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Part I Ing. Dietmar Schulze Schwedes + Schulze Schiittguttechnik, Consultants

Measuring your dry bulk powder’s flow properties, which helps determine the powder’s flowability, is the first step in designing a silo or other handling equipment that won’t develop arching, segregation, or other flow problems. This two-part article com- pares several methods for measuring flowability. Part I, which appeared in April 1996, provides the background you need for evaluating various flowa- bility test methods against the industry-standard shear test developed by A.W. Jenike. Part I1 can help you choose a test method that is simpler to per- form than the Jenike shear test but still provides the

then measure the sample’s strength in a defined manner. You also can’t compare the measurements from different tests without considering the test methods’ differences.

A test method that provides a quantitative flowability measurement meets seven requirements. The test method:

1. Provides a consolidation procedure and a correspond- ing strength measurement.

2. Consolidates the powder sample until it reaches steady- state shear (preshear).

qualitative or quantitative flowability measure- , ments you need. 3. Applies the load (major principal consolidation stress)

during consolidation and measures strength in the same direction.

efore discussing flowability test methods, it’s helpful to understand how to evaluate each method’s measuring accuracy.

4. Provides areproducible load application for (a) consol- idating the powder sample and (b) measuring its B strength.

Test method requirements for measuring flowability If you don’t need a quantitative flowability statement about your powder, in principle you can use any test in which the powder is somehow consolidated and the strength is somehow measured. Just be aware that various factors - such as wall friction and anisotropy - can re- duce the measurement accuracy.

5. provides average stresses in the working plane with possibly the most uniform stress distribution for (a) consolidating the powder sample and (b) measuring its strength.

6. Possibly varies the consolidation stresses.

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However, if you do need a quantitative statement about a powder’s flowability, you must consolidate a sample and

7. Possibly measures the increase in the powder sample’s strength over time (time consolidation).

18 Powder and Bulk Engineering, June 1996

To compare the quantitative flowability measurements of different test methods, primarily consider requirements 1, 4a, 4b, and, if you need information on time consolidation, 7. But to ensure greater accuracy, consider the other re- quirements too. If you need many time-consolidated mea- surements, such as to design a silo that will store powder for up to 7 days at a time, use a test method that allows more than one powder sample to be placed under load.’.2

A review of flowability test methods Information in this section reviews flowability test meth- ods that are relatively easy to apply, rather than those used only in re~earch.~.~ (For an overview of all methods, see references 9 and 10.) The test methods are discussed in sequence in Table I according to how well they measure flowability, as defined in Part I of this article, and how many of the seven test method requirements they meet (from the fewest requirements on up). Also included are test methods that don’t measure flowability according to the Part I information but can help you evaluate flow properties by other means.

You may find it helpful to choose a test method from those listed in Table I. (The information is based on references 11 through 41 .) But be aware that the information isn’t pre- cise in every respect: Apparently poor flowability results don’t always mean the test method can’t be used for a spe-

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cial purpose. If in doubt, review what you’ll measure be- fore you buy or build the test equipment and determine whether the equipment can yield the information you 0 cn

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need, such as stress levels, powder condition, and storage time. It’s best to experiment with different test methods.

In the table, each test method (named in the first column) is illustrated in a simplified schematic (second column) for the most important mealsurement value. The indices named in the second column are:

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Vis powder consolidation for the measurement.

*Mis the measured value (such as m for mass, Ffor force, M for moment, a for angle, and pb for bulk density).

Nis a normal force that’s constantly applied on the pow- der sample during the test.

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2. After a short description of each test method’s measuring principles (third column) are some general comments (fourth column) regarding how “correct” the method’s measurements are, as defined in Part I, compared with the Jenike shear test. The comments are based on the test method’s fulfillment of each of the seven requirements, indicated in the table’s last columns. A dash in any col-

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umn indicates the requirement doesn’t apply to this test method.

