written report-filtration

1

Written Report on

FILTRATION (Cake Filters)

Submitted to:

Engr. Arniel Ching Dizon Instructor

Submitted by:

Christel Joy Marciano

2

Diamaden Salinding Jr. Adolf Christian Velos Meriel Stephanie Zaragoza

March 14, 2011

Table of Contents

Introduction ------------------------------------------------------------------------ 2 Types of Filter --------------------------------------------------------------------- 2 a.

Cake Filter -------------------------------------------------------------- 2 i. Discontinuous Pressure Filters ------------------------------- 3

ii. Discontinuous Vacuum Filters -------------------------------- 8 iii. Continuous Vacuum Filters ---------------------------------- 10 iv. Centrifugal Filters ----------------------------------------------- 12 v. Filter Media ------------------------------------------------------ 14 vi. Filter Aids -------------------------------------------------------- 14

3 vii. Principles of Cake Filtration --------------------------------- 16 viii. Washing of Cake ---------------------------------------------- 17 b.

Clarifying Filter ----------------------------------------------------- 19

c.

Cross Flow Filter --------------------------------------------------- 19

Summary ------------------------------------------------------------------------- 20

I. Introduction The general problem of separating solids from liquids may be solved by a wide variety of operations depending upon the character of the solid and the proportion of solid to liquid in the mixture to be separated. When the amount of solid is relatively small as compared to the liquid, the process is usually called filtration. Filtration is the removal of solid particles from a fluid by passing the fluid through the filtering medium or the septum, on which the solids are deposited. Commercial or Industrial filtrations cover

4

a wide range of applications. The fluid can be a gas or liquid. The suspended solid particles can be very fine (in micrometer range) or much larger, very rigid or plastic particles, spherical or very irregular in shape, aggregates of particles or individual particles. The desired product may be the clear filtrate or the solid cake. The feed is or slurry solution may carry a heavy load of solid particles or a very small amount. When the concentration is very low, the filters can operate for very long periods of time before the filter needs cleaning. Often the feed is modified in some way by pre-treatment to increase the filtration rate, as by heating, recrystallizing, or adding a filter aid such as cellulose or diatomaceous earth. Most industrial filters are pressure filters, vacuum filters, or centrifugal separators. They also either continuous or discontinuous, depending on whether the discharge or filtered solids is steady or intermittent.

II. Types of Filters a.

Cake Filters

Filters that accumulate appreciable visible quantities of solids on the surface of a filter medium are called cake filters. The slurry feed may have a solids concentration from about 1 percent to greater than 40 percent. The filter medium on which the cake forms is relatively open to minimize flow resistance, since once the cake forms, it becomes the effective filter medium. The initial filtrate therefore may contain unacceptable solids concentration until the cake is formed. This situation may be made tolerable by recycling the filtrate until acceptable clarity is obtained or by using a downstream polishing filter (clarifying type). Cake filters are used when the desired product of the operation is the solids, the filtrate, or both. When the filtrate is the product, the degree of removal from the cake by washing or blowing with air or gas becomes an economic optimization. When the cake is the desired product, the incentive is to obtain the desired degree of cake purity by washing, blowing, and sometimes mechanical expression of residual liquid. Implicit in cake filtration is the removal and handling of solids, since the cake is usually relatively dry and compacted. Cakes can be sticky and difficult to handle; therefore, the ability of a filter to discharge the cake cleanly

5

is an important equipment-selection criterion. In the operational sense, some filters are batch devices, whereas others are continuous.

Discontinuous Pressure Filters Pressure can apply a large pressure differential across the septum to give economically rapid filtration with viscous liquids or fine solids. The most common types of pressure filters are filter presses and leaf filters. Filter Press It contains a set of plates designed to provide a series of chambers or compartments in which solids may collect. The plates may be circular or square, it is covered with a filter medium such as canvas. The slurry is admitted to each compartment under pressure; liquor passes through the canvas and out at the discharge pipe, leaving a wet cake of solids behind. The press is said to be jammed if there is no liquor or no longer flows out to the discharge and there is suddenly rise of filtration pressure. Plate-and-frame presses- This filters consist of plates and frames assembled alternatively with a filter cloth over each side of the plates. The square plates are usually 6 to 78 in. on a side alternate with open frames. The plates are ¼ to 2 in. thick. Plates and frames sit vertically in a rack metal, with cloth covering the face of each plate and are squeezed tightly together by a screw or a hydraulic ram. The feed slurry is pumped into the press and flows through the duct into each of the open frames so that slurry fills the frames. The filtrate flows through the filter cloth and the solids build up as a cake on the frame side of the cloth. The filter flows between the filter cloth and the face of the plate through the channels to the outlet. Filtration proceeds until the frames are completely filled with solids. The filter press has the advantage of simplicity, low capital cost, flexibility, and ability to operate at high pressure in either a cake-filter or a clarifying-filter application. Floor-space and headroom needs per unit of filter area are small, and capacity can be

