Size Separation

Chapter 3

SIZE SEPARATION

INTRODUCTION

• Large pieces of material are usually estimated visually but difficulties arise only when powders are to be estimated. The aim of any estimation, including particle size separation, is either analytical or preparative. Size separation is a unit operation that involves the separation of a mixture of various size particles into two or more portions by means of sieveing surfaces. Size separation is performed by processes known as sieving, sifting, segregation etc. Size separation is a method of classifying particles into fractions based on size. Granular mixtures separate according to particle size when shaken, with large particles rising; a phenomenon termed the 'Brazil-nut effect'. It is well known fact that differences in particle density affect size separation in mixtures of granular particles.

• Particles of many kinds and various sizes have played an important role in development and production of dosage forms. If particles are spherical or cubical, it would be easy to characterize them. Unfortunately, most of the particles or granules in pharmaceuticals are of irregular size and shape. Therefore, various methods and techniques have been developed to characterize such particles. Particle size analysis is important in studying particle behavior in a medium during industrial applications. Particle size analysis in various processes involves many concepts and techniques; however, this chapter is focused on the methods of particle size separation utilizing mechanical techniques.  

OBJECTIVES

•  Particles of many kinds and various sizes play an important role in performing specific function in development and use of dosage forms. If particles are spherical or cubical, it is easy to characterize them. Unfortunately, most of the particles employed in pharmaceuticals are of irregular size and shape. Therefore, it is desirable to develop methods and techniques to characterize particles of irregular size and shape, and this is the main objective of particle size separation. Apart from this other objective of size separation are:

• (i) Size separation is useful in grading powders or granules. Any solid materials after size reduction never give particles of the same size but contain particles of varying sizes. The size-reduced particles are then passed through sieves with the objective to get fractions of narrow size range.

• (ii) To control size variations in the materials. During tablet granulation the granules should be within narrow size range, otherwise, weight variation will take place during tablet punching. 

• (iii) To classify materials into different sizes for the desired purpose.

• (iv) To enhance performance (topical powders), efficacy (powders for inhalation) and stability of dosage forms (suspensions). 

• (v) To judge uniformity in a mixing material.

• (vi) To avoid variations in the bulk properties of materials. 

• (vii) To specify the quality parameter of a intermediate and finished dosage forms. 

APPLICATIONS

• Particle size separation is important in studying particle behavior in a medium as in many analytical sciences and industrial applications. Size separation has following major applications in pharmacy: 

• (i) Size separation has significance in formulating uniform dosage forms with respect to drug content. (ii) It can be used to obtain granules of required size. 

• (iii) It helps to ensure good flowability.

• (iv) It can also be useful in separating undesirable size solids from desired ones.

• (v) It has applications in determination of particle size and their distribution in the given samples. 

• (vi) It can also be used to investigate efficiency and validate size reduction equipments. 

• (vii) It is used to obtain minorized powders/particles which undergo least size segregation.

 OFFICIAL STANDARDS FOR POWDER SIZE

•  Standards for powders used in pharmaceuticals are reported in the British Pharmacopoeia (B.P.) which states that “the degree of coarseness or fineness of a powder is differentiated and expressed by the size of the mesh of the sieve through which the powder is able to pass”. The B.P. specifies five grades of powder and the number of the sieve through which all the particles must pass, Table 3.1.

• The B.P. specifies a use of smaller size of sieve for the coarser powders but states that not more than 40 % shall pass through. The relevant grades of powder and sieve number arepresented in the Table 3.2. Thus, the coarse powder is that wherein all the particles which pass through a No. 10 sieve and not more than 40 % through a No. 44 sieve. This is usually referred as a 10/44 powder. 

• The B.P. states that when a powder is described by a number, all particles must pass through the specified sieve and when a vegetable drug is being ground and sifted, none must be rejected. This classification is must if the character of a vegetable drug is compared with a chemical substance.

• The chemical materials are generally homogeneous. If a certain quantity of a powder is required, an excess may be ground so as to get a sufficient amount of the desired size range by sieving, and the oversize particles may be discarded. As we know vegetable drugs consists of a variety of tissues of different degrees of hardness that softer tissues are ground first and oversize mass is obtained by sifting contain a higher proportion of the harder tissues. In many cases, constituents are not distributed uniformly through vegetable tissues. For example, digitalis contains glycosides concentrated in the mid-rib and veins of the plant. Hence, if tailings are discarded while grinding and sifting the drug, it is likely that a high proportion of the active constituents may be lost.

• The British Pharmaceutical Codex has given a further grade of powder known as ultrafine powder. For these powders it is required that the maximum dimension of at least 90 % of the particles must be not greater than 5 µm and none must be greater than 50 µm. Determination of particle size for this grade is carried out by a microscopic method.

