Particle-Size-Analysis.pptx

7/29/2019 Particle-Size-Analysis.pptx

1/34

M.BILAL

ROLL NO.43

PARTICLE SIZE

ANALYSIS


2/34

What is a particle?

What is the importance of particle analysis?


3/34

Particle size analyzers measure the sizes of grains or particlesin a sample. They use methods such as light scattering,sedimentation, laser diffraction etc to calculate particle sizes.Particle size analyzers can measure the sizes of manyparticles in a sample very quickly and can provided data on

particle size distributions, which is of value to manyindustries.

Why to measure particle size of particles???

Particle size can affect

Final formulation: performance, appearance, stability

Processability of powder


4/34

Methods for determining particle size

Microscopy

Sieving

Sedimentation techniques Optical and electrical sensing zone method

Laser light scattering techniques


5/34

Choosing a method for particle

Nature of the material to be sized, e.g.

Estimated particlesize and particlesize range Solubility Ease of handling Toxicity Flowability Intended Use Cost

Capital Running Specification requirements Time restrictions


6/34

Microscopy

Optical microscopy (1-150m) Electron microscopy (0.001)

Being able to examine each particle individually has led tomicroscopy being considered as an absolute measurement ofparticlesize.

Can distinguish aggregates from single particles When coupled to image analysis computers each field can be

examined, and a distribution obtained.

Number distribution Most severe limitation of optical microscopy is the depth of focus

being about 10m at x100 and only 0.5m at x1000.

With small particles, diffraction effects increase causing blurring atthe edges - determination of particles < 3m is less and less certain.


7/34


8/34

For submicron particles it is necessary to use

either

TEM (Transmission Electron Microscopy) or

SEM (Scanning Electron Microscopy). TEM and SEM (0.001-5m)


9/34

Manual Optical Microscopy

Advantages Relatively inexpensive

Each particle individuallyexamined - detect

aggregates, 2D shape,color , melting point etc.

Permanent record -photograph

Small sample sizesrequired

Disadvantages Time consuming - high

operator fatigue - fewparticles examined

Very low throughput

No information on 3Dshape

Certain amount ofsubjectivity associatedwith sizing - operatorbias


10/34

Transmission and Scanning Electron Microscopy

Advantages

Particles are individuallyexamined

Visual means to see sub-micron specimens

Particle shape can bemeasured

Disadvantages

Very expensive

Time consuming samplepreparation

Materials such asemulsionsdifficult/impossible toprepare

Not for routine use


11/34

Sieving

Sieve analysis is performed using a nest or stack ofsieves where each lower sieve has a smaller aperturesize than that of the sieve above it.

Sieves can be referred to either by their aperture size

or by their mesh size (or sieve number). The mesh size is the number of wires per linear inch.

Approx. size range : 5m - ~3mm

Standard woven wire sieves Electroformed micromesh sieves at the lower end or

range (< 20m)

Punch plate sieves at the upper range.


12/34

British Pharmacopoeia

The degree of fineness of a powder may be expressed by reference to sieves t

hat comply with the specifications for non-analytical sieves.Where the degree of fineness of powders is determined by sieving, it is defined in relation to the sieve number(s) used either by means of the following terms or, where such terms cannot be used, by expressing the fineness of thepowder as a percentage m/m passing the sieve(s) used.

The following terms are used in the description of powders:Coarse powder: Not less than 95% by mass passes through a number1400 sieve and not more than 40 % by mass passes through a number 355sieve.Moderately fine powder: Not less than 95% by mass passes through anumber 355 sieve and not more than 40% by mass passes through a number

180 sieve.Fine powder: Not less than 95% by mass passes through a number 180sieve and not more than 40% by mass passes through a number 125 sieve.


13/34

Sieving may be performed wet or dry; by machineor by hand, for a fixed time or until powder passes

through the sieve at a constant flow rate

Wet sieving

Air-jet sieving

Weight distribution


14/34


15/34

Sieving

Advantages

Easy to perform

Wide size range

Inexpensive

Disadvantages

Known problems ofreproducibility

Wear/damage in use orcleaning

Irregular/agglomeratedparticles

Rod-like particles :overestimate of under-size

Labour intensive


16/34

Sedimentation techniques

Methods depend on the fact that the terminal velocity ofa particle in a fluid increases with size.

Stokes's diameter (dst) is defined as the diameter of thesphere that would settle at the same rate as the particle

The particlesize distribution of fine powder can be dete

rmined by examining a sedimenting suspension of

the powder.


17/34

If the particles are falling in the viscous fluid by their own weightdue to gravity, then a terminal velocity, also known as the settlingvelocity, is reached when this force combined with the buoyantforce exactly balance the gravitational force. The resulting settlingvelocity (or terminal velocity) is given by:[2]

where:

vs is the particles' settling velocity (m/s) (vertically downwards ifp > f, upwards ifp < f),

g is the gravitational acceleration (m/s2),p is the mass density of the particles (kg/m3), andfis the mass density of the fluid (kg/m3).