6

adjusted by adding or removing plates and frames. Filter presses are cleaned easily, and the filter medium is easily replaced. With proper operation a denser, drier cake compared with that of most other filters is obtained. There are several serious disadvantages, including imperfect washing due to variable cake density, relatively short filter-cloth life due to the mechanical wear of emptying and cleaning the press (often involving scraping the cloth), and high labor requirements. Presses frequently drip or leak and thereby create housekeeping problems, but the biggest problem arises from the requirement to open the filter for cake discharge. The operator is thus exposed routinely to the contents of the filter, and this is becoming an increasingly severe disadvantage as more and more materials once believed safe are given restricted exposure limits. Recessed-plate filter press- This press is similar to the plate-and-frame press in appearance but consists only of plates. Both faces of each plate are hollowed to form a chamber for cake accumulation between adjacent plates. This design has the advantage of about half as many joints as a plate-and-frame press, making a tight closure more certain. It shows some of the features of one type of recessed-plate filter which has a gasket to further minimize leaks. Air can be introduced behind the cloth on both sides of each plate to assist cake removal. Some interesting variations of standard designs include the ability to roll the filter to change from a bottom to a top inlet or outlet and the ability to add blank dividers to convert a press to a multistage press for further clarification of the filtrate or to do two separate filtrations simultaneously in the same press. Some designs have rubber membranes between plates which can be expanded when filtration is finished to squeeze out additional moisture. Some designs feature automated opening and cake-discharge operations to reduce labor requirements. Leaf Filters Sometimes called tank filters, they consist of flat filtering elements (leaves) supported in a pressure shell. The leaves are circular, arc-sided, or rectangular, and they have filtering surfaces on both faces. The shell is a cylindrical or conical tank. Its

7

axis may be horizontal or vertical, and the filter type is described by its shell axis orientation. Pressure leaf filters are available with filtering areas of 930 cm2 (1 ft2) (laboratory size) up to about 440 m2 (4734 ft2) for vertical filters and 158 m2 (1700 ft2) for horizontal ones. Leaf spacings range from 5 to 15 cm (2 to 6 in) but are seldom less than 7.5 cm (3 in) since 1.3 to 2.5 cm (0.5 to 1 in) should be left open between surfaces. A filter leaf consists of a heavy screen or grooved plate over which a filter medium of woven fabric or fine wire cloth may be fitted. Textile fabrics are more commonly used for chemical service and are usually applied as bags that may be sewed, zippered, stapled, or snapped. Wire-screen cloth is frequently used for filter-aid filtrations, particularly if a precoat is applied. The filter medium, regardless of material, should be as taut as possible to minimize sagging when it is loaded with a cake; excessive sag can cause cake cracking or dropping. Leaves may be supported at top, bottom, or center and may discharge filtrate from any of these locations. Pressure leaf filters are operated batchwise. The shell is locked, and the prefilt slurry is admitted from a pressure source. The slurry enters in such a way as to minimize settling of the suspended solids. The shell is filled, and filtration occurs on the leaf surfaces, the filtrate discharging through an individual delivery line or into an internal manifold, as the filter design dictates. Filtration is allowed to proceed only until a cake of the desired thickness has formed, since to overfill will cause cake consolidation with consequent difficulty in washing and discharge. The decision of when to end the filtering cycle is largely a matter of experience, guided roughly by the rate in a constant-pressure filter or pressure drop in a constant-rate filter. This judgment may be supplanted by the use of a detector which “feels” the thickness of cake on a representative leaf. If the cake is to be washed, the slurry heel can be blown from the filter and wash liquor can be introduced to refill the shell. If the cake tends to crack during air blowing, it may be necessary to displace the slurry heel with wash gradually so as never to allow the cake to dry. Upon the completion of filtration and washing, the cake is discharged by one of several methods, depending on the shell and leaf configuration. Leaf filters maybe:

