Size separation topic of Pharmaceutical Engineering

1. Prepared by: Ms. Kadam A. J.
Assistant Professor (Institute
of Pharmacy, Loni)

2.  CONTENT:
 Objective
 Applications
 Official standards of powders
 Sieves
 Size separation principle
 Sieve shaker machine
 Cyclon separator
 Air separator
 Bag filter
 Illustriation tank

3.  Size Separation
 A special technique is used to separate particles of specified
size which is known as the "process of size separation".
 Large pieces of material are usually estimated visually,
difficulties arising only in the estimation of powders. size
separation is a unit operation that involves the separation of
mixture of various size of particles into two or more portion
by means of screening surface also known as sieving, sifting,
classifying or screening.

4.  Objectives:
 To obtain powders of desired size distribution.
 To improve mixing
 To set a quality control parameter for raw material.
 To avoid the variations in physicochemical properties.
 Application:
1. Capsule filling: uniform size particles are easy to fill in the
capsule giving good flow properties.
2. Tablet formulation: uniform particle size ensures good flow from
hopper which avoids weight variation.
3. Removal of impurities on the basis of size.
4. Separation of solids from a gas suspension.

5.  Standards for powders for pharmaceutical purposes are laid
down principally in the Indian Pharmacopoeia 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 IP specifies five grades of powder and the number of the
sieve through which all the particles must pass.

6. Grade of powder
Sieve through which all
particles must pass
Coarse 10
Moderately coarse 22
Moderately fine 44
Fine 85
Very fine 120
The IP specifies a second, smaller size of sieve for the coarser
powders but states that not more than 40 per cent shall pass
through. The relevant grades of powder and sieve number are
shown in the table:

7. Grade of powder Sieve through
which all particles
must pass
Sieve through
which not more
than 40 per cent of
particles pass
Coarse 10 44
Moderately coarse 22 60
Moderately fine 44 85
Fine 85 Not specified
Very fine 120 Not specified

8.  1. Coarse powder : A powder of which al! the particles pass through a
sieve with nominal mesh aperture of 1.70 mm (No. 10 sieve) and not
more than 40.0 per cent through a sieve with nominal mesh aperture of
355 um (No. 44 sieve) is called coarse powder.
 2. Moderately coarse powder: A powder of which all the particles
pass through a sieve with nominal mesh aperture of 710 um (No. 22
sieve) and not more than 40.0 per cent through a sieve with nominal
mesh aperture of 250 um (No. 60 sieve) is called moderately coarse
powder.
3. Moderately fine powder : If all the particles of a powder pass
through a sieve with nominal mesh aperture of 355 um (No. 44 sieve)
and not more than 40.0 per cent through a sieve with nominal mesh
aperture of 180 um(No. 85 sieve), it falls in this group.

9.  4. Fine powder : In case all the particles pass through a sieve with a
nominal mesh aperture of 180 μm (No. 85 sieve), it is called fine
powder,
 5. Very fine powder : If all the particles of the powder pass through
a sieve with a nominal mesh aperture of 125 μm (No. 120sieve), it is
said to be very fine powder.
 SIEVES
 Sieves for pharmacopoeial testing are constructed from wire cloth
with square meshes, woven from wires of brass, bronze, stainless
steel or any other suitable material. The wires should be of uniform
circular cross-section and should not be coated or plated. There
should not be any reaction between the material of the sieve and the
substance which is being sifted from it.

10.  Types of Sieves:
 1. Woven Wire sieves: These are general purpose sieves and
widely used in pharmacy practice. Types: a) Plain Weave b)
Twilled Weave
 2. Bolting cloth Sieves: Silk, nylon and cotton are generally
used. These used for separation of fine powders.
 3. Bar screen: These are used in handling large and heavy
pieces of materials.
 4. Punched Plates (Perforated screen): These are used for
coarse sizing.

13.  Standards for sieves used for pharmacopoeial testing must
specify the following:
 1. Number of sieve : Sieve number indicates the number of
meshes in a length of 2.54 cm in each transverse direction
parallel to the wires.
 2. Nominal size of aperture : Nominal size of aperture
indicates the distance between the wires. It represents the
length of the side of the square aperture. The I.P. has given the
nominal mesh aperture size for majority of sieves in mm or
in cm.

