particle-size-analysis.pptx
TRANSCRIPT
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M.BILAL
ROLL NO.43
PARTICLE SIZE
ANALYSIS
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What is a particle?
What is the importance of particle analysis?
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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
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Methods for determining particle size
Microscopy
Sieving
Sedimentation techniques Optical and electrical sensing zone method
Laser light scattering techniques
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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
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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.
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For submicron particles it is necessary to use
either
TEM (Transmission Electron Microscopy) or
SEM (Scanning Electron Microscopy). TEM and SEM (0.001-5m)
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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
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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
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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.
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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.
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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
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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
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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.
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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).
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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
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Andreasen pipette
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Advantages
Equipment required can be relatively simple andinexpensive.
Can measure a wide range of sizes withconsiderable accuracy and reproducibility.
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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.
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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.
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The Air Elutriator
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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
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Coulter counter method
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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.
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Advantages:
Non-intrusive : uses a low powerlaser beam
Fast : typically
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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.
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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
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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.
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REFERENCES
-Unit Operations of Chemical Engineering.
by McCabe & Smith
-Mass Transfer Operations
by Treybal
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