Page 232 - Dust Explosions in the Process Industries

204 Dust Explosions in the Process Industries

3.3.3
INTERPARTICLE FORCES DUE TO LIQUIDS

It is a common experience from practice in industry that dry dusts are usually easier to
disperse than moist dusts (one exception can be heavily electrostatically charged dry plas-
tic powders). Even small quantities of adsorbed moisture can, in some cases, increase
the attraction forces between particles considerably. Adsorbed layers of up to 3 nm thick-
ness can adhere firmly to the particle surface and make it more smooth. This can appre-
ciably reduce the effective distance between two touching particles. Even for a spherical
particle as small as 1 pm diameter, the volume of a 3 nm layer of liquid water consti-
tutes only 2% of the particle volume. (The situation is different if the moisture is also
absorbed by the interior of the particle rather than just on its surface.)
The next stage is reached when the moisture content in the powder has become so
high that excess water starts to form liquid bridges between particles, as illustrated in
Figure 3.l(a). If the moisture content increases further, a transition range is reached that
is characterized by some of the space between particles being completely filled with water
(Figure 3.l(b)). Figure 3.l(c) illustrates the capillary range where the capillary under-
pressure is the main source of the cohesion. If the water content is increased beyond this
point, the system is transformed from a cohesive powder to a suspension of particles in
a liquid (Figure 3.1(d)).
To assess the strength of liquid bridges between particles in a powder (Figure 3.l(a)),
Schubert (1973) used the approximate relationship derived by Rumpf (1970) for the ten-
sile strength 0, of a bed of monosized spheres (see Section 3.4.1):
1--E F(E)
0T=--.-
E x2

Figure 3.1 Distribution of a liquid in a powder (From Schubert, 1973).