Powder and Bulk Engineering, August 1988 13

Dr. Jerry R Johanson JR Johanson, Inc.

Particle segregation is a common problem in solids handling and storage systems. This article identifies and illustrates five of the most prevalent segregation mechanisms and discusses small-scale tests designed to quantify segregation potential. Practical sugges- tions for reducing segregation in various systems are provided.

e segregation renders solids mixtures out of specifi- n and creates inconsistencies among batches. When

not resolved, segregation can ruin the taste of powdered soft drink mixes, foul up gas flow in chemical reactors, cause dos- age variations in pharmaceuticals, and create significant weight variations in consumer packages.

Segregation mechanisms Five segregation mechanisms are prevalent in most solids han- dling systems: sifting, angle of repose, two types of air entrain- ment (fines fluidization and air currents), and trajectory fiom chutes (Table I). These mechanisms act alone or in various com- binations to produce solids segregation.

Sifting. Sifting segregation occurs whenever free-flowing fines are small enough to fit between larger particles.'sZThe severity of sifting depends on the amount of fines available in the mixture. The worst cases are mixtures that contain more coarse particles than fines. Figure 1 shows a typical sifting pattern in a hnnel flow hopper. The binary, free-flowing mixture is made up of 20 percent salt and 80 percent mung beans. At the charging point, the fine salt is

Table 1 Prevalent segregation mechanisms

Important solids Causing Name flow properties action Description

Sifting Size differences. Cohesion of fines.

Coarse interparticle motion from bin flow or deposit onto a pile vibrator.

Fines sift between large particles to fill the void space.

Angle of Solids with greater angles of repose repose among solids in mixture. stationary pile. form a steep pile under the deposit

point, allowing solids with lesser angles of repose to slide or roll to the head of the pile.

Differences in angles of repose

Influenced by particle surface roughness and cohesion.

Deposit onto or removal from a

Fluidization Permeability. of fines Cohesion.

Sufficient amount of fines to form a fluidized layer. Sufficient amount of inertia in large particles to penetrate fines.

Fines enter the bin with sufficient entrained gas to fluidize top surface. Larger particles penetrate layer, pushing fines to bin top.

Air currents Air quantity. Sufficient air flowing with solids to Very fine particles become entrained in air currents, which become weaker as they move outward from the impact point; this pushes fines toward deposit at bin's outer edges.

High friction coefficient solids slide slower down chutes than low friction coefficient solids and assume different discharge trajectories.

Entrained particle velocity. Cohesion.

carry individual particles.

Trajectory Differences in solids friction from chutes coefficients.

Sliding of solids mixture on a chute, creating a single layer of particles at the chute's discharge end.

14 Powder and Bulk Engineering, August 1988

Fig. 1. Sifting segregation

Mixture: 20 percent salt 80 percent mung beans

Fig. 2 Funnel flow hopper (one-third empty)

Mixture: 20 percent salt 80 percent mung beans

Fig. 3 Screening results (sifting mechanism)

Mixture: 20 percent salt 80 percent mung beans

caught in a matrix of coarse beans; this leaves the sides void of fines. When the hnnel flow hopper emptie~,~ the mixture flows only in the center region (Fig. 2); the salt exits first followed by the beans. When the hopper is emptied in six equal portions and the salt is screened from the beans, the f is t two samplings contain almost equal amounts of salt and beans (Fig. 3); however, by the last sam- pling the amount of salt decreases to almost nothing. The fity- fity salt-bean mixture in the center region ofthe hopper contains just enough salt to fi the voids between the beans, preventing a maximum amount of salt from sifting. Sifting only occurs to the extent that voids between particles al- low retention of fines. Figure 4 shows a hopper loaded with 50 percent salt and 50 percent mung beans. Segregation is still prev- alent. However, because a sufficient amount of salt exists to fd the voids between the beans, segregation is limited to the extreme edges of the hopper. When the hopper is emptied and the mixture screened, the first four samplings (70 percent of the output) contain roughly equal amounts of salt and beans (Fig. 5). The last two samplings contain more beans than salt. Thus, the conclusion is that segregation won’t occur when a few more fines are added to the mixture. This is true, but only if sifting is the sole source of segregation.

