1 Lappeenranta University of Technology
Laboratory of Separation Technology
BJ02A3030 Fluid Solid-Liquid Separation
Theory and practice of centrifugal filtration
March 15, 2015
Centrifugation plays a vital role as a separator in solid-liquid separation process
In general ,They are subdivided to main categories, Batch and Continuous
1. Introduction On the whole , There are two kind of centrifuges which are utilized for dividing solids
from liquids .The first group which is known as sedimenting centrifuges, relies on a
discrepancy in density of the two phases, solid-liquid or liquid-liquid. The second class
which is the main goal of this study is filtering centrifuges. Separation of solid-liquid
systems can be conducted by filter centrifuges. Furthermore, they are used for washing
and dewatering of the filter cake. One crucial thing to keep in mind is that the liquid phase
freely passes through the filter medium acting like barrier and the solid phase is kept in
such filters. Centrifugal acceleration is considered as driving force in this system which
actually is an alternative driving force to the traditional ones. Previously, the driving force
was developed by gravity or partial vacuum on the other side of the cloth while nowadays,
centrifugal filters introduced a new approach which is able to produce a high pressure
difference through the filter cloth. In fact this driving force is created because of
centrifugal forces operating on the fluid. A rotating basket provided with a filter medium
is the base part of such filters. A general schematic of centrifugal filters is presented in
the Figure 1.
Figure 1. Schematic diagram of centrifugal filters (Svarovsky, 1977)
The need for density differences between solids and suspended liquid is not necessarily
required, however, if such differences exists, it can have positive effect on better
classification of particle size distribution in the cake. It means that larger particles
sediment faster and perform as a per-coat for smaller particles on the surface of filter
medium. The most prominent key factor which distinguishes centrifugal filters from
others is that the liquid passing through the cake exposure to centrifugal forces tending to
transport it outwards the cake which significantly leads to more efficient dewatering
applications of this kind of filters .Moreover, the particle size spectrum which are filtered
by them is another trait causing them to be unique. For example, they encompass an
intensive range of applications from 10 microns to 10 millimeters (Coarse particle size)
as can be seen in the figure 2.The only sedimenting centrifuges which enter to the domain
application of filtering centrifuges is Decanter .This overlap exclusively occurs when the
precipitation of particles is not actually the regnant principle. (Svarovsky, 1977)
In general, centrifuges owing to their appropriate functionality and high expenses is
considered as one of the most expensive solid-liquid separation techniques. High- speed
rotating parts are the common feature of all centrifuges imposing extra expenses for
maintenance. In addition they require high quality of engineering standards and unique
infrastructure to wipe out vibration problems.
Figure 2. Shows the different application of various centrifuges according
to particle size (Svarovsky, 1977)
2. General principle of Centrifugal Filtration It is an established-fact that theoretically estimation of solid-liquid mixture behavior is
much more challenging in a filtering centrifugal compared to other filtration like gravity
filtration. Both area of flow and driving force are the function of radius and both of them
increase with respect of radial distance from the centrifuge axis.
2.1 Flow Rate
In this stage in order to discuss about the behavior of solid-liquid mixtures we refer to
Darcy equation for an incompressible filter cake as follows:
.. . ( . .
where c is a dimensionless centrifugal acceleration and can be determined by the
where r and , respectively, the radial distance from the axis and the angular velocity; and g is the gravitational constant. According to this equation, it can be noticed that C is
actually the measurement of comparative centrifugal force in relation to gravitation of the
earth (Force of gravity) which is normally employed to evaluate the effectiveness of
separation in various centrifuges with diverse bowl radius. In the equation (1) the filter
area can be defined as follows:
= 2 . . . (3)
the volumetric flow rate can be measured by an integration over the filter cake and
solving the equation (1) for
(differential pressure over radius) with equation (3) as
.2 . .
. ln (
where the L is centrifuge bowl length, the centrifuge bowl radius, the fluid pool radius and is the radial distance to the filter cake surface. The equation (4) can be simplified as follows when the thickness of filter cake is bantam,
=2 .. .
2 . (2
In this stage, another assumption in addition to first one which is applied in Darcy
equation (incompressible filter cake) is presumed, the acceleration and rotation speed of
fluid in centrifuge bowl is not change. (Friedmann, 1999)
2.2 The Constituent of Fluid in Filter Cakes
The total void volume is saturated with fluid over the cake filtration and washing process.
Dewatering process (desaturation process) is carried out in the final stage of the process.