SIEVES

• A sieve, also called sifter, is a device for separating desired elements from unwanted material or for characterizing the particle size distribution of a sample. This is a device typically made-up of using a woven sieve such as a mesh or net or metal. The word "sift" is derived from "sieve". A sieve has very small holes. The sieving process is comparatively inexpensive, simple in concept and easy to use. Coarse particles are separated or broken-up by grinding against one-another and sieve openings. Sieves are the most commonly used devices for particle size analysis. Depending upon the types of particles to be separated, sieves with different types of holes are used. Sieving plays an important role in pharmaceutical and food industries where they (often vibrating) are used to prevent the contamination of the product by foreign bodies. The design of the industrial sieve is of primary importance. Each sieve has a specific number that denotes the number of meshes in a length of 2.54 cm (≈1 inch).

Construction of Sieves

• Based upon application specially constructed sieves are used. Various types of sieves includes electroformed sieves, perforated plate sieves, sonic sifters, air jet sieves, wet wash sieves etc. Generally pharmaceutical sieves are made up of stainless steel, brass, bronze etc. and are not coated with any material to avoid wear and tear as well as contamination in the products. Sieves should be non-reactive and resistant to corrosion. The most common choice of material for sieves is iron because it is cheap. Iron has limitation for its use when there is possibility of corrosion and contamination. This can be avoided by coating iron surfaces with galvanizing agent. Stainless steel, brass and phosphorus bronze due to their corrosion resistant, good strength and non-contaminating qualities are preferred as an alternative to iron. Even non-metals such as nylon and terylene are used if contamination is to be avoided. Sieves with different type of holes viz. size and shapes made in plates as perforations can be used as separating devices. Sieves made-up of woven cloth are used if fine powders are to be separated. This cloth can be made of cotton, nylon or silk. 

Types of Sieves

• Pharmaceutical sieves are extensively used for size separations from 300 mm down to around 38 µm. The efficiency of sieving decreases rapidly with fineness. Dry sieving is used for material above 5 mm in size and wet sieving is common in use down to 250 µm but is possible to about 40 µm. In its simplest form, a sieve is a surface having many apertures (holes), usually with uniform dimensions. Particles presented to that surface will either pass through or be retained, according to whether the particles are smaller or larger than the dimension of the apertures. There are numerous different types of industrial sieves available, Fig. 3.1. Some of the most common are vibrating sieves, static sieves, trommels, roller sieves, flip-flow sieves, circular sieves, linear sieves etc.

 (i) Vibrating Sieves:

• The commonest sieve type in industrial applications is the vibrating sieve. There are many subtypes of vibrating sieves in use for coarse- and fine-sieving applications. Vibrating sieves have a rectangular sieving surface with feed and oversize discharge at opposite ends. They are used in a variety of sizing, grading, scalping, dewatering, wet sieving, and washing applications. Sieves are vibrated in order to throw particles off the sieving surface so that they can again be presented to the sieve and to convey the particles along the sieve. Vibrating motion is generally produced by vibrating mechanisms based on eccentric rotating masses with amplitude of 1.5–5 mm and operating in a range of 700–1000 r.p.m. The right type of vibration also induces stratification of the feed material, which allows the finer particles to work through the layer of particles to the sieve surface while causing larger particles to rise to the top. Vibrating sieves of most types can be manufactured with more than one sieve deck. On multiple-deck systems, the feed is introduced to the top coarse sieve, the undersize falling through to the lower sieve decks, thus producing a range of sized fractions from a single sieve.

(ii) Inclined or circular motion sieves:  

• This type of vibrating sieve is widely used for sizing applications. A vertical circular or elliptical vibration is induced mechanically by the rotation of unbalanced weights or flywheels attached usually to a single drive shaft. The amplitude of throw can be adjusted by adding or removing weight elements bolted to the flywheels. The rotation direction can be contra flow or in-flow. Contra flow slows the material flow more and permits more efficient separation, whereas in-flow permits a greater throughput. Single-shaft sieves must be installed on a slope, usually between 15° and 28° angle to permit flow of material along the sieve.

(iii) Static sieves:  

• Very coarse material is usually sieved on an inclined sieve called grizzly sieve. Grizzlies are characterized by parallel steel bars or rails set at a fixed distance apart and installed in line with the flow of ore. The gap between grizzly bars is usually greater than 50 mm and can be as large as 300 mm, with feed top size as large as 1 m. The most common use of grizzlies in mineral processing is for sizing the feed to primary and secondary crushers. 

