Page 228 - Dust Explosions in the Process Industries

200 Dust Explosions in the Process Industries

Resuspension and redispersion of dust may either occur intentionally, such as by han-
dling and transport in various process equipment (powder mixers, bucket elevators,
pneumatic transport, etc.), or unintentionally, by bursting of sacks and bags that contain
powder, leaks of dust from process equipment, or sudden blasts of air generated by an
explosion that started elsewhere in the plant.
The characterizationof the “state” of a dust cloud is far more complicated than char-
acterizingthe “state” of a premixed quiescentgas mixture. For a quiescentgas, the ther-
modynamic state is completely defined by the chemical composition, the pressure, and
the temperature. For a dust cloud, however, the state of equilibrium is complete separa-
tion, with all the particles settled out at the bottom of the system.
In the context of dust explosions, the relevant state therefore always is dynamic. In
various industrial environments as well as in experiments with dust clouds, gravity and
other inertia forces act on the dust particles, giving rise to a complex dynamic picture.
In the ideal static dust cloud, all the particles are located in fixed positions, either ordered
or at random. The closest approximation to the ideal dust cloud that can be encountered
in practice is probably a cloud in which the particles are settling in quiescent gas under
the influence of gravity alone.

3.2
STRUCTURE OF THE PROBLEM

Formation of explosible dust clouds from powder deposits implies that particles origi-
nally in contact in the powder deposit must be separated and distributed in the air to give
concentrationswithin the explosible range. There are two aspects to consider. The first
is the spectrum of forces originally acting on and between the particles in the deposit,
resisting the separation of the particles. The second aspect is the forces and energy
required for the separation process under various conditions.
Eckhoff (1976) suggested that a global dispersibility parameter for a powder deposit
may be defined by considering these two aspects. A given mass of powder at equilib-
rium with the ambient atmosphere contains a finite number of interparticlebonds, each
of which requires a specificamount of work to be broken. The total minimum work W~n
needed to break all these bonds in one unit mass of powder could, in principle, be calcu-
lated by integratingthe work required for breaking all the individualinterparticlebonds.
The influence of gravity would depend on whether the particles have to be raised into
suspension or whether dispersion is downward. One could then define a theoretical
upper limit value of the dispersibilityfor that specific powder deposit by

When defined in this way, the “dispersibility”has the dimension mass per unit of energy
or work and is therefore a measure of the quantity of powder that can be completely dis-
persed by spending one unit of energy from external sources in the process. However,