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Unit:- 1 Size Separation PDF Download



    UNIT-I.                     BP304T (Pharmaceutical Engineering)

    1. Size Separation: Objectives, applications & mechanism of size separation, official
        standards of powders, sieves, size separation Principles, construction, working, uses,
        merits and demerits of Sieve shaker, cyclone separator, Air separator, Bag filter &
        elutriation tank.

    Definition: 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
    screening surfaces. Size separation is also known as sieving, sifting, screening.
    This technique is based on physical differences b/w the particles such as size,
    shape and density.
    Objective/Application of size separation:
    To prepare granulesof desired size to ensure good flowability.
    To seperate undesirable particles.
    To measure particle size and size distribution.
    To know the efficiency of size reduction equipments.
    To get uniform dosage form.
    During filling of capsule the particles must be uniform in weight and doses.

    Mechanism of size seperation

        1) AGITATION:- In this sieves are agitated in number of ways-
        a) Oscillation:- In this method sieve is oscillate at high speed either electrically or

        b) Vibration:- In this method sieve is vibrated at high speed either electrically or
        c) Gyration:- In this method sieve is rotated either clockwise or anticlockwise to the
        particles that help then to pass through the sieve.

        2) BRUSHING:- In this method, the brush is used to move the particles on the surface
        of the sieve and to keep the meshes clear.

        3) CENTRIFUGAL FORCE:- In This method the high speed motor is fixed inside a
        vertical cylinder sieve, so that during the rotation the particles are thrown out by the
        centrifugal force. The air stream can be generated by means of air jetin the
        equipment, which helps in the separation of particles.

In sieving process the particles that get remain on screening surface is known as oversize and
the material that pass through screening surface is known as under-size.
Generally powders are classified as :-

1) Coarse powder:- It is powder in which all the particles pass through sieve no. 10 and not and
not more that 40% pass through sieve no. 44.
It is usually referred to as 10/44 powder

Moderately coarse powder:- It is powder in which all the particles pass through sieve no. 22
and not more than 40% pass through sieve no. 60.
It is usually referred as 20/60 powder.

Moderately fine powder:- It is powder in which all the particles pass through sieve no. 44 and
not more than 40% pass through sieve no. 85.
It is usually referred as 44/85 powder.

Fine powder:- It is powder in which all the particles pass through sieve no. 85 .

Very fine powder:- It is powder in which all the particles pass through sieve no. 120 .

    Sieves are use for Size separation. Sieves are constructed from wire cloth with
    square meshes, woven from wires of brass, bronze, stainless steel etc. Each
    sieve is given a definite number which denotes number of the meshes present in
    length of 2.54cm or one inch. Generally, Iron wire is used as screen material
    because it is cheap but their disadvantage are corrosive nature and chance to
    contamination by Iron. This advantage cane be overcome by coating iron with
    galvanizing agent which makes it corrosion resistant.
    Brass, stainless steel, and non- metals are used due to their corrosion resistant,
    good strength and non contamination qualities.

    Holes in the screen is called Mesh.
    Mesh number indicate number of holes included in a length of 1 inch.
    Screen number denotes number of meshes in a linear length of 25.4mm.

                                                                           25.4 mm
     `         It is required that wire-mesh sieves                        (1 inch)
     will be made from wire of uniform, circular
     cross- section and for each sieve the                                                        wire
     following particulars are stated:                                                            diameter
     Number of sieve
     This is the number of meshes in a length of 25.4 mm
     25.4mm (i.e. 1 inch), in each direction.       (1 inch)

                                                                             5 mesh

    Nominal diameter of the wire
    The diameter of the wire is represented by Standard Wire Gauge.
Common standard use for sieves –
  a) Tyler Standard sieve :- It is U.S. standard sieves. They are available in
     Mesh no.4-325.
  b) British standard sieve :- It is available in Mesh no. 5-300. It is used in U.K   Nominal aperture
  c) F.S.S:- French standard sieve available in Mesh no. 17-38
  d) IP standard sieve:- It is commonly used in India.

     Sieve shaker is an instrument that used in particle analysis. It is
     used to shake a stack of test sieves which are placed in order
     (largest aperture on the top and smallest on the bottom),so that
     materials get sifted through according to particle sizes.
     Sieve shakers can replace manual hand sieving to conduct
     sample size reduction, material separation and assist particle

 Shaking screen consist of metal frame to which screen are fixed.
The standard sieves of different mesh number (as per IP and USP) are used.

Working: The sieve are arranged in such a way that the largest aperture will be at the topand
smallest aperture will be at the bottom. A sieve nest consist of 6 to 8 sieves powder sample having
weight of 50 gm is placed on top most sieve. Then close the sieve. Set and fixed it a mechanical
shaker apparatus. Then start shaking the sieve set for 20 min. The powder retained on each sieve is
collected and weighed.

        • Less power requirement
         •   Easy to use
         •   Using a sieve shaker is a more efficient method compared with manual hand sieving.
             Once the timer is set up the experiment will run automatically without needing users
             to be present.
         •   Shakers provide more flexibility than manual hand sieving,

     During shaking attrition may occurs.
     Chance to clogging of sieve if powder is not properly dry.

     In cyclone separator centrifugal force is used to separate solid from fluids. The separation process
     depends on particle size and particle density. It is also possible to allow fine particles to be carried with
     the fluid.

Construction                                                              Tangential
     It consists of a short vertical, cylindrical vessel with a           inlet
     conical base. The upper part of the vessel is fitted with a
     tangential inlet. The solid outlet is at the base. Fluid outlet is
     provided at the center of the top portion, which extends

     inwardly into the separator. Such an arrangement prevents                         Fig. Cyclone separator
     the air short-circuiting directly from the inlet to the outlet of the fluid.