 (iv) Horizontal low-head or linear vibrating sieves:

• These sieves have a horizontal or near-horizontal sieving surface and therefore need less headroom than inclined sieves. Horizontal sieves must be vibrated with a linear or an elliptical vibration. The accuracy of particle sizing on horizontal sieves is superior to that on inclined sieves; however, because gravity does not assist the transport of material along the sieve, they have lower capacity than inclined sieves. Horizontal sieves are used in sizing applications where sieving efficiency is critical and as drain-and-rinse sieves in heavy medium circuits.

(v) Banana or Multiscope sieves:

• These sieves become widely used in high-tonnage sizing applications where both efficiency and capacity are important. Banana sieves typically have a variable slope of around 40–30° at the feed end of the sieve, reducing to around 0–15° at the discharge end in increments of 3.5–5°. The steep sections of the sieve cause the feed material to flow rapidly at the feed end of the sieve. The resulting thin bed of particles stratifies more quickly and therefore has a faster sieving rate for the very fine material than would be possible on a slower moving thick bed. Toward the discharge end of the sieve, the slope decreases to slow down the remaining material, enabling more efficient sieving of the near-size material. The capacity of banana sieves is significantly greater and is reported to be up to three or four times that of conventional vibrating sieves.  

(vi) Rotary scrubbers:

• These sieves are cost-effective washing units that are an integral part of a material handling system to upgrade a wide variety of primary crushed hard rock and ore, including iron ore. A rotary scrubber is a cylindrical drum with internal lifters, typically supported by trunnion rollers at either end. These are high-capacity, high-retention time machines primarily used to remove water-soluble clays, deleterious materials, and coatings from a wide variety of hard rock and ore. Rotary scrubber designs include the solid shell and combination scrubber sieves and are available in a variety of diameters and lengths. Drive transmission choices include chain and sprocket, gear and pinion, or pedestal mount hydraulic motor.

(vii) Test Sieves  

• Test sieves are measuring devices used to determine the size and size distribution of particles in a material sample using wire mesh of different openings to separate particles of different sizes. Test sieves come in different materials such as brass, stainless steel, or brass frames with stainless steel mesh. Diameters include 3", 8", and 12" with mesh sizes ranging from 4 mm to 38 microns. When stacked on a sieve shaker, the top test sieve has the largest mesh size and the bottom one the smallest mesh size. The sieve stack consists of pan at the bottom and covers at the top.

Calibration of Sieves

• Sieves used for sieving are to be cleaned regularly. The frequent use can cause changes in mesh openings but much of the damage sustained to working sieves occurs during cleaning. Often, the operator hurries to clear the mesh of residual particles by strongly tapping the frame. This tapping can distort the mesh. Operators also use brushes to remove residual particles after use. This process often distorts sections of the sieve mesh. These alterations of the sieve may change the quality of products in terms of size hence sieves are calibrated intermittently.

  Standards for Sieves

• Sieving is the separation of fine material from coarse material by means of a meshed or perforated surface. The technique was used since early Egyptian days as a way to size grains. These early sieves were made of woven reeds and grasses. Today the sieve test is the technique used most often for analyzing particle-size distribution. Although at first look the sieving process appears to be elementary, in practice, there is a science and art involved in producing reliable and consistent results. To better understand sieving, there are several areas of sieve specifications that should to be explained and some of them are given Table 3.3. Sieves used in pharmaceuticals must comply with the standards given in Pharmacopoeia, Table 3.4.

•  Table 3.5 presents the different international sieve standards and the corresponding sieve types. There are several sieve aperture progression ratios commonly available depending on the different international standards. In the USA, a progression ratio of 21/2 is used. This ratio corresponds to successive particle groups of 2 : 1 particle surface ratio. The progression rate of 21/3 (100.1) has been adopted by the French which corresponds to successive particle groups of 2:1 particle volume ratio. The progression ratios of 100.1 and 100.05 are recommended for narrow size distributions. 

  MECHANISMS OF SIZE SEPARATION

• A single sieve or sieves in a set may be agitated in different ways:

• (i) Oscillation: In this case the sieve is mounted in inclined frame or rack that oscillates back and forth. This is most common and simple mechanism in which material rolls on the surface of sieve and sometimes forms a ball. This movement is achieved by ordinary eccentric movement of the rotating motor shaft.

• (ii) Vibration: Sieves in this mechanism are mechanically or electrically vibrated at high-speed allowing powder material to pass through. Vibration are either sinusoidal or gyratory. Sinusoidal vibration occurs at an angled plane relative to the horizontal. The vibration in a wave pattern are determined by frequency and amplitude. This motion can cause sieves to move-up and down or left to right. 

