Page 230 - Dust Explosions in the Process Industries

202 Dust Explosions in the Process Industries

elements inherent in the total problem complex. Some of this is reviewed in the following
sectionsin sufficient detail for the genuine nature of the various problems to become vis-
ible. This is considered important in a text on dust explosions because, in the past, dust
explosionresearch was often conducted without paying appropriateattention to the cen-
tralrole played by powder mechanics and particle technology. Section 9.2.2 in Chapter 9
reviews some further works on dust cloud generation processes.

3.3
ATTRACTION FORCES BETWEEN PARTICLES
IN POWDER OR DUST DEPOSITS

Two categories of interparticle forces exist, one that operates even in dry powders and
one due to the presence of a viscous liquid. Useful summarieshave been given by Green
and Lane (1964), Corn (1966), Rumpf (1974), Schubert (1979), and Enstad (1980).

3.3.1
VAN DER WAALS FORCES

The van der Waals force F, between two spherical particles has been estimated theo-
retically by integrating London-van der Waals forces over all interacting pairs of mole-
cules. The resulting expression is

where A is a constant, a the smallest distance between the sphere surfaces, and x1and x2
the diameters of the two spheres.
Van der Waals forces between particles are of significance as long as x < 100 nm. If
x1>> x2,the force is determined by the size of only the smallest particle, and equation
(3.3) reduces to

A
F,=-x 2 2 (3.4)
a
Most particles in real life are not smooth spheres but of irregular shape and surface
topography. Schubert (1979) showed that F, between a plane surface and a point on an
irregularparticle of diameterx,having a smallelevation of radius r that touches the plane
surface. is

(3.5)

The distance, ao,is the smallest distance that can exist between two bodies in touch, and
it is estimated at 0.4 nm.