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MECHANICAL SEPARATIONS. Introduction. Separations are divided into two classes : Diffusional operations , involves the transfer of material between phases
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Introduction • Separations are divided into two classes: • Diffusional operations, involves the transfer of material between phases • mechanical separations are applicable to heterogeneous mixtures based on physical differences between the particles such as size, shape, or density. They are applicable to separating solids from gases, liquid drops from gases, solids from solids, and solids from liquids. • mechanical separations methods: • Sieve or membrane, such as a screen or a filter, which retains one component and allows the other to pass • Utilization of differences in the rate of sedimentation of particles or drops as they move through a liquid or gas.
SCREENING • Separating particles according to sizealone • A single screen can make a single separation into two fractions • Industrial screens are made from woven wire, silk or plastic cloth, metal bars, perforated or slotted metal plates • Standard screens range in mesh size from 4 to 400-mesh, and woven metal screens with openings as small as 1 μm are commercially available. Screens finer than about I50-mesh are not commonly used, however, because with very fine particles other methods of separation are usually more economical. • In screens the particles drop through the openings by gravity, brush or centrifugal force. • Coarse particles drop easily through large openings in a stationary surface, but with fine particles the screen surface must be agitated in some way, such as by shaking, gyrating, or vibrating it mechanically or electrically
Motions of screens (a) gyrations in horizontal plane; (b) gyrations in vertical plane; (c) gyrations at one end, shaking at other; (d) shaking; (e) mechanically vibrated; if) electrically vibrated.
CAPACITY AND EFFECTIVENESS • Screen efficiency is a measure of the success of a screen in closely separating materials A and B. • Capacity of a screen is measured by the mass of material that can be fed per unit time to a unit area of the screen. • Capacity and effectiveness are opposing factors. In practice, a reasonable balance between capacity and effectiveness is desired. • The capacity of a screen is controlled simply by varying the rate of feed to the unit. • The overall chance of passage of a given undersize particle is a function of the number of times the particle strikes the screen surface and the probability of passage during a single contact. If the screen is overloaded, the number of contacts is small and the chance of passage on contact is reduced by the interference of the other particles.
Introduction Filtration • Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles. • The suspension of solid and liquid to be filtered is known as the slurry. The porous medium used to retain the solids is described as the filter medium; • The accumulation of solids on the filter is referred to as the filter cake, while the clear liquid passing through the filter is the filtrate. • The fluid may be a liquid or a gas. • the valuable stream from the filter may be the fluid, or the solids, or both. • They are also either continuous or discontinuous, Figure 1. Schematic diagram of filtration system
Rate of Filtration: The factors affecting rate of filtration is known as Darcy”s law and may be expressed as: dV/dt = KA ∆P /μL where V= volume of filtrate t = time of filtration K = constant for the filter medium and filter cake A = area of filter medium P = pressure drop across the filter medium and filter cake μ= viscosity of the filtrate l = thickness of cake.
Rate of Filtration The fluid passes through the filter medium, which offers resistance to its passage, under the influence of a force which is the pressure differential across the filter. rate of filtration= driving force/resistance
Rate of Filtration: Factors affecting rate of filtration 1- Permeability coefficient • The constant (K) represents the resistance of both the filter medium and the filter cake. • As the thickness of the cake increase, the rate of filtration and surface area of the particles will decrease. • the porosity of the cake, and rigidity or compressibility of the particles could affect the permeability of the cake. 2- Area of filter medium • The total volume of filtrate flowing from the filter will be proportional to the area of the filter. • The area can be increased by using larger filters. • In the rotary drum filter, the continuous removal of the filter cake will give an infinite area for filtration.
