Size Reduction and Size Separation
Syllabus:
Definition, objectives of size reduction and size
separation, factors affecting size reduction, laws governing energy and power
requirements of mills including ball mill, hammer mill, fluid energy mill etc.,
sieve analysis, standards of sieves, size separation equipment shaking and
vibrating screens, gyratory screens, cyclone separator, air separator, bag
filters, cottrell precipitator, scrubbers, size separators basing on
sedimentation theory.
Definition
Size reduction (or Comminution)
Size reduction or comminution is the process of reducing
substances to smaller particles.
Size separation (or Classification)
Size separation (or classification) is a process in which
particles of desired size are separated from other fractions.
Objectives
Objectives of size reduction
1. Size
reduction leads to increase of surface area.
Example-I: The rate of
dissolution of solid drug particles increases many folds after size reduction. Griseofulvin, an antifungal
drug, when administered in its micronized form shows around five times better
absorption.
Example-II: The absorptive power
of charcoal and kaolin increases after size reduction due to increase in
surface area.
2. Size
reduction produces particles in narrow size range. Mixing of powders with
narrow size range is easier.
3. Pharmaceutical
suspensions require finer particle size. It reduces rate of sedimentation.
4. Pharmaceutical
capsules, insufflations (i.e. powders inhaled directly into the lungs),
suppositories and ointments require particles size to be below 60mm
size.
Objectives of size separation
1. Any
solid materials, after size reduction, never gives particles of the same size
but contains particles of varying sizes. The size-reduced particles are then
passed through sieves to get fractions of narrow size range.
2. During
tablet granulation the granules should be within narrow size range, otherwise,
weight variation will take place during tablet punching.
Factors affecting size-reduction
The pharmaceutical industry uses a great variety of
materials, including chemical substances, animal tissues and vegetable drugs.
A. Factors related to the nature of raw materials
Hard materials:
Hard materials like pumice and iodine are most difficult to comminute. During
size reduction these types of materials will produce abrasive wear of milling
surfaces, which will then contaminate the material.
Fibrous materials:
Crude drugs obtained from plants like glycyrrhiza, rauwolfia, ginger etc. are
fibrous in nature and cannot be crushed by pressure. So they may be
size-reduced by cutter mill.
Friable materials:
Sucrose and dried filter cakes are friable (i.e. brittle) hence they are easy
to comminute by hammer mill or fluid energy mill.
Plastic materials:
Synthetic gums, waxes and resins become soft and plastic during milling. These
low melting substances should be chilled (made cold) before milling. These
types of materials are milled by using hammer mill and fluid energy mill.
Hygroscopic materials:
Hygroscopic materials absorbs moisture rapidly hence they must be comminuted
inside a closed equipment like ball-mill.
Thermolabile
materials: Thermolabile materials like vitamins and antibiotics are milled
inside chilled equipment.
Inflammable materials:
Fine dust, such as dextrin, starch and sulphur, is a potential explosive
mixture under certain conditions. All electrical switches should be explosive
proof and the mill should be earthed properly.
Particle size of the
feed: For a mill to operate satisfactorily, the feed should be of proper
size.
Moisture content:
Presence of more than 5% moisture hinders the milling process and produces a
sticky mass.
B. Factors related to the nature of the finished product
Particle size:
Moderately coarse powders may be obtained from various impact mill. If very
fine particles like micronized particles of griseofulvin may be obtained from
fluid energy mill.
Ease of sterilization:
When preparations are intended for parenteral (injection) purpose and
ophthalmic uses, size reduction must be conducted in a sterile environment.
Mills should be sterilized by steam before use.
Contamination of
milled materials: In case of potent drugs and low dose products,
contamination of the products should be avoided. Equipment free from wearing
(e.g. fluid energy mill) may be used in this case.
