Filtration.

Presentation on theme: "Filtration."— Presentation transcript:

1 Filtration

2 Dead-end or Conventional Filtration
FILTRATION defined as the separation of a solid from a fluid (The term fluid includes liquids and gases) by means of a porous medium that retains the solid but allows the fluid to pass. The principal pharmaceutical application of gas filtration is the treatment of air to produce sterile atmospheres. Feed Permeate Retentate Dead-end or Conventional Filtration Crossflow Filtration

3 The following terms must be concerned
The slurry is the suspension of solid and liquid to be filtered. The filter medium is the porous medium used to retain the solids. The filter cake is the accumulation of solids on the filter. The filtrate is the clear liquid passing through the filter.

4 FACTORS INFLUENCE THE CHOICE OF METHOD OF FILTRATION AND THE RATE OF FILTRATION
The unit operation of filtration is affected by the characteristics of the slurry, including: The properties of the liquid: such as density, viscosity, and corrosiveness. The properties of the solid: such as, particle shape, particle size, particle size distribution, and the rigidity or compressibility of the solid. Whether the objective is to collect the solid, the liquid, or both.

5 RATE OF FILTRATION Darcy's law correlating the rate factors as following: Where: V = volume of filtrate. t = time of filtration. K = constant for the filter medium and the filter cake. A = area of filter medium. ∆P = pressure drop across the filter medium and filter cake. η = viscosity of the filtrate. l = thickness of cake. =

6 Properties of the Filter Medium and Filter Cake
The constant (K) represents the resistance of the filter medium and filter cake. The resistance of the filter medium is of significance on the laboratory scale, it is relatively less important on the large scale and can usually be neglected. The magnitude of the resistance of the filter medium will change due to the early layers of solids which may block the pores or may form bridges over the entrances to the channels.

7 For this reason, the pressure should be kept low at the start, to avoid the pores `plugging', and increased as the cake builds up. The usual procedure is to filter at constant rate & increasing the pressure as necessary. When the normal working pressure is reached it is maintained, hence the rate of filtration decreases as the thickness of the cake increases. When the rate is uneconomically low, filtration is stopped, the filter cake removed, and filtration restarted.

8 The principal properties of the solid affecting the resistance of the cake are:
The surface area of the particles. The porosity of the cake. Rigidity or compressibility of the particles. Thus, any decrease in the resistance of the cake will show as an increased value of K, so increasing the rate of filtration. This can be done, for example, by using filter aids.

9 Area of Filter The total volume of filtrate flowing from the filter will be proportional to the area of the filter. Hence, this rate can be increased by: using larger filters using a number of small units in parallel.

10 In the rotary filter, the filter cake is removed continuously, giving, in effect, an infinite area for filtration.

11 Pressure Drop Reduced pressure Pressure Gravity Centrifugal force
The rate of filtration is proportional to the overall pressure drop across both the filter medium and filter cake. The pressure drop can be achieved in a number of ways: Reduced pressure Pressure Gravity Centrifugal force

12 1. REDUCED PRESSURE The pressure below the filter medium may be reduced below atmospheric pressure by connecting the filtrate receiver to a vacuum pump and creating a pressure differential across the filter. Advantage of reduced pressure is the safety, since, if part of the equipment fails, it will collapse and not explode. Disadvantage is that reduction of pressure lowers the boiling point of liquids so that it is possible for the filtrate to boil in the receiver causing loss of liquid, and the vapour may be damaging to the vacuum pump.

13 2. PRESSURE 4. CENTRIFUGAL FORCE 3. GRAVITY
The commonest method is to obtain a pressure difference by applying pressure to the surface of the slurry by pumping the slurry into the filter under pressure. 3. GRAVITY A simple method of obtaining a pressure difference is by maintaining a head of slurry above the filter medium. 4. CENTRIFUGAL FORCE The gravitational force can be used successfully in filtration processes.

14 Viscosity of Filtrate An increase in the viscosity of the filtrate will increase the resistance to flow, so that the rate of filtration is inversely proportional to the viscosity of the fluid. The rate of filtration may be increased by raising the temperature of the liquid, which lowers its viscosity. (This may not be practicable if thermolabile materials are involved or if the filtrate is volatile). Dilution is another alternative, but may not always be acceptable.