Factors affecting rate of filtration 3- Pressure drop • The rate of filtration is proportional to the pressure difference across both the filter medium and filter cake. • The pressure drop can be achieved in a number of ways: • Gravity: A pressure difference could be obtained by maintaining a head of slurry above the filter medium. The pressure developed will depend on the density of the slurry. • Vacuum: 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 difference across the filter. • Pressure: The simplest method being to pump the slurry into the filter under pressure. • Centrifugal force: The gravitational force could be replaced by centrifugal force in particle separation,
Factors affecting rate of filtration 4- Viscosity of filtrate • It would be expect that an increase in the viscosity of the filtrate will increase the resistance of flow , so that the rate of filtration is inversely proportional to the viscosity of the fluid. • This problem can be overcome by two methods: • a- The rate of filtration may be increased by raising the temperature of the liquid, which lowers its viscosity. However, it is not practicable if thermolabile materials are involved or if the filtrate is volatile. • b- Dilution is another alternative but the rate must be doubled. 5- Thickness of filter cake • The rate of flow of the filtrate through the filter cake is inversely proportional to thickness of the cake. • Preliminary decantation may be useful to decrease the amount of the solids.
Gravity filters • In a gravity filter the filter medium can be no finer than a coarse screen or a bed of coarse particles like sand. • Gravity filters depend upon atmospheric pressure to force the solids through the medium. • Sand filters, travelling belt filters and rotary drum gravity filters are found in this group. • Simple • zero direct running cost • the equipment is usually bulky • fairly coarse solids will still contain an appreciable amount of liquid after
Vacuum filters • A driving force of up to about 0.81 bar, which in many instances is sufficient to give vastly improved rates of filtration with all except the finest solids. • The vacuum is created behind the filter medium, which causes the atmospheric pressure in front of the filter medium to drive the slurry through the medium, filtering out suspended solids in the process. • Types of vacuum filters include: belt, horizontal pan, vertical disc, and drum varieties.
Pressure filters • produce a greater output per unit area, so enabling smaller equipment to fit easily into the process circuit. • it is sometimes difficult for continuous discharge of solids, such as cake. • The feed slurry is introduced into the filter under pressure and forced through the filter medium. • There are two basic designs used in pressure filtration: • plate-type filter presses, whereby a series of plates utilize a filter medium between Them • casing-enclosed pressure filters, which consist of filter medium covered elements enclosed in a pressure tank. • Another type of pressure filter utilizes two belts which move in the same direction.
Mechanisms of filtration (a) cake filter; (b) clarifying filter; (c) cross flow filter.
Mechanisms of filtration 1. surface filters • surface filters are used for cake filtration in which the solids are deposited in the form of a cake on the up-stream side of a relatively thin filter medium. • Cake filters separate relatively large amounts of solids as a cake of crystals or sludge, as illustrated. • the filter medium has a relatively low initial pressure drop and particles of the same size or larger than, the openings wedge into the openings and create smaller passages which remove even smaller particles from the fluid. • In order to prevent blinding of the medium, filter aids are used as a precoat which forms an initial layer on the medium. • Surface filters are usually used for suspensions with higher concentrations of solids, say above 1% by volume, because of the blinding of the medium (or of the precoat) that occurs in the filtration of dilute suspensions. This can, however, sometimes be avoided by an artificial increase of the input concentration, in particular by adding a filter aid as a body feed.
Mechanisms of filtration 2. depth filter • Clarifying filters remove small amounts of solids to produce a clean gas or a clear liquids. • The solid particles are trapped inside the filter mediumor on its external surfaces. • In a depth filter the particles are smaller than the medium openings and hence they proceed through relatively long and tortuous pores where they are collected by a number of mechanisms (gravity, diffusion and inertia) and attach to the medium by molecular and electrostatic forces. • Clarifying filters differ from screens in that the pores of the filter medium are much larger in diameter than the particles to be removed. • In a cross flow filter the feed suspension flows under pressure at a fairly high velocity across the filter medium. A thin layer of solids may form on the surface of the medium, but the high liquid velocity keeps the layer from building up.
FILTER MEDIA • The function of the filter medium is generally to act as a support for the filter cake, and the initial layers of cake provide the true filter. • filter must meet the following requirements: • 1. It must retain the solids to be filtered, giving a reasonably clear filtrate. • 2. It must not plug or blind. • 3. It must be resistant chemically and strong enough physically to withstand the • process conditions. • 4. It must permit the cake formed to discharge cleanly and completely. • 5. It must not be prohibitively expensive. • 6- Sterile filtration imposes a special requirement since the pore size must not exceed the dimension of bacteria or spores. • In industrial filtration a common filter medium is canvas cloth • Many different weights and patterns of weave are available for different services. Corrosive liquids require the use of other filter media, such as woolen cloth, metal cloth of stainless steel, glass cloth, or paper. • Synthetic fabrics like nylon, polypropylene, and various polyesters are also highly resistant chemically.
