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Objectives, applications & Theories in FILTRATION
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- 2. FILTRATION : Definitions •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 w/v, the process of its separation from liquid is called clarification. concentration, i.e., not exceeding 1.0% • Terms used in Filtration Slurry - Suspension to be filtered Filter medium - Porous medium used to retain solid Filter cake - Accumulated solids on the filter Filtrate - Clear liquid passing through the filter Process of filtration Pores of filter medium are smaller than size of particles to be separate. Rate of Filtration-Volume of filtrate collected in unit time.
- 3. Applications of filtration Productionof sterile products Production of bulk drugs Production of liquid dosage formulation Dewaxing of oils Removing suspended oils from aqueous solutions Removing of undesirable solids Clarifying the potable water Effluents and waste water treatment.
- 4. Mechanism of filtration: Themechanism whereby particles are retained by a filter is significant only in initial stages of filtration. 1. Straining - Similar to sieving, i.e., particles of larger size can’t pass through smaller pore size of filter medium. 2. Impingement - 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. 3. Entanglement - Particles become entwined (entangled) in the masses of fibres (of cloths with fine hairy surface or porous felt) due to smaller size of particles than the pore size.Thus solids are retained within filter medium. 4. Attractive forces - Solids are retained on the filter medium as a result of attractive force between particles and filter medium, as in case of electrostatic filtration.
- 5. Types of filtration 1.Surface/ screen filtration – It is screenings 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. 2. 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 of medium.
- 6. Theories of filtration: •Basic rules: that govern the flow of any liquid through the medium offering resistance. • Rate = driving force / resistance time (dv/dt). • The rate of filtration may be expressed as volume (litres) per unit • Driving force = pressure upstream – pressure downstream • 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 filtration process, since the resistance is minimum. solids continuously • After forming of filter cake, its surface acts as filter medium and deposit adding to thickness of the cake.
- 7. 1. Poiseullie’s Equation:Poiseullie considered that filtration is similar to the streamline flow of liquid under pressure through capillaries. • Poiseullie’s Equation is- V= 𝝅Δ𝑷𝒓𝟒/𝟖𝑳ƞ • Where, V = rate of flow, m3 /s (l/s), ΔP= Pressure difference across the filter, r = radius of capillary in the filter bed, L = thickness of filter cake (capillary length), 𝜂 = viscosity of filtrate. • 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. 2. Darcy’s Equation: The factors influencing the rate of filtration has been incorporated into an equation by Darcy, that mostly affected to the permeability, which is: V= KAΔ𝑷/ƞL • Where, K = permeability coefficient of cake, m2 A = surface area of porous bed (filter medium), m2surface area and K depends on characteristics of cake, such as porosity, specific 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 beds and various porous. • This equation is valid for liquids flowing through sand, glass media. This model is applied to filter beds or cakes and other types of depth filter. • 3. Kozeny-Carman (K-C) equation: Kozeny-Carman equation is widely used for filtration that mainly describes porosity of the cake. V= A/ȠS2*ΔP/KL*€3/(1-€)2 • Where, € = porosity of cake (bed) S = specific surface area of particles comprising the cake m2 / m3 K = Kozeny constant (usually taken as 5) • 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 throughout the bed, but it is sinuous or tortuous.
- 8. Factors influencing filtration 1.Surface area of filter medium – Rate of filtration is Directly proportional to surface area of filter medium (According to Darcy’s equation) Rate can be increased either using large filter or connecting a number of small units in parallel. 2. Pressure drop across the filter medium: According to K-C equation the rate of filtration is directly proportional to the overall pressure drop across both the filter medium and filter cake. 3. Viscosity of filtrate: According to K-C equation rate of filtration is inversely proportional to the viscosity of the fluid. This is an increase in the viscosity of the filtrate will increase the resistance of flow. 4. Temperature of liquid to be filtered: 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. 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 ofthe cake and decreases the rate of filtration.