15 Thickness of Filter Cake
The filtrate must flow through the filter cake formed on the filter medium as filtration progresses; hence the rate of flow will be inversely proportional to the thickness of the cake. Cake thickness will be affected by area of filtration, so that for a given amount of slurry, increase in area will decrease the cake thickness. Also, in the rotary filter the filter cake is removed continuously, so that the cake thickness is minimized.

16 Mechanisms of Filtration
STRAINING IMPINGEMENT ENTANGLEMENT ATTRACTIVE FORCES

17 1) STRAINING The simplest filtration procedure is straining, in which, like sieving, the pores are smaller than the particles, which are retained on the filter medium.

18 2) IMPINGEMENT As a flowing fluid approaches an object such as a cylinder, the flow pattern is displaced, as shown diagrammatically by the streamlines

19 Solids having force would cross the streamlines and strike the cylinder, but, in practice, some will follow the streamlines and will not be collected or will be re-entrained. Thus, particles between streamlines A and B will be collected, and the accumulation of solids will form a ridge, roughly triangular in section, as indicated in the figure.

20 3) ENTANGLEMENT If the filter medium consists of a cloth with a nap or is a porous felt, particles become entangled in the mass of fibers. Usually the particles are smaller than the pores, so that it is possible that impingement is involved.

21 4) ATTRACTIVE FORCES In certain cases, particles may collect on a filter medium as a result of attractive forces due to the electrostatic precipitation, where large potential differences are used to remove particles from air streams. In practice, the process may combine the various mechanisms, but the solids removal is mainly by a straining mechanism, once the first complete layer of solids has begun to form the cake on the filter medium.

22 Filter Media The filter medium may be responsible for the collection of the solids, or a support for the filter cake. The filter medium must be strong, have a low resistance to flow, and be unaffected by the substances in the slurry.

23 Materials used for filter media include:
Woven materials, such as felts or cloths in wool, cotton, silk, glass, metal or synthetic fibers (rayon, nylon, etc.). Perforated sheet metal. Beds of granular solids built up on a supporting medium. The solids may vary in size according to the needs of the process.

24 Typical examples of granular solids used for this purpose include gravel, sand, asbestos, paper pulp, and kieselguhr. The final choice of filter medium will depend on the chemical nature of the slurry.

25 Filter Aids The resistance to flow due to the filter medium itself is very low, but will increase as a layer of solids builds up, blocking the pores of the medium and forming a solid, impermeable cake. The object of the filter aid is: To prevent the medium from becoming blocked. To form an open, porous cake, so reducing the resistance to flow of the filtrate.

26 The filter aid must be a light, porous, inert solid and may be used in either or both of two ways.
Firstly, by forming a pre-coat over the medium by filtering a suspension of the filter aid sufficient to give a coating up to 0.5 kg/m2. Secondly, a small proportion of the filter aid (0.1 to 0.5 percent) is added to the slurry, ensuring that the filter cake has a porous structure.

27 Care must be taken in the choice of the filter aid, as materials with adsorptive properties (such as kaolin) could remove solutes from the filtrate; this may be done intentionally, as sugar solutions may be decolorised by means of charcoal. Kieselguhr is a very successful filter aid that 0.1 % added to slurry with 20 % solids, resulted in an increase in the rate of filtration of 5 times or more. Filter aids are limited to clarification processes when the solids are discarded.

28 Filter Leaf Industrial Filters
The filter leaf consist of a frame enclosing a drainage screen or grooved plate, the whole unit is covered with filter cloth. The outlet for the filtrate connects to the inside of the frame. The frame may be of any shape, circular, square or rectangular shapes.

29 Principle of work The filter leaf is immersed in the slurry
A receiver and vacuum screen system connected to the filtrate outlet. The slurry may be pumped under pressure. The leaf is constructed with ribs on both sides to allow free flow of filtrate towards the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake. Each leaf has a centrally located neck at its bottom which is inserted into a manifold that collects the filtrate.

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32 A typical example is the Sweetland filter, where it will be seen that the vessel is cylindrical and arranged so that it is supported by the upper part, and the lower part can be swung away. This permits the cake to be discharged by compressed air without removing the filter leaves from the vessel.

