Page 231 - Dust Explosions in the Process Industries

Generation of Explosible Dust Clouds 203

3.3.2
ELECTROSTATIC FORCES

When considering electrostatic forces, one distinguishes between electrically conduct-
ing and nonconducting particles. In the case of conducting particles, electrostatic inter-
particulate attraction between touching particles may occur even if the particles did not
initially carry any net excess charge, provided their electron work functions are differ-
ent. Electrons then are transferred from one particle to the other. Different electron work
functions can occur in particle systems of apparently identical materials, due to differ-
ences in impurities, oxide layers, and the like. Provided the smallest distance a between
the twso surfaces is shorter than 100 nm, that is, the particles are in electric contact, the
electrostatic contact attraction force between the two conducting particles is

Here eois the permittivity of vacuum and the dielectric constant of the gas surround-
ing the particles; U is the contact potential between the two particle surfaces.
For electrically nonconducting particles, such as plastics, the electrostatic contact
force is negligible. In this case, electrostatic attraction between particles is caused by
excess charges on the particle surfaces, acquired triboelectrically during the preceding
production and handling. The attraction force between two nonconducting particles
having total excess opposite charges on the surfaces of q1 and q2equals

(3.7)

For a >> (xl +x2),equation (3.7) reduces to Coulomb’s equation for attraction between
two opposite point charges. If a is much smaller than the diameter of the largest parti-
cle, Fe,nessentially is independent of a.
Equations (3.3)-(3.7) are concerned with the attraction between two single particles
under idealized conditions. It is clear, therefore, that these equations are of limited value
for predicting interparticle attraction forces in real powders and dusts, where many par-
ticles interact and the particle shape and surface properties may be complex. In the case
of electrostatic forces, realistic assessment of the particle charges q1 and q2is also diffi-
cult, even for idealized particle geometries.
In industrial practice the relative humidity of the air has different values, and this
influences the strength of the electrostatic attraction forces between particles in powders.
This influence was investigated by Nguyen and Nieh (1989). They proposed a general
mechanism of charge elimination in flowing powders in humid air by “hydrated ion
clusters” (H,O),H+ and (H20),0H- and their polymers.
Ross (1988), working with clouds of lycopodium in air, was able to significantly
reduce electrostatic agglomeration of particles, as well as electrostatic adhesion to the
wall of an experimental flame tube, when the air was ionized by means of an alpha emit-
ter mounted on the flame tube wall.