Filtration in process chemistry

This slide is present in types of filtration and contruction process

1. FILTRATION Presented by- Puja R. Basule Department of Pharmaceutical Chemistry Smt. Kishoritai Bhoyar College of Pharmacy, Kamptee, Nagpur

2. Contents: Definition Mechanism of filtration Types of filtration Theories of filtration Factors influencing filtration Pressure filtration – 1) plate and frame filter press 2) leaf filters 3) automatic belt filters 4) chamber filter press 7. Vacuum filtration - 1) leaf filters 2) continuous rotary vacuum filters 3) discontinuous vacuum filters

3. Definition Filtration It may be define 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. Clarification When solid are present in very low concentration i.e. not exceeding 1.0% w/v, the process of its separation from liquid is called clarification .

4. Terms used in Filtration

5. Mechanism of Filtration Straining Impingement Entanglement Attractive Forces Similar to sieving i.e. particles of larger size can’t pass through smaller pore size of filter medium. Solids having the momentum move along the path of streaming flow and strike (impinge) the filter medium. Thus the solids are retained on the filter medium. Particles become entwined (entangled) in the masses of fibers (of cloths with fine hairy surface or porous felt) due to smaller size of particles than the pore size. Thus solids are retained within fiber medium. Solids are retained on the filter medium as a result of attractive force between particles and filter medium, as in case of electrostatic filtration.

6. Types of Filtration Surface/screen filtration It is a screening action by which pores or holes of medium prevent the passage of solids Mechanism involved: straining and impingement For this, plates with holes or woven sieves are used Efficacy is defined in terms of mean or maximum pore size Depth filtration In this slurry penetrates to a point where the diameter of solid particles is greater than that of the tortuous void or channel Mechanism: Entanglement The solids are retained with a gradient density structure by physical restriction or by adsorption properties or medium

7. Theories of Filtration The flow of liquid through 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- Rate= driving force/resistance The rate of filtration may be expressed as volume(liters) per unit time(dv/dt) Driving force=pressure upstream – pressure downstream Resistance is not constant It is increase with an increase in the deposition of solids on the filter medium Therefore filtration is not a steady state

8. The rate of flow will be greatest at the beginning of filtration process, since the resistance is minimum After forming of filter cake, its surface acts as filter medium and solids continuously deposit adding to thickness of the cake Powder or granule bed visualized as a bundle of capillaries Upstream pressure, P1 Surface area Length (L) Downstream pressure, P2 Viscosity flow rate: m3/unit time Fig.: Filtration process parameters Resistance to flow is related to several factors given in fig. Resistance to movement=(pressure upstream – pressure downstream)/length of capillaries

9. Poiseullie’s Equation Poiseullie considered that filtration is similar to the streamline flow of liquid under pressure through capillaries Poiseullie’s Equation is- V Where, V= rate of flow, /s (l/s) = Pressure difference across the filter, Pa r= radius of capillary in the filter bed, m L= thickness of filter cake (capillary length), m η =viscosity of filtrate, Pa.s If the cake is composed of bulky mass of particles and the liquid flows through the interstice, then flow of liquids through these may be expressed by this equation  

10. Darcy’s Equation Poiseullie’s law assumes that the capillaries found in the filter are highly irregular and non-uniform Therefore, if the length of capillary is taken as the thickness of bed, a correction factor for radius is applied so that the rate is closely approximated and simplified The factors influencing the rate of filtration has been incorporated into an equation by Darcy, which is: V Where, K= permeability coefficient of cake, A= surface area of porous bed (filter medium), Other terms are same as previous equation K depends on characteristics of cake, such as porosity, specific surface area and compressibility  

11. 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 equation is valid for liquids flowing through sand, glass beds and various porous media This model is applied to filter beds or cakes and other types of depth filter This equation is further modified by including characteristics of K by Kozeny-Carman

12. Kozeny-Carman (K-C) Equation Kozeny-Carman equation is widely used for filtration V Where, =porosity of cake (bed) S= specific surface area of particles comprising the cake / K= Kozeny constant (usually taken as 5) Other terms are same as previous equations Limitations: It does not consider the fact that depth of granular bed is lesser than the actual path traversed by the fluid The actual path is not same through out the bed, but it is sinuous or tortuous  