14. 3. Nominal diameter of the wire : Wire mesh
sieves are
made from the wire having the specified diameter
in order to give a suitable aperture size and
sufficient strength to
avoid distortion of the sieve.
4. Approximate percentage sieving area : This
standard
expresses the area of the meshes a percentage of
the total area of the sieve. It depends on the size of
the wire used for any particular sieve number.
Generally the sieving area is kept within the range
of 35 to 40 per cent in order to give suitable
strength to the sieve.

15.  5. Tolerance average aperture size : Some variation in the
aperture size is unavoidable and when this variation is
expressed as a percentage, it is known as the 'aperture
tolerance average'.
 The term tolerance is used in engineering practice to mean the
limits within which a particular quantity or dimension can be
allowed to vary and still be acceptable for the purpose for
which it is required.

16. Effect of wire diameter on sieve mesh size.
Finer wires are likely to be subject to a greater proportional
variation in diameter than coarse.
fine meshes cannot be woven with the same accuracy as coarse
meshes. Hence, the aperture tolerance average is smaller for sieves
of 5 or 10 mesh than is the case for 300 mesh.

17.  SIEVING METHOD
 In this method, the fine powder is separated from the coarse
powder by using sieves of desired number. The degree of
fineness of a powder is known with the help of sieve through
which the powdered material is passed. Sieves are numbered
in order to distinguish from each other. Size separation of
powder is done by passing the powdered material through a
set of sieves. Sieves are arranged in descending order i.e.
sieve of larger size is at the top and the smallest one at the
bottom. The bottom sieve is attached to the receiving pan.

19.  The material is placed in the uppermost sieve.
The sieves are
shaken with the help of mechanical sieve shaker or
electromagnetic devices. It helps the particles to
pass through the sieves.
The working of mechanical sieving devices are
based on any of
the following methods.
1.Agitation 2. Brushing
3. Centrifugal
1. Agitation methods: Sieves may be agitated in a
number of
different ways, such as:
1. Oscillation :This sieve is mounted in a frame
that oscillates
back and forth. It is a simple method but the
material may roll on the surface of the sieve.

20. 2. Vibration : The sieve is vibrated at high speed by means of an
electric device. The rapid vibration is imparted to the particles
on the sieve which helps to pass the powdered material through it.
 3. Gyration : In this method, a system is made so that
sieve is on rubber mounting and connected to an eccentric fly wheel. This gives
a rotary movement of small amplitude to sieve which in turn gives spinning
motion to the particles that helps to pass them through a sieve. Agitation
methods are not continuous methods‘ but can be made so by inclination of the
sieve and the
provision of separate outlets for undersize and oversize particles.
 2.Brushing methods: In this case, a brush is used to move
the particles on the surface of the sieve and to keep the meshes clear. The brush
is rotated in the middle in the case of a circular sieve but spiral brush is rotated
on the longitudinal axis in case Of a horizontal cylindrical sieve.

21.  3. Centrifugal methods: In this method, a high speed rotor is fixed
inside the vertical cylindrical sieve, so that on rotation of rotor the
particles are thrown outwards by centrifugal force. The current of air
which is produced due to high speed of rotor helps in sieving the
powder. On shaking the powdered material in a mechanical or
electromagnetic device using any of the above methods, the weight of
powder retained on each sieve is determined. The percentage of each
fraction is then calculated. Sieving method is a rapid process and it
requires very little skill. The equipment used for sieving is not
expensive.
Advantages: 1. centrifugal methods are extremely useful in cases
where conventional sieving tends to blocks the sieves.
2. Useful for fine powder, because sieves have the limitation of mesh
size.

22.  Sieve Analysis- Testing of Powder
Size distribution analysis is important in different areas as:
1. quality control tool for the analysis of raw material
2. testing the efficiency of a size reduction equipment or process.
3. Optimising the process conditions such as method of agitation, time of
screening.
4. Selecting the sieve system for commercial equipment.
 Sieve Shaker Machine:
 Principle: The Powdered drug is separated according to its particle size
using a number of sieves in a nest. These are subjected to different types of
agitation, so that size separatipn is rapid.

