Page 340 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 309

35 I I I I I I I I I
30

o Gilsonite z
0 Polyethylene-graphite P
mixlures
iq r
3
u
8
20
-3 +
3c1
-60 5
m -
-70 g
-I
-80 a
i5
- 100 5
-90
E
z
- I50 -
- 200
- 300
- 500
I 103
0 IO 20 30 40 50 60 70 80 90 100
VOLATILITY I%]
Figure 4.28 Inverse and minimum explosible dust concentration versus content of volatiles for var-
ious dusts (From Cashdollar,Hertzberg, and Zlochower, 1988).
80
70 -
D TOTAL MASS
kE 60 - nCOMBUSTlBLE MASS /*'*
-
j 50 -
:: 30 -- ..-----a m - . - . - . - .
z /: .
D u
-.I 40 -
- ,
-
I
S
" L L Figure 4.29 Minimum explosibledust concen-
_I a.
0
polyester and epoxy resins. (From Eckhofi,
0 20 30 LO 50 60 tration versus non-combustible mass fraction for
NONCOMBUSTIBLE MASS FRACTION l%1 Pedersen, and Arvidsson, 1988).
concentration is similar to that through a premixed gas; that is, flame propagation takes
place in the combustiblegas evolved from the particles in the preheating zonejust ahead
of the flame. In accordance with this model, p being the mass fraction of noncorn-
bustibles in percent, the minimum explosible dust concentration(MEC) of this category
of dusts is
MEC = 32 g/m3 [100/(100-p)] (4.67)
This relationship gives the curved line in Figure 4.29, and it is seen that the agreement
with the experimental points is reasonable.Approximate estimates of MEC for various
contents of noncombustible material can be obtained by this relationship. However,
undue extrapolationbeyond the experimentalpoints give physically meaninglessresults
because MEC +w whenp + 100.
Buksowicz and Wolanski (1983) studied flame propagation near the minimum explosi-
ble dust concentration,in a 5.5 liter vertical cylinder of 150 mm diameter. By choosing