Chemical Engineering December 2010 - 34

Feature Report
Filtration Centrifuges:
An Overview
Peter Schmidt
Andritz KMPT GmbH
The first step
in selecting the
right centrifuge is
understanding what
types are available
T
o solve the task of solid-liquid
(S-L) separation in the chemical
process industries (CPI) a wide
variety of methods are invariably
used. Many of these methods are
not mutually exclusive and include a
selection of technologies that can be
expediently combined to provide an
efficient operating system.
A good understanding of the options
available and details of the individual
system modules can enhance
the efficiency of the overall concept.
For example, mechanical separation
using pusher centrifuges has proved
to be a very expedient intermediate
stage for the dewatering of sodium
bicarbonate downstream of the vacuum
filter and before calcination. By
reducing the residual moisture between
filter discharge and centrifuge
discharge, some 38% of the required
evaporation heat can be saved; and
just a fraction of the saved energy is
required to operate the centrifuge.
This helps to protect the environment
and also saves costs.
This article presents a basic overview
of different types of centrifuges
with descriptions of how they operate
and where they are applied in the CPI.
Some definitions
To begin with, a distinction is made
between thermal and mechanical separation
of solid products from liquids.
Whereas thermal S-L separators can
Sedimentation
Filtration
10,000 g
3,000 g
100 g
1 g
Separator
Centrifuge
Cyclone
Gravity
thickener
Chamber filter press
Belt filter press
Centrifuge
Drum filter
Disc filter
DCFF
Filtration thickener
Screen
Drum filter
Disc filter
Belt filter
Pan filter
0.2 bar
30.0 bar
15.0 bar
5.0 bar
1.0 bar
Figure 1. A classification scheme of the various types of equipment available for
solid-liquid separation is shown here
be grouped under the general term
" dryer " , it is not so easy to classify the
equipment used in the first stage of
mechanical S-L separation. For a finer
distinction it is necessary to consider
the flow directions of the solid and the
liquid phases. If these are both in the
same direction one talks of filtration;
in the case of opposing directions the
process is referred to as sedimentation
(Figure 1). There is also the special
case in which the flow directions of
the solid and liquid phases are at right
angles to each other, this is referred to
as cross flow filtration.
In most sedimentation processes,
the difference in density between the
solid and liquid phase is utilized, but it
is also possible to use electric or magnetic
fields for separating purposes.
To some extent, the natural sedimentation
in the earth's gravitational field
is used to this end, for example, by
gravity thickeners. This natural sedimentation
is enhanced by also super34
ChemiCal engineering www.Che.Com DeCember 2010
imposing a centrifugal field, as is done
by cyclones and centrifuges.
Filtration, on the other hand, uses
a filter medium that retains the solid
phase while allowing the liquid phase
to flow through.
Another way to distinguish filtration
methods is to consider the driving
potential that moves the liquid
phase, or filtrate, through the filter
medium. Separation can take place in
the simplest of ways: using a screen
in the earth's gravitational field or by
imposing a pressure gradient. Such
pressure gradients can be generated
by applying a vacuum to the filtrate
side. Here, however, natural limits are
quickly revealed.
Far greater potentials are to be
found when the pressure gradient is
applied to the solids side using overpressure.
This brings us to the focus
of the article, filtration centrifuges,
which are used when a cake-forming
filtration is required.
Driving force of separation = gravitation (g)
Vacuum Atmospheric Pressure
Driving force of separation = pressure (bar)
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Chemical Engineering December 2010

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