Page 234 - Dust Explosions in the Process Industries

206 Dust Explosions in the Process Industries

"0 02 0,I 0,6 0,8 1
18158861 FRACTION S OF VOIDS BETWEEN PARTICLES FILLED WITH LIQUID

Figure 3.3 Tensile strength 0,of a powder bed as a function of the fractions of the voids between
the particles that are filled with liquid. Experiments are with limestone of 70 pm particle diameter;
E = 0.4 15; and -, -- - and --- are theoretical calculations using different assumptions (From
Schubert, 1973).

bed in the region just before complete saturation is three times the maximum tensile
strength in the liquid bridge region.
However, as pointed out by Enstad (1980), the tensile strength of the powder bed in the
capillary underpressureregime can never exceed a pressure difference of 1atmosphere. In
the liquidbridgeregime, there is no suchlimitation;and for smallparticle diameters<<70 pm,
equation (3.9)can easily give tensile strengthscorrespondingto pressure differencesof sev-
eral atmospheres.In this range of particle sizes,the shape of the curve of or(S)differs from
that in Figure 3.3 by having its maximum in the liquid bridge range of S < 0.25.
Adding liquids to dusts is sometimes used intentionallyin industry for reducting dust
dispersibility. One application of this method is addition of soy bean oil to grain to pre-
vent the generation of grain dust clouds in grain storage plants. See Section 1.4.10 in
Chapter 1.

3.4
RELATIONSHIP BETWEEN INTERPARTICLEATTRACTION
FORCES AND STRENGTH OF BULK POWDER

3.4.1
THEORIES

The question arises whether it would be possible to deduce some measure of the inter-
particle forces in powder deposits from measurement of bulk powder properties such as
shear strength and tensile strength. As already mentioned, Rumpf (1970) developed the