Flow Properties of Powders andBulk Solids and Silo Design for Flow

Dietmar Schulze

The phrase "good flow behavior" usually means that apowder or bulk solid flows easily. Products are "poorlyflowing" if they experience flow obstructions (arches orpipes, figure 1), or consolidate during storage ortransport. The reason for the latter is the compressivestrength (unconfined yield strength) of a bulk solid.

Fig. 1:Arching (on the left), piping (on the right)

Fig. 2: Measurement of unconfined yield strength

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 byappropriate shear testers.

Fig. 4: Ring Shear Tester (shear cell, in principle)

Flowability

Ring shear tester

Figure 2 shows a hollow cylinder with frictionless walls,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 . Curve A shows 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 thefollowing classification is applied:

ff < 1 not flowing1 < ff < 2 very cohesive (to non-flowing)2 < ff < 4 cohesive4 < ff < 10 easy-flowing10 < ff free-flowing

Typical shear testers are the Schulze Ring ShearTesters (figures 4 and 5) [1,3] which are widely used inindustry and research.

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Schwedes + Schulze Schüttguttechnik GmbHConsultants (silo design and powder testing)Am Walde 3, D-38302 WolfenbüttelPhone +49 5331 8573-312, Fax -313E-Mail: mail@schwedes-and-schulze.comInternet: www.schwedes-and-schulze.com

Dr. Dietmar Schulze SchüttgutmesstechnikR ing shear tes te rs and so f twareAm Forst 20, D-38302 WolfenbüttelPhone +49 5331 9-35490, Fax -78001E-Mai l : ma i l@die tmar -schu lze .comInternet : www.d ie tmar-schu lze.com

The bulk solid specimen (figure 4) is contained in anannular shear cell ("ring") and loaded from the top witha 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 thebulk solid are calculated [1,2,3].

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With the automatic Ring Shear Tester RST-01.pc(figure 5, right) the test 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 alsooperated computer-controlled (figure 5, left).

Fig. 5: Computer-controlled Ring Shear TestersRST-01.pc and RST-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 defined physical quantities are measured [2,3].

When a silo is discharged, two different flow patternscan prevail: mass flow and funnel flow (figure 6) [3,4].In case of mass flow the whole silo contents, 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 remainsstationary thus forming stagnant zones.

The most frequent problems emerging when powdersor bulk solids are stored or discharged are [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 inthe silo for extremely long time periodsSegregation due to particle size, density or shape,which leads to an unsteady product composition atthe outlet in funnel flow silos

Fig. 6: Mass flow (on the left), funnel flow (on the 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 causeof flow problems.

For the design of a mass flow silo the slope of thehopper walls to ensure mass flow and the minimumoutlet dimension required to avoid stable arches 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 otherequipment exhibiting flow problems.

[1] Schulze, D.: Powder Handling & Processing8 (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)

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Flow of bulk solids in silos

Literature

(c) D. Schulze 2014. All rights reserved.Misprints and errors excepted.

RST-XS.s RST-01.pc