The two most significant factors impacting on the final remaining moisture are Material
and the process parameter. To comprehend clearly the mechanism of desaturation,
different fluid compounds in the filter cake should be identified in the filter cake which
is illustrated in figure 3 (Batel, 1956)
5 Figure 3. Fluid source in a filter cake: A free moisture, B: Capillary moisture, C: pendular
moisture, D: inherent (bound) moisture (Batel, 1956)
The free fluid (A) can be split into bulk fluid flowing out fast in the large pores and surface
fluid which drains stilly. The particle size which is one of the most crucial parameter in
the filtration process influences on the amount of residual fluid on the surface.
The existence of capillary fluid in a fine ceaseless pores greatly relies on the size of pores
and the characteristics of the fluid. The symbol is commonly known as capillary rise which can be specified by the interfacial forces performing on air-liquid-solid when a
forces balance is established between interfacial forces and hydrostatics forces on the fluid column.
The pendular fluid (C) which is maintained by surface tension and capillary forces can be
eliminated by high centrifugal forces. The pendular fluid exists in the contact point of two
The inherent or bounded fluid contains fluid within the particles or fluid bounded to solid
matrix by intermolecular forces and cannot easily removed by typical mechanical solid-
liquid separation process. (Friedmann, 1999)
2.3 Washing and Dewatering
The presence of solution in the final solid product is an undesirable outcome of the
process. Most of the time after filtration, the cake consist of a solution which impact on
the quality of our product, therefore, a fresh solution should be utilized to decline the
solute by washing that from the cake or at least reduce the amount of that as much as
possible. Afterwards, the cake is dewatered in order to eliminate the remained solvent
between the pores and the solid which creates the cake.
The term dewatered is commonly applied whether the solvent is water or not. Regarding
to the fact that the radius of the cake is stable over the washing stage, the time that is taken
to wash the cake can be defined as follows,
2 2 (02 2)/2 [ ln(
where is the volume of wash water during the time of washing; 0 and are respectively , the radius of the centrifuge and the inner liquid radial position .both Specific
resistance and cake concentration can be determined with the usual cake filtration
experiments and experimental equations such as the following expressions;
= 0(1 )
= 0(1 )
where 0,0, m and n are empirical constants.
Dewatering before washing is of crucial importance as it can decline the amount of
solution which is required to be removed from the cake in addition to decreasing the any
inhomogeneity which might be happen within the cake. The prominence of this action is
presented in the figure 4 which indicate the impact of using the initial dewatering prior to
washing. (Holdich, 2002)
Figure 4. Comparing the performance of dewatered cake and flooded cake(Holdich, 2002)
The differences between concentration of solute in filter and the amount of solute in
solution is assessed to recognize the quality and performance of washing.
3. Classification and Application of Filter Centrifuges
In terms of mechanism of filter cake, the filter centrifuges can be divided into two major
categories; batch and continuous filter centrifuges.
Most of the time in the industries, the requirement of a specific machine which is able to
simply conform itself with various the product conditions is sensible due to the changes
which are applied usually by industries to change the product specifications. There are
significant aspects which effect on the process of solid-liquid separation which the
selection of centrifuge type is relied on. The separation stage is established on special
parameters (e.g. particle size distribution, compressibility) and on favorable outcomes
(residual moisture of filter cake). According to what has been mentioned a schematic of
classification based on these two categories are represented in figure 5. (Yang, 2003)
Figure 5. Classification of filter centrifugal filters. (Yang, 2003)
3.1. Batch-Type Filter Centrifuges
Although the filter centrifuge with batch operation has been used for many years before
the advent of continuous type filter centrifuges, they still used in several solid-liquid
separation. Furthermore, in terms of quantity they are much more than continuous types.
The two significant privileges of this type of centrifuges are high performance of them in
separation and high purity of products. In addition their flexibility in adjustment of feed,
wash and dewatering condition which give them a unique features to change themselves
in different process and product condition are the others advantages of these kind of filter
centrifuges. On the other hand, there are some problems with these kind of centrifuges;
in the first sight , the thing which is clearly crystal is that they cannot operate unceasingly
accordingly, it is needed to have upstream and downstream of centrifuge. Secondly, the
remaining bed of solid which is left on the basket wall which is glazed after several
operations and should be eliminated to avoid decrement of filtration rate. The two most
popular and common batch-type centrifuge is investigated in this study. (Hottel, 2008);
(Yang, 2003); (Thermopedia.com, 2015); (Friedmann, 1999)
3.1.1. Three Column or Basket Filter Centrifuge
The most basic and normal type of discontinuous filter centrifuges which can be
abundantly seen in industries. They are simple in design; a cylindrical basket which is
suspended on three columns and due to this features it is familiar as the three-column
centrifuge. In this centrifuge the discharge is conducted either manually (through a valve
in the bottom part of the screen bowl) or by peeling mechanism. The application and
advantage and drawback of the basket centrifuge is demonstrated in Table I. (Friedmann,
Table I. The pro/contra and applications of Basket centrifuges (Friedmann, 1999)
advantage disadvantage applications
low cost time consuming discharge
mechanism small scale operation
product change possible in
no speed for discharge frequently changing product
risk of uneven cake build
up shear sensitive products
products requiring long
The different parts of three-column was indicated in the figure 6.
