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FlowPropertiesofPowders andBulkSolidsandSiloDesignforFlow

DietmarSchulze

The phrase "good flow behavior" usually means that apowder or bulk solid flows easily. Products are "poorly

flowing" if they experience flow obstructions (arches orpipes, figure 1), or consolidate during storage ortransport. The reason for the latter is the compressivestrength (unconfinedyieldstrength) ofa bulk solid.

Fig. 1:Arching(ontheleft), piping(on theright)

Fig. 2: Measurement ofunconfinedyieldstrength

ff = / (1)

Fig.3: Unconfined yield strength in dependence onconsolidation stress; lines of constantflowability ff

Additionally, in figure 3 the boundaries of the ranges ofthe classifications listed above are shown as straightlines. The ratio ff and thus the flowability of a specificbulk solid change with consolidation stress .Therefore, for flowability measurements testers arerequired which make possible the adjustment ofdefined consolidation stresses. This is fulfilled byappropriateshear testers.

Fig. 4: Ring ShearTester (shear cell, inprinciple)

Flowability

Ringshear tester

Figure2 shows a hollowcylinderwith frictionlesswalls,filled with a fine-grained, cohesive bulk solid. First thebulk solid is consolidated by the consolidation stress

. Subsequently the hollow cylinder is removed andthe cylindrical bulk solid specimen is loaded with anincreasing vertical compressive stress until thespecimen (f ls). The stress failure iscalled unconfined yieldstrength .

The unconfined yield strength typically increaseswith consolidation stress . CurveAshows the typicalincrease of unconfined yield strength in depend-ence on consolidation stress (figure 3). theratio ff of consolidation stress to unconfined yieldstrength is used to characterize flowability numeri-cally:

The larger ff , the better a bulk solid flows. Often thefollowingclassification is applied:

ff < 1 not flowing1 < ff < 2 very cohesive (tonon-flowing)2 < ff < 4 cohesive4< ff

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Schwedes + Schulze Schttguttechnik GmbHConsultants (silo design and powder testing)Am Wald e 3, D-38 30 2 Wolf en b tt elPhone +49 5331 8573-312, Fax -313E-Mail: mail@schwedes-and-schulze.comInternet: www.schwedes-and-schulze.com

Dr. Dietmar Schulze SchttgutmesstechnikR i ng s h e ar t e s te r s a n d s o f tw a reAm Forst 20 , D-38302 WolfenbttelPhone +49 5331 9-35490, Fax -78001E-Mai l : mai l@dietmar -schulze.comInternet : www.dietmar -schulze.com

The bulk solid specimen (figure 4) is contained in anannular shearcell ("ring") and loadedfrom thetopwitha vertically acting force N through the lid in order toadjust the stress level. During testing the shear cellrotates slowly in direction of arrow , while the lid isprevented from rotation by two tie rods. Thereby thebulk solid specimen is sheared. Forces (F +F ) actingin the tie rods are measured. From the results of aprescribed test procedure the flow properties of the

bulksolidare calculated [1,2,3].

1 2

With the automatic Ring Shear Tester RST-01.pc(figure 5, right) thetest procedure, including loading ofthe specimen and shearing, and also the evaluationare performed computer-controlled. For very smallsamples of fine powders as occurring, e.g., inpharmaceutical industry, the smaller automatic RingShear Tester RST-XS.s is available, which is alsooperatedcomputer-controlled (figure 5, left).

Fig.5: Computer-controlled RingShearTestersRST-01.pc andRST-XS.s

Advantages of Ring Shear Testers RST-01.pc andRST-XS.s are:

Measurement of well defined flow properties, e.g.,flowability, caking, bulk density .....Different stress levels.Easy operation.Automatic testing & evaluation.

Thus a Ring Shear Tester is an advantageousalternative to so-called simple flowability testers,

since it is easy to operate, whereby neverthelessclearly definedphysicalquantities aremeasured[2,3].

When a silo is discharged, two different flow patternscan prevail: mass flow and funnel flow (figure 6) [3,4].Incase ofmass flow thewholesilocontents, i.e., everyparticle, move during discharge. Mass flow is onlypossible if the hopper walls are sufficiently steepand/or smooth. If the latter is not the case, funnel flowprevails. In case of funnel flow, only a portion of thebulk solid in the silo moves downwards duringdischarge while the rest of the bulk solid remains

stationary thusformingstagnant zones.

The most frequent problems emergingwhen powdersorbulk solidsarestored ordischargedare[3,4]:

Arching (figure 1)Piping (figure 1)Irregular flow and floodingBroad residence time distribution: The bulk solid inthe stagnant zones of a funnel flow silo remains inthesilo forextremelylong time periods

Segregation due to particle size, density or shape,which leads to an unsteady product composition attheoutlet in funnel flow silos

Fig. 6: Mass flow (onthe left), funnel flow (onthe right)

In a funnel flow silo all problems listed above canappear, while in the case of mass flow only theproblem of arching must be considered.Also practicalexperience shows that funnel flow is a primary cause

of flowproblems.

For the design of a mass flow silo the slope of thehopper walls to ensure mass flow and the minimumoutlet dimension required to avoid stablearches haveto be determined. For this the flow properties of thebulk solid have to be measured with an appropriateshear tester [3]. With sufficient experience in theapplication of these quantities one can design newsilos as well as redesign existing silos or otherequipmentexhibiting flowproblems.

[1] Schulze, D.: Powder Handling & Processing

8 (1996) 3, pp. 221-226[2] Schulze, D.: Powder & Bulk Engineering 10

(1996) 4, pp. 45-61 and 10 (1996) 6, pp. 17-28[3] Schulze, D.: Powders and Bulk Solids -

Behavior, Characterization, Storage and Flow,Springer, Berlin Heidelberg New York (2008)

[4] Jenike, A.W.: Storage and Flow of Solids, Bull.123, Engng. Exp. Station, Univ. Utah, Salt LakeCity (1964)

Flowofbulksolids insilos

Literature

(c) D. Schulze 2014. All rights reserved.

Misprints and errors excepted.

RST-XS.s RST-01.pc