Chemical Engineering July 2016 - 58

Type
Table 3. characTerisTics of various Types of cenTrifuGes (adapTed from ref. 1)
Solids
discharge
Basket, cylindrical
Conical basket
Pusher
Tubular
Multichamber bowl
Disk, solid bowl
Disk, solids ejecting
Disk, nozzle
Disk, nozzle-valve
Decanter, solid bowl
Decanter, screen bowl
Washing
possibility
Intermittent Washable
Continuous Washable
Intermittent Washable
Batch
Batch
Batch
Intermittent
Continuous
Intermittent
Continuous
None
None
None
None
Some
None
Some
Continuous Washable
Dutya
cr, sc, ls
cr, sc, ls
cr, sc, ls
cr, ls, ll, lls
cr, cl, ls
cr, ls, ll, lls
cr, ls, lls
cr, ls, lls
cr, ls
cr, sc, ls, ll
G-force
600-1,800
600-1,800
200-2,500
Feed
solids, %
v/v
>5
>5
>35
12,000-62,000 <0.5
5,000-9,000
5,000-8,000
5,000-7,000
5,000-8,500
2-20
14,000-15,000 <10
2,000-5,000
cr, sc, ls, lls 2,000-5,000
2-60
5-60
1-5%
<1.0
<10
Particle
size, μm
>5
>50
>150
0.1-200
0.5-200
0.5-200
0.5-200
2-5,000
Feed rate,
L/min
Up to 800c
25-1,500
Up to 1,700
8-120
0.5-5,000 1.5-350
0.25-200 0.5-1,500
4-1,500
40-4,000
4-600
4-1,800
20-5,000 4-1,800
Notes:
a. cr: clarification; cl: classification; sc: solids concentration; ls: liquid-solid; ll: liquid-liquid; lls: liquid-liquid-solid
b. At 1,000G
c. Average over cycle
moval from the solids-ejecting disk
centrifuge is automatic. The bowl of
this type of centrifuge - also called
an opening-bowl disk centrifuge -
is divided horizontally into two parts,
sealed with a gasket. The bottom
piece acts as a sliding piston that
periodically drops, opening the bowl
to expose slots through which solids
are ejected. Water (called operating
water) pressure is used to open and
close the bowl, which remains open
for only 0.13-0.3 s per cycle. The
time between openings is usually
one to several minutes, but may be
as long as several hours.
The opening frequency can be
controlled by a timer, set based on
experience, or by a turbidity sensor
placed in the supernatant pipe. For
the latter method, when the solids
holding space is full, fine solids begin
to find their way into the supernatant,
increasing its turbidity. When a
preset value is reached, the operating
water valve automatically opens,
causing the bowl to open and solids
to eject. Ear protection is required,
because the opening and closing of
the bowl is quite loud.
Solids-ejecting machines handle
slurry solids as high as 10 vol.%,
although solids that tend to compact
may cause difficulty. The solids,
which cannot be washed, leave the
centrifuge as a paste, less dry than
the solids obtained with the solidbowl
units.
And, unlike the solid bowl version,
solids-ejecting centrifuges are
58
designed with clean-in-place (CIP)
capability, meaning they can be
cleaned without disassembly. This
feature makes this type of centrifuge
popular in food and pharmaceutical
applications, including clarification
of beverages (for instance, fruit
and vegetable juices, beer, wine)
and biomass removal from fermentation
broths.
Nozzle-disk centrifuge. With this
type of disk centrifuge, solids (in
the form of a concentrated slurry or
sludge) flow continuously through
nozzles that are situated around the
periphery of the bowl. Small nozzle
centrifuges contain as few as two
nozzles, while larger units have as
many as 24, with openings of 0.5 to >3
mm. To minimize erosion, the nozzles
are made from hard materials, such
as tungsten carbide or various ceramics.
Prefiltration of centrifuge
feed may be necessary to avoid
nozzle pluggage.
Like solids-ejecting centrifuges,
nozzle units can be cleaned in place,
again rendering them useful in food
and pharmaceutical plants. Nozzle
centrifuges are able to handle higher
solids levels (up to 20 vol.%) than
the solids-ejecting units. Moreover,
the nozzle units can process the
sticky, compacting solids that solidsejecting
machines find troublesome,
and some designs allow introduction
of wash liquids. On the other hand,
the solids stream is more dilute
than that obtained with the solidsejecting
centrifuge as it must be to
flow through the nozzles. In addition,
the nozzle unit requires about
twice the power input of the solidsejecting
version.
Nozzle-valve disk centrifuge.
These centrifuges should not be confused
with the nozzle-disk centrifuge
discussed above). These units are
similar to the solids-ejecting unit, except
that solids exit through elastomeric
valves rather than slots. Each
valve consists of an elastomeric disk
attached to the slide ring that seals
the bowl. A typical machine contains
12 valves. Operating water pressure
forces the slide ring downward,
opening the valves and discharging
the solids. With opening times of
only 0.07-0.10 s, these centrifuges
are faster than the solids-ejecting
units. The advantage is that less liquid
is lost with the discharged solids.
G-forces are as high as 15,000 -
double those achievable by the solids-ejecting
centrifuge, allowing the
nozzle-valve version to compete with
filters for applications that require high
clarity or fine-particle separations.
The main limitation is the inability to
handle solids that compact, a result
of the high G-forces at the periphery
of the bowl. Like the solids-ejecting
and nozzle-disk centrifuges, the
nozzle-valve units can be cleaned
in place. Feed solids are limited to
10 vol.%.
A disk centrifuge process with
two-stage countercurrent washing
of solids is shown in Figure 7. Solids
from a bank of solids-ejecting cenChemiCal
engineering www.Chemengonline.Com july 2016
Settled solids
consistency
Granular solid
Granular solid
Granular solid
Firm cake
Firm cake
Firm cake
Thick paste, flowable
Concentrated
slurry
or sludge,
flowable
Thick paste, flowable
Flowable
paste to
granular solid
Granular solid
Spin test
time, minb
0-2
0-2
0-2
2-20
2-20
1-10
1-10
1-10
1-10
0-3
0-2

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Chemical Engineering July 2016

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