Lecture 4 SIZE Separation 2015

8/18/2019 Lecture 4 SIZE Separation 2015

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Size Separation &Particle size analysis

after size reduction → particles of varying

sizes.

To control the particle size distribution, thesize-reduced material must be sifted to get

fractions of narrow size ranges.

size separation is usually an integral partof size reduction in industrial pharmacy.


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Size Separation &Particle size analysis

classify a powder Two important methods:

Sieving

by Sedimentation and

Elutriation

Sieving :The ability

of particle to pass

through an aperture.

Fluid classification :employs a drag forces on

a particle moving through

a fluid.


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Sieving

Sieving or sifting is a process

by which different grades of

powders are separated fromeach other

It is most widely used method

for particle size analysis

because it is inexpensive,

simple and rapid with little

variation between operators..


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SievesWire Mesh Sieves square apertures

No. of the sieve indicating the number ofmeshes included in 1 inch in each direction

(square inch)

↑ sieve No. → ↑ the number of meshes →pd size (e.g., very fine pd pass through sieve No. 120 )


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Sieving

British Pharrnaconoeia (BP) →standardization of powders by means of

sieves

The BP specifies five grades of powder

Sieve No.grades of powder

10/44Coarse

22/60Moderately Coarse44/85Moderately fine

85/nsfine

120/nsvery fine


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Mechanical Sieving Methods

Agitate the sieve

Brushing Methods

Centrifugal force

Wet sieving


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Agitation Methods:

Oscillation

o Sieve oscillates back and forth

o simple

o The material may roll on the surface of the

sieve, and fibrous materials in particular tend

to ‘ball’. (Disadv) Vibration using electric AC current

o the particles are less likely to ‘blind’ the mesh.


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Gyration

o the sieve is given a rotary movement of small

amplitude, but of considerable intensity, giving a

spinning motion to the particles.

o This increases the chances of a particle becoming

suitably oriented to pass through the mesh,

o The efficiency is usually considerably greater than that obtained with oscillating or vibratory

sieves.

o Gyration > Vibratory > Oscillating


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Brushing Methods

A brush can be used to move theparticles on the surface of the sieve and

to keep the meshes clear.


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Centrifugal Methodso Vertical cylindrical sieve

o with a high-speed rotor

o particles are thrown outwards bycentrifugal force.

o The current of air created by the movement

assists sieving also, and is especiallyuseful with very fine powders.


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Wet sieving:

wet sieving is more efficient than the

equivalent dry process, however the

process has obvious limitations.


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Materials used for Sieves

1. Metals such as iron, coated iron, copper,

alloys of copper, stainless steel. 2. Non-Metals:

o To avoid the risk of metallic contamination

o e.g, Nylon and Terylene,


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Size separation by Sedimentationand Elutriation (Fluid classification)

It depends on the differences in the rates

of settling of particles of different sizes.

Large scale Fluid classification methods:

Sedimentation Elutriation


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Continuous Sedimentation methods

A shallow tank is arranged with

inlet and outlet pipes

The settling rates of particles

depend both on their size and

their shapes.

particles are subjected to force

that is divided into 2 component:

horizontal component due to the

flow of the fluid that carries the

particle forward,

vertical component due to gravity,

which causes the particle to fall

towards the bottom of the tank.

faster settling particle

slower settling particle


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V sed= d2 (ρ s -ρ o) g (stokes law) 18 η

o

Where,

V sed = sedimentation velocity in cm / sec

d = Diameter of particle ρ s =density of disperse phase

ρ o =density of disperse media

g = acceleration due to gravity

η o = viscosity of disperse medium in poise


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Product can be collectedcontinuously

In some tanks,

partitions are

arranged on thefloor , enabling

particular size

fractions to becollected

continuously


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Elutriation Methods:

Elutriation depends on

the movement of a fluidagainst the direction of

sedimentation of theparticles.


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Multistage Elutriator

three tubes of

increasing

area of cross-section can beconnected in series→ separation into

more than onefraction

inlet

V1 >V2 >V3

Coarse Moderately fine fine

overflow


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Cyclone Separator

The cyclone separator

consists of a

cylindrical vesselwith a conical base.

Cyclones is most

common used with

suspensions of a solid

in a gas.


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The suspension is introduced tangentially

at fairly high velocity, so that movement

takes place within the vessel. The rotary flow within the cyclone causes

the particles to be acted on by centrifugal

force, solids being thrown out to the walls,

hence coarse powders fall to the conical

base and out through the solids discharge.

Fine pd.are carried with fluid to top and out

through fluid outlet


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Lecture 4 SIZE Separation 2015

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