are symmetrical in relation to the center fill point, but one outlet is right under the fill point, where it will see mainly fines. In Figure 6b, all four outlets are located symmetrically from the bin s fill point, so that each outlet will receive a similar mix of fine and coarse particles. An alternative to using a traditional mass-flow bin is to use a mass-flow insert inside an existing bin. 6, 7 The insert consists of a hopper within a hopper, and its bottom hopper controls the velocity pattern in the bin, making it possible to provide a completely uniform velocity profile that will minimize segregation. To solve a vertical segregation problem, where the mixture segregates in layers of different components (as shown in the test equipment in Figure 4) and discharges one layer at a time, another option is to change the existing bin s hopper geometry so that the center section of material moves faster than the outside section. This facilitates in-bin blending, eliminating the effect of vertical segregation at the bin discharge. Figure 6 Improper and proper multiple-outlet designs for center-filled bins (plan views) a. Nonsymmetrical outlets (segregated fines flow through center outlet) b. Symmetrical outlets (mixture flows through all outlets without segregating)
Figure 7 Distributor at bin inlet 3. ASTM Standard D6940-03 Standard Practice for Measuring Sifting Segregation Tendencies of Bulk Solids, ASTM International (www.astm.org). 4. ASTM Standard D6941-03 Standard Practice for Measuring Fluidization Segregation Tendencies of Powders, ASTM International (www.astm.org). 5. Sampling is typically done at different locations in the blender and at several intervals during the mixing cycle; for more information about sampling and troubleshooting, see J.K. Prescott and T.P. Garcia, A solid dosage and blend content uniformity troubleshooting diagram, Pharmaceutical Technology, March 25, 2001, pages 68-88. 6. H. Purutyan, B.H. Pittenger, and J.W. Carson, Solve solids handling problems by retrofitting, Chemical Engineering Progress, Vol. 94, No. 4, April 1998, pages 27-38. 7. BINSERT hopper-in-hopper insert, Jenike & Johanson, Inc., Tyngsboro, Mass. (www.jenike.com). You can also use a distributor at the bin inlet to reduce sifting segregation that creates a fines concentration in the bin center. This method is suitable for a bin with a relatively small diameter. As shown in Figure 7, the distributor produces multiple fill points and creates multiple piles inside the bin. Because the segregation that occurs during pile formation happens at a smaller scale, the result is less severe segregation at the bin discharge. Which of these material, process, and equipment solutions is best for your application depends on your plant s budget, time constraints, space limitations, and other factors. Work with your equipment supplier or bulk solids handling consultant for help choosing the right approach for your mixture and handling system. PBE Endnotes 1. J.C. Williams, The segregation of particulate materials: A review, Powder Technology, Vol. 15, 1976, pages 245-251. For further reading Find more information on mixture segregation and sampling in articles listed under Mixing, Solids flow, and Sampling in Powder and Bulk Engineering s comprehensive article index at www.powderbulk.com and in the December 2006 issue. Herman Purutyan is vice president of Jenike & Johanson, Inc., 400 Business Park Drive, Tyngsboro, MA 01879; 978-649-3300, fax 978-649-3399 (hpurutyan @jenike.com, www.jenike.com). He holds bachelor s and master s degrees in mechanical engineering from Worcester Polytechnic Institute, Worcester, Mass., and an MBA from Babson College, Wellesley, Mass. John W. Carson is president of the company and holds a PhD in mechanical engineering from Massachusetts Institute of Technology, Cambridge, Mass. 2. J.C. Williams and M.I. Kahn, The mixing and segregation of particulate solids of different particle size, Chemical Engineering (London), Vol. 19, 1973, page 269.