Figure 6. Illustrates the various parts of basket filter centrifuge (Yang, 2003)
3.1.2. Three Column or Basket Filter Centrifuge
In this type of filter centrifuge to eliminate the negative impact of gravity on the cake
during washing process, the vertical axis which is used in basket column is transformed
to horizontal axis. In this type of centrifuge, for filter cake discharge, a peeler knife is
moved into the cake. Besides, for the large sizes, a horizontal screw convey out is used to
remove the discharged cake from the basket. The major problem would be emerge is that
the cake layer remain on filter cloth after discharge. A schematic of such filter centrifuge
is shown in figure 7.The mechanism employed in this centrifuge is so-called siphon
mechanism counting as an advantage of this centrifuge due to additional pressure which
can be provided by this mechanism in this centrifuge. In addition, the speed of discharge
is very high (full speed) and the cycle time is relatively short. They are utilized in
dewatering and washing of solids specially chemicals and pharmaceuticals. The other
discontinuous filter centrifuges are Beaker, Pendulum and Inverting.
Figure 7. Peeler filter centrifuge (Svarovsky, 1977)
3.2. Continuous Operating Filter Centrifuges
Pusher centrifuge and Conical screen centrifuge in this category is considered as the most
regular basis design.
3.2.1. Pusher Filter Centrifuge
The term of pusher has been chosen for them as the mechanism which is utilized to take
solids to the basket is pushing mechanism. They consist of a cylinder basket with its axis
horizontal figures 8.
In this centrifuge, suspension is fed in the back of rotating bowl. The reciprocating pusher
plate pushes the newly foamed cake towards the outer (lipless of the edge).The
applications, advantages and disadvantages of Pusher filter centrifuge is given in the
Table II. The pro/contra and applications of Pusher centrifuges
advantage disadvantage applications
full speed for discharge over flow of unfiltered suspension over Crystalline materials(>100m)
good washing result should be prevented e.g. aspirin, lactose
manual discharge of cake after machine stop various polymer
e.g. polystyrene, polyethylene
Figure 8. Pusher filter centrifuge in backward stroke (a) and forward stroke (b) (Svarovsky,
3.2.2. Conical screen centrifuge filter centrifuge
They contain a council basket which turns and rotated in both vertical and horizontal axis
based on the application and process figure 9. In this type of continuous filter centrifuge
the suspension is fed in the back of the conical screen bowl and forced transportation of
solids by difference rotation between helical/screw conveyor (worm) and screen. They
can applied for fine and coal; fiber recovery and dewatering of pulp material for example
potato; lactose and citric acid crystals. This type of centrifuges is inconvenient for
The advantage of these centrifuge can specified as follows: high solid throughput; they
can handle varying feed composition; longer residence time of cake on screen.
Figure 9. Council screen filter centrifuge
The other continuous filter centrifuges are Vibratory and Sliding discharge.
In-text: (Hottel, 2008)
Bibliography: Hottel, H. (2008). Perry's chemical engineers' handbook. [New York]:
In-text: (Svarovsky, 1977)
Bibliography: Svarovsky, L. (1977). Solid-liquid separation. London: Butterworths.
In-text: (Yang, 2003)
Bibliography: Yang, W. (2003). Handbook of fluidization and fluid-particle systems.
New York: Marcel Dekker.
In-text: (Holdich, 2002)
Bibliography: Holdich, R. (2002). Fundamentals of particle technology. Shepshed:
Midland Information Technology and Pub.
In-text: (Thermopedia.com, 2015)
Bibliography: Thermopedia.com, (2015). A-Z Index. [online] Available at:
http://www.thermopedia.com/content/620/ [Accessed 24 Feb. 2015].
Friedmann, Thomas E.. Flow of non-Newtonian fluids through compressible porous media in
centrifugal filtration processing. Laboratory of Food Process Engineering, Swiss Federal
Institute of Technology (ETH) Zrich (1999)
Batel, W. (1956), Aufnahmevermgen krniger Stoffe fr Flssigkeiten, im Hinblick auf verfahrenstechnische Prozesse. Chemie Ingenieur Technik, 28: 343349.