Filtration may be defined as a process of separation of solids from a fluid by passing the same through a porous medium that retains the solids, but allows the fluid to pass through. ” The suspension to be filtered is known as slurry. The porous medium used to retain the solids is known as filter medium. The accumulated solids on the filter are referred to as filter cake, while the clear liquid passing through the filter is filtrate. When solids are present in a very low concentration i. e. , not exceeding 1. 0% w/v, the process of its separation from liquid is called ‘clarification’.
Process of filtration: The filtration operation is shown below in the figure The pores of the filter medium are smaller than the size of the particles to be separated. Filter medium (for eg: filter paper or muslin cloth) is placed on a support (a sieve). When slurry (feed) is passed over the filter medium, the fluid flows through the filter medium by virtue of a pressure differential across the filter. Gravity is acting on the liquid column. Therefore, solids are trapped on the surface of the filter medium Figure 1: filtration Once the preliminary layer of particles is deposited, further filtration is brought about wherein the filter medium serves only as a support. The filter will work efficiently only after an initial deposit. After a particular point of time, the resistance offered by the filter cake is high that virtually filtration is stopped. For this reason, a positive pressure is applied on the filter cake (upstream) or negative pressure (suction) is applied below the filter medium (downstream). Factors affecting the rate of filtration:
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The rate of filtration which depends on various factors can be written as: Rate of filtration = Area of filter X Pressure difference Viscosity X Resistance of cake and filter The rate of filtration depends on the following factors:
1. Pressure: The rate of filtration of liquid is directly proportional to the pressure difference between the ‘filter medium’ and ‘filter cake’. Thus, the rate of filtration can be increased by applying pressure on the liquid being filtered or by decreasing the pressure beneath the filter.
2. Viscosity: The rate of filtration is inversely proportional to the viscosity of the liquid undergoing filtration. Liquids which are very viscous get filtered slowly in comparison to liquids with low viscosity. Reduction of viscosity of a liquid by raising the temperature is frequently done in order to accelerate filtration. eg: syrups are more quickly filtered when hot and cold.
3. Surface area of filter media: The rate of filtration is directly proportional to the surface area of filter media. Pleating the filter paper or using a fluted funnel increases the effective surface area of filter paper for filtration. Filter press also works on the same principle.
4. Temperature of liquid to be filtered: Temperature plays an important role in the rate of filtration. Viscosity is reduced by a rise in temperature and the filtration of viscous oils, syrups etc is often accelerated by filtering them while they are still hot.
5. Particle size: The rate of filtration is directly proportional to the particle size of the solid to be removed. It is easier to filter a liquid having coarse particles than that having finely divided particles because coarse filtering medium can be used to filter liquid having coarse and hence it increases the rate of filtration. Therefore before filtration, some method should be adopted to agglomerate the finely divided particles into coarse particles or to increase the particle size by precipitation.
6. Pore size of filter media: The rate of filtration is directly proportional to the pore size of the filter media. The liquid having coarse particles requires a coarse filtering media to remove them. So, the rate of filtration is increased when a coarse filter medium is used for filtration.
7. Thickness of cake: The rate of filtration is inversely proportional to the thickness of the filter cake formed during the process of filtration. As the filtration process proceeds, the solid particles start depositing on the filter medium, and thus, it increases the thickness of the cake and decreases the rate of filtration.
8. Nature of the solid material: The rate of filtration is directly proportional to the porosity of the filter cake. The porosity of the filter cake depends on the nature of the solid particles to be removed from the liquid. * Filter aids are sometimes added to the filtering liquid to make a porous cake Theories of filtration
The flow of a liquid thorough a filter follows the basic rules that govern the flow of any liquid through the medium offering resistance. The rate of flow may be expressed as: Driving force Rate = -------------------- (equation 1) Resistance The rate of filtration may be expressed as volume (lit) per unit time (dv/dt). The driving force is the pressure differential between the upstream and downstream of the filter. The resistance is not constant.