• (iii) Gyration: The sieves are kept on rubber mounting which is attached to rotary eccentric flywheel. This produces a movement of desired amplitude with required intensity to induce motion in the material. Rotary motion allows particles to spin and pass through the meshes. Gyratory vibration occurs at near level plane at low angles in a reciprocating side-to-side motion.

Brushing Method

• Material to be processed for size separation using sieving is passed through sieves by use of brushes in different directions. For motion in circular sieves brush is rotated at Centre of sieve while in horizontal cylindrical sieves it rotates around its longitudinal axis. Brush helps to keep the meshes clean. This method is suitable for wet, greasy and sticky materials.

Centrifugal Method

• In this method vertical cylindrical type mechanical sieve is used. It consists of high-speed rotor inside the cylinder that throws particles on the sieve by centrifugal force. A current of air generated due to movement of rotor causes particle separation. To aid particle separation further an air jet can be fixed in the cylinder. . Other mechanisms of particle separation are gravitational force, density and electrostatic force. Gravity is a physical interaction in the sense that when the material is thrown from the sieve causes it to fall to a lower level. Gravity also pulls the particles through the sieve media. The density of the material relates to material stratification. The classification in this case is based upon weight of material. Electrostatic force can be applied to sieve when particles are extremely dry or wet that help to sieving.

PRINCIPLE, CONSTRUCTION, WORKING, USES, MERITS AND DEMERITS

• There are different techniques used in particle size separation. The most common mechanical techniques for particle size separation relevant to pharmaceutical sciences and industrial applications are: sieving, sedimentation (gravitational or centrifugal), elutriation, electrostatic precipitation, thermal precipitation, hydrodynamic chromatography and impaction. Some of the major equipment are briefly presented below.

 Sieve Shaker

• Sieving is most widely used technique in pharmaceutical industry for particle size analysis. The particles are classified based on their size, independent of any other particle characteristics such as density and surface properties. Micromesh sieves are used to classify particles of size range 5 - 20 µm, while particles of size range 20 -125 µm are classified in the standard woven wire sieves. Coarse particles (>125 µm) are classified in punched plate sieves. Punched plate sieves are commonly used in industrial applications where the openings are circular or rectangular. The sieves can take different configurations.

Principle:

• The principle of sieve shaker is based upon vibration, agitation or gyration. Efficiency of sieving by sieve shaker depends on the particles size distribution, sieve load, the method of sieve shaking, the dimension and shape of the particle, and the ratio of open area of sieve to total area. The sieving operation can be affected by the friability and cohesiveness of the powder. During sieving the sample is subjected to horizontal or vertical movement in accordance with the chosen method. This causes a relative movement between the particles and the sieve. Depending on size the individual particles either pass through the sieve mesh or are retained on the sieve surface. The likelihood of a particle passing through the sieve mesh is determined by the ratio of the particle size to the sieve openings, the orientation of the particle and the number of encounters between the particle and the mesh openings. Thus sieving depends on the sieve movement and the sieving time.

Construction:

• The sieve shaker consists basically of a cradle for holding the sieves, a power unit and a base. The cradle consists of a platform fastened to the lower ends of two vertical support rods, Fig. 3.2. The upper ends of which are shocked mounted to a horizontal support that is free to pivot about its mounting. A sieve holder, a retaining ring and nuts on the vertical support rods hold the top bar firmly against the nest of sieves. The standard sieves of different sieve numbers are used.

 Working:

• The sieving by sieve shaker is conducted using up to 11 sieves stacked with progressively larger aperture openings towards the top. The sample is placed on the top sieve. A closed pan (receiver) is placed at the bottom. There are several methods for shaking the sieves by mechanical or ultrasonic means. The residues in each sieve are recorded and expressed in percentage as cumulative values against the nominal sieve aperture values. The common methods of sieving are machine, wet, hand and air-jet sieving. Wet-sieving is recommended for material originally suspended in a liquid and is necessary for powders which form aggregates when dry-sieved. In such sieving the stack of sieves is filled with liquid and the sample is fed to the top sieve. Sieving is accomplished by rinsing, vibration, reciprocating action, vacuum, ultrasonication or a combination of these.

Manual and mechanical sieving: 

• Today, manual sieving is only used where no electricity supply is available, for example, for rapid on-site random checking for oversize and undersize. It is only used for orientation purposes. In contrast, sieve analyses in the laboratory and for quality assurance are carried out with sieve shakers. Modern sieve shakers are characterized by the fact that their mechanical parameters, such as sieving time and amplitude or speed, are carried out with exact reproducibility. In the laboratory a differentiation is made between horizontal sieve shakers and throw-action sieve shakers.