Laws governing energy and power requirements of mills
During size reduction energy is supplied to the equipment
(mill). Very small amount of energy (less than 2%) actually produce size
reduction. Rest of the energy is dissipated (wasted) in:
(i)
Elastic deformation of particles
(ii)
Transport of material within the milling chamber
(iii)
Friction between the particles
(iv)
Friction between the particles and mill
(v)
Generation of heat
(vi)
Vibration and noise.
(vii)
Inefficiency of transmission and motor.
Theories of milling
A number of
theories have been proposed to establish a relationship between energy input
and the degree of size reduction produced.
Rittinger’s theory
Rittinger’s theory
suggests that energy required in a size reduction process is proportional to
the new surface area produced.
where, E = energy required for size reduction
KR = Rittinger’s
constant
Si = initial specific
surface area
Sn = final specific
surface area
Application: It is
most applicable in size reducing brittle materials undergoing fine milling.
Bond’s theory
Bond’s theory states that the energy used in crack
propagation is proportional to the new crack length produced
where, E = energy required for size reduction
KB = Bond’s work index
di = initial diameter
of particles
dn = final diameter of
particles
Application: This law is useful in rough
mill sizing. The work index is useful in comparing the efficiency of milling
operations.
Kick’s theory
Kick’s theory states that the energy used in deforming
(or fracturing) a set of particles of equivalent shape is proportional to the
ratio of change of size, or:
where, E = energy required for size reduction
KK = Kick’s constant
di = initial diameter
of particles
dn = final diameter of
particles
Application: For
crushing of large particles Kick’s theory most useful.
Walker’s theory
Walker proposed a generalized differential form of the
energy-size relationship:
where E = amount of
energy (work done) required to produce a change
D = size of unit mass
K = Constant
n = constant
For n =1.0 Walker equation becomes Kick’s theory used for
coarse particles > 1 mm.
For n =1.5
Walker equation becomes Bond’s theory. This theory is used when neither Kick’s
nor Rittinger’s law is applicable.
For n =2.0 Walker equation becomes Rittinger’s theory used
for fine particles < 1 mm size.
Methods of size reduction
     Approximate
increase in
fineness of
product
|
Method
|
Diagram
|
Common Examples
|
|
Cutting
|
|
Scissors
Cutter mill
|
|
|
Compression
|
|
Roller mill
Crusher mill
|
|
|
Impact
|
|
Hammer mill
|
|
|
Attirtion
(Pressure
and friction)
|
|
File
Fluid energy mill
|
Table: Uses of size
reduction methods
|
Degree of size
reduction
|
Typical methods
|
Examples
|
|
Large pieces
|
Cutter or compression mills
|
Rhubarb
|
|
Coarse powders
|
Impact mills
|
Liquorice, cascara
|
|
Fine powders
|
Combined impact and attrition mills
|
Rhubarb , belladonna
|
|
Very fine powders
|
Fluid energy mills
|
Vitamins and antibiotics
|
CUTTER MILL
Method of size
reduction: Cutting
Construction and
working principle:
The equipment has two parts – one is rotor and another part
is the casing. Stationary knives are fitted on the casing and rotating knives
are fitted on the rotor. Feed enters through the top hopper. The rotor rotates
and both stationary and rotating knives cut the material into pieces. The lower
part consists of a screen, so that material is retained in the mill until
sufficient degree of size reduction has been effected.
Applications:
This method is used to obtain coarse degree of size
reduction of soft materials. Applied in size reduction of roots, peels or
woods, prior to extraction.
ROLLER MILL
Method of size
reduction: Compression
Construction and working principle:
The roller mill has two cylindrical rolls of stone or metal,
mounted horizontally, which are capable of rotating on their longitudinal axes.
One roll is rotated directly and the other rotates freely. When material is
placed above the rolls it is drawn in through the nip and the second roll is
rotated b y friction.
Diameter of the rolls:
Few centimeter up to several meters
The gap between the roll may be adjusted to control the
degree of size reduction.
Applications:
Used for crushing or cracking seeds prior to extraction of
fixed oils or bruising soft tissues (often after cutting) to aid solvent
penetration.