It increases with an increase in the deposition of solids on the filter medium. Therefore filtration is not a steady state. The rate of flow will be greatest at the beginning of the filtration process, since the resistance is minimum. Once the filter cake is formed, its surface acts as filter medium and solids continuously deposit adding to the thickness of the cake. The resistance to flow is related to several factors as mentioned below. Length of capillaries Resistance to movement = ------------------------------------------------------------ Poiseuille’s Equation:
Poiseuille’s considered that filtration is similar to the stream line flow of a liquid under pressure through capillaries. Poiseuille’s equation is ? pr4 V = ----------------- 8L? Where, V= rate of flow, i. e. , volume of liquid flowing in unit time, m3/s(1/s) p = pressure difference across the filter, pa r = radius of the capillary in the filter bed, m L = thickness of the filter cake (capillary length), m = viscosity of filtrate, pa s If the cake is composed of a bulky mass of particles and the liquid flows through the interstices (correspond to a multiplicity of capillary tubes), then the flow of liquids through these may be expressed by poiseulle’s equation. Darcy’s Equation: Poiseuille’s law assumes that the capillaries found in the filter are highly irregular and nonuniform. Therefore, if the length of a capillary is taken as the thickness of the bed, correction factor for radius is applied so that the rate equation is closely approximated and simplified.
The factor influencing the rate of filtration has been incorporated into an equation by Darcy, which is: KA P V = -------------------- ? L Where, K = permeability coefficient of the cake, m2 A = surface area of the porous bed (filter medium), m2 p = pressure difference across the filter, pa L = thickness of the filter cake (capillary length), m ? = viscosity of filtrate, pa s
The term K depends on the characteristics of the cake, such as porosity, surface area and compressibility. Permeability may be defined quantitatively as the flow rate of a liquid of unit viscosity across a unit area of cake having unit thickness under a pressure gradient of unity. This model relates not only to filter beds or cakes but also applies to other types of depth filter. Equipment is valid for liquids flowing through sand, glass beads and various porous media. Darcy’s equation is further modified by including characteristics of K by Kozeny-Carman. Kozeny-Carman Equation:
Poiseuille’s equation is made applicable to porous bed, based on a capillary type structure by including additional parameters. Thus the resultant equation, which is widely used for filtration is Konzeny- Carman equation. A p ? 3 ?S2 KL (1- ? )2 V = ------ ------- -------- Where, ? = porosity of the cake (bed) S = specific surface area of the particles comprising the cake, m2/m3 K = Konzeny constant p = pressure difference across the filter, pa L = thickness of the filter cake (capillary length), m ? = viscosity of filtrate, pa s
The Konzeny constant is usually taken as 5. The effect of compressibility of the cake on flow rate can be appreciated from equation (1), since the flow rate is proportional to ? 3/ (1- ? )2. A 10 percent change in porosity can produce almost 3-fold change inn V. Limitations of Kozeny Carman equation: Kozeny Carman equation does not take in to account of the fact that the depth of the granular bed is lesser than the actual path traversed by the fluid. The actual path is not straight throughout the bed, but it is sinuous or tortuous Mechanisms of filtration:
The mechanism whereby particles are retained by a filter is significant only in the initial stages of filtration. Some of the mechanisms are: Straining: Similar to sieving i. e. , the particles of larger size cannot pass through the smaller pore size of the filter medium. Impingement: Solids having momentum move along the path of streamline flow and strike (impinge) the filter medium. Thus, the solids are retained on the filter medium. Entanglement: Particles become entwined (entangled) in the mass of fibres (of cloth with a fine hairy surface or porous felt) due to smaller size of particles than the pore size.
Thus the solids are retained on the filter medium. Attractive forces: Solids are retained on the filter medium as a result of attractive forces between particles and filter medium, as in case of electrostatic precipitation.
Filter Media and Filter Aids
Filter media: The filter medium act as a mechanical support for the filter cake and is also responsible for the collection of solids. Filter medium should have the following characteristics:
1. It should have sufficient mechanical strength.
2. It must be inert; it should not show chemical or physical interaction.
3. It should not absorb the dissolved material.
4. It should allow the maximum passage of liquid, while retaining the solids.
It means that it must offer low resistance to flow. The magnitude of the resistance of the filter medium will change due to the layers of solids deposited earlier, which may block the pores or may form bridges over the entrances of the channels. Therefore, the pressure should be kept low at the beginning to avoid the plugging of the pores. The usual procedure is to filter at constant rate by increasing the pressure as necessary. When normal working pressure is reached, it is maintained.
On continued filtration, the thickness of the cake further builds up and hence the rate of filtration decreases. When the rate is uneconomical, filtration is stopped. The filter cake is removed and filtration is restarted. Materials: The following materials are used as filter media:
1. Woven materials such as felt or cloth: Woven material is made of wool, cotton, silk, glass, metal or synthetic fibres (rayon, nylon etc. ) Synthetic fibres have greater chemical resistance than wool or cotton, which are affected by alkali and acid respectively. The choice of the fibre depends on the chemical reactivity with the slurry.