Throw-action sieving:

• Throw-action sieve shakers are also known as vibratory sieve shakers. An electromagnetic drive sets a spring-mass system in motion and transfers the oscillations to the sieve stack. The sample is subjected to a 3-dimensional movement and is distributed uniformly across the whole area of the sieve, Fig. 3.3. The amplitude can normally be set continuously in the range from 0-2 mm or 0-3 mm. Modern instruments enable the required amplitude to be entered digitally. During the sieving process, a built-in measuring system and control unit performs a continuous comparison between the set and actual amplitude values. This provides the optimal preconditions for reproducible sieving parameters. Digital accuracy for the sieving time and the interval function is a matter of course. 

Horizontal sieving:

• In a horizontal sieve shaker the sieves move in horizontal circles in one plane, Fig. 3.4. Horizontal sieve shakers are preferably used for needle-shaped, flat, long or fibrous samples, as their horizontal orientation means that only a few disorientated particles enter the mesh and the sieve is not blocked so quickly.

• The AS 400 control permits the use of test sieves with a diameter up to 400 mm. The large sieving area makes it possible to sieve large amounts of sample, for example as encountered in the particle size analysis of coarse granules and aggregates. In addition, in the fine particle size range and for obtaining single fractions, ultrasonic and airjet sieving is used for special application.

Sieve set sieving:

• Sieve set sieving is the process in which a set of several sieves is used together with a collector pan. The tests sieves are arranged in a stack with the largest mesh openings at the top of the stack. The sample is placed on the top sieve and allowed to pass through in lower sieves.

Dry and wet sieving: 

• Most sieving processes are carried out on dry materials. However, there are many applications in which wet sieving cannot be avoided, for example, suspension or when a very fine sample that tends to agglomerate has to be sieved. As in dry sieving, a sieve stack is assembled on a sieve shaker. The sieving process is additionally supported by water from a spray nozzle located above the uppermost sieve. The sample is placed on the uppermost sieve in the form of a suspension. Rinsing is carried out until the sieving liquid leaving the sieve stack outlet is no longer turbid with solid particles. In wet sieving, the sieving liquid must not alter the physical or chemical properties of the sample.

Optimal sieving time and amplitude or speed

• The settings for the sieving time and the optimal amplitude or speed depend on the material to be sieved. National and international standards, internal regulations and standards normally provide detailed information about product-specific sieve analyses and their associated sieving parameters. The instruction manual for the sieve shaker should also provide guidelines for this. If this basic information does not exist then the sieving time and amplitude or speed must be determined experimentally. This is done by first selecting a relatively short sieving time (for example, 5 min) and carrying out sieving at various amplitudes or speeds to determine at which values the largest amount of sample passes through the sieves (optimal sieving quality).

Sieving Aids:

• Sieving aids are used for very fine samples that tend to adhere together. They are used to make the sample sievable. A differentiation is made between mechanical sieving aids (for example, rubber cubes, brushes, balls, chains) for eliminating molecular adhesive forces, and additives (for example, talcum, Aerosil) for greasy, sticky and oil-containing products. Antistatic sprays reduce electrostatic charges whereas surfactants reduce the surface tension in wet sieving. A complete sieving process includes the following steps which should be performed in a precise careful manner.

• (a) Sampling. 

•  (b) Sample division (if required). 

•  (c) Selection of suitable test sieves. 

•  (d) Selection of sieving parameters.

•  (e) Actual sieve analysis.

• (f) Recovery of sample material.

•  (g) Data evaluation. 

•  (h) Cleaning and drying the test sieves. 

Merits:  

Sieve shaker is simple to operate. 

• It separates samples rapidly. 

• It is suitable only for particle size up to 50 µm. 

• Requires less area for its installation. 

• Results of particle sizing are accurate and reproducible. 

• Cost of the instrument is lower than other methods.

Demerits:

• For materials finer than 100 mesh, dry sieving can be significantly less accurate. 

• Sieve analysis assumes that all particle will be round (spherical) or nearly so but in reality it is not the fact. 

• For elongated and flat particles a sieve analysis does not yield reliable mass-based results. 

• Not suitable for particles smaller than 50 µm. 

• There is a possibility of further reduction in size, which can cause errors. 

• Sieves could be clogged and distorted if not properly handled and maintained.

Applications: 

• Sieve shaker is used for particle size analysis of variety of materials. 

• It is suitable for coarse material down to 150 µm. (

• It can be used for wet sieve analysis where the material analyzed is not affected by the liquid - except to disperse it.

Cyclone Separator 

• Cyclone separators provide a method of removing particulate matter from air or other gas streams at low cost and low maintenance. This separator is mainly used for the separation of solids from the liquids. It mainly consists of tangential inlet to feed the materials inside the chamber. It has solid and fluid outlets which separate fluid through one side and solids through the other side.

