Page 229 - Dust Explosions in the Process Industries

Generation of Explosible Dust Clouds 201

no realistic dispersion process can be 100% efficient. This can be accounted for by
incorporatingan efficiency factor, K:

The particle size distribution of the powder has a great influence on W,, at a given
powder bulk density. It also is well known that powders consistingof smallparticles are com-
pressible. The reason is that the various interparticle forces other than gravity are stronger
than the gravity forces and therefore permit the formation of loosely packed particle
arrangementsthat would have collapsedhad gravity been the only force in operation. This
means that the number of interparticle bonds per unit mass of cohesive powder can be
increasedby compacting the powder, that is, by increasingthe bulk density of the powder
deposit. Therefore, W,, also increases with the degree of compaction. Moisture influ-
ences W,, by influencing the strength of certain types of interparticlebonds.
The logical link between W,, and the nature and number density of the interparticle
bonds in a powder has given rise to detailed studies of various types of interparticlebonds.
Attempts have further been made at predicting aggregated powder/mechanical strength
properties from microscopic interparticlestructure and forces. This kind of work is con-
cerned with the quantity D,,, (equation (3.1)).
However. the efficiency factor 0 <K< 1in equation (3.2) allowsDredto have any value
between 0 and D,,,, depending on the way in which the work W,,, is applied to the
powder to be dispersed. This, in turn, depends on the geometrical arrangement of the
powder and the form of the mechanical energy available for the dispersion process. If a
comparatively coarse noncohesive powder is charged into a silo from a hopper at the silo
top, for example, the potential energy of the powder, when transformed to kinetic energy
in the gravity field, may be sufficient to generate a well-dispersed explosibledust cloud in
the silo. The same applies if deposits of this powder falling from shelves and beams in
a factory workroom. However, very energetic airflows may be required to raise deposits
of such a powder on the factory floor into explosible suspensions.
When consideringthe other end of the scale, cohesive powders composed of very small
particles, interparticle forcesplay a major role and interparticle bonds may not be broken
unless the particle agglomerates are exposed to large shear forces.This means that com-
plete dispersion into primary particles is possible only in high-velocity flow fields or if
the particles are exposed to high-velocity impacts.
Consequently, the understanding of how explosible dust clouds can be generated
requires knowledgeboth of the nature of the powder (W,,) and of the actual dispersion
process (K). The dispersionprocess, in turn, depends very much on the actual industrial
situation, which is different in bucket elevators, pneumatic transport systems, fluidized
beds, various kinds of mills, driers, mixers, cyclones, filters, and silos. Therefore, inti-
mate knowledge of the nature of the industrial environment is required.
It has not been possible to trace any comprehensive theory of the generation of dust
clouds leading from the properties of the powder deposit via the nature of the energy avail-
able for dispersionto the structureof the dust cloud. However, in view of the wide vari-
ation in possible boundary conditions in industrialpractice, one would not expect to find
one single, unified theory covering all possible situations. On the contrary, each situa-
tion needs to be analyzed separately.Much work has been conducted on various limited