filtration
TRANSCRIPT
FILTRATION
Dr. O. Alkarim
Dr. O. Alkarim
Dr. O. Alkarim
DEFINITIONProcess where by a SOLID is separated from a LIQUID or gas by means of a POROUS MEDIUM.
POROUS MEDIUM which retain the solid but allows the fluid to pass.SLURRY the suspension of solid and liquid to be filteredFILTER CAKE the accumulation of solids on the filter FILTRATE the clear liquid passing through the filter
Note: Clarification : Solid present in small proportion
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Factors affecting process:1. Particle size.2. Physical state.3. Quantity of suspended matter.4. The speed of the operation.5. Characteristic of the fluid medium.
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PRINCIPLES
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PRINCIPLE
Note: if the viscosity of the liquid is high, the waiting period may increase
Generally liquid allows to stand in a suitable container until the suspended particles either SETTLED or RISEN.
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Failure!!!! If the particle in
colloidal size!!!! Approach densities
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Acceleration of processa. Centrifugeb. Filtrationc. Adsorption d. Absorption
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Filter media Selection depend on purpose of filtration and quantities. General properties of filters:1. Inert. 2. Maximum passage of liquid.3. Mechanical strength.4. Filter even negligible amount 5. Delivering a clear filtrate at a suitable production rate.
Dr. O. Alkarim
Classifications of media
Dr. O. Alkarim
Woven material these include:a. wire screening e.g. stainless steel is durable, resistance to
plugging and easily cleaned.b. fabrics of cotton, wool, and nylon. Cotton is a common filter Nylon is superior for pharmaceutical use, Because it is unaffected by mold, fungus or bacteria and has negligible absorption properties
Dr. O. Alkarim
Dr. O. Alkarim
Porous platesThese include perforated metal or rubber plates, natural porous materials such as stone, porcelain or ceramics, and sintered glass
Dr. O. Alkarim
Dr. O. Alkarim
Membrane filter These are basic tools for micro-filtration, useful in
the preparation of sterile solutions. These filters are made by casting of various esters of cellulose, or from nylon, Teflon, polyvinyl chloride. The filter is a thin membrane with millions of pores per square centimeter of filter surface.
Dr. O. Alkarim
Dr. O. Alkarim
Un woven material filter paper is a common filter medium since
it offers controlled porosity, limited absorption characteristic, and low cost.
Dr. O. Alkarim
Dr. O. Alkarim
Granular material with suitable support Include sand, gravels, diatomaceous, and charcoal
Dr. O. Alkarim
Dr. O. Alkarim
Through porous media
Mechanism of filtration
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Mechanism…
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Rate of Filtration
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Factors affecting rate of filtration Resistance Viscosity Pressure drop Size of the pore Blocking potential of
particle.
FILTER AID Usually, 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 cake.
The object of the filter aid is to prevent the medium from becoming blocked and to form an open, porous cake, so reducing the resistance to flow of the filtrate. The particles must be inert, insoluble, incompressible, and irregular shaped.
Dr. O. Alkarim
Dr. O. Alkarim
Filter aids may be used in either or both two ways:1. Pre- coating technique: by forming a pre-coat over the
filter medium by filtering a suspension of the filter aid .2. Body- mix technique: A small proportion of the filter
aid (0.1-0.5 %) is added to the slurry to be filtered. This slurry is re-circulated through the filter until a clear filtrate is obtained, filtration then proceeds to completion.
Dr. O. Alkarim
The following filter aids may be used:1. DIATOMITE ( Kieselguhr ): obtained from
natural siliceous deposits.2. PERLITE , it is an aluminum silicate.3. CELLULOSE and ASBESTOS.
Dr. O. Alkarim
Dr. O. Alkarim
FILTRATION EQUIPMENTFour groups may be listed:1. Gravity filters. 2. Vacuum filters3. Pressure filters.4. Centrifugal filters.
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Gravity filter Employing thick
granular beds widely used in water filtration e.g. Sand
Filter
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VACUUM FILTERS
Vacuum filters operate practically at higher pressure differentials than gravity filters.1. Leaf filter2. Rotary vacuum filter
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LEAF FILTER 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 frame may be circular, square or rectangular shapes.
The operation: The leaf filter is immersed in the slurry and a receiver and a vacuum system connected to the filtrate outlet.
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 not too high 5 % being a suitable maximum.6. Labor costs for operating the filter are comparatively
moderate.
Dr. O. Alkarim
Dr. O. Alkarim
ROTARY FILTER In large –scale operation, continuous operation is sometimes desirable and
it may be necessary to filter slurries containing a high proportion of solids. The rotary filter is continuous in operation and has a system for removing
the cake that is formed , so, it is suitable for use with concentrated slurries. It is a metal cylinder mounted horizontally, 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.
Dr. O. Alkarim
Dr. O. Alkarim
Operation…
The drum is immersed to the required depth in the slurry, which is agitated to prevent settling of the solids, and vacuum is applied to those sectors of the drum which is submerged.
A cake of the desired thickness is produced by adjusting the speed of rotation of the drum. Each sector is immersed in turn in the slurry and the cake is then washed and partially dried by means of a current of air. Finally, pressure is applied under the cloth to aid the removal of the cake. Removal of the washed and partially dried cake is affected by means of a doctor knife.
Dr. O. Alkarim
Dr. O. Alkarim
Summary of the process: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.
Dr. O. Alkarim
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.
Dr. O. Alkarim
DISADVANTAGES1. 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
USES1. The rotary filter for continuous operation on large
quantities of slurry.2. Suitable for slurry contains considerable amounts of
solids in the range 15-30%.Examples of pharmaceutical application 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
Dr. O. Alkarim
Dr. O. Alkarim
PRESSURE FILTERS 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 whilst the filter is under pressure.
Examples are the sweet-land filter, plate and frame filter press.
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PLATE AND FRAME FILTER PRESS
This press is made up of two units, known respectively as 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 grooved surface to support the filter cloth, and with an outlet for the filtrate
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Plate and Frame Press
Operation…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.
Dr. O. Alkarim
Dr. O. Alkarim
Channels for the slurry inlet and the filtrate outlet can be arranged by fitting eyes to the plates and frames. This has the advantages that the filtrate from each plate can be seen and, in the event of a broken cloth, the faulty plate can be isolated and the filtration continued with one plate less.
The thickness of the cake can be varied by using frames of different thickness and, in general, there will be an optimum thickness of filter cake for any slurry, depending on the solids content of the slurry and the resistance of the filter cake.
As filtration proceeds, the resistance of the cake increases and the filtration rate will decrease. At a certain point it will be preferable in terms of the overall output of the process, to stop and empty the press rather than to continue filtration at a very low flow rate.
Plates and frames may be made in various metals to provide resistance to corrosion or prevent metallic contamination of the product. Non-metals e.g. plastics is lighter, also varieties of wood are satisfactory materials of construction.
Plates and frames may be of considerable size, of about 1m square
ADVANTAGES1. 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.
Dr. O. Alkarim
DISADVANTAGES1. It is a batch filter, so it is a time consuming.2. 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.
3. Operation is critical, as the frames should be full, otherwise washing is inefficient and the cake is difficult to remove.
4. The filter press is used for slurries containing less about 5 % solids5. In view of the high labor costs , it is most suitable for expensive
materials .e.g. the removal of precipitated proteins from insulin liquors.