Chemical Engineering March 2016 - 46

Facts At Your Fingertips
Particle-sizing technology selection
Department Editor: Scott Jenkins
T
his column summarizes
the strengths and
limitations of some
widely used industrial particle-analysis
techniques.
Technique
Laser diffraction
Dynamic light scattering
Sieving
Sieving determines particlesize
distributions by measuring
the mass of material
that passes through progressively finer
meshes.The measurement range for
sieving is about 100 µm to 10 mm.
Key advantages. Sieving is a wellestablished
technology and forms the
original specification for many products.
Further, relative to other techniques,
it is simple, inexpensive, easy
to use and requires little training.
Limitations. Results are prone to
operator-to-operator variability, and
measurement times are relatively long
(5-10 minutes). The measurement
resolution can be poor, as the number
of size classes are few and wide
(defined by the number of sieves in
the stack), leaving the technique blind
to subtle differences in particles. Measurements
become more problematic
for finer particles because they tend
to agglomerate, causing sieve-blocking
and, possibly measuring agglomerates,
rather than primary particle
size. Rigorous sieve examination and
maintenance is essential for data integrity,
but is time-intensive.
Most common uses. Uses include
quality assurance (QA) and quality
control (QC) across the solids-handling
industries, most often in sectors
where profit margins are tight.
Electrophoretic light scattering
Automated imaging
Sedimentation
Electrozone sensing
Sieving
Table 1. CharaCTerisTiCs of ParTiCle-sizing TeChniques
Size Shape
Zeta
potential
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Dynamic
range
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to a minimum and at the same time,
increases repeatability and reproducibility.
Laser diffraction reports over
100 class sizes, providing good resolution
for detection of particle-sizedistribution
changes. Robust process
analyzers, for inline and online use,
enable application of the technique
from laboratory to process line, and
for automated process control.
Limitations. Samples must be diluted
for analysis, which may cause
particle-size changes (dilution shock).
The size-distribution calculation assumes
the measured particles are
spherical, so the results can be affected
by changes in particle shape.
Most common uses. The technique
is used as an alternative to traditional
manual methods in industries
from cement to pharmaceuticals, to
accelerate R&D, enhance product
quality, and to support automated
process control.
Laser diffraction
Laser diffraction generates particlesize
distributions from measurements
of the angular variation in intensity of
light scattered by a dispersed sample
when it passes through a laser beam.
The measurement range for laser diffraction
is about 0.01 µm to 3.5 mm.
Key advantages. Laser diffraction
measurements take less than a minute.
Calibration is not necessary, and
with a modern system, routine maintenance
requirements are minimal.
Full automation reduces manual input
46
Imaging
Automated imaging technology captures
images of individual particles,
and uses these to calculate numberbased
particle size and shape distributions.
The measurement range is
about 0.5 µm to 1 mm.
Key advantages. Automated imaging
produces microscope-quality
images of thousands of particles in
a few minutes. The added ability to
quantify shape permits the efficient
and robust differentiation of particle
types in a sample, (for example, agglomerates
from primary particles or
contaminants from product particles).
Compared with microscopy, imaging
is faster and less subjective.
Limitations. Automated imaging, a
laboratory technique, is slower than
techniques like laser diffraction, and
l l
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Rapid Resolution Sampling Wet Dry
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equipment costs are relatively high.
Most common uses. Uses include
product development, QC and process
troubleshooting, when size data
alone is insufficient, and in the development
of particle-sizing methods.
Dynamic light scattering
Dynamic light scattering (DLS) determines
the diffusion speed of particles
moving under Brownian motion from
measurements of light scattering intensity,
and converts this to a particlesize
distribution using the Stokes-Einstein
relationship. The measurement
range is about 0.3 nm to 10 µm.
Key advantages. DLS is non-invasive,
allows complete sample recovery
and offers fast, automated, high
throughput analysis. Required sample
volumes are very small (as little as
12µL) and with modern systems that
incorporate backscatter technology,
samples can be measured over a
wide range of concentrations. New
technology for online implementation
is an important advance for process
monitoring and automated control.
Limitations. Light scattering intensity
scales with particle size to the power
of six, so large particles in a sample
can dominate a result. Although the
accessible size range is good, the
measurement resolution can be poor,
especially when measuring polydisperse
(wide) size distributions.
Most common uses. DLS is used in
R&D, because of its ability to measure
at the nanoscale using small sample
volumes, but increasingly for process
monitoring as particle-size specifications
become finer, and for QC. Typical
samples include proteins, polymers,
emulsions and nanoparticles.
n
Editor's note: The material for this column was authored
by John Duffy, product marketing manager, Malvern Instruments
Ltd., (Malvern, U.K.; www.malvern.com).
ChemiCal engineering www.Chemengonline.Com marCh 2016
http://www.malvern.com http://www.Chemengonline.Com
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