33 Advantages: 1.The slurry can be filtered from any vessel and the cake can be washed simply by immersing the filter in a vessel of water. Removal of the cake is facilitated by the use of reverse air flow. 2. The pressure difference may be obtained with vacuum or by using pressures up to the order of 8 bars. 3. The area can be varied by employing a suitable number of units.

34 The leaf filter is most suitable if the solids content of the slurry is not too high, about 5 % maximum. A higher proportion results in excessive non-productive time while the filter is being emptied and, provided this is observed, with cost operating the filter. Application

35 (PLATE AND FRAME PRESS)
Filter Press (PLATE AND FRAME PRESS) This press is made up of two types of units, Plates and frames, with a filter medium, usually filter cloth, between the two. The frame is open, with an inlet for the slurry, while the plate has a grooved surface to support the filter cloth, and with an outlet for the filtrate.

36 Principle of operation
The slurry enters the frame (marked by 2 dots) from the feed channel the filtrate passes through the filter medium on to the surface of the plate (marked by 1 dot), while the solids form a filter cake in the frame. The filtrate then drains down the surface of the plate, between the projections on the surface and escapes from the outlet.

37 Filtration is continued until the frame is filled with filter cake, when the process is stopped, the frame emptied, and the cycle restarted. In practice, a large number of plates and frames are arranged alternately and clamped in a supporting structure. This gives a number of filtration units operating in parallel and a filtration area as large as necessary. The thickness of the cake can be varied by using frames of different thicknesses.

38 Plates and frames may be made in various metals to provide resistance to corrosion or prevent metallic contamination of the product. Non-metals avoid this problem and many have the advantage of lightness as reinforced plastics Certain varieties of wood are materials of construction, provided the press is kept wet at all times to avoid shrinkage.

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42 Advantages (a) Construction is very simple and a wide variety of materials can be used, as cast iron, stainless steel, hard rubber or plastics. (b) It provides a large filtering area in a relatively small floor space. (c) The capacity being variable according to the thickness of the frames and the number used.

43 (d) The strong construction permits the use of considerable pressure difference up to about 20 bars.
(e) Efficient washing of the cake is possible. (f) Operation and maintenance is simple, because there are no moving parts, filter cloths are easily renewable (g) Because all joints are external, any leaks are visible and do not contaminate the filtrate.

44 Disadvantages (a) It is a batch filter.
(b) The filter press is an expensive filter, the emptying time, the effort involved, and the wear and tear on the cloths resulting in high costs. (c) Operation is critical, as the frames should be full; otherwise washing is inefficient and the cake is difficult to remove.

45 (d) The filter press is used for slurries containing less than about 5 % solids;
In view of the high work costs, it is most suitable for expensive materials, as the collection of bismuth salts, the collection of precipitated antitoxins, and the removal of precipitated proteins from insulin liquors.

46 Rotary Filter

47 Principle of work In operation, The drum rotates slowly about its horizontal axis and is partially submerged in a slurry reservoir, so that each unit passes through the various zones. The surface of the drum is covered with the filter medium and a vacuum is maintained below the medium on the inside of the drum

48 Filtration is followed by cake washing and dewatering
Liquid is sucked through the filter medium and solids in the feed are retained on the medium Filtration is followed by cake washing and dewatering The outlet of the leaf is connected to compressed air, which detaches the cake from the filter medium. If the cake is not completely removed, this can be effected by scraping with a knife.

49 Rotary Filter Operation
Connected to Service Position Zone Filtrate receiver Vacuum Slurry trough Pick-up Drainage Wash water receiver Wash sprays Washing Wash receiver Drying Filter cake conveyor Compressed air Scraper knife Cake removal

50 Rotary filters Cake Formation Zone Cake Predrying Zone
Cake washing Zone Rotary filters Cake Final Drying Zone Cake Discharge Zone Cake Dead Zone

51 The commonest form in use in the pharmaceutical industry is the rotary drum filter
The filter units have the shape of longitudinal segments of the periphery of a cylinder. Each filter unit is rectangular in shape with a curved profile so that a number can be joined up to form a drum. Each unit has a perforated metal surface to the outer part of the drum and is covered with filter cloth. Appropriate connections are again made from each unit through a rotating valve at the centre of the drum.

