Chemical Engineering July 2016 - 56

Motor
Feed
Discharge
unit. Solids are driven to the wall,
while the heavy and light liquids form
concentric layers, with the heavy
liquid adjacent to the solids and
the light liquid closer to the axis
of rotation.
Each liquid spills over a weir at the
Heavy liquid
Light liquid
Rotating bowl
Solids
Feed
FIGURE 4. A tubular centrifuge is shown here
(Adapted with permission from Ref. 4)
new cake.
The subsequent forward stroke
pushes the cake the length of the
stroke, typically <5 cm. In this manner,
the cake moves along the length
of the basket, drying as it goes. Usually
the cake is washed somewhere
in the middle of its travel. Eventually
the cake reaches the end of the basket,
then falls to a vessel or conveyor
below. The reciprocation rate is typically
<100 cycles/min.
In addition to the single-stage unit
shown in Figure 3, there are also
multistage pusher centrifuges. These
contain multiple (typically 2-4) bowls
of increasing diameter placed in series
along the length of the housing.
The first and each alternate bowl
both rotate and reciprocate, while the
other bowls only rotate. The pusher
Feed
FIGURE 5. This multichamber bowl centrifuge
has six chambers (Adapted with permission from
Ref. 1)
pushes the cake from the first bowl,
then each successive bowl (except
the last one) pushes the cake that
forms on the bowl ahead of it. The
increasing bowl diameter with successive
stages provides increasing
G-force toward the discharge end,
as the cake becomes increasingly
dry and more difficult to dewater.
Other advantages over single-stage
units include less tendency of the
cake to buckle because each stage
is relatively short, and the ability to
implement countercurrent washing
for improved efficiency.
The pusher centrifuge is often an
excellent choice for concentrated
(>35 vol.% solids) slurries of large
(>150 μm), free-draining particles,
provided they form a cake with
mechanical strength sufficient to
withstand the pushing action. Thoroughly
washed cakes with low moisture
content are obtained with feed
rates as high as 100 ton/h [3].
Casing
Light liquid
Solids
Heavy liquid
Rotating bowl
FIGURE 6. Shown here is a solid bowl disk centrifuge
(Adapted with permission from [4])
56
Sedimenting centrifuges
Tubular centrifuge. This type of centrifuge,
shown in Figure 4, comprises
a long, narrow tube that rotates at
high speed within a cylindrical housing.
A typical unit uses a tube that
is 12 cm in diameter by 76 cm in
length, and rotates at speeds as high
as 15,000 rpm. Tubular centrifuges
can handle solid-liquid, liquid-liquid,
and liquid-liquid-solid applications.
The unit shown in Figure 4 is for
the most general case: two liquid
phases and a solid phase. Feed enters
the rotating tube through a stationary
nozzle at the bottom of the
top of the centrifuge and then exits.
At least one of the weirs is adjustable,
and there is an optimal position
that results in the best separation.
This is because the position of the
weirs determines the location of the
liquid-liquid interface - and in turn
the thickness of each liquid layer. The
preferred weir position depends on
whether removal of light liquid from
the heavy layer, or heavy liquid from
the light layer, is more difficult. For
the former, the weirs would be positioned
to increase the thickness of
the heavy layer, as this provides additional
residence time for the more
problematic separation. Conversely,
if removal of heavy liquid from the
light layer is more difficult, greater
thickness of the light liquid layer
is preferred.
Advantages of the tubular centrifuge
include high clarification ability,
good solids drying, high G-force,
and simple design and operation.
Disadvantages are the inability to
wash the solids, and the lack of a
mechanism for automatic solids
discharge. Regarding the latter, a
tubular centrifuge must be manually
disassembled and cleaned when the
solids holding space is full, meaning
solids removal is a batch operation.
Only feeds with low solids content
are suitable, to prevent the solidsholding
space from filling too quickly.
Tubular units work well for separation
of fine particles (0.1-200 μm)
from feeds containing no more than
0.5 vol.% solids, and for liquid-liquid
applications.
Multichamber bowl centrifuge.
This centrifuge contains a series of
concentric bowls, with baffles to direct
the flow. As shown in Figure 5,
feed enters the center of the unit at
the top, then flows outward. Because
the G-force increases with distance
from the center, successively finer
solids are deposited onto the bowl
walls as material moves toward the
periphery. Consequently, this type
of centrifuge provides classification
of solids into different size ranges -
one of the few designs that does so.
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Chemical Engineering July 2016

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