FLOWABILITY
With the term “good flowability” it usually is expressed that a powder or bulk solid is easily set to flow. Products are characterized as “non-flowing” if they build up stable arches or pipes when discharged from a silo (figure 1), or if they are caking at storage or transport. The reason for the latter is the compressive strength (unconfined yield strength) of a bulk solid.
Figure 2 shows a hollow cylinder with frictionless walls, filled with a fine-grained, cohesive bulk solid. First the bulk solid is consolidated by the consolidation stress rhop1. Subsequently the hollow cylinder is removed and the cylindrical bulk solid sample is exposed to an increasing compressive stress until the specimen breaks (flows). The stress acting at failure is called the unconfined yield strength rhopc.
The unconfined yield strength rhopc increases with increasing consolidation stress rhop1. Curve A shows a typical function of the unconfined yield strength rhopc in dependence on the consolidation stress rhop1 (figure 3). The flowability is the ratio ffc of the consolidation stress rhop1 to the unconfined yield strength rhopc:
ffc = rhop1/rhopc
ffc < 1 non-flowing
1 < ffc < 2 very cohesive (to non-flowing)
2 < ffc < 4 cohesive
4 < ffc < 10 easy flowing
10 < ffc free flowing
In figure 3 also the boundaries of the ranges due to the ranking listed above are plotted. The ratio ffc and with it also the flowability changes with the consolidation stress rhop1. Thus for comparative tests measurement devices are required which make possible the measu- rement of the above mentioned quantities at defined consolidation stresses (shear tester).
RING SHEAR TESTER
Typical shear testers are the Ring Shear Testers RST-01.0, RST-01.pc (figures 4 and 5) [1,4] and RST- XS, which are used in many areas of industry and research.
The bulk solid sample (figure 4) is contained in an annular shear cell and loaded from the top with a vertically acting force N (adjustment of the stress level) through the lid. At testing the shear cell rotates slowly in direction of arrow w, while the cover is prevented from rotation by two tie rods. Thereby the bulk solid specimen is sheared. The forces (F1+F2) acting in the tie rods are measured. From the results of a prescribed test procedure the flow properties of the bulk solid are calculated [1,2].
With the automatic Ring Shear Tester RST-01.pc (figure 5, right) the test procedure, including loading of the specimen and shearing, and also the evaluation are performed computer-controlled. For very small samples of fine powders as occurring e.g. in the pharmaceutical industry, the smaller Ring Shear Tester RST-XS.s is available, which is also operated automatically.
Advantages of the Ring Shear Testers are
Measurement of well defined flow properties, flowability, caking, bulk density …..
Different stress levels.
Simple operation.
Automatic testing & evaluation (RST-01.pc, RST-XS).
Thus a Ring Shear Tester is an advantageous alternative to so-called simple flowability testers, since it is easy to operate, whereby nevertheless clearly defined physical quantities are measured [2].
FLOW OF BULK SOLIDS IN SILOS
When a silo is discharged, two different flow patterns can prevail: mass flow and funnel flow (figure 6) [3]. At mass flow every particle within the silo begins to move when the outlet is opened. Mass flow is only possible, if the hopper walls are sufficiently steep and/or smooth. If the latter is not the case, funnel flow prevails. Thereby the material located in the so-called dead zones emerging near the silo wall is stationary.
The most frequent problems emerging at the storage of bulk solids in silos are:
Arching (figure 1).
Piping (figure 1).
Irregular flow and flooding.
Broad residence time distribution: The bulk solid in the dead zones of a funnel flow silo remains in the silo for extremely long time periods.
Segregation due to particle size, density or shape, which leads to an unsteady product composition at the outlet in funnel flow silos.
In a funnel flow silo all problems listed above can appear, while at mass flow only the problem of arching must be considered. Also practical experience shows, that funnel flow is the primary cause for flow problems.
For the design of a mass flow silo the necessary steepness of the hopper walls for mass flow and the minimal outlet dimension of the outlet opening required to avoid arching have to be determined. For this the flow properties of the bulk solid are to be measured (shear tester) [3]. With sufficient experience in the application of these quantities one can design new silos as well as redesign silos which are not working properly.
LITERATURE
[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] Schwedes, J.: Chem.-Ing.-Techn. 48 (1976) 4, pp. 294-300
[4] Schulze, D.: PARTEC 98,Preprints “1st Eur. Symp. Process Technology in Pharm. and Nutritional Science”, Nuremberg/Germany March 1998, pp. 157-166
