Page 142 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 7 15

The oil may be sprayed onto the grain stream by conventional spraying equipment used
in agriculture. The drop size should be sufficiently small to ensure even distribution of
oil across the entire grain stream but not so small that the oil becomes airborne (aerosol).
This would indicate an optimal drop size diameter somewhere in the range 0.1-1 .O mm.
The oil may wet and penetrate into the surface of the grain. This counteracts formation
of new fine dust by rubbing and impact. The oil layer on the grain surface may also act
as an adhesive for fine dust particles. The oil further causes agglomeration of the fine
primary dust particles to larger effective particles.
The relative significance of the two mechanisms, grain surface wetting and adhesion
of dust and dust agglomeration, to some extent depends on the way in which the oil is
applied. If oil is sprayed while the grain rests on a belt conveyor, the grain wetting mech-
anism may play a main role. If, however, the oil is applied inside the inclined feed duct
to a bucket elevator bottom, where the high turbulence in the airflow causes most of the
dust to be in suspension, direct agglomeration of dust particles is the more likely main
result of adding the oil.
The latter configuration was used successfully by Johansen (1989). When handling var-
ious kinds of grain (wheat and barley, containing 700-800 g dust per tonne of grain), appli-
cation of only 100 g soybean oil per tonne of grain was sufficient to reduce the dust level
outside the process equipment substantially. The amount of dust, per tonne of grain, col-
lected in the dust filters for the process stream was nearly the same as with no oil added.
This was because of high airflow rates in the dust extraction system, which ensured col-
lection of practically all the dust. However, the content of fine, unagglomerated parti-
cles in the collected dust, was considerably reduced by adding oil, as shown in Table 1.10.

Table 3.10 Influence of treatment of wheat grain with soybean oil on the effective particle size,
ignitability, and explosibility of the grain dust resulting from handling the grain (1 15-230 g of oil per
tonne of grain)
I I I Minimum electric I Minimum ignition I I I
Weight % of particles spark ignition temperature 03 Kst Pmax
425 p I 43 pm I <32 pm I <10 p energy [mJ] dust cloud [“C] [bar\c.m/s] [bat(g)]
Withoutoil I 75 I 60 I 50 I 25 I 10-1 00 430 I 115 I 7
Withoil I 50 40 30 10 I 100-1000 I 430 80 7

The oil treatment method does not eliminate the dust explosion hazard. However, it
reduces the hazard significantly in two ways. First, the quantity of the airborne fine dust
that normally escapes from the process equipment and accumulates in workrooms, gal-
leries, and the llke is substantiallyreduced. Second, the clouds of agglomerated dust inside
the process equipment have lower ignition sensitivity and explosibility than the clouds
of unagglomerated dust that would be inside the equipment in the absence of oil treat-
ment. Some figures for dust collected in grain handling plants with and without oil treat-
ment are given in Table 1.10. The independence of the minimum ignition temperature
on oil treatment is in accordance with this parameter being rather insensitive to changes
in particle size for organic dusts.
According to Johansen (1989), the oil spraying dust control method, when applied to
a grain storage and handling plant, in fact reduced the running cost of the plant, in addi-
tion to reducing the dust explosion hazard.