Principle: 

• In the cyclone separator the centrifugal force is used to separate solids from the fluids (liquid or gas/air). The separation depends on the particle size as well as on the density of the particles. Hence depending on the fluid velocity the cyclone separator can be used to separate all types of the particles. Especially it is used to remove only coarse particles and allows fine particles to be carried through with the fluid. It simply transforms the inertia force of a gas particle to a centrifugal force by means of a vortex generated in the cyclone body.

• In a cyclone, the air containing particulate material is forced along the tangential axis. A helical flow pattern is set up within the chamber. The centrifugal force causes the particles to migrate to the outside of the chamber. Here they fall down to the bottom of the cyclone by gravity. The collected particulates are allowed to exit out an underflow pipe while the gas phase reverses its axial direction of flow and exits out through the vortex finder (gas outlet tube). The air moves-up the center of the cyclone and reaches the top. 

Construction:  

• Cyclone separators are a bit complicated in design with better particle removal efficiency. Cyclones are basically centrifugal separators. It consists of a cylindrical vessel referred to as the barrel with the conical base, Fig. 3.5. The upper part of the vessel is fitted with a tangential inlet and a fluid outlet and at the base it is fitted with the solid outlet. Cyclones have no moving parts and are available in many shapes and sizes. For example, cyclone separators of 1 to 2 cm diameter is used for particle size analysis and upto 5 m diameter is used after wet scrubbers, but the basic separation principle remains the same.

Working: 

• In the suspensions of a solid-gas usually air is introduced tangentially at a very high velocity so that rotary movement takes place within the vessel. The fluid (gas) is removed from the central outlet at the top. The rotatory flow within the cyclone separator causes the particles to be acted on by centrifugal force. The solids are thrown out to the walls forming an outer vortex, thereafter it falls to the conical base and discharged out through solids outlet. The increasing air velocity in the outer vortex results in a centrifugal force on the particles separating them from the air stream. When the air reaches the bottom of the cone, it begins to flow radially inwards and out at the top as clean air/gas while the particulates fall into the dust collection chamber at the bottom of the cyclone.

• The collection efficiency of cyclones varies as a function of density, particle size and cyclone design. Cyclone efficiency generally increases with increase in particle size and/or density, inlet duct velocity, cyclone body length, number of gas revolutions in the cyclone, ratio of cyclone body diameter to gas exit diameter, inlet dust loading, smoothness of the cyclone inner wall etc. Similarly, cyclone efficiency may decrease with increases in the parameters such as gas viscosity, cyclone body diameter, gas exit diameter, gas inlet duct area, gas density, leakage of air into the dust outlet etc. 

Merits:  

• Cyclone separator requires low capital investment. 

• It has high efficiency for 5 - 200 µm particles. 

• It produces high volume flow rate. 

• A lack of moving parts reduces wear and tear.

• It can be operated on continuous or batch process. 

• It requires virtually no downtime for maintenance or recovery. 

• It has versatile applications.

• It is small in size relative to other separation equipment. 

• It can be operated at a wide range of temperatures and pressures.

Demerits: 

• It shows reduced efficiency when overloaded than its capacity.

• It finds difficulty in obtaining good separation of substances with similar densities. 

• It cannot handle viscous flow. 

• The extremely high velocities cause abrasive wear.

• Clogging of the dust outlet is common in reverse flow cyclones.

Applications:

A cyclone separator is most often used to separate "heavies" from a liquid mixture originating at a centrifugal pump or some other continuous source of pressurized liquid. A cyclone separator is most likely to be the right choice for processes where "lights" are the greater part of the mixture and where the "heavies" settle fairly easily. These may be used to separate solids from water.

• Cyclone separator is used to separate the suspensions of a solid in a gas or air. It can be used with the liquid's suspensions of solids. 

• It is used in the production of active pharmaceutical ingredients (API) through drying systems such as spray or fluid bed drying.

• It can also be used for separation of particles based upon size, specific gravity, porosity and concentration from solids, liquids and gases. 

• In pharmaceutical industry it is mostly used for separation of fines from coarse granules.

• In tablet compression cyclone is used to extract waste powder before it reaches the central extraction system.

• It can be used in air-handling systems to produce particle free clean air. (vii) It is used in oil industry to separate oil from water or vice versa.

Air Separator 

• An air separator is usually a mechanical device with large metal box containing a blower, heating or cooling elements and filter racks or chambers, sound attenuators, and dampers. Air separators are usually connected to a duct of a ventilation system that distributes the air and fine particles.