- 9. Material used asfilter media • Woven material: Made up of wool, silk, metal or synthetic fibres (rayon, nylon etc.). These include a- wire screening and b- fabrics of cotton, wool, nylon. Wire screening e.g. stainless steel is durable, resistance to plugging and easily cleaned. Cotton is a common filter, however, Nylon is superior for pharmaceutical use, since it is unaffected by mold, fungus or bacteria and has negligible absorption properties. The choice of fibre depends on chemical reactivity with the slurry • Perforated sheet metal: Stainless steel plates have pores which act as channels as in case of meta filters. • Membrane filter media: These are cartridge units and are economical and available in pore size of 100 μm to even less than 0.2 μm. Can be either surface cartridges or depth type cartridges. • Surface cartridges: These are corrugated and resin treated papers and used in hydraulic lines. Ceramic cartridges and porcelain filter candles are examples. Can be reuse after cleaning. • Depth type cartridges: Made up of cotton, asbestos or cellulose. These are disposable items, since cleaning is not feasible.
- 10. Classification of filtrationequipments- Based on application of external force: • Pressure filter – Plate and frame filter press, metafilter, • Vacuum filters – Filter leaf, Centrifugal filters Based on operation of filtration • Continuous filtration - Discharge and filtrate are separated steadily and uninterrupted • Discontinuous filtration - Discharge of filtered solid is intermittent. Filtrate is removed continuously. Operation should be stopped to collect solid. Based on nature of filtration • Cake filters - Remove large amount of solids • Clarifying filters - Remove small amounts of solids • Cross-flow filters - Feed of suspension flows under pressure at fairly high velocity across the filter medium.
- 11. Plate and framefilter press • Principle: Mechanism is surface filtration. The slurry enters the frame by pressure and flows through filter medium. The filtrate is collected on the plates and send to outlet and A number of frames and plates are used so that surface area consequently large volumes of slurry can be processed simultaneously with or without washing. • Uses: Sterile filtrate can by obtain by using asbestos and cellulose filter press (for this whole filter press and filter medium have been sterilized previously). Filtration of viscous liquid can also be done by incorporating heating/cooling coils in filter the press. • Advantages: Construction of filter press is very simple and a variety of materials can be used. Provide large filtration area in relatively small floor space. The capacity being variable according to thickness of frames and number used. Sturdy construction permits the use of considerable pressure Kilopascals normally used. Efficient washing of cake is possible. Operation and maintenance is easy. It produces dry cake in form of slab. • Disadvantages: It is a batch filter, so it is a time consuming. The filter press is an expensive filter, the emptying time, the labour involved, and the wear and tear on the cloths resulting in high costs. Washing is inefficient when frame is full with filterate. Operation is critical, as the frames should be full, otherwise and the cake is difficult to remove. The filter press is used for slurries containing less about 5 % high labour costs , it is most suitable for expensive materials e.g. the removal of precipitated proteins from insulin liquors.
- 12. Construction • The Filterpress is made of two types of units, plate and frames. • Usually made of aluminium alloy. • Sometimes, these are also lacquered for protection against corrosive chemicals and made suitable for steam sterilization. Frame- • It contains a open space inside wherein the slurry reservoir is maintained for filtration and an inlet to receive the slurry. • It is indicated by two dots in description. • Frames of different thickness are available. • It is selected based on the thickness of cake formed during filtration. • Optimum thickness of frame should be chosen. Plate • The plate has a studded or grooved surface to support the filter cloth and an outlet. • It is indicated by one dot in description. • Plate supports the filter medium, receiving the filtrate and outlet.The filter medium usually cloth is interposed between plate and frame. • Plate, filter medium, frame, filter medium and plate are arranged in sequence and clamed to a supporting structure. • It is normally described by dots as 1.2.1.2.1 so on. • A number of plates and frames are employed so that the filtration area is as large as necessary. • Number of filtration units are operated in parallel. • Channels for slurry inlet and filtrate outlet can be arranged by fitting eyes to the plates and frames, these join together to form a channel. • In some types only one inlet channel is formed, while each plate outlets controlled by valves.
- 13. Working :Working canbe divided into two steps 1. Filtration operation • Frame- marked by 2 dots • Plate – marked by 1 dot • Slurry enters the frame from the feed channel and passes through the filter medium on the surface of the plate. • The solid forms a filter cake and remain in the frame. • The thickness of the cake is half of the frame thickness, because on each side of frame filtration occurs Thus two filter cakes are formed , which meet eventually in the centre of the frame. • The filtrate drains between the projections of the surface of the plate and escape from the outlet As filtration proceeds, the resistance of the cake increases and filtration rate decrease At a certain point process is stopped and press is emptied and cycle is restarted. • Washing of cake (if desirable) When washing of cake is also required modified plate and frame filter is used. • For this purpose an additional channel is included called as washing plate and are identified by 3 dots. • Water cannel to the In the half of the washing plate, there is a connection from wash surface of plate. • The sequence of arrangement of plates and frames can be represented by dots as 1.2.3.2.1.2.3.2.1 so on (between 1 and 1, 2.3.2 must be arranged. Procedure for washing the press: • Filtration proceeds in the ordinary way until the frames are filled with cake. • To wash the filter cake, the outlets of washing plates are closed. • Wash water is pumped in the washing channel. The water enters through the inlets on the surface of washing plate. • Water passes through the filter cloth and enters frame which contains the cake. • Then water washes the cake, passes through the filter cloth and enters the plate down the surface. • Finally washed water escapes through the outlet of that plate.