13. Factors influencing filtration Properties of solid Temperature Objectives Properties of solids in slurry Properties of liquids Particle shape Particle size Particle charge Density Particle size distribution Rigidity or compressibility of solids under pressure Tendency of particle to flocculate or adhere together Density Viscosity C orrosiveness Rate of formation of filter cake especially in early stages of filtration Whether the solids or liquid or both are to be collected Temperature of suspension

14. Pressure Filtration Pressure filters can apply a large pressure differential across the septum to give economically rapid filtration with viscous liquids or fine solids. Due to the formation of cakes of low permeability, many types of slurry require higher pressure difference for effective filtration than can be applied by vacuum techniques. Pressure filters are used for such operations. However , high operational pressures, may prohibit continuous operation because of the difficulty of discharging the cake while the filter is under pressure. Example is plate and frame filter press, Filter press, leaf filters, automatic belt filters, chamber filter pressure etc.

15. Principle: According to Kozeny-Carman equation, rate of filtration is directly proportional to the overall pressure drop across filter medium and filter cake. The most common method of obtaining a pressure difference is applying pressure on the surface of the slurry i.e. pumping the slurry onto the filter. However, i n the early stages of filtration, pressure difference should be less. This is to prevent the pores of the filter medium from clogging or plugging, which subsequently increases resistance to the flow.

16. Advantages Cakes are obtained with very low moisture content. Cakes may be disposed and flattened in layers provided they are not thixotropic and sufficiently stable to support a tractor. Intensive soluble recovery or removal of contaminants from the cake may be achieved. Clean filtrates may be produced by recirculating the filtrate for 1-2 minutes or by precoating if a clear filtrate is required right from the start. Solutions may be polished to a high degree of clarity. Except for Filter presses a gas tight construction may be provided. Except for Filter presses it is possible to jacket the tanks with steam if heat preservation is required. This is of particular importance for saturated brines. The filter bodies and internals may be constructed from a wide variety of alloys including synthetic materials for filter presses. Pressure Filters are available in a wide level of automation from labor intensive operator controlled up to fully automatic machines .

17. Disadvantages Cloth washing is difficult and if the solids are sticky a 3 mm precoat from diatomite or perlite is required. - This adds another step prior to filtration and if not done carefully and a too thin precoat is formed it may leave areas with exposed cloth which will blind rapidly. Conversely, if a too thick precoat is formed since the operator seeks to be on the safe side, such a precoat consumes effective cake volume. The operator can hardly see the forming cake and is unable to carry out an inspection while the filter is in operation. The internals are difficult to clean and this may be a problem with food grade applications.

18. 1. Plate and Frame Filter Press A filter press contains a set of plates designed to provide a series of chambers or compartments in which solids may collect. The plates are covered with a filter medium such as canvas. Slurry is admitted to each compartment under pressure; liquor passes through the canvas and out a discharge pipe, leaving a wet cake of solids behind. Construction Consist of plates and frames assembled alternately with a filter cloth over each side of the plates The plates have channels cut in them so that clear filtrate liquid can drain down along each plate 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

19. The filtrate flows between the filter cloth and the face of the plate though the channels of the outlet The filtration proceeds until the frames are completely filled with solids Fig. Filter press. Assembly of plates and frame

20. Principle of operation

21. Working : The slurry enters the frame from the feed channel The filtrate passes through the filter medium on to the surface of the plate while the solids form a filter cake in the frame. The filtrate then drained down the surface of the plate , between the projections on the surface and escapes from the outlet. Filtration is continued until the frame is filled with filter cake, when the process is stopped , the frame emptied, and the cycle re-started .

22. Advantages &disadvantages Advantages: 1. Construction is very simple and a wide variety of materials can be used. 2. It provides a large filtering area in a relatively small floor space. 3. It is versatile, the capacity being variable according to the thickness of the frames and the number used. 4. The construction permits the use of considerable pressure difference. 5. Efficient washing of the cake is possible. 6. Operation and maintenance is straightforward , because there no moving parts , filter cloths are easily renewable and, because all joints are external, any leaks are visible and do not contaminate the filtrate.