Principle:   

• The air separator works on the same principle of cyclone separator. The cyclone separators sometimes are not efficient in separating powders that contain very fine size particles. In such situations an additional provision is made that combines current of air with the centrifugal force. This causes fines to separate along with air and coarse particles to thrown away by centrifugal force which is collected at the bottom.

Construction:

• It consists of vertical metal cylindrical vessel with conical base at the bottom, Fig. 3.6. The feed inlet is fitted tangentially at the upper part of vessel. The outlet for collected solids is at the base of conical portion where as fluid outlet is at the canter of the top portion. The fluid outlet pipe extends down below inlet section to avoid air short-circuiting directly from the inlet into the outlet. The rotating disc and rotating blades are fitted on shaft is placed at the center of the vessel. It has two separate outlets at bottom for finer and coarser/heavy particles. 

Working:

• The feed enters through the inlet tangentially in the upper part of vessel. Feed falls on the rotating blades. The rotating blades produce an air jet in the direction indicated in. The fine particles are blown away on the walls by centrifugal force generated with the air jet and are collected at the bottom. The coarser particles due to their large mass travel less distance from the center of the separator and falls in the coarse particle collection zone which is collected at its discharge.

Merits:  

• It is easy to install. 

• The rotor speed as well as air flow is adjustable. 

• It has high product output. 

• It is easy to clean and maintain. 

• It is superior to sieving method in terms of output and quality of products. (vi) It is used in batch as well as continues mode.

Demerits:  

• If particles are too fine (<5 µm) its efficiency decreases. 

• It is not suitable for wet and sticky materials.

Applications: 

• It is used as dust collectors in many processes to either recover valuable granular solid or powder from process streams. 

• It can be used to separate sub-micron size particles those cannot be handled by sieving. 

• It is often used as an air pollution control device to maintain or improve air quality in pharmaceutical production areas. 

• It can be used as mist collectors to remove particulate matter in the form of find liquid droplets from the air to improve or maintain the quality of air in the workplace environment. 

• It can be used to remove granular solid pollutants from exhaust gases prior to venting to the atmosphere. 

• It can be used for the collection of metal working fluids, and coolant or oil mists.

Bag Filter  

• The bag filter is a mechanical device used in quality of air in pharmaceutical production and other allied areas. In pharmaceutical production very potent drugs such as hormones, vitamins, antibiotics etc generates lot of dust during processing which may be life threatening to the operators. These processes include screening, blending, mixing, drying, granulating, tableting, compression, packaging etc. It causes air pollution and chemical hazards. In order to avoid these unwanted situations, the air is needed to be maintained clean. This is achieved by use of filter bags. A fabric filter is a dust collection device made using a woven or non-woven filter bag that filters and collects the dust in process gas. When the filter cloth is made into a cylindrical-shaped bag and suspended, it is referred to as a bag house or a fabric filter.

Principle:  

•  The purpose of filter bag to collect dust is based upon the principle of filtration. The dust layer adheres to and is deposited on the surface of the filter bag and the interior of the filter cloth (the primary dust layer) filters and collects the dust contained in the process gas. The fabric provides surface for dust particles to get accumulated. The accumulation or collection takes place by inertial or electrostatic interaction, interception and Brownian movement. Together these mechanism results in formation of the dust cake on fabric surface.

Construction: 

• This equipment consists of a big metal vessel (bag house) with series of fabric bags in compartments, The bags are made-up of woven cotton, wool, membranes, sintered metal fibers or ceramic cartridges. The selection is based upon the operating temperature and pressure and stability of filter medium to these conditions. Filter bags are suspended in invert position in the vessel. The length of bags varies from 2 to 10 m with a diameter up to 40 cm. The open ends of the bags are attached to the manifold. The number of bags in a vessel varies from 100 to 1000 or more depending on bag house. In the bottom portion hopper is provided to collect dust held by the filter.

Working: 

• rking: The processing gas enters through the inlet pipe that strikes the baffle plate. This striking causes large particles to fall down due to gravity in the hopper at the bottom. Carrier gas then flows in upward direction and leaves through the bags leaving behind fine particulate matter on its interior surface. Normally the filtration velocity of the process gas passing through a filter cloth is about 0.3-2 m/min, and the pressure loss is 1-2 kPa. As the dust layer collected on the surface of the filter cloth becomes thicker, the pressure loss of the filter cloth increases, so the collected dust is intermittently removed. Removing the dust layer is carried out by mechanical shaking, reverse pressure, or pulse jet. In most cases the dust collection efficiency of fabric filters is 99% or higher, and the dust concentration at the outlet is less than 10 mg/m³. In order to achieve better efficiency filter bags are cleaned, maintained and changed intermittently.