52 Special attachments may be included for particular purposes;
Cake compression rollers may be fitted if the cake shrinks and cracks as it dries out. It compress the cake to a homogeneous mass to improve the efficiency of washing as the cake passes through the washing zone, or to aid drainage of wash water as the cake passes through the drying zone.

53 A pre-coat filter may be used Where the solids of the slurry block the filter cloth.
The pre-coat of filter aid is deposited on the drum prior to the filtration process. Thus the scraper knife removes the solid filtered from the slurry together with a small amount of the pre-coat, the knife advancing slowly as the pre-coat is removed.

54 The string discharge rotary filter is especially useful if removal of the cake presents problems for certain pharmaceutical applications, particularly for filtering the fermentation liquor in the manufacture of antibiotics where the mould is difficult to filter by ordinary methods because it forms a felt-like `cake'. The string discharge filter is operated by means of a number of loops of string which pass round the drum, and cause the cake to form over the strings.

55 The strings are in contact with the surface of the drum up to the cake removal zone, where they leave the surface and pass over additional small rollers before returning to again contact the drum. In operation, the strings lift the filter cake off the filter medium, and the cake is broken by the sharp bend over the rollers so that it is easily collected while the strings return to the drum.

56 Advantages (a) The rotary filter is automatic and is continuous in operation, so that costs are very low. (b) The filter has a large capacity, the area of the filter as represented by A of Darcy's law is infinity. (C) Suitable for use with concentrated slurries since the system removes the cake continuously. (d) Variation of the speed of rotation enables the cake thickness to be controlled, and for solids that form an impenetrable cake the thickness may be limited to less than 5 mm. If the solids are coarse, forming a porous cake, the thickness may be 100 mm or more.

57 Disadvantages (a) The rotary filter is a complex equipment, with many moving parts, and is very expensive. (b) The cake tends to crack due to the air drawn through by the vacuum system, so that washing and drying are not efficient. (c) Being a vacuum filter, the pressure difference is limited to 1 bar and hot filtrates may boil. (d) The rotary filter is suitable only for straight­forward slurries, but if the solids form an impermeable cake or will not separate cleanly from the cloth it will be less satisfactory.

58 USES OF THE ROTARY FILTER
The rotary filter is most suitable for continuous operation on large quantities of slurry, especially if the slurry contains considerable amounts of solids, that is, in the range 15 to 30 per cent. Examples of pharmaceutical applications include the collection of calcium carbonate, magnesium carbonate, and starch, and the separation of the mycelium from the fermentation liquor in the manufacture of antibiotics.

59 Edge Filters Filters such as the leaf or press act by presenting a surface of the filter medium to the slurry. Edge filters use a pack of the filter medium, so that filtration occurs on the edges. Forms using packs of media such as filter paper can be used, but in the pharmaceutical industry greatest use is made of the Metafilter.

60 METAFILTER (a) surface view of ring (b) section through filter
The metafilter consists of a grooved drainage rod on which is packed a series of metal rings. These rings, usually of stainless steel, are about 15 mm inside diameter, 22 mm outside diameter, and 0.8 mm in thickness, with a number of semicircular projections on one surface. (a) surface view of ring (b) section through filter

61 (a) surface view of ring (b) section through filter
When the rings are packed together, all the same way up, and tightened on the drainage rod with a nut, channels are formed that taper from about 250µm down to 25µm, One or more of these packs is mounted in a vessel, and the filter may be operated by pumping in the slurry under pressure or, by the application of reduced pressure to the outlet side. (a) surface view of ring (b) section through filter

62 Advantages (a) The metafilter possesses strength that high pressures can be used, with no danger of bursting the filter medium. (b) As there is no filter medium, the running costs are low, and it is a very economical filter. (c) The metafilter can be made from materials that can provide excellent resistance to corrosion and avoid contamination of the most sensitive product. (d) Removal of the cake is effectively carried out by back-flushing with water.

63 USES OF THE METAFILTER The small surface area of the metafilter restricts the amount of solid that can be collected. This, together with the ability to separate very fine particles, means that the metafilter is used almost for clarification purposes. The strength of the metafilter permits the use of high pressures (up to 15 bars) making the method suitable for viscous liquids. It can be constructed in materials appropriate for corrosive substances. Specific examples of pharmaceutical uses include the clarification of syrups, of injection solutions, and of products such as insulin liquors.