23. Disadvantages: It is a batch filter, so it is a time consuming. The filter press is an expensive filter, the emptying time, the labor involved, and the wear and tear on the cloths resulting in high costs. Operation is critical, as the frames should be full, otherwise washing is inefficient and the cake is difficult to remove. The filter press is used only for slurries containing less about 5 % solids

24. 2. Leaf filters Filter leaf is an apparatus consisting of a longitudinal drainage screen covered with a filter cloth. The mechanism is surface filtration and acts as sieve or strainer. The leaf filter was developed for larger volumes of slurry and more efficient washing Each leaf is a hollow wire framework covered by a sack of filter cloth A number of these leaves are hung in parallel in a closed tank The slurry enters the tank and is forced under pressure through the filter cloth, where the cake deposits on the outside of the leaf The filtrate flows inside the hollow framework and out a header The wash liquid follows the same path as the slurry. Hence, the washing is more efficient than the through wishing in plate-and-frame filter presses

25. Construction The core of the leaf, whether round or rectangular is a piece of heavy, coarse-mesh double-crimped wire screen. Often over this is a backing of finer screen, but still too coarse to act as a filter medium. Around the outside edges of this assembly is a light piece of metal of U-shaped cross section that binds the edges. To this binding is attached a discharge nipple that serves both to remove filtrate and suspend the leaf. Over the leaf is drawn a sack of filter cloth, sewn tightly around the edge of the leaf and caught under a collar around the discharge nipple.

26. The nipple passes through a collar on the filter body and may be tightened by cap nut. Gasket between the nipple and the inside of the filter shell and between the cap nut and the nipple make tight joints. Connections between the nipple and the discharge manifold include valves and sight glass, the latter being held between stuffing boxes. From the upper valve a connection leads to the discharge manifold.

27. 3. Automatic belt filter It 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 the other. A belt of filter cloth passes through 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 a flexible diaphragm in the chamber ceiling, squeezing the cake and mechanically expressing some of the liquid. With the diaphragm released, wash water may be passed through the cake and the cake recompressed by the diaphragm if desired. Finally air is blown through the cake to remove additional liquid .

28. Fig. Larox automatic pressure filter showing path of filter belt and mechanism of cake discharge.

29. 4. Chamber filter presses Construction and working A heavy fixed head of cast iron is mounted on a suitable frame and has the necessary pipe connections on it. Extending from this head are two horizontal bars, supported at the other end by the end frame. These supports the plates of the press by lungs. In the center of each plate is a hole, which is in line with a connections on the head of the press where the feed is introduced. Over each plate is thrown a sheet of filter cloth with a hole cut in the center to register with the hole in the plate. The filter cloth is fastened to the plate at this point by means of rings, called grommets, which either screw together or lock with a bayonet lock.

30. Lugs Lugs Feed channel Outlet ports Front elevation of plate and a typical section through along the line MM When all the plates have been so dressed, a heavy follower plate is placed behind the whole assembly and the plates are pressed tightly together by means of a heavy screw or a hydraulic pressure device. Under pressure the cloths are forced back against the face of the plate. When sufficient cake has accumulated to fill the chambers, the closing screw is released, the follower is drawn back along the supporting rails, the plates are drawn back, and the cakes discharged, one at a time.

31. Vacuum filtration F iltration technique where suction is applied to the mixture of solid and liquid being filtered so as to pull the liquid through the filter.  This technique is much faster than gravity filtration.  Vacuum filtration usually involves the use of a vacuum filter flask, a filter trap, a water pump, and either a Buchner funnel or sintered glass crucible.  The liquid mixture is poured into the filter (Buchner or sintered glass), the liquid is sucked down into the filter flask by the action of the water pump, and any excess liquid that overflows out of the filter flask is caught in the filter trap.  The solid matter is left on the surface of the filter .

32. Principle: The pressure underneath the filter medium may be reduced below atmospheric pressure by connecting the filtrate receiver to a vacuum pump. This creates a pressure differential across the filter. In   vacuum filters , the driving force for filtration results from the application of suction on the filtrate side of the medium. Although the theoretical pressure drop available for vacuum filtration is 100 kPa, =1 Kg/cm2 in practice it is often limited to 70 or 80 kPa or even less.

33. A dvantages : 1. Gravity settling can take place before the vacuum is applied. In many cases, this may prevent excessive blinding of the cloth due to action of a pre-coat formed by the coarser particles. 2. Heavy or coarse materials can be filtered because if they settle out from the feed they do so onto the filter surface. 3. Fine particle penetration through the medium can be tolerated because the initial filtrate can be recycled back onto the belt. 4. Top-feed filters are ideal for cake washing, cake dewatering and other process operations such as leaching. 5. A high degree of control can be exercised over cake formation. Allowances can be made for changed feeds and/or different cake quality requirements. This is particularly true of the horizontal belt vacuum filters. With these units the relative proportions of the belt allocated to filtration, washing, drying, etc., as well as the belt speed and vacuum quality, can be easily altered to suit process changes. D rawbacks : Such filters usually require large floor areas. 2. Their capital cost is high.