8

Horizontal Pressure Leaf filters- In these filters the leaves may be rectangular leaves which run parallel to the axis and are of varying sizes since they form chords of the shell; or they may be circular or square elements parallel to the head of the shell, and the entire same dimension. The leaves may be supported in the shell from an independent rack, individually from the shell, or from a filtrate manifold. Horizontal filters are particularly suited to dry-cake discharge. Most of the currently available commercial horizontal pressure filters have leaves parallel to the shell head. Cake discharge may be wet or dry; it can be accomplished by sluicing with liquid sprays, vibration of the leaves, or leaf rotation against a knife, wire, or brush. If a wet cake discharge is allowable, the filters will probably be sluiced with high-pressure liquid. If the filter has a top or bottom filtrate manifold, the leaves are usually in a fixed position, and the spray header is rotated to contact all filter surfaces. If the filtrate header is center mounted, the leaves are generally rotated at about 3 r/min and the spray header is fixed. Some units may be wet-cake-discharged by mechanical vibration of the leaves with the filter filled with liquid. Dry-cake discharge normally will be accomplished by vibration if leaves are topor bottom-manifolded and by rotation of the leaves against a cutting knife, wire, or brush if they are center-manifolded. In many designs the filter is opened for cake discharge, and the leaf assembly is separated from the shell by moving one or the other on rails. For processes involving toxic or flammable materials, a closed filter system can be maintained by sloping the bottom of the horizontal cylinder to the drain nozzle for wet discharge or by using a screw conveyor in the bottom of the shell for dry discharge. Vertical Pressure Leaf Filter-These filters have vertical, parallel, rectangular leaves mounted in an upright cylindrical pressure tank. The leaves usually are of such different widths as to allow them to conform to the curvature of the tank and to fill it without waste space. The leaves often rest on a filtrate manifold, the connection being sealed by an O ring, so that they can be lifted individually from the top of the filter for inspection and repair. A scavenger leaf frequently is installed in the bottom of the shell to allow virtually complete filtration of the slurry heel at the end of a cycle. Vertical filters are not convenient for the removal of dry cake, although they can be used in this service if they have a bottom that can be retracted to permit the cake to fall into a bin or hopper

9

below. They are adapted rather to wet-solids discharge, a process that may be assisted by leaf vibration, air or steam sparging of a filter full of water, sluicing from fixed, oscillating, or traveling nozzles, and blowback. The advantages of pressure leaf filters are their considerable flexibility (up to the permissible maximum, cakes of various thickness can be formed successfully), their low labor charges, particularly when the cake may be sluiced off or the dry cake discharged cleanly by blowback, the basic simplicity of many of the designs, and their adaptability to quite effective displacement washing. Their disadvantages are the requirement of exceptionally intelligent and watchful supervision to avoid cake consolidation or dropping, their inability to form as dry a cake as a filter press, their tendency to classify vertically during filtration and to form misshapen non-uniform cakes unless the leaves rotate, and the restriction of most models to 610 kPa (75 psig) or less. Pressure leaf filters are used to separate much the same kinds of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of liquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. Automatic Belt Filter The Larox belt filter is a discontinuous pressure filter that separates, compresses, washes and automatically discharges the cake. Filtration takes place in from 2 to 20 horizontal chambers, set one above other. With the belt of filter cloth passes though the filter chambers in turn. With the belt held stationary, each chamber is filled with solids during the filtration cycle. High pressure water is then pumped behind flexible diaphragm in the chamber ceiling, squeezing the cake and mechanically expressing some of the liquid.