18/34

Andreasen Pipette

Size distribution is determined by allowing a homogeneoussuspension to settle in a cylinder and taking samples from thesettling suspension at a fixed horizontal level at different intervalsof time.

Each sample will contain a representative sample of the suspension,with the exception of particles greater than a critical size, all of

which will have settled below the level of the sampling point. The concentration of solid in a sample taken at time t is determined

by centrifugation of the sample followed by drying and weighing . This concentration expressed as a percentage of the initial

concentration gives the percentage (w/w) of particles whose fallingvelocities are equal to or less than h/t. Substitution in the equationabove gives the corresponding Stokes' diameter.

A d i


19/34

Andreasen pipette


20/34

Advantages

Equipment required can be relatively simple andinexpensive.

Can measure a wide range of sizes withconsiderable accuracy and reproducibility.


21/34

Disadvantages

Sedimentation analyses must be carried out atconcentrations which are sufficiently low for interactiveeffects between particles to be negligible so that theirterminal falling velocities can be taken as equal to those ofisolated particles.

Large particles create turbulence, are slowed and arerecorded undersize.

Careful temperature control is necessary to suppressconvection currents.

The lower limit of particle size is set by the increasingimportance of Brownian motion for progressively smaller

particles. Particle re-aggregation during extended measurements. Particles have to be completely insoluble in the

suspending liquid.


22/34

The Air Elutriator

In the air elutriator particles are ejected from a smallfluidised bed into a flow of air and carried over into acollecting vessel where they can be periodicallyremoved and weighed. The air velocity is increasedincrementally so that larger and larger particles arecarried over. The maximum size carried over at anyair velocity can be calculated from Stoke's law.


23/34

The Air Elutriator


24/34

Electrical sensing zone method CoulterCounter

Instrument measures particle volume which can beexpressed as dv : the diameter of a sphere that has thesame volume as the particle.

The number and size of particles suspended in anelectrolyte is determined by causing them to passthrough an orifice an either side of which is immersedan electrode.

The changes in electric impedance (resistance) asparticles pass through the orifice generate voltagepulses whose amplitude are proportional to the

volumes of the particles.

Volume distribution


25/34


26/34

Coulter counter method


27/34

Laser diffraction

Particles pass through a laser beam and the lightscattered by them is collected over a range ofangles in the forward direction. The angles of diffraction are, in the simplest case inversely

related to the particlesize. The particles pass through an expanded and collimated laser

beam in front of a lens in whose focal plane is positioned aphotosensitive detector consisting of a series of concentric

rings. Distribution of scattered intensity is analysed by computer to

yield the particlesize distribution.


28/34


29/34

Advantages:

Non-intrusive : uses a low powerlaser beam

Fast : typically


30/34

PCS

Large particles move more slowly than small particles, so that the rateof fluctuation of the light scattered from them is also slower.

PCS uses the rate of change of these light fluctuations to determine thesize distribution of the particles scattering light.

Comparison of a "snap-shot" of each speckle pattern with another takenat a very short time later (microseconds).

The time dependent change in position of the speckles relates to thechange of position of the particles and hence particlesize.

The dynamic light signal is sampled and correlated with itself atdifferent time intervals using a digital correlator and associatedcomputer software.

The relationship of the auto-correlation function obtained to timeintervals is processed to provide estimates of the particlesizedistribution.


31/34

PCS

ADVANTAGES:

When the particle isasymmetrical, the intensityof scattered light varies with theangle of observation,permittingan estimation of the shape & size

of particle.

light scattering has been used tostudy proteins,syntheticpolymers, associationcolloids & lyophobic sols.

this method is applicable formeasuring the particle sizeranging from 5nm to approx3m.

DISADVANTAGES:

PCs however cannotcharacterize systems havingbroadly distributed particles.

Vibration, temperaturefluctuations can interfere withanalysis

Restricted to solid in liquid orliquid in liquid samples

Expense Need to know R.I. values and

viscosity


32/34

Comparison of Different MethodsThe method used to determine size distribution will depend onmany factors;the coarseness of the material, the precision required, timeavailable, equipment available. One thing which is important toremember is that in most cases size distributions are notreproducible using different techniques. Thus sieve analysis willnot produce the same mean particle diameter as a Coulter counter

This is partly due to the different methods measuring differenttypes of diameter (see above). Some devices will calculatedistributions based on area, volume etc which can aidcomparisons. However in general only measurements using the

same equipment should be compared directly. The method usedwill also depend on the purpose of the measurement, for examplesometimes the amount of under or oversize in a sample isimportant, in which case a single sieve may give the requiredinformation.


33/34

REFERENCES

-Unit Operations of Chemical Engineering.

by McCabe & Smith

-Mass Transfer Operations

by Treybal


34/34

Date post:	03-Apr-2018
Category:	Documents
Upload:	gautamahuja
View:	218 times
Download:	0 times

Particle-Size-Analysis.pptx

Documents