34. 1. The leaf filter : -The leaf filter is consisting of a frame enclosing a drainage screen or grooved plate, the whole unite being covered with filter cloth. -The outlet for the filtrate connects to the inside of the frame, the general arrangement is shown in the Fig. which represents a vertical section through the leaf. The frame may be circular, square or rectangular shapes. Working : The leaf filter is immersed in the slurry and a receiver and a vacuum system connected to the filtrate outlet. Slurry passed through the filter cloth and finally filtrate enters the Drainage canal and goes through outlet into the receiver. Air is passed to flow in reverse direction which facilitates removal of cake

35. Advantages: 1- The slurry can be filtered from any vessel. 2-The cake can be washed simply by immersing the filter in a vessel of Water. 3- Removal of the cake is facilitated by the use of reverse air flow . 4- The filter can be modified by employing a suitable number of unites. 5- The leaf filter is most satisfactory if the solids content of the slurry is no too high, 5 % being a suitable maximum. 6- Labor costs for operating the filter are comparatively moderate. An alternative method is to enclose the filter leaf in a special vessel into which the slurry is pumped under pressure . A number of leaves are connected to a common outlet, to provide a large area for filtration e.g. Sweet land filter.

36. 2 . Continuous Rotary Vacuum Filter :

37. Rotary vacuum filter (Rotary filter) The most common type of continuous vacuum filter. The rotary drum filter in which liquor is sucked through a moving septum to deposit a cake of solids. Cake is moved out of the filtering zone, washed, sucked dry and dislodged from the septum . Construction It is consisting on a sheet metal drum. A vacuum system is also attached. The surface of the drum is the divided into different section by stripes. Filter medium is hold by metal strips on drum and covered by filter cloth. In each section there are many pipes which open inside the cover plates. To remove the cake “Doctor knife” is provided.

38. Rotary vacuum filter

39. Working 1. Before filtration rotary drum filter is dip in tank of filter aid for precoating (e.g. diatomaceous earth, perlite or purified wood cellulose). 2. Drum is rotated in slurry tank at a speed of 1 to 2 rpm. 1/3 of drum is dip in slurry tank and vacuum is applied. 3. Cake formation. 4. As , the panel leave the cake formation zone it enters to washing zone. 5. As , the panel leave the cake formation zone it enters to washing zone. 6. Cake removed in cake removal zone.

40. Industrial Application These filter are developed to overcome large problem of filter press and leaf filter. It is very versatile filter. It is used generally for those operation in which the volume is very large. It is used where colloidal particles. It is used for production of wallboard from gypsum.

41. Advantages: 1- The rotary filter is automatic and is continuous in operation, so that the labor costs are very low. 2- The filter has a large capacity , so it is suitable for the filtration of highly concentrated solutions. 3- Variation of the speed of rotation enables the cake thickness to be controlled. 4- Pre-coat of filter aid could used to accelerate the filtration rate. Disadvantages: 1- The rotary filter is a complex piece of equipment , with many moving parts and is very expensive,. 2- In addition to the filter itself, some accessories are connected , e.g, a vacuum pump, vacuum receivers , slurry pumps and agitators are required . 3 - The cake tends to crack due to the air drawn through by the vacuum system, so that washing and drying are not efficient. 4- Being a vacuum filter, the pressure difference is limited to 1 bar and hot filtrates may boil. 5 - It is suitable only for straight- forward slurries.

42. 3. Discontinuous vacuum filters A discontinuous vacuum filter, however, is sometimes a useful tool. A vacuum nutsch is little more than a large Buchner funnel, 1 to 3m (3-10 ft) in diameter and forming a layer of solids 100 to 300 mm (4-12 in.) thick. Because of its simplicity, a nutsch can readily be made of corrosion-resistant materials and is valuable where experimental batches of a variety of corrosive materials are to be filtered. Nutsches are uncommon in large-scale processes because of the labor involved in digging out the cake; they are, however, useful as pressure filters in combination filter-dryers for certain kinds of batch operations.

43. Thank you!