1 Lappeenranta University of Technology

Laboratory of Separation Technology

BJ02A3030 Fluid Solid-Liquid Separation

Literature review

Theory and practice of centrifugal filtration

Student N.N

0445024

March 15, 2015

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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

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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:

=

.. . ( . .

) (1)

where c is a dimensionless centrifugal acceleration and can be determined by the

following equation:

= .2

(2)

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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

. (2

2).2

.2 . .

. ln (

) (4)

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

2).

2 (5)

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

particles.

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(

0

) +

] (6)

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 )

(7)

= 0(1 )

(8)

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)

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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)

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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

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advantage and drawback of the basket centrifuge is demonstrated in Table I. (Friedmann,

1999)

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

short time

no speed for discharge frequently changing product

risk of uneven cake build

up shear sensitive products

products requiring long

desaturation cycles

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

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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.

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

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Figure 8. Pusher filter centrifuge in backward stroke (a) and forward stroke (b) (Svarovsky,

1977)

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

mechanical repair.

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.

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4. Conclusion

References:

In-text: (Hottel, 2008)

Bibliography: Hottel, H. (2008). Perry's chemical engineers' handbook. [New York]:

McGraw-Hill.

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.

(Friedmann, 1999)

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