10

Discontinuous Vacuum Filters Nutsche Filters-is one of the simplest batch filters. It is a tank with a false bottom, perforated or porous, which may either support a filter medium or act as the filter medium. The slurry is fed into the filter vessel, and separation occurs by gravity flow, gas pressure, vacuum, or a combination of these forces. The term “nutsche” comes from the German term for sucking, and vacuum is the common operating mode. The design of most nutsche filters is very simple, and they are often fabricated by the user at low cost. The filter is very frequently used in laboratory, pilot-plant, or smallplant operation. For largescale processing, however, the excessive floor area encumbered per unit of filtration area and the awkwardness of cake removal are strong deterrents. For small-scale operations, cake is manually removed. For large-scale applications, cake may be further processed by reslurrying or redissolving; or it may be removed manually (by shovel) or by mechanical discharge arrangements such as a movable filter medium belt. Thorough displacement washing is possible in a nutsche if the wash solvent is added before the cake begins to be exposed to air displacement of filtrate. If washing needs to be more effective, an agitator can be provided in the nutsche vessel to reslurry the cake to allow adequate diffusion of solute from the solids. Filtration The filter is charged with slurry and pressure is applied to displace the filtrate leaving the cake retained over the filter medium. For slurries with a wide distribution of coarse fast settling solids and slow settling fines there is a risk of segregation with the finer fraction settling over the coarse fraction. When this happens the fines seal the cake and slow down the cake formation so keeping the slurry in suspension with rotating arms during filtration assists in forming a homogeneous cake. Cake Washing In the washing stage a spray ring or connections on top of the cover introduce the wash liquid over the cake. This displaces the mother solution with the wash liquid but with

11

such in-situ washing the efficiency may be quite low if the cake forms with an uneven thickness. One of the advantages of the Nutsche Filter is the ability to smoothen the cake's surface prior to applying spray wash so that the entire bed is washed evenly. Washing efficiency may be further improved if air or gas is not allowed to enter the cake in a multi-washing system so the wash liquids always displace the solutions in a "piston" like manner. This is achieved by a special detector that monitors the surface of the cake for moisture and once air or gas start entering the bed a signal is transmitted to close the filtrate valve and reopen it once next washing commences. Cake Repulping Many processes require high washing efficiency to remove the contaminating liquid from the product and washing the cake by repulping yields the most efficient product purity. This is done by resuspending the cake with the paddle arms for thorough mixing with the wash solution. During resuspension the rotating arms are moving slowly downwards and are "shaving" the bed gradually layer after layer until the entire cake enters the slurry. Pressure Drying In the drying stage air or gas purges the cake until the captive moisture is reduced to an asymptotic level and in practical terms the cake is considered to be as dry as possible. To obtain minimum moisture the cake is smoothened by reversing the rotation of the paddle or auger arms and exerting controlled pressure on its surface with the hydraulic system. This seals cracks in the cake so that air or gas will not bypass the bed. Vacuum Drying Further reduction in cake moisture may be obtained by slowly rotating and lowering the paddle arms to scrape and delump the cake. To take advantage of the drying ability of

12

the Nutsche Filter it is worth considering the option of heating components such as the vessel, filter floor and paddles to enhance drying. Cake Discharge Once all the stages are completed the cake discharge valve opens and the paddle arms on the smaller machines or the auger arms on the larger ones are rotated and lowered to convey the dry cake towards the center. The same procedure also applies to side discharge machines however it should be noted that in this case the cake comes out intermittently and not continuously. This may have a layout impact on the downstream facility such as the conveyor that handles the product to storage. On some filters the cloth or woven mesh screen may be backwashed with water to dislodge and remove any cake residue that adhered to the medium after cake discharge. Continuous Vacuum Filters In a continuous filter the discharge of both solids and fluid is uninterrupted as long as the equipment is in operation. In all continuous vacuum filters liquor is sucked through a moving septum to deposit a cake of solids. The cake is moved out of the filtering zone, washed, sucked dry, and dislodged from the septum. Some part of the septum is in the filtering zone at all times, part is in the washing zone, and part is being relieved of its solids, so that the discharge of both solids and liquids from the filter is uninterrupted. Types of continuous vacuum filters: 1. Rotary-drum filter It is the most common type of continuous vacuum filter where a horizontal drum with a slotted face turns at a certain revolution in an agitated slurry trough. A filter medium, such as canvas, covers the face of the drum, which is partly submerged in the liquid. Under the slotted cylindrical face of the main drum is a second smaller drum with a solid

13

surface. Between the two drums are radial partitions dividing the annular space into separate compartments, each connected by an internal pipe to one hole in the rotating plate of the rotary valve. Vacuum and air are alternately applied to each compartment as the drum rotates. A strip of filter cloth covers the exposed face of each compartment. Continuous rotary vacuum filters are sometimes adapted to operate under positive pressures up to about 15 atm for situations in which vacuum filtration is not feasible or economical. This maybe the case when solids are very fine and filter very slowly or when the liquid has a high vapor pressure, has a viscosity greater than 1 P, or is a saturated solution that will crystallize if cooled at all. Disadvantages:  High cost and complex.  Their small size limits their application to special problems.