HAMMER MILL
Method of size reduction: Impact
Construction
and working principle:
Hammer mill
consists of a stout metal casing, enclosing a central shaft to which four or
more hammers are attached. These are
mounted with swivel joints, so that
the hammers swing out to a radial position when the shaft is rotated. The lower
part of the casing consists of screen through which materials can escape, when
sufficiently size reduced. The material is collected in a container placed
below the screen.
·
The screen can be changed according to the
particle size required.
·
According to the purpose of operation the
hammers may be square-faced, tapered to a cutting form or have a stepped-form.
·
The interior of the casing may be undulating in
shape, instead of smooth circular form for better impact.
·
The rotor operates at a speed of 80cycles per
second.
Advantages:
(a)
It is rapid in action, and is capable of grinding many
different types of materials.
(b)
The product can be controlled by variation of rotor
speed, hammer type and size and shape of mesh.
(c)
Operation is continuous.
(d)
No surface moves against each other so very little
problem of contamination of mill materials.
Disadvantages:
(a)
High speed of operation generates heat that may affect
thermolabile materials or drugs containing gum, fat or resin.
(b)
The rate of feed should be controlled otherwise the
mill may be choked.
(c)
Because of high speed of operation, the hammer mill may
be damaged if some foreign materials like stone, metal pieces etc. are present
in the feed.
Applications: Powdering of crystals and
filter cakes.
BALL MILL
Construction
The ball mill
consists of a hollow cylinder rotated on its horizontal axis. Inside the
cylinder balls or pebbles are placed.
Cylinder:
·
Cylinder may be made up of metal, porcelain or
rubber.
·
Rubber reduces the abrasion. Diameter of the
cylinder ranges from 1 to 3m in pharmaceutical practice.
Balls:
·
Balls occupy about 30 to 50% of the volume of
the cylinder.
·
Diameter of the balls depends on the feed size
and diameter of the cylinder. The diameter of balls ranges from 2cm to 15cm.
·
Balls may be of metal, porcelain or pebbles.
Working Principle:
Larger particles
are fed through an opening of the cylinder. The opening is closed. The cylinder
is rotated at the critical speed of ball mill. The optimum size reduction in a
ball mill depends o the following factors:
Feed quantity:
Too much feed will
produce cushioning effect and too little feed will produce loss of efficiency
of the mill.
Speed of rotation of the cylinder:
·
At low speed the mass of
balls will slide or roll over each other and only a negligible amount of size
reduction will take place.
At low speed the mass of
balls will slide or roll over each other and only a negligible amount of size
reduction will take place.
·
At high speeds, balls will be thrown out to the
wall of the cylinder due to centrifugal force and no grinding will occur.
·
At 2/3rd speed at which
centrifugation just occurs is called the critical speed of the ball mill. At
this speed the balls are carried almost to the top of the mill and then fall in
a cascade across the diameter of the mill. By this means the maximum size
reduction is obtained by impact of the particles between the balls and by
attrition between the balls. Generally it is 0.5 cycles per seconds (cps).
Advantages
1.
It is capable of grinding a wide variety of materials
of differing hardness.
2.
It can be used in completely enclosed form, which makes
it suitable for use with toxic materials.
3.
It can produce very fine powders.
4.
It is suitable both for dry and wet milling. Wet
milling is required for preparation of pharmaceutical suspensions.
Disadvantages
1.
Wear occurs from the balls and the inside surface of
the cylinder hence there is possibility of contamination of product with mill
material. Abrasive materials increase wear.
2.
Soft or sticky materials may cause problems by caking
on the sides of the mill or by holding the balls in aggregates.
3.
The ball mill is a very noisy machine, particularly if
the cylinder is made of metal.
Applications:
Large ball mills
are used to grinding ores prior to manufacture of pharmaceutical chemicals.
Smaller ball mills are used for grinding of drugs or excipients or for grinding
suspensions.
