process for green pellets is carried out in two zones of the dedicated machine (Fig.1a).
In the first zone, the layer is blown with the heat transfer agent supplied from the
bottom to the top. Hot gases, which are fed through the grate, first heat the pellet bed
or the layer, and then acts on green pellets. When moving wet gases, moisture
condenses in the upper horizons of the layer, causing overmoisturazing. Consequently,
pellets lose their mechanical strength, deform, and worsen the permeability of the layer
and the quality of finished products.
In the second zone of drying, the heat transfer agent in the layer comes from
above, and the process of gases cooling and the moisture condensation occurs in the
lower layers of pellets, with the only difference that the material is already heated. This
prevents the secondary condensation of moisture on the pellets and on the grate which
eliminates pellets destruction because of overmoisturazing. The heat flow density from
gases to pellets is determined by the temperature and the heat transfer agent rate. In
order to intensify the production process of pellets, it is necessary to increase the
temperature and rate of the gases supplied to the layer.
When drying the pellets through the top down, the actions of reducing the
moisture content in pellets are capable of increasing the rate of heat supply to the pellet
layer along with increasing the rate of the heat transfer agent filtration and raising its
temperature. The possible gradual increase in gas temperature at the room over the
pellets layer occurs in agreement with the decrease in the moisture amount within the
pellets. However, at the beginning of the process it is possible that the unstoppable
overmoisturazing below the pellets to occur, which causes the pellets strength loss as
much as 30–40%, and this limits the gas filtration rate and the height of the layer 0.8–
1.2 m/s and 350–400 mm, respectively. Drying conducted from below brings
advantages over draining down-sweeping because it provides 14-15% greater gas
permeability for the layer in subsequent zones. Increasing the filtration rate can
significantly reduce the drying time. With the increase in the gas rate from 0.6 m/s to
1.5 m/s, the drying time significantly reduces. Further increase in the filtration rate up
to 2–2.5 m/s is not reasonable, as the drying time actually does not decrease more. The
drying rate increases with the growth of the temperature of the heat transfer agent: if
the filtration rate is of 0.8–1.0 m/s, the temperature increases from 200 °C to 450 °C
reducing the drying time. It can be reported that the pellets are ready for induration,
when the moisture in the upper third of the their layer thickness approaches the value
of the equilibrium moisture.
Considering the materials presented above it can be easily deduced that the
principle parameters of the drying process are determined as follows: the temperature
of the gas drained through the pellets and the gas flow rate during the process of pellets
drying.
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