Chemical Engineering December 2010 - 36

Feature Report
Figure 5. This photo shows the basket
of a baffle ring centrifuge
cally with pressure blows or hydraulically
by dissolving and breaking up
the cake through swirling action.
Siphon peeler. The siphon-peeler
centrifuge (Figure 4) is a design variant
of the peeler centrifuge that enhances
the performance. This design
uses a rotary siphon that enables the
residual heel to be backwashed from
the filtrate side, which frees the capillaries
of fine particles. In this way,
the residual heel can be used much
longer without necessitating its timeconsuming
removal.
A further advantage of the siphon
variant is that, besides the applied
filtration pressure generated by the
mass of the liquid, there is an additional
vacuum created behind the
filter medium (through the low level
in the siphon chamber) that increases
the filtration rate.
Because it is possible to extend the
length of the siphon-peeler pipe, feeding
into a liquid pool is also possible
with this type of machine.
Horizontal peeler centrifuges -
both standard and with siphon basket
- are very flexible in use. They can
be used for starch, herbicides or fine
chemicals, to name just a few application
examples.
Vertical-basket peeler. Another
variant of the peeler centrifuge is the
vertical-basket peeler centrifuge. The
operating principle is practically the
same as the horizontal peeler, except
that the product is not discharged
after peeling via a chute or conveyor
screw, but rather drops through the
openings in the bottom of the basket
during the peeling process. To do this,
the centrifuge is decelerated to enable
Figure 6. The basket of a tumbler centrifuge is mounted on a U-joint, which results
in a tumbling motion that enables continuous product discharge
a controlled product discharge. This,
however, extends the batch processing
time and consequently reduces
the throughput rate.
Nevertheless, this vertical variant
reduces capital costs due to its compact
design. A typical application of
vertical-basket peeler centrifuges is
for the dewatering of gypsum produced
in fluegas desulfurization units.
In general, the throughput of peeler
centrifuges is somewhat restricted in
comparison with the other types of filtration
centrifuges. This is where the
continuously operating centrifuges,
in which all process steps are carried
out simultaneously, enables a huge increase
in the throughput rate.
Continuous centrifuges
In continuous centrifuges a further
distinction is made with regard to the
conveying mechanism used for discharging
product.
Sliding discharge. The sliding discharge
centrifuge uses the simplest
possible conveying mechanism. Instead
of a cylindrical shell common to peeler
centrifuges, the horizontally arranged
basket has a conical shell that opens
out towards the discharge of the centrifuge.
The solids in the basket have a
product-specific friction angle that has
36 ChemiCal engineering www.Che.Com DeCember 2010
to be taken into account when selecting
the opening angle of the basket.
Through the slope resistance of the
centrifugal force and the pressure of
the following solids, the product slides
through the basket without requiring
any mechanical tool. To keep the friction
angle of the product and the basket
angle small, a fine-hole sheets or
slotted screens rather than a mesh,
are used in continuous centrifuges. The
slotted screens are made of profiled
wires between which the screen slots
point in the transport direction.
Baffle ring. A special variant of the
sliding discharge centrifuge is the
baffle ring centrifuge (Figure 5). Here
it is not the cake transport as a closed
product layer that is intended, but
rather a dewetting of individual particles.
To this end, steps are arranged
on the shell surface so that the particles
are in a free fall for a short period
of time. When the individual particles
hit the next step, the surface moisture
is spun off the particle. If, in the closed
cake layer, there is still interstitial
water between the particles that are
in direct contact, this impact effect can
attain cake residual-moisture levels of
less than 0.1 wt.%. This application
example, however, is almost solely restricted
to coarse synthetic granules.
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Chemical Engineering December 2010

Table of Contents for the Digital Edition of Chemical Engineering December 2010
