Page 60 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 33

BRIGHTENER

5 10 25 SO 100 250 500
MEDIAN PARTICLE SIZE BY MASS Iuml 1 1 1 1 1 1 1 1 1 1
0 50 100
Figure I .30 Minimum electric spark igni- MEDIAN PARTICLE SIZE BY MASS [vml
tion energy of clouds in the air of an optical
brightener, polyethylene and aluminum, as Figure 1.31 Influence of median particle
functions of median particle size (From size of mass on the minimum ignition energy
Bartknecht, 1987)and theoretical line for poly- of clouds of methyl cellulosein air. Experiments
ethylene (From Ka/kert and Schecker, 1979). performed at Chr.Michelsen Institute, Bergen.

Eckhoff and Mathisen (1977/1978) investigated the influence of the degree of dis-
persion of maize starch grains on the rate of pressure rise during explosions in a 1.2liter
Hartmann bomb (see Chapter 7). As shown in Figure 1.22, maize starch consists of
fairly monosized, close-to-spherical grains of typical diameters 10-15 pm. The degree
of dispersion of the individual starch grains in the Hartmann bomb was studied by
mounting a microscope slide with a double-sticky tape inside a specially made 1.5 liter
dummy vessel that fitted to the dust dispersion cup of the Hartmann bomb (see Figures
7.4 and 7.5). Microscopic analysis of the dust deposited on the tape revealed a consid-
erable fraction of stable agglomerates,which were probably formed during production
of the starch. It was found that various qualities of maize starch had different degrees of
ag,glomeration.This was reflected in differences in combustion rate, in agreement with
Figure 1.32. Figure 1.33 shows a scanning electron micrograph of typical stable maize
starch agglomeratesfound in a commercial maize flour purchased in Norway. Figure 1.34
shows the results of Hartmann bomb experiments with this flour, as purchased and after
removal of the agglomerates retained by a 37 pm sieve, and a maize starch purchased
in the United States, all of which passed a 37 pm sieve. Figure 1.34 shows a consistent
increase of (dP/df),,, as the effective particle size decreases.