2. Perforated sheet metal: For eg: stainless steel plates have pores which act as channels as in case of meta filter (edge filter).
3. Bed of granular solid built up on a supporting medium: In some processes, a bed of graded solids may be formed to reduce the resistance to the flow. Typical examples of granular solids are gravel, sand, asbestos, paper, pulp and keiselguhr. The choice of solids depends on the size of the solids in the process.
4. Prefabricated porous solid unit: Porous solids prefabricated into a single unit are being increasingly used for its convenience and effectiveness. Sintered glass, sintered metal, earthenware and porous plastics are some of the materials used for the fabrication.
5. Cartridge filter media: Cartridge units are economical and available in pore size of 100µm to even less than 0. 2 µm. These can be used either as surface cartridges or depth type cartridges.
6) Surface type cartridges: These are corrugated and resin treated papers. These are used in hydraulic lines. Ceramic cartridges are advantageous in cleaning for reuse by back flushing or firing. Porcelain filter candles are used for sterile filtration. ) Depth type cartridges: These are made of cotton, asbestos or cellulose. These are disposable items, since cleaning is not feasible. Filter Aids: Filter aid forms a surface deposit which screens out the solids and also prevents the plugging of the supporting filter medium. The important characteristics of the filter aids are:
1. Chemically inert to the liquid being filtered and free from impurities.
2. Low specific gravity, so that filter aids remain suspended in liquid.
3. Porous rather than dense, so that previous cake can be formed.
4. Recoverable Justification:
The object of the filter aid is to prevent the medium from becoming blocked and to form an open, porous cake, hence reducing the resistance to flow of the filtrate. a) Usually low resistance is offered by the filter medium itself, but as layers of solid built up the resistance will be increased. The cake may become impervious by blocking of the pore in the medium. Flow rate is inversely proportional to the resistance of the solid cake. b) Slimy or gelatinous material and highly compressible substances form impermeable cakes. The filter medium gets plugged and the flow of filtrate stops.
Disadvantages: The filter aids remove the coloured substances by absorbing them. Sometimes active principles such as alkaloids are absorbed on the filter aid. Rarely, filter aids are a source of contaminants such as soluble iron salts, which can provoke degradation of sensitive ingredients. Liquid retained in the pores of the filter cake is lost in the manufacturing process. Example of filter aids: Keiselguhr, Talc, Charcoal, Asbestos, Paper pulp, Bentonite, Fullers earth Activated charcoal is used for removal of organic and inorganic impurities. Keiselguhr is a successful filter aid and as little as 0. 1% can be added to the slurry. The rate of filtration is increased by 5 times or more, at the above concentration, though the slurry contains 20% solids. Handling of filter aids: Filter aids are mostly used for clarification processes, i. e. , where solids are discarded. Different flow rates can be achieved depending on the grade of the aids. Low flow rate (fine solids) - fine grade filter aids –mainly intended for clarity. Fast flow rate (coarse solids) -coarse grade filter aids –acceptable filtrate.
The filter aid can be employed in either one or both ways. a) Firstly, a pre coat is formed over the medium. For this purpose, a suspension of the filter aid is filtered to give a coating up to 0. 5/m2. b) Secondly, a small proportion of filter aid (0. 1-0. 5% of total batch weight) is purposely added to the slurry. So the filter cake has a porous structure and filtration can be efficient. The filter aid of 1-2 parts per each part of contaminant is mixed in the feed tank. This slurry is re circulated through the filter until a clear filtrate is obtained. Filtration then proceeds to completion.
The body mix method minimises equipment requirement and cross contamination potentials. Sterile Filtration: Sterile filtration is carried out for removal of microorganisms from fluids. It is a cheap and satisfactory method for sterilizing heat-sensitive (thermolabile) materials. The method implies the use of membrane filters which do not impart any particulate matter, fibers, or chemical reaction to the filtrate unlike unglazed porcelain candles, asbestos pads and other filters. In addition, no pretreatment is required, cleaning is no problem and the filters can be autoclaved or gas sterilized after assembly in its holder.