- 15. Filter leaf Principle: • Itis an apparatus consisting of a longitudinal drainage screen covered with a filter cloth. • The mechanism is surface filtration and acts as sieve or strainer. • Vacuum or pressure can be applied to increase the rate of filtration. Use: Use for the filtration of slurry which do not contain high solid content, about 5%, i.e. dilute suspensions. Advantages: • Simplest form of filter used for batch process. • A number of units can be connected in parallel to increase the surface area of filtration. • Pressure difference can be obtained either with vacuum or using pressure up to the order of 800 kilopascals. • Labour costs for operating the filter leaf are fairly moderate. • The efficiency of washing is high. • The slurry can be filtered from any vessel.
- 16. Construction: The leaffilter is consisting of a frame enclosing a drainage screen or grooved plate. The frame may be any shape circular, square or rectangular. The whole unite being covered with filter cloth. The outlet for the filtrate connects to the inside of the frame through suction. Working The filter leaf is immersed in the slurry. Vacuum system is connected to the outlet. The slurry passes through the filter cloth. Finally filtrate enters the drainage canal and goes through the outlet into receiver. Air is passed to flow in reverse direction which facilitates removal of cake.
- 18. Metafilter/ Edge filters Principle: •Mechanism is surface filtration. In this, metal rings contain semicircular projections, which are arranged as a nest to form channels on the edges. This channel offers resistance (strainer) to the flow of solids (coarse particles). The clear liquid is collected into receiver from the top. A number of leaves are connected to a common outlet, to provide a large area for filtration. • Uses: Clarification of syrups Filtration of injection solutions, Clarification of insulin liquors Filtration of viscous liquids can be achieved by applying pressure. • Advantages: Can be used under high pressures, without any danger of bursting the filter medium. Running cost is low, as separate filter medium is not used. Can be constructed from a material that can provide excellent resistance to corrosion and avoid contamination of sensitive products. It is extremely versatile filter because fine as well as large both type of particles can be separated. Removal of cake can be carried out by simply back- flushing with water. Change over from one batch to another or one product to another is easy. Sterile products can be handled.
- 19. Construction • Meta filterconsists of a series of metal rings. • These are threaded so that a channel is formed on the edges. • It contains a grooved drainage column on which a series of metal rings are packed. • These rings are usually made up of stainless steel and have dimensions of about 15.0 mm internal diameter and 22.0 mm external diameter. • Each metal ring has a number of semicircular projections (0.8 mm in thickness) on one side of surface. • The projections are arranged as a nest to form channels on the edges. • These rings are tightened on the drainage column with a nut. Working: • Filters are placed in a vessel. • Slurry is pumped under pressure or occasionally by applying reduced pressure to the outlet side. • Slurry passes through the channels formed on the edges between the rings. • The clear liquid rises up and collected from the outlet into receiver . • For separation of fine particles, a bed of suitable materials such kieselguhr is first built up. • The pack of rings serves essentially as a base on which the true filter medium is supported.
- 20. Cartridge filter Principle: • Itis a thin porous membrane in which pre filter and membrane filter are combined in a single unit. • The filtration action is mainly sieve like and particles are retained on the surface. Uses: • Particularly useful for preparation of particulate free solutions for parenterals and ophthalmic uses. • This filter holder will process 1000 – 15000 litres of sterile solution per hour. Advantages: • Autoclaving can be done for sterile operations due to stainless steel construction. • Cartridges with self cleaning devices are advantageous. • Rapid disassembling as well as reusing of filter medium is possible. • Cartridges are not brittle, when they are dry. • Used as in-line continuous filtration, this reduces handling of solution. • It minimizes chances of contaminations. Disadvantages: • A number of manufactures provide the components, which are generally not interchangeable between suppliers. • Cost of disposable elements offsets the labour saving in terms of assembly and cleaning of cartridge clarifiers.