Powder and Bulk Engineering, June 1996

For information in the “General comments” column, you’ll need to understand these terms:

Quantitative statement means these measurements have the same values for strength as those from the Jenike shear test.

Limited quantitative statement means the measured val- ues are somewhat different from those of the Jenike shear test and one of the seven requirements isn’t com- pletely fulfilled.

Qualitative statement means that at least two of the re- quirements aren’t fulfilled, so the measured values show a larger difference from those of the Jenike shear test and thus can’t be used for quantitative design of handling or storage equipment, such as a silo.

Based on the seven requirements, Table I indicates that the Jenike shear test is the most accurate for measuring a powder’s flowability. However, because correctly operat- ing the Jenike test equipment requires training and experi- ence, the ring shear and torsional shear tests are reasonable options for comparative flowability measure- ments. If properly applied, the ring shear test is more likely to match the results of the Jenike shear test; 37-39,41 re- sults from the torsional shear test may show a larger dif- ference from those of the Jenike shear t e ~ t . ~ ’ , ~

As the table shows, for best results, the measured values achieved by a test method shouldn’t be affected by small external influences such as the operator’s skill in filling or preparing the powder sample, the wall friction, or the test equipment’s dimensions. Above all, the test method should address all the issues regarding important limiting conditions, such as stress levels, time consolidation, and characterizing flowability (as defined in the Part I section, “Using a specific characteristic value for flowability”).

PBE

Acknowledgment The author and Powder and Bulk Engineering thank Hendrik Colijn of H. Colijn and Associates, Pisgah For- est, N.C., for translating the author’s original paper from German.

References Additional information on the references is available from the author.

1. A.W. Jenike, Storage and Flow of Solids, Bulletin 123, Engineering Experiment Station, University of Utah, Salt Lake City, 1970.

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2. The Institution of Chemical Engineers (publisher), Standard Shear Testing Technique for Particulate Solids Using the Jenike Shear Cell, d

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1989.

3. J. Harder, Ermittlung der FlieJeigenschaften kohasiver Schiittgiiter

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mit einer Zweiaxialbox, Diss. Technical University of Braunschweig, Germany, 1985.

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4. A.H. Genitsen, The Mechanics of Cohesive Powders, Diss. Rijksuniv of Groningen, Netherlands, 1982. ca

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5. J. Schwedes, Scherverhalten leicht verdichteter, kohasiver Schiittgiitel; Diss. University of Karlsruhe, Germany, 1971. Q

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6. G. Haaker and W.J.A. Wiersma-van Schendel, Bulk Solids Handling, Vol. 13,1993, No. 1, pages 129-133.

7. J.R.F. Arthur, T. Dunstan, and G.G. Enstad, International Journal of rn E.

Bulk Solids Storage in Silos, Vol. 1, 1985, No. 2, pages 7-10. 3

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8. M. Nowak and J. Schwedes, Proc. Reliable Flow of Particulate Solids ZZ, August 1993, Oslo, Norway, EFChE F’ubl. Ser. No. 96,

2. pages 285-305.

9. J. Schwedes and D. Schulze, Powder Technology, Vol. 61, 1990, pages 59-68.

10. J. Schwedes, Preprints PARTEC, Nuremberg, Germany, 1979, pages 278-299.

11. Degussa AG, “AEROSIL zur Verbesserung des Flieherhaltens pulverformiger Substanzen,” Schriftenreihe Pigmente, No. 31, 1981.

12. German code DIN 53916, “Bestimmung der Rieselfahigkeit von Pulvern und Granulaten,” Verfahren nach Pfrengle (Priifung von Tensiden).