Various types of ball mills:
Hardinge mill: In this type of ball mills the cylinder has a conical end towards a
discharge point. In this mill the larger balls remain within the cylinder and
the smaller balls are collected in the
conical portion. As a result, coarser grinding takes place in the cylinder
portion and finer grinding takes place in the apex of the conical portion. The
product is more finer and uniform than general cylindrical ball mill.
Tube mill: They consist of long cylinder and can grind to a finer product than
the conventional ball mill.
Rod mill: Instead of balls they contains rods, which extends the length of the
mill. This rods are useful with sticky materials since rods do not form
aggregates like balls.
Vibration mill: In this type of mills vibratory movements are given instead of
rotation. The cylinder is mounted on springs which sets up vibration. The
cylinder moves through a circular path with an amplitude of vibration up to
abut 20mm and a rotational frequency of 15 to 50 per second.
FLUID ENERGY MILL
Construction:
It consists of a
loop of pipe, which has a diameter of 2 to 20cm. The height of the loop may be
up to 2m. Several nozzles are fitted at the bottom of the pipe. A classifier is
fitted at the product collection point.
Working principle
A fluid usually
air, is injected at very high pressure through nozzles at the bottom of the
loop. This gives rise to a high velocity of circulation that produce
turbulence. Solids are introduced into the stream through the feed inlet. As a
result of high degree of turbulence, impacts and attrition occur between the
particles. A classifier is fitted in
the system so that only finer size particles are collected as products and the
larger size particles are again sent to the stream of air for further size
reduction.
The feed to the
mill is previously size reduced and passed through a 100mesh screen.
The size of the
product may be 5mm
or below.
Advantages:
1.
The particle size of the product is smaller than that
produced by any other method of size reduction.
2.
Expansion of gases at the nozzles lead to cooling,
counteracting the usual frictional heat that can affect heat-sensitive
(thermolabile) materials.
3.
Since the size reduction is by inter-particulate
attrition there is little or no abrasion of the mill and no contamination of
the product.
4.
For oxygen or moisture sensitive materials inert gases
like nitrogen can be used instead of normal air.
5.
This method is used where fine powders are required
like micronization of griseofulvin (an antifungal drug), antibiotics etc.
SIEVE ANALYSIS
Equivalent diameter
Sieve diameter, 
, is the particle dimension that passes through a square
aperture (length = x).
, is the particle dimension that passes through a square
aperture (length = x).Range of analysis
The International Standards
Organization (ISO) sets lowest sieve diameter of 45mm. Powders are usually
defined as particles having a maximum diameter of 1000mm, so this is the upper
limit. In practice sieve analysis can be done over a range of 5 to 125000mm.
ISO Range : 45 to
1000 mm
Range available in practice: 5 to 125000 mm
Sample preparation
·
Powders in dry state is usually used.
·
Powders in liquid suspension can also be
analyzed by sieve.
Principle of measurement with sieve
Sieve analysis utilizes a set of sieves. Each sieve is a
woven, punched or electroformed mesh, often in brass or stainless steel, with
known aperture diameter which form a physical barrier to particles. In sieve analysis
a set of sieves (known as ‘stack’ or ‘nest’ of sieves) are arranged in such a
way that the smallest aperture will be at the bottom and the largest aperture
will be at the top.
1. A
sieve-nest usually comprises 6 to 8 sieves with an aperture progression based
on Ö2
or 2Ö2
change in diameter between adjacent sieves.
2.
Initial weight (W0) of powder sample was
taken on the first sieve (i.e. topmost sieve). The sieve-set was closed and
shaking was started. After shaking for a stipulated time, the sieve-set was
taken out. All the sieves were disassembled.
3.
The powder retained on each sieve was collected on a
paper (bearing the mesh number) and weighed.