Angle of repose. Particles also segregate in mixtures containing materials with different angles of repose. Figure 6 shows a hopper f i e d with 80 percent salt and 20 percent mung beans. Obviously, there is enough salt to fill the voids between the beans. However, segregation is still evident due to the dlfference in each material‘s angle of repose. The fine, angular salt has a steeper angle of repose than the large, round mung beans (bottom of Fig. 6). Thus, the beans tend to roll and slide down the salt’s steeper angle ofrepose, causing the beans to accumulate at the hopper’s edge.

Fig. 4 Sifting segregation

Mixture: 50 percent salt 50 percent mung beans

When the funnel flow hopper is emptied and the mixture screened, the first four samplings (70 percent of the output) con- tain small amounts of beans (Fig. 7). This indicates that sifting did not occur in the central region. The bean accumulation in the last two samplings is caused by the angle of repose mechanism.

Finesfiidi@wn and air currents. Segregation also occurs when a fine/coarse mixture enters a bin. The f i e s readily fluidize and al- low the coarse material to pass through them. Air currents also play a role, carrying the fines away fiom the impact point. Figure 8 shows a binary mixture of 50 percent fine polypropylene powder and 50 percent mung beans dropped quickly onto a sim-

Fig. 5 Screening results (Sifting mechanism)

Mixture: 50 percent salt 50 percent mung beans

Fig. 6 Angle of repose segregation

Mixture: 80 percent salt 20 percent mung beans

Powder and Bulk Engineering, August 1988 15

Fig. 7 Screening results (angle of repose mechanism)

Mixture: 80 percent salt 20 percent mung beans

Fig. 8 Fluidization and air current segregation

Mixture: 50 percent polypropylene powder 50 percent mung beans

Fig. 9 Fluidization and air current segregation with subsequent layers

Mixture: 50 percent polypropylene powder 50 percent mung beans

16 Powder and Bulk Engineering, August 1988

ulated halfpile. (The solids are dropped at the left edge ofthe slice model and flow to the right edge.) The powder fluidizes, allowing the beans to penetrate and congregate on the bottom of the pile. This leaves a fluidized powder layer on the surface.

Note the accumulation of powder at the bin's right edge, away h m the impact point. The powder is carried here by air currents generated by the solids impact. Air currents reaching the vertical wall are forced to flow upward, leaving entrained solids behind by impinging them against the wall.

Figure 9 shows a second and third layer of the same mixture. The beans penetrate the powder so completely that there is no evi- dence of the initial powder surface layer (indicated by blue line). In addition, the powder layer near the impact point has grown with subsequent layers; there is no evidence that powder sepa- rates the bean layers. Also, the powder depth at the right edge has

Fig. 10 Fluidization and air current segregation in a hopper

Mixture: 50 per cent polypropylene powder 50 percent mung beans

Fig. 11 Fluidization and air current segregation

Mixture: 20 percent polypropylene powder 80 percent mung beans

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almost doubled with each new layer, a result of the cumulative ef- fect of the air current segregation mechanism. All excess powder is squeezed to the top and sides of the bin, only the powder neo essary to f i the voids between the beans is present in the lower part. Figure 10 shows this same mixture (50 percent powder, 50 per- cent mung beans) deposited into a hopper. Note the lack ofbeans in the upper part of the hopper. The flat repose angle of the mix- ture indicates that the powder at the top fluidized upon deposit; the center depression indicates that the beans penetrated the powder and pushed it aside. Figure 1 1 shows a mixture of 20 percent fine polypropylene pow- der and 80 percent mung beans. Although the fluidized top layer is now minimal, air currents still deposit a layer of fine powder at the bin's outer edge. (Fine particle concentration at the edge of a bin is typical, even ifthe material contains only a small amount of fme particles capable of becoming airborne.) Because there is in- sufficient powder to fii the voids between the beans, the sifting mechanism causes beans to accumulate in the outer portion of the pile. A subsequent layer of the same mixture (Fig. 12) shows a concen- tration of beans between layers, an increase in bean accumula- tion in the outer portion of the pile, and an increase of fines at the wall. In this case, segregation is caused by a combination of sift- ing, fluidization, and air current mechanisms.