     The solids to be separated are suspended in a stream of fluid (usually air or water). Such feed
     is introduced tangentially at a very high velocity, so that rotary movement takes place within
     the vessel. The centrifugal force throws the particles to the wall of the vessel. As the speed of
     the fluid (air) diminishes, the particles fall to the base and collected at the solid outlet. The
     fluid (air) can escape from the central outlet at the top.
High efficiency 5-200 micron particles

Can not handle viscous flow
Clogging of the dust particles is common in reverse flow cyclones.

Uses                                                                                              Rotating
      1. Cyclone separators are used to separate solid particles                                  shaft

         from gases.                                                           Feed        Feed
                                                                                                             Fan blade

      2. It is also used for size separation of solids in liquids.                                           Settling
      3. It is used to separate the heavy and coarse fraction from                                           chamber

         fine dust.                                                                                        Rotating

Principle                                                                                  Fine particle
      The cyclone separator alone cannot carry out size separation
                                                                       Coarse Fig.   Air separator
      on fine materials. For such separations a current of air         particle

combined with centrifugal force is used. The finer
particles are carried away by air and thecoarser particles
are thrown by centrifugal force, which fall at the

      It consists of a cylindrical vessel with a conical
      base. A rotating plate is fitted on a shaft placed at
      the center of the vessel. A set of fan blades are
      also fitted with the same shaft. At the base of the
      vessel two outlets are provided: one for the finer
      particles and the other for coarse particles.

      The disc and the fan are rotated by means of a
      motor. The feed (powder) enters at the center of
      the vessel and falls of the rotating plate. The
      rotating fan blades produce a draft (flow) of air in
      the direction as shown in the diagram. The fine
      particles are picked up by the draft of air and
      carried into space of settling chamber, where the
      air velocity is sufficiently reduced so that the fine
      particles are dropped and removed through the
      fine particle outlet.
Particles too heavy to be picked up by the air stream are removed at
the coarse particle outlet.
Advantage:- Easy Installation and rotor speed is adjustable.
                High product capacity

Uses                                                                              Discharge

  Air separators are often             Bell-Crank
  attached to the ball mill or         Assembly                              Metal
  hammer mill to separate and
  return over sized particles for                                            Filter
  further size reduction.
                                                                    Feed                               Air
                                                                    inlet                              outlet

Principle                                                                   Dust hopper

  In a bag filter, size separation              Product
  of fines (or dust) from the                   (Closed)                                             (Opened)
                                                (a) Filtering Period                 (b) Shaking Period
  milled powder is achieved in                                       Fig. Bag filter
  two steps. In the first step, the
  milled powder is passed through a bag (made from cloth) by applying suction on the opposite
  side of the feed entry. This facilitates the separation. In the next step, pressure is applied in
  order to shake the bags so that powder adhering to the bag falls off, which is collected from
  the conical base.
  It consists of a number of bags made of cotton or wool fabric. These are suspended in a metal
  container. A hopper is arranged at the bottom of the filter to receive the feed. At the top of the
  metal container, a provision is made for vacuum fan and exhaust through discharge manifold.
  At the top of the vessel a bell-crank lever arrangement is made to change the action from
  filtering to shaking.
  (a) Filtering period: During this period the vacuum fan produce a pressure lower than the
      atmospheric pressure within the vessel. Gas to be filtered enters the hopper, passes
      through the bags, and out of the top of the apparatus. The particles are retained within the
  (b) Shaking period: During this period the bell-crank lever first close the discharge manifold
      and air enters through the top so the vacuum is broken. At the same time it gives a violent
      jerking action to the bags so that they are freed from the dust. The fine particles are
      collected at the conical base.
  1. Bag filters are used along with other size separation equipment, e.g. a cyclone separator.
  2. They are use on the top of fluidized bed dryer for drying to separate the dusts.
  3. They are used to clean the air of a room.
  4. Household vacuum cleaner is a simple version of bag filter.

  Size separation by sedimentation utilizes the differences in settling velocities of the particles
  with different diameter (d) and these can be related to Stoke’s law.

Stoke’s law
  When a solid particle is suspended in a liquid the particle settles downward at a velocity, V.
  This velocity is called sedimentation rate. It is found that this rate of sedimentation depends
  on the diameter of the particle, density of the liquid and particle, viscosity of the liquid and
  the acceleration due to gravity. All this parameters can be combined in the form of Stoke’s
                                                                Where d = diameter of the particle
                           d 2 (ρ - ρ2 )g
                     V=                                                 1 = density of the particle
                              18                                        2 = density of the liquid
                              η                                       g = acceleration due to gravity
                                                                         = viscosity of the liquid.

                                                      Liquid Inlet                                          Liquid Outlet

A shallow tank is arranged with inlet and outlet
pipes as shown in the figure.
 Particles entering the tank will be acted upon by a force that can be divided
into two components:
    (i) a horizontal component due to the flow of liquid carrying the particles forward and
    (ii) a vertical component due to gravity, which causes the particles to fall towards the bottom of the
    This component is governed by Stoke’s law
    so that the velocity of sedimentation is proportional to the square of the diameter of the particles.
Thus the particles will settle at the bottom of the tank in such a way that the coarsest (largest)
particles will settle near to the inlet of liquid and the finest particles near to the outlet of the
liquid. Partitions are arranged at the floor of the tank to enable collection of different size
fraction particles.
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Pharmaceutical Engineering
1st sem
Bachelor of Pharmacy