Size separation

Size Separation
B. Pharm 2nd year, IIIrd Semester
Subject:- Pharmaceutical Engineering
Prepared by:
Mrs. Kokare Pratima. S.

Content
• 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
Size Separation by- Kokare Pratima S.
2

Introduction
• 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 sieving surfaces.
• Size separation is performed by
processes known as sieving, sifting,
segregation etc.
• 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.
3

Objectives
Useful in grading powders or granules1
To control size variations in the materials2
To enhance performance (topical powders), efficacy (powders
for inhalation) and stability of dosage forms (suspensions)
3
To judge uniformity in a mixing materials4
To avoid variations in the bulk properties of materials5
To specify the quality parameter of a intermediate and finished dosage
forms
6
4

Applications
To prepare granules of desired size to ensure good flowability1
To separate undesirable particles2
Size separation has significance in formulating uniform dosage forms with respect to
drug content3
It can be used to obtain granules of required size.4
It has applications in determination of particle size and their distribution in the given
samples.5
It can also be used to investigate efficiency and validate size reduction equipments.6
It is used to obtain monosized powders/particles which undergo least size segregation.7
5

Mechanism of size separation
• Size separation is done by following methods:
Mechanisms
Agitation
Oscillation Vibration Gyration
Brushing
Centrifugal
force
Other
Gravitational
force
Density
Electrostatic
force
6

Agitation Method
Oscillation
▪ Sieve is mounted in
inclined frame or rack
that oscillates back and
forth.
▪ Material rolls on the
surface of sieve and
sometimes forms a ball.
▪ Movement is achieved by
ordinary eccentric
movement of the rotating
motor shaft.
Vibration
▪ Vibration are either
sinusoidal or gyratory.
▪ This motion can causes
sieves to move-up and
down or left to right.
▪ Sieves in this mechanism
are mechanically or
electrically vibrated at
highspeed allowing
powder material to pass
through.
Gyration
▪ Sieves are kept on rubber
mounting which is
attached to rotary
eccentric flywheel.
▪ Rotary motion allows
particles to spin and pass
through the meshes.
7

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 center 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.
8

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.
• To aid particle separation further
an air jet can be fixed in the
cylinder.
9

Other Mechanisms
• Gravitational force- 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.
• Density- The density of the material relates to material
stratification. The classification in this case is based upon
weight of material.
• Electrostatic force- Electrostatic force can be applied to
sieve when particles are extremely dry or wet that help to
sieving.
10

Official Standards For Powder Size
• In sieving process the particle that get remain on screening
surface are known as oversize & the material that pass through
screening surface are known as undersize.
• The Indian Pharmacopoeia has laid down the standards for
powders for pharmaceutical purposes. The I.P. specifies five
grades of powder which are as follows:
Grade of powder
Sieve number through which all
particles must pass
Coarse 10
Moderately coarse 22
Moderately fine 44
Fine 85
Very fine 120
11

• The degree of coarseness or fineness is determined with
reference to nominal mesh aperture size.
• Classification of Powders as per I.P.
Grade of powder
Sieve number through which all
particles must pass
Sieve number through which
not more than 40 % of
particles pass
Coarse 10 44
Moderately coarse 22 60
Moderately fine 44 85
Fine 85 Not specified
Very fine 120 Not specified
12

• A powder of which all the particles pass through a sieve with
nominal mesh aperture of 1.70 mm (No. 10 sieve) and not more than
40.0% through a sieve with nominal mesh aperture of 355 nm (No.
44 sieve) is called coarse powder.
Coarse
Powder
• A powder of which all the particles pass through a sieve with
nominal mesh aperture of 710 nm (No. 22 sieve) and not more than
40.0% through a sieve with nominal mesh aperture of 250 nm (No.
60 sieve) is called moderately coarse powder.
Moderately
coarse
Powder
• If all the particles of a powder pass through a sieve with nominal
mesh aperture of 355 µm (No. 44 sieve) and not more than 40.0 %
through a sieve with nominal mesh aperture of 180 µm (No. 85
sieve), it falls in this group.
Moderately
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.
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.
Very fine
powder
13

Sieves
• Device for separating desired elements
from unwanted material or for
characterizing the particle size
distribution of a sample
• A sieve, also called Sifter.
• Most of sieves used for pharmaceutical
purpose are made up of cloth with
square meshes, woven from wires of
brass, bronze, stainless steel or any
other suitable material.
• Each sieve is given a definite number
which denotes number of meshes
present in a length of 2.54 cm or one
inch.
14

Types of Sieves
• Sieves commonly used in pharmaceutical
processing include:
15
Sieves
Woven wire
sieve
Bolting cloth
sieves
Closely spaced
bars (screens)
Punched
plates

Woven wire sieves
• These are general purpose sieves
& widely used in the pharmacy
practice
• 2 types of woven wire sieves:
1. Plain weave
2. Twilled weave
• For fine sieving- metal wire oven
sieves are used.
• In case of coarse sieves, the wire is
generally given a double crimp to
preserve the alignment of the wire.
16

Bolting cloth sieves
• Silk, nylon & cotton are generally
woven from twisted multi- strand
fibers.
• Nylon cloths generally designated
by their micrometer opening &
available in different grades.
These are used for the separation
of fine powders.
17

Bar screens
• Generally used in handling large
& heavy pieces of materials.
• The bars are fixed in parallel
position & held by cross bars &
spacers.
18