Eajectotyfim &I&. Segregation is also a problem when mix- tures discharge from chutes because particles with different sur- face fiiction coefficients take different trajectories. As a rule, fine particles have higher fi-iction coefficients, and are usually depos- ited near the chute. This is true even in mixtures containing solids with similar chemical compositions. The trajectory mechanism also occurs with cohesive solids and is often responsible for side- to-side solids segregation in bins and on conveyor belts. Figure 13 shows two solids of different surface friction coeffi- cients being introduced into a bin via a sloping chute. The differ- ent surface friction coefficients create a short trajectory for the

Fig. 12 Fluidization and air current segregation with subsequent layers

Mixture: 20 percent polypropylene powder 80 percent mung beans

fme, more frictional salt and an extended trajectory (indicated by blue dashes) for the round, less frictional mung beans. The result is severe segregation in the pile.

Measuring segregation potential To determine the segregation potential of any solids mixture, you must simulate the full-scale factors affecting the segregation mechanism you are investigating. Sifting and angle of repose mechanisms have the best chance of being simulated in small- scale experiments; small-scale simulation of fluidization, air cur- rent, and trajectory mechanisms is harder.

Simuhtings@ingandangkof repose mechanhm. Figure 14 shows a typical small-scale tester used to measure the sifting segregation mechanism. The inclined plane of the tester is coated with salt (the test solid) to roughen the plane and prevent the salt fiom slid- ing except at its angle of repose. With the plane set at the salt’s an- gle of repose, a well-mixed sample of 20 percent salt and 80 per- cent mung beans is poured at the upper end. The mixture is allowed to slide on itselfto form a nearly d o r m layer of solids on the plane. The layer is then sampled in sections to quantify the segregation. Compare Fig. 14 with Fig. 15, which shows a tester containing a mixture of 80 percent salt and 20 percent mung beans. Segregation is evident in both cases. However, in Fig. 14 the cause is sifting; in Fig. 15, the cause is the difference in the ma- terials’ angles of repose. The small-scale test results of sifting and angle of repose mecha- nisms in free-flowing solids can be scaled to full-size piles. How- ever, the results are unrepresentative when quantifying these mechanisms in cohesive solids. With cohesive solids, a fine particle’s ability to stick to a coarse particle is affected by the number and severity of impacts the coarse particle has as it descends on the pile. Consequently, the hstribution of segregation is signdkantly affected by the size of the pile and the impact velocity of the solid onto the pile.

Fig. 13 Chute segregation from different trajectories

Mixture: salt and mung beans

Powder and Bulk Engineering, August 1988 17

Figure 16 shows the tester being used with a cohesive solid, pro- duced by adding oil to the 20 percent salt/80 percent bean mix- ture used in Fig. 14. Segregation is much less pronounced but still evident. However, this small-scale experiment is not representa- tive: ifthe cohesive solid was placed on a M-size pile, the segre- gation pattern would probably look similar to the one in Fig. 14 due to the large number of impacts individual beans would have as they moved down the extended length of the pile.

Measuringjhidiz&m, air current, and trajectov mechanhms. In general, fluidization, air current, or trajectory mechanisms are not easily simulated on a small scale. For instance, when investi- gating air currents, the small-scale setup must simulate the quan- tity and velocity of air entrained with the falling solids relative to the quantity and velocity of solids. Because gravitational accel- eration during falling can’t be scaled, the experimental fall height must duplicate the fidl-scale height. The air quantity introduced by any pneumatic conveying system also must be duplicated.

Fig. 14 Measuring segregation potential (sifting)

Mixture: 20 percent salt 80 percent mung beans

Fig. 15 Measuring segregation potential (angle of repose)

Mixture: 80 percent salt 20 percent mung beans

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Similarly, to quantify trajectory segregation you must duplicate the thickness of solids leaving the full-scale chute, otherwise the results are insigtllficant. This means you must control and scale the chute angles, the fiiction coefficients, and the rate of solids in- troduced to the chute to represent full-scale conditions.

