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Solid liquid separation- Centrifugal filtration

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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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  • 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

  • 2

    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

  • 3

    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)

  • 4

    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)

  • 6

    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)

  • 7

    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

  • 8

    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

  • 9

    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

  • 10

    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.

  • 11

    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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