TABLE - 1
|
Sieve
number
(Passed/
Retained)
|
Size
of
opening
(Passed
/
Retained)
|
Arithmetic
mean
Size
of openings
d (mm)
|
Weight
Retained
on
smaller sieve
(gm)
|
%
Retained
on
smaller
sieve
|
Cumulative
%
oversize
|
|
M1/M2
M2/M3
M3/M4
--
--
--
|
d1/d2
d2/d3
d3/d4
--
--
--
|
½(d1
+ d2)
½(d2
+ d3)
½(d3
+ d4)
--
--
--
|
W1
W2
W3
--
--
--
|
p1=100w1/W
p2=100w2/W
p3=100w3/W
--
--
--
|
p1
p1+p2
p1+p2+p3
--
--
--
|
|
|
|
|
Total
= W
|
Total
= 100
|
|
TABLE – 2: Plot |
log d
|
Weight % retained
|
Cumulative % oversize
|
|
|
|
|
STANDARDS OF SIEVES
It is required that wire-mesh
sieves will be made from wire of uniform, circular cross-section and for each
sieve the following particulars are stated:
Number of sieve
This is the number of meshes in a
length of 25.4mm (i.e. 1 inch), in each direction.
Nominal size aperture
This is the distance between the
wires, so that it represents the length of the side of the square aperture.
N.B. While it is the diameter of the largest sphere that
would pass the mesh, it is not necessarily the maximum dimension of the
particle, plate like particles will pass through diagonally and long fibrous
particles require only suitable orientation.
Nominal diameter of the wire
The wire diameter is selected to give a suitable aperture
size. It is also required to give the necessary strength to avoid distortion.
The diameter of the wire is represented by Standard Wire
Gauge.
Approximate screen area
This standard expresses the area of the meshes as a
percentage of the total area of the sieve.
It is governed by the diameter of the wire. It is generally
kept within 35 to 45% of the total area of the sieve.
This represents the useful area of a sieve. Greater screen
area is preferred.
Aperture tolerance average
Some variation in the aperture size is unavoidable and this
variation is expressed as a percentage, known as aperture tolerance average. It
is the maximum limit within which the dimension of meshes can be allowed to
vary and still be acceptable for sieving.
Finer wires are likely to be subject to a greater
proportional variation in diameter than coarse mesh. Hence, the aperture
tolerance average is smaller for sieves of 5 to 10 mesh than in case of
300mesh.
Tyler Standard Screen
Scale
|
Mesh
|
Clear Opening, mm
|
Wire Diameter, mm
|
|
3
|
6.680
|
1.778
|
|
4
|
4.699
|
1.651
|
|
6
|
3.327
|
0.914
|
|
8
|
2.362
|
0.813
|
|
10
|
1.651
|
0.889
|
|
14
|
1.168
|
0.635
|
|
20
|
0.833
|
0.437
|
|
28
|
0.589
|
0.318
|
|
35
|
0.417
|
0.310
|
|
48
|
0.295
|
0.234
|
|
65
|
0.208
|
0.183
|
|
100
|
0.147
|
0.107
|
|
150
|
0.104
|
0.066
|
|
200
|
0.074
|
0.053
|
SIZE SEPARATION EQUIPMENTS
SHAKING SCREEN
Principle:
Particles of different sizes are
separated by passing them through a sieve, which oscillates to-and-fro
continuously.
Construction:
Shaking screen consists of metal
frame to which a screen is fixed at the bottom. The screen cloth may be riveted
directly or fitted by using a removable bolted frame. The metal frame is
suspended by hanger rods, so that it can move freely. The metal frame may be
suspended either horizontally or in inclined position. One side of the frame is
attached with an ordinary eccentric on a rotating shaft. The entire frame
experiences a reciprocating (to-and-fro) motion.
Working:
The screen is
allowed to shake in a reciprocating motion. The feed (material to be screened)
is introduced on to the screen from a side. . Fine particles are screened off
initially. The remaining materials moves forward and the over-sized particles
are collected at the other end.
Advantages:
It requires low-head room and low power
requirement.
Disadvantages:
High cost of maintenance of screens
and supporting structures.
Its capacity is low.