 Works poorly or not at all when the feed contains coarse fast-settling particles of solid. The coarse particles cannot be suspended well in the slurry trough, and the cake that forms often will not adhere to the surface of the filter drum.

Precoat filter - It is a rotary drum filter modified for filtering small amounts of fine gelatinous solids that ordinarily plug a filter cloth. 2. Horizontal belt filter It is used when the feed contains coarse fast-settling particles of solid. It resembles a belt conveyor, with a transversely ridged support or drainage belt carrying the filter cloth (also in the form of an endless belt). Central openings in the drainage belt slide over a longitudinal vacuum box, into which the filtrate is drawn. Feed slurry flows unto the belt from a distributor at one end of the unit; filtered and washed cake is discharged from the other.

14

Advantage:  Useful in waste treatment, since the waste often contains a very wide range of particles sizes.

Centrifugal Filters Another industrial filters aside from pressure filters and vacuum filters are the centrifugal filters. Slurry is fed to a rotating basket having a slotted wall covered with a filter medium (canvas or metal cloth). Pressure resulting from the centrifugal action forces the liquor through the filter medium, leaving the solids behind. Centrifugal filters can leave the solids much “drier” than those from a filter press or vacuum filter. Main types of Centrifugal Filters: 1. SUSPENDED BATCH CENTRIFUGES a. Top suspended batch centrifuge 

Perforated basket range from 30-40 in, in diameter (18-30in. deep) and

turn at speeds between 600 and 1800 r/min. 

For the process: Basket is held at the lower and of free-swinging vertical

shaft driven from above. Filter medium lines the perforated wall of the basket. Feed slurry enters the rotating basket through an inlet pipe or chute. Liquor drains through the filter medium into the casing and out a discharge pipe: the solid form a cake inside the basket, soluble material is remove ant the cake is then spun as dry as possible. Motor is shut-off and with the basket slowly turning, the solids are discharged with an unloader knife.  Use extensively in sugar refining. b. Bottom

15

 Driven from the bottom, with the driv motor, basket, and casing all suspended from vertical legs mounted on a bas plate.  Solids are unloaded by hand through the top of the casing.

2. AUTOMATIC BATCH CENTRIFUGES In this machine the basket (between 20 and 42 in.) rotates at constant speed about a horizontal axis. Feed slurry, wash liquid, and screen rinse are successively spayed into the basket. The basket is unloaded while turning at full speed by a heavy knife. Automatic centrifuges have high productive capacity with free draining crystals. Usually they are not used when the feed contains many particles finer than 150-mesh Disadvantages:  Cannot handle slow-draining solids, which would give uneconomically long cycles, or solids that do not discharge cleanly through the chute.  There is also considerable breakage or degradation of the crystals by the

unloader knife. 3. CONTINUOUS FILTERING CENTRIFUGES a. Reciprocating-conveyor centrifuge  Baskets ranges from 12-48 in. in diameter  For the process: A rotating basket with a slotted wall is fed through a

revolving feed funnel (accelerate the feed slurry gently and smoothly). Feed enters the small end of the funnel from a stationary pipe at the axis of rotation of the basket. It travels toward the large end of the funnel, gaining speed as it goes, and when it spills off the funnel into the wall of the basket, it is moving in the same direction as the wall, which may be covered with a woven metal cloth. A

16

layer of crystals (1-3 in. thick) is formed. This layer is moved over the filtering surface by a reciprocating pusher. When the crystals reach the lip of the basket, they fly outward into a large casing and drop into a collector chute. Filtrate and any wash liquid that is sprayed on the crystals during their travel leave the casing through separate outlets.