Classification of filter media 1- Woven filters: these include a- wire screening . 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 . 2- Non- woven filters: Filter paper is a common filter medium since it offers controlled porosity, limited absorption characteristic, and low cost. 3- Membrane filters: 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. 4-Porous plates: These include perforated metal or rubber plates, natural porous materials such as stone, porcelain or ceramics, and sintered glass.
CAKE FILTERS • At the start of filtration in a cake filter some solid particles enter the pores of the medium and are immobilized, but soon others begin to collect on the septum surface. After this brief initial period the cake of solids does the filtration, not the septum • a visible cake of appreciable thickness builds up on the surface and must be periodically removed. • Except as noted under bag filters for gas cleaning, cake filters are used almost entirely for liquid-solid separations. • may operate with above-atmospheric or with vacuum • continuous or discontinuous, but because of the difficulty of discharging the solids against a positive pressure, most pressure filters are discontinuous.
FILTER AIDS • Slimy or very fine solids that form a dense, impermeable cake quickly plug any filter medium that is fine enough to retain them. Practical filtration of such materials requires that the porosity of the cake be increased to permit passage of the liquor at a reasonable rate. • 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. • filter aids: diatomaceous silica, perlite, wood cellulose, or other inert porous solid • The filter aid may subsequently be separated from the filter cake by dissolving away the solids or by burning out the filter aid. If solids have no value, they and the filter aid are discarded together.
FILTER AIDS • 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 recirculated through the filter until a clear filtrate is obtained, filtration then proceeds to completion. • Precoats prevent gelatinous solids from plugging the filter medium and give a clearer filtrate. • The precoat is really a part of the filter medium rather than of the cake.
DISCONTINUOUS PRESSURE FILTERS • Pressure filters can apply a large pressure differential across the septum to give economically rapid filtration with viscous liquids or fine solids. • Pressure filters are usually discontinuous; vacuum filters are usually continuous. • The most common types of pressure filters are • Filter presses • Shell-and-Ieaf filters • AUTOMATIC BELT FILTER
Industrial Filters • Four groups may be listed: A- Gravity filters. B- Vacuum filters C- Pressure filter D- Centrifugal filters. A. Gravity filters: Employing thick granular beds Are widely used in water filtration e.g. Sand Filter
B- Vacuum filters • Vacuum filters operate practically at higher pressure differentials than gravity filters. • Rotary vacuum filter • leaf filter
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 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- Labour costs for operating the filter are comparatively moderate.
Rotary vacuum filter (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. • Uses • 1- 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%.
Continuous rotary vacuum filter • 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.
Advantages & disadvantages of rotary drum filter Advantages 1-The rotary filter is automatic and is continuous in operation, so that the labour 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.
C- 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. • High operational pressures, may prohibit continuous operation because of the difficulty of discharging the cake whilst the filter is under pressure. • Examples are the sweetland filter, plate and frame filter press.
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. • 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.
Advantages of the filter press (a) Because of its basic simplicity the filter press is versatile and may be used for a wide range of materials under varying operating conditions of cake thickness and pressure. (b) Maintenance cost is low. (c) It provides a large filtering area on a small floor space and few additional associated units are needed. (d) Most joints are external and leakage is easily detected. (e) High pressure operation is usually possible. (f) It is equally suitable whether the cake or the liquid is the main product. Disadvantages of the filter press (a) It is intermittent in operation and continual dismantling is apt to cause high wear on the cloths. (b) Despite the improvements mentioned previously, it is fairly heavy on labour.
Washing • Simple washing • the wash liquid is fed in through the same channel as the slurry although, as its velocity near the point of entry is high, erosion of the cake takes place. • The channels which are thus formed gradually enlarge and uneven washing is usually obtained. • Simple washing may be used only when the frame is not completely full. • thorough washing, • the wash liquid is introduced through a separate channel behind the filter cloth on alternate plates, known as washing plates and flows through the whole thickness of the cake, first in the opposite direction and then in the same direction as the filtrate. • Even with thorough washing some channelling occurs and several inlets are often provided so that the liquid is well distributed. • If the cake is appreciably compressible, the minimum pressure should be used during washing, and in no case should the final filtration pressure be exceeded. • After washing, the cake may be made easier to handle by removing excess liquid with compressed air.