Powder and Bulk Engineering, August 1988

Fig. 16 Measuring the segregation potential of cohesive material

Mixture: 20 percent salt 80 percent mung beans oil

Eliminating segregation in bins, conveyors, and batch processes

Sifting and angle of repose mechanisms produce a symmetric ra- dial pattern from the point of impact. These mechanisms are par- ticularly difficult to eliminate in free-flowing solids. However, they can be reduced by using bins designed to discharge in a fust- in/fist-out sequence, which ensures that the exit stream has the same proportion of material as the fill stream. To reduce the effects of sifting and angle of repose, use a mass flow hopper that has a tall, vertical bin section. As long as the ma- terial level is maintained above the point where the top surface is

Fig. 17 Cone-in-cone hopper configuration I

affected by the velocity gradient in the hopper (the place where a dimple occurs in a level surface2), a first-in/fiist-out sequence is closely approximated. If your process requires the bin to empty, the Johanson cone-inxone design4 (Fig. 17) produces uniform ve- locity almost to the end of the emptymg cycle. However, keep in mind that even if your bin maintains a first-in/ first-out sequence, the solids still discharge with the same radial segregation pattern introduced at the top of the bin. Fortunately, segregation occurs in the outlet’s small cross-section and, ifneo essary, can be easily mixed. For instance, ifthe bin uses a feeder, the material is mixed as the feeder draws solids from across the entire bin outlet. The air entrainment mechanisms generally produce topto-bot- tom segregation. The effects ofair entrainment worsen when sol- ids discharge from mass flow or uniform velocity bins. The air carried with or introduced into the solids is the segregation source; consequently, any modification that reduces entrained air reduces segregation. For instance, ifpneumatic conveying is used, entrained air can be reduced by installing a cyclone at the bin top, using a deflection plate that solids can hit, or tangentially entering the pneumatic line into the bin (using the bin as a cyclone). With belt conveyors and other en masse conveying systems, air is entrained as the solid stream f d s off the conveyor into the bin. To reduce this, decrease the stream’s free-fall height. Or, use an in- clined open chute or an external loading chute which remains 111 during loading to ensure that the solids contact a surface as the stream descends. When batch blending several different particle sizes, topto-bot- tom segregation is possible within each batch introduced to the surge bin. If the bin contains three or more batches, you can elim- inate segregation in the bin’s output by using the cone-inane design2 (Fig. 17) to convert the bin to a mass flow, in-bin blender. The velocity in the center should be about 1.3 times the velocity at the edge. This will remix about two batches and approximate a first-in/first-out flow sequence. Side-to-side segregation prob- lems will also be minimized.

Conclusion More than 80 percent of the segregation problems encountered in solids handling and storage systems are due to sifting angle of re- pose, fluidization of fines, air currents, and trajectory of chutes mechanisms. Sometimes several segregation mechanisms work at once. Generally, the segregation problems created by these mechanisms can be solved by modifying equipment or operating procedures, steps which usually don’t require large capital ex- penditure~.~ PBE

References 1. Johanson, J.R., “Particle Segregation and What to Do About It,” ChernicalEn-

gineering 182-188 (May 1978).

2. Johanson, J.R., “Segregation: Its Causes and Solutions” Video tape instruction: JR Johanson Presents Solids Flow Theory and Applications Presentation No. 7.

3. Johanson, J.R., “Know Your Material - How to Predict and Use the Properties of Bulk Solids,’’ Chemical Engineering 9-17 (October 30, 1978).

4. Johanson, J.R., “Controlling Flow Patterns in Bins by Use of an Insert,” Bulk Solids Handling 2(3) ( 1982).

5. Johanson, J.R., “Solids Segregation - Case Histories and Solutions,” BulkSol- ids Handling 7(2) (1 987).

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Dr. &wy R. hhanson is president of JR Johanson, Inc, 2975 Hawk Hill Lane, San Luis Obispo, CA 93401. El: (805) 544-3775. The firm specializes in solving solidsflow and processingproblems. I