(708b) Alternative Method for Bulk Solids Time Flow Function Estimation

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. 3^rd 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. 4^th 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)