Filter media Filter media are required for a filter in order to retain solid. The filter medium must meet the following specifications: 1. the ability to bridge solids across its pores quickly after the feed is started 2. low rate of entrapment of solids within its interstices 3. minimum resistance to filtrate flow 4. resistance to chemical attack 5. sufficient strength to support the filtering pressure

6. acceptable resistance to mechanical wear 7. ability to discharge cake easily and

cleanly

8. ability to conform mechanically to the kind of filter with which will be

used 9. minimum cost Filter media includes canvass cloth, the most common filter medium, and for corrosive liquids, woolen cloth, metal cloth of monel, glass cloth, and filter paper are used. Furthermore, synthetic fibers are also used because of its high resistance chemically.

17

Filter Aids When problems like slow filtration rate, rapid medium blinding, or unsatisfactory filtrate arise, filter aids are added to the slurry before filtration. It is to increase the porosity of the cake to permit passage of liquor at a reasonable rate. Filter aids should be capable of forming a highly permeable filter cake in which very fine solids or slimy deformable flocs may be trapped. It allows the use of much more permeable filter medium than the required to produce filtrate of the same quality. Filter aids should be of low bulk density to minimize settling and to have a good distribution on filter-medium surface for non-horizontal surfaces. Furthermore, it should be chemically inert to the filtrate. Filter aids can be separated from the filter cake by dissolving away the solids or burning out the filter aid. In cases where the cake is not desired, both can be disregarded. The most common filter aids are the diatomaceous silica and the perlite. The diatomaceous silica has dry bulk density of about 128-320 kg/m3 and contains particles mostly smaller than 50micro meter. The produced cake has a porosity range of 0.9. Similarly, perlite is lower in bulk density but contain a higher fraction of particles with 50150 micro meter range. Diatomaceous silica will withstand slightly more extreme pH levels than perlite and it is said to be less compressible. Filter aids are added to form a layer of second medium which protects the basic medium of the system this termed as precoat. In addition, this is also to improve the flow rate by decreasing the cake compressibility and increasing the cake’s permeability or termed as admix.

18

Figure 1 shows the mechanism of filtration with filter aids.

Principles of Cake Filtration Filtration is a special example of flow through porous media for cases in which the resistances to flow are constant. In filtration the flow resistances increase with time as the filter medium becomes clogged or a filter cake builds up. The chief quantities of interest are the flow rate through the filter and the pressure drop across the unit. As time passes during filtration, either the flow rate diminishes or the pressure drop rises. In what is called CONSTANT-PRESSURE FILTRATION the pressure drop is held constant and the flow rate allowed to fall with time; less commonly, the pressure drop is progressively increased to give what is called CONSTANT-RATE FILTRATION.

19

In cake filtration the liquid passes through two resistances in series: that of the cake and that of the filter medium. The filter-medium resistance, which is the only resistance in clarifying filters, is normally important only during early stages of cake filtration. The filter medium resistance may vary with the pressure drop, since the higher liquid velocity caused by a large pressure drop may force additional particles of solid into the filter medium. It also varies with age and cleanliness of the filter medium, but since it is important only during the early stages of filtration, it is nearly always satisfactory to assume that it is constant during any given filtration. On the other hand, the cake resistance is zero at the start and increases with time as filtration proceeds. If the cake is washed after it is filtered, both resistances are constant during the washing period and that of the filter medium is usually negligible.

Figure 2. Pressure Vs. Flow Rate. Most batch-type filters operate under pressure. Generally the pumping mechanism determines the filtration flow characteristics and serves as a basis for the following three categories.

20

1. Constant-pressure filtration. The actuating mechanism is compressed gas maintained at a constant pressure. In constant-pressure filtration, the pressure drop is held constant and the flow rate diminishes with time. 2. Constant-rate filtration. Positive-displacement pumps of various types are employed. In constant rate filtration, the flow rate is held constant and the pressure drop increases. 3. Variable-pressure, variable-rate filtration. The use of a centrifugal pump results in this pattern. In this mechanism, the flow rate decreases with increasing back pressure. PRINCIPLES OF CENTRIFUGAL FILTRATION The basic theory of constant-pressure filtration can be modified to apply to filtration in a centrifuge. The treatment applies after the cake has been deposited and during flow of clear filtrate or freshwater through the cake. ASSUMPTIONS ARE MADE • Effects of gravity and of changes in kinetic energy of the liquid are neglected • Pressure drop from centrifugal action equals the drag of the liquid flowing through the cake • The cake is completely filed with liquid • The flow of the liquid is laminar • Resistance of the filter medium is constant • The cake is nearly incompressible, so as an average specific resistance can be used as a constant

Washing filter cakes After its formation, the cake may contain a significant amount of entrapped liquid. When the liquid is the product of interest, this entrapment represents a loss of yield. When the cake is itself the product, the entrapped liquid represents the presence of impurity. The entrapped liquid can be removed by cake washing.