Shell-and-Ieaf filters For filtering under higher pressures than are possible in a plate-and-frame press, to economize on labor, or where better washing of the cake is necessary
Horizontal belt filter When the feed contains coarse fast-settling particles of solid, a rotary-drum filter works poorly or not at all. The coarse particles cannot be suspended well in the slurry trough, and the cake that forms often will not adhere to the surface of the filter drum. In this situation a top-fed horizontal filter may be used.
Centrifugal Filters • Solids that form a porous cake can be separated from liquids in a filtering centrifuge. • Slurry is fed to a rotating basket having a slotted or perforated wall covered with a filter medium such as canvas or metal cloth. Pressure resulting from the centrifugal action forces the liquor through the filter medium, leaving the solids behind. If the feed to the basket is then shut off and the cake of solids spun for a short time, much of the residual liquid in the cake drains off the particles, leaving the solids much "drier" than those from a filter press or vacuum filter. • When the filtered material must subsequently be dried by thermal means, considerable savings may result from the use of a centrifuge.
Bag filters • Bag filters have now been almost entirely superseded for liquid filtration by other types of filter, although one of the few remaining types is the Taylor bag filter which has been widely used in the sugar industry. • A number of long thin bags are attached to a horizontal feed tray and the liquid flows under the action of gravity so that the rate of filtration per unit area is very low. • It is possible, however, to arrange a large filtering area in the plant of up to about 700 m2. • The filter is usually arranged in two sections so that each may be inspected separately without interrupting the operation. • Bag filters are still extensively used for the removal of dust particles from gases and can be operated either as pressure filters or as suction filters.
D- Centrifugal Filters • A centrifuge consists of a basket in which mixture of solid and liquid , or mixture of two liquids is rotated at high speed so that it is separated into its constituents by the action of centrifugal force. • Types of baskets: • Imperforated, in which the liquid is removed through a skimming tube, while the solid particles, sediment to the wall. In pharmacy, the centrifuge is commonly used for drying crystals and for separating emulsions into their constituent liquids. • Perforated basket, in which the liquid passes out through the holes.
Advantages & Disadvantages • Advantages of a centrifuge 1- It is very compact, occupying very little floor space, 2- It is capable of handling slurries with high proportions of solids . 3- The final product has generally, a very low moisture content if compared to a filter cake of a similar material. • Disadvantages 1- Batch process 2- It involves a considerable labour cost, making the process expensive.
Equipment selection There are a number of product – related factors that should be considered when selecting a filter for a particulate process. 1- The chemical nature of the product. Interactions with the filter medium may lead to leaching of the filter components, degradation or swelling of the filter medium or adsorption of components of the filtered product on the filter. 2- The volume to be filtered and the filtration rate required. 3- The operating pressure needed. This is governing the filtration rate . 4- The amount of material to be removed. Pre-filters (decantation) may be required or filter where the cake can be continuously removed. 5- The degree of filtration required. This affect the chosen pore size of membrane filters or the filter grade to be used. 6- If sterility is required , then the equipment should itself be capable of being sterilized, and must ensure that contamination does not occur after the product has passed the filter. 7- The product viscosity and filtration temperature. A high product viscosity may require elevated pressure to be used.
Application of solid/liquid filtration 1- Improvement of the appearance of solutions, mouth washes, etc..... 2- Removal of potential irritants, e.g. from eye drop preparations or solutions applied to mucous membranes. 3- Recovery of desired solid material from suspension or slurry ,e.g. to obtain drug after crystallization process. 4- Certain operations, such as the extraction of vegetable drugs with a solvent, may yield a turbid product with a small quantity fine suspended colloidal matter; this can be removed by filtration. 5- Detection of microorganisms present in liquids, This can be achieved by analyzing a suitable filter on which the bacteria are retained .This method can also be used to assess the efficiency of preservatives.
