Chemical Engineering August 2022 - 28

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feed, can be tolerated. Solids
that are compressible, fibrous
or unstable under pressure are
also easier to process.
All in all, the feed conditions
for pusher centrifuges are more
demanding than for a screenscroll
centrifuge, and even
technical innovations, such as
the prefiltration stages [5], are
not always able to completely
eliminate this difference.
FIGURE 3. This cross sectional view shows the various
components of a screen-scroll centrifuge
well as on the properties of the liquid,
especially viscosity. Basically, the
smaller the particles and the higher
the viscosity, the higher the filtration
resistance. High filtration resistance
calls for slow pre-dewatering and,
therefore, limited machine capacity.
Second, the solids must be able
to form a solid cake. Solids that are
compressible, fibrous or unstable
under pressure often do not meet
the criteria, so they cannot be processed
at all or only in a special version
of the pusher centrifuge.
Screen-scroll centrifuge. Looking
at the screen-scroll centrifuge, the
situation in the feed area is completely
different. A filter cake is also
formed, but it slides in the direction
of the solids discharge right from the
start due to the conical shape of the
basket. The result is that no cake ring
in the proper meaning of the term is
formed. Usually the scroll in the feed
area initially acts as a brake to prevent
solids from sliding down uncontrollably.
This results in a significantly
lower cake height, and the conditions
for transporting the filter cake,
such as adding further suspension
and pre-dewatering the cake, are no
longer needed.
In practice, this means that screenscroll
centrifuges can run with higher
liquid quantities and higher viscosities,
and also that even strong fluctuations
in the centrifuge feed, up
to and including interruptions in the
28
Cakes, moisture & washing
The previous section showed
that the structure of the filter
cake in the two types of machines
is very different. But
what role does this different
structure play in the quality of
the solids at the discharge?
Compact, relatively high cake rings
are formed in the pusher centrifuge
that are pushed through the machine
by newly forming cake rings. In the
screen-scroll centrifuge, a compact,
uniform cake covering is only formed
in absolutely exceptional cases due
to the interaction of sliding and conveying.
Viewed in cross-section, its
filter cake has a roughly triangular
profile (Figure 3), while the one from
the pusher centrifuge has a roughly
rectangular profile (Figure 2). In addition,
the cake height is significantly
lower with the screen-scroll centrifuge
than with the pusher centrifuge,
as mentioned before.
Dewatering. It not only takes increased
centrifugal
acceleration
but,
above all, time to dewater the
solid material. In its simplified form,
this residence time is calculated
from the cake volume in the machine
divided by the solids volume
flow to the machine. Since the filter
cake in the screen-scroll centrifuge
has a much lower cake height than
in the pusher centrifuge, the overall
cake volume is smaller and thus
the residence time is much shorter.
While it is not uncommon for pusher
centrifuges to have a residence time
of 20-40 s, it is more common for
screen-scroll centrifuges to have a
residence time of 2-6 s. This effect
is weakened by the fact that a lower
cake height has a positive effect on
the dewatering behavior, because
the liquid in the upper layers of the
cake only has to travel a short distance
to be removed by centrifugal
force. However, it remains to be said
that, all other physical conditions
being equal, better dewatering can
be achieved on pusher centrifuges
than on the screen-scroll centrifuge.
To compensate for this effect,
screen-scroll centrifuges are usually
operated at higher speed than
pusher centrifuges.
Nevertheless, it should be noted
at this point that the remaining residence
time will of course no longer
improve the residual moisture,
if the minimum residual moisture
of a product is reached after only
six seconds at a certain centrifugal
acceleration, and thus in special
cases, the residual moisture that
can be achieved with the two technologies
may be the same. This is
often the case with very coarse particles,
which are easy to dewater.
Another special case in which the
above statement does not apply is
in applications involving sticky solids
or solids with liquid inclusions. The
scroll conveyor of the screen-scroll
centrifuge breaks up such structures
to a greater extent, which can
sometimes result in lower residual
moisture than expected in screenscroll
centrifuges.
Washing. The second common
quality criterion for solids discharge
is product purity. This can be influenced
by cake washing, because
the aim of cake washing is to wash
residues of mother liquor or other
impurities out of the solids as efficiently
as possible. To make this
possible, the filter cake must be
penetrated as completely and uniformly
as possible by a washing
liquid. Since liquid is known to follow
the path of least resistance, it
is forced to flow through the thinner
areas of the cake in the screenscroll
centrifuge, whereas there is
less flow through the thicker side of
the cake (Figure 3). Accordingly, the
cake is not washed very thoroughly
on the thicker side - if at all - because
there is very little or no contact
with the washing liquid.
Compared to the pusher centrifuge,
which has a mostly flat cake, the
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