21

To wash soluble material that may be retained by the filter cake after a filtration, a solvent miscible with the filtrate may be used as a wash. Water is the most common wash liquid. The rate of flow of the wash liquid and the volume of liquid needed to reduce the solute content of the cake to a desired degree are important in the design and operation of a filter. The volume of wash liquid required is related to the concentration-time history

of the wash liquid leaving the filter. Figure 3. Washing of Filter Cake The first portion of the recovered liquid is represented by segment ab. The effluent consists essentially of the filtrate that was left on the filter, which is swept out by the first wash liquid without appreciable dilution. This stage of washing, called displacement wash is the ideal method of washing a cake. The volume of the wash liquid needed for a displacement wash is equal to the volume of filtrate left in the cake, or έAL, where L is the cake thickness and έ is the average porosity of the cake. The second stage of washing in bc is characterized by a rapid drop in concentration of the effluent. The second stage of washing is characterized by a rapid drop in concentration of the effluent. The volume of wash liquid used in this stage is also of the order of magnitude of that used in the first stage. The third stage is shown by segment cd. The concentration of solute in the effluent is low, and the remaining solute is slowly leached from the cake. If sufficient wash liquid is used, the residual solute in the cake can be reduced to any desired point, but the washing should be stopped when the value of the unrecovered solute is less than the cost of recovering it. In most filters the wash liquid follows the same path as that of the

22

filtrate. The rate of flow of the wash liquid is, in principle, equal to that of the last of the filtrate. If the viscosities of filtrate and wash liquid differ, correction for this difference can be made. b.

Clarifying Filters

Clarifying filters are used to separate liquid mixtures which contain only very small quantities of solids. When the solids are finely divided enough to be observed only as a haze, the filter which removes them is sometimes called a polishing filter. The prefilt slurry generally contains no more than 0.10 percent solids, the size of which may vary widely (0.01 to 100 mm). The filter usually produces no visible cake, sometimes because the amount of solids removed is so small, sometimes because the particles are removed by being entrapped within rather than upon the filter medium. Compared with cake filters, clarifying filters are of minor importance to pure chemical-process work, their greatest use being in the fields of beverage and water polishing, pharmaceutical filtration, fuel- and lubricating-oil clarification, electroplating-solution conditioning, and dry-cleaning-solvent recovery. They are essential, however, to the processes of fiber spinning and film extrusion; the spinning solution or dope must be free of particles above a certain size to maintain product quality and to prevent the clogging of spinnerets. Clarification differs from screening in that pores in the filter medium are larger sometimes much larger than the particles to be removed.

c.

Cross flow Filters

It can be applied to concentrate suspension of fine particles or colloidal material or to fractionate solutions of macromolecules. It involves microfiltration, size ranges from .1 to 5 micrometer, ulrafiltration, 1 down to molecules about 10-3 micrometer, and hyperfiltration that involve reverse osmosis.

23

III. Summary Filtration is the removal of solid particles from a fluid by passing the fluid through the filtering medium or the septum, on which the solids are deposited. Classifications of filters include cake, clarifying, and cross-flow. For cake filters the filtering media must meet certain requirements for it to be effective, it must be capable of retaining the solids to be filtered, it must not plug or blind, it must be resistant chemically and strong physically, it must permit clean and complete discharge, and must be of minimum cost. Filter aids are added when problems like slow filtration rate and rapid medium blinding occurs. Most common filter aids are diatomaceous silica and perlite. Principles of cake filtration describe constant-pressure filtration and constant-rate filtration. As the rate of filtration is held constant, there is an increase in pressure drop. If the pressure drop is held constant, the rate of filtration will vanish with time. In washing filter cakes, common medium used is water. Types of cake filters include the following: Discontinuous pressure filters, Discontinuous vacuum filters, Continuous vacuum filters, and Centrifugal filters.