435847 Alternative Method for Bulk Solids Time Flow Function Estimation

Thursday, November 12, 2015: 12:51 PM
252A/B (Salt Palace Convention Center)
Rodrigo Condotta, Engenharia Química, Centro Universitário da FEI, São Bernardo do Campo, Brazil and Alain de Ryck, Centre RAPSODEE, Mines Albi, Albi, France

There are several methods and devices to estimate bulk solid flow function, which is an indicative of powder flowability. They are described in a large number of articles [1-5]. Each of them prescribe strict test procedures in order to be reliable [6, 7]. 

All these studies are based on the pioneering work developed by Jenike on granular shear testing in order to obtain the physical relevant parameters to properly design equipment, including industrial silos. Since the properties of powders involve with time storage, [1, 2], Jenike has also developed a methodology to measure these temporal evolutions.

The dependence of powder shear strength with time storage was object of some studies [8, 9] and a logarithmic ageing was embedded in the phenomenological state-and-rate model formulated by Dieterich, Rice and Ruina to describe rock-rock friction: the static coefficient of friction increases logarithmically with the contact time between the two solid bodies.

Regarding the design of silos, some authors admit that Jenike technique works very well, others argue that it overestimate the diameter of silo's outlet [10]. On the other hand, observation at industrial site showed that a silo dimensioned according to Jenike technique could not be unloaded when a bulk material remained stored for more than 24 hours. The same phenomenon was observed with a silo built on a small scale [11]. This is believed due to the physical characteristics of the affected bulk material, reducing its flowability [5].

To verify the changes in the properties of powders at different storage times, an organic granular powder was tested on a Schulze ring shear cell tester for the determination of the instantaneous and temporal flow functions. The results have not shown a significant difference between these two functions.

  However, this powder showed flow interruptions when stored for more than 8 hours on a small-scale silo. To investigate this discrepancy between the prediction of the standard procedure for temporal flow functions and experimental observations of time consolidation during storage, the standard protocol of temporal shear test was modified. While in the classical protocol the shear stress must be unloaded during powder rest, in the new methodology proposed, the shear stress is maintained during powder ageing. The modified temporal flow functions hence obtained display a more significant difference between instantaneous and temporal flow functions, as showed in Figure 1.

Figure 1. Instantaneous and time yield loci (experimental and extrapolated) for consolidation stress of 2,5 kPa.

Keeping the shear stress during the temporal ageing seems coherent with the real situation in silos stress conditions, but the weak point of this methodology is that the shear cell cannot be removed to the classical time-benches and, therefore, the shear cell remains unavailable during a large period, which is not good for industries and consulting labs.

To overcome that, we develop a new methodology to extract the temporal flow functions from a small series of temporal shear experiments. Once incipient shear stress is time dependent, which could be well described by logarithmic model, such as Dieterich, Rice and Ruina, this work links this predicted behaviour to the prorating technique normally used to minimize the scatters during shear experiments. Then, a couple of shear-holding experiments at relative short period of hold was used to obtain Dieterich, Rice and Ruina equation parameters. For each desired storage time, the time yield locus was extrapolated by prorating the steady-state shear flow of the instantaneous yield locus with the time incipient shear stress corrected by Dieterich, Rice and Ruina equation.

The extrapolated results agreed with the time yield locus obtained by the modified methodology (with shear stress maintained) proposed to estimate time flow functions. Differences between both methodology are not negligible, but are smaller than results scatter observed using the same classical method when carried out by different laboratories [4]. Both results are able to predict the flow disturbances observed after 8 hours of storage.

Thus, the present work has tested a new methodology to obtain time flow functions based originally on the classic methodology of Jenike. This alternative methodology allow to observe a more relevant change in the physical properties during storage time. Furthermore, a procedure to extrapolate the flow functions of large periods from shear experiments of short period using Dieterich, Rice and Ruina friction model showed similar results. This could be a useful tool for industries to obtained temporal flow functions of long aging period at a relative short time.

[1]     JENIKE A.W. Storage and Flow of Solids, Bulletin N° 123. Utah Engineering Experiment Station. 1964. [2]     JENIKE A.W. A Measure of Flowability for Powders and Other Bulk Solids. Powder Technology, v. 11. 1975.

[3]     SCHWEDES, J. Testers for Measuring Flow Properties of Particulate Solids. Powder Handling & Processing, v. 12, n° 4. 2000.

[4]     OSE, S.; de SILVA, S.R. Preliminary Results from an International Project on Comparative Characterization of Powders. 3rd Israeli Conference for Conveying and Handling of Particulate Solids, Israel, 2000.

[5]     FURLL, C.; HOFFMANN, T. The Influence of the granulometric Condition on the flow characteristics of shredded grain products in their dependence on the duration of storage. Powder Technology, v. 235. 2013.

[6]     Standard Shear Testing Technique for Particulate Solids Using Jenike Shear Cell. The Institution of Chemical Engineers, 1989. [7]     ASTM - American Society for Testing and Materials Committee. Standard Shear Testing Method for Bulk Solids Using the Jenike Shear Cell, Designation D 6128 – 97, 1998.

[8]     LUBERT, M.; de Ryck, A.; DOODS, J.A. Evaluation of the Mechanical Properties of Powder for Storage. Handbook of Conveying and Handling of Particulate Solids. 2001.

[9]     de RYCK, A.; CONDOTTA, R.; DODDS, J.A. Rheology of bulk solids: response to shear flow interruption. 4th European Congress of Chemical Engineering. Granada. 2004.

[10]  DESCHER, A.; WALTERS, A.J.; RHOADES, C.A. Arching in Hoppers: II- Arching Theories and Critical Outlet Size. Powder Technology, v. 85. 1995.

[11] CONDOTTA, R. Coulabilité des poudres cohésives: mesures aux faibles contraintes, granulaires humides et application a une poudre industrielle. INP Toulouse, France, 2005. (thesis)





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