Even when sterility is not warranted but ‘polishing’ (removal of particulate matter including live or dead bacterial cells in order to obtain high purity and clarity) is desired in products like oral or topical antibiotic preparations, membrane filters are the best choice. The following filters are used for bacterial filtration:
1. Candle filter
2. Seitz filter
3. Edge filter
4. Sintered glass filter
5. Membrane filter
Candle filters: Candle filters are made of unglazed porcelain and are available in various porosity grades, either cylindrical or in the shape of the flanged test tube.
Normally the filtration is so carried out that the liquid flow is from is from outside inwards and greater filtration surface is available to the incoming liquids. Candle filters can be sterilized by steaming, by hot moist air, or by autoclaving. Cleaning may be affected by drawing a large volume of distilled water through the candle filter thereby completely washing the previous solution from the pores. Thus the surface of the filter should be gently scrubbed with a soft brush, rinsed well with water and finally ignited in a muffle furnace. The main disadvantage of such filters s that the pores become plugged with organisms and debris which necessitate a very thorough cleaning. Sietz filter: It consists of an asbestos pad. The pads are available in several porosities that make them valuable for ‘polishing’ of solutions as well as removal of bacteria. Unless however the filter is backed with nylon mesh or sintered stainless steel: fibers occasionally get into the solution. The lower edge is fitted with a broad flat flange and the upper part is cylindrical. A perforated plate fitted into a lower part of the funnel supports the asbestos pad.
As the pads are meant only for single use, the cleaning of filter media is no problem. Each time a fresh pad is to be used. The apparatus is simple in operation but suitable mostly for small quantities of liquids. Sintered glass filters: These are made of borosilicate glass. Borosilicate glass is finely powdered, sieved and particle of desired size are separated. It is then packed in to a disc mould and heated to a temperature at which adhesion takes place between the particles. The disc is then fused to a funnel of suitable shape and size. The sintered glass filters are available in different pore size.
Hence the funnel with a sintered filter is numbered according to the pore size. The filtration is carried out under reduced pressure. These funnels are used for bacterial filtration. Sintered filters are also available in stainless steel which has a greater mechanical strength. However these are very much liable to attack by the solutions passing through them. Edge filters: In edge filters a pack of the filter media used and filtration is done edges by passing the liquid or slurry between and not through the media. Such filtration must be conducted under pressure or under partial vaccum system.
Meta filter and stream line filter are two types of edge filters but the former is of greater use in pharmaceutical industry. Meta filters: Meta filters are useful in those manufacturing processes where filter presses are not frequently suitable. It requires no cloth, gauges, paper etc. and may be used at any pressure and temperature and for any liquid. It can be thoroughly cleaned after each operation. In its simplest form, meta filters consists of a grooved drainage rod or guide tube on which a series of rings are packed. On keeping the pack and finds its way along the grooves in drainage rod and ultimately to the receiver.
These may be operated with pressure or under vaccum system. The rings are usually of stainless steel, of about 15mm inside diameter, 22mm outside diameter and 0. 8mm in thickness, with a number of semicircular projections on one surface. These pressure filters can be used for the filtration of very viscous liquids such as syrups or oils by fitting a steam jacket and rendering the liquids less viscous. They are also useful in the clarification of injection solutions and products such as insulin liquids. This type of filter can be cleaned easily by back-flushing with water or steam.
Because of the shape of the pores in the ring, back-flushing will wash away the filter bed completely. Meta filters are very economic in use. Streamline filters: Operation wise and also geometrically, the streamline filter is similar to meta filter but the cylindrical filter pack consists of compressed paper discs. The liquid flow takes place radially inwards through the small space between individual papers and through the papers themselves. Membrane filters: Ultra filtration methods have become popular in recent years mainly due to increased refinement of various membranes. Cellulose and cellulose derivatives are mostly commonly used materials for these filters. They are available in a wide range of pore sizes, ranging from 8µ down to 0. 22µ.
However, for sterile filtration, membranes with pore size of 0. 22 to 0. 45µ are usually specified. As such fine porosity of membranes may get clogged rapidly, a prefilter is used to remove colloidal matter in order to extend the filtration cycle. The filter primarily acts as a simple screen and retains on its surface all particles of size greater than the pore size of the filter (resembling sieving action). Due to an enormous number of very fine pores, the pore volume approximates 80% of the total volume of the membrane. The action of the filter is mainly due to the combined forces of gravity and van Der Waals forces. Membrane efficiency can be predicted in terms of its bubble point which is a characteristic function of porosity. It is defined as the pressure required to push air through a liquid saturated filter. Filter pores retain liquid until this point is reached.
