Page 518 - Dust Explosions in the Process Industries
P. 518
Assessment of lgnitability 485
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u1
w -
w -.-SETTLING VELOCITY APPARATUS
LL
LL
-4- SLOW DEPOSITION
-A- FAST DEPOSITION
I I , 1 1 1 1
1 10 100
DUST LAYER DENSITY [g/rn*I
Figure 7.1 0 Free stream air velocity required for 50% removal of maize starch as a function of mass
of dust m2 of layer, for three different dust deposition methods (From Ural, 7 989a).
The layer bulk density was highest with the fast deposition method and lowest when
the layer was produced using the settling velocity apparatus illustrated in Figure 7.7. As
Figure 7.10 shows, the average critical air velocity for liftoff decreased with decreasing
bulk density and.with increasing mass of dust per m2of dust layer.
Further development of the test procedures may make Ural’s tests an attractive can-
didate for a standard method both for acquiring fundamental data and for relative rank-
ing of dispersibility and dustability of dusts.
7.4.3
POWDER MECHANICAL PROPERTIES
The relationshipsbetween the mechanical bulk properties of a powder and the ease with
which it can be dispersed into a dust cloud are discussed in detail in Chapter 3. Cohesion
and tensile strength are two parameters often used for characterizing the cohesiveness
of bulk powders. Test apparatusesin use are shown in Figures 3.5 and 3.6. However, both
cohesion and tensile strength vary with the bulk density, or the degree of compaction,
of the powder, and therefore,just a single figure may not be useful unless the method of
preparing the powder sample is specified.
Other, relative powder mechanical test parameters that may be related to dispersibility
include compressibility,angle of repose, angle of fall, and angle of difference(Ural, 1989a).
7.4.4
MOISTURE CONTENT
As discussed in Chapter 3 and illustratedby Figure 3.3, moisture in a powder can increase
the powder cohesiveness considerably. Section 1.3.1 illustrates the strong influence of
the dust moisture content on both ignitability and explosibility of the dust.