SHAKING AND VIBRATING SCREENS (ROTEX
SCREEN)
Principle: Rotex screen works on oscillating agitation (to-and-fro
motion) by means of an eccentric mechanism. Further vibrations are caused by
rubber balls.
Construction:
This equipment
consists of a set of screens, which are slightly inclined at 5 degrees with
horizontal axis. Each screen set is double layered. The upper screen is of a
fixed size and the lower one is coarser screen, which is a supporting sieve.
Between these two screens, wooden blocks are placed at different intervals.
Between the wooden blocks rubber balls are placed. This two-sieve system represents one unit.
Several such units are arranged in the descending order, i.e. sieve or larger
size opening remains at the top and finer size opening remains at the bottom.
The overall assembly of screens is supported on sliding contacts at the lower
end. The upper end of the screen system is connected to an eccentric pin on a
flywheel.
Working:
The screen system
is allowed to agitate with the help of eccentric. The shaking motion of the
screen causes the balls to fly between the screens. As they strikes the
inclined surface of the wooden blocks, the balls deflect upward and strike the
screen cloth and thus prevents blocking of the mesh. The feed is introduced at
the higher end of the screen. The material passes through the upper screen and
reaches the next screen. This process continues until all the materials are
separated into fractions. The fractions are collected separately at the outlet
point.
Uses:
Rotec screen is used for handling a variety of dry powders, granules and dry
foods.
GYRATORY SCREENS (FINEX)
Principle:
The Finex shifting
machine consists of a horizontal screen to which a gyratory movement of great
intensity and small amplitude is imparted by eccentric mechanism. Every
particle is given a rotary movement on one axis and a second rotary movement on
an axis at right angles to the first.
Construction:
Screens are fitted
horizontally. The topmost has the largest opening and the bottom one will have
finer opening. The overall assembly is given a gyratory motion by eccentric
mechanism. Clearing gates are fitted at one side of the periphery.
Working:
The feed is
introduced at the center of the screen and the gyratory motion drives the
larger sizes to the periphery of the screen where it is discharged through a
clearing gate set in upright side of the sieve. In some devices a spiral strip
of metal is welded to the screen. The larger size passes progressively from the
center to the periphery.
CYCLONE SEPARATOR
Principle
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
It consists of a
short vertical, cylindrical vessel with a 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 the air short-circuiting
directly from the inlet to the outlet of the fluid.
Working
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.
Uses
1. 
Cyclone separators are
used to separate solid particles from gases.
Cyclone separators are
used to separate solid particles from gases.
2. It
is also used for size separation of solids in liquids.
3. It
is used to separate the heavy and coarse fraction from fine dust.
AIR SEPARATOR
Principle
The cyclone
separator alone cannot carry out size separation on fine materials. For such
separations a current of air combined with centrifugal force is used. The finer
particles are carried away by air and the coarser particles are thrown by
centrifugal force, which fall at the bottom.
Construction
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.
Working
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.
Uses
Air separators are
often attached to the ball mill or hammer mill to separate and return over
sized particles for further size reduction.
BAG FILTER
Principle
In a bag filter,
size separation of fines (or dust) from the milled powder is achieved in 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.
Construction
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.
Working
(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 bags.
(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.
Uses
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.
COTTRELL
PRECIPITATOR
Principle
If a gas is
subjected to a strong unidirectional electrostatic field, the gas become
ionized and drifts toward one electrode. If a finely divided solid particle (or
liquid droplet) is suspended in the gas, the particle (or droplet) will become
charged and will drift toward the same electrode as the ionized gas.
Construction
There will be two
electrodes: (i) Discharge electrode and (ii) Collecting electrode.
·
The discharge electrode is usually a wire,
chain, wire screen or other arrangement with a large surface.
·
The collecting (or smooth) electrode may be
parallel plate or pipe.
·
In case of parallel plate design the gas flows
parallel to plates.
Plate dimensions: Length = 10 to 18 ft Width = 3 to 6 ft.