Each membrane has specific bubble point which depends on the liquid wetting the membrane. An obvious disadvantage of membrane filter is their brittleness when dry and this makes handling difficult. The use of filters in cartridge form, overcomes this problem. Apart from the small laboratory models, large models are available for pilot plant and small scale production to handle up to about 25litres/minute of liquid through a 0. 45µ pore size membrane. Membrane filters find extensive use in filtration and sterilization of a variety of pharmaceutical products such as ophthalmic and intravenous solutions, other aqueous products, biological preparations, hormones and enzymes. In conjunction with a suitable pipette syringe, it is very useful in dispensing measured volumes of sterile fluids. This assembly is often utilized for handling of pharmaceutical, biological and bacteriological preparations which can be damaged by metallic contact. Centrifugation Centrifugation is a unit operation employed for separating the constituents present in the dispersion with the aid of the centrifugal force. Equipment used for centrifugation is centrifuge. Centrifugal force is used to provide the driving force for the separation. It replaces the gravitation force in the sedimentation.
Centrifugation is particularly useful when separation by ordinary filtration is difficult. Centrifugation provides convenient method of separating two immiscible liquids or solid from liquid. Centrifugation is a separation process which uses the action of centrifugal force to promote accelerated settling of particles in a solid-liquid mixture. If particles size in the dispersions is 5 micro meter or less, they undergo Brownian motion, hence they do not Sediment under gravity, therefore a stronger force, centrifugal force is applied in order to separate
Two distinct major phases are formed in the vessel during centrifugation: The sediment Usually does not have a uniform structure. The centrifugate or centrate which is the supernatant liquid. Process of centrifugation: The centrifuge consists of a container in which mixture of solid and liquid or two solids is placed and rotated at high speeds. The mixture is separated into it’s constituent parts by the action of the centrifugal force on their densities. A solid or liquid with higher specific gravity is thrown outward with greater force & it is retained at the bottom of the container leaving a clear supernatant liquid.
The speed of the centrifuge is commonly expressed in terms of number of revolutions per minute.
Theory of centrifugation: If a particle (mass = m kg) spins in a centrifuge (radius r, m) at a velocity (v, m s-1) then the centrifugal force (F, N) acting on the particle equals m v2/r. The same particle experiences gravitational force (G, Newton) = m g (where g = acceleration due to gravity) Centrifugal force = f = mv2/r Centrifugal effect (C) = F/G = mv2 /mgr (v = 2 ? r n ) c = (2? r n)2/ g r = 4 ? 2r n2/ g (d= r/2) = 2 ? 2 d n2/ g (g = 9. 807) C = 2. 013 d n2 Centrifugal effect, C= 2. 013 n2d n= speed of rotation( revolution per second of centrifuge) * d= diameter of rotation
So Centrifugal effect is directly proportional to diameter of rotation Centrifugal effect is directly proportional to (speed of rotation)2 There are two main types of centrifuge used to achieve separation on an industrial scale, Filtration centrifuge: Those using perforated baskets, which perform a filtration-type operation (work like a spin-dryer) and Sedimentation centrifuge : Those with a solid walled vessel, where particles sediment towards the wall under the influence of the centrifugal orce Perforated basket centrifuge: Figure: Perforated Basket Centrifuge In this type of centrifuge, a basket is mounted above a driving shaft. This type of centrifuges are used for batch processes. Principle: Perforated basket (bowl) centrifuge is a filtration centrifuge. The separation through a perforated wall based on the difference in the densities of solid and liquid phases. The bowl contains a perforated side wall. During centrifugation, the liquid phase passes through a perforated wall, while solid phase is retained in the bowl.
The solids are removed after stopping the centrifuge. Construction: It consists of a basket, made of steel (sometimes covered by vulcanite or led) or copper. The material of construction should be such that it offers greatest resistance to corrosion. The basket may have diameter of 0. 90 meters and capacity of 0. 085 meter cube. The diameter of perforations must be based on the size of crystals to be separated. The basket is suspended on a vertical shaft and is driven by a motor using suitable power system.