Merits: 

• Filter bag is best method amongst allfor removing fines from the air.
• Electricity consumption is low.
• It helps to maintain and protect healthy environment. 
• They are simple in construction and operation.
• It has versatility and effective design. 
• It helps to reduce housekeeping and better product quality. 
• Filter bags has effective design according to American Ventilation System Standards
• High quality filter bags have trouble free operation. 
• It has robust construction.

Demerits: 

• It has limitations for its operation due to high gas temperature and high humidity. 
• The maintenance cost is high as fabric used is costly. 
• The characteristics of fabric change with operating parameters. 
• Comparatively it is large in size. 
• Condensation of vapours and presence of hygroscopic material reduces its efficiency.

Applications: 

• Bag filters are used in industries to separate dust particles from the air.
• These are used to clean the air in working areas. 
• They are extensively used in large industries that produce different kind of dust such as metals, cement, chalk and lime, ceramics, flour and foundries. 
• It is most commonly used in fluidized bed dryer.

Elutriation Tank

• An elutriator is a simple device which can separate particles into two or more groups. Elutriation depends on the movement of a fluid against the direction of sedimentation of the particles. Elutriation is based on particle size, shape and density. The air elutriator is mainly used for particles smaller than 1 µm. The smaller or lighter particles rise to the top (overflow) because their terminal sedimentation velocities are lower than the velocity of the rising fluid. The terminal velocity of any particle in any medium can be calculated using Stokes' law if the particle's Reynolds number is below 0.2. In elutriator particles separation is based upon densities. The heavier particles settle due to gravitational force.

Principle: 

• In the process of elutriation, particles falling in a rising fluid can be classified into two sizes. When the fluid in a sorting column is rising with a certain velocity, the particles having terminal velocities higher than this velocity settle at the bottom of the sorting column and the particles with lower terminal velocities are lifted to the top of the sorting column and are carried away to the next tube. Terminal velocities of the particles falling in a fluid can be calculated using the Stokes' law equation for Reynolds number. Re = 2r × Vm × (ρ − ρo) η …  Vm = 2r2 × g × (ρ − ρo) 9η …Where, ρ = specific gravity of the particle, ρo specific gravity of the fluid medium, r is radius of the particle, η = viscosity of the medium, Vm = terminal velocity of the particle and g = gravitational acceleration. In the elutriation, when the volumetric flow rate of rising fluid is constant, the velocity of the rising fluid in the columns depends on their diameters. The narrow diameter column gives high velocity and the one with wide diameter gives low fluid velocity. Higher velocities of the rising medium allow coarser particles to settle while lower velocities allow finer particles to settle. Various size classes of particles can be obtained when the sample is separated in columns of increasing diameters connected in series. Upper size limit of the particles follows Stokes' law.

Construction: 

• For the gravitational system, the apparatus consists simply of a vertical column with an inlet near the bottom for the suspension, an outlet at the base for coarse particles, and an overflow near the top for fluid, One column will give a single separation into two fractions, but it must be remembered that this will not give a clear-cut separation, since there is a velocity gradient across the tube, resulting in the separation of particles of different sizes according to the distance from the wall. If more than one fraction is required, a number of tubes of increasing area of cross-section can be connected in series.

• With the same overall flowrate, the velocity will decrease in succeeding tubes as the area of cross-section increases, giving a number of fractions.

Working:  

• The material whose particles are to be separated is first levigated and the paste is transferred to elutriation tank. A large amount of water is added to tank so as to make independent particle settling. The contents in the tank are stirred to obtain uniform particle distribution. If left aside undisturbed coarse particles settle at the bottom whereas small size particles remain suspended in liquid. These fines can be transferred to next elutriator in connection wherein similar process of separation takes place to obtain further fractions of fines.

Merits: 

• The process is continuous. 

• The separation is quicker than with sedimentation.

• It has feasibility to add many columns based upon fractions required. 

• It needs no skilled operators. 

• It is a fast process than sedimentation.

Demerits:

• The suspension has to be dilute, which may sometimes be undesirable. 

• It separates particles based on their sedimentation property but not on specific features (for example, surface or shape). 

• It cannot separate different types of particles which have similar sedimentation properties.

Applications:

• Both simple and multiple elutriators are used for similar purposes following a size reduction process, with the object of separating oversize particles, which may be returned for further grinding, used for other purposes, or discarded according to the circumstances.

• With liquids, it can be used to separate insoluble solids, such as kaolin or chalk, which are often subjected to wet grinding followed by sedimentation or elutriation with water. (iii) 

• With gases, it can be applicable to finer solids that would separate too slowly in liquids, to water-soluble substances, or where dry processing is required.