Page 254 - Dust Explosions in the Process Industries

226 Dust Explosions in the Process Industries

line, and the tangent of the angle between this line and the vertical force axis was defined
as the internal friction factor 9.When comparing this approach with the comprehensive
approach described in Section 3.4.2, it seems that a measure of the degree of consoli-
dation of the powder sample, either in terms of the porosity E, the bulk density, or the
major principal consolidation stress o,,was lacking in this early work of Gutterman and
Ranz (1959).
To determine z, from equation (3.26),C, x Re2was first calculatedfrom equation (3.25),
after that Re was found by trial and error from the universal C,(Re) graph (Figure 3.13).
Gutterman and Ranz also conducted wind tunnel experiments with different powder
types, and found reasonable agreement between the critical experimental z,for the onset
of particle movement on the powder surface and the theoretical values from equations
(3.25) and (3.26). Reasonable agreement was also found between the corresponding
theoretical and experimental critical gas velocities for initial particle movement. Initial
bulk movement (fluidization) of the powder surface was the result of a cascade process,
starting with a particle upstream being lifted into the airflow.When this particle impinged
on the bed surface, one or more new particles were ejected from the bed, and their return
to the bed surface ejected further particles, and so on.
Bagnold (1960) largely limited his studies to silica sand in the noncohesive range of
particle diameters >40 pm. He was fully aware of the strong influence of cohesion on
the range of smaller particles but found that the knowledge of the nature of interparticle
forces was insufficientto allow him to conduct any systematic studies. He nevertheless
carried out an entrainmentexperiment with a smooth layer of fine, uncompressed cement
in a wind tunnel. Even at a wind speed of 36 ds, measured 10 cm above the powder
layer, there was no continuing disruption of the powder surface. However, as also implied
by Bagnold, deposits of fine, cohesive powders can be easily disrupted if the character-
istic surface roughness is considerably larger than the particle size and the laminar
boundary layer. This is particularly so if the surface topography of the bed is character-
ized by sharp edges rather than rounded contours.
Figure 3.21 illustrates how agglomerates of fine cohesive particles can be entrained
by and carried along with the airflow as apparent single “particles.” As long as the
agglomerate is not exposed to shear or tensile stresses that exceed its cohesive strength,
it will not be broken down further.

PARTICLE
MAIN AIRFLOW AGGLOMERATE
DISLODGEMENT

Figure 3.21 Rough surface topography of a deposit of fine, cohesive particles.

In the case of powders having very wide particle size distributions, the entrainment
of the large particles can include mechanical disturbance of the fine ones and facilitate
their deagglomeration (breaking of cohesive interparticle bonds) and entrainment. This
process is called saltation.