Page 259 - Dust Explosions in the Process Industries

Generation of Explosible Dust Clouds 23 I

Figure 3.24 Grade emission curves for a chalk
powder at various fluidization velocities (From
Schofield et ai., 7 9791.

The grade emission curve represents a useful empirical concept, which permits rela-
tive comparisons of the “dustability” of various powders and dusts. Schofield et al.
(1979) give results illustrating the effect on the grade emission curve of dust moisture
content and particle size distribution of the initial dust bed.
In the fluidized bed studies of cohesive powders by Geldart and Wong (19841, the
expansion of the entire powder bed due to airflow through the bed was used as an indi-
cation of the strength of the interparticle forces, or the cohesive strength of the powder.
The data were analyzed using the Richardson-Zakiequation:

-
V,,€ - V,.,=l €IZ (3.32)
where is the superficialgas velocity through the bed, and v,,,=~is the minimum super-
ficial velocity needed for dispersing the entirebed; E is the porosity of the bed and a direct
measure of the bed expansion; and n is an empirical constant. For laminar liquidholid
systems,n has been found to equal 4.65. For gadsolid systems, n is generally higher than
4.65 and therefore d4.65 > 1. Geldart and Wong (1984) correlated the ratio d4.65 with
the ratio between the tapped and loose bulk density of a range of cohesive powders and
found
4.16
n
4.65 (3.33)

Both the loose and the tapped densities are sensitive to the methods of sample prepara-
tion, and Geldart and Wong (1984) specify detailed experimental procedures for deter-
mining the two densities.