Punched plates (Perforated
plates)
• Used for coarse sizing.
• The screens are prepared by using a
sheet metal of varying thickness
with perforated holes. The holes
may be round, oval, square or
rectangular.
19

Standards of sieves
20
• Holes in the screen
Mesh
• Number of holes included in a length
of 1 inchMesh number
• Clear space between wires of screen
opening & it is given in terms of mm.
Aperture of
screen
• Denotes number of meshes in a
linear length of 25.4 mmScreen number
• Indicates gap between two adjacent
wires
Nominal
aperture size

• The sieves may be designated by opening size. The common
standards used for sieves are
A. Tyler standards sieve series: It is U. S. standard used for sieves.
B. U. S. Standard sieve series: It is used in U. S. A.
C. British Standard Sieve Series: It is available in mesh number 5-
300. It is extensively used in UK.
D. U. S. ASTM: It is “American Society of Testing Materials”
standard sieve series. It is available in mesh number 4-325.
E. German DIN (Deutsche Industrienormein): It is commonly
used in Europe & Germany.
F. F. S. S. : It is French Standard Sieve. It is available in mesh
number 17-38.
G. IP standard sieve series: It is commonly used in India.
H. International test sieve series (ISO): It is used world wide.
21

Principle, Construction, Working,
Uses, Merits & Demerits
• The milled material is subjected to size separation in order
to obtain the powder of desired size.
• 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.
22

Sieve Shaker
• Most widely used technique in pharmaceutical industry
for particle size analysis
❖ Principle: The principle of sieve shaker is based upon
vibration, agitation or gyration. 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.
❖ Construction:
✓ It consists basically of a cradle for holding the sieves (up
to 11 sieves stacked with progressively larger aperture
openings towards the top), a power unit and a base.
✓ The cradle consists of a platform fastened to the lower
ends of two vertical support rods. The upper ends of
which are shock mounted to a horizontal support that is
free to pivot about its mounting. Size Separation by- Kokare Pratima S.
23

❖ Working:
✓ A sample (50g) is placed on the top sieve.
✓ This sieve set is fixed to the mechanical shaker apparatus & shaken for a certain period of time
(20 min.).
✓ Then all sieves are disassembled. The powder retained on each sieve is collected & weighed.
❖ Advantages :
✓ It require less power
✓ Inexpensive
✓ Easy to use
❖ Disadvantages:
✓ During shaking, attrition may occurs
✓ Chances of clogging of sieve if powder is not properly dry.
❖ 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.
24

Cyclone separator
❖ Principle: 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.
❖ Construction:
✓ It consists of a cylindrical vessel referred to as the barrel
with the conical base.
✓ 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.
✓ The outlet for air or vapor is provided at the center of top.
❖ 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 centrifugal force & vortex throw the solids to the wall.
The coarse solid particles fall into the conical bottom & are
collected through solid outlets. The air or vapor can escape
from central outlet at the top. Size Separation by- Kokare Pratima S.
25

❖Merits:
✓ Cyclone separator requires low capital investment.
✓ It has high efficiency for 5 -200 µm particles.
✓ It produces high volume flow rate.
✓ It can be operated on continuous or batch process.
✓ 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.
✓ The extremely high velocities cause abrasive wear.
❖Applications:
✓ Used to separate solids from fluids
✓ Used for separating heavy or coarse fraction from dust. Size Separation by- Kokare Pratima S.
26

Air Seperator
❖ Principle: The air separator works on the same principle of cyclone
separator. For size separation of fine cyclone separator alone is not
efficient. Material for such separations an air stream combined with
centrifugal force efficient.
❖ Construction:
✓ It consists of vertical metal cylindrical vessel with conical base at the
bottom.
✓ 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 centre of the top portion.
✓ The rotating disc and rotating blades are fitted on shaft is placed at the
centre 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.
✓ 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 centre of
the separator and falls in the coarse particle collection zone which is
collected at its discharge.
27

28
❖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 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: can be used for dry powders in micro-sized that
could not be separated by traditional sieves.

Elutriation tank
❖ Principle- the principle of separation is based on elutriation. In elutriation
method the fluid flows in opposite direction to the sedimentation movement.
❖ Construction:
✓ It consist of 2 cones of different sizes which are placed one in another.
✓ At the top on the outside surface of the outer cone discharge launder is fitted.
✓ Feed enters through inner cone present at the top. At the bottom of the
equipment hydraulic water inlet is provided.
✓ Inside apparatus mixing port is also fitted . At the bottom a collector is
provided to collect the coarse particles.
✓ The inner cone is shorter than the outer cone. The inner cone can e rotated
along its vertical axis.
❖ Working:
✓ The feed that enters the inner cone & the water is introduced through the
water inlet at the bottom.
✓ The particles enters from the inner cone & encounter an upward stream of
water at the lower end.
✓ The fine particles pass upwards stream of water at the lower end. The fine
particles pass upwards & are collected in the discharge launder.
✓ The coarse particles are deposited in the collector & coarse material are
removed at the intervals. Size Separation by- Kokare Pratima S.
29

❖ 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:
✓ With liquids, it can be used to separate insoluble solids, such as kaolin or chalk,
✓ 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.
30

Assignment Questions
1. What is size separation? What are its objectives?
2. Explain various applications of size separation.
3. Discuss various sieving mechanisms involved in size separation.
4. What are pharmaceutical sieves? Write about its construction.
5. Discuss principle, construction working, uses, merits and demerits
of:
✓ Cyclone separator.
✓ Air Separator
✓ Bag filter
✓ Elutriation tank.
31

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Size separation

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