Perforated basket is kept in a casing which collects the filtrate and discharges it through outlet. Working: The material to be separated kept in the basket. The loading of material must be done to give an even distribution. The power is applied to run the basket at speed of 1000 rpm. During centrifugation the liquid passes through the perforated wall and solid phase retaind in the basket. Uses: Perforated basket centrifuge is extensively used for separation of crystalline drugs (aspirin) from mother liquor. Sugar crystals are separated using the perforated basket centrifuge. Precipitated proteins from insulin can be separated. Advantages: The process is rapid The final product has low moisture content It cam handle slurries with high proportion of solids even those having paste like consistency Dissolved solids from cake can be separated. Disadvantages: On prolonged operation solids may form hard cake. It is a batch process. Non-Perforated Centrifuge: Principle This is sedimentation centrifuge.
The separation is based on the difference in the densities of solid and liquid phases without a porous barrier. The bowl contains a non perforated side wall. During centrifugation, solid phase is retained on the sides of the basket and liquid remains at the top removed by skimming tube. Construction: It consists of a basket, made of steel (sometimes covered by vulcanite or led) or copper. The material of construction should be such that it offers greatest resistance to corrosion. The basket is suspended on a vertical shaft and is driven by a motor using suitable power system Working: The feed is continuously introduced into the centrifuge while the liquid (centrate) is continuously removed from an overflow weir inside the centrifuge.
Solids build up during centrifugation forming a cake that must be periodically discharged Figure: Non-Perforated Basket Centrifuge After the basket becomes filled with solids the centrifuge slows down and "skimming" (the removal of the top semi-liquid soft cake layer) takes place Skimming typically removes 5 to 15% of the bowl solid volume The bulk of the cake is discharged using a ploughing knife moving into the slowly rotating cake. The solid is discharged centrally at the bottom of the centrifuge Solid accumulation is typically up to 60 to 85% of the maximum available depth. This type of centrifuge is typically operated at low centrifugal forces and has a relatively low solid handling capacity. The imperforated basket centrifuge is the only basket centrifuge commonly used for typical sludge dewatering applications. High solid recovery can be achieved with this centrifuge even without chemical additives. Uses: Non-perforated basket centrifuge is useful when deposited solids offer high resisttance to the flow of liquid. Conical disc centrifuge: Principle: It is a sedimentation centrifuge. The separation is based on the difference in the the densities between phases under the influence of centrifugal force. In this a number of cone shaped plates are attached to the central shaft (which has provision for feed) at different elevations.
During centrifugation, the dense solids are thrown outwards to the underside of cone shaped casing. While lighter clarified liquid passes over bowl and collected from top of the cone. Construction: It consists of shallow form of bowl containing series of conical discs attached to the central shaft at different elevations. The discs are made up of thin sheet of metal or plastic separated by narrow spaces. A concentric tube is placed surrounding the central drive shaft.
Working: The feed is introduced into the concentric tube surrounding the drive shaft. The feed flows down and enters the spaces between the discs. The solids and heavier liquids thrown out ward and move underside of the discs.
Low speed and short time of centrifugation is sufficient to give high degree of clarification.
Uses: Two immiscible liquids can be easily separated by continuous process after liquid-liquid extraction in manufacture of antibiotics. Precipitated proteins in manufacture insulin can be clarified. Advantages: Conical disc centrifuge is compact and occupies very less space. By controlling speed of rotation and rate of flow, particles are separated into two sizes. Separating efficiency is very high. Disadvantages: Capacity of conical disc centrifuge is limited Construction is complicated Not suitable if sediment of solids form hard cake. Figure: Conical Disc Centrifuge Tubular bowl centrifuge.
The tubular bowl centrifuge has been used for longer than most other designs of centrifuge. It is based on a very simple geometry: it is formed by a tube, of length several times its diameter, rotating between bearings at each end. The process stream enters at the bottom of the centrifuge and high centrifugal forces act to separate out the solids. The bulk of the solids will adhere on the walls of the bowl, while the liquid phase exits at the top of the centrifuge. As this type of system lacks a provision of solids rejection, the solids can only be removed by stopping the machine, dismantling it and scraping or flushing the solids out manually. Tubular bowl centrifuges have dewatering capacity, but limited solids capacity. Foaming can be a problem unless the system includes special skimming or centripetal pumps. Figure: Tubular bowl centrifuge This type of centrifuge can also be used to separate immiscible liquids. Rate of sediment can be control by controlling the inlet rate. The uses of centrifugal sedimenters include liquid/liquid separation, e. g. during antibiotic manufacture and purification of fish oils, the removal of very small particles, the removal of solids that are Compressible and which easily block the filter medium, The separation of blood plasma from whole blood (need C =3000).
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