·
In case of pipe design the pipes are placed
vertically and a wire (discharge electrode) is fitted at the center of the
pipe. Gas flows from the bottom to the top of the pipe. At the bottom of the
pipe a hopper is given to collect the particles. Pipe dimensions: Height 6 to 15 ft.
Working
AC current is first stepped up with a step up transformer
to raise the potential difference to 50,000 to 60,000 volts. Then the voltage
is made unidirectional by a motor-disc assembly where the motor is rotating at
a speed similar to the frequency (i.e.
cycles per second or Hz) of the AC current. This results in a pulsating but
unidirectional electrostatic field.
The particles will be charged and
precipitated on the smooth plate or pipe, which is then collected through the
hopper.
Use
The Cottrell process is
successfully used for the removal of fine dusts from all kind of waste gases.
SCRUBBERS
Principle
Solid and liquid particles
suspended in gas can be removed (or scrubbed) by introducing water from the top
of the equipment and gas through the bottom of the equipment, i.e. water and
gas flow in counter-current flow.
Construction
This equipment consists of a
cylindrical shell with conical
bottom. The gas containing the suspended particles enters through a tangential
inlet at the bottom. Deflector cones
are placed on stationary vanes. These vanes are placed on annular shelves. One deflector cone,
vanes and the annular ring as a whole called a contact. There are several such contacts placed inside the shell.
The liquid inlet is placed at the
second top contact. The topmost contact is called entrainment separator. Dust collector outlet is placed at the
bottom of the equipment. The gas is taken out through the gas outlet placed at
the top of the separator.
Working
The gas carrying the particles
(or droplets) are introduced through the gas inlet at the bottom and passes
upward through the vanes under deflector cones. Liquid (e.g. water) is
introduced from the top. It falls on the conical deflector and flows over the
vanes to produce a curtain of liquid. The gas passes through these curtains.
Particles or droplets are retained in the liquid and gas passes to the next
contact. After passing through several such contacts the gas passes through the
entrainment separator where small droplets of water (known as entrainment) are
separated and the gas leaves the separator through the gas outlet. The
discharged gas goes to a cyclone separator where the entrained water (if any)
are finally removed from the gas.
Use
To clean the dust of air before entering inside a room or
before discharging a gas in the environment from an industry.
SIZE SEPARATORS BASING ON SEDIMENTATION THEORY
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 equation:
CONTINUOUS
SEDIMENTATION TANK
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 tank. 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.
DOUBLE CONE CLASSIFIER
Principle
The separation takes place by
elutriation principle.
N.B. In sedimentation method
the fluid is stationary and the separation is taking place by the velocity of
the particles. In elutriation method the liquid is flowing to the opposite
direction of the sedimentation of the particle.
In elutriation
method the fluid flows in opposite direction to the sedimentation movement. In
this equipment the liquid moves upward. If the sedimentation velocity of a
particle is less than the velocity of the liquid then the particle will move
upward. If the sedimentation velocity of the particle is more than the velocity
of the liquid then the particle will move downward. So if the velocity of the
liquid is kept within the sedimentation velocities of the coarse and fine
particles then the finer particles (has lesser sedimentation velocity) will
move upward and the coarser particles (have greater sedimentation velocity)
will move downward.
Construction
Two cones of different sizes are
placed one in another. At the top of the outer cone a discharge launder is fitted at the outside surface of the outer
cone. Water inlet is provided at the bottom of the equipment. The inner cone is
shorter than the outer cone. The inner cone can be moved along its vertical
axis with the help of a hand wheel. At the bottom a collecting box is provided
to collect the coarse particles.
Working
The feed enters
the inner cone and water is introduced through the water inlet at the bottom.
The particles settle from the inner cone meet a rising stream of water at lower
end of inner cone. The fine particles pass upward and are collected in the
discharge launder. The coarse particles settle into the collecting box for
coarse material and are drawn off at intervals.
The degree of
separation is regulated by the velocity of water supply and the height of the
inner cone regulated by the hand-wheel.