2. Dryer description
Fig. 1 shows a view of the industrial dryer. It is a
co-current rotary dryer of 3 m diameter, 30 m long.
The olive waste is mixed with dried product and introduced
by means of a continuous solid conveyor. A digger
makes the mixture and, therefore, there is not a good
control of solid inlet moisture content. This is one of the
main disturbances in the process. The solid advances impelled
by a lot of internal flights that also shower it
through the gas stream.
The drying gas pass through the dryer sucked in by a
fan. It has been previously heated up to 700–800 C in a
furnace fuelled with olive stone. The olive stone is fed by
means of two screws and burnt on a stationary grate and
two fans blow in through the grate primary air. The ash
is extracted manually to clean the grate and, when cleaning,
the drying gas temperature increases suddenly, representing
another important disturbance for the control
system.
The product leaves the dryer through a rotary valve
at the bottom of the collecting box and it is sent to a
second dryer by means of a belt conveyor. The exhaust
gas is dedusted by two cyclone separators and belched
out through the stack. The periodic cleaning of the collecting
box represents another main disturbance in the
process.
Orujo is a sticky solid difficult to handle through the
rotary dryer that besides requires a long residence time
to dry. The internal flights lift and shower the wet material
through the hot gas to allow a good contact between
them. However, these internals, among with orujo stickiness,
have an adverse effect on dryer operation because
they tend to produce plugs of solid, with cyclic cylinder
charges and discharges, that affects both the production
and the moisture content of the outlet solid. When a
plug occurs, it is necessary to reduce the feed of wet solid
to avoid overload, and when it breaks, a sharp discharge
takes place and the second dryer and the transport devices
can not treat all the solid flow. Moreover, if there
is a severe blockage, the overload of the cylinder may
stop the motor, and a high fire risk occurs because the