Chemical Engineering July 2023 - 38

C. Minnich
FIGURE 3. The decision tree shown here indicates the selection of measurement position for process
analytical technologies (adapted from Ref. 9)
passed through a sample of the particles
of interest and their sizes are
calculated from the diffraction signal.
Specifically, the intensity of scattered
light at different angles is measured.
Large particles scatter light at
smaller angles relative to the laser,
while smaller particles scatter light at
larger angles.
Applications with high particle
concentrations, like in many industrial
processes, lead to multiple scattering,
which is still not well understood.
Therefore, laser diffraction
has to be applied in a bypass line
(discussed later) if applied to an industrial
production processes. Still
significant measurement errors were
reported using LD in industrial applications.
Grumbach and Czermak
[7] report that online-applied LD
responded differently than the offline
system. The bypass or online
system was more sensitive to larger
particles and predicted a larger particle
size and a wider distribution
than the laboratory equipment. Constant
bias correction worked only as
long as the d50 remained constant.
Especially the error at the edges of
the size distribution remained high.
Furthermore, the tested bypass systems
were more sensitive to process
and raw-material variability, so the
pre-filtering of data, as well as slopebased
bias correction, were necessary
to align with the offline system
[8]. LD technology is considered a
one-dimensional (1D) technology, as
it does not provide any information
about the shape of the particles.
Photosedimentometry. Photosedimentometry
is a technique for measuring
the size and distribution of
particles in suspensions. It works by
shining a light (laser, X-ray or both)
through the suspension and measuring
the amount of light scattered
by the particles. Based on this measurement,
the size and distribution
38
of the particles
are approximated.
The advantages of photosedimentometry
are its simplicity and low
cost. The instrumentation is relatively
simple and does not require specialized
training, making it easy to operate
and maintain. However, note that
photosedimentometry has some significant
limitations as well. It provides
a very coarse signal, which has to be
interpreted by the operator or analysis
software. Therefore, the absolute
values and their precision are very
limited and the system is usually only
used as a trending tool rather than a
full particle size and shape distribution
measurement system.
Ultrasound attenuation spectroscopy.
Ultrasound attenuation
spectroscopy (UAS) can be used to
measure particle-size distribution in
colloids, dispersions and emulsions.
UAS works by transmitting highfrequency
sound waves through the
suspension of particles and measuring
the attenuation or weakening of
the signal as it passes through the
suspension. An ultrasonic transducer
is placed in the reaction vessel, and a
pulse of sound is transmitted through
the suspension of particles. The signal
received by the transducer is
then analyzed to determine the attenuation
caused by
the particles. From
these data, the particle-size
distribution
can be calculated
using mathematical
models. In cases
where the measurement
gap (distance
between emitter and
sensor) is affected
by strong fouling, a
bypass solution is
often necessary to
successfully apply
UAS. The UAS technology
is considered
Shutterstock
FIGURE 4. Real-time particle sizing has become a crucial tool in the polymer
industry because of its numerous advantages, including better control
of the polymerization process
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2023
a 1D technology, because it does not
provide any information about the
shape of the particles.
Laser backscattering. Laser backscattering
(LBS) technology works
by emitting a focused laser beam
into the reaction vessel and measuring
the backscattered light as particles
pass through the beam. As particles
pass through the beam, they
scatter the light, creating a unique
signal that can be analyzed to determine
the chord length distribution.
The probes can usually be easily
placed in the reaction vessel using
a standard flange. The challenge is
to correctly differentiate background
scattering signals from any specific
particle signal, and then to interpret
the chord length into a particle size.
As the laser beam might interact only
with a short edge of the particle, the
chord length distributions are usually
undersizing the actual particlesize
distribution. LBS technology is
also considered a 1D technology, as
it does not provide any information
about particle shape.
Inline microscopy. The last
discussed
technology in this article are
process implementations based on
inline microscopy (IM), coupled with
automated image analysis. Inline
microscopy allows for the real-time
monitoring of particle size and shape
during the production process. This
enables operators to make adjustments
to reaction conditions in real
time, which helps to ensure consistent
product quality. The technology
nowadays usually comes with cameras
ready for high-resolution images.
On these, individual particles
are identified and evaluated as oversized
particles, agglomerates, or

http://WWW.CHEMENGONLINE.COM

Chemical Engineering July 2023

Table of Contents for the Digital Edition of Chemical Engineering July 2023