- 21. Construction: • It hascylindrical configuration made with disposable or changeable filter media. • Made up of either plastic or metal. • Consist of two membrane filters (sieve like) made of polypropylene: prefilter and actual filter for filtration. • A protective layer surrounds them. • The cartridge is housed in a holder and a number of cartridges can be placed in a same housing. • The housing is closed with the lid. • Housing has provisions for slurry inlet and outlets. Working: • Slurry is pumped into cartridge holder. • It passes through cartridge filter unit by straining. • The clear liquid passes through the centre • Moves up to collect through outlet.
- 23. Rotary drum filter Principle: •Slurry filtered through sieve like mechanism on the rotation drum surface, under the condition of vacuum. • In addition compression, drying (using hot air), and removing the filter cake (using knife) are possible. Uses: • The rotary filter for continuous operation on large quantities of slurry. • Suitable for slurry contains considerable amounts of solids in the range 15-30%. • An example of pharmaceutical application includes the collection of calcium carbonate, magnesium carbonate, and starch. • The separation of the mycelium from the fermentation liquor in the manufacture of antibiotics. Advantages: • The rotary filter is automatic and is continuous in operation, so that the labour costs are very low. • The filter has a large capacity, so it is suitable for the filtration of highly concentrated solutions. • Variation of the speed of rotation enables the cake thickness to be controlled. • Pre-coat of filter aid could used to accelerate the filtration rate. Filter has large surface area. Disadvantages: • The rotary filter is a complex piece of equipment, with many moving parts and is very expensive. • In addition to the filter itself, some accessories are connected, e.g., a vacuum pump, vacuum receivers, slurry pumps and agitators are required. • The cake tends to crack due to the air drawn through by the vacuum system, so that washing and drying are not efficient. • Being a vacuum filter, the pressure difference is limited to 1 bar and hot filtrates may boil. • It is suitable only for straight- forward slurries.
- 24. Construction: It consists ofa metal cylinder mounted horizontally. The drum may be up to 3 meters in diameter and 3.5 meters in length and gives surface area of 20 meter square. The curved surface being a perforated plate, supporting a filter cloth. Internally, it is divided into several sectors and a separate connection is made between each sector and a special rotary valve. Working The drum is dipped into the slurry and vacuum applied to the outlet, which is connected to the filtrate receiver. When the cake has formed, the cake drained or partially dried by vacuum. The drum is sprayed with water to wash the cake. Retaining the vacuum connection drains the cake and produces partial dryness then, removed by a doctor knife. When the solids of the slurry are too much that the filter cloth becomes blocked with the particles, a pre-coat filter may be used. A pre-coat of filter aid is deposited on the drum prior to the filtration process.
- 26. Membrane filters • Membranefilters act as a barrier to separate contaminants from water, or they remove the particles contaminating the water. Reverse osmosis, ultrafiltration, and nanofiltration all use a membrane in their different filtration processes. • Uses: These filters are mainly used for sterilization of both aqueous and oily liquids. The membrane filters cannot be used for filtration of organic solvents, such as alcohols, ketones, esters and chloroform. • Construction: Membrane filters are made of thin and flat membranes of cellulose derivatives, such as, cellulose acetate and cellulose nitrate. These filters are brittle when in dry condition and can be stored for an indefinite period. The filters are between 50 and 150 μ thick and are available in sizes upto 60 cm2 . • Working: A membrane filter has 400 to 500 million pores per square centimetre of filter surface. The pores are absolutely uniform in size and occupy about 80% of filter volume. To avoid rapid clogging of a membrane, pre-filtration is often required. The selection of a membrane filter for a particular application depends on the particles to be removed.
- 27. SEITZ FILTER Principle: Itis based on filtration of asbestos pad filter disc. Adsorption. Advantages: • No risk of contaminating the filtrate. • Apparatus is very simple to use. • For viscous solution they are more suitable. Disadvantages: • Asbestos may shed loose fibres. • Pad may absorb sufficient amount of medicament. Construction: It consists of two parts. Lower part fitted with a perforated plate over which compressed asbestos pad is placed. Upper part has a value through which pressure can be applied. Both parts joined together by winged nuts. A valve is present on the upper part through which vacuum is applied. The asbestos pads may yield alkali and cause precipitation of alkaloids. It may shed fibres into the filtrate and absorb drug from solution.
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