24.  Construction: Standard sieves of different mesh numbers are available
commercially as per IP specification. These Sieves are fixed in a
mechanical shaker apparatus.
 Working: Sieves are arranged in a nest with the coarest at the top. A
sample (50g) of the powder is placed on the top sieve. This sieve set is
fixed to the mechanical shaker apparatus and shaker for a certain period of
time (20 min). The powder retained on each sieve is weighed.
 Uses: It can be used for handling a variety of dry powders, granules and
dry foods.
 Advantages: It is inexpensive,, simple and rapid with reproducible results.
 Disadvantages:1. Lower limit of particle size is 50 um.
2. If powder is not dry, apertures get clogged with particles, leading to
improper sieving.

25. Principle: In cyclone separator,
the centrifugal force is used to
separate solids from fluids. The
separation depends not only
on the particle size but also on
density of particles. Hence
depending on the fluid velocity,
the cyclone separator can be
used to separate all types of
particles or to remove only
coarse particles and allow fine
particles to be carried through
with the fluid.

26.  Construction: It consists of a cylindrical vessel with a conical
base. In the upper part of the vessel is fitted with a tangential
inlet and a fluid outlet and at the base it is fitted with solid
outlet.
 Working: The suspension of a solid in gas (usually air) is
introduced tangentially at a very high velocity, so that rotary
movement takes place within the vessel. The fluid is removed
from a 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,
thereafter it falls to the conical base and discharged out through
solids outlet.

28.  Uses: 1.Cyclone separators are used to separate the suspension
of a solid in a gas (air).
2. It can be used with liquid suspensions of solids.
3. It is used in oil refineries to separate oils and gases.
 Advantages: 1. low capital cost.
2. Relative simplicity and few maintenance problems.
3. Low pressure drop.
4. Dry collection and disposal.
5. Relatively small space requirement.
6. No moving part.
7. Can be used under extreme processing conditions.
 Disadvantages: inability to handle sticky material.

29.  AIR SEPARATOR
Principle: It works on the same principle as that of cyclone separator.
But in this case the air movement is obtained by means of rotating disc and
blades. To improve the separation, the stationary blades are used. By
controlling these blades and the speed of rotation, it is possible to vary the
size at which separation occurs

30.  Construction: It consists of a cylindrical vessel with a conical base. In the
upper part of the separator the vessel is fitted with feed inlet, and at the base
there are two outlets, one for line particles and other for heavy particles.
The rotating disc and rotating blades are attached to the central shaft, to
 produce air movement

31.  Working : As shown in the Fig. 6-3, the sample powder is passed through
the feed inlet, which falls on the rotating disc. The rotating blades are
attached to the same shaft. These produce a current of air as shown by the
arrows. The fine particles are picked up and are carried into space, where
air velocity is sufficiently reduced. The fine particles are dropped and
ultimately collected at an outlet meant for fine particles. The heavy
particles which fall downward are removed at an outlet meant for heavy
particles.
Uses: Air separator is often attached to the ball mill or hammer mill to
separate and return oversized particles for further size reduction.
Advantages: 1. it gives efficient separation in smaller apparatus.
2. The waste does not interact with the generator of the air flow, avoiding
wear and clogging.
3. Low maintenance.
4. High reliability.
5. Low pressure drop.
Disadvantages: 1. Low separation yield.
2. Unsuitable for separating smaller particles.

32.  Filter Bag:
 Principle: In a Bag filter, size separation of fines from the
milled powder is achieved in two steps.
A) the milled powder is passed through a bag filter by applying
the suction on the opposite of the feed entry.
B) 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.
 Construction: It consists of number of bags made of cotton or
wool fabric. These are suspended in a sheet 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
the exhaust. Adjacent to this, a bell cranck lever arrangement
is made to bring the filters to normal atmospheric conditions.

34.  Working: A) Filtering period- during this period the vaccum fan
produces 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 bags.
B) Shaking period- during this period the bell-cranck lever first close
the discharge manifold and air enters through the top so the vaccum 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.
 Uses: 1. it is 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.
 Advantages: 1. bag filter is extremely useful for removing fines,
which cannot be separated by other methods.
2. These can be used to remove dust.
3. No high voltage requirments.

35.  Disadvantage: 1. it has a high resistance which is about 600-
1200 P.
 2. collection of hygroscopic materials or condesation of
moisture can lead to fabric plugging loss of cleaning
efficiency, large pressure losses.