13. W. Imse, Zement Kalk Gips, Vol. 25,1972, No. 3, pages 147-149.

14. D. Mayerhauser, D., Die Chemische Produktion, Special issue, October 1989, pages 24-3 1.

15. Hosokawa Micron Powder Systems, “Powder characteristics tester” operating instructions.

16. R.R. Kammler, Aufbereitungs-Technik, 1985, No. 3, pages 136-141.

17. Brabender OHG, Duisburg, “Flowability test,” specification sheet No. 2124.1982.

18. P.C. Knight and S.H. Johnson, Powder Technology, Vol. 54, 1988, pages 279-283.

19. L.P. Maltby and G.G. Enstad, Bulk Solids Handling, Vol. 13, 1993, No. 1, pages 135-139.

20. J. Kozler and J. Novosad, Bulk Solids Handling, Vol. 9, 1989, No. 1, pages 43-48.

21. J.C. Williams, A.H. Birks, and D. Bhattacharya, Powder Technology, Vol. 4, 1970-1971, pages 328-337.

22. R. Beckhaus, W. Felgner, and J. Runge, Chem. Ing. Tech., Vol. 64, 1992, No. 3, pages 292-293.

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23. D.E. Keller, Experimental student project, Institute of Mechanical Process Engineering, Technical University of Braunschweig, Germany, 1988 (not published).

Powder and Bulk Engineering, June 1996

24. I.A.S.Z. Peschl, “Bulk Handling” seminar, University of Pittsburgh, December 1975.

25. I.A.S.Z. Peschl and H. Colijn, Journal of Powder & Bulk Solids Technology, Vol. 1, 1977, No. 3, pages 55-60.

26. Sankyo Dengyo Co. Ltd., Tokyo, “Powder bed tester,” catalog no. 1 110 18903.

27. T. Oshima and M. Hirota, KONA, Vol. 3, 1985, pages 63-68.

28. M. Hirota and T. Oshima, Powder Technology, Vol. 53, 1987, pages 49-54.

29. J.R. Johanson, Bulk Solids Handling, Vol. 12, 1992, pages 237-240.

30. J.R. Johanson, Proceedings of Reliable Flow of Particulate Solids II, August 1993, Oslo, Norway, E F C E Publ. Ser. No. 96, pages 11-32.

31.T.A. Bell, B.J. Ennis, R.J. Grygo, W.J.F. Scholten, and M.M. Schenkel, BulkSolids Handling, Vol. 14, 1994, No. 1, pages 117-125.

32. D.A. Ploof and J.W. Carson, Bulk Solids Handling, Vol. 14, 1994, No. 1, pages 127-132.

33. J. Schwedes, FlieJverhalten von Schiittgiitern in Bunkern, Verlag Chemie, Weinheim, 1968.

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34. J. Schwedes, Chem. Ing. Tech., Vol. 48,1976, NO. 4, pages 294-300.

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35. P. Martens (publisher), Silohandbuch, Verlag Ernst & Sohn, Berlin, 1988.

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cn 37. D. Schulze, Aujbereitungs-Technik, Vol. -35, 1994, No. 10, pages s

524-535. 3 ca 38. D. Schulze, Proceedings of the First International Particle 3

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Germany, pages 45-56. Q m

Technology Forum, August 1994, Denver, pages 11-16.

39. D. Schulze, Preprints PARTEC: Third European Symposium on Storage and Flow of Particulate Solids, March 1995, Nuremberg,

40. G. Miinz, Entwicklung eines Ringschergerates zur Messung der FlieJeigenschufen von Schiittgiitem und Bestimmung des Einflusses der TeilchengroJ’enverfeilung auf die FlieJeigenschafen kohasiver

7 i rn

Kalksteinpulvel; Diss. University of Karlsruhe, Germany, 1976. E.

3, pages 397-400. 3

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41. P.C. Arnold and A.R. Reed, Bulk Solids Handling, Vol. 7, 1987, No. 2. ra

Ins. Dietmur Schulze is a partner in Schwedes + Schulze Schuttguttechnik, Consultants, Rebenring 33, 0-381 06 Braunschweig, Germany: fax 011 -49-531 -380-4462. This paper is adaptedfrom a paper the author published in Chem. Ing. Tech., Vol. 67,1995, No. 1.